Photosynthesis and Respiration

Where does the food come from?

Think about your last meal. Perhaps it was banku with okro stew, or rice and beans, or kelewele. Every bit of it can be traced back to a plant: the maize in the banku, the rice, the plantain, even the fish, which ate smaller creatures that ate tiny floating plants.

But where did the plant get its food? A cassava plant does not eat. Nobody feeds it. Yet it grows from a stick pushed into the ground to a bush with heavy roots full of starch.

The answer is one of the most important processes on Earth: photosynthesis. Plants make their own food out of air, water and sunlight.

And there is a second process, running in every living cell including yours right now, that releases the energy stored in that food: respiration.

This book is about both. By the end you will be able to write the word equation for each, explain what a leaf needs, test a leaf for starch, and say why neither process could exist for long without the other.

Living things need energy

Every living thing uses energy all the time:

• to move – a fish swimming, a goat running, your heart beating
• to grow and repair – new cells, healing a cut
• to keep warm – your body stays at about 37 °C even on a cool harmattan night
• to send messages – nerves carrying signals to your brain
• to make new substances – proteins, enzymes, the walls of plant cells

Where does the energy come from? From food. Food contains chemical energy locked up in substances such as glucose (a sugar). When a cell breaks glucose down, the energy is released and the cell can use it.

Animals get glucose by eating. Plants cannot eat, so they must make glucose themselves. That is photosynthesis. Both plants and animals then release the energy by respiration.

So the two big questions are: how is glucose made? and how is its energy released?

What is photosynthesis?

Photosynthesis is the process by which green plants make glucose from carbon dioxide and water, using the energy of sunlight.

The name tells you what it means: photo = light, synthesis = making. Making with light.

It happens mainly in the leaves, inside tiny green structures called chloroplasts, which contain the green pigment chlorophyll.

Here is the whole thing in one sentence:

The plant takes in carbon dioxide from the air and water from the soil; using light energy trapped by chlorophyll, it joins them to make glucose, and oxygen is given off.

Glucose is the plant's food. Some is used straight away for energy; some is turned into starch and stored; some is used to build the plant itself: cellulose for cell walls, and with minerals from the soil, proteins and other substances.

Every time you breathe in, the oxygen comes from this process.

The word equation for photosynthesis

Scientists summarise a chemical change in a word equation. On the left are the substances that go in (the reactants); on the right, what comes out (the products).

carbon dioxide + water → glucose + oxygen

And because this change needs energy and a helper, we write them above or beside the arrow:

carbon dioxide + water —(light energy, chlorophyll)→ glucose + oxygen

Say it aloud until you can write it from memory. In an exam, you must get these right:

• the two reactants: carbon dioxide and water
• the two products: glucose and oxygen
• the two conditions: light and chlorophyll

Light and chlorophyll are not used up, so they do not appear as reactants; they make the reaction possible.

Try this

Cover the page and write the word equation. Then check it, one word at a time.

What a plant needs: light

Light is the energy source. Without it, no photosynthesis happens at all.

Evidence you can see:

• A plant grown in a dark room becomes pale and spindly, and eventually dies, even if it is watered.
• Grass under a mat or a board turns yellow in a few days. Lift the board and it is pale where the light was blocked.
• Plants on a windowsill lean towards the window.
• Farmers space maize plants so each gets enough sun; crowded plants give a poor harvest.

Sunlight is strongest around midday, which is when photosynthesis is fastest. On a cloudy day in the rainy season, plants make less glucose than on a bright day.

In Ghana we rarely lack light; our plants' problem is more often water. But a seedling under a thick tree canopy is light-starved, which is why the forest floor has few plants and why farmers clear and thin trees.

Try this

Place a stone on a patch of grass. After five days, lift it and describe what you see.

What a plant needs: water

Water is one of the two reactants. It enters the plant through the roots, travels up the stem in narrow tubes called xylem, and reaches the leaves.

Only a small part of the water taken up is used in photosynthesis. Most of it evaporates from the leaves through tiny pores. This evaporation, called transpiration, pulls more water up from the roots, like sucking through a straw, and also keeps the leaf cool.

