Nutrition in Plants and Animals | Form One - High School Biology

1. Autotrophism: mode of nutrition through which living organisms manufacture their own food from simple inorganic substances in the environment such as carbon (IV) oxide, water and mineral ions. Organisms that make their own food through this mode are autotrophs.
2. Heterotrophism: mode of nutrition in which living organisms depend on already manufactured food materials from other living organisms. Heterotrophs are the organisms that feed on already manufactured food materials.

- In this mode of nutrition, organisms manufacture their own food from readily availablematerials in the environment.
- These organisms use energy to combine carbon (IV) oxide, water and mineral salts in complex reactions to manufacture food substances.
- Depending on the source of energy used to manufacture the food, there are two types of autotrophism:
A) Chemosynthesis
- This is the process whereby some organisms utilize energy derived from chemical reactions in their bodies to manufacture food from simple substances in the environment.
- This nutrition mode is common in non green plants and some bacteria which lack the sun trapping chlorophyll molecule.
B) Photosynthesis
- This is the process by which organisms make their own food from simple substances in the environment such as carbon (IV) oxide and water using sunlight energy.
- Such organisms often have chlorophyll which traps the required sunlight energy.
- This mode of nutrition is common in members of the kingdom Plantae.
- Some protoctists and bacteria are also photosynthetic.

Importance of Photosynthesis

• Photosynthesis helps in regulation of carbon (IV) oxide and oxygen gases in the environment.
• Photosynthesis enables autotrophs make their own food, thus, meet their nutritional requirements.
• Photosynthesis converts sunlight energy into a form (chemical energy) that can be utilized by other organisms that are unable to manufacture their own food.

- Externally, the leaf has a petiole through which it attaches to the leaf branch or stem, lamina- the broad flat surface, margin- the outline and the leaf apex.
- The leaf margin can be smooth, dentate, serrated or entire.
- The size of a leaf depends on its environment.
- Plants in arid areas have small sized leaves with some leaves reduced to needle like shape. This helps reduce the rate of water loss in such plants.
- However, the plants in areas of water abundance have broad leaves to enable them lose the excess water.

1. Cuticle

- This is the outermost layer of the leaf.
- It is a thin non-cellular, waxy, transparent and waterproof layers that coats the upper and lower leaf surfaces.
Functions of the Cuticle

1. Being waterproof, it minimizes water loss from the leaf cells to the environment through transpiration and evaporation.
2. It protects the inner leaf tissues from mechanical damage.
3. It prevents entry of pathogenic microorganisms into the leaf.

- This is the outermost one cell thick layer covering upper and lower leaf surfaces.
- Its cells are flattened and lack chloroplasts.
Functions of the Epidermis

1. It protects the leaf from mechanical damage.
2. It also protects the leaf from entry of disease-causing microorganisms.
3. It secretes the cuticle.

- There are many small pores on the epidermis known as stomata (singular-stoma) through which exchange of materials occur.
- The opening and closing of the stomata is controlled by the guard cells.
- Each stoma is controlled by two guard cells.
- The guard cells have chloroplasts and are bean shaped. They have thicker inner cell wall and thinner outer cell wall.
Adaptations of the guard cells

• They have differentially thicker walls to enable them bulge as they draw water through osmosis from the neighbouring cells making them to open the stomata.
• They contain chloroplasts that manufacture sugars which increase osmotic pressure of the guard cells. As they draw water through osmosis, they bulge making the stomata to open.

- This is the chief photosynthetic tissue in plants. Its cells are regular in shape.
- Its cells contain numerous chloroplasts for photosynthesis.
- Their close packing and location just below the epidermis enables them to trap maximum sunlight for photosynthesis.
- Location of palisade layer on the upper surface explains why upper leaf surfaces are greener than the lower surfaces.

- This layer contains loosely arranged irregular cells.
- This leaves large airspaces between the cells which permits free circulation of gases carbon (IV) oxide and oxygen into the photosynthetic cells.
- Spongy mesophyll cells contain fewer chloroplasts compared to palisade cells.

- This is found in the midrib and leaf veins.
- Vascular bundle is made of phloem and xylem tissues.
- Xylem tissues conduct water and some dissolved mineral salts from the roots to other plant parts while phloem translocates manufactured food materials from photosynthetic areas to other plant parts.

- This is the organelle in which photosynthesis takes place. It is an oval shaped double membrane bound organelle.
- Internally, it is made up of membranes called lamellae suspended in a fluid filled matrix called stroma.
- Lamellae forms stacks at intervals called grana (singular-granum). Chlorophyll molecules are contained in the grana
- Within the stroma, fat droplets, lipid droplets and starch grains are found.
- The strona contains enzymes and forms the site where light independent reactions take place.

• The leaf has a flat and broad lamina to increase surface area for trapping sunlight energy and for gaseous exchange.
• The leaf has numerous stomata through which photosynthetic gases diffuse.
• The leaf is thin to reduce the distance through which carbon (IV) oxide has to diffuse to the photosynthetic cells.
• The palisade mesophyll cells contain numerous chloroplasts which contain chlorophyll molecules which trap sunlight energy for photosynthesis.
• The photosynthetic mesophyll is located towards the upper surface for maximum absorption of sunlight energy.
• The leaf has an extensive network of veins composed of xylem which conducts water to the photosynthetic cells and phloem to translocate manufactured food materials to other plant parts.
• The epidermis and cuticle are transparent to allow light to penetrate to the photosynthetic cells.

Raw materials for photosynthesis: Water and Carbon (IV) oxide
Conditions for photosynthesis: Light energy and Chlorophyll