Photosynthesis Notes

Oxford AQA IGCSE Biology: Revision notes

Key facts

  • Photosynthesis: carbon dioxide + water → glucose + oxygen, using light absorbed by chlorophyll in chloroplasts.
  • Carbon dioxide enters leaves by diffusion through the stomata; water is absorbed by the roots.
  • The rate is limited by light, carbon dioxide or temperature: whichever is in shortest supply.
  • Greenhouses raise these factors, but only if the extra yield is worth the cost.
  • Glucose is used for respiration, starch, fat or oil, cellulose and proteins.

The photosynthesis equation

Plants and algae use light energy to make glucose from carbon dioxide and water, releasing oxygen as a by-product.

Photosynthesis needs light, absorbed by chlorophyll in chloroplasts. In leaves, the palisade mesophyll does most of it.

6 COX2+6 HX2O→CX6HX12OX6+6 OX2\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}

Light is the energy source, written above the arrow, not a reactant.

  • Word equationcarbon dioxide + water → glucose + oxygen
  • Symbol equation6 COX2+6 HX2O→CX6HX12OX6+6 OX2\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}
  1. 1

    Light energy

    absorbed by chlorophyll in chloroplasts

  2. 2

    Carbon dioxide and water

    carbon dioxide from the air, water from the soil

  3. 3

    Glucose and oxygen

    glucose is the food; oxygen is released

What goes in and what comes out

Which substance is a by-product of photosynthesis?

Carbon dioxide and water

Carbon dioxide diffuses into the leaf through the stomata; roots absorb water and mineral ions.

Root hairs increase the surface area of the roots for absorbing water and mineral ions. The flattened shape of leaves gives a large surface area to volume ratio, and air spaces in the spongy mesophyll help gases diffuse.

Stomata let carbon dioxide in and oxygen out. They close if the plant loses water faster than the roots replace it, to prevent wilting.

Labelled cross-section of a leaf
Cross-section of a leaf.
  1. 1

    Air

    contains carbon dioxide

  2. 2

    Stomata

    carbon dioxide diffuses in

  3. 3

    Air spaces

    gases diffuse to the cells in the spongy mesophyll

  4. 4

    Palisade cells

    carbon dioxide is used in photosynthesis

How carbon dioxide reaches the cells that use it

How does carbon dioxide enter a leaf?

Limiting factors

The rate is limited by the factor in shortest supply: light, carbon dioxide or temperature. Raising any other factor will not help.

These factors interact, and any one may be the limiting factor at a given time.

On a graph, the rate rises as the limiting factor increases, then levels off when another factor becomes limiting. When a graph levels off, the factor on the x-axis is no longer limiting.

1234567891012345xyrate of photosynthesis
The rate levels off when another factor becomes limiting.

Light

  • Less energy absorbed by chlorophyll

Carbon dioxide

  • Less of this reactant available

Temperature

  • Enzymes work slowly at low temperature

A graph levels off as light increases. Which is true?

Greenhouses

Growers raise temperature, carbon dioxide and light to speed photosynthesis, but the extra crop must pay for the cost.

Raising the rate of photosynthesis increases growth. Each change has a cost, so economics matter: the extra glucose, and so the extra crop, must be worth more than the heating, gas and electricity.

Raising a factor that is not limiting wastes money.

123456789102468101214xyNormal carbon dioxideExtra carbon dioxide
Illustrative: where light is no longer limiting, adding carbon dioxide (dashed) raises the rate of photosynthesis.

Temperature

  • Heaters keep the greenhouse warm

Carbon dioxide

  • Add it by burning a fuel or using a generator

Light intensity

  • Artificial lighting extends the hours of light and brightens dull days

Heating a greenhouse gives no extra growth. The best explanation is

Uses of glucose

Glucose is used for respiration, or converted into starch, fat or oil, cellulose and proteins.

Plants use glucose as a source of energy, or convert it into larger molecules. Insoluble starch is the storage form, and being insoluble it does not affect the water balance of cells.

To make proteins, plants also need nitrate ions from the soil, which provide the nitrogen that glucose lacks.

  1. 1

    Respiration

    Releases energy for life processes

  2. 2

    Starch

    Insoluble, so stored without affecting water balance

  3. 3

    Fat or oil

    Also used for storage

  4. 4

    Cellulose

    Strengthens the cell wall

  5. 5

    Proteins

    Glucose plus nitrate ions from the soil

What plants do with the glucose they make.

Product

Respiration:
Energy released
Starch:
Insoluble starch
Fat or oil:
Fat or oil
Cellulose:
Cellulose
Proteins:
Proteins

Purpose

Respiration:
Life processes
Starch:
Storage
Fat or oil:
Storage
Cellulose:
Strengthens the cell wall
Proteins:
Needs nitrate ions from the soil

Which ions do plants need to make proteins?

Investigating the rate

Count the oxygen bubbles from pondweed in a set time, changing one factor and keeping the others constant.

Place pondweed in water in a beaker with a lamp at a measured distance. Count bubbles of oxygen in a set time, or collect and measure the gas. Repeat and calculate a mean.

Change one variable only, and use the same piece of pondweed. A heat shield or water bath stops the lamp changing the temperature.

  1. 1

    Set up

    pondweed in water, lamp at a measured distance

  2. 2

    Measure

    count oxygen bubbles in a set time

  3. 3

    Change one variable

    then repeat the measurement

  4. 4

    Calculate a mean

    from the repeats

Pondweed method

Light intensity

  • Move the lamp closer or further away

Carbon dioxide

  • Add different amounts of sodium hydrogencarbonate

Temperature

  • Use a water bath at different temperatures

Moving the lamp closer changes the

Try an exam question

Explain how a grower could increase the rate of photosynthesis in a greenhouse, and why this might not be worthwhile.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.