Photosynthesis and respiration
Walk through a Wicklow forest on a sunny day and the trees look as if they are doing nothing. In fact they are making food from air and water, and at the same moment they are releasing energy from that food, just as you do when you run for the bus. How can one living thing both build glucose and break it down, and why do the two jobs matter for the air we breathe?
Photosynthesis: making food with light
Plants do not eat. They make their own food by photosynthesis. It happens in the leaves, inside the chloroplasts of the cells.
Chloroplasts contain chlorophyll, the green pigment. Chlorophyll absorbs light energy, and the plant uses that energy to build glucose.
The word equation is:
$$\text{carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{light energy}} \text{glucose} + \text{oxygen}$$The chemical equation is $6\mathrm{CO_2} + 6\mathrm{H_2O} \to \mathrm{C_6H_{12}O_6} + 6\mathrm{O_2}$. You only need the word equation at this level.
Where do the raw materials come from? Carbon dioxide enters the leaf from the air through tiny holes called stomata. Water is taken up by the roots and carried to the leaf. The oxygen leaves through the stomata.
State the name of the green pigment that absorbs light for photosynthesis, and name the two raw materials a plant takes in and the two products it makes.
State the name of the green pigment that absorbs light for photosynthesis, and name the two raw materials a plant takes in and the two products it makes.
Respiration: releasing energy
Every living cell needs energy to move, grow and repair itself. Respiration releases that energy from glucose. It happens in all living cells, day and night, in plants and animals.
$$\text{glucose} + \text{oxygen} \to \text{carbon dioxide} + \text{water} + \text{energy}$$Aerobic respiration takes place in the mitochondria of cells. Photosynthesis takes place in the chloroplasts, which only plant cells have.
Anaerobic respiration
When there is no oxygen, some cells can still release a smaller amount of energy. This is anaerobic respiration.
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In yeast: glucose → ethanol + carbon dioxide (+ a little energy). This is fermentation. The carbon dioxide forms bubbles that make bread dough rise.
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In human muscle cells during intense exercise: glucose → lactic acid (+ a little energy). Lactic acid builds up and can make muscles feel sore.
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the glucose is only partly broken down.
| Aerobic | Anaerobic (yeast) | Anaerobic (muscle) | |
|---|---|---|---|
| Oxygen needed? | Yes | No | No |
| Products | $\mathrm{CO_2}$, water | Ethanol, $\mathrm{CO_2}$ | Lactic acid |
| Energy released | Large | Small | Small |
a) Write the word equation for photosynthesis. b) Write the word equation for aerobic respiration. c) State where in a plant cell photosynthesis takes place and where…
- a) Write the word equation for photosynthesis.
- b) Write the word equation for aerobic respiration.
- c) State where in a plant cell photosynthesis takes place and where respiration takes place.
Yeast cells respire in a sugar solution in a warm kitchen in Sligo, and the bread dough rises. a) Write the word equation for anaerobic respiration in yeast. b) Explain…
Yeast cells respire in a sugar solution in a warm kitchen in Sligo, and the bread dough rises.
- a) Write the word equation for anaerobic respiration in yeast.
- b) Explain why the dough rises.
- c) Compare the amount of energy released per glucose molecule in aerobic respiration and in anaerobic respiration.
- d) Name the product of anaerobic respiration in human muscle cells during intense exercise.
Factors that affect the rate of photosynthesis
A plant is like a factory. If any one of its inputs is short, production slows down. Three factors matter most:
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Light intensity. More light means more energy for chlorophyll, so a faster rate, up to a point.
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Carbon dioxide concentration. More carbon dioxide means more raw material, so a faster rate, up to a point.
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Temperature. Photosynthesis is controlled by enzymes. Warmer temperatures speed up the reactions, up to an optimum (about 30 °C in the graph below). Above that, the enzymes are damaged (denatured) and the rate falls sharply.
Water is also needed. If the plant is short of water, the stomata close, so less carbon dioxide enters and the rate drops.
When light or carbon dioxide is increased, the rate rises and then levels off. At that stage another factor is limiting: it is in short supply and holds the rate back, so adding more light or carbon dioxide makes no difference.
To make this a fair test, change only the independent variable (lamp distance). Keep the others constant, for example temperature (use a water bath or heat-absorbing tank), carbon dioxide (the same amount of sodium hydrogencarbonate in the water), the same pondweed and the same time period. Repeat and take an average to improve reliability.
Name three factors that can affect the rate of photosynthesis in a plant.
Name three factors that can affect the rate of photosynthesis in a plant.
