More about photosynthesis
In this topic you will:
- find out where photosynthesis happens in plants
- learn why plants need magnesium and nitrate.
Key words
- fertiliser
- stomata (singular: stoma)
- yield
Chloroplasts and chlorophyll
You saw that chlorophyll is essential for photosynthesis. Chlorophyll captures energy from sunlight. The energy helps water and carbon dioxide to react together.
Chlorophyll is kept inside chloroplasts, inside plant cells.
This means that photosynthesis happens inside chloroplasts. Not all plant cells have chloroplasts, so not all of them can photosynthesise.
In most plants, the cells in the leaves have the most chloroplasts. Inside the cells in a leaf, carbon dioxide and water are made to react, to produce carbohydrates and oxygen. We can think of the field of lettuces in the photograph as a giant carbohydrate factory.
On warm, sunny days, plants can make more carbohydrate than they need to use immediately. So they store some of it for use later on – perhaps at night, or at a time of year when there is less sunlight. Plants store carbohydrates as starch. They store the starch inside the chloroplasts in their cells. One way to check whether a leaf has been photosynthesising is to test it for starch.
Quick Fact
Only the green parts of a plant—like leaves—contain chloroplasts. These are the only parts where photosynthesis happens.
Important Concept
Photosynthesis takes place only in plant cells that contain chloroplasts. Chloroplasts hold the green pigment chlorophyll, which absorbs sunlight and drives the reactions that convert water and carbon dioxide into carbohydrates and oxygen.
Common Mistake
Not all plant cells carry out photosynthesis. It’s a mistake to assume that every plant cell contains chloroplasts—only green parts like leaves usually do. Root cells, for example, do not have chloroplasts.
Questions
Show Answer
Chlorophyll absorbs energy from sunlight and uses it to help carbon dioxide and water react to form carbohydrates during photosynthesis.
Show Answer
Only plant cells that contain chloroplasts can carry out photosynthesis. Some cells, such as those in roots, do not have chloroplasts and therefore cannot photosynthesise.
Think Like a Scientist
Testing a leaf for starch
You are going to use a test you may already know — the iodine test for starch — to find out if a leaf has been photosynthesising.
You will need: a healthy plant that has been kept in sunlight, a burner (e.g. Bunsen or spirit burner), tripod and gauze, a beaker (e.g. 250 cm³), a test tube, ethanol, forceps, iodine solution with a dropper, and a tile.
Safety:
- You will be using hot water, so stand up while working and keep safe in case of spills.
- Be very careful to turn off the flame before using ethanol — it is flammable.
Method
- Put water in a beaker, heat it until it boils.
- Pick a healthy leaf and drop it in the boiling water.
- After ~2 minutes, turn off the burner (important before using ethanol).
- Put ethanol in a test tube.
- Use forceps to transfer the leaf into the test tube with ethanol.
- Place the tube in hot water. The green colour will start to leave the leaf.
- Remove the leaf gently and place it briefly in warm water to soften it.
- Spread it on a tile and add iodine. If starch is present, it will go blue-black.
Questions
Show Answer
Starch is stored in the chloroplasts of plant cells.
Show Answer
Boiling the leaf breaks open the cell membranes so that iodine can enter the cells and reach the starch.
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Removing the green chlorophyll makes it easier to see the blue-black colour change if starch is present.
Show Answer
Yes, if the leaf turned blue-black, it contained starch made during photosynthesis using sunlight, water, and carbon dioxide.
Show Answer
Only the green parts would contain starch because only green cells have chloroplasts that carry out photosynthesis.
Inside a leaf
Photosynthesis happens inside chloroplasts, which are inside some of the cells in a leaf.
The diagram opposite shows a magnified view of the inside of a leaf.
On the diagram, you can see that chloroplasts are mostly inside the cells in the middle layers of the leaf. Leaves are very thin, so it is easy for sunlight to reach these cells.
Chloroplasts also need plenty of water and carbon dioxide, because these are used in photosynthesis. Water is brought to the cells in the leaf along the veins. You can read more about this in Unit 4.
Carbon dioxide diffuses into the leaf from the air. If you look at the diagram, you can see that there are tiny holes in the leaf, which allow gases to diffuse in and out. These holes are called stomata (singular: stoma). The gases can easily diffuse through the air spaces between the cells inside the leaf.
The diagram shows what a leaf looks like if you cut it across, and then look at the cut edge.
