Some Properties and Functions of the Cell – SS1 Biology Lesson Note (First Term)

Nutrition is how organisms obtain food. Autotrophs, mainly green plants, make their own organic food from inorganic materials such as water and carbon dioxide, by photosynthesis or chemosynthesis. Heterotrophs, including all animals, fungi, protozoa and some bacteria, depend directly or indirectly on autotrophs. Plants also need mineral elements from the soil for normal growth.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Define autotrophic and heterotrophic nutrition.
2. Describe the heterotrophic modes of feeding (holozoic, parasitic, saprophytic, symbiotic and carnivorous/insectivorous) and give examples of each.
3. Write the equation for photosynthesis and state its features and importance.
4. Explain what chemosynthesis is and name organisms that carry it out.
5. List the macronutrients and micronutrients that plants need, with their importance and deficiency symptoms.

What is autotrophic nutrition?

Plants are referred to as autotrophs, that is, they are able to manufacture their food (organic compounds from inorganic materials such as water and carbon dioxide).

Autotrophs generally have two modes of nutrition, which are chemosynthesis and photosynthesis (holophytic).

What is heterotrophic nutrition?

This is a type of nutrition in which organisms depend directly or indirectly on autotrophs for food. All animals, fungi, protozoa and some bacteria belong to this group and they are referred to as heterotrophs.

Holozoic nutrition

This is the mode of feeding in which food is ingested in the form of complex organic substances, which are digested and assimilated into the body. Holozoic organisms are grouped into carnivores, herbivores, omnivores and scavengers.

Parasitic nutrition

This is the mode of feeding in which certain organisms (parasites) feed on other organisms (hosts). Parasites can be endoparasites (tapeworm, liver fluke) or ectoparasites (lice, ticks), depending on whether they live inside or outside the host. Plant parasites include dodder and mistletoe. Parasites have special structures such as haustoria, suckers, hooks and muscular lips.

Saprophytic nutrition

This is a feeding process in which certain organisms (saprophytes) feed on non-living organic matter, that is, plant and animal remains. Saprophytes secrete digestive enzymes into the dead organic matter to digest the food (extracellular digestion), and the soluble food substances are then absorbed into their body. Examples include fungi such as mucor, mushroom and yeast, and some bacteria.

Symbiotic nutrition

This is a mode of feeding in which two organisms of different species, called symbionts, live together and derive mutual benefits from each other, for example nutrients or food, shelter and moisture. Examples are the nitrogen-fixing bacteria called Rhizobia, which live in the root nodules of legumes, and the algae and fungi in a lichen.

Carnivorous / insectivorous nutrition

This is a feeding process in which some plants have special devices that trap and digest insects. These plants grow in poor soil, and the proteins obtained from the insects are used to supplement their nitrogenous compound requirement. They secrete enzymes which help in digesting their victims. Examples include bladderwort (Utricularia), pitcher plant, sundew and Venus flytrap.

What are the modes of autotrophic nutrition?

Autotrophs make their food by photosynthesis or by chemosynthesis.

Photosynthesis

This is the process by which green plants synthesise organic compounds from inorganic raw materials in the presence of chlorophyll and light.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The reaction takes place in the presence of light and chlorophyll. Carbon dioxide and water are the inorganic materials, C₆H₁₂O₆ is the organic compound (sugar), and oxygen is the by-product.

The main product of photosynthesis is sugar (carbohydrate), while the oxygen given up is a by-product which is released into the atmosphere.

Features (characteristics) of photosynthesis

1. Photosynthesis occurs in the chloroplasts of cells in the presence of light.
2. The raw materials required are low-energy inorganic compounds, that is, CO₂ and H₂O.
3. The final product is a high-energy compound (C₆H₁₂O₆). Photosynthesis is therefore an anabolic process (a metabolic building-up process).
4. The energy required is obtained from solar energy through chlorophyll (the green pigment present in the chloroplasts of green plants).

Carbon dioxide and oxygen diffuse in and out (respectively) between the surrounding air and the leaf, passing through the stomata on the leaf surface. The sugar made is stored in insoluble form (as starch) until the day is dark. Then it is converted back to sugar and transported to other parts of the plant where it is needed. The movement of the sugar is called translocation.

The leaf is the main photosynthetic organ of the plant, although all green parts of a plant are photosynthetic. The main site of photosynthesis in a leaf is the palisade mesophyll.

Importance of photosynthesis

The energy-rich product of photosynthesis is used by plants and animals in the following ways:

1. It is the source of energy for their metabolic activities.
2. It is needed in the food cycle, being the only process that can trap sun energy.
3. All animals and other heterotrophic plants are directly or indirectly dependent on green plants for food.
4. Photosynthesis assists in the purification of the environment by removing carbon (IV) oxide from it while oxygen is released into it.
5. It is the basis for manufacturing other complex organic compounds such as lipids and proteins.

Chemosynthesis

This is the process by which non-green plants (e.g. Nitrosomonas, Nitrobacter, iron bacteria etc.) synthesise organic compounds from inorganic materials (carbon dioxide and water). The energy used in this process is obtained from inorganic substances such as ammonia and hydrogen sulphide.

2H₂S + O₂ → 2S + 2H₂O + chemical energy

CO₂ + H₂O → CH₂O (using the chemical energy released)

What are the mineral requirements of plants?

Mineral elements are required for the normal growth and development of green plants. In addition to carbon, oxygen and hydrogen, other mineral requirements are discussed below. Unlike glucose and other carbohydrates, which green plants make from carbon dioxide and water, mineral salts are obtained from the soil.

Macronutrients

These are mineral nutrients that are required by plants in large quantities. They include the following:

Micronutrients

These are mineral nutrients required by plants in small quantities. They include the following:

Evaluation

1. Define heterotrophic nutrition.
2. Give two examples of endoparasites and two examples of ectoparasites.
3. Write the equation for photosynthesis.
4. State three ways in which photosynthesis is important.
5. What is a saprophyte? Give two examples.

Assignment

1. The main photosynthetic organ of the plant is —– (A) leaf (B) seed (C) fruit (D) root
2. Which of these is not a macro element needed for plant growth? (A) potassium (B) magnesium (C) sodium (D) molybdenum
3. Yellowing of leaves is a symptom associated with deficiency of (A) iron, calcium and magnesium (B) nitrogen, sulphur and potassium (C) sulphur, phosphorus and iron (D) iron, magnesium and nitrogen
4. Trace elements are required by plants mainly for the (A) formation of pigments and enzymes (B) production of energy and hormones (C) manufacture of carbohydrates (D) manufacture of protein
5. The dark phase of photosynthesis occurs in the ——- of chloroplast (A) cells (B) tissues (C) fluid (D) stroma
6. What are macronutrients?
7. List three macro elements that are essential for protein synthesis.
8. What is autotrophic nutrition?
9. (a) List the two modes of autotrophic nutrition. (b) Differentiate between the two modes listed above.