Cellular Respiration SS2 Biology Lesson Note

DEFINITION/PHASES OF RESPIRATION

The process of respiration involves the taking in of oxygen, its transport within the body of the organism, its exchange in the cells and the eventual release of energy in the form of ATP, water and carbon(IV)oxide. The energy is utilized by the cells for their daily activities. Respiration can therefore be defined as a biochemical activity of the cell in which glucose is broken down in a series of reactions controlled by enzymes to release energy.

The following are the different phases of respiration:

1. External Respiration (Breathing)

This is the taking in of oxygen (inhalation)into the respiratory organ(e.g. lungs or gills) and breathing out (exhalation) of carbon(IV)oxide and water vapour.

2. Internal (Tissue) Respiration 

This is the oxidation of food substances within the cells leading to the release of energy, carbon(IV)oxide, and water. This is made possible by the oxygen taken in through the breathing process. This can be represented by the following equation

C6H12O6 +6O6 →6H2O+6C02+Energy (ATP)

CONDITIONS NECESSARY FOR RESPIRATION

For the effective exchange of gases to take place, the following conditions must be met:

1. Respiratory Medium: This refers to the environment from which the organism picks up oxygen e.g. air and water.
2. Respiratory Organ: This refers to the organ possessed by the organism needed to pick oxygen from the environment and pass out CO2 and water vapour e.g. lungs in mammals and gills in fishes, etc.
3. Transport Medium: This is needed to transport dissolved oxygen to the various cells of the body which in turn picks up CO2 and other waste products for the elimination process e.g. blood in mammals.
4. Ventilation: This is the movement of air in the direction over the respiratory surface. This ensures the replacement of used oxygen and the elimination of waste products. An increase in the rate of gaseous exchange is facilitated by ventilation mechanisms e.g. breathing in humans.
5. Respiratory surface: This refers to the actual surface of the body where gaseous exchange takes place e.g. alveoli in mammals and cell membrane in amoeba

CHARACTERISTICS OF RESPIRATORY SURFACE

The following characteristics must be exhibited by all respiratory surfaces be it in plants or animals:

1. The respiratory surface must be moistened because gases diffuse in solution through them.
2. It must be permeable to allow gas to pass in and out of them.
3. It must be thin-walled to make diffusion easier and faster.
4. It must have an adequate supply of transport medium e.g. blood.
5. The surface must be large to aid the easy diffusion of gases.
6. It must be highly vascularized i.e. it must have lots of capillaries or similar networks to bring in and take away gases.

CELLULAR (INTERNAL) RESPIRATION

The oxidation of glucose to release energy is known as cellular respiration and it occurs in the mitochondria (powerhouse) of all living cells.

Within the cytoplasm of the cells, one molecule of 6-carbon sugar is broken down into two molecules of 3-carbon pyruvate catalysed by the enzymes in the cytoplasm. 

This process does not require oxygen. Each pyruvic acid is further oxidized completely to carbon dioxide and water in the mitochondria. The breakdown of glucose to pyruvic acid is termed glycolysis while a series of chemical reactions occurring within the mitochondrion, responsible for the final breakdown of food molecules to form carbon dioxide, water, and energy carried out by seven enzymes is known as the Krebs cycle (citric acid cycle). Most of the ATPs are generated in the Krebs Cycle (36 ATP). A total of 38 ATP molecules are formed when one molecule of glucose is completely oxidized.

KREB CYCLE

AEROBIC AND ANAEROBIC RESPIRATION

In most cells, cellular respiration takes place in the presence of oxygen and this is known as aerobic respiration. The largest amount of ATP possible is generated through it from one molecule of glucose (38 ATP).

In some other organisms, the cells get energy from breaking down glucose in the absence of oxygen, this is known as anaerobic respiration. Only two ATPs are produced. Lactic acid often results from anaerobic respiration instead of pyruvic acid in animals which makes it useful in the production of yoghurt. In plants, alcohol and carbon(IV)oxide are produced. 

RESPIRATION IN PLANT

There is no special respiratory organ in the plant. Gases move in and out of the plant through the stomata and lenticels.

1. Stomata:They are tiny pores in the lower epidermis of leaves. Each stoma is enclosed within two bean-shaped cells known as guard cells. It regulates the opening  and closing of the stomata,
2. Lenticels: These are breathing pores or tiny openings found in the bark of older stems. Lenticles consist of a loose mass of small thin-walled cells which permits easy diffusion of gasses in and out of the plant.

MECHANISM OF GASEOUS EXCHANGE IN PLANTS

Oxygen, carbon dioxide, and water vapour are released by a simple diffusion process in plants. Oxygen diffuses into plants through the stomata and lenticels while CO2 and water vapour diffuses out of the plant through the same opening. This is facilitated by the difference in the concentration gradient of these gases. 

The plant takes in oxygen mostly during the night and gives out carbon dioxide and water vapour during the day due to the photosynthetic activities of the plant. Oxygen is the by-product of photosynthesis.

The opening and closing of the stomata are regulated by the guard cells. When the guard cell is turgid, the stomata open but when the cells become flaccid the stomata are closed.

ASSIGNMENT

1. During anaerobic respiration, how many ATP are produced?  2 ATP B. 3 ATP C. 4 ATP D. 5 ATP

2. The cartilaginous flap which prevents food from entering the windpipe is  glottis B. epiglottis C. larynx D. oesophagus

3. The gaseous exchange in mammals takes place in the trachea Bronchus C. alveolus D. lungs

4. When the guard cell is turgid, the stomata opens B. closes C. shrinks D. breaks

5. In strenuous activities in the absence of oxygen, glucose is broken down into pyruvic acid B. carbon dioxide C. lactic acid D. alcohol

6. Explain fermentation.
7. Differentiate between internal and external respiration.