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 overall summary equation is:

6CO2 + 6H2O + light energy -> C6H12O6 + 6O2

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.

Worked example

A bean plant (C3) and a maize plant (C4) are grown side by side at 35 degrees Celsius, high light and low soil water. The bean plant closes its stomata to conserve water; O2 builds up inside the leaf, RuBisCO fixes O2 (photorespiration), and photosynthetic rate falls. The maize plant uses PEP carboxylase to keep CO2 concentrated in the bundle sheath, so RuBisCO continues fixing CO2 efficiently and growth rate stays higher.

Common traps

Saying photosynthesis happens "in the chloroplast" without specifying the substructure. Light-dependent reactions occur on the thylakoid membrane; the Calvin cycle occurs in the stroma.

Calling the Calvin cycle the "dark stage." It does not need to happen in the dark; it just does not need light directly. Use "light-independent stage" or "Calvin cycle".

Forgetting where O2 comes from. Oxygen comes from the photolysis of water, not from CO2.

Mixing up NADP+ and NAD+. NADP+ is reduced to NADPH in photosynthesis. NAD+ is reduced to NADH in cellular respiration.

Saying glucose is the immediate product of the Calvin cycle. The direct product is G3P; glucose and other carbohydrates are made from G3P afterwards.

In one sentence

Photosynthesis converts light energy into chemical energy by splitting water and generating ATP and NADPH on the thylakoid membrane, then using those products plus CO2 in the stroma to fix carbon through RuBisCO into G3P and glucose, with C4 and CAM plants adding specialised CO2-concentrating steps to avoid photorespiration.