eChapters

1.1. What is Economics?
If we see our day-to-day activities, most of which are related to earning one’s own living and to the manner of satisfying one’s own wants. These human activities, which are generally called economic, are mainly related to production, distribution, consumption and exchange of goods and services. The scientific study of various problems arising out of these economic activities is called economics.
1.2. Basic Economic Problem
The basic economic problem is Human wants are many; but means to satisfy these wants are limited. These limited means are capable of alternative uses and hence there is a need of satisfying maximum wants with limited resources. In reality most resources are scarce. Similarly, Individuals and nations possess boundless desires, yet they are constrained by a finite pool of resources, including labor, machinery, factories, and raw materials. These scarce resources can be allocated in multiple ways to fulfill various needs. Likewise, individuals have limited financial means but an array of wants they aspire to satisfy, necessitating careful decision-making regarding resource utilization.
For instance, a farmer has the option to cultivate crops such as paddy, sugarcane, banana, or cotton on his garden land. However, the choice of crop must be made based on the availability of irrigation water, as different crops have varying water requirements. More example are shown in figure 1.

Every science has its own language. Economics has its own language. There are certain terms which are used in a special sense in economics. So we must understand the meaning of some basic concepts in the field of economics are discussed below.
2.1. Goods
Anything that can satisfies a human want is called a good in economics. For example: Foodgrain, pulse, oilseeds, machinery, seeds, fertilizer, pen, book, etc.
2.2. Services
A service refers to an act or performance that one party provides to another. Services are characterized by their intangible, non-material nature, as well as their inseparability, variability, and perishability. Examples include the services offered by doctors, teachers, lawyers, engineers, and laborers.

3.1. Agriculture
• The term agriculture originates from the Latin words ager, meaning soil, and cultura, meaning cultivation.
• In its broadest sense, agriculture refers to the cultivation of crops and the rearing of livestock for production.
• It is essentially synonymous with farming, involving the field-based production of food, fodder, and industrial organic materials.
3.2. Economics • Economics is the science that studies as how people choose to use scare productive resources (like land, labour, capital and organiser) to produce various goods and to distribute these goods to various members of the society for their consumption.
3.3. Agricultural Economics
• Agricultural economics is a branch of applied economics that applies the principles of choice to the efficient use of scarce resources such as land, labor, water, seeds, and fertilizers in farming and related activities.
• It focuses on guiding farmers in the optimal utilization of land, labor, and capital.
• Its significance lies in providing practical solutions to help farmers maximize income while managing limited resources effectively.
3.4. Definitions
1. Prof. Gray has defined agricultural economics as “The science in which the principles and methods of economics are applied to the special conditions of agricultural industry”.

4.1. Introduction
The agricultural sector plays a vital role in a nation’s economic development. It has been a key driver of prosperity in advanced economies and remains essential for the progress of developing nations. In regions with low per capita real income, agriculture and other primary industries are given high priority. As Dr. Bright Singh emphasizes, “An increase in agricultural production, along with a rise in the per capita income of the rural population, fosters industrialization and urbanization, thereby boosting demand for industrial goods.”
The historical trajectory of England provides clear evidence that the Agricultural Revolution preceded and laid the foundation for the Industrial Revolution. Similarly, in the United States and Japan, agricultural advancement has played a pivotal role in fostering industrialization. Many developing nations, in their pursuit of economic growth, have recognized the limitations of an excessive focus on industrialization as a sole strategy for achieving higher per capita real income.
Industrial and agricultural development are not substitutes but rather complementary forces that mutually reinforce each other in terms of both inputs and outputs. Empirical observations indicate that enhanced agricultural productivity and output significantly contribute to a nation’s overall economic progress. Therefore, prioritizing the further development of the agricultural sector is both rational and essential.

5.1. Meaning
Demand in economics means a desire to possess a good supported by willingness and ability to pay for it. If you have a desire to buy a tractor, but do not have the adequate means to pay for it, it will simply be a wish, a desire or a want and not demand. Demand is an effective desire, i.e., a desire which is backed by willingness and ability to pay for a commodity in order to obtain it. In the words, “Demand means the various quantities of a good that consumer willing and able to purchase at various price during a particular period of time in a given place/market”. Thus, demand in economics implies both the desire to purchase and the ability to pay for a good.
5.2. Definitions
1. According to Prof. J M Keynes, “emphasized the importance of effective demand, which is the total amount of goods and services that consumers are willing and able to purchase at various price levels, taking into account their income and other factors”.
2. According to Alfred Marshall, “the amount demanded of a commodity is the amount that buyers are able and willing to purchase at a given price in a given time period.”
3. According to John Maynard Keynes: “demand as the total amount of goods and services that individuals, businesses and the government are willing and able to purchase at various price levels”.
4. According to Milton Friedman “demand as the quantity of a good or service that people are willing to buy at a specific price, influenced by their utility and income”.
5. According to Paul Samuelson “demand as the quantity of a product that consumers are willing to buy at different prices, holding other factors constant”.

6.1. Concept of Utility
In the ordinary language, ‘utility’ means ‘usefulness’. In Economics, utility is defined as the power of a commodity or a service to satisfy a human want. Utility is a subjective or psychological concept. The same commodity or service gives different utilities to different people. For a vegetarian, mutton has no utility. Warm clothes have little utility for the people in hot countries. So utility is determined by the consumer and his need for the commodity.
Total Utility
Total Utility is denoted by the sum of utilities of all units of a commodity consumed. For example, if a consumer consumes ten biscuits, then the total utility is the sum of satisfaction of consuming all the ten biscuits.
Marginal Utility
Marginal Utility is the addition made to the total utility by consuming one more unit of a commodity. For example, if a consumer consumes 10 biscuits, the marginal utility of 10th unit is nothing but the total utility of 10 biscuits minus the total utility of 9 biscuits.
6.2. Law Of Diminishing Marginal Utility
The law of diminishing marginal utility explains an ordinary experience of a consumer. If a consumer takes more units of a commodity, the additional utility he derives from an extra unit of the commodity goes on falling. Thus, according to this law, the marginal utility decreases with the increase in the consumption of a commodity. When marginal utility decreases, the total utility increases at a diminishing rate.

7.1. Introduction
The concept of consumer’s surplus was first introduced by Alfred Marshall. When a consumer plans to buy a commodity, they assess the utility they will gain from its consumption. A rational consumer always compares the utility derived from a good with the price they must pay for it. If the utility exceeds the price, the consumer chooses to buy it; otherwise, they do not. The excess utility gained over the price paid is known as consumer’s surplus.
In nutshell, Consumer’s surplus is the difference between the amount a
consumer is willing to pay for a good and the amount they actually pay.
Consumer’s surplus = Price that a consumer is willing to pay - Price he actual pays
Suppose a consumer wants to buy a shirt and is willing to pay `250 for it, but the actual price is only `200. In this case, the consumer gains a surplus of `50. This difference, known as consumer’s surplus, represents the extra benefit the consumer receives from the purchase.
Consumer’s surplus is often experienced with commodities that are highly useful but relatively inexpensive, such as newspapers, salt, matchboxes, and postage stamps. In these cases, consumers are willing to pay more than the actual price, resulting in a surplus of utility

8.1. Meaning
The Law of Demand states that when the price of a good increases, its quantity demanded decreases, assuming all other factors remain constant. However, the law does not specify how much the quantity demanded will decrease in response to a price increase or how sensitive demand is to price changes. This information is crucial for businesses and policymakers. To address this, Alfred Marshall developed the concept of elasticity of demand, which measures the responsiveness of quantity demanded to changes in factors such as price, income, and the price of related goods.
8.2. Type of Elasticity of Demand
There are three types of elasticity of demand viz., 01) Price Elasticity of Demand 02) Income Elasticity of Demand 03) Cross elasticity of Demand
1. Price elasticity of Demand
Price elasticity measures the responsiveness of the quantity demanded of a commodity to changes in its price, while keeping all other factors constant (ceteris paribus).

