C. Metabolic Processes · Metabolic Processes

Comparing Cellular Respiration and Photosynthesis: Matter and Energy Flow

C3.4 C3.3 C3.1 C3.2 C2.1 A1.11 A1.12About 70 minutes

Learning goals

I can describe, compare and illustrate with flow charts the matter and energy transformations in aerobic respiration, anaerobic respiration (fermentation) and photosynthesis.
I can explain the roles of oxygen, mitochondria and chloroplasts in these processes.
I can use the laws of thermodynamics to compare energy transfer in respiration and photosynthesis.
I can explain the light compensation point and why plant cells carry out both processes.

Words to know

light compensation point
The light intensity at which a plant's rate of photosynthesis exactly equals its rate of cellular respiration, so there is no net exchange of CO₂ or O₂.

light saturation point
The light intensity above which a further increase in light no longer increases the rate of photosynthesis.

limiting factor
The factor in shortest supply (such as light, CO₂ or temperature) that limits the rate of a process.

flow chart
A diagram that shows the steps of a process in order, with arrows showing inputs, outputs and the direction of change.

autotroph
An organism that makes its own organic molecules from inorganic ones (e.g. by photosynthesis).

heterotroph
An organism that gets organic molecules by eating other organisms or their products.

Two halves of one story

A grade 12 student breathing on a houseplant and the houseplant "breathing" back is a tidy picture: the student's mitochondria make CO₂ and water; the plant's chloroplasts turn them back into sugar and O₂. But the plant has mitochondria too, and it's respiring right now, day and night. Expectation C3.4 asks you to describe, compare and illustrate (using flow charts) the matter and energy transformations in both processes, including the roles of oxygen, mitochondria and chloroplasts. This lesson pulls together everything in the unit so far.

Remember from Lessons 4 to 14

Respiration: glycolysis (cytosol) → pyruvate oxidation → Krebs cycle (matrix) → ETC and chemiosmosis (inner membrane). Without O₂: fermentation. Photosynthesis: light reactions (thylakoid membranes) → Calvin cycle (stroma). Both use ETCs, H⁺ gradients and ATP synthase.

The overall picture: matter cycles, energy flows

Matter cycles. The products of photosynthesis (glucose, O₂) are the reactants of aerobic respiration, and the products of respiration (CO₂, H₂O) are the reactants of photosynthesis.
Energy flows one way. Light energy enters, is stored as chemical energy in glucose, is transferred to ATP, and is used for work, and at every step some is released as heat (second law). Heat can't be turned back into light or chemical energy by cells, so ecosystems need a constant input of sunlight.
Thermodynamics: photosynthesis is endergonic (ΔG = +2870 kJ/mol per glucose), driven by light; respiration is exergonic (ΔG = −2870 kJ/mol). The first law is satisfied (energy is conserved overall); the second law explains why neither process is 100% efficient (respiration about 34%; the light-to-sugar efficiency of photosynthesis in a crop field is typically only about 1 to 2%).

Matter and energy flow between chloroplasts and mitochondria. Glucose and O₂ from photosynthesis fuel respiration; CO₂ and H₂O from respiration feed photosynthesis. Energy enters as light and leaves as heat.

Flow charts of the three processes

Aerobic respiration (eukaryotes)

glucose → glycolysis (cytosol) → 2 pyruvate + 2 ATP + 2 NADH → pyruvate oxidation (matrix) → 2 acetyl-CoA + 2 CO₂ + 2 NADH → Krebs cycle (matrix) → 4 CO₂ + 2 ATP + 6 NADH + 2 FADH₂ → ETC and chemiosmosis (inner membrane, O₂ as final acceptor) → about 26 to 28 ATP + H₂O

Anaerobic: fermentation (cytosol only)

glucose → glycolysis → 2 pyruvate + 2 ATP + 2 NADH → NAD⁺ regenerated → 2 lactate (animals, some bacteria) or 2 ethanol + 2 CO₂ (yeast)

Photosynthesis (chloroplast)

light + H₂O → light reactions (thylakoid membranes) → O₂ (released) + ATP + NADPH → Calvin cycle (stroma) + CO₂ → G3P → glucose, sucrose, starch

When you draw these as flow charts, put each stage in a box, write its location under the box, and show inputs entering and outputs leaving with arrows. Mark where ATP is made and where O₂ is used or released.