Leaves are so thin that it is difficult to imagine they contain several layers of cells. It is the cells in the middle of the leaf that carry out photosynthesis
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A waxy layer on the leaf surface stops the leaf cells from drying out.
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The upper epidermis protects the cells inside the leaf.
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The palisade layer contains cells that do most of the photosynthesis.
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The spongy layer has lots of air spaces. The cells in the spongy layer do a small amount of photosynthesis.
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A stoma (plural: stomata) is a tiny hole in the lower epidermis. These holes let carbon dioxide from the air get into the leaf.
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The lower epidermis protects the cells inside the leaf.
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A vein carries water to the cells in the leaf.
Quick Fact
The palisade layer contains most of the leaf’s chloroplasts and is the main site of photosynthesis.
Important Concept
Photosynthesis happens in chloroplasts, which are mainly found in the middle layers of the leaf, especially in the palisade cells. These cells receive sunlight easily because the leaf is thin and flat, allowing efficient light penetration.
Common Mistake
It’s a common mistake to think that all cells in a leaf carry out photosynthesis. In reality, only cells with chloroplasts—mainly in the palisade and spongy layers—can perform photosynthesis. Cells in the upper and lower epidermis do not contain chloroplasts.
Questions
Show Answer
Carbon dioxide moves from an area of high concentration outside the leaf to a lower concentration inside the leaf. It diffuses through the stomata and into the air spaces between cells, then into the cells that use it for photosynthesis.
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The oxygen diffuses out of the leaf through the stomata into the air. Some of it may also be used by the plant for respiration.
Minerals and plant growth
The farmer in the picture is adding fertiliser to a field of wheat.
Farmers add fertiliser to their fields because it makes the crops grow larger and produce a higher yield. Yield is the quantity of crop that the farmer harvests.
Fertilisers contain minerals. Like the minerals that we need in our diet, plants need only quite small quantities of minerals. They get these minerals from the soil, through their roots. But sometimes the soil does not contain enough of certain minerals. This stops the plants growing to their full potential.
Two important minerals for plants are magnesium and nitrate.
Magnesium is needed to make the green pigment, chlorophyll. If a plant does not have enough magnesium, its leaves look yellow instead of green. It cannot grow well, because it does not have much chlorophyll to absorb energy from sunlight, and so it cannot photosynthesise as much as it should.
Nitrate contains nitrogen atoms. These are needed so that the plant can convert carbohydrates to proteins. Proteins are essential for making new cells, so that the plant can grow well. Without enough nitrogen, leaves die and the plant stays small, like these maize (corn) plants. Nitrogen is also needed to make chlorophyll.
Farmers can test the soil in their fields to find out exactly which minerals are lacking in each part of the field. This tells the farmer where they need to add fertiliser, and where it is not needed.
Farmers who can afford the latest technology can use global positioning satellites (GPS) with their machinery.
The screen in the tractor cab shows the farmer exactly where he is in the field, as well as the results of the soil tests. The farmer can easily control how much fertiliser is added in each part of the field.
Questions
Show Answer
Plants absorb nitrate from the soil and use it to make amino acids, which are then built into proteins. Animals eat food that contains protein and break it down into amino acids, which their bodies use to build new proteins.
Show Answer
Both magnesium and protein deficiencies can cause poor growth and yellow leaves. The symptoms look similar, so it is hard to tell which mineral the plant is lacking just by looking at it.
Think Like a Scientist
Plan and carry out an investigation to explore how fertiliser affects the growth rate of duckweed, a small aquatic plant with floating leaves and roots that dangle into water.
You may choose to test different types or amounts of fertiliser, and grow your duckweed in small containers like Petri dishes. To measure growth, count the number of new leaves.
Investigation Steps
Step 1: Decide on a hypothesis to test.
Step 2: Choose how you will vary your independent variable (e.g. type or amount of fertiliser).
Step 3: Determine how you will measure your dependent variable — the growth of duckweed (e.g. number of leaves).
Step 4: Identify the variables you will keep the same (e.g. container type, light, temperature).
Step 5: List the equipment you will need.
Step 6: Assess the risks and explain how you will control them.
Step 7: Predict the outcome and justify why you expect that result.
Follow-Up Questions
Show Answer
Yes, I had to adjust the amount of fertiliser added because the initial amount was too concentrated. I realised this when the duckweed began to yellow after the first day.
Show Answer
My results showed that the duckweed grew faster with a medium amount of fertiliser, which supports my hypothesis that fertiliser improves growth up to a certain point.