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Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in organic molecules. It is the fundamental process that sustains almost all life on Earth, providing both the organic compounds and the oxygen upon which heterotrophic organisms depend. The overall equation is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Key Definition: Photosynthesis is the process by which light energy is used to convert carbon dioxide and water into glucose and oxygen. It is an endergonic (energy-requiring) process in which light energy is converted to chemical energy in organic molecules.
Photosynthesis takes place in the chloroplasts and consists of two main stages: the light-dependent reactions (on the thylakoid membranes) and the light-independent reactions or Calvin cycle (in the stroma).
A described diagram of a chloroplast would show a lens-shaped organelle approximately 5–10 µm long, bounded by a double membrane (envelope). Inside, stacks of flattened membrane-bound sacs called thylakoids are arranged in piles known as grana (singular: granum). The grana are connected by extensions of thylakoid membrane called intergranal lamellae (or stroma lamellae). The fluid-filled space surrounding the grana is the stroma, which contains enzymes, small circular DNA molecules, 70S ribosomes, lipid droplets, and starch grains. The interior space within each thylakoid is called the thylakoid lumen.
| Structure | Function |
|---|---|
| Outer membrane | Permeable to small molecules; forms part of the chloroplast envelope |
| Inner membrane | Contains transport proteins; controls what enters/exits the stroma |
| Thylakoid membranes | Contain photosystems (I and II), electron carriers, and ATP synthase; site of light-dependent reactions |
| Grana (stacks of thylakoids) | Increase surface area for light absorption; contain high density of photosynthetic pigments |
| Thylakoid lumen | Enclosed space where H⁺ ions accumulate during chemiosmosis |
| Stroma | Fluid-filled matrix; site of the Calvin cycle; contains enzymes (including RuBisCO), DNA, 70S ribosomes |
| Intergranal lamellae | Connect thylakoids in different grana; maximise thylakoid surface area |
Exam Tip: When asked to describe adaptations of the chloroplast for photosynthesis, always refer to: (1) the large surface area of thylakoid membranes for pigments and electron carriers, (2) the small volume of the thylakoid lumen for rapid H⁺ accumulation, and (3) the stroma containing all necessary Calvin cycle enzymes. Relate structure to function.
Key Definition: An absorption spectrum shows the wavelengths of light absorbed by a pigment. An action spectrum shows the rate of photosynthesis at different wavelengths of light.
A described graph comparing the two spectra would show wavelength of light (nm) on the x-axis (from 400 to 700 nm). Two curves are plotted: the absorption spectrum (showing peaks in the blue and red regions with a trough in the green) and the action spectrum (closely following the same pattern but slightly broader, indicating accessory pigments contribute at additional wavelengths).
These occur on the thylakoid membranes and require light energy.
flowchart LR
W["H₂O
(Photolysis)"] -->|"2e⁻ + 2H⁺ + ½O₂"| PSII["Photosystem II
(P680)
Light excites e⁻"]
PSII -->|"Excited e⁻"| ETC["Electron Transport Chain
(Plastoquinone → Cyt b6f → Plastocyanin)
Pumps H⁺ into thylakoid lumen"]
ETC -->|"e⁻ passed to"| PSI["Photosystem I
(P700)
Light re-excites e⁻"]
PSI -->|"e⁻ via Ferredoxin"| NADPR["NADP⁺ Reductase
NADP⁺ + H⁺ → NADPH"]
ETC -.->|"H⁺ gradient drives"| ATP["ATP Synthase
ADP + Pi → ATP"]
Products of the light-dependent reactions: ATP, NADPH (reduced NADP), and O₂.
Key Definition: Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate using light energy. It can be non-cyclic (involving both PSI and PSII, producing ATP and NADPH) or cyclic (involving only PSI, producing ATP only).
These occur in the stroma and do not directly require light, but depend on the ATP and NADPH produced by the light-dependent reactions.
flowchart TD
CO2["CO₂"] -->|"Carbon fixation
(RuBisCO)"| RuBP["RuBP (5C)"]
RuBP --> unstable["Unstable 6C intermediate"]
unstable --> GP["2x GP (3C)"]
GP -->|"Reduction
uses ATP + NADPH"| GALP["2x GALP (3C)"]
GALP -->|"5 out of 6 GALP
(uses ATP)"| RuBP
GALP -->|"1 out of 6 GALP
(net product)"| Products["Glucose, amino acids,
fatty acids, glycerol"]
For one molecule of glucose (C₆H₁₂O₆), 6 CO₂ molecules must be fixed. This requires 6 turns of the Calvin cycle.
Per turn: 1 CO₂ + 1 RuBP → 2 GP → 2 GALP (using 2 NADPH and 3 ATP for reduction and regeneration).
For 6 turns:
Of the 12 GALP produced, 2 GALP are used to make one glucose (2 × 3C = 6C), and the remaining 10 GALP are rearranged to regenerate 6 RuBP (10 × 3C = 30C → 6 × 5C = 30C).
