bioenergetics

Photosynthesis word equation

Carbon dioxide + water → (light + chlorophyll) → glucose + oxygen.

Photosynthesis symbol equation

6CO2 + 6H2O → C6H12O6 + 6O2 (light energy required, chlorophyll catalyses).

Photosynthesis is endothermic

Energy is transferred FROM the environment (light) TO the chloroplasts. Light energy is converted to chemical energy in glucose.

Where photosynthesis happens

In the chloroplasts of plant cells, mainly in leaf palisade cells. Chlorophyll absorbs light energy.

Limiting factor

A factor in shortest supply that limits the rate of a reaction. Increasing it increases rate; further increase has no effect once another factor becomes limiting.

Limiting factors of photosynthesis

Light intensity, CO2 concentration, temperature, and chlorophyll amount (e.g. due to disease or mineral deficiency).

Light intensity graph

Rate rises proportionally with light then plateaus when another factor becomes limiting.

CO2 concentration graph

Rate rises with CO2 then plateaus when another factor (light/temperature) is limiting.

Temperature graph

Rate increases up to optimum, then drops sharply as enzymes (e.g. Rubisco) denature.

Inverse square law

Light intensity ∝ 1 ÷ distance². Doubling the distance from a light source reduces intensity to a quarter.

Photosynthesis required practical

Place pondweed (e.g. Cabomba) in water with NaHCO3 (CO2 source). Vary distance from lamp. Count oxygen bubbles per minute. Use 1/d² for light intensity. Keep temperature constant using a heat shield/water beaker.

Variables in pondweed practical

Independent: distance from lamp (light intensity). Dependent: bubbles per minute. Control: temperature, CO2 conc, same pondweed, same time period.

Why temperature is controlled

Heat from the lamp could increase enzyme activity and confound the result — use a heat shield or beaker of water between lamp and plant.

Greenhouse commercial use

Farmers control limiting factors: paraffin heaters add CO2 and heat, artificial light extends growing time, shading prevents overheating. Costs must be balanced against profit.

Uses of glucose in plants

1. Respiration (release energy). 2. Converted to insoluble starch for storage. 3. Used to make cellulose (cell walls). 4. Used to make amino acids (with nitrate ions) → proteins. 5. Stored as lipids in seeds.

Why store glucose as starch

Insoluble, so doesn't affect water potential or move out of storage. Compact.

Compensation point

The light intensity at which photosynthesis rate = respiration rate, so no net gas exchange. Below this, plants release more CO2 than they absorb.

6-marker tip: limiting factors

State factor → describe shape of graph → explain why (link to either enzymes, chlorophyll, or rate of chemical reactions) → name what becomes limiting next.

4b Respiration & Metabolism

Aerobic respiration equation

Glucose + oxygen → carbon dioxide + water (+ energy). C6H12O6 + 6O2 → 6CO2 + 6H2O.

Respiration is exothermic

Energy is transferred FROM the chemical reaction TO the environment/cell. It is a continuous process in all living cells.

Site of respiration

Mainly in mitochondria (aerobic). Some occurs in the cytoplasm (anaerobic).

Uses of energy from respiration

Movement (muscle contraction), keeping warm (mammals/birds), active transport, protein synthesis (building amino acids into larger molecules), cell division.

Anaerobic respiration in muscles

Glucose → lactic acid. Releases LESS energy than aerobic because oxidation is incomplete. Builds up during vigorous exercise.

Anaerobic respiration in yeast/plants

Glucose → ethanol + carbon dioxide. Called fermentation. Used in brewing (alcohol) and bread-making (CO2 makes dough rise).

Why anaerobic releases less energy

Glucose is not fully broken down — lactic acid/ethanol still contain chemical energy.

Aerobic vs anaerobic — comparison

Aerobic: needs O2, produces CO2 + H2O, lots of energy, in mitochondria. Anaerobic: no O2, produces lactic acid (animal)/ethanol+CO2 (plant/yeast), less energy, in cytoplasm.

Oxygen debt

The extra oxygen needed after exercise to break down accumulated lactic acid (transported to the liver and converted back to glucose). Causes you to keep breathing heavily after exercise.

Response to exercise

Heart rate increases (more blood to muscles), breathing rate and depth increase (more O2 in, more CO2 out), muscles respire more.

If insufficient oxygen during exercise

Muscles respire anaerobically → lactic acid builds up → muscle fatigue and cramp.

Why heart rate increases during exercise

To supply muscles with more oxygenated blood and glucose for respiration, and remove CO2 and lactic acid faster.

Metabolism

The sum of all the chemical reactions in a cell or the body. Includes building large molecules (anabolism) and breaking down molecules (catabolism).

Examples of metabolic reactions

Glucose → starch/glycogen/cellulose. Glycerol + fatty acids → lipids. Glucose + nitrate → amino acids → proteins. Breakdown of excess proteins → urea (excreted).

Glycogen

The animal storage form of glucose, stored in liver and muscles.

Why we need urea

Excess amino acids cannot be stored. The liver deaminates them, forming ammonia → urea, which is excreted by the kidneys in urine.

Why mitochondria are abundant in some cells

Cells with high energy demand (muscle, sperm, liver, root hair cells) have more mitochondria to release more energy from respiration.

6-marker tip: aerobic vs anaerobic

Compare oxygen requirement, products, energy yield, location (mitochondria vs cytoplasm), and when each occurs.