B4 Bioenergetics

Photosynthesis is the process where plants and algae make glucose from carbon dioxide and water, releasing oxygen as a by-product. The reaction is endothermic because energy is transferred from the environment by light.

It happens in chloroplasts, which contain a green pigment called chlorophyll that absorbs the light.

The word equation for photosynthesis is:

carbon dioxide + water → glucose + oxygen

Energy is transferred by light. The balanced symbol equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Carbon dioxide diffuses into the leaf through the stomata. Water is taken up by the roots and carried to the leaf in the xylem.

Uses of the glucose: respiration; storage as insoluble starch, or as fats and oils in seeds; cellulose to strengthen cell walls; and, with nitrate ions from the soil, amino acids that are built into proteins.

Light provides the energy for the reaction. Oxygen diffuses out, but some is used for respiration by the plant.

A large oak tree contains several tonnes of wood, mostly cellulose. Explain where the carbon in the wood came from.

Carbon dioxide from the air diffused into the leaves through the stomata.

In the chloroplasts, photosynthesis used it (with water) to make glucose.

The tree converted glucose into cellulose to build its cell walls, so the carbon came from the air, not the soil.

Plants get their food (glucose) by making it themselves, not from the soil.

Glucose is stored as insoluble starch, not as glucose, because starch is insoluble and does not affect the water balance in cells.

Nitrate ions for making proteins come from the soil, not from the air or from glucose alone.

Plants respire all the time (day and night), not just when there is no light.

The rate of photosynthesis is affected by four main factors: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll.

For light and carbon dioxide, as the factor increases, the rate of photosynthesis also increases until it levels off. It levels off because another factor holds the rate back.

For temperature, the rate rises to an optimum as the particles collide more often and with more energy. Above this optimum temperature, the rate falls steeply because the enzymes that control photosynthesis are denatured.

Less chlorophyll, for example due to a magnesium deficiency or in the pale areas of a variegated leaf, means less light is absorbed and the rate is lower.

The rate is measured as the amount of oxygen produced per unit time or carbon dioxide used per unit time.

Rate = amount ÷ time.

The rate rises and then levels off for light and carbon dioxide; for temperature it rises to an optimum (30 °C here) and then falls as enzymes are denatured.

A piece of pondweed produces 18 cm³ of oxygen in 4 hours. What is the mean rate of photosynthesis in cm³/h?

Rate = amount ÷ time

Rate = 18 ÷ 4 = 4.5 cm³/h

High temperatures do not 'kill' enzymes; they denature them.

The rate does not keep increasing forever as light increases; it will eventually level off.

Photosynthesis produces oxygen; it does not use it up.

Cellular respiration is an exothermic reaction which goes on continuously in every living cell, day and night. It transfers energy to the surroundings.

The energy transferred supplies all the energy needed for living processes, such as building larger molecules from smaller ones, muscle contraction for movement, and keeping mammals and birds warm.

Aerobic respiration uses oxygen. It transfers a large amount of energy because the glucose is completely oxidised. The word equation is:

glucose + oxygen → carbon dioxide + water

The balanced symbol equation is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration happens without oxygen. The oxidation of glucose is incomplete, so it transfers much less energy than aerobic respiration.

In animal cells (like muscles during vigorous exercise), the equation is:

glucose → lactic acid

In plant and yeast cells, the equation is:

glucose → ethanol + carbon dioxide

Anaerobic respiration in yeast cells is called fermentation. It is economically important for making bread (the carbon dioxide makes the dough rise) and alcoholic drinks.

A muscle cell and a yeast cell both respire anaerobically. Give two differences in the products they make.

The muscle cell produces only lactic acid.

The yeast cell produces ethanol and carbon dioxide.

Respiration is not the same as breathing! Breathing is the ventilation of the lungs, while respiration is a chemical reaction in cells.

Remember that plants respire all the time, just like animals. They do not only photosynthesise.

Energy is transferred, not 'made' or 'created'.

Learn the different products for anaerobic respiration: muscles make lactic acid (no carbon dioxide), but plants and yeast make ethanol and carbon dioxide.

During exercise, muscles contract more frequently and need more energy. To supply this, the rate of cellular respiration must increase.

The body responds by increasing heart rate, breathing rate, and breath volume, delivering more oxygen and glucose to muscles and removing extra carbon dioxide.

During vigorous exercise, if oxygen supply cannot keep up, muscles also respire anaerobically. This produces lactic acid and causes an oxygen debt. Over time, muscles become fatigued and stop contracting efficiently.

Investigate the effect of exercise by measuring pulse and breathing rates at rest, and again immediately after a set exercise.

Volume of air breathed per minute = breathing rate × breath volume.

Heart rate rises during exercise, levels off, and stays raised for several minutes of recovery while extra oxygen is supplied to deal with the lactic acid.

Explain why a sprinter's leg muscles make lactic acid during a hard 400 m race.

The muscles contract very fast, so they need energy faster than aerobic respiration can supply it.