Exam Tip: Learn the numbers 18 ATP and 12 NADPH per glucose. A common exam question asks you to explain why the Calvin cycle stops when light is removed — the answer is that ATP and NADPH from the light-dependent reactions are no longer being produced, so GP cannot be reduced and RuBP cannot be regenerated.
| Condition Change | Effect on GP | Effect on RuBP | Explanation |
|---|---|---|---|
| Light intensity decreases | Increases | Decreases | Less ATP and NADPH produced → GP cannot be reduced → GP accumulates; less RuBP regenerated |
| CO₂ concentration decreases | Decreases | Increases | Less CO₂ to fix with RuBP → less GP formed → RuBP accumulates as it is still regenerated but not used |
| Temperature increases (to a point) | Decreases then increases | Variable | Enzymes work faster up to optimum; beyond optimum, RuBisCO denatures |
Exam Tip: Questions on GP and RuBP levels are very common. Always think about what happens to the step that produces the molecule and the step that uses it up. If production stays the same but use decreases, the molecule accumulates.
Key Definition: The compensation point is the light intensity (or CO₂ concentration) at which the rate of photosynthesis exactly equals the rate of respiration. At this point, there is no net gas exchange — CO₂ produced by respiration is used in photosynthesis, and O₂ produced by photosynthesis is used in respiration.
The rate of photosynthesis is determined by whichever factor is closest to its minimum value (law of limiting factors, proposed by F.F. Blackman).
| Factor | Effect | Biological Explanation |
|---|---|---|
| Light intensity | Increases rate up to a plateau | Light provides energy for photolysis and excitation of electrons; at the plateau, all photosystems are saturated or another factor is limiting |
| CO₂ concentration | Higher CO₂ increases the rate until a plateau | CO₂ is the substrate for RuBisCO in carbon fixation; at the plateau, RuBisCO is saturated or another factor limits |
| Temperature | Increases rate up to an optimum (~25–30 °C); then rate drops sharply | Higher temperature increases kinetic energy of molecules and enzyme–substrate collisions; beyond the optimum, RuBisCO and other enzymes denature |
A described graph of rate of photosynthesis against light intensity at two different CO₂ concentrations would show two curves, both rising steeply at low light intensity. The curve at higher CO₂ concentration continues to rise to a higher plateau than the curve at lower CO₂ concentration. Both plateau because at high light intensity, another factor (CO₂ or temperature) becomes limiting. The initial slopes of both curves are identical because at very low light intensity, light is the limiting factor regardless of CO₂ concentration.
A student investigates the effect of light intensity on the rate of photosynthesis using an aquatic plant (Elodea). She counts oxygen bubbles produced per minute at different light intensities:
| Distance from lamp (cm) | Light intensity (arbitrary units, proportional to 1/d²) | Bubbles per minute |
|---|---|---|
| 5 | 400 | 45 |
| 10 | 100 | 40 |
| 20 | 25 | 22 |
| 30 | 11 | 12 |
| 40 | 6.25 | 6 |
Note: Light intensity is proportional to 1/d², where d is the distance from the lamp.
At low light intensities (large distances), the rate increases steeply with light intensity — light is the limiting factor. At high light intensities (short distances), the rate begins to level off, suggesting that another factor (temperature or CO₂ concentration) is now limiting.
Exam Tip: Always state that light intensity is proportional to 1/d² (the inverse square law), not 1/d. This is a common error. When plotting a graph, plot rate against 1/d² (not distance) to obtain a meaningful relationship.
This apparatus measures the rate of photosynthesis by collecting and measuring the volume of oxygen gas released by an aquatic plant.
A simpler method involves counting the number of oxygen bubbles released by Elodea per unit time. This is less accurate because bubble size may vary, but it provides a quick estimate.
Key Definition: The Hill reaction demonstrates that the light-dependent reactions of photosynthesis can occur in isolated chloroplasts when provided with an artificial electron acceptor.
A student separates photosynthetic pigments using thin-layer chromatography. The solvent front moves 12.0 cm from the origin. The pigments move the following distances:
| Pigment | Distance moved (cm) | Rf value |
|---|---|---|
| Carotene | 11.4 | 11.4 / 12.0 = 0.95 |
| Xanthophyll | 8.4 | 8.4 / 12.0 = 0.70 |
| Chlorophyll a | 6.0 | 6.0 / 12.0 = 0.50 |
| Chlorophyll b | 4.8 | 4.8 / 12.0 = 0.40 |
Carotene has the highest Rf value because it is the most soluble in the non-polar solvent (it is a non-polar hydrocarbon). Chlorophyll b has the lowest Rf value because it is the least soluble in the solvent (most polar of the four pigments). Rf values can be compared with reference values to identify unknown pigments.