Chapter 6 of 8

All 8 chapters
  1. 1Cell structure in detail, readunderstood
  2. 2Microscopes and size, readunderstood
  3. 3Diffusion, osmosis and active transport, readunderstood
  4. 4Cell division, readunderstood
  5. 5Enzymes, readunderstood
  6. 6Photosynthesis, readunderstood
  7. 7Testing photosynthesis, readunderstood
  8. 8Respiration, readunderstood

S4.1.6 Chapter 6

Photosynthesis

A tree grows from a seed to many tonnes of wood. Where does all that material come from?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Write the word and symbol equations for photosynthesis
  • Explain how light, carbon dioxide and temperature limit the rate

In the 1600s Jan van Helmont grew a young willow tree in a pot of dried soil. He gave it only water. After five years the tree had gained about 74 kg. He weighed the dried soil again. It had lost only about 60 g.

So the tree did not build itself from the soil. Van Helmont thought the water had become wood. He was partly right. About half of a living tree is water. Most of the rest, the dry mass, comes from carbon dioxide in the air, which the leaves take in. The process is photosynthesis.

Photosynthesis makes glucose from carbon dioxide and water, using the energy of light. It happens in the chloroplasts, which contain the green substance chlorophyll. Chlorophyll absorbs the light.

A plant with five labelled arrows. The labels are light in, carbon dioxide in, water from the roots, oxygen out and glucose made.light incarbondioxide inwaterfrom therootsoxygen outglucosemade
Light, carbon dioxide and water go in. Glucose is made in the leaf. Oxygen goes out. Carbon dioxide and oxygen pass through tiny holes in the leaf called stomata.

Photosynthesis takes in energy. The energy of light is stored as chemical energy in the glucose. This is why photosynthesis is the start of almost every food chain.

The word equation for photosynthesis. A box labelled carbon dioxide + water, then an arrow labelled light, chlorophyll, then a box labelled glucose + oxygen.carbon dioxide + waterlight, chlorophyllglucose + oxygen
Read it from top to bottom. Light and chlorophyll are the conditions, so they are written beside the arrow. Light gives the energy. Chlorophyll is not used up.

The same equation in symbols uses the formulae. Carbon dioxide is CO₂, water is H₂O, glucose is C₆H₁₂O₆ and oxygen is O₂.

The symbol equation for photosynthesis. A box labelled 6CO₂ + 6H₂O, then an arrow labelled light, chlorophyll, then a box labelled C₆H₁₂O₆ + 6O₂.6CO₂ + 6H₂Olight, chlorophyllC₆H₁₂O₆ + 6O₂
The numbers in front are how many molecules take part. A molecule of glucose needs 6 molecules of carbon dioxide and 6 of water.

The equation is balanced when each side has the same number of atoms of every element. Count them. The numbers in front multiply the whole formula.

A table with the columns Atom, Left and Right. Carbon: 6 and 6. Hydrogen: 12 and 12. Oxygen: 18 and 18.AtomLeftRightcarbon66hydrogen1212oxygen1818
The two sides match for every element, so the equation is balanced. No atoms are lost or made. They are rearranged.

Worked example

How many oxygen atoms are on the left of the equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂?

  1. Count the oxygen in the carbon dioxide. 6CO₂ is 6 molecules with 2 oxygen atoms each: 6 × 2 = 12.
  2. Count the oxygen in the water. 6H₂O is 6 molecules with 1 oxygen atom each: 6 × 1 = 6.
  3. Add them: 12 + 6 = 18.

Answer: 18 oxygen atoms

The plant uses the glucose in several ways. It releases energy from it by respiration. It stores some as starch. It makes cellulose for cell walls. With nitrates from the soil it makes proteins.

How fast does photosynthesis go? Its rate depends on three things: light, carbon dioxide and temperature. If one of these is in short supply, it limits the rate. It is called the limiting factor.

Light first. In dim light, more light gives a higher rate. But a plant with little carbon dioxide soon stops gaining. The line goes flat, because carbon dioxide has now become the limiting factor.

A line graph with two lines. The x axis is light as % of full sun, from 0 to 100 in steps of 25. The y axis is rate as % of the highest, from 0 to 100 in steps of 20. The line labelled high CO₂ rises steadily to 100 at 100. The line labelled low CO₂ rises to 40 at 50 and stays there. The two lines are the same up to 25.0255075100020406080100high CO₂low CO₂light as % of full sunrate as % of the highest
CO₂ is carbon dioxide. At low light both lines are the same, because light is the limiting factor. Then low CO₂ goes flat. High CO₂ keeps rising.

Worked example

At 75% of full sun, how much higher is the rate with high carbon dioxide than with low carbon dioxide?

  1. Read the high CO₂ line at 75. It is at 75% of the highest rate.
  2. Read the low CO₂ line at 75. It is at 40% of the highest rate.
  3. 75 − 40 = 35, so the rate is 35% of the highest rate higher.

Answer: 35% of the highest rate

Temperature acts in two ways. A warmer leaf has faster enzymes, so the rate rises. But above a certain temperature the enzymes begin to be denatured, and the rate falls fast. The best temperature differs from one plant to another.