Feature	Aerobic respiration	Fermentation	Photosynthesis
Organisms	Almost all eukaryotes; many prokaryotes	Yeast, muscle cells (short-term), many bacteria	Plants, algae, cyanobacteria
Organelle (eukaryotes)	Cytosol + mitochondrion	Cytosol only	Chloroplast
Reactants	C₆H₁₂O₆ + 6O₂	C₆H₁₂O₆	6CO₂ + 6H₂O + light
Products	6CO₂ + 6H₂O + ATP + heat	2 lactate, or 2 ethanol + 2 CO₂; 2 ATP	C₆H₁₂O₆ + 6O₂
Role of O₂	Final electron acceptor (reduced to H₂O)	Not used	By-product (from water)
Redox	Glucose oxidized; O₂ reduced	Glucose partly oxidized; pyruvate (or acetaldehyde) reduced	H₂O oxidized; CO₂ reduced
Electron carrier	NAD⁺/NADH, FAD/FADH₂	NAD⁺/NADH	NADP⁺/NADPH
ATP made by	Substrate-level + oxidative phosphorylation	Substrate-level only	Photophosphorylation (used in Calvin cycle)
ΔG per glucose	−2870 kJ/mol	about −200 kJ/mol (lactate: −196)	+2870 kJ/mol
H⁺ gradient	Intermembrane space → matrix	None	Lumen → stroma

A comparison of the three main metabolic processes in this unit.

Plants do both: the compensation point

Plant cells have both chloroplasts and mitochondria. All living plant cells respire all the time; green cells also photosynthesize when lit. What we measure (CO₂ uptake or O₂ release) is the net result.

In the dark, only respiration happens: the plant releases CO₂.
As light increases, photosynthesis speeds up. At the light compensation point, photosynthesis exactly balances respiration: no net gas exchange.
Above it, there is net CO₂ uptake and net O₂ release, and the plant can grow.
At high light, the rate levels off (light saturation): another factor, such as CO₂ concentration or temperature, becomes limiting.

Shade-tolerant plants on a forest floor (such as many ferns and the seedlings of sugar maples) have low compensation points, so they can make a net gain in dim light.

Net CO₂ uptake vs light intensity. Below the light compensation point, respiration exceeds photosynthesis (net CO₂ release); above it, the plant takes up CO₂ until the rate levels off at light saturation.

Worked example

Let's draw a comparison flow chart

Draw a single flow chart that shows how one carbon atom could travel from CO₂ in the air into a sugar maple leaf, then into a deer that eats the leaf, and back into the air. Name each process, its organelle and the form of energy at each step.

Step 1: CO₂ enters the leaf through stomata. In the chloroplast (stroma), the Calvin cycle fixes it into G3P, using ATP and NADPH from the light reactions (light energy → chemical energy).
Step 2: G3P becomes glucose, then sucrose or starch (stored chemical energy).
Step 3: The deer eats the leaf. Digestion releases glucose, which enters glycolysis in the cytosol of a deer cell → pyruvate.
Step 4: In the mitochondrion, pyruvate oxidation or the Krebs cycle releases the carbon as CO₂ (chemical energy → NADH, FADH₂ → ATP, plus heat). O₂ is the final electron acceptor in the ETC.
Step 5: The deer exhales the CO₂ into the air. The cycle can begin again.

CO₂ → (chloroplast: light reactions + Calvin cycle) → G3P → glucose/starch → (deer: glycolysis in cytosol → Krebs in mitochondrion) → CO₂. Energy: light → chemical (glucose) → ATP → work + heat.

Worked example

Let's interpret gas-exchange data

A potted maple seedling in a sealed chamber shows these net CO₂ changes per hour: dark: +12 mL (released); dim light: 0 mL; bright light: −40 mL (taken up). Estimate the seedling's gross rate of photosynthesis in bright light, assuming respiration stays the same.

In the dark, only respiration: respiration releases 12 mL CO₂/h.
In dim light, net = 0: this is the compensation point (photosynthesis uses 12 mL/h, equal to respiration).
In bright light, net uptake = 40 mL/h. Gross photosynthesis = net uptake + respiration = 40 + 12 = 52 mL CO₂/h.
This assumes respiration is constant; in reality it can change slightly with temperature.

