Photosynthesis in Higher Plants — Revision Notes

⚡ 30-Second Revision

Overall Equation: — $6CO_2 + 6H_2O xrightarrow{\text{Light}} C_6H_{12}O_6 + 6O_2$
Light Reactions (Thylakoids):

- Inputs: Light, $H_2O$, ADP, NADP$^+$ - Outputs: ATP, NADPH, $O_2$ - Key processes: Photolysis ($H_2O$ splitting), Electron Transport Chain (Z-scheme), Chemiosmosis (ATP synthesis) - Photosystems: PSII (P680), PSI (P700)

Dark Reactions (Calvin Cycle, Stroma):

- Inputs: $CO_2$, ATP, NADPH - Outputs: Glucose (sugars), ADP, NADP$^+$ - Phases: Carboxylation ($CO_2$ + RuBP by RuBisCO), Reduction (3-PGA to G3P), Regeneration (RuBP) - Energy cost per $CO_2$: 3 ATP, 2 NADPH - Energy cost per Glucose: 18 ATP, 12 NADPH

C4 Pathway (Kranz Anatomy):

- Primary $CO_2$ acceptor: PEP (mesophyll cells) - Primary enzyme: PEPcase (mesophyll cells) - First stable product: OAA (4-C) - Calvin cycle in bundle sheath cells (high $CO_2$ concentration) - Minimizes photorespiration; higher efficiency in hot/dry conditions.

Photorespiration: — Wasteful process in C3 plants when RuBisCO binds $O_2$ instead of $CO_2$.

2-Minute Revision

Photosynthesis is the process converting light energy into chemical energy (glucose) using $CO_2$ and $H_2O$, releasing $O_2$. It occurs in chloroplasts and has two main stages. The light-dependent reactions happen on thylakoid membranes, where chlorophyll captures light.

Water is split (photolysis) to release $O_2$ and electrons. These electrons flow through Photosystem II (PSII) and Photosystem I (PSI) in a 'Z-scheme', generating ATP via chemiosmosis and NADPH. These ATP and NADPH then power the light-independent reactions (Calvin cycle) in the stroma.

Here, $CO_2$ is fixed by RuBisCO onto RuBP, forming 3-PGA. This 3-PGA is reduced to G3P using ATP and NADPH, and G3P is used to synthesize glucose or regenerate RuBP. C3 plants follow this direct pathway.

C4 plants, adapted to hot, dry climates, use a two-step process with Kranz anatomy: PEPcase fixes $CO_2$ in mesophyll cells, forming a 4-carbon compound, which is then transported to bundle sheath cells where $CO_2$ is released for the Calvin cycle.

This strategy minimizes photorespiration, a wasteful process where RuBisCO binds $O_2$ instead of $CO_2$. Factors like light, $CO_2$, temperature, and water limit the rate of photosynthesis.

5-Minute Revision

Photosynthesis is the anabolic process converting light energy into chemical energy, primarily glucose, using carbon dioxide and water, releasing oxygen. This occurs in chloroplasts, specifically within the thylakoids for light reactions and the stroma for dark reactions.

Light-Dependent Reactions: These occur on the thylakoid membranes. Light energy is captured by chlorophyll 'a' and accessory pigments (chlorophyll 'b', carotenoids) organized into Photosystem II (PSII, P680) and Photosystem I (PSI, P700).

PSII absorbs light, exciting electrons. Water is split (photolysis) to replace these electrons, releasing $O_2$ and $H^+$ into the thylakoid lumen. The excited electrons from PSII pass through an electron transport chain (ETC) to PSI, generating a proton gradient across the thylakoid membrane.

This gradient drives ATP synthesis via ATP synthase (chemiosmosis). Electrons are re-excited at PSI and then reduce NADP$^+$ to NADPH. This entire non-cyclic electron flow is called the Z-scheme, producing both ATP and NADPH.

Cyclic photophosphorylation, involving only PSI, produces only ATP.

Light-Independent Reactions (Calvin Cycle / C3 Pathway): Occurring in the stroma, this cycle uses the ATP and NADPH from light reactions to fix $CO_2$. It has three phases:

1
Carboxylation: — $CO_2$ combines with RuBP (5-C) catalyzed by RuBisCO, forming two molecules of 3-PGA (3-C).
2
Reduction: — 3-PGA is converted to Glyceraldehyde-3-phosphate (G3P, 3-C) using ATP and NADPH.
3
Regeneration: — Most G3P regenerates RuBP, consuming ATP, to continue the cycle. For every $CO_2$ fixed, 3 ATP and 2 NADPH are consumed. To make one glucose, 6 $CO_2$ are fixed, requiring 18 ATP and 12 NADPH.

