B. Biology: Sustainable Ecosystems and Climate Change · Sustainable Ecosystems and Climate ChangePhotosynthesis and Cellular Respiration: Two Sides of One Cycle
B2.3 B2.2 A1.2 A1.5About 75 minutes
Learning goals- I can write the word and chemical equations for photosynthesis and cellular respiration.
- I can compare and contrast the two processes: where, when, in which organisms, and what happens to energy and matter.
- I can explain how their complementary relationship keeps oxygen and carbon dioxide in dynamic equilibrium.
- I can plan and carry out a fair test of a factor that affects the rate of photosynthesis.
Words to know
photosynthesis
The process in which producers use light energy to turn carbon dioxide and water into glucose and oxygen.
cellular respiration
The process in which cells break down glucose with oxygen to release usable energy, producing carbon dioxide and water.
chloroplast
The organelle in plant and algae cells where photosynthesis happens; it contains the green pigment chlorophyll.
mitochondrion (plural: mitochondria)
The organelle where cellular respiration releases energy from glucose.
glucose
A sugar (C₆H₁₂O₆) that stores chemical energy.
ATP
Adenosine triphosphate, the molecule cells use to carry energy for their activities.
reactant / product
Reactants are the substances that go into a chemical reaction; products are the new substances that come out.
complementary
Fitting together so that each completes the other.
Remember from Grade 8
In Grade 8 Cells you learned that chloroplasts carry out photosynthesis and mitochondria release energy from food, and that plant cells have both. In Grade 7 you learned the basic idea of photosynthesis and respiration. Now you will write and balance the equations, compare the two processes point by point, and explain how together they keep the gases of the atmosphere in balance.
Let's start with a question
A sealed glass bottle garden can live for years without ever being opened. Where do the plants get carbon dioxide? Where does the oxygen for the soil microbes come from? The answer is that two processes run in a loop. Plants make sugar and oxygen. Plants and microbes use that sugar and oxygen and give back carbon dioxide and water. Each process makes what the other needs.
Photosynthesis
Producers (plants, algae and some bacteria) use light energy to build glucose:
carbon dioxide + water → (light energy, chlorophyll) → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Where: in chloroplasts, mostly in leaf cells. CO₂ enters through tiny pores called stomata; water comes up from the roots.
- When: only when there is light.
- Energy: light energy is changed into chemical energy stored in glucose.
- Matter: carbon from the air becomes part of living things; oxygen is released.
The glucose is used for energy and to build other molecules, such as cellulose (cell walls) and starch (stored food).
Cellular respiration
All living cells, including plant cells, release the energy stored in glucose:
glucose + oxygen → carbon dioxide + water + energy (ATP and heat)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
- Where: mostly in mitochondria.
- When: all the time, day and night.
- Energy: chemical energy in glucose is transferred to ATP, which powers cell activities; some is released as heat.
- Who: plants, animals, fungi and most microorganisms.
Some organisms, such as yeast, can also release energy without oxygen (anaerobic respiration or fermentation), but this releases much less energy per glucose molecule.
| Feature | Photosynthesis | Cellular respiration |
|---|
| Organelle | Chloroplast | Mitochondrion |
| Organisms | Producers only (plants, algae, some bacteria) | Almost all living things, including plants |
| When | Only in light | All the time |
| Reactants | CO₂ + H₂O (+ light energy) | C₆H₁₂O₆ + O₂ |
| Products | C₆H₁₂O₆ + O₂ | CO₂ + H₂O + energy (ATP, heat) |
| Energy change | Light → stored chemical energy | Stored chemical energy → ATP + heat |
| Effect on air | Removes CO₂, adds O₂ | Adds CO₂, removes O₂ |
| Role in carbon cycle | Carbon sink process | Carbon source process |
The complementary relationship and dynamic equilibrium
Look at the two equations. The products of photosynthesis (glucose and oxygen) are the reactants of respiration, and the products of respiration (carbon dioxide and water) are the reactants of photosynthesis. Together they form a cycle of matter.
This cycle keeps ecosystems in dynamic equilibrium:
- Over the whole planet, photosynthesis and respiration (including decomposition) have been close to balanced for long periods, which kept the oxygen and CO₂ in the air fairly steady.
- In a lake, algae photosynthesize during the day, raising dissolved oxygen. At night, everything respires and oxygen drops. Over 24 hours the levels swing up and down around a steady average.
- In a healthy aquarium with plants, fish and plants exchange gases in the same way.
But energy does not cycle. Light energy goes in; heat comes out. That is why the bottle garden needs a sunny window to survive.