Gross photosynthesis in bright light ≈ 52 mL CO₂ per hour (net 40 + respiration 12).

Just for fun

Plants are the ultimate multitaskers: every green cell in a leaf runs photosynthesis and cellular respiration at the same time, in different organelles. It's like charging your phone while using it, except the plant also built the charger.

Hands-on activity

Build a matter-and-energy flow chart poster

You need: a large sheet of paper (or 4 printer sheets taped together), coloured markers or pencils (use one colour for matter and another for energy), sticky notes.

Draw a large chloroplast on the left and a large mitochondrion on the right; draw the cytosol between them.
Using sticky notes, place each stage in the right compartment: light reactions, Calvin cycle, glycolysis, pyruvate oxidation, Krebs cycle, ETC/chemiosmosis, lactate fermentation, alcohol fermentation.
Draw matter arrows (one colour) for CO₂, H₂O, O₂, glucose, pyruvate, acetyl-CoA, G3P, lactate, ethanol.
Draw energy arrows (second colour) for light, ATP, NADH, NADPH, FADH₂ and heat.
Mark every place where O₂ is used or produced with a star.
Explain your poster to someone at home in 3 minutes, using the words oxidized, reduced, chemiosmosis, endergonic and exergonic.

What to look for: O₂ is produced only by the light reactions (thylakoid) and used only at the end of the ETC (inner mitochondrial membrane). Glycolysis and fermentation are in the cytosol. Heat arrows should leave from every process. Matter arrows form a loop; energy arrows go in one direction, from light to heat.

Your turn

Explain the statement "matter cycles but energy flows" using respiration and photosynthesis.

Compare the role of O₂ in aerobic respiration and in photosynthesis.

Name the location of each stage: glycolysis, Krebs cycle, ETC of respiration, light reactions, Calvin cycle.

Which electron carriers are used in respiration and in photosynthesis?

Give two similarities between chemiosmosis in mitochondria and chloroplasts.

What is the light compensation point?

Use thermodynamics to explain why the total energy captured in ATP from respiration is much less than the energy of the light that made the glucose.

More practice: warm-up

Is photosynthesis endergonic or exergonic?

Do plant cells carry out cellular respiration?

Where in the cell does fermentation take place?

Which process releases O₂: photosynthesis or respiration?

What is a limiting factor?

More practice: on your own

Compare the redox changes in aerobic respiration and photosynthesis.

Explain why a houseplant kept in a very dim corner may slowly lose mass over several months.

In a sealed jar with a plant and a mouse in the light (Priestley's idea), both can survive for some time. Explain using both processes, and predict what happens in the dark.

Using the table, explain why fermentation releases so much less energy than aerobic respiration, even though both start with glucose.

A seedling's net CO₂ release in the dark is 8 mL/h. In bright light, net uptake is 30 mL/h. Calculate gross photosynthesis.

Challenge

Create a single balanced equation showing that photosynthesis followed by aerobic respiration of the glucose produced has no net change in matter, and explain what changes.

Explain why a plant's light compensation point rises on a hot day.

Evaluate this claim: "Since plants make O₂ and animals use it, plants don't need oxygen." Use evidence from both processes.

Check your understanding

Quiz

Which statement best describes how matter and energy move between photosynthesis and respiration?

In which pair of locations is ATP made by chemiosmosis?

Which is correct for both aerobic respiration and photosynthesis?

At the light compensation point:

What is the role of O₂ in aerobic respiration?

A plant releases 10 mL CO₂/h in the dark and takes up 25 mL CO₂/h in bright light. Its gross rate of photosynthesis in bright light is about:

For parents and teachers

Big idea: C3.4 asks students to describe, compare and illustrate (with flow charts) the matter and energy transformations of aerobic respiration, fermentation and photosynthesis, including the roles of O₂, mitochondria and chloroplasts. C3.3 links them through thermodynamics.

How to help: The poster activity is a great way to review the whole unit; ask your teen to explain it to you using the key words. Check that O₂ appears in only two places (released by the light reactions; used at the end of the respiratory ETC).

Common difficulties:

Thinking plants photosynthesize instead of respiring.
Saying energy cycles. Energy flows; matter cycles.
Mixing up where each stage occurs.
Forgetting that fermentation is a partial oxidation with only 2 ATP.