Photosynthesis is the process by which green plants convert light energy into chemical energy stored in glucose. Four things are required for it to happen, and a Life Sciences answer that names all four and explains the role of each is the answer that earns full marks.
This page sets out the requirements, the conditions that limit the rate, and how each requirement is demonstrated in the standard school practicals.
Requirements at a Glance
What photosynthesis requires.
- Carbon dioxide — taken in from the air through the stomata in the leaf
- Water — absorbed by the roots and transported to the leaves in the xylem
- Light energy — usually sunlight, absorbed by the pigment
- Chlorophyll — the green pigment in the chloroplasts that absorbs the light
- A suitable temperature, since the process is enzyme-controlled
- Chloroplasts, the organelles in which the process takes place
- Living plant tissue containing photosynthetic cells, mainly in the leaf mesophyll
Raw Materials Required
The balanced overall equation summarises the process: carbon dioxide plus water, in the presence of light energy and chlorophyll, produces glucose and oxygen. In symbols, 6CO2 + 6H2O → C6H12O6 + 6O2. Learn the equation and be able to say what each term is for — that is what most questions actually test.
Carbon dioxide enters through the stomata, the pores mostly on the underside of the leaf, controlled by guard cells. It supplies the carbon that becomes the carbon skeleton of glucose.
Water is absorbed by the root hair cells by osmosis and moves up through the xylem to the leaf. It supplies the hydrogen used to reduce carbon dioxide, and its splitting is where the oxygen released comes from.
The oxygen released comes from the water, not from the carbon dioxide. This is a favourite examination point and it is frequently answered wrongly.
Glucose is the immediate product, and it is used for respiration, converted to starch for storage, converted to cellulose for cell walls, and combined with mineral salts to make proteins, fats and oils.
Light, Chlorophyll and Conditions Required
Light provides the energy that drives the reaction. Both the intensity and the wavelength matter — chlorophyll absorbs mainly red and blue light and reflects green, which is why leaves look green.
Chlorophyll is the absorber. Without it, light energy cannot be captured, which is why the white parts of a variegated leaf do not photosynthesise — the standard practical demonstration of this requirement.
Temperature affects the rate because photosynthesis is controlled by enzymes. The rate rises with temperature to an optimum and then falls sharply as the enzymes denature.
Limiting factors determine the actual rate. When light intensity, carbon dioxide concentration or temperature is in short supply, that factor limits the rate no matter how plentiful the others are. Increasing a factor that is not limiting produces no further increase — this is what the graph questions test.
Water shortage closes the stomata, which cuts off the carbon dioxide supply and so reduces photosynthesis indirectly as well as directly.
Chlorophyll production needs magnesium, so a magnesium deficiency causes chlorosis — yellowing leaves — and reduced photosynthesis.
Products and What the Plant Does With Them
Where the products go.
- Glucose is used immediately in cellular respiration to release energy
- Starch is the storage form, made from glucose and stored in leaves, roots, stems, seeds and tubers
- Cellulose is made from glucose and forms the plant cell wall
- Sucrose is the transport form, moved through the phloem to other parts of the plant
- Proteins are made by combining glucose products with nitrates absorbed from the soil
- Fats and oils are made from glucose products and stored, particularly in seeds
- Oxygen is released through the stomata as a by-product, and is used by the plant in respiration
How the Requirements Are Tested in Practicals
Destarching first. Every starch test practical begins by keeping the plant in the dark for a day or two so that existing starch is used up. Without this control the result proves nothing, and marks are given for stating it.
Testing a leaf for starch. Boil the leaf in water to kill it and break down cell membranes, boil it in ethanol in a water bath to remove the chlorophyll, rinse it in warm water to soften it, then add iodine solution. A blue-black colour shows starch is present.
Ethanol is heated in a water bath, never over a flame, because it is highly flammable. Safety points are marked.
Showing chlorophyll is required: use a variegated leaf. Only the parts that were green turn blue-black with iodine.
Showing light is required: cover part of a destarched leaf with foil, leave the plant in light, then test. Only the uncovered part turns blue-black.
Showing carbon dioxide is required: enclose a destarched leaf in a flask with sodium hydroxide, which absorbs carbon dioxide, alongside a control without it. The leaf deprived of carbon dioxide gives a negative starch test.
Measuring the rate: count the bubbles of oxygen released by an aquatic plant such as Elodea at different light intensities, keeping temperature and carbon dioxide constant. Always state the variables held constant.
Frequently Asked Questions
What are the four requirements for photosynthesis?
Carbon dioxide, water, light energy and chlorophyll, at a suitable temperature. Carbon dioxide enters through the stomata, water comes from the roots, light is absorbed by chlorophyll in the chloroplasts.
Where does the released oxygen come from?
From the splitting of water, not from carbon dioxide. This is a common examination point and is frequently answered wrongly.
What is a limiting factor?
The factor in shortest supply, which determines the rate no matter how plentiful the others are. Increasing a factor that is not limiting produces no further increase in rate.
Why is the plant destarched before a starch test?
So that any starch found afterwards must have been made during the experiment. Without that control the result proves nothing, and marks are awarded for stating it.
Curriculum content, prescribed practicals and examination requirements are set in the national Life Sciences curriculum and assessment policy and are revised. Confirm the current requirements with your teacher and the Department of Basic Education’s published curriculum documents.