Bioenergetics

Photosynthesis is the process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. It is essential because it stores light energy as chemical energy in glucose and provides organic material for plant growth.

The word equation for photosynthesis is: carbon dioxide + water → glucose + oxygen. The balanced chemical equation is $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$. Students should recognise the symbols $CO_2$, $H_2O$, $O_2$ and $C_6H_{12}O_6$.

Photosynthesis mainly occurs in chloroplasts. Chloroplasts contain chlorophyll, which absorbs light energy. Palisade mesophyll cells in leaves contain many chloroplasts and are therefore particularly well adapted for photosynthesis.

Photosynthesis is an endothermic reaction because energy is transferred from the surroundings into the plant. The energy source is light, which is absorbed by chlorophyll and used to drive the chemical reactions that produce glucose.

The rate of photosynthesis is affected by light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll present. A limiting factor is the factor that is in shortest supply and therefore prevents the rate from increasing further.

At low light intensity, increasing light usually increases the rate of photosynthesis. At higher light intensities the rate may level off because another factor, such as carbon dioxide concentration or temperature, becomes limiting. Increasing carbon dioxide concentration has a similar effect: the rate rises until another factor becomes limiting.

Temperature affects photosynthesis because the process is controlled by enzymes. Increasing temperature initially increases the rate because molecules have more kinetic energy and enzyme-controlled reactions occur faster. Above the optimum temperature, enzymes may denature and the rate falls.

The rate of photosynthesis can be investigated using aquatic plants such as pondweed. Oxygen production can be estimated by counting bubbles or, more accurately, by measuring the volume of oxygen produced over a fixed time. Rate is calculated using $rate = change$ in quantity ÷ time.

In a photosynthesis investigation, the independent variable is the factor deliberately changed, such as light intensity, distance from a lamp, carbon dioxide concentration or temperature. The dependent variable is the measured response, such as oxygen bubbles per minute or oxygen volume produced. Control variables should be kept constant to ensure a fair test.

Useful control variables in photosynthesis experiments include the species and length of pondweed, temperature, carbon dioxide concentration, wavelength or colour of light, volume of water and measuring time. Repeats should be carried out and a mean calculated to improve reliability.

Glucose produced by photosynthesis has several important uses. It may be used immediately in respiration, converted into insoluble starch for storage, converted into cellulose for cell walls, converted into fats and oils for storage, or combined with nitrate ions absorbed from the soil to make amino acids and then proteins.

Cellular respiration is an exothermic process that transfers energy from glucose for use by living cells. Energy released by respiration is required for movement, active transport, maintaining body temperature and building larger molecules from smaller ones.

Aerobic respiration requires oxygen and is represented by the word equation glucose + oxygen → carbon dioxide + water. Most aerobic respiration in eukaryotic cells occurs in mitochondria. Cells that require large amounts of energy, such as muscle cells, often contain many mitochondria.

Anaerobic respiration occurs when oxygen is unavailable or insufficient. In animal muscle cells glucose is incompletely broken down to lactic acid. Less energy is transferred than in aerobic respiration because glucose is not completely oxidised. In yeast and plant cells anaerobic respiration produces ethanol and carbon dioxide and is called fermentation.

During vigorous exercise, heart rate, breathing rate and breath volume increase to deliver more oxygen and glucose to muscles. If oxygen supply cannot meet demand, anaerobic respiration occurs and lactic acid accumulates. Oxygen debt is the extra oxygen required after exercise to deal with accumulated lactic acid. Metabolism is the sum of all chemical reactions in cells and includes respiration, synthesis of carbohydrates, lipids and proteins, conversion of glucose into storage molecules and breakdown of excess amino acids.