You have studied both processes separately. Now comes the harder and more important question: how do they interact within a single cell, across a whole organism, and across the entire biosphere — and what happens when that balance is disrupted?
A student writes the following in their notes:
"Plants photosynthesise during the day and respire at night. During the day, plants release O₂ and absorb CO₂. At night, they release CO₂ and absorb O₂. Animals only respire — they never photosynthesise. The products of photosynthesis and respiration are completely separate — photosynthesis makes food, respiration uses it, and that's it."
This paragraph contains at least four errors or oversimplifications. Without reading on, identify as many as you can and write a correction for each.
You will return to this at the end of the lesson.
Plants photosynthesise during the day and respire at night.
Plants respire continuously — 24 hours a day — because all living cells need ATP at all times. Photosynthesis only occurs in the light. During the day, both processes run simultaneously; at night, only respiration occurs.
During the day, plants only release O₂ and absorb CO₂.
During the day, plants are doing both: respiration consumes O₂ and releases CO₂; photosynthesis consumes CO₂ and releases O₂. We observe net O₂ release and net CO₂ uptake because photosynthesis is faster than respiration in bright light — but both processes are happening simultaneously.
Animals never photosynthesise.
True for almost all animals — but some exceptions exist. Sea slugs (Elysia chlorotica) incorporate chloroplasts from algae they eat and can photosynthesise for weeks. Spotted salamander embryos host symbiotic algae inside their cells. These are exceptions — but the sweeping claim "animals never photosynthesise" is an oversimplification.
The products of photosynthesis and respiration are completely separate.
They are tightly linked. The O₂ produced by photosynthesis is used by mitochondria in the same cell for aerobic respiration. The CO₂ produced by mitochondria is used by chloroplasts in the same cell for the Calvin cycle. The glucose made by photosynthesis is the substrate for respiration. These are not separate pipelines — they are deeply interconnected within a single plant cell.
Core Content
In a plant cell in sunlight, both processes run at the same time in different organelles:
When we measure gas exchange in a plant, we measure the net result of both processes running simultaneously. This distinction is critical:
| Condition | What's happening | What we observe | Why |
|---|---|---|---|
| Bright light | Photosynthesis faster than respiration | Net O₂ release; Net CO₂ uptake | Photosynthesis produces more O₂ than respiration consumes; consumes more CO₂ than respiration produces |
| Dim light (compensation point) | Photosynthesis rate = Respiration rate | No net gas exchange | All O₂ produced is immediately consumed; all CO₂ produced is immediately fixed. Plant appears to be doing nothing. |
| Darkness | Only respiration occurs | Net O₂ uptake; Net CO₂ release | No photosynthesis — respiration's gases are not balanced by any other process |
The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point, all O₂ produced by photosynthesis is used by respiration, and all CO₂ produced by respiration is fixed by photosynthesis. There is no net gas exchange with the environment.
Shade-adapted plants have a lower compensation point than sun-adapted plants — they can achieve net photosynthesis (and therefore grow) at lower light intensities. This is an evolutionary adaptation to their environment.
Zooming out from a single cell to the biosphere, carbon moves between organisms and the atmosphere through photosynthesis, respiration, and decomposition:
Carbon fixation (atmosphere → organic molecules): Photosynthesis removes CO₂ from the atmosphere and incorporates ("fixes") carbon into glucose and other organic molecules. Autotrophs are the entry point for carbon into food webs.
Carbon release (organic molecules → atmosphere): All organisms release CO₂ via cell respiration. When organisms die, decomposers (bacteria and fungi) break down organic molecules via decomposition — also releasing CO₂. This completes the carbon cycle.
Long-term carbon storage: Some carbon is buried in sediments over millions of years and compressed into fossil fuels (coal, oil, natural gas). This carbon is effectively removed from the active cycle — until humans extract and combust it.