9.1. Production
Production means creation of value in the goods, i. e. creation of utility. Production activity helps in transforming a set of inputs into goods and services. It essentially means transforming of one set of goods into another. The output generated from production possesses greater utility than the combined inputs used in the production process. The inputs that are utilized in production of goods may be provided by the nature and/ or by other industries.
9.2. Factors of Production
These mean the production resources required to produce a given product. Fraser defined factor of production as “a group or class of original productive resources”. The factors of production have been traditionally classified as land, labour, capital and organization.
Land
According to Marshall, land means “the materials and forces which nature gives freely for man’s aid, in land and water, in air and light and heat.” In Economics the term land has a very broad meaning. It includes all free gifts of nature available to mankind viz., air, water and land. It includes all types of land surfaces such as mountains, valley, plains, forests etc. It includes all types of water resources such as rivers, oceans, etc. Thus the concept of land includes all natural resources on, above and below the earth’s surface.

10.1. Introduction
In the production process, the farmers combine the required input factor in various proportion. This type of usage of input by the farmer gives way for the operation of law of returns. In the production process, when the single input factor is varied keeping other required factor constant, the relationship take place between single variable input and the consequent output pertain to either one or a combination of the following relationship.
1. Law of increasing returns;
2. Law of constant returns; and
3. Law of decreasing returns.
10.1.1. Law of increasing returns
According to the law of increasing returns, each additional (or marginal) unit of input leads to a proportionally larger increase in output compared to the previous unit. In other words, the productivity of inputs rises as more units are employed.

11.1. Introduction
The input-output relationship is commonly referred to as the Classical Production Function.
The Classical Production Function is also known as the Law of Diminishing Returns or the Law of Increasing Costs.
It is sometimes termed the “Law of Life”, as it serves as a fundamental principle of economics upon which several other economic laws are based.
This law analyzes the relationship between a single variable factor and output while keeping all other factors constant.
It represents a short-run production function.
11.2. Definition
The Law of Increasing Cost states that the proportion of one factor in a combination of factors increases till a certain point but after that first the marginal product then the average product will diminish.
According to Marshall “An increase in capital and labour applied in the cultivation of land causes in general less than proportionate increase in the amount of produce raised, unless it happens to coincide with the improvements in the arts of agriculture”.
According to Earl O. Heady “If the quantity of one of productive service is increased by equal increments, with the quantity of other resource services held constant, the increments to total product may increase at first but will decrease after certain point”.

12.1. Meaning
The rate at which output change as the quantity of all input are change. It is based on long run law in which all factor are variable and factor proportion do not change.
Stage of Return to Scale
1. Constant return to scale
2. Increasing return to scale
3. Decreasing return to scale
1. Constant return to scale
• In constant return, Change in proportion of input is equal to the change in proportion of output.
• A constant return to scale is also known as linear homogenous production function of the first degree.
• Example: Industry.

13.1. Introduction
Knowledge regarding various relationships existing between costs and output is necessary to comprehend the concepts of equilibrium conditions of different firms under different market conditions. Basically we require data on output, fixed costs, variable costs and the prices of inputs and outputs. From this data we finally derive all the seven cost concepts, viz.,TFC, TVC, TC, AFC, AVC, AC and MC. These cost concepts would have implications for output expansion of the firms and equilibrium position of the firms in different time periods. In the cost theory, economists use different names for cost concepts under different context. They are money costs or nominal costs, real costs, opportunity costs, economic costs, implicit costs, explicit costs, deflated costs, social costs, short run costs, long run costs, separable costs, etc.
Nominal Costs or Money Costs
These are usually expressed in money terms at current prices. Nominal costs of production refer to per unit cost of production of output at current market prices.
Real Costs
When the costs of inputs and input services are expressed at constant prices they become real costs.

14.1. Meaning
Supply in agriculture refers to the availability of agricultural products, both raw and processed, for distribution to markets, consumers, or other industries. This includes crops, livestock, dairy products, fruits, vegetables, and other agricultural goods. “Supply is the amount of a commodity that sellers are able and willing to offer for sale at different price per unit of time”.
In the words of Meyer “Supply is a schedule of the amount of a good that would be offered for sale at all possible price at any period of time; e.g., a day, a week, and so on”.
14.2. Stock
Stock in agriculture refers to the quantity of inputs and resources available for farming, such as seeds, fertilizers, pesticides, machinery, and livestock. It encompasses all the assets and materials that a farmer or agricultural operation has on hand to support the production process.
“Stock is meant the total quantity of a commodity this exists in a market and can be offered for sale at a short notice”. 14.3. Stocks Vs Supply
• Supply may be defined as the active stock (actual quantity) which is made available for the consumers at the prevailing market prices.
• Supply is flow concept.
• Stock is the quantity of different commodities which are stored; they are made available to the consumer when there are favourable prices.

15.1. Meaning
While doing production of goods, the producer coordinates different factors of production i.e., land, labour, capital and management. In the process of distribution the returns obtained through the production activity are apportioned to these factors that are employed in the production process. Consequently land gets rent, labour gets wages and interest is paid to capital and finally organization is rewarded with profit. Such an apportionment of returns among different factors of production is called distribution. It is also called factor pricing.
15.1.2. Rent
Rent is the return for the fertility status of the land. In fact the land is defined in a broad sense. All the natural resources existing on the surface and beneath the surface of land like mines, rivers, etc., are also treated as land, from which rent is received. Some resources are publicly owned, while others are privately owned. Rent is almost zero for publicly owned resource because one cannot use it for one’s own purpose. These are meant for public welfare. Rent is expressed in two forms i.e., one is economic rent and the other is contract rent.
15.1.3. Economic Rent
Economic rent is the rent received exclusively from the use of land only. We use the term, exclusively because the rent of a building referred as the return obtained by the owner on the capital invested in the construction of building as well as the land. It is the return obtained from the combined values of both. In that case, it no longer qualifies as rent. In farming the rent paid by a tenant to the landlord is not economic rent.

16.1. Meaning
National Income is nothing but the aggregate money value of all goods and services produced in a country in a year. It can also be viewed as income distributed among the factors of production in the form of rent (to land), wages (to labour) interest (to capital) and profits (to entrepreneurs).
According to Alfred Marshal National Income is defined as the labour and capital of a country acting on its natural resources produce annually a certain net aggregate of goods and services.
16.1.1. Limitations
• In an economy in which numerous goods and services are produced, it is really a difficult proposition to estimate them correctly to arrive at the national income. Coming to the farming, farmers do retain some part of production for personal consumption and the rest is only marketed. In such a case, the methodology to evaluate the amount of produce not reached the market has not been given.
• Double counting: It implies the same good being counted twice: say, cotton may be counted in agricultural production and the cotton cloth in industrial production.
Marshall approached national income from production end. On the other hand, some other economist approached from consumption end. At the same time this approach in reality poses problems. Consumers who number in millions, consumes the same good at different places, and the estimation of their total consumption is a difficult proposition. Hence, there are problems in measurement of national income.

