Photosynthesis: Overview and Importance

Energy Flow in Ecosystems

Photosynthesis is the process by which autotrophs convert light energy into chemical energy, forming the foundation of most ecosystems. Energy flows from the sun to producers and then to consumers.

• Producers (Autotrophs): Organisms that make their own food using light or chemical energy. Examples: Plants, algae, some bacteria.

• Consumers (Heterotrophs): Organisms that cannot make their own food and must consume other organisms. Examples: Animals, fungi, some bacteria, some protists.

Leaf and Chloroplast Structure

Leaf Structure

• Stomata: Pores on the leaf surface that allow gas exchange (CO2 in, O2 out).

• Chloroplasts: Organelles where photosynthesis occurs.

Chloroplast Structure

• Outer membrane: Fairly permeable to small molecules and ions.

• Inner membrane: More selective, controlling entry of substances.

• Thylakoid membrane: Highly selective; contains chlorophyll and is the site of the light reactions.

• Granum: Stack of thylakoids.

Stages of Photosynthesis

Light Reactions (Thylakoid Membrane)

The light reactions use solar energy to split water, releasing oxygen, and generating ATP and NADPH.

• Step 1: Light hits Photosystem II, splitting water into protons, electrons, and O2.

• Step 2: Electrons travel through the electron transport chain, pumping protons into the thylakoid lumen.

• Step 3: Light hits Photosystem I, re-energizing electrons, which are used to reduce NADP+ to NADPH.

• Step 4: Protons flow back through ATP synthase, generating ATP (chemiosmosis).

Inputs: H2O, light, NADP+, ADP Outputs: O2, NADPH, ATP

The Calvin Cycle (Stroma)

The Calvin Cycle uses ATP and NADPH from the light reactions to fix CO2 into organic molecules (sugar).

• Carbon Fixation: CO2 is attached to RuBP by the enzyme Rubisco.

• Reduction: ATP and NADPH are used to convert 3-phosphoglycerate into G3P (a sugar building block).

• Regeneration: Some G3P is used to regenerate RuBP, allowing the cycle to continue.

Inputs: CO2, ATP, NADPH Outputs: G3P (sugar), ADP, NADP+

Light and Pigments in Photosynthesis

Visible Light and the Electromagnetic Spectrum

Visible light is a small part of the electromagnetic spectrum and is composed of various wavelengths (ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet). Different pigments absorb different wavelengths.

Chlorophyll and Absorption Spectrum

Chlorophyll is the main pigment in plants, absorbing blue and red light strongly and reflecting green light, which is why leaves appear green.

• Chlorophyll a: Absorbs light in the blue-violet and red regions.

• Chlorophyll b: Absorbs light in the blue and orange regions.

Absorption spectrum: The range of wavelengths absorbed by a pigment. Action spectrum: The effectiveness of different wavelengths in driving photosynthesis.

Diversity of Photosynthetic Organisms

Photosynthetic Pigments in Algae

Different algae have evolved pigments to match the light available at different ocean depths:

• Green algae: Shallow water, use chlorophyll.

• Brown algae: Moderate depths, use fucoxanthin.

• Red algae: Deep water, use phycoerythrin to absorb blue-green light.

Alternative Mechanisms of Carbon Fixation

C3, C4, and CAM Pathways

Plants have evolved different pathways to fix carbon, adapting to various climates:

• C3 plants: Use the ancestral pathway; Rubisco fixes CO2 directly. Best in cool, wet climates.

• C4 plants: Pre-concentrate CO2 in special cells, reducing photorespiration. Adapted to warm, dry climates.

• CAM plants: Collect CO2 at night and run the Calvin cycle during the day, minimizing water loss. Adapted to hot, arid climates.

Photorespiration and Rubisco

Rubisco can bind O2 instead of CO2, leading to photorespiration, which wastes energy and releases CO2. C4 and CAM pathways evolved to minimize this problem.

Factors Affecting Photosynthesis

• Light intensity

• CO2 concentration

• Temperature

• pH

These factors influence enzyme activity, which in turn affects the rate of photosynthesis.

Photosynthesis in Prokaryotes

Oxygenic vs. Anoxygenic Photosynthesis

• Oxygenic photosynthesis: Uses water as the electron donor, produces oxygen (e.g., cyanobacteria).

• Anoxygenic photosynthesis: Uses other electron donors (e.g., hydrogen sulfide), does not produce oxygen (e.g., purple sulfur bacteria).

Photosynthesis and Ecosystems

Global Productivity

Photosynthetic productivity is highest in tropical rainforests near the equator due to abundant sunlight, heat, and water. Deserts and polar regions have low productivity.

Life Without Photosynthesis: Movile Cave

Movile Cave in Romania is an ecosystem isolated from sunlight, relying on chemosynthesis (bacteria oxidizing sulfide and ammonium) instead of photosynthesis.

Photosynthesis and Biotechnology

Improving Photosynthetic Efficiency

Projects like RIPE aim to increase crop yields by enhancing the expression of genes involved in photosynthesis.

Leaf vs. Solar Panel Efficiency

Leaves convert light to energy at about 4% efficiency, while solar panels can reach 18% efficiency.

Summary Table: Comparison of Photosynthetic Pathways