When water is short:

• the plant closes its leaf pores to save water
• but closing them also shuts out carbon dioxide
• so photosynthesis slows or stops
• the leaves wilt and the plant stops growing

That is why a maize field in a dry spell stops growing even though the sun is shining. And it is why irrigation matters: around the Volta, farms watered from the lake can grow crops in the dry season when rain-fed farms cannot.

Water is also what carries dissolved minerals from the soil into the plant, such as nitrates for making proteins.

What a plant needs: carbon dioxide

Carbon dioxide (CO₂) is the other reactant. It is a gas, and only about 0.04 % of the air, yet plants pull enough of it in to build every leaf, stem and root.

It enters the leaf through tiny openings called stomata (one is a stoma), mostly on the underside of the leaf. Inside, it dissolves in the thin film of water around the cells and diffuses into the chloroplasts.

Where does the carbon dioxide come from? From respiration by animals, plants and microbes; from burning wood, charcoal, petrol and gas; and from decay.

Here is a surprising fact: most of the mass of a tree does not come from the soil. It comes from the air. The carbon in the wood was once carbon dioxide drifting past the leaves. A huge odum tree is, in a sense, made of captured air and water.

Try this

Breathe out onto a cool mirror. The mist is water, but your breath also carries carbon dioxide that a plant could use. What else do you add to the air when you breathe out?

Chlorophyll: the green trap

Why are leaves green? Because they contain chlorophyll, a pigment inside the chloroplasts of the leaf cells.

Chlorophyll's job is to absorb light energy. It absorbs red and blue light strongly but reflects green light, which is why we see leaves as green. (If plants absorbed all colours, they would look black.)

The energy absorbed is used to split water and to drive the building of glucose. No chlorophyll, no trapping of energy, no photosynthesis.

Facts to remember:

• Chloroplasts are found mostly in the cells of the upper half of the leaf, where light is strongest.
• Each cell may contain dozens of chloroplasts.
• Plants with red or purple leaves still have chlorophyll; other pigments mask the green.
• In the dry season many trees lose their leaves; the chlorophyll breaks down first, which is why leaves turn yellow before they fall.

Chlorophyll is to a leaf what a solar panel is to a house: the part that catches the light.

The leaf: a factory built for the job

A leaf is shaped by its work. Look at any leaf and you will see:

Broad and flat – a large surface to catch as much light as possible.

Thin – light reaches all the cells, and gases do not have far to travel.

Veins – tubes that bring water (xylem) and carry away the glucose made (phloem).

Stomata – pores, mostly underneath, that let carbon dioxide in and oxygen and water vapour out. Each is opened and closed by two guard cells.

A waxy layer (cuticle) on top – stops the leaf drying out.

Palisade cells – tall cells packed with chloroplasts just under the upper surface, where most photosynthesis happens.

Spongy cells with air spaces – below the palisade layer, letting gases move freely.

Try this

Hold a leaf up to the sun. Trace the veins with your finger. Turn it over and feel the difference between the shiny top and the duller underside. Which side do you think has more stomata, and why?

Stomata: the leaf's doors

Stomata are tiny pores on the surface of leaves. A single leaf may have hundreds of thousands. Through them:

• carbon dioxide comes in for photosynthesis
• oxygen goes out (made by photosynthesis)
• water vapour goes out (transpiration)

Each stoma is bordered by two curved guard cells. When the guard cells are full of water they bow apart and the pore opens; when they lose water they go limp and the pore closes.

Stomata are usually open in the day (when light is available for photosynthesis) and closed at night. They also close in very hot, dry weather to save water, even in daylight; that is why plants slow down in the heat of the afternoon.

Most stomata are on the underside of the leaf. It is cooler there and less water is lost.

Try this

Paint a thin layer of clear nail varnish on the underside of a leaf, let it dry, peel it off and look at it with a hand lens. The small marks are the stomata.

Recap: what photosynthesis needs

Photosynthesis makes glucose in the chloroplasts of green plants.

carbon dioxide + water —(light, chlorophyll)→ glucose + oxygen

Needed	Where from	How it gets in
Light	the Sun	absorbed by chlorophyll
Water	soil	roots, xylem, veins
Carbon dioxide	air	stomata
Chlorophyll	made by the plant	inside chloroplasts

The leaf is built for the job: broad, thin, veined, with stomata underneath and palisade cells on top.

