Important
Metabolic Processes
Nothing alive works for free. Every heartbeat of a sardine, every centimetre a kelp frond grows, every flick of a tube worm on a volcanic vent is paid for in one currency: ATP. This topic is about the three machines that handle that currency — photosynthesis and chemosynthesis, which earn it, and respiration, which spends it. Two of them build food. Only one of them needs the sun.
In 1977 the submersible Alvin dropped 2 500 m into total darkness on the Galápagos Rift. The scientists on board expected bare volcanic rock. Instead their lights swept across a garden — metre-long worms with blood-red plumes, white crabs, clams the size of dinner plates, packed around a smoking chimney. Every textbook said food chains start with sunlight. There is no sunlight down there. So what were they all eating?
Bacteria that eat rotten-egg gas. The chimneys pour out superheated water loaded with hydrogen sulphide (H₂S) — the chemical that makes rotten eggs stink and that is poisonous to most life. Bacteria and archaea living there strip energy out of that H₂S and use it to build sugars from carbon dioxide. No sun required. The process is called chemosynthesis, and those microbes are the producers at the base of an entire food chain that has never once seen daylight.
The tube worms don't even have mouths. They farm the bacteria inside their own bodies and live off what the bacteria make. Until this discovery — and CAPS notes the process was thought insignificant right up to the early 1990s — biologists were certain that all life on Earth ultimately ran on sunlight.
So there are two ways for life to make food: the sun-powered one that runs the sunlit ocean, and the chemical-powered one that runs the abyss. And there is one way that every living thing — bacterium, kelp, sardine, you — releases that stored energy again: respiration.
Earn It, Store It, Spend It
Every living organism needs a constant supply of chemical potential energy in a biologically usable form. Building it is photosynthesis or chemosynthesis. Releasing it is respiration. The molecule that carries it is ATP.
Think of ATP as a rechargeable battery. Its full name is adenosine triphosphate — adenosine with three phosphate groups clipped onto it. Snapping the last phosphate off releases a usable packet of energy exactly where the cell needs it: to contract a muscle, drive a pump in a membrane (Topic 14), build a protein, or make a firefly squid glow. Photosynthesis and chemosynthesis charge the battery. Respiration discharges it. Nothing in biology happens without it.
Producers and consumers. Organisms that make their own food are autotrophic — and because they feed everything else, we call them producers. In the sea those are the phytoplankton (Topic 17), the seaweeds and the sea grasses in the light, and the chemosynthetic bacteria in the dark. Everything that cannot make its own food is heterotrophic — a consumer — and must eat producers, or eat something that ate a producer.
The catch that shapes the whole ocean: producers only work where their energy source exists. Sunlight runs out with depth (you measured exactly how fast in Topic 20), so photosynthesis is confined to a thin skin at the top of the sea. That single limit explains why roughly 90% of all marine life lives in the sunlit layer — and why the deep-sea vent communities, running on chemistry instead, were such a shock.
The Engines of Marine Life
One water column, from the sunlit surface to a volcanic vent on the seabed. Four metabolic engines are running in it right now, plus the invisible line that separates the lit world from the dark one. Tap each numbered node.
Photosynthesis — the Solar Refinery
Plants, seaweeds, single-celled algae and many bacteria take in carbon dioxide and water, and use the energy in sunlight to bind them into glucose. Oxygen comes off as a by-product — the by-product that made animal life on this planet possible.
CAPS asks you to describe the process in four steps: ① intake of the raw materials (CO₂ and H₂O), ② trapping and storing of the sun's energy, ③ formation of food and other important organic compounds and their storage, and ④ release of oxygen. The whole thing happens inside a chloroplast (Topic 14), where the solar energy is converted into chemical energy, and every step is controlled by enzymes.
Light-dependent phase
The grana — stacked discs inside the chloroplast — hold chlorophyll and the other photosynthetic pigments. They capture light and use it to make energy-rich ATP and to strip hydrogen off water molecules. The leftover oxygen is expelled as waste.
makes ATP + H · releases O₂Light-independent phase
Enzymes in the stroma use the ATP from phase one to bond that hydrogen to carbon dioxide, building glucose — a molecule packed with chemical potential energy. (This phase has a full name, the Calvin cycle; its individual steps are outside the depth CAPS asks for here.)
makes glucoseHow deep does this topic go? Exactly this deep.
