Photosynthesis Quiz: Mechanisms and Functions

Photosynthesis Overview

• Photosynthesis is a process converting atmospheric carbon dioxide (CO2) and water (H2O) into carbohydrates.
• Solar energy is captured as ATP and NADPH.
• ATP and NADPH are used to convert carbon dioxide to hexose phosphates.
• Phototrophs are photosynthetic organisms, including some bacteria, algae, and higher plants.

Photosynthesis 1 - Light Reactions

• Light and dark reactions are parts of photosynthesis.
• Light energy is captured for biochemical processes.
• Specific light wavelengths are important for photosynthesis.
• Photosynthesis takes place in chloroplasts.
• The Z-scheme is a model of the electron flow in light reactions.

Light Reactions and Dark Reactions

• Light-dependent reactions are membrane-bound.
• Light reactions release oxygen from water molecules.
• Light reactions produce ATP and NADPH.
• Dark reactions occur in the solution (stroma).
• Dark reactions convert carbon dioxide to carbohydrates.
• Dark reactions use energy from NADPH and ATP.

Chloroplast Structure

• Chloroplasts are 4-8 mm diameter organelles in plant cells.
• Chloroplasts have two surrounding membranes.
• Internal membranes are stacked as thylakoid membranes.
• Thylakoids are stacked into grana.
• The space surrounding the thylakoids is called stroma.

Chlorophyll and Other Pigments

• Chlorophyll is a major, abundant light-harvesting pigment.
• Tetrapyrrole ring structure is similar to heme but with magnesium (Mg2+) instead of iron.
• Other pigments like carotenoids, phycoerythrin, and phycocyanin are antenna pigments.
• Antenna pigments capture light energy, especially in the green spectrum.
• Antenna pigments transfer light energy to chlorophylls.

Light Absorption and Photosystems

• Light absorption occurs in two photosystems (PSI and PSII).
• Photosystems are groups of proteins and pigments in the thylakoid membrane.
• PSI is paired for P700 (absorbs light maximally at 700 nm).
• PSII is paired for P680 (absorbs light maximally at 680 nm).
• A Z-scheme model illustrates the electron flow.
• PSII to PSI to NADP+.

Z-Scheme

• Z-scheme describes the path of electron flow and reduction potentials of components in photosynthesis.
• Light energy converts P680 and P700 to excited molecules that are better electron donors.
• Electron flow drives electron transfer uphill.
• NADP+ is the final electron acceptor.
• Water is oxidized to produce oxygen (O2), while NADPH is produced.

Chemiosmosis

• Chemiosmosis theory explains energy conversion during photosynthesis.
• The formation of a transmembrane electrochemical gradient is key.
• Protons (H+) are transferred across a membrane to create this gradient.
• The gradient is used to drive protons through ATP synthase.
• ATP synthase creates ATP from ADP and Pi.

Photophosphorylation — H+ ATP Synthase

• Photophosphorylation is an ATP synthesis process dependent on light energy.
• Chloroplast ATP synthase has two major parts, CF0 and CF1.
• CF0 spans the membrane, acting as a proton channel.
• CF1 protrudes into the stroma, catalyzing ATP synthesis.
• ATP is made from ADP and Pi.

Cyclic Electron Flow and Phosphorylation

• Cyclic electron flow produces ATP, bypassing NADPH production.
• Energy for CO2 reduction generates 2 NADPH and 3ATP.
• Cyclic flow increases proton motive force for greater ATP production.

Summary of Photosynthesis

• Photosynthesis uses light energy to fix carbon into sugar.
• Light reactions capture light energy to reduce NADP+ and phosphorylate ADP.
• Photosynthesis takes place in chloroplasts.
• ATP and NADPH are the energy sources for cellular reactions.