17.1. Theories of Population
The theory of population is studied because the supply of labour depends upon population and its growth. Observing the abnormal growth of population and consequent fall in the standard of living, Thomas Malthus (1760 – 1834) an English Clergyman first studied population growth in various countries of Europe. Later he wrote a book entitled “An Essay on the Principles of Population” in 1798. His observations compelled him to foresee a gloomy future for the human race and hence emphasized the immediate need to keep the population growth under check. Now his theory is popularly known as Malthusian theory of population. To quote the theory in his own words “By nature human food increases in a slow arithmetic ratio, man himself increases in a quick geometric ratio unless want and vice stop him.”
17.1.2. Propositions of Malthusian Theory
These are briefly presented below:
1. Population is necessarily limited by means of subsistence (food supply). According to Malthus in a country the size of population depends on its food production. Greater the food production, larger would be the size of population, which a country can support and smaller the food production then the country would be in a position to support smaller population only.
2. The power of population is infinitely greater than the power in the earth to produce subsistence for men. He further adds that population growth always overtakes food production, since agricultural production is constrained by the law of diminishing returns. This prevents the food production to grow as faster as that of population. As such, there is no limit to the growth of population.
3. Population increases in geometric progression, whereas food production increases in arithmetic progression. This means that population tends to grow in quick geometric progression i.e., 1, 2, 4, 8, 16, 32, etc., while,

18.1. Meaning
When the human civilization was not developed, people used to exchange those goods which they produced for those which others produced. Such an act of exchanging goods for goods is called barter. But as the years rolled by and when the social organization became more complex, barter system was found to be not practicable. The following are the main difficulties which were found in the barter system.
1. Double coincidence of wants
2. Lack of a standard unit of account
3. Impossibility of subdivision of goods
4. Lack of information
5. Production of large and very costly goods not feasible
The difficulties in barter system were replaced with the introduction of money. Money has been defined as the medium of exchange. According to Robertson money is defined as “Anything which is widely acceptable in discharge of obligations”. The different stages in the development of money are as follows.
1. Commodity Money: The earliest form of money consisted of goods like rice, wheat, cattle, skins, elephant tusks etc. These were accepted as they were all desired by all the people.
2. Metallic Money: As the civilization advanced, people found it difficult to carry on the exchange transaction with commodities, as they were found to be very inconvenient. Commodity money gave way for the metals to be used as money. These metals include gold, silver, copper, bronze, etc. From the beginning of their introduction, Government kept the right to issue coins and certify their weight and quality. Metals were converted into coins for this purpose.

An economy is a human-made system developed or adapted to fulfill societal needs. There are three main types of economic systems namely mixed economy, capitalist economy, and socialist economy. The following are some key features of an economy:
• The type of an economy is determined by the ownership of resources and the means of production.
• Resources or means of production may be privately owned, allowing individuals the freedom to use them for profit, or they may be collectively owned and controlled by the government for the overall welfare of society.
• The three types of economic systems are classified based on the extent of individual freedom and the role of profit motive in each. 19.1. Capitalist Economy The capitalist, or free-market, economy is the most traditional form, where government involvement in economic activities is minimal.
Main features of a capitalist economy
1. Private property: In a capitalist economy, individuals have the right to own property, acquire assets, and utilize them for personal gain. There are no limitations on owning land, machinery, mines, or factories for profit and wealth accumulation.
2. Freedom of enterprise: Individuals have the freedom to choose any occupation they wish to pursue. The concept of free enterprise means that businesses are free to acquire resources and utilize them to produce any goods or services. Additionally, firms have the liberty to sell their products in any market of their choice, while workers have the right to select their employer.

20.1. Introduction
• The exchange of goods and services (trade) among countries across national boundaries is called as international trade.
• No Country can be self-sufficient in producing all the goods and services required by it.
• Thus, Countries need to trade to obtain commodities, they cannot produce themselves or they can purchase elsewhere at a lower price.
• The act of selling domestically produced goods to international consumer is known as exportation. India Export Refined Petroleum, Jewellery, Rice, etc.
• The act of purchasing goods from foreigne country in order to sell them domestically is known as importation. India import Crude oil, Coal, Gold, Electronics item etc.
20.2. Why International Trade?
• Countries cannot domestically produced all the goods and services that each individual consumer wants and needs.
• Because each country has different factor of production (Land, Labour, and Capital).
• Each country is endowed with different combination of these factors and therefore each country is capable of producing different combination of goods and services.
• When domestic production is greater than domestic demand for a good, excess production is traded on the international market

21.1. Meaning
Balance of payments is an accounting statement that provide a systematic record to all the economic transaction, between resident of a country and the rest of the world, in a specific time frame.
21.2. Economic Transaction
1. Visible Items: These include all types of physical goods which are imported and exported as they are made of some matter that can be seen, touched, measured.
2. Invisible Items: Invisible items of trade refers to all types of services like shipping , banking , etc…these are called invisible goods as they can’t be felt , seen ,and measured
3. Unilateral Items or transfer: Unilateral transfer include gifts, remittance,etc.. since these transaction donot involve any claims for repayments.
4. Capital transfer: Capital transfer relate to capital receipts (through borrowings or sale of assets)and capital payments (through capital repayments or purchase of assets)

22.1. Meaning
GST is a tax imposed on the purchase of goods and services. It is designed to be a single, comprehensive tax that replaces multiple smaller indirect taxes on consumption, such as service tax.
• The taxes are included in the final price and are to be paid by the consumers at point of sale and passed on to the government by the seller.
• The GST is a common tax used by the majority of countries globally.
• It is a unified tax applied to the supply of goods and services from the manufacturer to the final consumers.
• More than 160 developed countries implement this system of taxation.
• The GST is usually taxed at a single rate across the nation.
22.2. Main Features of Goods and Services Tax (GST)
The main features of Goods and Services Tax are as follows:
• Dual GST- India has implemented a dual GST system, consisting of two components: Central GST (CGST) and State GST (SGST). The central government collects CGST, while states and union territories collect SGST/UTGST on all supply transactions of goods and services. Following the 101st Constitutional Amendment, both the central government and union territories have the authority to levy taxes simultaneously on the supply of goods and services.
• Applicable on supply side- GST is applicable on “supply” of goods or services as against the old concept on the manufacturer of goods or on sale of goods or on provision of services.
• Destination based taxation- GST is based on the principle of destination-based consumption taxation as against the present principle of origin-based taxation.