CAPS is explicit about the depth required here: "Mention light and dark phases in relation to chloroplast. No biochemical details of light and dark phases are required." Respiration carries the same instruction — "No biochemical detail of process required; each phase is mentioned with the resultant products." So what you must be able to do is say where each phase happens and what it produces — not the chemistry inside it. If you have the Two Oceans booklet, notice that it works through the Calvin cycle and the Krebs cycle step by step. That is exactly the biochemical detail the instruction above excludes. Read it for interest if it grabs you; the line you are accountable for is the one CAPS draws.
Why it matters — the three CAPS reasons
Produces oxygen in the ocean
Every aerobic organism in the sea — from a copepod to a great white — depends on dissolved oxygen that was put there by a producer. Marine phytoplankton alone generate 60–70% of Earth's oxygen (Topic 17).
Takes up CO₂ and nutrients
Photosynthesis pulls carbon dioxide out of the water and the atmosphere, and drives the cellular uptake of the other nutrients producers need. It is the ocean's carbon pump — and the reason the sea buffers climate change.
Makes food & organic compounds
Glucose can be stored as-is, or converted to starch and fats for long-term reserves, or built up — with other elements — into amino acids, nucleic acids and vitamins. This is where the food chain literally begins.
Producers need more than CO₂ and water
To build amino acids and nucleic acids, a producer needs nitrogen, phosphorus, iron and other elements too. Land plants pull these out of the soil through roots. Phytoplankton and seaweeds absorb them straight out of the surrounding sea water — which is exactly why nutrient-rich upwelling zones like the Benguela explode with life, and why the clear blue tropical open ocean, however sunny, is a biological desert by comparison.
What Controls the Rate — and Where it Stops
CAPS names three variables that set how much photosynthesis can happen in any part of the ocean.
Light
Different wavelengths penetrate to different depths (Topic 20). That dictates which algae can grow at which depth — chlorophyll uses red and blue light, but the accessory pigments in red and brown seaweeds respond mainly to blue, which is why they can live deeper. And the producers present dictate the entire consumer community above them.
Carbon dioxide
Given enough light, the rate of photosynthesis rises and falls with the amount of dissolved CO₂ available in the water. Below a certain concentration, CO₂ becomes the factor holding the whole process back.
Temperature
There is an optimal temperature for photosynthesis. Go higher or lower and the enzymes controlling it work poorly — the process slows down or stops completely. This is why warming seas change which producers can live where.
The compensation depth
Go deep enough and a producer reaches a point where it is just breaking even: over 24 hours, the oxygen and carbohydrate it produces by day exactly equals what it consumes at night. That depth is the compensation depth. There is no surplus left to store or grow with, so no producer can survive permanently below it. At this depth roughly 1% of surface light remains — the same 1% figure you met at 150 m in Topic 20 — and it marks the bottom of the euphotic zone. In very clear open ocean it can reach about 200 m; in turbid coastal water it may be only a few metres down.
Careful — two things called "the 1% depth"
In Topic 20 you learned that about 1% of surface light reaches 150 m in the clearest water. Here that same light level is given a biological name: the compensation depth, the deepest a producer can survive. Same physics, different question. Topic 20 asked "how far does the light get?"; Topic 21 asks "how far down can something still make a living off it?" And because the answer depends on water clarity, the compensation depth is shallow in murky coastal water and deep — up to about 200 m — in clear open ocean.
Respiration — Cashing the Cheque
Making glucose is only half the job. Cellular respiration is the process in the cells of every living organism where organic compounds are broken down to release energy — transferred to wherever the cell needs it, as ATP. Note that this is not the same thing as breathing: breathing delivers the oxygen; respiration is what the cells do with it.
Aerobic respiration — the three-step engine
"Aerobic" means in the presence of oxygen. It happens in the cytoplasm and the mitochondria, and CAPS wants each phase named with its products — no biochemistry beyond that.
Glycolysis
One glucose molecule is broken down, releasing 2 ATP and energised hydrogen atoms. What is left of the glucose — called pyruvic acid — is passed into a mitochondrion.