C4 Pathway: An adaptation in plants like maize and sugarcane for hot, dry climates. They exhibit Kranz anatomy (bundle sheath cells around vascular bundles). $CO_2$ is initially fixed in mesophyll cells by PEP carboxylase (PEPcase) into a 4-carbon compound (e.

g., OAA). This C4 acid is transported to bundle sheath cells, where it's decarboxylated, releasing a high concentration of $CO_2$ for the Calvin cycle. This minimizes photorespiration, a wasteful process in C3 plants where RuBisCO binds $O_2$ instead of $CO_2$ under low $CO_2$/high $O_2$ conditions.

Limiting Factors: Photosynthesis rate is affected by light intensity, $CO_2$ concentration, temperature, and water, as per Blackman's Law. For example, in C3 plants under high light, $CO_2$ is often limiting.

Prelims Revision Notes

1
Photosynthesis Overview: — Anabolic process, light energy to chemical energy. $6CO_2 + 6H_2O xrightarrow{\text{Light}} C_6H_{12}O_6 + 6O_2$. Occurs in chloroplasts.
2
Chloroplast Structure: — Double membrane, stroma (fluid), thylakoids (sacs), grana (stacks of thylakoids). Thylakoid lumen is the space inside thylakoids.
3
Pigments: — Chlorophyll a (reaction center, absorbs blue/red), Chlorophyll b, Carotenoids (accessory pigments, broaden spectrum, photoprotection).
4
Light Reactions (Thylakoid Membrane):

* Photosystems: PSII (P680, water splitting), PSI (P700, NADP$^+$ reduction). * Photolysis: $H_2O \rightarrow 2H^+ + 2e^- + \frac{1}{2}O_2$. Occurs at PSII, releases $O_2$ into atmosphere, $H^+$ into lumen.

* Electron Transport Chain (Z-scheme): Non-cyclic flow from $H_2O \rightarrow$ PSII $ightarrow$ ETC $ightarrow$ PSI $ightarrow$ NADP$^+$. Produces ATP and NADPH. * Chemiosmosis: Proton gradient across thylakoid membrane (lumen high $H^+$).

$H^+$ flow through ATP synthase (CF0-CF1) generates ATP. * Cyclic Photophosphorylation: Only PSI, electrons cycle back to PSI. Produces only ATP, no NADPH, no $O_2$.

1
Dark Reactions (Calvin Cycle / C3 Pathway, Stroma):

* Carboxylation: $CO_2$ + RuBP (5-C) $xrightarrow{\text{RuBisCO}}$ 2 molecules of 3-PGA (3-C). RuBisCO is the most abundant enzyme. * Reduction: 3-PGA $xrightarrow{\text{ATP, NADPH}}$ Glyceraldehyde-3-phosphate (G3P). * Regeneration: G3P $xrightarrow{\text{ATP}}$ RuBP. * Energy Cost: Per $CO_2$: 3 ATP, 2 NADPH. Per Glucose ($6,CO_2$): 18 ATP, 12 NADPH.

1
Photorespiration (C3 plants): — RuBisCO binds $O_2$ instead of $CO_2$ (oxygenase activity). Wasteful, consumes ATP, releases $CO_2$, no sugar production. Occurs under high $O_2$/low $CO_2$/high temperature.
2
C4 Pathway (Hatch-Slack, C4 Plants):

* Kranz Anatomy: Mesophyll cells (loose) surrounding bundle sheath cells (large, thick walls, few intercellular spaces, rich in chloroplasts). * Mesophyll Cells: Primary $CO_2$ acceptor: PEP (3-C).

Enzyme: PEPcase (high $CO_2$ affinity, no $O_2$ affinity). First stable product: OAA (4-C). OAA converted to malate/aspartate, transported to bundle sheath. * Bundle Sheath Cells: Decarboxylation of C4 acid releases $CO_2$.

Calvin cycle occurs here (RuBisCO present). Concentrates $CO_2$ to minimize photorespiration. * Energy Cost: Per $CO_2$: 5 ATP, 2 NADPH (extra 2 ATP for PEP regeneration). Per Glucose: 30 ATP, 12 NADPH.

* Adaptation: Hot, dry climates, high light intensity.

1
CAM Pathway: — Temporal separation of $CO_2$ fixation (night) and Calvin cycle (day). Succulents. Stomata open at night.
2
Limiting Factors (Blackman's Law): — Light, $CO_2$, Temperature, Water. The factor in shortest supply limits the rate.

Vyyuha Quick Recall

For the Calvin Cycle's phases: Carbon Really Regenerates.

Carbon Fixation
Reduction
Regeneration