1. Introduction
Aquaculture is rapidly emerging as one of the most important sectors in global food production, supplying an increasing share of the world’s demand for high-quality protein. As global populations rise and overfishing depletes wild fish stocks, aquaculture has become essential in addressing the global food security challenge. The expansion of aquaculture, however, presents a new set of challenges, particularly in the formulation and delivery of efficient and sustainable aquafeeds. Fish and crustaceans, like all living organisms, require precise nutrition for optimal growth, immune function, and reproduction. In commercial aquaculture, ensuring that farmed species receive the right balance of nutrients through formulated feeds is crucial to achieving high productivity and minimizing environmental impact.
`Traditional aquafeeds are designed to meet the nutritional needs of farmed species; however, they face several obstacles when it comes to efficient nutrient delivery. The aquatic environment poses unique challenges for feed stability and nutrient bioavailability, including:
• Water Stability: Aquatic feeds must resist rapid breakdown when exposed to water, ensuring that nutrients remain intact until they are ingested by fish or crustaceans.
• Nutrient Leaching: In water, nutrients can leach out of feeds before they are consumed, reducing their availability and increasing waste.
• Complex Dietary Requirements: Aquatic species have diverse and often complex dietary needs, which must be met through balanced and targeted feed formulations.

1. Introduction
Aquaculture, the farming of aquatic organisms such as fish, crustaceans, mollusks, and aquatic plants, plays an essential role in global food security and economic development. As the demand for sustainable and efficient fish farming practices grows, the need to optimize fish health, growth, and feed efficiency becomes increasingly crucial. One emerging area of interest in aquaculture nutrition is the use of bioactive peptides, which offer a natural and effective alternative to synthetic feed additives.
Bioactive peptides are short chains of amino acids, typically ranging from 2 to 20 residues, which are derived from protein sources. These peptides are typically inactive within the parent protein but become bioactive once they are released through specific processes, including enzymatic hydrolysis, microbial fermentation, or gastrointestinal digestion. The peptides exhibit a wide range of biological activities that can directly benefit fish and shellfish health. Their functional properties include antimicrobial, antioxidant, immunomodulatory, antihypertensive, and growth-promoting activities, making them valuable components of aquafeeds.
The integration of bioactive peptides into aquaculture systems offers an innovative and environmentally sustainable approach to enhancing the health and performance of aquatic species. By replacing or complementing conventional feed additives, such as antibiotics and synthetic growth promoters, bioactive peptides can improve feed efficiency, disease resistance, and immune function, thereby promoting better overall productivity.

1. Introduction
Aquaculture, the farming of aquatic organisms, has grown rapidly over the past few decades to meet the rising global demand for seafood. As wild fish stocks continue to decline due to overfishing and environmental pressures, aquaculture is playing a crucial role in food production. However, like other forms of agriculture, aquaculture faces significant challenges, with feed management being one of the most critical. Feed represents the single largest operational cost in aquaculture, often accounting for 50-70% of the total production expenses. Efficient feed utilization is therefore central to the economic viability of aquaculture operations. Yet, traditional feeding methods are often inefficient, leading to feed wastage, nutrient pollution in water bodies, and suboptimal growth in farmed fish.
In conventional aquaculture, feed is typically dispensed at set intervals and in quantities based on estimations of fish appetite and growth requirements. However, this approach has several inherent inefficiencies. Overfeeding or underfeeding is common due to the lack of real-time monitoring and precision. Excess feed that remains uneaten sinks to the bottom of ponds or cages, decomposing and contributing to nutrient pollution. This not only represents a direct economic loss but also creates environmental problems such as eutrophication, which can lead to algal blooms, oxygen depletion, and poor water quality. On the other hand, underfeeding can result in stunted growth, poor health, and reduced productivity of fish. Both overfeeding and underfeeding, therefore, represent significant barriers to achieving sustainability and profitability in aquaculture.

1. Introduction
Aquaculture has become one of the most significant contributors to global food production, responding to the escalating demand for fish and seafood. As a rapidly growing industry, aquaculture plays a crucial role in ensuring food security, nutrition, and livelihoods. According to the Food and Agriculture Organization (FAO), aquaculture production has expanded at an average annual rate of 5.3% over the past 30 years. This growth has positioned aquaculture as a primary source of protein, essential fatty acids, and micronutrients for billions of people worldwide. Despite its rapid expansion, the aquaculture industry faces significant environmental challenges, primarily due to the large amounts of organic waste generated by fish farming. This waste includes uneaten feed, fish excreta, and the remnants of dead organisms, all of which can contribute to nutrient pollution when not properly managed. Nutrient pollution is associated with serious ecological consequences such as eutrophication, where excess nutrients lead to algal blooms, oxygen depletion, and the degradation of aquatic habitats. These environmental issues threaten biodiversity and undermine the sustainability of aquaculture systems.
Nutrient recycling has emerged as an innovative and sustainable solution to address these challenges. This approach transforms waste into valuable resources, particularly in the form of nutrient-rich feed ingredients. By harnessing nutrients such as nitrogen, phosphorus, and organic matter from aquaculture waste, nutrient recycling not only reduces environmental impact but also improves the economic efficiency of aquaculture operations. Nutrient recycling promotes a circular economy model, where waste from fish farming is reintroduced into the system, closing nutrient loops and reducing the reliance on external inputs.

1. Introduction
Aquaculture, the farming of aquatic organisms such as fish, shellfish, and aquatic plants, has become one of the fastest-growing food production sectors globally. This growth has been driven by the increasing demand for high-quality protein to feed a growing global population, coupled with the depletion of wild fish stocks due to overfishing. As the aquaculture industry expands, so too does the need for sustainable practices that balance productivity with environmental and animal welfare considerations. One of the critical challenges faced by modern aquaculture is the need to maintain fish health and growth performance while minimizing the industry’s ecological footprint. Traditionally, synthetic additives such as antibiotics, chemical growth promoters, and other artificial substances have been used to improve fish health, enhance growth rates, and prevent disease outbreaks. However, the excessive use of these chemicals has led to several issues, including antibiotic resistance, chemical residues in fish products, and environmental pollution. These concerns have spurred a shift toward more sustainable, eco-friendly alternatives in aquaculture production systems.
In this context, phytogenics—natural, plant-based additives—have garnered increasing interest as a promising solution. Phytogenics are bioactive compounds extracted from various plant sources, including herbs, spices, and essential oils, that possess multiple beneficial properties. These compounds have been used in traditional medicine and animal husbandry for centuries due to their antimicrobial, antioxidant, anti-inflammatory, and immunomodulatory effects. By leveraging the natural bioactive compounds in plants, phytogenics offer a range of advantages for aquaculture, including enhanced growth performance, improved feed efficiency, strengthened immune responses, and reduced disease susceptibility. The need for sustainable and natural alternatives to synthetic additives is becoming more urgent as the global consumer base becomes more health-conscious and environmentally aware. Consumers today increasingly demand seafood products that are free from chemical additives

1. Introduction
Reproduction is one of the most critical processes in the life cycle of fish, directly influencing population sustainability, species survival, and aquaculture productivity. Successful reproduction hinges on several interconnected factors, including environmental conditions (such as water quality, temperature, and photoperiod), genetic predisposition, physiological health, and, most importantly, nutrition. Among these, nutrition has a profound and direct influence on reproductive performance in both wild and cultured fish populations.
In aquaculture, the reproductive success of broodstock—mature fish that produce offspring—is paramount to the production of high-quality fry (young fish) and larvae, which in turn affects the overall yield and profitability of fish farms. Ensuring that broodstock are provided with an optimal diet tailored to their reproductive needs can significantly enhance fecundity (the number of eggs produced), fertilization rates, and the viability of offspring. Proper nutrition not only contributes to better gamete (egg and sperm) quality but also plays a vital role in boosting the health, vitality, and resilience of both broodstock and their progeny.
Nutritional deficiencies or imbalances during the reproductive phase can lead to a variety of problems, such as reduced spawning frequency, poor egg and sperm quality, low hatching rates, and poor larval survival. Conversely, well-nourished broodstock with access to essential nutrients are more likely to produce high-quality eggs and sperm that result in healthier, more viable offspring.