Quick quiz

1. Name the two reactants of photosynthesis.
2. Name the two products.
3. Which pigment absorbs the light?
4. Through what do gases enter and leave the leaf?
5. Why do leaves look green?
6. Give one reason a plant in a dry spell stops growing even in bright sunshine.

Testing a leaf for starch

How do we know a leaf has been photosynthesising? Glucose is quickly turned into starch and stored in the leaf. Starch turns iodine solution from yellow-brown to blue-black. So if a leaf contains starch, it has been making food.

The method (your teacher will handle the hot water and ethanol):

1. Boil the leaf in water for one minute. This kills it and softens it.
2. Put it in a tube of ethanol standing in the hot water (not over a flame; ethanol catches fire). The ethanol removes the chlorophyll. The leaf turns pale; the ethanol turns green.
3. Dip the leaf in warm water to soften it again.
4. Spread it on a white tile and add a few drops of iodine solution.
5. Blue-black = starch present. Yellow-brown = no starch.

Removing the chlorophyll in step 2 matters: without it, the green would hide the colour change.

Fair tests: light and starch

To show that light is needed for photosynthesis:

1. Destarch a potted plant by leaving it in a dark cupboard for 48 hours. It uses up its stored starch, so we start from zero.
2. Cover part of one leaf with a strip of black paper or foil on both sides, held with paper clips.
3. Put the plant in bright light for a day.
4. Remove the leaf, take off the paper and test it for starch.

Result: the parts that were in the light turn blue-black; the covered strip stays yellow-brown.

Conclusion: starch (and so glucose) was only made where light reached. Light is needed for photosynthesis.

This is a fair test because everything about the two parts of the leaf is the same (same leaf, same water, same air, same chlorophyll) except the one thing we changed: light.

Fair tests: chlorophyll and starch

Some plants have variegated leaves: part green, part white or cream. The white parts have no chlorophyll. They are perfect for a test.

1. Destarch the plant for 48 hours.
2. Place it in bright light for a day.
3. Draw a careful picture of one variegated leaf, marking which parts are green and which are white.
4. Remove the leaf and test it for starch.

Result: only the parts that were green turn blue-black. The white parts stay yellow-brown, even though they received the same light, water and carbon dioxide.

Conclusion: chlorophyll is needed for photosynthesis.

Try this

A pupil says: "The white part of the leaf had no starch because it was in the shade." What is wrong with this reasoning? How does the method make sure the white and green parts had the same light?

Fair tests: carbon dioxide and starch

To show that carbon dioxide is needed, we must take it away from one plant and not the other.

1. Destarch two similar potted plants.
2. Cover each with a clear plastic bag, sealed at the pot.
3. In bag A, put a small dish of soda lime or sodium hydroxide, which absorbs carbon dioxide from the air inside.
4. In bag B, put a dish of water (no effect), so that both bags are set up the same way.
5. Leave both in bright light for a day, then test a leaf from each.

Result: the leaf from plant B turns blue-black; the leaf from plant A stays yellow-brown.

Conclusion: without carbon dioxide, no starch is made. Carbon dioxide is needed for photosynthesis.

Plant B is the control: it shows that it was the missing carbon dioxide, not the bag or the dish, that stopped photosynthesis. Every good experiment has one.

Showing oxygen is given off

Water plants make this easy. A pond weed such as Elodea or Hydrilla, which you can find in slow streams and ponds, gives off bubbles when it photosynthesises.

1. Put a sprig of pond weed in a beaker of water.
2. Cover it with an upside-down funnel, and over the funnel's stem place a test tube full of water.
3. Put the beaker in bright light.

Bubbles rise from the cut stem and collect in the test tube, pushing out the water. After a day or two there is enough gas to test.

4. Lift the tube out with your thumb over the end, and put a glowing splint into it. The splint relights.

Only oxygen relights a glowing splint. So the gas the plant gave off is oxygen.

You can also use this set-up to compare rates: count bubbles per minute in bright light, then in dim light. More light, more bubbles, faster photosynthesis.

