CELLULAR RESPIRATION Lesson Note

Learning Objectives

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

Cellular respiration is the chemical process by which living cells break down food substances, especially glucose, to release energy for various life activities.

The energy released is stored temporarily in a special energy-carrying substance called ATP (adenosine triphosphate).

Respiration takes place continuously in living organisms because cells require energy for activities such as:

*Movement and locomotion

*Growth and development

“Active transport

*Cell division

*Synthesis of substances

*Maintenance of body temperature

*Repair of damaged tissues

*Germination and growth in plants

General equation for aerobic respiration:

Glucose + Oxygen → Carbon dioxide + Water + Energy

Or:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

There are two major types:

Aerobic respiration is the complete breakdown of glucose in the presence of oxygen to release a large amount of energy.

It occurs mainly in the mitochondria of cells, although the first stage begins in the cytoplasm.

Equation:

Glucose + Oxygen → Carbon dioxide + Water + Large amount of energy

Aerobic respiration produces much more energy than anaerobic respiration.

Stages of Aerobic Respiration

Aerobic respiration can be considered in two major stages:

Takes place in the cytoplasm.

One molecule of glucose is broken down into smaller molecules called pyruvate.

A small amount of ATP is produced.

Takes place mainly in the mitochondria. Pyruvate is further broken down. Carbon dioxide is produced. Oxygen is required for the final stages.A large amount of ATP is produced.

The mitochondrion is therefore often described as the site of aerobic respiration.

Anaerobic respiration is the breakdown of glucose to release energy without the use of oxygen. It releases less energy than aerobic respiration. Anaerobic respiration occurs in some microorganisms and can also occur temporarily in the muscles of animals when oxygen supply is insufficient.

In yeast and some plant tissues, glucose is converted into ethanol, carbon dioxide and a small amount of energy.

Equation:

Glucose → Ethanol + Carbon dioxide + Small amount of energy

This process is also called alcoholic fermentation.

IMPORTANCE OF FERMENTATION

Fermentation by yeast is useful in:

“Bread making

*Production of some beverages

*Production of ethanol

*Some industrial processes

When vigorous exercise occurs, the muscles may require energy faster than oxygen can be supplied.

Under such conditions, muscle cells can obtain some energy anaerobically.

Glucose is converted to lactic acid, releasing a small amount of energy.

Equation:

Glucose → Lactic acid + Small amount of energy

The accumulation of lactic acid in muscles is associated with muscle fatigue.

After exercise, breathing may remain faster and deeper for a period so that the body can obtain sufficient oxygen and process the accumulated products of anaerobic respiration. This increased need for oxygen is commonly referred to as oxygen debt.

Aerobic Respiration  Anaerobic Respiration

*Requires oxygen Does not require oxygen

*Complete breakdown of glucose  Incomplete breakdown of glucose

*Produces a large amount of energy Produces a small amount of energy

*Produces carbon dioxide and water May produce ethanol and carbon dioxide in

yeast, or lactic acid in muscles

*Mainly associated with mitochondria Does not require mitochondria for

after glycolysis the energy-releasing pathway

*Can continue efficiently when   usually occurs when oxygen is oxygen is available absent or insufficient

ATP (adenosine triphosphate) is the main immediate energy carrier in cells.

Energy released during respiration is used to form ATP. When ATP is broken down, energy is released for cellular activities.

ATP provides energy for:

*Active transport

*Muscle contraction

*Movement of cells

*Protein synthesis

*Cell division

*Growth

Repair of tissues

*Synthesis of important substances

Therefore, respiration is essential because it provides the energy needed to produce and maintain ATP.

The rate of respiration can be affected by several factors.

Respiration is controlled by enzymes. Increasing temperature within a suitable range generally increases the rate of respiration. Very high temperatures can damage enzymes and reduce respiration.

For aerobic respiration, an adequate supply of oxygen is necessary. A shortage of oxygen can reduce aerobic respiration and encourage anaerobic respiration.

Water is necessary for many metabolic reactions. Lack of water can reduce the rate of respiration, especially in plant tissues.

The availability of glucose and other respiratory substrates can affect the rate of respiration.

Young, growing tissues generally have high metabolic activity and may respire faster than older tissues.

In animals, increased physical activity increases the demand for energy. This causes respiration and breathing rates to increase.

Cellular respiration is important because it:

SUMMARY

Cellular respiration is the process by which cells release energy from food substances. It may be aerobic, requiring oxygen and producing a large amount of energy, or anaerobic, occurring without oxygen and producing a smaller amount of energy. Aerobic respiration mainly involves the mitochondria, while anaerobic respiration can occur in the cytoplasm. The energy released is used to form ATP, which powers many activities of living cells.