1. Introduction
Enzymes are specialized proteins that act as biological catalysts, facilitating and accelerating biochemical reactions within living organisms. They are essential for various physiological processes, including digestion, metabolism, and energy production. In fish and other aquatic animals, enzymes play a pivotal role in breaking down complex nutrients found in their diets into simpler, absorbable components. This enzymatic action is vital for maximizing nutrient utilization, promoting growth, and ensuring overall health. The inclusion of enzymes in aquaculture feeds has garnered significant attention over recent years, particularly as the industry strives to meet the increasing global demand for fish and seafood products. With aquaculture now accounting for over 50% of the fish consumed worldwide, there is an urgent need for innovative strategies that enhance the sustainability and efficiency of fish farming operations. One such strategy is the supplementation of feeds with exogenous enzymes, which has been shown to improve feed digestibility, promote better growth performance, and mitigate environmental impacts associated with aquaculture.
1.1. The Role of Enzymes in Nutrient Digestion
Fish diets often consist of a combination of protein, carbohydrates, and lipids sourced from both animal and plant materials. However, many of these feed ingredients, especially plant-based sources, contain complex macromolecules that can be difficult for fish to digest. Fish possess a limited capacity for endogenous enzyme production, which can hinder their ability to effectively break down these complex nutrients. As a result, poorly digested feed not only leads to inefficient nutrient absorption but also contributes to increased waste production, further exacerbating water quality issues in aquaculture systems.

1. Introduction
The early life stages of fish, particularly the larval and juvenile phases, are critical periods in aquaculture characterized by rapid physiological changes, high metabolic demands, and heightened vulnerability to external stressors. Mortality rates during these stages can be alarmingly high, with reports indicating that up to 70% or more of fish larvae and juveniles may not survive to reach adulthood. These losses are typically caused by a combination of nutritional deficiencies, environmental stress, suboptimal husbandry practices, and disease outbreaks. Addressing these challenges is paramount for improving survival rates and ensuring the success of aquaculture systems. Nutritional management during the larval and juvenile stages plays a pivotal role in determining the overall health, growth performance, and survival of fish. Fish larvae and juveniles have distinct dietary requirements compared to adults due to their rapid growth, underdeveloped digestive systems, and the need for specific nutrients to support organ development, skeletal formation, and immune system maturation. A comprehensive understanding of these nutritional needs is essential for developing feeding strategies that optimize fish health and maximize growth potential.
1.1. Importance of Early Nutrition in Fish Development
The nutritional status of larval and juvenile fish significantly influences their physiological development and long-term survival. During the early stages of life, fish undergo rapid cell proliferation and differentiation, especially in tissues such as the muscle, digestive organs, and immune system. Proper nutrition is essential to support these processes, ensuring that fish achieve optimal growth rates and develop strong resistance to environmental stressors and diseases.

1. Introduction
Aquaculture has emerged as a vital sector in global food production, with the increasing demand for fish and seafood as a primary source of protein for a rapidly growing population. According to the Food and Agriculture Organization (FAO), aquaculture contributes significantly to global fisheries production, accounting for nearly 50% of the total fish consumed worldwide. This remarkable growth trajectory is driven by the need to meet food security challenges while providing sustainable and nutritious food sources. Despite its importance, intensive aquaculture practices present numerous challenges that can adversely affect fish health and productivity. Fish raised in aquaculture systems are often subjected to a variety of environmental stressors, including poor water quality, high stocking densities, rapid growth rates, and fluctuations in temperature. These stressors lead to increased metabolic activity in fish, which can elevate the production of reactive oxygen species (ROS) — highly reactive molecules that can cause cellular damage. Under normal physiological conditions, the production of ROS is counterbalanced by the antioxidant defense mechanisms in fish. However, when the production of ROS exceeds the capacity of these defense systems, oxidative stress ensues.
Oxidative stress is a detrimental condition characterized by an imbalance between ROS generation and the antioxidant capacity of an organism. In fish, this condition can manifest in various forms, including cellular damage, lipid peroxidation, protein oxidation, and DNA damage. The consequences of oxidative stress are severe, resulting in impaired growth, compromised immune responses, increased susceptibility to diseases, and ultimately higher mortality rates. Understanding the underlying mechanisms of oxidative stress in aquaculture is critical for developing effective strategies to mitigate its impact and improve fish welfare.

1. Introduction
The aquaculture industry has experienced unprecedented growth in recent decades, emerging as a critical sector in global food production. According to the Food and Agriculture Organization (FAO), aquaculture is the fastestgrowing food production system worldwide, with an annual growth rate of approximately 5-6%. This remarkable expansion is primarily driven by the increasing demand for seafood, fueled by factors such as population growth, urbanization, and changing dietary preferences towards healthier protein sources. As seafood consumption continues to rise, the aquaculture sector faces the challenge of producing fish that meet consumer demand while ensuring sustainable practices that protect marine ecosystems.
A key component of fish production is nutrition, which directly influences fish growth, health, and overall productivity. Among the essential nutrients required by fish, lipids play a pivotal role in providing energy, supporting metabolic functions, and delivering vital fatty acids that are crucial for growth and development. Traditionally, fishmeal and fish oil derived from wild-caught fish have served as the primary lipid sources in aquaculture feeds. These sources are rich in high-quality protein and essential omega-3 fatty acids (n-3 PUFA), which are crucial for the optimal health and growth of various fish species.

1. Introduction
Feed formulation for carnivorous fish species is a pivotal aspect of aquaculture that significantly impacts growth performance, health, and the sustainability of production systems. As the global demand for seafood continues to rise, driven by population growth and changing dietary preferences, optimizing feed formulations becomes increasingly essential for aquaculture operators. Efficient feed formulations are crucial not only for enhancing fish growth rates and feed conversion efficiency but also for ensuring the economic viability of fish farming operations.
Carnivorous fish species, such as salmon, trout, tilapia, and catfish, have specific nutritional requirements that must be met to achieve optimal performance. These requirements typically include high-quality protein sources, essential fatty acids, vitamins, and minerals. The formulation of diets that adequately supply these nutrients while minimizing costs is a complex challenge that aquaculture practitioners face. Traditional feed ingredients, including fishmeal and fish oil, are often expensive and subject to supply fluctuations, prompting a need for innovative alternatives that can provide similar nutritional profiles at a lower cost.
This chapter delves into the intricacies of feed formulation for carnivorous fish species. It begins by outlining the nutritional requirements specific to these fish, emphasizing the importance of protein and fatty acid sources. The chapter then discusses various feed ingredients, both traditional and alternative, that can be used to formulate cost-effective diets without compromising fish health and performance. Additionally, it addresses the challenges associated with formulating diets for carnivorous species, including ingredient variability, palatability, and the potential for anti-nutritional factors.