Worked exampleLet's check that the photosynthesis equation is balanced
Count the atoms of each element on both sides of 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Carbon: left 6 × 1 = 6; right 6 (in C₆H₁₂O₆).
- Hydrogen: left 6 × 2 = 12; right 12.
- Oxygen: left (6 × 2) + (6 × 1) = 12 + 6 = 18; right 6 + (6 × 2) = 6 + 12 = 18.
- Every element has the same number of atoms on both sides, so atoms are conserved. Matter is not created or destroyed; it is rearranged.
Balanced: 6 C, 12 H and 18 O on each side. The respiration equation uses the same numbers in reverse.
Worked exampleLet's analyse a yeast experiment
Three sealed bottles each hold 250 mL of warm water and 1 package of yeast. Bottle A has no sugar, B has 1 tablespoon of sugar, C has 3 tablespoons. A balloon on each bottle collects gas. After 30 minutes the balloons measure: A 0 cm, B 15 cm, C 24 cm around. What process produced the gas, and what do the results show?
- The yeast are breaking down sugar to release energy: cellular respiration (in a sealed bottle it soon becomes mostly anaerobic fermentation). The gas is carbon dioxide.
- Independent variable: amount of sugar. Dependent variable: balloon size (amount of CO₂).
- Bottle A (no sugar) made no gas: yeast need glucose (sugar) as the reactant.
- More sugar (C) gave more CO₂ than less sugar (B) in the same time.
The gas is CO₂ from respiration. More sugar (more reactant) gave more CO₂, showing glucose is used up and CO₂ is produced, just as the respiration equation predicts.
The floating leaf disk investigation. Oxygen made by photosynthesis collects in the disks and makes them float. The faster they float, the faster the rate of photosynthesis.Hands-on activityFloating leaf disks: what affects the rate of photosynthesis? (A1.2, A1.5)
You need: fresh spinach leaves, a single-hole paper punch (or a sturdy drinking straw), baking soda, dish soap, water, a plastic syringe with no needle (from a pharmacy, often sold for children's medicine), 3 clear cups, a desk lamp or bright window, a timer.
- Make bicarbonate solution: 1/8 teaspoon of baking soda in 300 mL of water, plus one small drop of dish soap. (Baking soda supplies CO₂ for photosynthesis; soap helps the solution soak into the disks.)
- Punch 10 disks from each of 3 spinach leaves, avoiding big veins.
- Remove the syringe plunger, put 10 disks inside, replace the plunger and draw in about 5 mL of solution. Hold the tip up, push out the air, then cover the tip with your finger and pull back the plunger gently for 10 seconds. This pulls air out of the leaf spaces. Repeat 2 or 3 times until the disks sink.
- Put the sunk disks in a cup with solution about 3 cm deep. Do the same for two more cups.
- Test one variable: for example, put the cups at 10 cm, 30 cm and 60 cm from the lamp (keep everything else the same).
- Every minute, count how many disks are floating. Stop at 20 minutes.
- Make a table and a line graph (time on the x-axis, number floating on the y-axis) for each distance.
- Conclude: which cup's disks floated fastest? What does that tell you about light and photosynthesis? Put one floating cup in a dark cupboard for 20 minutes: what happens, and why?
What to look for: disks closest to the light float first, because more light = faster photosynthesis = more oxygen bubbles in the disks. In the dark, disks slowly sink again, because respiration uses up the oxygen.
Safety (A1.5): use a syringe with no needle. Baking soda is safe, but don't drink the solution. A desk lamp gets hot: don't touch the bulb, keep water away from the cord and plug, and have a grown-up nearby. Wash up when done.
Just for funRiddle: I take in what you breathe out, and you take in what I give off. We've never met, but we can't live without each other. What am I?
A plant! (And, to be fair, plants breathe out CO₂ too, especially at night.)
Your turn
1Write the word equation for photosynthesis.
carbon dioxide + water → (light energy) → glucose + oxygen
2Write the chemical equation for cellular respiration.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
3Name the organelle for each process.
Photosynthesis: chloroplast. Cellular respiration: mitochondrion.
4Do plants carry out cellular respiration? When?
Yes, all the time, day and night, like all living cells.
5Explain what "complementary" means for these two processes.
The products of one are the reactants of the other: photosynthesis makes glucose and O₂, which respiration uses; respiration makes CO₂ and H₂O, which photosynthesis uses.
6Describe the energy change in each process.
Photosynthesis: light energy → chemical energy (in glucose). Respiration: chemical energy in glucose → ATP (usable energy) and heat.
