Photosynthesis Quiz

Photosynthesis Overview

• Photosynthesis is the process used by plants, algae, and some bacteria to convert light energy into chemical energy.
• This process harnesses energy from sunlight to produce carbohydrates.
• Two types of photosynthesis exist: oxygenic and anoxygenic.

Oxygenic Photosynthesis

• The most common type, found in plants, algae, and cyanobacteria.
• Light energy transfers electrons from water (H₂O) to carbon dioxide (CO₂), producing carbohydrates.
• In this process, CO₂ is reduced, and water is oxidized.
• Ultimately, oxygen (O₂) is produced along with carbohydrates.

Anoxygenic Photosynthesis

• Typically occurs in certain bacteria like purple bacteria and green sulfur bacteria.
• Uses electron donors other than water.
• Produces sulfur when hydrogen sulfide (H₂S) is used as an electron donor.

Significance of Photosynthesis

• Converts solar energy into organic matter.
• Consumes atmospheric carbon dioxide and produces oxygen.
• Maintains atmospheric oxygen levels, crucial for respiration in other living organisms.
• Provides the building blocks (carbohydrates, organic acids, proteins, etc.) for other biological molecules.
• Provides energy for various metabolic activities and movements in living organisms.

Photosynthetic Requirements

• Photosynthetic eukaryotic organisms contain plastids (organelles) in their cytoplasm.
• Plastids are surrounded by a double membrane and can contain pigments or store nutrients.
• Different types of plastids include chloroplasts (with chlorophyll), chromoplasts (with carotenoids), and leucoplasts (non-pigmented, storing starch and fat).

Chloroplast Structure

• Chloroplasts are double-membrane-bound organelles performing photosynthesis.
• The grana are the innermost portion, consisting of stacked disc-shaped thylakoid membranes.
• Thylakoids contain light-harvesting complexes where electron transfer takes place.
• The empty spaces between grana are the stroma.

Thylakoids

• The thylakoid is the structural unit of photosynthesis.
• The photosynthetic membrane is vesicular, with an outer stroma phase and an inner lumen.
• The photosynthetic membrane contains most of the pigments and proteins needed for light reactions.

Pigments

• Pigments bestow color to plants, algae, and bacteria and are crucial for trapping sunlight.
• Different pigments absorb different light wavelengths.
• Chlorophylls (a, b, and c) absorb blue and red light, carotenoids absorb bluish-green light, and phycobilins absorb light wavelengths not well absorbed by other pigments (found in cyanobacteria and red algae).

Absorption of Light Energy

• Photosynthesis is primarily driven by visible light (400-700 nm).
• Light is captured by pigment molecules (chlorophyll a and b, carotenoids) in light-harvesting protein complexes within the photosynthetic membrane.
• Light-harvesting complexes surround the reaction centers that act as antennas.
• A photon's energy is absorbed by an antenna molecule, causing a transition to an excited state.
• Excited state decays within femtoseconds (10⁻¹⁵ s) to the first excited singlet state, and energy is transferred to other nearby molecules before ending at the reaction center.

Chlorophyll Fluorescence

• Excitons (excited electrons) are converted back to photons and emitted as fluorescence (a longer wavelength).
• Used as an indicator of photosynthetic energy conversion performance in plants, algae, and bacteria.
• Fluorescence can also indicate plant stress or damage to the photosynthetic apparatus.

Chlorophyll Phosphorescence

• A type of photoluminescence related to fluorescence.
• A phosphorescent material takes time to re-emit the absorbed radiation.
• Phosphorescence involves radiative electronic transitions from triplet to ground states in pigment molecules.

Light-Dependent Reactions (Light Reactions)

• Light-dependent reactions require direct light energy and use captured light energy from chlorophyll a to make energy carrier molecules (ATP and NADPH).
• Water is split, releasing oxygen (a byproduct) during these reactions.
• This occurs on the thylakoid membranes. Major protein complexes involved: PSII, cytochrome b6f complex, PSI, and ATP synthase.

