the inputs, outputs and locations of the light-dependent and light-independent stages of photosynthesis in plants (C3); the factors that affect the rate of photosynthesis; differences between C3, C4 and CAM plants

What this dot point is asking

VCAA wants the two stages of photosynthesis, their location in the chloroplast, their inputs and outputs, the factors that change the rate, and the C3, C4 and CAM comparison.

The answer

Photosynthesis is the process by which plants, algae and some bacteria convert light energy into chemical energy stored in glucose. The diagram shows the two stages: the light-dependent reactions on the thylakoid membrane that produce ATP and NADPH, and the light-independent Calvin cycle in the stroma that fixes CO₂ into glucose.

It happens in two linked stages inside the chloroplast.

Light-dependent stage (thylakoid membrane)

Location

The thylakoid membranes and the lumen they enclose, inside the chloroplast.

Inputs

Light energy (absorbed by chlorophyll in photosystems II and I), water, ADP and inorganic phosphate (Pi), and NADP+.

Outputs

Oxygen, ATP and NADPH.

Key steps

1. Light absorption. Chlorophyll in photosystem II absorbs light, exciting electrons.
2. Photolysis of water. Water is split (2H2O -> 4H+ + 4e- + O2), replacing the lost electrons. O2 is released as a waste product.
3. Electron transport chain. Excited electrons pass along membrane-bound carriers between photosystem II and photosystem I, pumping H+ from the stroma into the thylakoid lumen.
4. Chemiosmosis. H+ flows back through ATP synthase down its gradient, generating ATP.
5. Reduction of NADP+. At photosystem I, electrons are re-energised and used to reduce NADP+ to NADPH.

The ATP and NADPH produced are passed to the next stage.

Light-independent stage (stroma, Calvin cycle)

Location

The stroma of the chloroplast (the fluid surrounding the thylakoids).

Inputs

CO2 (from the atmosphere through stomata), ATP and NADPH (from the light-dependent stage), and the five-carbon acceptor RuBP.

Outputs

G3P (which leaves to form glucose and other carbohydrates), ADP + Pi and NADP+ (recycled back to the light-dependent stage).

Key steps

1. Carbon fixation. The enzyme RuBisCO attaches CO2 to RuBP, forming an unstable 6-carbon intermediate that immediately splits into two molecules of 3-PGA (a 3-carbon compound).
2. Reduction. ATP and NADPH convert 3-PGA into G3P (glyceraldehyde-3-phosphate, a 3-carbon sugar).
3. Regeneration. Most G3P is recycled, using ATP, to regenerate RuBP. For every six CO2 fixed, one G3P leaves the cycle to make glucose, sucrose, starch, amino acids or lipids.

Factors affecting the rate of photosynthesis

Light intensity

Rate rises with intensity until another factor becomes limiting. Very high intensity can damage chlorophyll (photoinhibition).

CO2 concentration

Rate rises with CO2 up to a plateau. CO2 is typically the limiting factor at high light.

Temperature

Rate rises with temperature up to an optimum (around 25 to 35 degrees Celsius for most C3 plants), then falls as enzymes such as RuBisCO denature. At high temperatures RuBisCO also reacts with O2 (photorespiration), wasting fixed carbon.

Water availability

Low water closes stomata, reducing CO2 entry.

Chlorophyll and wavelength

Chlorophyll absorbs red and blue light strongly and reflects green; rate is highest under red and blue light.

C3, C4 and CAM plants

C3 plants (for example, wheat, rice, most temperate species). RuBisCO fixes CO2 directly in mesophyll cells, producing a 3-carbon intermediate (3-PGA). They photorespire when hot and dry.

C4 plants (for example, sugar cane, maize, sorghum). CO2 is first fixed by PEP carboxylase in mesophyll cells into a 4-carbon compound (oxaloacetate, then malate). Malate is shuttled to bundle sheath cells, where CO2 is released around RuBisCO at high concentration. This minimises photorespiration in hot, dry conditions.

CAM plants (for example, cacti, pineapple, succulents). Stomata open only at night, fixing CO2 into malate via PEP carboxylase and storing it in vacuoles. During the day, stomata close and the stored malate releases CO2 for the Calvin cycle. CAM saves water in arid environments at the cost of slower growth.

Examples in context

Example 1. C4 photosynthesis in sugarcane at the Burdekin Sugar Region. Australian sugarcane (Saccharum officinarum) grown in the Burdekin and Queensland cane regions uses C4 photosynthesis, an adaptation to high light and temperature. CO2 is first fixed in mesophyll cells by PEP carboxylase (which does not bind oxygen, avoiding photorespiration), producing oxaloacetate then malate. Malate is shuttled to bundle-sheath cells where it releases CO2 at high concentration around RuBisCO, which then runs the Calvin cycle efficiently. Net effect: sugarcane converts about 7 percent of incident solar energy to biomass under tropical sun, twice the efficiency of C3 wheat in the same conditions. The Mackay Sugar Mill processes the resulting sucrose for export.

Example 2. Spinifex CAM-like adaptations in central Australia. Triodia (spinifex) and many desert succulents at Alice Springs Desert Park use CAM (crassulacean acid metabolism) to survive desert heat. Stomata open at night when transpiration is low, taking up CO2 and fixing it as malate stored in vacuoles. During the day, stomata close (preventing water loss), and stored malate releases CO2 internally to the Calvin cycle. Researchers at the University of Adelaide measured stomatal conductance through a 24-hour cycle and confirmed CAM behaviour. CAM plants grow slowly (about half the rate of C3) but they can fix carbon in environments where C3 plants would lose too much water to survive.

Try this

Q1. State the inputs, outputs and location of the light-dependent and light-independent stages of photosynthesis in a C3 plant. [4 marks]

• Cue. Light-dependent: thylakoid membranes; inputs water, light; outputs O2, ATP, NADPH. Light-independent (Calvin cycle): stroma; inputs CO2, ATP, NADPH; output glucose.

Q2. A C3 plant and a C4 plant are grown side-by-side at 35 degrees C and high light. The C4 plant grows faster. Explain in terms of photorespiration and RuBisCO. [3 marks]

• Cue. At high temperature, RuBisCO binds O2 (photorespiration) more than CO2 in C3 plants, wasting fixed carbon; C4 plants concentrate CO2 around RuBisCO, suppressing photorespiration; growth rate higher.

Q3. Refer to factors affecting photosynthesis. (a) Sketch the expected rate response to increasing light intensity. (b) Explain why rate plateaus despite further light increase. (c) Predict the effect of doubling atmospheric CO2 on a C3 wheat crop. [2+2+2 marks]

• Cue. (a) Rate rises then plateaus with light. (b) Another factor (CO2 concentration, temperature, RuBisCO availability) becomes limiting. (c) Rate increases substantially because RuBisCO is CO2-limited under current atmosphere.