1. Introduction
Aquaculture has emerged as one of the fastest-growing sectors of food production worldwide, driven by the increasing demand for fish and seafood as essential sources of protein, healthy fats, and micronutrients. As wild fisheries face overexploitation and environmental degradation, aquaculture is seen as a solution to meet the growing global need for seafood. However, the sustainability of aquaculture itself is under scrutiny due to its reliance on traditional feed ingredients, particularly fishmeal and fish oil derived from wild-caught fish. Fishmeal has long been considered an ideal protein source for aquafeeds because of its excellent amino acid profile, high digestibility, and essential fatty acid content. Yet, the continued use of fishmeal places immense pressure on wild fish stocks, particularly small pelagic species that are harvested specifically for fishmeal production.
The overexploitation of these fish stocks raises significant ecological concerns, threatening marine biodiversity and destabilizing food webs. The reliance on fishmeal is also economically challenging, as fluctuating supply and increasing demand have led to rising costs and price volatility. Moreover, as the aquaculture industry continues to expand, the demand for fishmeal will surpass the sustainable supply, exacerbating these environmental and economic issues. In response to these challenges, researchers and industry stakeholders have been actively seeking sustainable and cost-effective alternatives to fishmeal. These innovative protein sources are not only essential for reducing the environmental impact of aquaculture but also for ensuring the long-term viability of the industry. The development of alternative protein sources must consider several factors, including their nutritional composition, availability, cost-effectiveness, and environmental footprint.

Introduction
Over the past few decades, fisheries nutrition has undergone a remarkable transformation, driven by the rapid expansion of global aquaculture and the increasing need for sustainable seafood production. As wild fish stocks have become overexploited and the demand for protein-rich aquatic food sources continues to rise, aquaculture has emerged as a vital solution for meeting the growing global need for fish and seafood. By 2014, aquaculture production had surpassed wild-caught fisheries in providing fish for human consumption, marking a significant shift in how we obtain seafood. This growth, however, brings with it numerous challenges, particularly regarding the nutrition of farmed fish.
Fisheries nutrition, as a specialized field, plays a pivotal role in supporting aquaculture’s ability to provide healthy, high-quality fish for human consumption while maintaining environmental balance. As aquaculture intensifies, the demand for high-quality, cost-effective feed ingredients has surged. Traditional reliance on marine-derived ingredients like fishmeal and fish oil is becoming increasingly unsustainable, as these resources are finite and face depletion due to overfishing. This has resulted in rising costs for these inputs, placing economic pressures on aquaculture producers. Additionally, marine-derived ingredients contribute to ecological imbalances, prompting the need for alternative solutions in feed formulation.
At the same time, the environmental impacts of aquafeeds have garnered increased scrutiny. The production of feed ingredients such as soybeans, corn, and other crops can contribute to deforestation, water pollution, and greenhouse gas emissions, undermining efforts to achieve environmental sustainability in aquaculture. Nutrient waste from uneaten feed and fish excreta can also lead to eutrophication in aquatic ecosystems, damaging water quality and biodiversity. Therefore, fisheries nutrition must address not only the nutritional requirements of fish but also the broader ecological footprint of aquafeed production and usage.

1.1 Carbohydrates Biochemistry
Carbohydrates are essential for life, both in plants and in animals. They are organic chemical compounds made of C, H, and O. These elements get arranged in different ways so that several simple, and stored complex compounds (starch in plants and glycogen in animals) are available in abundance on the Earth. They are involved in enormous biological and nonbiological functions. Their biological functions, found in all living things, are the chief energy source for men and animals, even for herbivorous fishlike carp and rohu fingerlings. Carbohydrates are also essential structural components like nucleic acids, containing genetic information. One must recognize the protective function in cells, plants, and human health management. Even bacteria form a glycocalyx and biofilm for protection. Knowledge about their involvement in blood groups helped to identify universal donors, universal recipients, and genuine donors for the recipient to save a life. Carbohydrates are found as a storage structure for energy in emergency conditions.
Upon combustion, they yield carbon dioxide and water. Autotrophs (plants) synthesize them from carbon dioxide and water to grow, reproduce, and store, but heterotrophs (humans and other animals) use them, too. In plants, they occur as fibers, supporting structures, and simple sugars. The plant carbohydrates are used for preparing adhesives, fermentation, foods, and papers. Cotton fibers are used to make clothes to cover the body and protect humans and animals against adverse climatic conditions.

Carbohydrates are the most abundant class of organic compounds found in living organisms. Carbohydrates are also called sugars and are found in plants and animals. Plants synthesize them through photosynthesis (endothermic) by fixing carbon, hydrogen, and oxygen in the presence of solar energy and chlorophyll pigments.
nCO2 + nH2O + energy → CnH2nOn + nO2
Carbohydrates are stored as starch in plants as a source of energy. They are also stored in animals as glycogen in muscles and the liver. They are also found in skeletal structures of cell walls in plants (cellulose), bacteria (peptidoglycans), insects, and crustaceans. This stored food is consumed by animals and man, and upon their oxidation in the body, energy is released for various metabolic activities (work, growth, and various productive functions). Compositionwise, plants are more prosperous in carbohydrates (30%) than animals (1%), with water at 60% and ash at 4%. Plants are poor in proteins (5%) and lipids (1%) as compared to proteins (20%) and lipids (15%) in animals.
2.1 Introduction to Carbohydrates
Carbohydrates or saccharides (sakcharon=sugar or sweetness) are a chemical term applied to a large group of substances, which are organic compounds containing elements Carbon, Hydrogen, and Oxygen in a ratio of 1:2:1. Hydrogen and Oxygen atoms are in the ratio of two to one as in water molecules. As the ratio of hydrogen to oxygen is like H2O, they are also known as “hydrates of carbon” and are represented as (CH2O)n. But there is an exception to this general formula. Deoxyribose (C5H1OO4) and rhamnose (C6H12O5) do not have the required ratio of 2:1 for hydrogen to oxygen. On the other side, some compounds possess this required ratio but are not carbohydrates, e.g., formaldehyde (CH2O), acetic acid (C2H4O2), and lactic acid (C3H6O3). Further, some carbohydrates have nitrogen(glucosamine), phosphorus, and sulfur but do not satisfy the above general formula. The simplest sugars are called monosugars or monosaccharides. There are two main functional groups in

Hydrates carbons (saccharides) found in nature (animals and plants) are classified into three groups based on the number of monosaccharides found or released upon acid hydrolysis. Monosaccharides the groups of carbohydrates.
3.1 Formulation of Monosaccharides
The molecular formula of any monosaccharide (mono=one and saccharide=sugar) is Cn(H2O)n or (CH2O)n. Rhamnose and Deoxyribose are exceptions to the general formula of carbohydrates. Acetic acid (C2H4O2) and formaldehyde (CH2O) are not carbohydrates but have the formula Cn(H2O)n or (CH2O)n. Fitting and Baeyer proposed the formula CHO-(CHOH)4-CH2OH, which indicates the presence of aldehyde and five hydroxyl groups.
More than one monosaccharide molecule unites to form disaccharides, oligosaccharides, and polysaccharides. Monosaccharides, being simpler units, cannot be broken down further. Generally, they have two to ten carbon atoms per molecule (dioses to decoses). The monosaccharides may have either aldehyde or ketone groups. The one possible aldodiose (CH2OH.CHO) is glycolaldehyde (2-hydroxyethanal), but ketodiose is not possible since there are only two carbons. The glucose molecular formula C6H12O6 represents the number of C, H, and O atoms but not groups. However, the proposed formula, CHO-(CHOH)4-CH2OH, indicates the presence of an aldehyde and five hydroxyl groups. These simple sugars or monosaccharides have carbon backbones linked by single bonds with hydrogen and hydroxyl groups attached in an unbranched manner.

