Introduction to Photosynthesis

Introduction to Photosynthesis

• Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water.
• It is a crucial process for life on Earth, as it produces oxygen and serves as the foundation of most food chains.
• Photosynthesis occurs in chloroplasts, organelles specifically adapted for this purpose.

Stages of Photosynthesis

• Photosynthesis is generally divided into two main stages: light-dependent reactions and light-independent reactions (also called the Calvin cycle).

Light-Dependent Reactions

• These reactions occur in the thylakoid membranes of chloroplasts.
• Light energy is absorbed by chlorophyll and other pigments, initiating a series of electron transfer reactions.
• Water is split, releasing oxygen as a byproduct.
• ATP and NADPH, energy-carrying molecules, are produced.
• The absorbed light energy is converted into chemical energy stored in ATP and NADPH.
• These high-energy molecules are then used in the light-independent reactions.

Light-Independent Reactions (Calvin Cycle)

• These reactions occur in the stroma of the chloroplasts.
• Carbon dioxide from the atmosphere is incorporated into an existing organic molecule, often a 5-carbon sugar.
• This process, called carbon fixation, is catalyzed by the enzyme RuBisCo.
• Energy from ATP and NADPH produced during the light-dependent reactions is used to convert the initial molecule into a simple sugar.
• This simple sugar is often a 3-carbon molecule (G3P), a crucial precursor for the synthesis of glucose and other carbohydrates.

Factors Affecting Photosynthesis

• Light Intensity: Increased light intensity generally increases the rate of photosynthesis until a maximum rate is reached. Beyond this point, further increases in light intensity do not affect photosynthesis.
• Carbon Dioxide Concentration: Higher carbon dioxide concentrations lead to an increase in the rate of photosynthesis until a maximum is reached.
• Temperature: Optimal temperature ranges are necessary for photosynthesis. Higher temperatures can damage enzymes and decrease the rate of photosynthesis.

Products of Photosynthesis

• Glucose (a simple sugar): Serves as an energy source for the plant and a building block for larger carbohydrates
• Oxygen: Released into the atmosphere as a byproduct. This is crucial for the respiration of many organisms.
• Water: A product of photosynthesis in the light-dependent reactions, playing roles in cellular processes.

Importance of Photosynthesis

• Oxygen Production: Photosynthesis is the primary source of atmospheric oxygen.
• Food Source: Photosynthesis forms the base of most food chains.
• Carbon Cycle: Photosynthesis plays a crucial role in the global carbon cycle, absorbing carbon dioxide from the atmosphere.
• Energy Storage: Converts light energy into usable chemical energy in the form of sugars.

C3, C4 and CAM Plants

• Different plant types have evolved different photosynthetic pathways.
• C3 plants are the most common type, but C4 and CAM plants have evolved adaptations to thrive in conditions where C3 photosynthesis is less efficient (e.g. hot, dry climates).
• C4 plants have a specialized photosynthetic pathway that minimizes water loss by concentrating CO2 around RuBisCo.
• CAM plants open their stomata at night to take in CO2, storing it for use during the day when photosynthesis occurs.

Chlorophyll and Accessory Pigments

• Chlorophyll a and b are the primary pigments absorbing light energy for photosynthesis.
• Accessory pigments, such as carotenoids, absorb light energy at different wavelengths and transfer the energy to chlorophyll. This broader spectrum of captured light increases the efficiency of photosynthesis.
• Chlorophyll absorbs mainly blue and red light, reflecting green light, giving plants their characteristic green color.