Recap: the tests

To show that…	What we do	Positive result
a leaf has made food	iodine test	blue-black colour
light is needed	cover part of a leaf	covered part stays brown
chlorophyll is needed	use a variegated leaf	white part stays brown
CO₂ is needed	soda lime in a sealed bag	that leaf stays brown
oxygen is given off	collect gas from pond weed	glowing splint relights

Remember destarching first (48 hours in the dark) so that any starch found was made during the experiment, and a control so that only one thing differs.

Quick quiz

1. Why is the leaf boiled in ethanol before adding iodine?
2. Why must ethanol never be heated over a flame?
3. In the carbon-dioxide test, what is the purpose of the dish of water in bag B?
4. Which gas relights a glowing splint?

What is respiration?

Now the second half of the story. Plants have made glucose. How does a cell, in a plant or in you, get the energy out of it?

Respiration is the process by which living cells break down glucose, using oxygen, to release energy. Carbon dioxide and water are produced.

Do not confuse it with breathing. Breathing is the movement of air in and out of the lungs. Respiration is a chemical reaction inside every cell: in your muscles, your brain, your skin; in a leaf, a root, a seed; in a fish, a fly, a mushroom and a bacterium.

It never stops. Even when you are asleep, every cell is respiring to keep you alive. A plant respires day and night, even while it photosynthesises.

The energy released is used for all the things living things do: movement, growth, warmth, repair, making new substances.

Respiration is, in a way, a slow and controlled kind of burning: glucose is the fuel, oxygen is needed, and energy and carbon dioxide are given out. But it happens at body temperature, in small steps, without any flame.

The word equation for respiration

glucose + oxygen → carbon dioxide + water (+ energy released)

Compare it with photosynthesis:

carbon dioxide + water → glucose + oxygen (light energy taken in)

They are the same equation backwards. Photosynthesis builds glucose and stores energy; respiration breaks glucose and releases it.

We write energy in brackets because energy is not a substance; it is not a product you could pour out. It is released as the bonds in glucose are broken and new ones formed in carbon dioxide and water.

The kind of respiration described here needs oxygen and is called aerobic respiration. (There is also anaerobic respiration, without oxygen, which releases much less energy; you will meet it in Basic 9. It is what makes your legs ache in a sprint, and what yeast does when it makes bread rise or palm wine ferment.)

Try this

Write both word equations side by side. Draw arrows to show which substance in one becomes which in the other.

Where respiration happens

Respiration happens inside cells, in tiny structures called mitochondria (one is a mitochondrion). You could think of them as the cell's power stations.

Cells that need a lot of energy have many mitochondria:

• muscle cells, which contract constantly
• liver cells, which carry out hundreds of chemical jobs
• sperm cells, which must swim
• the growing tip of a plant root

Cells that need less energy have fewer.

The glucose reaches the cells through the blood (in animals) or the phloem (in plants). The oxygen reaches animal cells from the lungs or gills via the blood; in plants it enters through stomata and the spaces between cells, and in roots from air in the soil.

That last point explains something farmers know: a field that stays waterlogged kills crops. Water fills the air spaces in the soil, the roots cannot get oxygen, the root cells cannot respire, and the plant dies from the bottom up.

Evidence for respiration

How do we know respiration is happening?

Carbon dioxide is given out. Breathe out through a straw into limewater. It turns milky. Limewater is the test for carbon dioxide. Germinating seeds in a sealed jar also turn limewater milky; the seeds are respiring.

Heat is given out. Put germinating beans in a vacuum flask with a thermometer; put boiled (dead) beans in another flask as a control. After a day, the live beans' flask is warmer. Energy released by respiration has escaped as heat. (A heap of damp grass or cocoa husks warming up as it rots is the same thing: microbes respiring.)

Oxygen is used up. Candle flames in a sealed jar with germinating seeds go out sooner than in a jar without seeds.

Mass is lost. A seed loses mass as it germinates in the dark, because the glucose in its stores is being broken down to carbon dioxide and water, which leave.

Try this

Why are boiled seeds used as the control in the heat experiment, rather than an empty flask?

Recap: respiration

Respiration releases energy from glucose in every living cell, all the time.

glucose + oxygen → carbon dioxide + water (+ energy)

• It is not breathing; breathing just brings oxygen to the blood.
• It happens in mitochondria.
• The energy is used for movement, growth, warmth, repair and making new substances.
• Evidence: carbon dioxide turns limewater milky; germinating seeds get warm, lose mass and use up oxygen.