Oligosaccharides comprise 2 to 10 units of monosaccharide molecules, which are released upon hydrolysis. The oligosaccharides are classified depending upon the number of monosaccharide units, e.g., Disaccharides (two monosaccharide units), Trisaccharides (three monosaccharide units), Tetrasaccharides (four monosaccharide units). Disaccharides are the most common oligosaccharides found in nature.
4.1 Disaccharides
Disaccharides are formed when glycosidic linkages covalently bond two monosaccharides together. A glycosidic bond is formed when the hydroxyl group on one of the sugars reacts with the monomeric carbon on the second sugar. A glycosidic bond can be alpha or beta, depending on the steric configuration at carbon one on the monosaccharide unit. During the union of two monosaccharide units, water molecules are eliminated (dehydration reaction or condensation reaction, or dehydration synthesis), and the formation of a glycosidic bond occurs, through which the units are linked via an oxygen bridge.

Polysaccharides comprise several monosaccharide units (more than ten) linked by glycosidic bonds; therefore, they are high molecular weight carbohydrates. They are also called polyanhydrides of simple sugars. The most common monosaccharide unit of polysaccharides is D-glucose. The polysaccharides may differ in their monosaccharide units, molecular weight, chain nature, glycosidic bond types (alpha, beta), and linkages (1-2, 1-3, 1-4, or 1-6). They are tasteless, colorless, amorphous powders that are a little water-soluble. Some may form colloidal solutions. The polysaccharides provide structural stability (structural polysaccharides) to cells, prevent cells and tissues from drying out due to their water-binding properties, and reserve food for the future. Reserved polymeric carbohydrates are osmotically less active and can be stored in large quantities within the cell.
The right and left end of polysaccharides, respectively, are called the reducing and non-reducing ends. Based on the functional aspects, they are grouped in two ways.
• The first way depends on its digestibility—1. Nutrient polysaccharides are (digestible) starch, dextrin, and glycogen, and 2. Structural polysaccharides are (indigestible), chitin and cellulose.
• The second way, depending upon the constituent monosaccharide units, they are 1. Homoglycans (same units), and 2. Heteroglycans (different units).

6.1 Metabolism
Dietary components like carbohydrates, lipids, and proteins are continuously broken down and synthesized through various chemical reactions (exergonic or endergonic) that are concerned with energy. These processes not only break them down but also interconvert them for their synthesis. Dietary components, when utilized, are known as nutrients the body. The number of reactions occurring in living cells maintains the body cells and fluid’s homeostasis and dynamic status. To keep the dynamic state of a cell, many reactions are going on to synthesize or break down the different types and sizes of molecules. The total of these reactions is known as metabolism.
The metabolism may be anabolic or catabolic. Further, it may be aerobic or anaerobic, depending on oxygen availability. These four terms are explained below:
I. Aerobic and Anaerobic Metabolism
• Aerobic metabolism-- In aerobic metabolism, the carbohydrates (substrate) in the presence of oxygen are metabolized to release energy, carbon dioxide, and water. The released energy is converted to ATP by two processes in the mitochondria (the Krebs cycle and oxidative phosphorylation). In oxidation, the electrons released from a glucose molecule are transferred to reduced NAD+ and FAD. The oxygen (electron acceptor) is reduced to water. Water formation [from H2(g) and O2(g)] is an exothermic reaction. The energy released from the chemical bond of glucose is captured as ATP, and heat is released.

Glycolysis involves a series of reactions to extract energy from glucose to
release two molecules with three carbons called pyruvate. The process is not
dependent on oxygen but can occur under aerobic and anaerobic conditions,
releasing different end products. Under aerobic conditions, the pyruvate formed
is then oxidized in the mitochondria to CO2 and H2O. The energy released in
glycolysis is in the form of ATP (High Energy Bond). All the reaction steps
occur in the cytoplasm, as it is the extramitochondrial pathway. In anaerobic
conditions and the absence of mitochondria (RBC), the end product of
glycolysis is lactic acid.
The net reaction of Glycolysis (anaerobic) may be summarized as under
Glucose + 2ADP + 2Pi → 2 Lactate + 2ATP
7.1 Steps in EMP Cycle or Glycolytic Pathway
Table 7.1: Steps and enzymes of glycolysis or EMP cycle.

The Hans Krebs cycle, known by different names like the citric acid or tricarboxylic acid cycle, was named after the scientist Hans Krebs. The citric acid cycle is named after the citrate product formed in the cycle. The name tricarboxylic acid is given as citrate itself, which is a tricarboxylic acid due to the presence of three carboxyl functional groups (-COOH).
The tricarboxylic acid (TCA) cycle occurs in aerobic conditions and within mitochondrial ETC, involving various sequential oxidoreductase reactions. The cycle requires Acetyl-CoA (C2 molecule) and oxaloacetate molecule (C4 molecule) to begin with the formation of a citrate molecule (C6 molecule). Acetyl-CoA is generated from the pyruvate (glycolysis), the fatty acids (lipolysis), or by oxidative deamination of specific amino acids. The complete oxidation of a glucose molecule results in the end products like energy (ATP), CO2 and H2O. A molecule of Acetyl-CoA has two carbons. It is formed from a three-carbon pyruvic acid molecule generated from the glycolysis of a sixcarbon glucose molecule. Acetyl-CoA is the substrate for this cycle and is an energy-rich molecule having a thioester bond between coenzyme A and acetic acid.
8.1 Formation of acetyl-CoA from Pyruvic Acid
Pyruvate is obtained by glycolysis or the oxidation of glucogenic amino acids. The pyruvic acid produced in the cytoplasm crosses the inner membrane of the mitochondrion to enter the mitochondrial matrix, where it undergoes oxidative decarboxylation, forming Acetyl-CoA. Acetyl-CoA carries (carrier molecule) the acetyl group to the citric acid cycle. It also links various metabolic products with the TCA cycle. Such products can be from the EMP cycle, beta-oxidation, and oxidative deamination of amino acids. Oxidative decarboxylation of pyruvic acid needs a complex enzyme system known as pyruvate dehydrogenase

This alternative pathway for glucose oxidation has enzymes in the cytosol through which glucose is degraded to carbon dioxide. It is also called a shunt because carbon atoms from glucose 6-phosphate take a long route before they proceed down the EMP or glycolytic pathway. This pathway is anabolic in nature as it produces many intermediates. In this pathway, two NADPH are generated from one glucose 6-phosphate molecule. No ATP is generated or consumed.
Overall, this pathway needs three glucose-6-phosphate molecules to complete all the reactions. Therefore, one cycle run produces 2 mol of fructose-6- phosphate, 1 mol of glyceraldehyde-3-phosphate, and 3 mol of carbon dioxide from 3 mol of glucose-6-phosphate with the reduction of 6 mol of NADP+ to NADPH.
Overall reactions of the HMP pathway are as follows:
3 Glucose-6-P + 6 NADP+→ 3 ribulose-5-P + 3 CO2 + 6 NADPH
3 Ribulose-5-P → 2 xylulose-5-P + Ribose-5-P
2 Xylulose-5-P + Ribose-5-P → 2 fructose-6-P + Glyceraldehyde-3-P

The uronic acid pathway is a minor and alternative pathway for glucose oxidation and for the production of uronic acid sugar. Uronic acids of sugars are formed when the terminal carbon’s (C6) hydroxyl group (CH2OH) of sugar gets oxidized to a carboxylic acid. Uronic acid names depend on the name of the parent sugars. Uronic acid of glucose is glucuronic acid, hexose uronic acids are hexuronic acids, and pentose uronic acids are named penturonic acids. The uronic acid pathway is located in the cytosol of the liver and adipose tissue.
The glucuronic acid should not be confused with gluconic acid (aldonic acid) and glucaric acid (aldaric acid or dicarboxylic acid). Gluconic acid (aldonic acid or “onic” acid) is obtained from D-glucose by oxidizing its aldehyde group at C1 to a carboxyl group. Glucaric acid (aldaric or dicarboxylic acid) is formed upon oxidation of both the terminal hydroxyl groups (C1 and C6) and the aldehyde. They have two carboxylic acid groups. This pathway yields pentoses & ascorbic acid or vitamin C (not in primates & guinea pigs) from glucose.

