Photosynthesis: C3, C4, and CAM Pathways, Pigments, and Factors

Plant Photosynthesis: C3, C4, and CAM Pathways

C4 Plants

C4 metabolism has a higher energy cost than C3, requiring 5 ATP and 2 NADPH per CO2 fixed. C4 plants are adapted to tropical, subtropical, relatively dry environments with bright illumination and can withstand high and low relative humidity. They fix CO2 using PEP carboxylase and then pump it to the bundle sheath cells.

C3 Plants

C3 plants use 3 ATP and 2 NADPH and are adapted to temperate climates with moderate temperatures and high relative humidity.

Exogenous Factors Affecting Photosynthesis

• Light
• Concentration of CO2 and O2
• Temperature
• Availability of water and nutrients

Endogenous Factors Affecting Photosynthesis

Type of plant (C3, C4, or CAM), stomatal density, leaf area, leaf age, hormone levels, and other cellular metabolic processes (respiration and photorespiration). Net photosynthesis (FN) = Gross Photosynthesis (FB) - (Photorespiration (FR) + Mitochondrial Respiration (MR))

FN: Net photosynthesis

FB: Gross Photosynthesis

FR: Photorespiration

RM: Mitochondrial Respiration

Photosynthetic Pigments

Phycoerythrin: An accessory pigment found in blue and red algae, along with phycocyanin. These are tetrapyrrole compounds.

Terrapyrrole: The basic structure of porphyrins, including chlorophyll, hemoglobin, and cytochromes.

The characteristic chlorophyll chromophore is due to the conjugated double bond system formed by the union of pyrrole rings by methine groups.

Phytol: A 20-carbon chain with a double bond. The phytol tail gives chlorophyll its waxy nature, explaining its water insolubility.

Hydrophobic: Water-insoluble.

Porphyrin: The core has a hydrophobic character.

Energy Loss and Reaction Centers

Excitation energy may be lost:

1. As heat when the electron returns to baseline.
2. As fluorescence and heat.
3. Used for photosynthesis.

Reaction Center: Consists of a chlorophyll a molecule associated with specific membrane components. Photosynthesis has two reaction centers: Photosystem I and Photosystem II.

• Energy migrates from pigment to pigment until reaching the reaction center.
• The spectrum of action identifies the pigments.
• All species have major peaks in the red region and a minor peak in the blue region.
• Photosystems I and II use light energy to oxidize H2O and transfer electrons to NADP+, forming NADPH.
• Aspartic or malic acid is decarboxylated to produce CO2 and pyruvic acid.

endo ---> is absorbed; exo ----> released; Gonic = energy.

Emerson Effect

Providing red light of short wavelength along with longer wavelengths results in a greater rate of photosynthesis than either wavelength alone.

P680

A dimer of reactive molecules that pass its electron to a primary electron acceptor in Photosystem II.

Reduction of CO2

ATP and NADPH are used to reduce carbon from CO2 to a simple sugar. This process results in the formation of a carbon skeleton. Carbon sequestration can occur without light. The chemical transformation is not directly linked to the two light reactions.

Calvin Cycle

Occurs in the chloroplast stroma. It is a set of reactions in which CO2 is reduced, resulting in carbohydrates. It is the second stage of photosynthesis.

Metabolism

First, CO2 joins with pyruvic acid or pyruvate to form a 4-carbon compound.

Oxaloacetic Acid

An intermediate in the Krebs cycle.

Anaerobic

Fermentation without O2.

Autotrophic

A cell or organism that does not need an external supply of oxidizable matter.

Krebs Cycle

A stage of cellular respiration in which acetyl groups are degraded to CO2, and ATP is formed.

Absorption Spectrum

The characteristic pattern of wavelengths of light that a pigment absorbs.

Action Spectrum

Identifies the wavelengths that evoke a particular reaction.

Phytochrome

A plant pigment that acts as a photoreceptor for red or infrared light, involved in flowering, dormancy, and germination.

Phytoplankton

Microscopic, photosynthetic, free-living organisms in water.

Photon

Elementary particles of light.

Hydrophilic

Has an affinity for water. Applicable to polar molecules.

Hydrophobic

Has no affinity for water.

Pigment

A substance that absorbs light in a certain band of wavelengths.

Plasto

Cytoplasmic organelles, often pigmented, in cells.