| Process | Direction of carbon movement | Organisms involved |
|---|---|---|
| Photosynthesis | Atmosphere → organic molecules (CO₂ fixed into glucose) | Plants, algae, cyanobacteria |
| Cell respiration | Organic molecules → atmosphere (CO₂ released) | All living organisms |
| Consumption | Producer → consumer (carbon transferred via food) | Animals, fungi, bacteria eating producers |
| Decomposition | Dead organic matter → atmosphere (CO₂ released) | Bacteria, fungi (decomposers) |
| Fossil fuel combustion | Long-term storage → atmosphere (CO₂ released rapidly) | Human industrial activity |
This lesson is an integration point. Here is how the major concepts of Module 1 link together through energy and matter:
| Concept | Links to | How |
|---|---|---|
| Cell membrane & transport (L06) | Cell requirements (L12) | Transport mechanisms deliver required matter (O₂, glucose, ions) to cells and remove wastes (CO₂, urea) |
| Photosynthesis (L10) | Cell respiration (L11), carbon cycle (L16) | Outputs of photosynthesis (glucose, O₂) are inputs of respiration; fixes atmospheric CO₂ |
| Cell respiration (L11) | Cell requirements (L12), waste removal (L12) | Produces ATP (energy requirement met); produces CO₂ and H₂O (wastes that must be removed) |
| Enzymes (L13–14) | Photosynthesis and respiration (L10–11) | Every step of both pathways is catalysed by specific enzymes; enzyme function depends on optimal conditions |
| SA:V and exchange surfaces (L09, L15) | Transport systems (L07–08), cell requirements | Exchange surfaces deliver required matter (O₂, nutrients) to circulatory systems that serve all body cells |
| Transport systems (L07–08) | Exchange surfaces (L15), waste removal (L12) | Circulatory systems carry O₂ from exchange surfaces to cells and return CO₂ and wastes to removal sites |
Cell requirements → exchange surfaces → transport → enzymes → photosynthesis & respiration → waste removal → carbon cycle. All connected through energy and matter.
Activities
A researcher measures the net gas exchange of a plant at different light intensities. At 0 lux (darkness), the plant releases 4 μmol CO₂/hr. At 1000 lux, there is no net gas exchange. At 3000 lux, the plant absorbs 8 μmol CO₂/hr.
Write your responses here or in your book.
In your book, construct a concept map that links all of the following Module 1 concepts. Use arrows with brief labels to show how each concept connects to at least two others. There is no single correct map — the goal is to show that you understand the relationships.
Concepts to include: cell membrane transport, photosynthesis, cell respiration, ATP, glucose, O₂, CO₂, enzymes, active site, SA:V ratio, exchange surfaces, circulatory system, cell requirements (energy + matter), waste removal, carbon cycle, Fick's Law.
After drawing your map, write two sentences describing the most important relationship you identified — one you might not have seen clearly before completing this module.
Write your two sentences here.
Assessment
1. A plant is placed in complete darkness for 24 hours. Which of the following correctly describes what occurs during this period?
2. At the compensation point of a plant, which of the following is true?
3. A scientist measures the O₂ concentration inside a sealed container with a plant in bright light over 24 hours. During daylight hours O₂ rises; during darkness O₂ falls. Which of the following correctly explains the O₂ rise during daylight?
4. Which of the following human activities DIRECTLY reduces the amount of CO₂ removed from the atmosphere each year?
5. A single plant cell in bright sunlight contains both chloroplasts and mitochondria. Which of the following correctly describes the flow of CO₂ within that cell?