Quick quiz

1. What is the difference between breathing and respiration?
2. Name the two reactants and two products of aerobic respiration.
3. Which part of the cell carries out respiration?
4. Give the test for carbon dioxide and its result.
5. Why do waterlogged crops die?

Two processes, one cycle

Put the two equations together and a beautiful pattern appears.

Photosynthesis uses carbon dioxide and water, makes glucose and oxygen, and stores energy from the Sun.

Respiration uses glucose and oxygen, makes carbon dioxide and water, and releases that energy.

What one process makes, the other needs. The oxygen you breathe was made by a plant. The carbon dioxide you breathe out will be taken into a leaf. The glucose in your waakye was built in a rice plant from carbon dioxide you, or a goat, or a fire, once released.

This is the carbon and oxygen cycle. Carbon atoms circle for ever between air and living things, with energy from the Sun driving the loop.

Without photosynthesis, oxygen would run out and food would run out. Without respiration, no living thing could use the food. Both matter, always.

Do plants respire too?

Yes, always. A common mistake is to say that plants photosynthesise and animals respire. In fact:

• Animals respire only.
• Plants respire and photosynthesise.

In the day, a plant does both. Photosynthesis is usually much faster, so overall the plant takes in carbon dioxide and gives out oxygen. The carbon dioxide its own respiration makes is used straight away for photosynthesis.

At night, there is no light, so photosynthesis stops. The plant still respires: it takes in oxygen and gives out carbon dioxide, just like you.

Over a whole day, a healthy plant makes much more glucose than it breaks down. The extra is what becomes new leaves, roots, fruit and stored starch: the plant's growth, and our food.

Try this

Draw two pictures of a leaf, one labelled "day" and one "night". On each, draw arrows for the gases going in and out.

Why both matter: the balance of gases

The air has stayed roughly 21 % oxygen for a very long time. That is no accident. Photosynthesis adds oxygen; respiration and burning remove it. The two have been roughly in balance.

Carbon dioxide, however, is rising. Burning fossil fuels (petrol, diesel, coal, gas) releases carbon that was locked away for millions of years, far faster than plants can take it up. Cutting down forests removes the very leaves that would absorb it.

Carbon dioxide traps heat, so the Earth is warming. In Ghana this shows up as hotter dry seasons, less predictable rains and coastal erosion.

What helps:

• Planting trees and protecting forests such as Kakum and the Atewa Range
• Using less fuel: walking, sharing transport, efficient stoves
• Clean energy: solar panels on roofs do, in a way, what leaves do: catch sunlight

Understanding these two equations is the first step to understanding one of the biggest problems of our time.

Food chains start with a leaf

A food chain shows who eats whom. Every food chain begins with a producer: a plant, or a tiny floating alga in the sea or a lake, which makes its food by photosynthesis.

grass → grasshopper → lizard → hawk maize → goat → human algae → tilapia → human

Everything after the producer is a consumer. It gets glucose by eating. At every step, each creature respires and uses most of the energy it took in for moving and keeping warm, so only a small part passes to the next eater. That is why there are many grasshoppers but few hawks.

The energy in every chain was once sunlight, trapped by chlorophyll. Even the energy you used to turn this page came, by a long road, from the Sun.

Try this

Write two food chains that end with you. Start each with the plant.

Photosynthesis and farming

Farmers are photosynthesis managers, whether or not they use the word. Everything they do increases how much glucose their crops can make and store.

Spacing – plants too close shade one another; too far apart wastes land. Maize is often planted about 75 cm between rows.

Weeding – weeds steal light, water and minerals.

Watering – without water, stomata close and photosynthesis stops.

Fertiliser and manure – add minerals such as nitrates so the plant can turn glucose into proteins for growth.

Timing – planting at the start of the rains gives seedlings water when they need it most.

Shade for cocoa – cocoa is a forest plant; young cocoa grows better under a light shade of plantain or taller trees, which cut harsh midday sun and keep moisture in.

Greenhouses – in some vegetable farms near Accra and Kumasi, plastic houses keep water in and allow year-round growing.

