Modes of Nutrition in Plants and Animals

Meaning of Nutrition

Nutrition is the process by which living organisms obtain and utilise food for growth, energy, repair of worn-out tissues, and other life activities. All living things require nutrients to survive, but the way organisms obtain their nutrients differs. Broadly, nutrition in living things is classified into two major types: autotrophic nutrition, found mainly in green plants, and heterotrophic nutrition, found in animals and some other organisms.

Autotrophic Nutrition in Plants

Autotrophic organisms, also called autotrophs, are able to manufacture their own food from simple inorganic substances using energy from sunlight or chemical reactions. Green plants are autotrophs because they carry out photosynthesis, the process by which plants use sunlight, carbon dioxide, and water to manufacture glucose and release oxygen as a by-product.

The equation for photosynthesis is: 6CO₂ + 6H₂O + light energy (in the presence of chlorophyll) → C₆H₁₂O₆ + 6O₂.

Photosynthesis occurs in two main stages:

• Light reaction: Takes place in the grana of the chloroplast, where light energy is absorbed by chlorophyll and used to split water molecules, releasing oxygen and producing energy carriers (ATP and NADPH).
• Dark reaction (Calvin cycle): Takes place in the stroma of the chloroplast, where carbon dioxide is fixed and combined with hydrogen from the light reaction to form glucose.

Factors affecting the rate of photosynthesis include light intensity, carbon dioxide concentration, temperature, and availability of water and chlorophyll. These are often called limiting factors because a shortage of any one of them can slow down the overall rate of photosynthesis.

Heterotrophic Nutrition in Animals

Heterotrophic organisms, or heterotrophs, cannot manufacture their own food and must obtain nutrients by consuming other organisms or organic matter. Animals, including humans, are heterotrophs. There are several types of heterotrophic nutrition:

• Holozoic nutrition: The organism takes in solid food, digests it internally, and absorbs the nutrients. This is the type of nutrition found in humans and most animals.
• Saprophytic (saprotrophic) nutrition: The organism feeds on dead and decaying organic matter by secreting digestive enzymes externally and absorbing the digested products. Examples include fungi such as mushrooms and moulds.
• Parasitic nutrition: The organism (parasite) lives on or inside another living organism (the host) and obtains nutrients from it, often causing harm to the host. Examples include tapeworms and mistletoe.
• Symbiotic (mutualistic) nutrition: Two different organisms live together and both benefit nutritionally from the relationship, such as the relationship between termites and the bacteria in their gut that help digest cellulose.

Methods of Feeding in Heterotrophic Organisms

• Filter feeding: The organism filters small food particles from water, as seen in some aquatic animals like clams.
• Fluid feeding: The organism feeds by sucking fluids such as blood, nectar, or plant sap, as seen in mosquitoes and aphids.
• Deposit feeding: The organism feeds on organic matter deposited in soil or sediment, as seen in earthworms.
• Bulk feeding: The organism takes in large pieces of food at once and digests them internally, as seen in most higher animals including humans.

The Human Digestive System and Nutrition

In holozoic organisms like humans, food undergoes ingestion, digestion, absorption, assimilation, and egestion. The alimentary canal (mouth, oesophagus, stomach, small intestine, and large intestine) along with accessory organs (liver, pancreas, and salivary glands) work together to break down food into simpler, absorbable units. Carbohydrates are broken down into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol.

Classes of Food Nutrients

A balanced diet must contain the following classes of nutrients in the right proportions:

• Carbohydrates: Provide energy for body activities. Sources include rice, yam, and bread. Deficiency leads to lack of energy and weight loss.
• Proteins: Needed for growth and repair of body tissues. Sources include beans, meat, fish, and eggs. Deficiency causes kwashiorkor, especially in children.
• Fats and oils: Provide concentrated energy and help absorb fat-soluble vitamins. Sources include groundnut oil, butter, and palm oil.
• Vitamins: Needed in small amounts for proper body function; for example, Vitamin C prevents scurvy and Vitamin D prevents rickets.
• Mineral salts: Such as calcium, iron, and iodine are needed for bone formation, blood production, and thyroid function respectively.
• Water: Essential for all metabolic processes, temperature regulation, and transport of nutrients.
• Fibre (roughage): Aids digestion and prevents constipation, though it is not digested or absorbed.

Balanced Diet

A balanced diet is one that contains all the classes of food nutrients in the correct proportions needed by the body for growth, energy, and repair. A balanced diet helps prevent nutritional deficiency diseases such as kwashiorkor, marasmus, scurvy, and rickets, and supports overall healthy living.

Summary

Nutrition is essential for the survival of all living organisms. Plants obtain their food through autotrophic nutrition, mainly by photosynthesis, while animals depend on heterotrophic nutrition, which includes holozoic, saprophytic, parasitic, and symbiotic feeding. Understanding the classes of food nutrients and their functions is important for maintaining good health and is a key topic frequently examined in WAEC and NECO Biology papers.