Chapter 2: Apply nutrition in plants and animals

Nutrition is fundamental to the survival and functioning of all living organisms, including the plants and animals that science laboratory technologists study and work with in Kenya. Understanding how organisms obtain and utilize nutrients is critical in fields such as agriculture, animal husbandry, food technology, and biomedical research. This chapter explores the types of nutrition in plants and animals, along with the physiological processes of digestion, absorption, assimilation, and egestion, providing laboratory professionals with essential knowledge to analyze and interpret biological samples accurately.

2.1 Types of Nutrition in Plants and Animals

In biological systems, nutrition refers to the processes by which organisms acquire and use food substances to support growth, repair, and energy production. In Kenya, laboratory technologists often analyse plant and animal tissues to assess nutritional status, disease conditions, or metabolic health. Differentiating between autotrophic and heterotrophic nutrition is fundamental to understanding organismal biology and ecological interactions.

2.1.1 Autotrophic Nutrition

Autotrophic nutrition is the ability of an organism to produce its own food from inorganic substances, typically using light or chemical energy. This mode of nutrition is predominant in plants, algae, and some bacteria, which convert carbon dioxide and water into carbohydrates through photosynthesis or chemosynthesis.

Characteristics of Autotrophic Nutrition

• Self-sufficiency in food production: autotrophs synthesise organic molecules without consuming other organisms, enabling them to sustain themselves independently.
• Use of inorganic raw materials: they utilise carbon dioxide, water, and minerals from the environment as raw materials for food synthesis.
• Energy conversion: photosynthetic autotrophs, such as maize and tea plants common in Kenyan agriculture, harness solar energy to drive chemical reactions.
• Chlorophyll presence: most autotrophic plants contain chlorophyll pigments that capture light energy, a feature critical for photosynthesis.
• Oxygen release: during photosynthesis, oxygen is released as a by-product, contributing to atmospheric oxygen levels essential for aerobic organisms.

Photosynthesis as the main route of autotrophic nutrition

Photosynthesis involves two major stages: the light-dependent reactions and the light-independent (Calvin cycle) reactions. In the light-dependent stage, chlorophyll absorbs sunlight, generating ATP and NADPH. These energy carriers then fuel the Calvin cycle, where carbon dioxide is fixed into glucose molecules. Both stages are taught in full in Process of photosynthesis.

In Kenyan science laboratories, assays measuring chlorophyll content or photosynthetic rates in crops like maize or sugarcane inform agronomic practices and fertiliser application.

Autotrophic Nutrition in Non-Photosynthetic Organisms

Certain bacteria obtain energy through chemosynthesis, which is the oxidation of inorganic molecules. Nitrifying bacteria used in soil fertility studies oxidise ammonia to nitrite and nitrite to nitrate, while sulphur bacteria oxidise hydrogen sulphide. These bacteria play vital roles in Kenyan ecosystems by recycling nutrients.

2.1.2 Heterotrophic Nutrition

Heterotrophic nutrition involves obtaining organic molecules by consuming other organisms or organic matter. This mode is typical of animals, fungi, and many bacteria, which depend on autotrophs or other heterotrophs for food. The three main forms studied here are holozoic, parasitic and saprophytic nutrition, and they differ in what the organism feeds on and where digestion takes place.

Symbiotic nutrition

In symbiotic nutrition both partners benefit from nutritional exchange. Nitrogen-fixing bacteria in the root nodules of legumes such as beans receive sugars from the plant, and in return they supply the plant with nitrogen compounds that improve soil fertility. Symbiosis is therefore a partnership, unlike parasitism, in which only one partner gains.

Importance of Heterotrophic Nutrition in Laboratory Context

Laboratory technologists analyse digestive enzymes, nutrient levels, and microbial flora in animal samples to diagnose nutritional deficiencies or infections. Understanding heterotrophic mechanisms informs the interpretation of such test results.

2.1.2.1 Holozoic

Holozoic nutrition is the mode in which an organism takes in solid or liquid food, digests it inside the body, absorbs the soluble products and removes the undigested remains. It is the usual mode of animals, from the single-celled amoeba, which engulfs food particles, to humans and livestock such as dairy cattle. Laboratory analysis of digestive enzymes aids veterinary diagnostics. The steps that follow ingestion are taught in Process of digestion, absorption, assimilation, and egestion.