7Why does dissolved oxygen in a pond go up in the afternoon and down before sunrise?
In daylight, algae and plants photosynthesize faster than everything respires, so oxygen rises. At night only respiration happens, so oxygen is used up and falls until sunrise.
More practice: warm-up
1What green pigment captures light?
2What gas do leaves take in for photosynthesis?
3What are the products of cellular respiration?
Carbon dioxide, water and energy (ATP and heat).
4Through what tiny pores do gases enter and leave a leaf?
The molecule cells use to carry energy for their activities.
More practice: on your own
1In the leaf disk activity, why does adding baking soda matter? Predict what would happen in plain water.
Baking soda supplies dissolved CO₂ (a reactant). In plain water there is very little CO₂, so photosynthesis would be slow and the disks would float much later or not at all.
2Name the independent variable, dependent variable and two controlled variables in the lamp-distance version.
Independent: distance from the lamp. Dependent: number of disks floating over time. Controlled: same solution, same number and size of disks, same leaf type, same water depth, same temperature, same lamp.
3A sealed jar with only a mouse would soon become unsafe for the mouse; a sealed jar with only a plant in the dark would also fail. Explain both. (Note: experiments like this must never be done with live animals.)
Mouse alone: respiration uses up O₂ and builds up CO₂, and nothing replaces the O₂. Plant in the dark: no photosynthesis, so the plant can only respire, using up its stored glucose and the O₂, until it dies. Both processes, plus light, are needed for the cycle.
4Use the equation to explain where most of the mass of a tree comes from.
From carbon dioxide in the air (plus water). Photosynthesis builds CO₂ and H₂O into glucose and then cellulose (wood). Only a small amount comes from soil minerals.
5How does the complementary relationship help keep ecosystems in dynamic equilibrium?
Because each process uses the products of the other, CO₂ and O₂ are constantly recycled. When one speeds up (e.g., summer photosynthesis), the gas levels shift, but they swing back as conditions change, keeping overall levels fairly steady.
Challenge
1At a certain dim light level, a plant's photosynthesis rate exactly equals its respiration rate. What would you observe in a sealed container with that plant, and why can't the plant grow at this light level?
The CO₂ and O₂ levels would stay constant, because CO₂ used = CO₂ released. The plant makes only enough glucose to replace what it uses in respiration, so there is no extra glucose to build new tissue, and it cannot grow. (This light level is called the compensation point.)
2A student says, "Respiration is just photosynthesis backwards, so energy is recycled too." Explain what is right and what is wrong.
Right: the matter is recycled: the reactants and products are reversed. Wrong: energy is not recycled. Light energy goes in during photosynthesis; respiration transfers it to ATP and finally heat, which leaves the ecosystem. New light is always needed.
3Design a test to find out whether the colour of light affects photosynthesis, using the leaf disk method. Include a hypothesis and a way to make the test fair.
Hypothesis (example): disks under blue or red light will float faster than under green light, because chlorophyll absorbs red and blue light best and reflects green. Method: cover identical lamps (or one lamp in turns) with red, blue and green transparent plastic or cellophane; keep the same distance, number of disks, solution and temperature; count floating disks every minute for 20 minutes; repeat each colour three times and average. Fairness: change only the colour.
Quiz
1. Which is the correct chemical equation for photosynthesis?
2. Where does cellular respiration mainly take place?
3. Which organisms carry out cellular respiration?
4. What happens to energy in cellular respiration?
5. How many oxygen atoms are on each side of the balanced photosynthesis equation?
6. In the floating leaf disk activity, why do the disks float?
7. Which statement best describes the complementary relationship?
Big idea (B2.3): photosynthesis and cellular respiration are complementary: matter (C, H, O) cycles between them, while energy enters as light and leaves as heat. Their balance keeps O₂ and CO₂ in dynamic equilibrium in ponds, forests and the whole atmosphere. The floating disk investigation is a full experimentation process (A1.2) with safe practices (A1.5).
How to help: The leaf disk activity works best with fresh spinach and a bright lamp; it takes about 45 minutes. Help your teen control variables and graph the results. If disks will not sink, they need a few more pulls with the syringe.
Common difficulties:
- Thinking plants only photosynthesize and animals only respire. Plants do both.
- Thinking respiration means breathing. Breathing moves gases in and out of lungs; cellular respiration is a chemical reaction in every cell.
- Saying energy is recycled. Only matter is.
- Balancing equations. Remember to multiply the subscript by the coefficient (6O₂ has 12 oxygen atoms).