Photosynthesis Overview and Light Reactions

Photosynthesis Overview

• Photosynthesis is the process by which photosynthetic organisms capture light energy and convert it into chemical energy in the bonds of glucose.
• Photosynthetic organisms are the foundation of food chains.
• Photosynthesis is an endergonic process, meaning it requires energy input.
• Light energy is converted to chemical energy in glucose.
• Photosynthesis is an anabolic process (building up complex molecules).

Light Dependent Reactions of Photosynthesis

• These reactions occur in the thylakoid membranes of chloroplasts.
• They require two photosystems (Photosystem I and Photosystem II).
• Photosystems are collections of pigments (like chlorophyll) surrounding a reaction center.
• Chlorophyll a is the main pigment, capable of transferring excited electrons to a primary electron acceptor.
• Accessory pigments (e.g., chlorophyll b, carotenoids) absorb other wavelengths of light and transfer the energy to chlorophyll a. This helps capture a wider range of light.
• Water molecules are split, releasing oxygen.
• Excited electrons are passed along the electron transport chain generating ATP and NADPH. These are energy carriers used in the Calvin Cycle.

Photosynthesis: Where it Takes Place

• Photosynthesis happens within cells of leaves in chloroplasts.
• Chloroplasts are specialized organelles found in plant cells that contain pigments, primarily chlorophyll, which trap light.

Photosynthesis Background Information

• Solar radiation is measured in terms of wavelength.
• Photons are packets of light energy.
• Visible light is the portion of the electromagnetic spectrum used for photosynthesis (PAR).
• Maximum absorption of light occurs in the blue-violet and red portions of the spectrum.
• Different wavelengths of light are absorbed by different pigments (chlorophyll a, chlorophyll b, carotenoids).

Chlorophyll Structure

• Chlorophyll has a porphyrin ring and a phytol tail.
• The tails are non-polar and the heads are polar. This accounts for its positioning in the thylakoid membrane.
• Chlorophyll contains a magnesium atom at its centre.
• This magnesium atom is crucial for chlorophyll's role in capturing light.

Chlorophyll Pigments

• Chlorophyll a is the primary pigment for photosynthesis.
• Chlorophyll a and b and carotenoids absorb light of different wavelengths.
• Chlorophyll b and carotenoids transfer light energy to chlorophyll a.
• Chlorophyll a absorbs violet-blue and red light best.
• Chlorophyll b absorbs blue and orange-red light and transmits green light, thus the colour of the leaves.

Light Dependent Reactions Summary

• A photon strikes a pigment molecule.
• Energy is converted from light to chemical energy (ATP and NADPH).
• Redox reactions and an electron transport chain (ETC) are involved.

Chemiosmosis in Chloroplasts

• Energy released from electrons is used to pump protons (H+) into the thylakoid space.
• This creates a proton gradient.
• Protons flow through ATP synthase to generate ATP.
• This process is facilitated diffusion.

Light-Dependent vs. Dark-Dependent (Light Independent)

• Light Stage: Photosynthesis' light-dependent phase traps light energy to produce ATP and NADPH.
• Dark Stage (Calvin Cycle): The light-independent reactions (Calvin Cycle) use the ATP and NADPH produced in the light-dependent phase to convert carbon dioxide into glucose.

Chloroplast vs. Mitochondria

• Chloroplasts and mitochondria both use chemiosmosis to make ATP.
• Chloroplasts have highly permeable outer membranes, while mitochondria have less permeable inner membrane.
• Chloroplasts have a stroma which is a large space inside, analogous to the matrix in mitochondria.
• The stroma contains chloroplast DNA, ribosomes and enzymes for metabolic processes.