Photosynthesis
In this topic you will:
- practise using the word equation for photosynthesis
- learn what photosynthesis is, how it happens and why it is important.
Key Words
chlorophyll, light intensity, photosynthesis
How plants make food
Plants make food by photosynthesis. ‘Photo’ means ‘light’. ‘Synthesis’ means ‘making’. So photosynthesis means ‘making with light’.
In photosynthesis, plants use:
- water, which they get from the soil
- carbon dioxide, which they get from the air
- energy, which they get from sunlight
The energy in sunlight is captured by a green pigment, called chlorophyll, which is inside some of the cells in the leaves of plants.
The plants use the energy to make the water and carbon dioxide combine together in a chemical reaction.
Two new substances are made in the reaction. These are glucose and oxygen.
You can write the word equation for photosynthesis like this:
water + carbon dioxide → glucose + oxygen
Photosynthesis happens in the leaves of a plant. You will find out more about this in the next topic.
Quick Fact
Photosynthesis takes place in the leaves of plants, where chlorophyll absorbs light energy to power the food-making process.
Important Concept
Photosynthesis converts light energy into chemical energy stored in glucose. This process also produces oxygen, making it vital for both plant growth and life on Earth.
Common Mistake
It’s incorrect to think that plants only need sunlight to make food. They also require water from the soil and carbon dioxide from the air for photosynthesis to occur.
Questions
Show Answer
The reactants are water and carbon dioxide.
Show Answer
The products are glucose and oxygen.
Think like a scientist
Collecting the gas produced in photosynthesis
You are going to use a plant that grows in water for this experiment. This makes it easy to collect the gas that it releases.
- the apparatus shown in the diagram
Collecting the gas produced in photosynthesis
If you do not have pond water, you can use ordinary tap water. You can use any plant that grows underwater (e.g. pond weed, often sold in pet shops for fish tanks).
Method
- Set up the apparatus shown in the diagram.
- Put the apparatus where the plant can get sunlight. Leave it for at least one day.
- When gas has collected in the test tube, place your hand into the beaker of water.
- Carefully remove the test tube from the funnel while keeping its opening underwater.
- Put your thumb over the test tube’s end to trap the gas.
- Remove the test tube from the beaker while keeping it upside down. Place a stopper into the tube.
- Light a splint, then blow it out so it is glowing (not burning).
- Test the gas with the glowing splint. If the gas contains oxygen, the splint will relight.
Questions
- Explain why it was best to use a water plant in this experiment.
- Explain why it was important to leave the apparatus where it would get plenty of sunlight.
Show answers
- Q1: Water plants can photosynthesise underwater, so the gas they produce (oxygen) can be trapped and collected easily.
- Q2: Sunlight provides the energy for photosynthesis. Without it, the plant wouldn't produce oxygen gas.
Think like a scientist
Investigating how light intensity affects the rate of photosynthesis
This experiment uses a water plant to observe how light intensity affects photosynthesis. By changing the distance of a lamp, you will measure how many bubbles are produced as a proxy for the rate of photosynthesis.
- a test tube containing water
- a piece of water plant
- a lamp
- a beaker to hold the test tube
- a ruler to measure distance
- a timer
Safety
It is very important to keep electrical wires, plugs and the lamp away from the water.
Method
- Set up your equipment. Place the lamp close to the test tube. Wait 5 minutes for the plant to adjust.
- Measure the distance from lamp to test tube and record it.
- Start the timer and count the number of bubbles produced in 1 minute.
- Repeat step 3 two more times and calculate the mean.
- Move the lamp further away and record the new distance.
- Repeat steps 3 and 4.
- Repeat steps 5 and 6 for at least two more distances.
Questions
- Calculate the mean number of bubbles for each distance of the lamp from the test tube.
- Plot a graph of your results:
- x-axis: distance from the lamp
- y-axis: mean bubbles per minute
- Draw a trend line to show the pattern
- Light intensity means the amount of light. Complete this sentence:
As the distance from the test tube increases, the light intensity ………………… - Now write a conclusion:
As the light intensity increases, the rate of photosynthesis …………………
Show answers
- Q1: Calculate mean by adding 3 trials and dividing by 3 for each distance.
- Q2: Graph with distance (x-axis) and mean bubbles per minute (y-axis). Trend line should show decrease as distance increases.
- Q3: As the distance increases, light intensity decreases.
- Q4: As light intensity increases, the rate of photosynthesis also increases.
Peer assessment
Exchange your chart with a partner. Score them 0–2 on each of these:
- Used a ruler to draw a clear results chart
- Included headings with correct units
- Included a column/row for mean number of bubbles
- The chart is neat, clear and easy to interpret
Max score: 10. If they got less, explain how they could improve.
Why is photosynthesis important?
First, photosynthesis provides energy, in the form of chemical energy in nutrients, for most other organisms.
Plants use the energy in sunlight to make glucose and other carbohydrates. These carbohydrates contain some of the energy that was originally in the sunlight.
Think about what you have learnt about food chains. When animals eat food, they get some of the energy that was captured by plants. Most of the energy in all the food in the world comes from plants. A food chain shows us how this energy is passed along from one organism to another.
The second reason that photosynthesis is so important is that it provides oxygen for the Earth’s atmosphere. Animals and plants, of course, need oxygen for respiration.
Oxygen is a waste product of photosynthesis. It is released from the leaves of plants and mixes with the other gases in the atmosphere. About 20% of the air around us is oxygen.
Quick Fact
Photosynthesis is the ultimate source of energy for nearly all organisms on Earth. It also produces the oxygen that makes animal life possible.
When the Earth was first formed, about 4.6 billion years ago, there was almost no oxygen in the Earth’s atmosphere. Scientists think that oxygen first began to collect in the air when bacteria first began to photosynthesise. (There were no plants at that time – they did not appear on Earth until about 4.7 million years ago.)
If photosynthesis had never begun on Earth, then no animals would ever have been able to live here.
Important Concept
Photosynthesis is the process by which green plants and some bacteria use sunlight to produce glucose and oxygen from carbon dioxide and water.
Questions
Explain how the lizard relies on photosynthesis to provide it with energy.
Show Answer
The grasshopper gets its energy from eating grass, which uses photosynthesis to make food. The lizard eats the grasshopper and gets energy that originally came from the grass. So the lizard relies on photosynthesis indirectly to obtain energy.
Show Answer
Plants produce oxygen during photosynthesis. Without plants, there would be no oxygen in the air, and the lizard would not be able to breathe or survive.