+2 ATPKrebs cycle
The pyruvic acid releases more energised hydrogen atoms. What is left of it is excreted as carbon dioxide — the CO₂ you breathe out, and the CO₂ a fish passes out across its gills.
releases CO₂ + HOxidative phosphorylation
The energised hydrogen atoms release their energy to produce 36 ATP. The depleted hydrogen is then bonded to free oxygen atoms to form water.
+36 ATPWhy oxygen is non-negotiable
Look at the last step. Oxygen's job is to collect the depleted hydrogen atoms at the end of the chain and carry them off as water. With no oxygen there is nowhere for the hydrogen to go, the chain jams, and the mitochondrion cannot make its 36 ATP — leaving the cell with the meagre 2 ATP from glycolysis. That is why breathing, gaseous-exchange surfaces (gills, lungs, skin) and the transport of oxygen to the cells are matters of life and death for aerobic organisms. It is also why a fish suffocates in warm, low-oxygen water even though it is surrounded by H₂O: the oxygen it needs is the dissolved O₂, not the oxygen locked inside the water molecule.
Anaerobic respiration — the emergency setting
"Anaerobic" means in the absence of oxygen. Prokaryotes have no mitochondria at all and use other energy pathways permanently; many need no oxygen whatsoever. Some larger animals switch to it temporarily. Only glycolysis can run, so the glucose residue (pyruvic acid) takes up the released hydrogen and becomes lactic acid in animals — or ethanol and CO₂ in yeast, which is alcoholic fermentation. The energy released is far less than when the pyruvic acid carries on into the mitochondrion.
| Aerobic respiration | Anaerobic respiration | |
|---|---|---|
| Oxygen needed? | Yes — it collects the depleted hydrogen atoms | No |
| Where in the cell | Cytoplasm and mitochondria | Cytoplasm only |
| Waste products | CO₂ + H₂O | Lactic acid (animals) · ethanol + CO₂ (yeast) |
| Energy yield per glucose | ≈38 ATP — high efficiency | 2 ATP — low efficiency |
| Marine examples | Most fish, plankton and surface-dwelling life | Deep-diving seals and whales (temporarily); many prokaryotes (permanently) |
The diving animal's bargain
A southern elephant seal at Marion Island — South Africa's sub-Antarctic territory — routinely dives to 400–600 m and stays down for 20–30 minutes on one breath. Long before it surfaces, the oxygen in its blood and muscle is gone, so its swimming muscles switch to working anaerobically. The bargain is brutal: it gets only 2 ATP per glucose instead of 38, so it burns through fuel roughly nineteen times faster; and the lactic acid it produces is potentially harmful and builds up in the muscle. That is why the animal must surface — once it can take in oxygen again, the lactic acid is converted back to a harmless form. Anaerobic respiration is a short-term survival option, never a way of life for a large animal.
Two halves of one loop
Look at the two equations together and something clicks: the products of one are the raw materials of the other. Photosynthesis and respiration form the two halves of a simple carbon cycle in every ecosystem — carbon, oxygen and hydrogen endlessly re-formed into different compounds, with the whole loop driven by the sun.
The trap in this diagram
A very common exam mistake is to write that "plants photosynthesise and animals respire". Producers respire too — all day and all night, exactly like you do. During daylight a healthy phytoplankton cell photosynthesises faster than it respires, so there is a surplus to grow on and oxygen to spare. In the dark, only respiration continues. Hold that thought and the definition of compensation depth suddenly makes complete sense: it is the depth where the day's surplus has shrunk to exactly zero.
Chemosynthesis — Food Without Sun
Certain bacteria make use of the energy in locally abundant chemicals to manufacture energy-rich compounds for their own use and storage — a bit like photosynthesis, but powered by a chemical source instead of the sun.
The organisms use small amounts of oxygen to break down hydrogen sulphide, which is abundant in their habitats — a chemical process called oxidation (a substance gains oxygen, or loses electrons and hydrogen). The energy released is used to combine CO₂ and hydrogen into a carbohydrate, and a sulphate ion is given off as waste. It is performed by bacteria and extremophiles — usually archaea, organisms able to live in conditions of extreme temperature, pressure and corrosiveness that were never thought capable of sustaining life at all.
Three habitats, not one
CAPS names all three: submarine hydrothermal vents, marine sediments, and even the sea-floor crust itself. Remotely operated vehicles (ROVs) are what finally made this world reachable — and revealed that chemosynthesis, dismissed as a curiosity until the early 1990s, is a genuinely significant part of how the ocean works.