Try this

Visit a farm or a garden. Find three things the farmer does that help photosynthesis, and one that helps roots respire.

What the plant does with glucose

Glucose is sweet, soluble and quickly made. A plant does five things with it.

1. Respires it – some glucose is broken down at once to release energy for the plant's own work.

2. Stores it as starch – starch is insoluble and compact, ideal for storing in leaves by day, and for the long term in roots (cassava, yam), tubers (potato), stems (sugar cane stores sugar itself) and seeds (maize, rice, beans).

3. Builds cellulose – thousands of glucose units joined into tough fibres make the cell wall of every plant cell. Wood, cotton and paper are mostly cellulose.

4. Makes proteins – glucose plus nitrates from the soil gives the amino acids that build proteins for growth and enzymes.

5. Makes oils and fats – stored in seeds such as groundnut, palm kernel and shea.

When you eat gari, chew sugar cane, wear a cotton shirt or cook with palm oil, you are using glucose that a leaf made from air, water and sunlight.

Minerals from the soil

Photosynthesis needs only carbon dioxide and water, but a plant is more than glucose. To build proteins, chlorophyll and new cells it needs minerals, dissolved in soil water and taken in by the roots.

Mineral	Needed for	If it is short
Nitrates	making proteins for growth	small plant, yellow older leaves
Magnesium	making chlorophyll	yellow leaves, poor photosynthesis
Phosphates	roots, energy transfer	poor roots, purple-tinged leaves
Potassium	healthy leaves and flowers	brown edges on leaves

Farmers add these as fertiliser (bags marked NPK: nitrogen, phosphorus, potassium) or as manure and compost, which release minerals slowly as they rot.

Growing beans and groundnuts also adds nitrates: bacteria in lumps on their roots turn nitrogen gas from the air into nitrates. That is why farmers rotate maize with legumes.

Try this

A pupil's maize plants have yellow leaves. Give two possible mineral causes, and say how you could tell them apart.

Life in the water

Photosynthesis is not only for land plants. In the sea, lakes and the Volta, tiny floating algae (phytoplankton) and larger seaweeds photosynthesise just as leaves do: they contain chlorophyll, take in carbon dioxide dissolved in the water, and give off oxygen.

The numbers are enormous. Algae in the oceans make about half of all the oxygen produced on Earth. Every second breath you take comes from the sea.

They are also the producers at the bottom of every water food chain:

algae → tiny animals → small fish → tilapia or tuna → people

Fishermen at Elmina, Tema and on Lake Volta depend on this invisible photosynthesis.

Algae need light, so they live near the surface. Muddy water from erosion, or water choked with weeds, blocks the light, so fewer algae grow and fish become scarce. Protecting rivers from silt and pollution protects the whole chain from the first leaf-like cell to the last plate of fish.

Seeds: packed lunches

A seed is a baby plant with a food store, wrapped in a coat. The store is starch, protein or oil made by the parent plant from the glucose of photosynthesis.

When a seed germinates:

1. It takes in water and swells.
2. Enzymes turn the stored starch back into glucose.
3. The cells respire that glucose to release energy for growth.
4. A root grows down and a shoot grows up.

All this happens in the dark, underground, with no photosynthesis. The seedling lives entirely on its packed lunch until the first green leaves open above the soil. Then it starts to make its own food, and the store is no longer needed.

This explains two things:

• A germinating seed loses mass and gives out carbon dioxide, because it respires but does not photosynthesise.
• A seed planted too deep uses up its store before the shoot reaches the light, and dies.

Try this

Soak some beans overnight, wrap them in damp paper and keep them in the dark. Record what happens each day for a week.

How fast? Limiting factors

Photosynthesis does not always run at full speed. Its rate depends on light, carbon dioxide and temperature. Whichever of these is in shortest supply sets the speed. It is called the limiting factor.

Think of a chop bar with three cooks: one for rice, one for stew, one for serving. If the stew cook is slow, it does not matter how fast the others work; plates leave at the stew cook's pace.

• Early morning: light is dim, so light limits the rate. As the sun rises, photosynthesis speeds up.
• Bright midday: light is plentiful, so now carbon dioxide (only 0.04 % of air) limits the rate.
• Cool harmattan dawn: low temperature slows the enzymes, so the rate is low even in sunshine.

