Photosynthesis

Definition and Importance

Photosynthesis is the process by which photosynthetic organisms use sunlight to manufacture carbohydrates from inorganic molecules. This process is fundamental to life on Earth, as it provides the primary source of energy and organic matter for most living organisms.

• Autotrophs ("self-feeders"): Organisms that produce their own food from ions and molecules using photosynthesis.

• Heterotrophs ("different-feeders"): Organisms that must obtain sugars and organic molecules from other organisms.

Photosynthesis Harnesses Light to Make Carbohydrates

Conversion of Energy

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

• Overall reaction:

• Cellular respiration (opposite reaction):

Electron Transfer in Photosynthesis

Redox Reactions

During photosynthesis, electron transfer reactions reduce carbon dioxide and oxidize water, resulting in the formation of carbohydrates and oxygen. Electrons are transferred from water to carbon dioxide, increasing the potential energy of the products.

• Reduction: Gain of electrons (CO2 to carbohydrate)

• Oxidation: Loss of electrons (H2O to O2)

Components of Photosynthesis

Linked Reactions

Photosynthesis consists of two major sets of reactions:

• Light-capturing reactions: Occur in the thylakoid membranes, use sunlight to produce ATP and NADPH, and release O2 from H2O.

• Calvin cycle: Occurs in the stroma, uses ATP and NADPH to fix CO2 and produce sugars.

Diagram: Sunlight → Light-capturing reactions → ATP/NADPH → Calvin cycle → Sugar

Chloroplast Structure

Organization and Function

Photosynthesis occurs in chloroplasts, which are specialized organelles found in plant and algal cells.

• Outer and inner membranes: Enclose the chloroplast.

• Thylakoids: Flattened, vesicle-like structures where light-capturing reactions occur.

• Grana: Stacks of interconnected thylakoids.

• Lumen: Space inside a thylakoid.

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

Pigments and Light Absorption

Types and Functions

Thylakoid membranes contain pigments that absorb specific wavelengths of light.

• Chlorophylls (a and b): Absorb red and blue light; reflect green light, giving plants their color.

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

Pigments appear colored because they reflect or transmit wavelengths not absorbed.

Light as Electromagnetic Radiation

Light behaves both as a wave and as particles called photons. The energy of a photon is inversely proportional to its wavelength; shorter wavelengths have more energy.

Absorption Spectrum

The absorption spectrum measures how the wavelength of photons influences the amount of light absorbed by a pigment. Chlorophylls are most effective at absorbing blue and red light, which drives photosynthesis.

Pigment Structure

• Isoprenoid tail: Interacts with proteins in the thylakoid membrane.

• Head (porphyrin ring): Contains a magnesium atom; site of light absorption.

Accessory Pigments

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

Carbon Dioxide Capture

Stomata and Gas Exchange

Plants are covered with a waxy cuticle that prevents water loss but also restricts gas exchange. Stomata are pores formed by two guard cells that allow CO2 to diffuse into photosynthetic tissues and O2 to diffuse out.

The Calvin Cycle

Carbon Fixation and Sugar Production

The Calvin cycle is the set of reactions that fix carbon dioxide and produce sugars. It occurs in the stroma of the chloroplast and consists of three phases:

1. Fixation phase: CO2 reacts with ribulose bisphosphate (RuBP) to produce two molecules of 3-phosphoglycerate (3PGA).

2. Reduction phase: 3PGA is phosphorylated by ATP and reduced by NADPH to produce glyceraldehyde-3-phosphate (G3P).

3. Regeneration phase: Some G3P is used to regenerate RuBP, allowing the cycle to continue.

One turn of the Calvin cycle fixes one molecule of CO2.

Role of Rubisco

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that catalyzes the fixation of CO2 to RuBP. It is the most abundant enzyme in leaves and is found in all photosynthetic organisms that use the Calvin cycle.

• Rubisco is slow and inefficient because it can catalyze the addition of either CO2 or O2 to RuBP.

• CO2 and O2 compete at Rubisco's active site, which can slow the rate of CO2 reduction.

Reactions catalyzed by Rubisco:

• With CO2:

• With O2 (photorespiration):

Regulation of Photosynthesis

Environmental and Cellular Control

• Light triggers production of photosynthetic proteins.

• High sugar levels inhibit production of photosynthetic proteins and stimulate production of proteins that process and store sugars.

• Rubisco is activated by regulatory molecules when light is available and inhibited when CO2 is low (favoring photorespiration).

Fate of Photosynthetic Sugars

Glucose, Sucrose, and Starch

G3P molecules produced by the Calvin cycle are used to make glucose and fructose via gluconeogenesis, which can be combined to form sucrose. When sucrose is abundant, glucose is polymerized to form starch.

• Starch production: Occurs inside the chloroplast.

• Sucrose synthesis: Takes place in the cytosol.

Virtually every carbon present in organic molecules can be traced back to photosynthesis.

Table: Comparison of Photosynthetic Pigments