The heart and lungs cannot deliver oxygen quickly enough, so the muscles also respire anaerobically.

Anaerobic respiration only partly oxidises glucose, making lactic acid, which builds up in the muscles.

Lactic acid is produced because oxygen is scarce, not when it is plentiful.

Breathing is ventilation, whereas respiration is the chemical reaction releasing energy in cells.

After exercise, heart and breathing rates stay high to supply oxygen for clearing lactic acid. Fitter people have a lower resting heart rate and recover faster.

Metabolism is the sum of all the chemical reactions that happen in a cell or in the whole body.

These reactions are controlled by enzymes. Many of them need a continuous supply of energy, which is transferred by respiration.

Metabolism includes synthesis reactions (building large molecules from smaller ones) and breakdown reactions (breaking large molecules into smaller ones).

Examples of synthesis reactions include:

• Small glucose molecules joining together to form starch (for storage in plants), glycogen (for storage in animals) or cellulose (to build plant cell walls).

• One glycerol molecule joining to three fatty acid molecules to form a lipid.

• Glucose combining with nitrate ions to make amino acids, which are then joined together to make proteins.

Examples of breakdown reactions include:

• Respiration, which breaks down glucose to transfer energy.

• Excess amino acids being broken down to form urea, which is then excreted in urine.

One molecule of glycerol joins to three molecules of fatty acids to synthesise a lipid molecule.

After a meal, a muscle cell takes in extra glucose and stores it. Name the metabolic reaction and explain why the cell needs respiration to carry it out.

The cell joins many glucose molecules together to make glycogen, a synthesis reaction controlled by enzymes.

Building a large molecule needs energy, and that energy is transferred by respiration in the cell's mitochondria.

Remember that metabolism is all the chemical reactions in the body, not just digestion or how fast you use up food.

Plants store glucose as starch, whereas animals store it as glycogen.

Urea is a waste product made from the breakdown of excess amino acids, not from glucose or fat.

The rate of photosynthesis depends on light intensity, carbon dioxide (CO₂) concentration and temperature. These factors interact: at any moment the factor in shortest supply is the limiting factor. Raising a factor that is not limiting has no effect.

Oxygen debt: lactic acid made by anaerobic respiration is carried by the blood to the liver and converted back to glucose. The oxygen debt is the extra oxygen needed after exercise to react with this lactic acid and remove it, so breathing stays fast and deep.

Inverse square law: light intensity is inversely proportional to the square of the distance d from the lamp: light intensity ∝ 1 ÷ d².

Doubling the distance gives one quarter (1 ÷ 2²) of the intensity; halving it gives four times the intensity.

On the shared rising part, light is limiting the rate. Where each line flattens out, CO₂ concentration or temperature is limiting.

A lamp is 12 cm from a plant. It is moved to 4 cm from the plant. How many times greater is the light intensity?

The distance is now 12 ÷ 4 = 3 times smaller.

Light intensity ∝ 1 ÷ d², so the intensity is 3² = 9 times greater (not 3 times).

Square the distance factor: moving a lamp from 5 cm to 20 cm is 4 times the distance, so the intensity becomes 1 ÷ 4² = 1/16 of the original, not 1/4.

Growers can add heat, light and CO₂, but it is only worth paying to raise the factor that is limiting, and only while the extra income from the crop is greater than the cost.

Lactic acid is converted back to glucose in the liver, not in the muscles or kidneys, and it is not breathed out.

Aim: find out how light intensity affects the rate of photosynthesis by measuring the oxygen given off by pondweed.

1. Put a cut piece of pondweed in a boiling tube of water containing a little sodium hydrogencarbonate (supplies carbon dioxide).

2. Place a lamp a measured distance away (metre ruler), with a beaker of water between them as a heat shield.

3. Leave the pondweed for a few minutes to adjust.

4. Count the oxygen bubbles in one minute, or collect the gas and measure its volume.

5. Repeat, then repeat at other distances, and calculate a mean for each.

Independent variable: light intensity (changed by the lamp distance).

Dependent variable: rate of photosynthesis (bubbles or cm³ of oxygen per minute).

Control variables: temperature (heat shield or LED lamp), carbon dioxide (same amount of sodium hydrogencarbonate), the same pondweed and lamp.

Hazards: a filament lamp gets hot and can burn, so do not touch it. Keep water away from the lamp and its plug to avoid electric shock. Take care when cutting the pondweed with a scalpel.

The beaker of water absorbs heat from the lamp so the temperature of the pondweed stays constant.

A student records 30 bubbles per minute with the lamp at 10 cm, 18 at 20 cm and 9 at 30 cm. Describe and explain the pattern.

The rate falls as the lamp moves further away, because the light intensity is lower, so less light energy is absorbed for photosynthesis.

Measuring the gas volume is more accurate than counting bubbles, which vary in size and can be missed.

Turn off room lights so the lamp is the only light source.

Rate = bubbles (or volume) ÷ time; leave out anomalous repeats from the mean.