Exchange Surfaces in Biology

Exchange Surfaces

• Exchange surfaces are specialized areas in organisms that facilitate the transfer of substances between the organism and its environment.
• Examples include gills in fish, lungs in mammals, and leaves in plants.
• Efficient exchange surfaces have a large surface area to volume ratio, thin membranes for short diffusion distances, and a transport system to move substances away from the exchange surface.
• The rate of exchange is affected by the surface area available, the thickness of the exchange surface, and the concentration gradient between the two sides of the surface.
• Efficient exchange surfaces often have adaptations such as folded membranes or numerous small projections to increase the surface area, and high blood flow rates in animals helps maintain steep concentration gradients.
• The concentration gradient is the difference in concentration of a substance between two areas. A steeper gradient leads to faster diffusion.

Photosynthesis

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose.

• The overall reaction is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

• Photosynthesis takes place in chloroplasts, organelles containing chlorophyll, a green pigment that absorbs light energy.

• Photosynthesis occurs in two main stages:

  • Light-dependent reactions: Light energy is absorbed by chlorophyll and used to split water (photolysis) into oxygen, hydrogen ions, and electrons. ATP and NADPH are produced, both carrying energy.
  • Light-independent reactions (Calvin cycle): The ATP and NADPH produced in the light-dependent reactions provide the energy to combine carbon dioxide with hydrogen to form glucose.

• Factors affecting photosynthesis include light intensity, carbon dioxide concentration, and temperature.

Respiration

• Respiration is a set of metabolic processes that break down nutrients to release energy in the form of ATP (adenosine triphosphate), the cell's main energy currency.
• Respiration occurs in both plants and animals.
• Respiration can be aerobic (using oxygen) or anaerobic (without oxygen).
• Aerobic respiration involves a series of reactions:
  • Glycolysis: A glucose molecule is broken down into two pyruvate molecules. A small amount of ATP is produced.
  • Link reaction: Pyruvate is converted to Acetyl CoA.
  • Krebs cycle (Citric Acid Cycle): Acetyl CoA enters a cycle, producing further ATP, NADH, and FADH₂.
  • Electron transport chain: NADH and FADH₂ release electrons in a series of reactions, pumping protons to generate a proton gradient. The gradient drives ATP synthesis. Oxygen is the final electron acceptor.
• The overall equation for aerobic respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP).
• Anaerobic respiration (fermentation) produces less ATP, and generates byproducts such as lactic acid (in animals) or ethanol and carbon dioxide (in yeast).
• The rate of respiration is affected by factors such as temperature, substrate availability, and oxygen levels.