Small returns, huge consequences
Chemosynthesis yields much less energy than photosynthesis — far too little for a large organism to live on directly. But the sheer number of chemosynthetic microbes in these habitats makes them food for filter feeders, and so the producers of a complex deep-ocean food chain. Many of the large animals now being found in the deep are not relying on dead matter sinking from the surface at all.
Local connection — the giant sulphur bacterium of the Namibian shelf
You do not have to go to the Galápagos to find chemosynthesis. The Benguela upwelling along the West Coast of southern Africa is so productive that huge volumes of dead plankton settle into the seabed mud, where decomposing bacteria strip out the oxygen and generate hydrogen sulphide. Every few years that gas erupts through the water column off Walvis Bay, turning the sea milky-turquoise and killing fish across enormous areas. Living in exactly that sediment is Thiomargarita namibiensis — "the sulphur pearl of Namibia" — discovered in 1997 and, at up to 0.75 mm across, one of the largest bacteria ever found: big enough to see with the naked eye. It makes its living by oxidising hydrogen sulphide, the same reaction as the vent microbes in the hook. Chemosynthesis is not only an exotic deep-sea trick — it is running right now, a few hundred kilometres up our own coastline.
Words and Symbols — Build the Equations
CAPS asks you to describe all three processes "using words and symbols", and it is the fastest place to lose easy marks. Pick a process, tap a gap, then tap the chip that belongs in it. Get all three right and you have the whole topic in three lines.
In the Lab — the CAPS Investigations
CAPS gives this topic 8 hours: 4 of theory and 4 of practical. These are the investigations you are expected to have done or seen demonstrated. Each one isolates a single variable — exactly the method you learned in Topic 05.
Starch is produced during photosynthesis
De-starch a leaf by keeping the plant in the dark for 48 h, then expose it to light. Boil the leaf, decolourise it in alcohol, and test with iodine solution.
Result: the illuminated leaf turns blue-black — starch is present. The de-starched control stays brown.
Light is necessary for photosynthesis
Cover part of a de-starched leaf with foil or a cardboard shape, leave it in light for several hours, then run the same iodine starch test.
Result: only the exposed part goes blue-black. The covered shape stays brown — no light, no starch.
Chlorophyll is necessary
Use a variegated leaf (green and white patches). Photograph it first so you know which parts were green, then de-starch, decolourise and test with iodine.
Result: only the parts that were green go blue-black. No chlorophyll, no photosynthesis.
Carbon dioxide is necessary
Seal one de-starched plant in a bag with soda lime (which absorbs CO₂) and an identical plant in a bag without it. Give both the same light, then test both with iodine.
Result: only the plant with CO₂ available makes starch.
Oxygen is produced during photosynthesis
Put pondweed (or a marine alga) under an inverted funnel and test tube of water, in bright light. Collect the gas that bubbles up and test it with a glowing splint.
Result: the splint relights — the gas is oxygen. Counting bubbles per minute at different light intensities gives you rate data to graph.
Respiration uses O₂ and produces CO₂
Seal germinating seeds or snails in a tube with hydrogencarbonate indicator, in the dark, alongside an empty control tube.
Result: the indicator turns from red to yellow — CO₂ has been produced. Living things respire in the dark, when no photosynthesis can mask it.
What an examiner is really testing here
Not the recipe — the method. For every one of these you should be able to state the independent variable (what you changed: light, CO₂, chlorophyll), the dependent variable (what you measured: starch, bubbles, indicator colour), the controlled variables (everything kept the same: temperature, plant species, time), and the control (the set-up you compare against). De-starching first matters for the same reason: without it, you cannot prove the starch you find was made during the experiment. Straight out of Topic 05.
The practical sheet · five investigations, with results supplied for every one
- 🟡 pharmacy What a leaf needs — the iodine starch tests for light, chlorophyll and carbon dioxide, with a safe way to heat methylated spirits.
- 🟡 aquarium shop Oxygen, and what limits the rate — count pondweed bubbles at four light levels and five temperatures, and find the limiting factor.
- 🟢 kitchen Respiration in a jar — germinating beans and red cabbage indicator show carbon dioxide given out; a tea-light shows the oxygen used, the half card 6 above leaves out.
