IB Biology HLPhotosynthesisPaper 1 & 2~12 min read
The Process of Photosynthesis
Respiration took glucose apart to make ATP. Photosynthesis runs the same logic backwards: it uses light to make ATP, then spends that ATP building glucose. Almost every idea from the last topic reappears here, in reverse.
📚 What you need to know
Photosynthesis converts light energy into chemical energy stored in organic molecules. It is an anabolic process.
It happens in chloroplasts, which have their own double envelope, internal membranes and stroma.
Thylakoids are flattened membrane sacs; stacks of them are grana, joined by intergranal lamellae. The fluid around them is the stroma.
The light-dependent reactions take place on the thylakoid membranes and need light directly.
The light-independent reactions (the Calvin cycle) take place in the stroma and do not use light directly.
The link between them is ATP and reduced NADP, made in the light-dependent stage and spent in the Calvin cycle.
Water is split (photolysis) to supply electrons; the oxygen released is a waste product.
Overall: carbon dioxide + water → glucose + oxygen, using light energy.
What photosynthesis is doing
Light is energy, but it is not a form a cell can use directly. You cannot build a sugar out of sunlight any more than you can pay a bill with sunshine. So the chloroplast does the conversion in two stages: first turn light into chemical energy carriers, then use those carriers to build sugar.
That single equation hides everything interesting. It does not tell you where the oxygen comes from, why light is needed at only one stage, or why the process stops in the dark even though carbon dioxide is still available. The rest of this topic unpacks it.
One detail worth fixing straight away: the oxygen released does not come from the carbon dioxide. It comes from water, split apart to supply electrons. Students lose marks on this every year, and it is a single fact.
The chloroplast
The structure of a chloroplast is not decoration — every feature exists to serve one of the two stages.
Compare this with a mitochondrion. Both organelles solve the same problem the same way: a folded inner membrane for the electron transport work, and a fluid space for the enzyme-driven cycle.
Structure
What it is
What it is for
Double envelope
Two membranes around the outside
Separates the chloroplast contents from the cytoplasm
Thylakoid
A flattened membrane sac
Holds the photosystems and pigments for the light-dependent reactions
Granum
A stack of thylakoids
Packs in a huge surface area for light absorption
Lamella
A membrane joining one granum to another
Connects the stacks so products can move between them
Stroma
The fluid filling the chloroplast
Contains the enzymes of the Calvin cycle
Starch grains
Insoluble carbohydrate stores
Store the products of photosynthesis without affecting water potential
Two stages, one hand-off
The whole topic hangs on understanding what passes between the two stages, and in which direction.
Note the red arrows. The Calvin cycle does not just consume ATP and NADPH, it returns the empty carriers — and the light reactions need them back.
🧠
Two names for the same trap
“Light-independent” is a badly chosen name. It means does not use light directly. It stops within seconds of darkness because the ATP supply dries up. Never write “the Calvin cycle happens at night”.
Photosynthesis and respiration side by side
Feature
Photosynthesis
Respiration
Type of process
Anabolic — builds molecules
Catabolic — breaks molecules down
Energy
Absorbs light energy (endergonic overall)
Releases free energy (exergonic overall)
Organelle
Chloroplast
Mitochondrion (and cytoplasm)
Gases
Takes in CO2, releases O2
Takes in O2, releases CO2
Hydrogen carrier
NADP
NAD and FAD
Which organisms
Plants, algae and some bacteria
Every living organism, including plants
Plants respire too — all the time. A plant photosynthesises only in the light but respires day and night. At low light the two roughly cancel out, and the light intensity at which they exactly balance is the compensation point. That idea comes back on the limiting factors page.
Worked examples
WE 1
Relate structure to function
Explain how two features of a chloroplast adapt it for photosynthesis. (4 marks)
Feature 1: thylakoids stacked into grana
This provides a large surface area of membrane holding photosystems and pigments, so more light can be absorbed for the light-dependent reactions.
Feature 2: the stroma
The fluid stroma contains the enzymes of the Calvin cycle, keeping them concentrated close to where ATP and reduced NADP are produced.
Big membrane area for light; an enzyme-rich fluid for the cyclegive the feature AND the function each time — a description alone scores half the marks
WE 2
Explain a dark period
A plant is moved into complete darkness. Explain why sugar production stops almost immediately, even though carbon dioxide is still available. (3 marks)
Point 1: the first stage stops
Without light the light-dependent reactions cannot occur, so no ATP or reduced NADP is produced.
Point 2: the knock-on effect
The Calvin cycle needs both of these to reduce its intermediates, so it cannot continue once the existing supply is used up.
Point 3: the conclusion
Carbon dioxide can still be fixed briefly, but without ATP and reduced NADP no sugars are formed.
No light means no carriers, and no carriers means no Calvin cyclethis is why “light-independent” does not mean “works in the dark”
💡 Exam tips
Give locations precisely: thylakoid membrane and stroma, not just “the chloroplast”.
State that the oxygen comes from the splitting of water, not from carbon dioxide.
Name the two products that link the stages: ATP and reduced NADP.
Remember the carriers go back the other way as ADP + Pi and NADP.
Use NADP for photosynthesis and NAD for respiration — the extra P matters.
Photosynthesis is anabolic; respiration is catabolic.
⚠ Common mistakes
Saying the oxygen comes from carbon dioxide. It comes from water.
Saying the light-independent reactions happen at night. They stop when ATP runs out.
Writing NAD instead of NADP in a photosynthesis answer.
Saying plants only respire at night. They respire constantly.
Confusing granum, thylakoid and lamella. A granum is a stack of thylakoids; lamellae join grana together.
Calling photosynthesis the opposite of respiration. The overall equations mirror each other, but the pathways are not simply reversed.
Up next: Separating Photosynthetic Pigments (Skills). Before looking at what the pigments do, the next page shows how you prove there is more than one of them in the first place.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