Photosystem II (PSII)

• The first protein complex in the light-dependent reactions of oxygenic photosynthesis.
• Catalyzes the capture of light photons to energize electrons.
• Electrons are transferred through various coenzymes and cofactors to reduce plastoquinone to plastoquinol.
• Water is oxidised to replenish lost electrons, releasing oxygen.

Cytochrome b6f complex

• Transfers electrons from Photosystem II to Photosystem I (PSI).
• It pumps protons into the thylakoid lumen, creating an electrochemical gradient.

Photosystem I (PSI)

• The second photosystem.
• The electron gets energy from another photon.
• The final electron acceptor is NADP+.
• This creates NADPH.

ATP Synthase

• Creates the energy storage molecule, ATP, using the proton gradient formed by the electron transport chain.
• It comprises two parts: FO embedded within the thylakoid membrane and F1 outside.

Non-cyclic and Cyclic Photophosphorylation

• Non-cyclic: the primary electron flow from PSII to cytochrome b6f then to PSI to create ATP and NADPH.
• Cyclic: ATP is produced, but no NADPH. The electron flow goes from PSII to cytochrome b6f to PSI, then back to the cytochrome b6f complex.

Calvin Cycle (Light-Independent Reactions or Dark Reactions)

• Plants and algae use the Calvin cycle to convert carbon dioxide into sugars.
• It occurs in the stroma of the chloroplasts.
• It involves four main steps: carbon fixation, reduction, carbohydrate formation, and regeneration.
• The cycle uses ATP and NADPH (created during light reactions) to fuel the reactions.

Carbon Fixation

• The enzyme RuBisCO catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP, a 5-carbon compound) by carbon dioxide (CO₂) in a two-step reaction.
• Carbon dioxide is captured, producing a 6-carbon intermediate that then splits into two 3-carbon molecules (3-PGA).

Carbon Reduction

• Enzyme phosphoglycerate kinase phosphorylates 3-PGA using ATP, producing 1,3-bisphosphoglycerate (1,3-BPG) and ADP
• Enzyme glyceraldehyde-3-phosphate dehydrogenase catalyzes the reduction of 1,3-BPG by NADPH. This produces glyceraldehyde-3-phosphate (G3P) and oxidised NADP+.

Regeneration Phase

• Molecules involved (fructose-1,6-bisphosphate and dihydroxyacetone phosphate) in converting five G3P molecules to three 5-carbon molecules of RuBP, also using ATP.

Photorespiration

• RuBisCO sometimes reacts with oxygen (O₂) instead of carbon dioxide (CO₂), a process called photorespiration.
• It's more active at higher temperatures and involves a loss of carbon dioxide.
• The byproducts are converted into other compounds.

C4 Carbon Fixation

• Evolved to minimize photorespiration in plants in hot, dry climates.
• CO₂ is concentrated around RuBisCO, creating higher CO₂ substrate levels to reduce the competition for O₂ and CO₂ by RuBisCO.
• Involves carbon fixation in mesophyll cells followed by transport to bundle-sheath cells, concentrating CO₂ around RuBisCO.
• Multiple enzyme systems in the mesophyll and bundle sheath cells. Variations include malate or aspartate as the 4-carbon intermediates transported from mesophyll to bundle sheath.

C4 Leaf Anatomy

• Kranz anatomy: leaf structure with concentric rings of cells surrounding the vascular bundles, consisting of mesophyll and bundle sheath cells.
• Bundle sheath cells are rich in chloroplasts, often lacking grana, and have high concentration of CO₂, low O₂ levels.

C4 Pathway

• The series of reactions of C4 pathway which involves creation of 4-carbon organic acids in mesophyll cells
• These molecules then are transported to other cells for decarboxylation and carbon dioxide release to further enter Calvin Cycle. This pathway is used by tropical and warm climate plants.

CAM (Crassulacean Acid Metabolism)

• Plants use this pathway to minimize water loss in hot, dry environments.
• Carbon dioxide is fixed at night when stomata are open, during this time it is converted into organic acids (such as malate) and stored in vacuoles. During the day the stomata are closed, organic acids release carbon dioxide for further use in Calvin Cycle.