- 🟢 desk The compensation depth — light-and-dark-bottle oxygen data from six depths, and the four light zones drawn.
- 🟢 desk Life without sunlight — the CAPS research on extremophiles and chemosynthesis, with research cards if you have no internet.
→ Open the Topic 21 practical sheet — printable, with supplied data for every task.
The Engines Running Off Our Coast
Local connection — the Great African Seaforest and the Benguela's oxygen debt
From Cape Point up the West Coast lies the Great African Seaforest: dense underwater forest of Ecklonia maxima, the sea bamboo kelp, some plants 10 m tall. Cold, nutrient-rich Benguela water plus strong sunlight makes it one of the most productive photosynthetic ecosystems on Earth — and the base of a food web running all the way up to abalone, rock lobster, seals and great whites.
But productivity has a bill attached, and it is paid in respiration. When one of the Benguela's enormous plankton blooms dies, the sinking cells are broken down by bacteria — and decomposition is respiration, which consumes oxygen. Strip enough oxygen out of the bottom water and you get a low-oxygen (hypoxic) event. Rock lobsters at Elands Bay and St Helena Bay respond by crawling out of the sea to escape it: the famous "walkouts", where thousands of Jasus lalandii strand themselves on the beach rather than suffocate. It is the perfect illustration of this topic: the same solar energy that grows the bloom, released again by respiration a few weeks later, can pull the oxygen out from under an entire fishery.
Key Vocabulary
Tap each card to flip it. These eight definitions are examinable and integral to understanding the topic.
The process in which producers take in carbon dioxide and water and, using light energy, chemically bind them to produce glucose, releasing oxygen as a by-product. Happens in the chloroplast in two phases: light-dependent (grana) and light-independent (stroma).
A process in the cells of living organisms where organic compounds are broken down to provide energy. That energy is transferred to the parts of the cell that need it in the form of ATP. In eukaryotes it normally takes place in the mitochondria.
Adenosine triphosphate — the energy carrier for metabolism. It is the biologically available form of chemical potential energy: made by photosynthesis, chemosynthesis and respiration, and spent on every life function a cell performs.
Any process which takes place in the presence of oxygen. Aerobic respiration runs in the cytoplasm and mitochondria and yields about 38 ATP per glucose molecule, with CO₂ and water as waste.
Any process which takes place in the absence of oxygen. Only glycolysis can run, so the yield is just 2 ATP per glucose, and the waste is lactic acid in animals or ethanol + CO₂ in yeast (alcoholic fermentation).
The depth at which the amount of CO₂ used and O₂ produced by a photosynthetic organism during the day is identical to the amount of CO₂ produced and O₂ used at night. No surplus energy is left for storage. About 1% of surface light remains; it marks the bottom of the euphotic zone.
A process in which certain bacteria make use of the energy in locally abundant compounds to manufacture energy-rich compounds for their own use and storage. Typically the oxidation of hydrogen sulphide: CO₂ + H₂S + O₂ → (CH₂O)ₓ + SO₄.
Organisms, normally Archaea, capable of living in extreme conditions not normally considered able to sustain life — including very high temperatures, pressures and corrosive environments. They are the chemosynthetic producers of the deep sea.
Also on the examinable list
Glycolysis: the breakdown of a glucose molecule releasing two ATP molecules and energised hydrogen atoms, and transferring the glucose residue (pyruvic acid) to a mitochondrion. · Lactic acid: an acid formed from the combination of glucose residues and hydrogen (C₃H₆O₃). · Oxidation: a chemical process in which a substance gains oxygen, or loses electrons and hydrogen. · Photic or euphotic zone: the upper region of the ocean, exposed to sufficient light for photosynthesis to take place. ⚠️ Careful — this is one definition covering two words. Your booklet treats photic and euphotic as the same thing, and that is what to write if a paper asks. In standard scientific use they are not the same: euphotic is where there is enough light to photosynthesise, while photic is the whole lit column — euphotic plus disphotic — which is how the zone diagram higher up this page labels it. Know both, and answer as the paper asks. · Producers: sources of food to heterotrophic organisms — the autotrophs that manufacture their own food.
Quick Check
Question 1
Well done!
Next: Cyanobacteria, Phytoplankton & Microalgae — meet the organisms doing most of the photosynthesis you just learned about.