A line graph. The x axis is temperature in °C, from 0 to 60 in steps of 10. The y axis is rate as % of the highest, from 0 to 100 in steps of 20. The rate rises from 5 at 0 °C to 100 at 40 °C, then falls to 70 at 45 °C, 20 at 50 °C and 0 at 60 °C.0102030405060020406080100temperature in °Crate as % of the highest
The rate climbs to a peak near 40 °C for this plant, which grows in a hot place. Many plants work best between about 25 °C and 35 °C. Above the peak the enzymes start to be denatured.

Growers use this in a greenhouse. They can add light at dusk, raise the carbon dioxide, or keep the air warm. Only the factor that is in shortest supply helps. Adding more of another gives no gain.

Try it

Does a leaf need light?

You need: a living plant with soft leaves, growing in soil or a pot, a strip of dark paper, such as brown paper, a paper clip or tape, a pencil and paper.

  1. Choose one leaf that gets light for most of the day. Draw it.
  2. Cover a strip across the middle of the leaf with dark paper, front and back. Fix it with a paper clip or tape. Leave the rest of the leaf uncovered.
  3. Leave the plant where it was for a week.
  4. Take the paper off. Compare the covered strip with the part that was uncovered.

What you will see: The covered strip is usually paler, because a leaf needs light to keep making chlorophyll. In a lab, scientists test the leaf for starch with iodine. They find none in the covered strip, which shows it made no glucose there. That test uses hot alcohol and is not done at home. The effect is smaller on some plants, so a plant with soft leaves is best.

If you cannot try it, this is what happens.
A table with the columns Part of leaf and After a week. The covered strip is pale. The uncovered part is green.Part of leafAfter a weekcovered strippaleuncoveredgreen

A wrong idea many people have

Plants get their food from the soil.

Why it feels right: We feed plants with manure and fertiliser, and we see roots going into the soil.

The test that breaks it: Van Helmont weighed his willow tree and the soil. The tree gained about 74 kg. The soil lost only about 60 g. The soil could not supply the mass.

What is true: A plant makes its own food by photosynthesis. The soil supplies water and a few minerals, such as nitrate. Most of the plant is built from carbon dioxide and water.

Key words

photosynthesis
the process in which a plant uses light to make glucose and oxygen from carbon dioxide and water.
chlorophyll
the green substance in chloroplasts that absorbs light.
glucose
a sugar that the plant makes and uses as food and as a building material.
limiting factor
the thing in shortest supply, which sets the rate.
stomata
tiny holes in a leaf that let gases in and out.

Back to your guess

The answer is B: Mostly from carbon dioxide in the air and from water.

Most of the mass of a plant comes from carbon dioxide taken from the air, and from water. Photosynthesis uses light to join them into glucose, and the plant builds itself from glucose.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which is the word equation for photosynthesis?

    Tap your answer.

    1. Not this one. That is the equation for respiration. It goes the other way round. Go to S3.6.6. Try another.

    2. Yes, that is right. A plant takes in carbon dioxide and water. It makes glucose and gives out oxygen.

    3. Not this one. Oxygen is made in photosynthesis, not used. Glucose and oxygen are the products. Try another.

    See the answer

    carbon dioxide + water → glucose + oxygen

    A plant takes in carbon dioxide and water. It makes glucose and gives out oxygen.

    • If you chose A: That is the equation for respiration. It goes the other way round. Go to S3.6.6.
    • If you chose C: Oxygen is made in photosynthesis, not used. Glucose and oxygen are the products.

  2. Where does most of the mass of a growing tree come from?

    Tap your answer.

    1. Yes, that is right. Photosynthesis joins carbon dioxide and water into glucose. The plant builds itself from glucose.

    2. Not this one. Van Helmont showed the soil lost only about 60 g while the tree gained about 74 kg. Go to S2.1.5. Try another.

    3. Not this one. Light gives the energy, but light is not material. The mass comes from carbon dioxide and water. Try another.

    See the answer

    Carbon dioxide from the air, and water

    Photosynthesis joins carbon dioxide and water into glucose. The plant builds itself from glucose.

    • If you chose B: Van Helmont showed the soil lost only about 60 g while the tree gained about 74 kg. Go to S2.1.5.
    • If you chose C: Light gives the energy, but light is not material. The mass comes from carbon dioxide and water.

  3. Which equation for photosynthesis is balanced? Equation A is CO₂ + H₂O → C₆H₁₂O₆ + O₂. Equation B is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 12O₂. Equation C is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

    Tap your answer.

    1. Not this one. Equation A has 1 carbon on the left and 6 on the right. The numbers in front are missing. Go to S4.7.2. Try another.

    2. Not this one. Equation B has 18 oxygen atoms on the left and 30 on the right. Too many. Go to S4.7.2. Try another.

    3. Yes, that is right. Equation C has 6 carbon, 12 hydrogen and 18 oxygen atoms on each side.

    See the answer

    Equation C

    Equation C has 6 carbon, 12 hydrogen and 18 oxygen atoms on each side.

