Lesson 7 Photosynthesis

Overview of Photosynthesis

Photosynthesis is the biochemical process by which green plants, algae, and certain bacteria transform light energy into chemical energy stored in the form of glucose. This essential process not only provides the primary energy source for all living organisms but also releases oxygen into the atmosphere, which is crucial for the survival of aerobic life forms.

Key Concepts

Cellular Respiration Review

• Process: Glucose (C6H12O6) + Oxygen → Carbon Dioxide + Water + Energy (ATP)

• Type: Exergonic (energy is released as a byproduct).

• Location: In eukaryotic cells, cellular respiration occurs in the mitochondria; for prokaryotes, the process takes place across the cell membrane.

• ATP Production: Aerobic respiration results in the production of significantly more ATP (30-38 ATP) compared to fermentation, which produces only 2 ATP.

Photosynthesis Definition

• Equation: Carbon Dioxide + Water + Light Energy → Glucose + Oxygen

• Type: Endergonic (energy is required to synthesize glucose from simpler molecules).

• Organelles Involved:

  • In eukaryotic cells, photosynthesis occurs in chloroplasts containing chlorophyll, which captures light energy.

  • In prokaryotic organisms, such as cyanobacteria, the reactions take place in the cytoplasm due to the absence of chloroplasts.

Roles of Producers and Consumers

• Producers (Autotrophs): Organisms, primarily plants and some algae, that synthesize their own food using light energy (photoautotrophs) or chemical energy (chemoautotrophs).

• Consumers (Heterotrophs): Organisms that rely on consuming other living entities (plants, animals) to obtain their energy. They cannot produce their own food.

• Decomposers: Fungi and bacteria that break down dead organic matter, recycling nutrients back into the ecosystem and enriching the soil.

Light Energy & Photosynthesis

• Photon: The basic unit of light energy absorbed by chlorophyll during photosynthesis, initiating the conversion of light energy into chemical energy.

• UV Radiation: The ultraviolet spectrum comprises three types - UVA, UVB, and UVC; while UVC is absorbed by the ozone layer, UVA contributes to skin aging and increases the risk of skin cancer.

Plant Structure Related to Photosynthesis

• Stomata: Tiny openings on the leaf surface that facilitate gas exchange; allowing carbon dioxide to enter and oxygen to exit during photosynthesis.

• Chloroplasts Structure:

  • Outer and Inner Membranes: Protective layers surrounding the chloroplast.

  • Thylakoids: Membrane-bound sacs where the light-dependent reactions transpire; these are stacked in structures known as grana.

  • Stroma: A semi-fluid surrounding thylakoids where light-independent reactions (Calvin Cycle) take place.

Photosynthesis Phases

1. Light-Dependent Reactions:

   • Capture light energy using chlorophyll.

   • Convert this energy into chemical energy in the form of ATP and NADPH.

   • Water molecules are split (photolysis), releasing electrons for energy production and oxygen as a byproduct.

2. Light-Independent Reactions (Calvin Cycle):

   • Utilize ATP and NADPH generated from light-dependent reactions to convert carbon dioxide into glucose.

   • Carbon fixation occurs through the enzyme Rubisco, which catalyzes the reaction between carbon dioxide and ribulose bisphosphate.

Energy Relationships

• Both cellular respiration and photosynthesis feature electron transport chains that play a pivotal role in ATP production.

• Light-Dependent Reaction Products: Produce ATP and NADPH that are essential for the Calvin Cycle.

• Calvin Cycle Requirements: Needs ATP and NADPH generated during light-dependent reactions to drive the formation of glucose.

Important Terms

• Carbon Fixation: The process of converting inorganic carbon (CO2) into organic compounds during the Calvin Cycle, critical for forming sugars.

• Rubisco: The prominent enzyme in photosynthesis that catalyzes the initial reaction of the Calvin Cycle. It is crucial for integrating CO2 into organic compounds.

• G3P (Glyceraldehyde 3 Phosphate): A three-carbon sugar produced in the Calvin Cycle; it serves as a building block for glucose and other carbohydrates.

Environmental Impact

• Increasing CO2 levels driven by human activities, such as fossil fuel combustion and deforestation, are major contributors to climate change.

• Plants, especially trees, play a vital role in sequestering CO2 from the atmosphere, and their removal exacerbates environmental issues such as global warming and habitat loss.