Pondweed in a beaker of water produces bubbles of oxygen. A student counts the bubbles per minute with a lamp at different distances from the beaker: | Distance of lamp…
Pondweed in a beaker of water produces bubbles of oxygen. A student counts the bubbles per minute with a lamp at different distances from the beaker:
| Distance of lamp (cm) | Bubbles per minute |
|---|---|
| 10 | 48 |
| 20 | 30 |
| 40 | 12 |
| 80 | 3 |
- a) State the relationship between light intensity and rate of photosynthesis shown by the results.
- b) Calculate the percentage drop in bubbles per minute when the lamp is moved from $10$ cm to $40$ cm.
- c) Name one other factor that should be kept constant in the experiment.
- d) Explain why the rate would eventually stop increasing even if the light became very bright.
A greenhouse owner in Co. Wexford grows tomatoes. She heats the greenhouse and adds extra carbon dioxide. a) Explain how increasing the carbon dioxide concentration can…
A greenhouse owner in Co. Wexford grows tomatoes. She heats the greenhouse and adds extra carbon dioxide.
- a) Explain how increasing the carbon dioxide concentration can increase the rate of photosynthesis.
- b) Explain why raising the temperature too high would reduce the rate of photosynthesis, referring to enzymes.
- c) Suggest why she also needs to water the plants well.
A graph of the rate of photosynthesis against temperature rises steadily from $5^\circ\mathrm C$ to a maximum at about $30^\circ\mathrm C$, and then falls sharply to…
A graph of the rate of photosynthesis against temperature rises steadily from $5^\circ\mathrm C$ to a maximum at about $30^\circ\mathrm C$, and then falls sharply to almost zero at $50^\circ\mathrm C$.
- a) Explain the rise from $5^\circ\mathrm C$ to $30^\circ\mathrm C$.
- b) Explain the fall above $30^\circ\mathrm C$.
- c) On the same axes, predict the shape of the curve if the light intensity were halved, and justify your prediction.
Two opposite processes in balance
Compare the two equations. The products of one are the reactants of the other.
| Photosynthesis | Respiration | |
|---|---|---|
| Reactants | carbon dioxide, water | glucose, oxygen |
| Products | glucose, oxygen | carbon dioxide, water |
| Energy | light energy is stored in glucose | energy is released from glucose |
| Where | chloroplasts, only in light | mitochondria, all the time |
During the day a green plant does both. If photosynthesis is faster than respiration, the plant takes in carbon dioxide and gives out oxygen. In the dark only respiration happens, so it takes in oxygen and gives out carbon dioxide.
At one particular light intensity the two rates are equal. The carbon dioxide made by respiration is used up by photosynthesis, so the carbon dioxide level in a sealed container stays constant. This is the compensation point. Below the compensation point the plant uses glucose faster than it makes it, so it cannot grow and gradually uses up its stores.
On a global scale the two processes keep the levels of oxygen and carbon dioxide in the atmosphere steady. Cutting down large areas of rainforest removes plants that take in carbon dioxide, so less is absorbed and the level in the air rises.
Photosynthesis is also the start of nearly every food chain. In Dublin Bay, phytoplankton capture light energy and store it in glucose. Small fish eat the phytoplankton (or animals that did), and seals eat the fish. The energy in each link originally came from light captured by photosynthesis.
A sealed container holds a green plant in the light for several hours. The carbon dioxide concentration in the container is measured and found to stay constant. a)…
A sealed container holds a green plant in the light for several hours. The carbon dioxide concentration in the container is measured and found to stay constant.
- a) Explain what this tells you about the rates of photosynthesis and respiration.
- b) Predict, with a reason, what would happen to the carbon dioxide concentration if the container were placed in darkness.
- c) Define the term "compensation point" in this context, and state what happens to the plant's growth if it is kept permanently below it.
Photosynthesis and respiration are described as "opposite processes" that together keep the levels of oxygen and carbon dioxide in the atmosphere steady. a) Compare the…
Photosynthesis and respiration are described as "opposite processes" that together keep the levels of oxygen and carbon dioxide in the atmosphere steady.
- a) Compare the two processes in terms of reactants, products and energy change.
- b) Explain how the destruction of large areas of rainforest could upset this balance.
- c) Explain why the energy stored by photosynthesis is the original source of energy for almost all food chains, including one in Dublin Bay with phytoplankton, small fish and seals.
Back to the start: the trees in Wicklow build glucose in their chloroplasts using light, and release energy from it in their mitochondria. Together with all other plants they keep the oxygen and carbon dioxide levels in the air balanced.