Cellular Energy: Photosynthesis and Respiration

Photosynthesis and Cellular Respiration

Photosynthesis

Photosynthesis is the process by which plants convert carbon dioxide and water into glucose and oxygen, using light energy. This endothermic reaction can be summarized by the following equation:

6CO₂ + 6H₂O -> C₆H₁₂O₆ + 6O₂

The rate of photosynthesis is influenced by several factors, often referred to as limiting factors. These include:

• Light intensity: Increased light levels generally lead to a higher rate of photosynthesis until an optimum level is reached.
• Carbon dioxide concentration: Higher levels of carbon dioxide can enhance the rate of photosynthesis, up to a certain point.
• Temperature: Plants in their ideal temperature range perform photosynthesis more efficiently than those outside this range.

Understanding these factors can help improve crop yields and make farming more efficient.

Experiment: Investigating the Effect of Light Intensity on Photosynthesis

1. Place a boiling tube 10 cm away from an LED light source.
2. Fill the boiling tube with a sodium hydrogen carbonate solution, which provides carbon dioxide.
3. Insert a piece of pondweed into the boiling tube with the cut end facing up.
4. Allow the setup to adjust for 5 minutes.
5. Count the number of oxygen bubbles produced in 1 minute using a stopwatch.
6. Repeat the experiment three times and calculate the average number of bubbles.
7. Repeat steps 1-6, placing the boiling tube at distances of 20 cm, 30 cm, 40 cm, and 50 cm from the light source.
8. Calculate the light intensity (1/d²) and plot a graph to show the relationship between light intensity and the rate of photosynthesis.

Uses of Glucose in Plants

Glucose produced during photosynthesis is used in various ways:

• Respiration: To release energy.
• Cellulose production: To build cell walls.
• Starch storage: As a form of energy reserve.
• Fat/oil production: For storage.
• Amino acid synthesis: Combined with nitrates from the soil to produce proteins.

Cellular Respiration

Cellular respiration is a chemical reaction that occurs in all living organisms to release energy from glucose. This process is essential for movement, growth, and maintaining body temperature.

Aerobic Respiration

Aerobic respiration requires oxygen and is essentially the reverse of photosynthesis:

C₆H₁₂O₆ + 6O₂ -> 6CO₂ + 6H₂O + energy

Most reactions in aerobic respiration occur in the mitochondria. This process takes place continuously in both plants and animals. Cells with high energy demands, such as muscle, sperm, egg, and liver cells, contain more mitochondria.

Anaerobic Respiration

Anaerobic respiration occurs when there is insufficient oxygen. It is a short-term process that produces less energy than aerobic respiration. In humans, anaerobic respiration results in the production of lactic acid, which can cause muscle cramps. In plants and microorganisms, it leads to fermentation:

• Animals: Glucose -> Energy + Lactic acid
• Plants and Microorganisms: Glucose -> Carbon dioxide + Ethanol + Energy (Fermentation)

Both aerobic and anaerobic respiration use the same amount of glucose, but anaerobic respiration is less energy-efficient.

Oxygen Debt

Oxygen debt is the amount of oxygen required to break down the lactic acid produced during anaerobic respiration. The longer the period of anaerobic respiration, the greater the oxygen debt.

Responses to Exercise

During exercise, the body undergoes several physiological changes:

• Sweating: To cool the body through evaporation.
• Increased breathing rate: To supply more oxygen to the blood.
• Increased heart rate: To deliver more glucose and oxygen to muscles and remove lactic acid.
• Vasodilation: Widening of blood vessels to increase blood flow to the skin and lower body temperature.

Metabolism

Metabolism refers to the sum of all chemical reactions in an organism. Catabolic reactions break down larger molecules into smaller ones.