If the limiting factor is increased, the rate rises until something else becomes limiting.

Try this

In the pond-weed experiment, moving the lamp closer increases bubbles up to a point, after which they stop increasing. Which factor has become limiting, and how could you test your answer?

Breathing, exercise and respiration

When you run, your muscles respire faster to release more energy. They need more glucose and more oxygen, and they make more carbon dioxide.

Your body responds:

• Breathing becomes faster and deeper, to take in oxygen and remove carbon dioxide.
• Your heart beats faster, to carry glucose and oxygen to the muscles quickly.
• You feel warm and sweat, because respiration releases energy partly as heat.

If you sprint flat out, the oxygen cannot arrive fast enough. Muscles switch to anaerobic respiration for a short time, producing lactic acid, which is what makes your legs burn and ache. After the race you keep panting to pay back the oxygen debt.

Try this

Count your breaths for one minute sitting still. Then do 30 star jumps and count again. Record both. Explain the difference using the word respiration.

Recap: putting it together

	Photosynthesis	Respiration
Where	chloroplasts, mainly leaves	mitochondria, all cells
When	daylight only	all the time
Who	green plants, algae	all living things
Takes in	carbon dioxide, water	glucose, oxygen
Gives out	glucose, oxygen	carbon dioxide, water
Energy	stored (from light)	released

The two processes are opposites, and together they form the carbon and oxygen cycle that keeps the air breathable and every food chain fed.

Quick quiz

1. Which process happens in plants at night?
2. Why is a forest called "the lungs of the Earth", and why is the phrase not quite accurate?
3. Name one farming practice that increases photosynthesis.
4. Why do you breathe harder after running?

Experiment design: your own fair test

You now know enough to design an investigation. Suppose you want to find out whether the colour of light affects how fast pond weed photosynthesises.

Question: Does pond weed make more bubbles in red, green or blue light?

Variables:

• Independent (what I change): colour of the light (use coloured cellophane over a lamp).
• Dependent (what I measure): bubbles per minute.
• Controlled (kept the same): same sprig of weed, same lamp, same distance, same water, same temperature, same time of counting.

Method: set up the pond weed as before; cover the lamp with red cellophane; wait two minutes; count bubbles for one minute; repeat three times and average. Repeat for green and blue.

Prediction: green light will give the fewest bubbles, because chlorophyll reflects green.

Repeating and averaging makes the result more reliable. Changing only one thing makes it a fair test.

Reading results

Here are results from the coloured-light experiment.

Light colour	Bubbles per minute (3 trials)	Average
Red	24, 22, 26	24
Green	6, 8, 7	7
Blue	20, 23, 20	21

What the results show: red and blue light gave many bubbles; green gave few. The prediction was right: chlorophyll absorbs red and blue light well and reflects green, so green light is of little use for photosynthesis.

Evaluating:

• Bubble size varied; a better method would collect the gas and measure its volume.
• The lamp also warms the water a little; a glass of water between lamp and weed would absorb the heat.
• Three trials is a reasonable start; five would be better.

Being able to say what was good and what could be improved is part of being a scientist, and it earns marks in examinations.

Common mistakes

"Plants breathe in carbon dioxide and breathe out oxygen." Plants do not breathe. They take in carbon dioxide through stomata by diffusion, and they also respire, taking in oxygen, all the time.

"Plants get their food from the soil." They get water and minerals from the soil. Their food (glucose) is made in the leaves from carbon dioxide and water.

"Respiration is breathing." Breathing moves air. Respiration is a chemical reaction in cells.

"Only animals respire." Every living cell respires, including every plant cell, day and night.

"Chlorophyll is a reactant." It is not used up. It is a pigment that traps light. Write it on the arrow, not on the left.

"The iodine turned blue, so the leaf has glucose." Iodine tests for starch, which the plant makes from glucose.

"Photosynthesis gives the plant energy." Photosynthesis stores energy in glucose. Respiration releases it.

Test yourself

Section A

1. Write the word equation for photosynthesis, including the conditions.
2. Write the word equation for aerobic respiration.
3. State three ways a leaf is adapted for photosynthesis.
4. Explain why most stomata are on the underside of leaves.
5. Describe how you would test a leaf for starch, giving the safety point about ethanol.