1. A potted plant is placed on a windowsill. During the day it gains mass; at night it loses mass. Explain these observations with reference to photosynthesis, respiration, and the compensation point. (3 marks)
1 mark: daytime — photosynthesis exceeds respiration (above compensation point); net glucose production → glucose converted to starch and other organic molecules → mass increases; 1 mark: night — only respiration; consumes stored glucose → mass decreases; 1 mark: overall daily mass gain only if daytime photosynthesis exceeds total daily respiration (i.e. average light intensity exceeds compensation point for enough hours)
2. Trace a carbon atom from a CO₂ molecule in the atmosphere through the following pathway, describing what happens to it at each stage: (i) uptake by a plant, (ii) transfer to a herbivore, (iii) respiration by the herbivore, (iv) return to the atmosphere. (3 marks)
1 mark: CO₂ enters stomata → fixed in Calvin cycle → incorporated into glucose → stored as starch/cellulose; 1 mark: herbivore eats plant → digestion breaks starch to glucose → glucose absorbed into blood → used in herbivore's cellular respiration; 1 mark: glucose + O₂ → CO₂ + H₂O + ATP in Krebs cycle → CO₂ diffuses into blood → transported to lungs → exhaled back to atmosphere
3. Ocean acidification is caused by increased atmospheric CO₂ dissolving in the ocean. Using your knowledge of photosynthesis, respiration, and the carbon cycle, explain (a) why atmospheric CO₂ is increasing, and (b) why a reduction in phytoplankton photosynthesis due to acidification would form a positive feedback loop that accelerates climate change. (3 marks)
1 mark: atmospheric CO₂ increasing because combustion of fossil fuels releases stored carbon rapidly; deforestation reduces photosynthetic fixation; rate of release now exceeds rate of fixation by autotrophs; 1 mark: phytoplankton fix ~50% of global CO₂; if reduced, less CO₂ removed from atmosphere; 1 mark: less CO₂ removed → higher atmospheric CO₂ → more dissolves in ocean → more acidification → further phytoplankton decline → further reduction in fixation → even higher CO₂ — positive feedback loop
Answers
SA1: During daylight hours, the plant is above its compensation point — photosynthesis proceeds faster than respiration. The net result is that more glucose is produced by photosynthesis than is consumed by respiration. This surplus glucose is converted to starch and other organic molecules (cellulose, amino acids, lipids) that are incorporated into the plant's structure, increasing its dry mass. At night, photosynthesis stops (no light). Only respiration continues, consuming stored glucose (and other organic molecules) to produce ATP, releasing CO₂ and H₂O. The plant's organic mass decreases as stored carbohydrates are broken down. For the plant to show overall growth across days, the total carbon fixed by photosynthesis over daylight hours must exceed the total carbon lost by respiration over 24 hours — that is, the plant must spend enough hours above its compensation point to generate a surplus of organic molecules.
SA2: (i) The CO₂ molecule diffuses from the atmosphere into the leaf through stomata, down its concentration gradient. Inside the chloroplast stroma, the Calvin cycle enzyme RuBisCO incorporates ("fixes") the carbon atom from CO₂ into an organic molecule. Over successive turns of the Calvin cycle, this carbon becomes part of glucose (C₆H₁₂O₆). The glucose may be stored as starch in the leaf, or converted to sucrose and transported to other tissues, or used to build cellulose in the cell wall. (ii) A herbivore eats the plant. During digestion in the gut, amylase and other enzymes hydrolyse starch back to glucose. Glucose is absorbed across the villi of the small intestine into the blood and transported to all body cells. (iii) Inside the herbivore's cells, glucose enters glycolysis in the cytoplasm, then the Krebs cycle in the mitochondrial matrix. During the Krebs cycle, the carbon atom is incorporated into CO₂ — this CO₂ is the waste product of aerobic respiration. (iv) CO₂ diffuses out of the mitochondria and into the bloodstream, where it is carried as bicarbonate ions (HCO₃⁻) to the lungs. In the alveolar capillaries, CO₂ diffuses across the thin alveolar walls down its concentration gradient into the alveolar air and is exhaled back into the atmosphere — completing the cycle.
SA3: (a) Atmospheric CO₂ is rising because human activities — primarily the combustion of fossil fuels (coal, oil, natural gas) and deforestation — are releasing carbon into the atmosphere much faster than autotrophs can remove it through photosynthesis. Fossil fuels represent carbon that was removed from the atmosphere and stored in geological deposits over hundreds of millions of years; combustion releases this stored carbon in decades. Simultaneously, deforestation removes the trees that would otherwise fix that CO₂. The global rate of carbon release now substantially exceeds the rate of photosynthetic fixation, causing net accumulation of CO₂ in the atmosphere. (b) Phytoplankton (microscopic marine photosynthesisers) are responsible for fixing approximately 50% of all CO₂ removed from the atmosphere globally each year. If ocean acidification reduces phytoplankton productivity — by disrupting their enzyme function, calcification, or reproduction — less CO₂ is removed from the atmosphere each year. Higher atmospheric CO₂ then dissolves more readily into the ocean, further increasing acidification. Increased acidification further reduces phytoplankton productivity, which further reduces CO₂ fixation, which further increases atmospheric CO₂, which further acidifies the ocean. This is a positive feedback loop: each step amplifies the next, accelerating both ocean acidification and atmospheric CO₂ accumulation beyond what combustion and deforestation alone would cause.
Return to the student's paragraph from the start of the lesson. Here are the four errors and their corrections:
How many did you identify? Update your original response with anything you missed.