These are the things that plants need for photosynthesis:

  • carbon dioxide
  • water
  • Light
  • glucose
  • oxygen

We can show photosynthesis as this word equation: carbon dioxide + water (+ light energy) --> glucose + oxygen (Light energy is shown in brackets because it is not a substance.)
This is the symbol equation for photosynthesis:

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Where does this process take place in the plant?

Photosynthesis takes place in the chloroplasts in plant cells. Chloroplasts contain a green substance called chlorophyll. This absorbs the light energy needed to make photosynthesis happen. Plants can only photosynthesise in the light.

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Palisade cells in leaves have lots of chloroplasts

Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots.

Light energy comes from the sun.

The glucose produced can be turned into other substances, such as starch, which is used for storage. The oxygen produced is released into the air from the leaves.

Demonstrating photosynthesis
Animals eat to get food, but green plants make their own food. The process plants use is called photosynthesis. We say that plants can photosynthesise.

These are the things that plants need for photosynthesis:
  • carbon dioxide
  • water
  • lightThese are the things that plants make because of photosynthesis:
  • glucose
  • oxygen

We can show photosynthesis as this word equation:

carbon dioxide + water (+ light energy) --> glucose + oxygen

(Light energy is shown in brackets because it is not a substance.)

Where does this process take place in the plant?

Photosynthesis takes place in the chloroplasts in plant cells. Chloroplasts contain a green substance called chlorophyll. This absorbs the light energy needed to make photosynthesis happen. Plants can only photosynthesise in the light.

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Palisade cells in leaves have lots of chloroplasts

Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots.

Light energy comes from the sun.

The glucose produced can be turned into other substances, such as starch, which is used for storage. The oxygen produced is released into the air from the leaves.

Demonstrating photosynthesis

Looking for starch

Remember that glucose made by photosynthesis is turned into other substances, including starch.
It is easier to show that photosynthesis happens by showing starch stored in a leaf, rather than by looking for glucose. There is a simple test for starch, using iodine solution - this turns blue-black when it makes contact with starch.

We need to treat the leaf first for the test to work well. We can't detect if you have flash or not, please go to the [[#|non-flash alternative]]

Notice that the leaf that was in the light turned blue-black when iodine solution was dripped on to it, because there was starch in it. The leaf had been able to photosynthesise, so it was able to make glucose, some of which was then stored as starch.

The leaf that was in the dark did not turn blue-black when iodine solution was dripped on to it, because there was no starch in it. The leaf had not been able to photosynthesise, so it had used up its store of starch.

Looking for oxygen

Rate of photosynthesis is affected by several factors, including:
  • the amount of light (the brighter the light, the faster the rate of photosynthesis)
  • the concentration of carbon dioxide in the air (the higher the concentration, the faster the rate)
  • the temperature of the plant (the rate of photosynthesis is highest at 30°C for most land-based plants)
  • the amount of water (a lack of water slows down photosynthesis, and may also cause a loss of leaves or the death of the plant)
Remember that oxygen is produced by photosynthesis - so another way to prove that photosynthesis happens is to show how water plants such as pond weed, lying under water, give off oxygen during the process.

Pond weed gives off oxygen as bubbles of gas. It is easy to collect the oxygen. The faster the rate of photosynthesis, the faster the bubbles come off the pond weed, and the more oxygen we collect in a given time.

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We can show that the gas collected is oxygen by applying a simple test - a glowing wooden splint relights in a tube of oxygen.

Getting carbon dioxide, light and water

Carbon dioxide

Plants get carbon dioxide from the air through their leaves. The carbon dioxide diffuses through small holes in the underside of the leaf called stomata. We can't detect if you have flash or not, please go to the [[#|non-flash alternative]]
The lower part of the leaf has loose-fitting cells, to allow carbon dioxide to reach the other cells in the leaf. This also allows oxygen to leave the leaf easily. ==Light== A leaf usually has a large surface area, so that it can absorb a lot of light. Its top surface is protected from water loss, disease and weather damage by a waxy layer.

The upper part of the leaf is where the light falls, and it contains a type of cell called a palisade cell. This is adapted to absorb a lot of light. It has lots of chloroplasts and is shaped like a tall box.

Water

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Plants get the water they need for photosynthesis through their roots. The roots have a type of cell called a root hair cell - these project out from the root into the soil. Roots have a big surface area and thin walls, which allow water to pass into them easily.

Note that root cells do not contain chloroplasts, as they are normally in the dark and cannot photosynthesise anyway.
The water absorbed by the root hair cells passes through the plant in xylem tubes into the leaves. If a plant does not absorb enough water, it will wilt or go floppy. Without water it may also not photosynthesise quickly enough, and it may die.

Minerals Plants get the following crucial elements from the carbon dioxide and water they use during photosynthesis.
  • carbon
  • oxygen
  • hydrogen
These, however, are not the only elements that plants need to grow. They also need elements such as nitrogen, phosphorus and potassium. These other elements allow the plant to make other substances. For example, nitrogen is needed to make proteins.

Soil contains elements such as these as minerals, dissolved in water. Plants absorb the minerals from the soil water, through their root hair cells.

Farmers and gardeners often add minerals to their soil, using artificial fertilisers or manure, to help their plants grow better.

Respiration in plants
Plants get the following crucial elements from the carbon dioxide and water they use during photosynthesis.
  • carbon
  • oxygen
  • hydrogen
These, however, are not the only elements that plants need to grow. They also need elements such as nitrogen, phosphorus and potassium. These other elements allow the plant to make other substances. For example, nitrogen is needed to make proteins.

Soil contains elements such as these as minerals, dissolved in water. Plants absorb the minerals from the soil water, through their root hair cells.

A plant grown in soil containing these dissolved minerals grows well. Soil containing the minerals a plant needs for healthy growth is called fertile soil.

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A plant grown in soil without these dissolved minerals grows poorly. Soil without the minerals a plant needs for healthy growth is called infertile soil.

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Farmers and gardeners often add minerals to their soil, using artificial fertilisers or manure, to help their plants grow better.

Respiration in plants

Respiration

Living cells respire. Aerobic respiration is the chemical reaction used to release energy from glucose. It is called aerobic because oxygen from the air is also needed.
Here is the word equation for aerobic respiration:

glucose + oxygen --> carbon dioxide + water (+ energy)

(Energy is put in brackets because it not a substance.)

Notice that the word equation for respiration is the reverse of the word equation for photosynthesis. Check back if you are not sure of this.

Plants

Plant cells respire, just as animal cells do. If they stop respiring, they will die. Remember that respiration is not the same as breathing, so take care - plants do not breathe.
Plants respire all the time, whether it is dark or light. They are always taking in oxygen and releasing carbon dioxide. But they also photosynthesise when they are in the light - and remember that plants take in carbon dioxide and release oxygen when they photosynthesise.

So, what happens to a plant overall, over any given period, depends on whether it is in the dark or the light, and how bright the light is.


Photosynthesis usually results in a net food gain, once respiration has been accounted for. This means that there is an increase in the biomass of the plant.

Plants that lose their leaves in winter store food produced during the summer by photosynthesis. They store enough food to last them over winter, and to provide energy reserves for new growth in the spring.