Plants, with the help of photosynthesis, convert simple molecules (like carbon dioxide and water) into more complex molecules (anabolism) through various chemical reactions. Animals synthesize more complex molecules (anabolism) from monomer units through multiple chemical reactions. Anabolism refers to a biosynthesis process involving the sequences of biochemical reactions by which complex molecules are formed in living cells from available monomer units. Anabolic processes include the biosynthesis of complex biomolecules like carbohydrates, proteins, and lipids. The anabolic processes require energy from high-energy bond compounds like ATP.
The anabolic processes related to the biosynthesis of carbohydrates include
(1) Gluconeogenesis and
(2) Glycogenesis
11.1 Gluconeogenesis
Gluconeogenesis is the synthesis of new glucose molecules from noncarbohydrate molecules. The non-carbohydrate molecules that are used for gluconeogenesis are derived from glycolysis, lipolysis, and proteolysis. Examples are pyruvate, lactate, glycerol, and glucogenic amino acids. The liver and kidneys are the preferred organs for the gluconeogenesis process. It also occurs to some extent in the brain. Blood glucose levels are maintained at physiological concentrations through the process of gluconeogenesis in humans and ruminants. The blood glucose drops below normal levels during various conditions like fasting, starvation, pregnancy, lactation, fever, and low carbohydrate diets.

Glycogenesis helps to maintain blood glucose, as it is a primary source of energy for every cell. An inadequate supply of glucose to the body may lead to the failure of the vital organs. Glucose, when in excess, is synthesized into glycogen and stored (glycogenesis), mainly in the liver and muscles (including the uterus). It is also found in other tissues, such as the kidney, brain, heart, and adipose tissues. The stored liver glycogen is broken down (glycogenolysis) into blood glucose when needed. Still, the glycogen stored in muscle is broken down to glucose 1-phosphate and converted to blood glucose later. Either the synthesis or breakdown pathway is primarily active at a time in tissues. Glycogen synthesis and its breakdown are not reversible processes.
The percentage of glycogen stored in skeletal muscles is less ( 1% to 2%) than in the liver (10%). Still, the total quantity of glycogen is greater in muscles due to their greater mass than an animal’s total liver mass. Muscle mass, on average, ranges from 40 to 45 % of body weight for the most mature mammals.
Glycogen synthesis (glycogenesis) is an anabolic process forming a branched homopolysaccharide made of several chains of alpha D-glucose units linked by α-1,4 glycosidic bonds in a linear chain and nearly every ten glucose residues; the chain branches off via α-1,6 glycosidic linkages. The α-glycosidic bonds give rise to a helical polymer structure. The number of glucose units in a single glycogen molecule varies (1,700 to -600,000 units). The glycogen granules are formed on the glycogenin protein in the center, so it acts as a primer, and their number determines the total number of glycogen particles.

Glycogen breakdown is known as glycogenolysis. This breakdown occurs in the cytoplasm of cells of the organs, wherein glycogen is stored. The end product of glycolysis is glucose in the liver, which is glucose-1-phosphate in skeletal muscles.
Glycogen is a polysaccharide synthesized from glucose monomers. Glycogen is stored in the cell instead of glucose because glucose requires more water, whereas glycogen can be stored with much less water. Most of the stored glycogen is in the liver (5% of body weight) and muscle (1 to 2 % of body weight). Muscle and liver glycogen undergo glycolysis for different purposes. Muscle glycogen provides energy for muscular activity, whereas liver glycogen helps to maintain the blood glucose level.
Glycogen contains many glucose residues (up to 120,000) linked by alpha 1-4 glycosidic bonds, forming a straight chain. Glycogen has branches at alpha 1-6 glycosidic bonds at every ten glucose residues. At the reducing end, the carbon one of glucose is free, whereas it is not accessible in the case of non-reducing ends.

Carbohydrates (glucose) are the primary energy source for a body; when digested, glucose is the end product in human beings and simple stomach animals, but it yields volatile fatty acids in ruminants. Precise regulation of levels of blood sugars/glucose in the body is the net result of various processes like digestion, absorption, and metabolism operating under the interaction of various a) enzymes and b) hormones secreted by the different organs of the body. An abnormal secretion of any of the enzymes or hormones associated with the metabolism of carbohydrates disrupts the precise regulation of blood sugar, leading to various disorders.
14.1 Enzyme Deficiency and Disorders in Carbohydrate Metabolism
Usually, various enzymes are involved in the breakdown of carbohydrates, and if their secretion is insufficient, missing, or does not work correctly, it leads to different disorders.
I. Glycogen storage diseases (GSDs, glycogenosis, and dextrinosis)
Glucose is needed by the cells for the generation of energy and other cellular activities. If glucose happens to be in excess, it is stored as glycogen, mainly in the liver and muscles, for future needs. The stored glycogen can be converted back to glucose when the need arises. Many enzymes are related to this interconversion of glucose and glycogen. Deficiencies of one or more enzymes may lead to abnormal glycogen metabolism, leading to glycogen accumulation of normal or abnormal chemical structure in tissue, collectively called glycogen storage diseases (GSDs). Roman numerals identify different types of glycogen storage conditions. They are also classified under various categories like inherited metabolic diseases, autosomal recessive type, and X-linked type (VIII/IX type).

The number of hormones secreted by the body exhibits control over the physiological regulation of glucose sugar level (homeostasis) in the manner needed by the body. Depending upon the situation, hormones may increase or decrease the level of glucose sugar present in the blood or tissues. The glucose is oxidized to yield energy, carbon dioxide, and water.
Glucose, more than the body’s needs, is stored or converted mainly to glycogen and, to some extent, protein and fat. If glucose is less than the need of the body, its denovo synthesis takes place from amino acids or fatty acids, or is formed from the breakdown of stored glycogen. Various hormones are secreted by multiple endocrine glands to control the blood glucose level. If it fails, it leads to hypoglycemia or hyperglycemia conditions. Gluconeogenesis is not developed at birth in the case of a baby pig. Therefore, hypoglycemia in baby pigs is observed within 2 days of age if starved, leading to hypoglycemia, weakness, confusion, and death.
15.1 The Hormones Involved in Homeostatic Regulation of Blood
Glucose Level
The hormones influencing carbohydrate metabolism are explained below: