Chapter 2: Apply nutrition in plants and animals

Nutrition is a fundamental concept in applied biology, underpinning the growth and development of both plants and animals. For professionals working in Kenya’s agricultural and biological sectors, understanding how nutrition operates at the physiological level enables better management of crops and livestock, enhancing productivity and sustainability. This chapter focuses on plant nutrition, emphasizing the leaf’s structure and the photosynthesis process, which are vital for crop yield and ecosystem balance. Mastery of these topics supports innovations in farming practices and environmental conservation efforts.

2.1 Plant Nutrition

Green plants are autotrophs: they make their own organic food from simple inorganic substances, using light energy. Carbon dioxide from the air and water from the soil are built into sugars by photosynthesis, and the sugars are then used for respiration or turned into starch, cellulose, proteins and oils. The leaf is the organ that carries out most of this work, so its structure explains how plants feed. In a nursery, a screenhouse or a crop plot at a field station, a technician who knows how a leaf captures light and gases and how photosynthesis proceeds can judge whether a crop is short of light, water or mineral nutrients.

2.1.1 Structure and Functions of the Leaf

A typical foliage leaf is a thin, flat organ that gives a large surface for light capture while keeping every photosynthetic cell close to the air and to a vein. Its structure can be studied from the outside and in cross section under the microscope.

External structure of the leaf

• Blade (lamina): the broad, flat part that gives a large surface for absorbing light. Its thinness lets light reach all the cells and lets gases diffuse over a short distance.
• Petiole: the stalk that joins the blade to the stem. It holds the blade in a position to receive light and carries xylem and phloem between stem and leaf. Leaves without a petiole are called sessile.
• Midrib and veins: the main vein and its branches carry water and mineral salts in the xylem and take sugars away in the phloem, and they also support the blade. Dicotyledon leaves have a net-like arrangement of veins, and monocotyledon leaves such as maize have veins running parallel.
• Margin: the edge of the blade, which may be smooth, toothed or lobed.
• Leaf base: the point of attachment to the stem. In grasses it forms a sheath around the stem.

Internal structure of the leaf

A cross section of a dicotyledon leaf shows these layers from top to bottom:

• Cuticle: a waxy layer on the outer surface of the epidermis that reduces water loss.
• Upper epidermis: a single layer of transparent cells with no chloroplasts, which lets light pass to the cells beneath.
• Palisade mesophyll: tightly packed, elongated cells, rich in chloroplasts, arranged at right angles to the surface. This is the main site of photosynthesis.
• Spongy mesophyll: loosely arranged, rounded cells with large air spaces that let carbon dioxide and oxygen move through the leaf.
• Vascular bundle: xylem, which brings water and minerals, and phloem, which removes the sugars made.
• Lower epidermis: a single layer of cells containing stomata, the pores through which gases and water vapour pass. Each stoma is bordered by two guard cells that change shape to open and close it.

Functions of the leaf

• Photosynthesis: the leaf converts light energy into the chemical energy of sugars.
• Gas exchange: carbon dioxide enters and oxygen leaves through the stomata and spaces of the spongy mesophyll.
• Transpiration: loss of water vapour through the stomata, which pulls water up the plant and cools the leaf.
• Storage: the fleshy leaves of succulent plants store water, and bulb scales are leaves that store food.
• Protection and support: some leaves are modified into spines or tendrils that protect the plant or help it climb.

Adaptations of leaves to the environment

• Thick cuticle: reduces water loss in plants of dry places.
• Sunken stomata and hairs: trap a layer of humid air next to the stomata and reduce evaporation.
• Small or rolled leaves: reduce the surface area exposed to the sun and the wind. Rolled leaves are seen in some grasses in dry weather.
• Leaf orientation: vertical leaves take less direct midday sun.
• Sun and shade leaves: leaves in full sun are thicker with more palisade cells, while shade leaves are thinner and broader, with more chlorophyll per unit of leaf area, so that they capture more of the weak light.

Example: a technician preparing a transverse section of a leaf for a teaching laboratory cuts a very thin slice with a razor blade, stains it and views it at low then high power to identify the cuticle, epidermis, palisade, spongy mesophyll, vein and a stoma.

2.1.2 Photosynthesis

Photosynthesis is the process in which chlorophyll in chloroplasts absorbs light energy and uses it to make glucose from carbon dioxide and water, releasing oxygen. The overall equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll)

The process takes place in the chloroplasts, mainly of the palisade and spongy mesophyll cells. A chloroplast has a double membrane enclosing the stroma, a fluid containing enzymes. Within the stroma are stacks of flattened sacs called grana, made of thylakoids, whose membranes carry the pigments. Chlorophyll a and chlorophyll b absorb mainly red and blue light and reflect green light, which is why leaves look green, and carotenoids absorb other wavelengths and protect the chlorophyll.

Example: to show that light and chlorophyll are needed, a teaching laboratory may take a plant with variegated leaves, expose it to light, dip the leaf in boiling water, boil it in ethanol to remove the chlorophyll, and test it with iodine solution. Only the green areas turn blue-black, showing that they made starch.

Factors affecting photosynthesis

The rate of photosynthesis is limited by whichever factor is in shortest supply.

• Light intensity: the rate rises with light up to a saturation point and then levels off.
• Carbon dioxide concentration: the air contains only about 0.04 per cent, so more carbon dioxide often increases the rate until another factor limits it. Growers of crops in greenhouses sometimes raise it.
• Temperature: the reactions of the light-independent stage depend on enzymes, so the rate increases with temperature up to an optimum and then drops sharply as enzymes are damaged.
• Water: water is a raw material, and shortage closes the stomata, which also cuts off the carbon dioxide supply.
• Chlorophyll content: leaves with little chlorophyll capture less light. Shortage of nitrogen, magnesium or iron causes yellowing of the leaves, and the plant photosynthesises less.

Importance of photosynthesis

• Food for the plant and for other organisms: the sugars are the starting point of almost all food chains.
• Oxygen: photosynthesis releases the oxygen that most organisms use in respiration.
• Carbon dioxide removal: it takes up carbon dioxide from the air.
• Crop yield: the dry mass of a crop is built largely from products of photosynthesis, so practices that improve light capture, such as correct spacing, weeding and pruning, raise the yield.

2.1.2.1 Light and Dark Stages

Photosynthesis takes place in two linked stages.

The light stage (light-dependent reactions) occurs in the thylakoid membranes. Chlorophyll in photosystems absorbs light and excites electrons. Light energy is used in photolysis to split water: 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen is released and the electrons replace those lost by the chlorophyll. The excited electrons pass along a chain of carriers, releasing energy that is used to make ATP from ADP and phosphate (photophosphorylation). At the end of the chain the electrons and hydrogen ions reduce NADP⁺ to NADPH. The products carried to the next stage are ATP and NADPH, and oxygen is a by-product.

The dark stage (light-independent reactions) occurs in the stroma. It does not use light directly, but needs the ATP and NADPH made in the light stage. Carbon dioxide is fixed and reduced to sugar in a cycle of reactions, the Calvin cycle. In the dark, the supply of ATP and NADPH soon runs out and the stage stops, so it depends on the light stage even though it needs no light itself.

2.1.2.2 Calvin Cycle

The Calvin cycle is the series of reactions in the stroma that builds sugar from carbon dioxide. It has three phases:

1. Carbon fixation: the enzyme rubisco joins carbon dioxide to the five-carbon sugar ribulose bisphosphate (RuBP). The six-carbon product splits at once into two molecules of the three-carbon compound 3-phosphoglycerate.
2. Reduction: ATP and NADPH from the light stage convert 3-phosphoglycerate into the three-carbon sugar triose phosphate.
3. Regeneration: most of the triose phosphate is used, with more ATP, to rebuild RuBP so that the cycle can continue. The remainder leaves the cycle and is used to make glucose, starch, sucrose and other compounds.

For every three carbon dioxide molecules fixed, the cycle makes six triose phosphate molecules, one of which leaves the cycle while five are used to regenerate three RuBP. The cycle uses 9 ATP and 6 NADPH for each three carbon dioxide molecules. Making one glucose molecule needs six turns of the cycle, which fix six carbon dioxide molecules and use 18 ATP and 12 NADPH.

Variations of carbon fixation in some crops

In C3 plants, such as beans and wheat, rubisco fixes carbon dioxide directly in the Calvin cycle. In C4 plants, such as maize, sorghum and sugarcane, the first fixation takes place in mesophyll cells, using a different enzyme, and the carbon is passed to bundle sheath cells where the Calvin cycle runs. This keeps carbon dioxide concentrated around rubisco and suits hot, bright conditions. In CAM plants, such as pineapple and sisal, the stomata open at night to take in carbon dioxide and store it as an acid, and the Calvin cycle runs by day with the stomata closed, which saves water in dry conditions.

Chapter Summary

This chapter explored the fundamentals of nutrition in both plants and animals, beginning with the structure and functions of the leaf, which plays a crucial role in plant nutrition by facilitating gas exchange and photosynthesis. The process of photosynthesis was detailed, including its light and dark stages and the Calvin cycle, which together enable plants to convert light energy into chemical energy. In animal nutrition, the digestive system and its enzymes were examined to understand how food is broken down into absorbable nutrients. The chapter also described the process of digestion and absorption, highlighting how nutrients are assimilated into the body. A practical dissection was included to reinforce knowledge of animal digestive anatomy. Finally, the chapter addressed nutritional deficiency diseases such as marasmus, kwashiorkor, scurvy, and beri-beri, explaining their causes and effects on health. This comprehensive overview provides a solid foundation for understanding the essential roles of nutrition in living organisms.