Photosynthesis: Mechanisms and Significance in Plants

Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the process by which autotrophic organisms, such as plants, algae, and some bacteria, convert sunlight into chemical energy stored in carbohydrates. This process is fundamental to life on Earth, as it provides the organic molecules and oxygen required by most living organisms.

• Autotrophs ("self-feeders"): Organisms that produce their own food from inorganic substances using light or chemical energy.

• Heterotrophs ("different-feeders"): Organisms that obtain their food by consuming other organisms.

10.1 Photosynthesis Harnesses Sunlight to Make Carbohydrates

Photosynthesis converts electromagnetic energy (sunlight) into chemical energy (carbohydrates). The overall process requires sunlight, carbon dioxide (CO2), and water (H2O), and produces oxygen (O2) as a by-product.

• Overall reaction:

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

• This reaction is the reverse of cellular respiration.

Photosynthesis: Two Linked Sets of Reactions

Photosynthesis consists of two main sets of reactions:

• Light-capturing reactions: Occur in the thylakoid membranes, where sunlight is absorbed, water is split, and ATP and NADPH are produced.

• Calvin cycle reactions: Occur in the stroma, where ATP and NADPH are used to reduce CO2 to carbohydrates.

Photosynthesis Occurs in Chloroplasts

Chloroplast Structure

Photosynthesis takes place in chloroplasts, which are organelles found in plant and algal cells. Chloroplasts have a double membrane and contain internal structures called thylakoids, which are stacked into grana. The fluid surrounding the thylakoids is the stroma.

• Thylakoid: Flattened, vesicle-like structure where light reactions occur.

• Grana: Stacks of thylakoids.

• Stroma: Fluid-filled space between thylakoids and inner membrane; site of the Calvin cycle.

Pigments in Photosynthesis

Thylakoid membranes contain pigments that absorb specific wavelengths of light. The most common pigment is chlorophyll, which reflects green light, giving plants their color.

• Chlorophylls (a and b): Absorb red and blue light; reflect green.

• Carotenoids: Absorb blue and green light; reflect yellow, orange, and red.

Absorption of Light by Pigments

Different pigments absorb different wavelengths of light, which is crucial for maximizing the energy captured from sunlight. Chlorophylls have a long isoprenoid tail and a "head" with a magnesium atom, where light absorption occurs.

Role of Accessory Pigments

Carotenoids and xanthophylls are accessory pigments that extend the range of light absorption and protect chlorophyll from damage by stabilizing free radicals.

Photosystems and Light Energy Conversion

Photosystems

Chlorophyll molecules and accessory pigments are organized into photosystems within the thylakoid membrane. Each photosystem contains 200–300 pigment molecules that function as antenna pigments, funneling energy to a central reaction center.

Electron Transport Chain (ETC)

The thylakoid electron transport chain (ETC) is similar to the mitochondrial ETC. Redox reactions transport protons across the membrane, creating a proton-motive force that drives ATP synthesis via ATP synthase.

• Photophosphorylation: The process of using light energy to produce ATP in chloroplasts.

$\text{ADP} + \text{P}_i \xrightarrow{\text{light energy}} \text{ATP}$

How Do Photosystems Work?

• Electrons from Photosystem II produce a proton-motive force for ATP synthesis.

• Electrons from Photosystem I reduce NADP+ to NADPH.

• Noncyclic electron flow: Electrons move linearly from water to NADP+, producing both ATP and NADPH.

• Cyclic electron flow: Electrons cycle back to the ETC, producing additional ATP but not NADPH.

Carbon Fixation and the Calvin Cycle

Stomata and Gas Exchange

Plants are covered with a waxy cuticle that prevents water loss but also restricts gas exchange. Stomata (pores formed by pairs of guard cells) allow CO2 to enter and O2 to exit the leaf.

The Calvin Cycle

The Calvin cycle is the set of reactions that fix carbon dioxide into organic molecules. The enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the addition of CO2 to ribulose bisphosphate (RuBP), forming two molecules of 3-phosphoglycerate (3-PGA).

• Carbon fixation: Incorporation of inorganic CO2 into organic molecules.

• Rubisco: Most abundant enzyme on Earth; catalyzes the first step of the Calvin cycle.

Photorespiration

When Rubisco reacts with O2 instead of CO2, a process called photorespiration occurs, which consumes oxygen and releases fixed carbon, reducing the efficiency of photosynthesis.

Mechanisms for Increasing CO2 Concentration

In hot, dry conditions, plants may close their stomata to conserve water, which limits CO2 uptake and increases photorespiration. Some plants have evolved mechanisms to increase CO2 concentration near Rubisco:

• C4 pathway: Initial carbon fixation occurs in one cell type, and the Calvin cycle occurs in another, spatially separating the processes to increase CO2 concentration.

• CAM pathway (Crassulacean Acid Metabolism): Carbon fixation occurs at night, and the Calvin cycle occurs during the day, temporally separating the processes to conserve water.

Fate of Photosynthetic Products

Utilization of G3P

The Calvin cycle produces glyceraldehyde-3-phosphate (G3P), which can be used to synthesize glucose and fructose (via gluconeogenesis). These sugars can be combined to form sucrose or polymerized into starch for storage.

• Starch: Stored in chloroplasts.

• Sucrose: Synthesized in the cytosol and transported throughout the plant.

Virtually all organic carbon in living organisms can be traced back to photosynthesis.