Section B

6. A pupil destarches a plant, covers half a leaf with foil and leaves it in the light. After the starch test, the whole leaf is yellow-brown. Suggest two possible reasons.
7. Explain why a plant in a sealed, well-lit glass jar can survive for weeks, but a mouse in the same jar cannot.
8. A farmer's field floods for a week. Explain, using the word respiration, why the maize dies.
9. Why does a plant give out carbon dioxide at night but not (overall) by day?
10. Give one reason the two processes in this book matter for the Earth's climate.

Test answers: Section A

1. carbon dioxide + water —(light, chlorophyll)→ glucose + oxygen.
2. glucose + oxygen → carbon dioxide + water (+ energy).
3. Any three: broad and flat; thin; veins for water and glucose; stomata for gases; palisade cells packed with chloroplasts near the top; air spaces for gases; waxy cuticle to stop drying.
4. Cooler and shaded underneath, so less water is lost by evaporation while gases can still move in and out.
5. Boil in water; remove chlorophyll in ethanol warmed in a water bath (never over a flame: ethanol is flammable); soften in warm water; spread on a tile; add iodine; blue-black = starch.

Test answers: Section B

6. The plant was not in light long enough; the leaf was tested wrongly (for example the chlorophyll was not removed, or the iodine was old); the plant was unhealthy; the "light" was too dim to drive photosynthesis.
7. The plant photosynthesises by day, making oxygen and glucose, and respires using some of each; its gases cycle inside the jar. The mouse only respires: it uses up the oxygen and produces carbon dioxide, with nothing to replace the oxygen.
8. Water fills the air spaces in the soil; roots cannot get oxygen; root cells cannot respire to release energy; roots die and the plant with them.
9. At night there is no light, so no photosynthesis, but respiration continues and releases carbon dioxide. By day photosynthesis uses carbon dioxide faster than respiration makes it, so overall the plant takes it in.
10. Photosynthesis removes carbon dioxide from the air; cutting forests and burning fuels adds it faster than plants can take it up, trapping heat and warming the planet.

Scoring: 9–10 correct, excellent; 6–8, good, revisit the recaps; below 6, work through the quizzes again before moving on.

Glossary

Photosynthesis — the process by which green plants make glucose from carbon dioxide and water using light energy.

Respiration — the release of energy from glucose in living cells; aerobic respiration uses oxygen.

Chlorophyll — the green pigment that absorbs light energy.

Chloroplast — the structure in a plant cell where photosynthesis happens.

Mitochondrion — the structure in a cell where respiration happens.

Glucose — a simple sugar; the plant's food and the fuel for respiration.

Starch — a storage substance made from glucose; turns iodine blue-black.

Stomata — pores in the leaf, each between two guard cells, through which gases pass.

Transpiration — loss of water vapour from leaves.

Xylem / phloem — tubes carrying water up / glucose around the plant.

Producer — an organism that makes its own food; the start of every food chain.

Fair test — an experiment in which only one thing is changed.

Control — the part of an experiment that shows what happens when the one thing is not changed.

Learn more

• Khan Academy: Photosynthesis in plants — videos and practice at a slightly higher level, good for stretching.
• Khan Academy: Photosynthesis and cellular respiration (middle school) — exactly this topic, with quizzes.
• Science Learning Hub: Photosynthesis and respiration — a clear article comparing the two processes.
• Wikipedia: Photosynthesis — for the curious; the first sections are readable, the later ones are for much later.

Watch more

Think about it

1. A tree can weigh many tonnes, yet the soil under it is not noticeably lower. Where did most of its mass come from?
2. If all the plants on Earth died tomorrow, what would happen to the oxygen in the air, and how quickly? What would happen to animals first: lack of oxygen or lack of food?
3. Why do you think photosynthesis is slower on a cloudy afternoon in Kumasi than at noon on a clear day, even though both are warm?
4. Both a candle and a mouse "use up" oxygen and make carbon dioxide. In what ways is respiration like burning, and in what ways is it different?
5. A friend says, "Plants are good for the air because they breathe in CO₂ and breathe out oxygen." Rewrite the sentence so that it is scientifically correct.