    • If you chose A: Equation A has 1 carbon on the left and 6 on the right. The numbers in front are missing. Go to S4.7.2.
    • If you chose B: Equation B has 18 oxygen atoms on the left and 30 on the right. Too many. Go to S4.7.2.

  4. A greenhouse has bright light and a warm temperature, but the air has little carbon dioxide. What helps the plants most?

    Tap your answer.

    1. Not this one. Light is not the limiting factor here. Carbon dioxide is in shortest supply, so more light gives no gain. Try another.

    2. Yes, that is right. The factor in shortest supply sets the rate. Here it is carbon dioxide, so adding it raises the rate.

    3. Not this one. The temperature is already good, and a much higher one could denature the enzymes. Try another.

    See the answer

    More carbon dioxide

    The factor in shortest supply sets the rate. Here it is carbon dioxide, so adding it raises the rate.

    • If you chose A: Light is not the limiting factor here. Carbon dioxide is in shortest supply, so more light gives no gain.
    • If you chose C: The temperature is already good, and a much higher one could denature the enzymes.

  5. Look at the graph. With low carbon dioxide, what limits the rate at 75% of full sun?

    A line graph with two lines. The x axis is light as % of full sun, from 0 to 100 in steps of 25. The y axis is rate as % of the highest, from 0 to 100 in steps of 20. The line labelled high CO₂ rises steadily to 100 at 100. The line labelled low CO₂ rises to 40 at 50 and stays there. The two lines are the same up to 25.0255075100020406080100high CO₂low CO₂light as % of full sunrate as % of the highest
    Tap your answer.

    1. Not this one. The line is flat from 50 to 100. More light does not raise the rate, so light is not the limit. Try another.

    2. Yes, that is right. The low CO₂ line is flat. Adding light gives no gain, but the high CO₂ line shows that more carbon dioxide would raise the rate.

    3. Not this one. The high CO₂ line reaches 75 at this light, but the low CO₂ line stays at 40. It is limited. Try another.

    See the answer

    Carbon dioxide

    The low CO₂ line is flat. Adding light gives no gain, but the high CO₂ line shows that more carbon dioxide would raise the rate.

    • If you chose A: The line is flat from 50 to 100. More light does not raise the rate, so light is not the limit.
    • If you chose C: The high CO₂ line reaches 75 at this light, but the low CO₂ line stays at 40. It is limited.

  6. Why does the rate of photosynthesis fall fast at 50 °C?

    Tap your answer.

    1. Not this one. Temperature does not change the light. The enzymes are what change. Try another.

    2. Not this one. Heat does not use up carbon dioxide in the air. The enzymes are what change. Try another.

    3. Not this one. The problem is not too much energy. Heat changes the shape of the enzymes. Try another.

    4. Yes, that is right. The reactions of photosynthesis need enzymes. High temperature changes their shape, so they stop working.

    See the answer

    The enzymes are denatured.

    The reactions of photosynthesis need enzymes. High temperature changes their shape, so they stop working.

    • If you chose A: Temperature does not change the light. The enzymes are what change.
    • If you chose B: Heat does not use up carbon dioxide in the air. The enzymes are what change.
    • If you chose C: The problem is not too much energy. Heat changes the shape of the enzymes.

  7. From an earlier chapter

    Carbon dioxide gets into leaf cells from the air, where there is more of it than inside the cells. How does it move?

    Tap your answer.

    1. Yes, that is right. Carbon dioxide moves from a higher concentration outside to a lower one inside, so it diffuses. No energy is needed.

    2. Not this one. Active transport moves particles from a lower to a higher concentration. Here the carbon dioxide goes down the gradient. Go to S4.1.3. Try another.

    3. Not this one. Osmosis is the movement of water, not carbon dioxide. Go to S4.1.3. Try another.

    See the answer

    By diffusion

    Carbon dioxide moves from a higher concentration outside to a lower one inside, so it diffuses. No energy is needed.

    • If you chose B: Active transport moves particles from a lower to a higher concentration. Here the carbon dioxide goes down the gradient. Go to S4.1.3.
    • If you chose C: Osmosis is the movement of water, not carbon dioxide. Go to S4.1.3.

  8. How many oxygen atoms are on the right side of the equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂?

    See the answer

    18 oxygen atoms

    C₆H₁₂O₆ has 6 oxygen atoms. 6O₂ is 6 × 2 = 12. 6 + 12 = 18, the same as on the left.

  9. In photosynthesis 6 molecules of carbon dioxide make 1 molecule of glucose. How many molecules of glucose can be made from 30 molecules of carbon dioxide?

    See the answer

    5 molecules

    30 ÷ 6 = 5, and each 6 molecules of carbon dioxide make 1 molecule of glucose.

Keep it

Copy this into your exercise book.

Carbon dioxide + water → glucose + oxygen, using light and chlorophyll. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The factor in shortest supply, light, carbon dioxide or temperature, limits the rate.

Draw: A leaf with arrows for light, carbon dioxide and water going in, and glucose and oxygen coming out.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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