Photosynthesis: Light and Dark Reactions

4.1 Capturing Solar Energy!

• Photosynthesis is the process by which photosynthetic organisms serve as the ultimate source of food for life.
• Photosynthesis is an endergonic process.
• Photosynthesis transforms light energy into chemical energy stored in glucose.
• Photosynthesis is an anabolic process.

Photosynthesis Overview

• Sunlight provides energy for photosynthesis.
• Photosynthetic organisms are the base of food chains.
• The process of photosynthesis converts carbon dioxide and water into glucose and oxygen.
• The balanced chemical equation summarizing the process is: 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂
• It is an endergonic process, meaning it requires energy input.
• It is an anabolic process, meaning it builds complex molecules from simpler ones.

Where Does Photosynthesis Take Place?

• Photosynthesis primarily takes place in the leaves of plants.
• Leaves contain chloroplasts.
• Chloroplasts are the site of photosynthesis.
• A chloroplast is composed of an outer membrane, inner membrane, stroma, and grana.
• Grana is made of thylakoid membranes.
• Thylakoids contain chlorophyll molecules.
• Each cell in a leaf contains many chloroplasts, from 40-300 per cell.

Photosynthesis Background Information

• Solar radiation is described by wavelength.
• Photons are discrete packets of radiant energy.
• Visible light, part of the electromagnetic spectrum, is captured by plants for photosynthesis.
• The portion of visible light useful for photosynthesis is called PAR (photosynthetically active radiation).
• Only 42% of solar radiation reaches the earth's surface. most as visible light

Light May Be

• Transmitted: Passed through matter.
• Reflected: Bounced off matter.
• Absorbed: Changed from light energy into another form.
• Pigments absorb certain wavelengths of visible light, then reflect other colors, determining the color we observe.

Chlorophyll Structure

• Chlorophyll is a pigment found in chloroplasts.
• It has a "porphyrin ring" and a "phytol tail".
• The "phytol tails" are non-polar, and the heads are polar, accounting for chlorophyll's position in the thylakoid membrane.
• Magnesium is at the center of the chlorophyll molecule, contributing to its instability.

Photosynthetic Pigments

• Chlorophyll a is the primary pigment in photosynthesis, absorbing red and blue-violet light and reflecting green light.
• Chlorophyll b is an accessory pigment, absorbing light in different wavelengths than chlorophyll a to extend the spectrum that can drive photosynthesis.
• Carotenoids are accessory pigments, that absorb light in different wavelengths than chlorophyll a and b.
• Different plants may have different combinations and amounts of accessory pigments based on their surroundings.

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 accessory pigments.
• Chlorophyll a is a pigment located in the reaction center of a photosystem.
• A photon of light excites an electron in chlorophyll a.
• The excited electron is passed to a primary electron acceptor.
• The electron acceptor is a protein containing a PEA.

Photosystems I and II

• Photosystem II (PSII) absorbs light at 680nm.
• Photosystem I (PSI) absorbs light at 700nm.
• Chlorophyll b and carotenoids, which help absorb other wavelengths, act like antennae to capture sunlight and transfer energy to chlorophyll a.

Light-Dependent Summary

• Light energy is converted into chemical energy as ATP and NADPH.
• A series of redox reactions and an electron transport chain (ETC) "package" the energy.

Chemiosmosis in the Chloroplast

• The gradient of H+ ions, formed by the ETC, is used to power the synthesis of ATP.
• The process of ATP synthesis is called photophosphorylation.

Light Stage (Requirements & Products)

• Requirements: Sunlight, water, chlorophyll, NADP+, ADP+P
• Products: Oxygen, NADPH, ATP

Dark Stage (Requirements & Products)

• Requirements: NADPH from light, ATP, CO₂
• Products: PGAL (a form of glucose) , ADP+P, NADP+

Chloroplast vs. Mitochondria

• Both use chemiosmosis but differ structurally and in function.
• Chloroplasts have a highly permeable outer membrane, a less permeable inner membrane, and a narrow intermembrane space.