Photosynthesis: Pigments, Light, and Carbon Fixation in Plants
Photosynthesis: Pigments, Light, and Carbon Fixation
Over 90% of a plant’s weight is water. CO2 in the atmosphere moves by diffusion through the ostiole, then the walls of the mesophyll, and finally reaches the chloroplasts. This process is proportional to the concentration of CO2 in the atmosphere (0.03%).
Chlorophyll: The Green Pigment
Chlorophyll is the green pigment that allows plants to absorb light, primarily in the violet, blue, and red spectrum, while reflecting green light. Chlorophylls are tetrapyrrole compounds.
- Chlorophyll A: Blue-green, directly involved in transforming light energy into chemical energy.
- Chlorophyll B: Yellowish-green.
- Chlorophyll C: Found in brown algae, lacking the phytol tail and hydrogen atoms in the IV ring at positions 7 and 8.
- Chlorophyll D: Found in algae.
- Bacteriochlorophyll: Typical of phototropic bacteria.
Starch is derived from chlorophyll function.
Light: Wave and Particle
Light behaves like both a wave and a particle. The distance between wave crests is measured in nanometers (nm). Violet light has a shorter wavelength, while red light has a longer wavelength.
Carotenoids: Red, Orange, and Yellow Pigments
Carotenoids are red, orange, or yellow pigments found in green leaves, often masked by chlorophyll. They can absorb light at different wavelengths than chlorophyll A. Carotenoids are long-chain hydrocarbons, insoluble in water but soluble in fat solvents. They are divided into two groups:
- Carotene: Unsaturated hydrocarbons.
- Xanthophyll: Oxygenated derivatives of unsaturated hydrocarbons.
Primary carotenoids participate in photosynthesis. Secondary carotenoids are found in flowers and fruits, as well as in chloroplasts and heterotrophic organisms like bacteria, yeasts, and fungi. In photosynthetic organisms, they can also be a product of poor mineral nutrition.
Phycocyanin and Phycoerythrin: Blue and Red Pigments
Phycocyanin (blue-green) and phycoerythrin (red-purple) are pigments found in blue-green algae (cyanobacteria) and red algae (Rhodophyceae). They are related to chlorophyll and carotenoids and belong to the tetrapyrrole family.
Differences Between Chlorophyll A and B
Chlorophyll A differs from Chlorophyll B by the substitution of a methyl group (-CH3) on carbon 3 in the second pyrrole ring with an aldehyde group (-CHO). This difference causes a change in coloration and the absorption spectrum of the molecule.
The absorption spectra of chlorophylls A and B show maximum absorption in the red and blue regions, while green and far-red light are poorly absorbed.
Photosynthesis: Location and Light Capture
The majority of photosynthesis takes place in the palisade parenchyma cells, elongated cells located below the upper epidermis in the mesophyll. These cells have large central vacuoles and numerous chloroplasts that move to optimize light exposure. Light is captured in the thylakoid membranes within the chloroplasts.
Chlorophyll B, carotenoids, and phycobilins absorb a wider range of light wavelengths and can transfer this energy to chlorophyll A, increasing the total amount of light captured.
Types of Pigments
- Chlorophyll: A, B, C, D, and E
- Carotenoids: Carotene and Xanthophyll
- Phycobilin: Phycocyanin and Phycoerythrin
Light Absorption by Pigments
- Chlorophyll A: Red and blue-violet
- Carotenoids: Red and blue
- Phycobilin: Orange, green, and blue
- Bacteriochlorophyll A: Far-red and infrared
Photosynthesis: Two Stages
Stage 1: Light-dependent reactions. This phase produces ATP from ADP and reduces NADP, initiating the electron transport chain.
Stage 2: Light-independent reactions (dark phase). ATP and NADPH, formed in stage 1, are used to reduce CO2 into simple carbohydrates. The chemical energy stored in ATP and NADPH is transferred to molecules specialized in transport and storage. This is known as carbon sequestration.
CAM Plants: Adaptation to Arid Environments
CAM plants (Crassulacean Acid Metabolism) are found in desert or semi-desert environments. They can withstand intense light and high temperatures with limited water availability. They are adapted to extreme arid conditions. Their photosynthetic tissue is homogeneous, without a distinct sheath, and they lack a lattice-like cloaca. They open their stomata at night and close them during the day. Carbon fixation and reduction require more ATP than in C3 and C4 plants. Their photosynthetic performance per unit of time is lower, resulting in slower growth. The mechanisms regulating the balance between transpiration and photosynthesis are directed to minimize water loss.
- Night: Stomata open, and CO2 fixation occurs via PEP carboxylase in the cytosol.
- Day: Stomata close, malic acid leaves the vacuole and is decarboxylated to pyruvate. This reaction releases CO2, which enters the chloroplasts to initiate the Calvin cycle.