• Ingestion: food is taken into the body, by a mouth in most animals and by engulfing in the amoeba.
• Internal digestion: large food molecules are broken into small soluble ones inside a gut or a food vacuole, by enzymes.
• Feeding groups: herbivores such as goats and cattle feed on plants, carnivores such as lions feed on other animals, and omnivores such as humans and pigs feed on both.
• Adaptations: the teeth, jaws and gut of a holozoic animal are shaped to its diet. Herbivores have broad grinding teeth and a long gut, while carnivores have sharp teeth for tearing flesh and a shorter gut.
• Ruminants: some animals, like ruminants in Kenyan pastoral systems, have specialised digestive systems to break down complex plant materials efficiently. Cattle, sheep and goats have a stomach with chambers in which microorganisms digest cellulose, and the animal chews the food again as cud.

2.1.2.2 Parasitic

Parasitic nutrition is the mode in which a parasite lives on or in a living organism, the host, and obtains its food from it, usually harming the host. Parasites that live on the outside of the host are ectoparasites, such as ticks, lice and fleas. Parasites that live inside the host are endoparasites, such as tapeworms, roundworms and blood parasites. Blood parasites like Trypanosoma, which cause sleeping sickness, are studied in laboratories to develop control strategies.

• Tapeworm: it lives in the gut of its host and absorbs digested food through its body surface. It has hooks and suckers to hold on, and no digestive system of its own.
• Trypanosoma: it lives in the blood of mammals, is passed on by the tsetse fly, and takes nutrients from the blood plasma. It causes sleeping sickness in people and nagana in livestock.
• Ticks: they feed on the blood of livestock through the skin and also pass disease organisms between animals.
• Plant parasites: witchweed (Striga) attaches to the roots of maize and sorghum and draws water and nutrients from them, and dodder (Cuscuta) twines round a host stem.
• Adaptations: parasites have attachment organs, a thick outer covering that resists the host's enzymes, and a high rate of reproduction that makes up for the many young that never find a host.
• Laboratory identification: parasites are found by examining stained blood smears or faecal samples under a microscope.

2.1.2.3 Saprophytic

Saprophytic nutrition is the mode in which an organism feeds on dead organic matter externally. It secretes digestive enzymes onto the material, and then absorbs the soluble products that result from the decomposition. Fungi growing on decaying plant material in Kenyan tea plantations exhibit this nutrition. Saprophytes are important decomposers in nature and return minerals to the soil for plants to use.

• Extracellular digestion: the enzymes are released outside the body, so that digestion takes place on the food and not inside the organism.
• Fungal structure: a fungus such as a bread mould (Rhizopus) grows a network of thread-like hyphae through its food. The hyphae give a large surface for secreting enzymes and absorbing the products.
• Examples: bread moulds, mushrooms and yeasts are saprophytic fungi, and many soil bacteria also feed this way.
• Conditions for growth: saprophytes grow where there is moisture, warmth and a supply of dead organic matter, which is why damp, warm stores spoil grain with mould.
• Importance: their activity recycles carbon and minerals from dead plants and animals, and it is used in making compost and in fermentation.
• Laboratory culture: saprophytic fungi are grown on agar in Petri dishes, where the spread of the hyphae can be observed and the colony diameter measured.

Chapter Summary

This chapter explored the various types of nutrition in plants and animals, distinguishing between autotrophic nutrition, where plants produce their own food, and heterotrophic nutrition, which includes holozoic, parasitic, and saprophytic modes in animals. The processes involved in nutrition such as digestion, absorption, assimilation, and egestion were examined to understand how organisms utilize food for energy and growth. Nutritional deficiency diseases were discussed, highlighting the health impacts of lacking essential nutrients. The morphology and functions of the leaf were detailed, emphasizing its role in photosynthesis and gas exchange. The process of photosynthesis was explained in two stages: the light stage, where light energy is captured, and the dark stage, where energy is used to synthesize food. Additionally, the chapter covered the C3 and C4 pathways, describing different biochemical routes plants use to fix carbon during photosynthesis. Together, these topics provide a comprehensive understanding of nutrition and energy flow in living organisms.