Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the biological process by which light energy from the sun is converted into chemical energy in the form of carbohydrates. This process is fundamental to life on Earth, as it provides the organic molecules and oxygen required by most organisms.

• Photosynthesis: The use of sunlight to manufacture carbohydrates from carbon dioxide and water.

• Autotrophs ("self-feeders"): Organisms, such as plants and algae, that produce their own food from inorganic substances.

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

Photosynthesis Harnesses Light to Make Carbohydrates

Overview of the Photosynthetic Process

Photosynthesis converts electromagnetic energy into chemical energy, requiring sunlight, carbon dioxide, and water, and producing oxygen as a by-product. The overall chemical reaction is the reverse of cellular respiration.

• Overall reaction of photosynthesis:

• Overall reaction of cellular respiration (opposite):

Redox Reactions in Photosynthesis

During photosynthesis, electrons are transferred, reducing carbon dioxide to carbohydrate and oxidizing water to oxygen. This transfer increases the potential energy stored in the products.

Components and Structure of Photosynthesis

Two Linked Stages of Photosynthesis

Photosynthesis consists of two main stages: the light-capturing reactions and the Calvin cycle.

• Light-capturing reactions: Use sunlight to produce ATP and NADPH, releasing O2 as a by-product.

• Calvin cycle: Uses ATP and NADPH to reduce CO2 and synthesize sugars.

Diagram: Two Linked Components

• Sunlight (light energy) is absorbed by chlorophyll in the chloroplasts.

• Light-capturing reactions occur in the thylakoid membranes, producing ATP and NADPH.

• The Calvin cycle occurs in the stroma, using ATP and NADPH to fix CO2 into sugars.

Chloroplast Structure and Function

Chloroplast Anatomy

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

• Chloroplasts: Surrounded by double membranes.

• Thylakoids: Flattened, vesicle-like structures inside the chloroplast; site of the light-capturing reactions.

• Grana: Stacks of thylakoids.

• Lumen: The internal space within a thylakoid.

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

Pigments and Light Absorption

Role of Pigments in Photosynthesis

Pigments are molecules that absorb specific wavelengths of light. The most common pigment in thylakoids is chlorophyll, which reflects green light and is responsible for the green color of plants.

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

• Carotenoids: Accessory pigments that absorb blue and green light; reflect yellow, orange, and red light.

Light as Electromagnetic Radiation

Light behaves both as a wave and as a particle (photon). The energy of a photon is inversely proportional to its wavelength; shorter wavelengths have more energy.

• Visible light: The portion of the electromagnetic spectrum that humans can see and that drives photosynthesis.

Absorption Spectrum and Action Spectrum

The absorption spectrum shows which wavelengths are absorbed by pigments, while the action spectrum shows which wavelengths are most effective at driving photosynthesis.

• Chlorophylls absorb mainly red and blue wavelengths.

• Carotenoids extend the range of light absorption and protect chlorophyll from damage.

Structure of Chlorophyll

• Isoprenoid tail: Anchors the molecule in the thylakoid membrane.

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

Carbon Dioxide Capture and Fixation

Stomata and Gas Exchange

Plants are covered with a waxy cuticle that prevents water loss but also restricts gas exchange. Specialized pores called stomata, formed by two guard cells, allow CO2 to enter and O2 to exit the leaf.

The Calvin Cycle and Carbon Fixation

The Calvin cycle is the set of light-independent reactions that fix carbon dioxide into organic molecules. The process occurs in the stroma of the chloroplast.

• Carbon fixation: The incorporation of CO2 into an organic compound, catalyzed by the enzyme Rubisco.

• Ribulose bisphosphate (RuBP): The five-carbon compound that reacts with CO2 in the first step of the Calvin cycle.

Rubisco: The Key Enzyme

• Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase): The most abundant enzyme on Earth; catalyzes the fixation of CO2 to RuBP.

• Rubisco is slow and can also react with O2, leading to photorespiration, which reduces the efficiency of photosynthesis.

Phases of the Calvin Cycle

The Calvin cycle consists of three main phases:

1. Fixation: CO2 reacts with RuBP to form two molecules of 3-phosphoglycerate (3PGA).

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

3. Regeneration: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

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

Regulation and Fate of Photosynthetic Products

Regulation of Photosynthesis

• Light triggers the production of photosynthetic proteins.

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

• Rubisco is activated by regulatory molecules in the presence of light and inhibited when CO2 is low (favoring photorespiration).

Fate of G3P and Sugar Synthesis

• G3P produced by the Calvin cycle is used to make glucose and fructose via gluconeogenesis.

• Glucose and fructose can be combined to form sucrose (in the cytosol) or polymerized into starch (in the chloroplast).

• Starch serves as a storage form of carbohydrate in plants.

Note: Nearly all organic carbon in living organisms can be traced back to photosynthesis.

Summary Table: Key Components of Photosynthesis