Exchange substances official content revision notes

Understanding Exchange Substances

Size and Exchange Requirements

• Larger organisms have a lower surface area to volume ratio, leading to increased demands for exchange of substances.
• Smaller organisms can rely on diffusion alone due to their high surface area to volume ratio.

Surface Area to Volume Ratio

• The surface area to volume ratio decreases as the size of an organism increases. This affects the efficiency of gas exchange and nutrient uptake.
• Calculation: Surface area = length x width x number of sides; Volume = length x width x height.

Specialized Exchange Surfaces

• Adaptations include:
• Thin walls to reduce diffusion distance.
• Large surface area to increase exchange rate.
• Moist surfaces to facilitate gas exchange.

Multicellularity and Mass Transport Systems

• Multicellular organisms require specialized transport systems (e.g., circulatory system) to efficiently move substances over longer distances.

Gas Exchange in Insects

• Insects use a tracheal system for gas exchange, with spiracles allowing air to enter and exit.
• Adaptations include a network of tubes that deliver oxygen directly to tissues.

Counter-Current Gas Exchange in Fish Gills

• Fish gills utilize a counter-current exchange mechanism to maximize oxygen absorption from water.
• Water flows over gill filaments in one direction while blood flows in the opposite direction, maintaining a concentration gradient.

Plant Gas Exchange

• Stomata on leaves regulate gas exchange, opening and closing to minimize water loss while allowing CO2 in for photosynthesis.
• Leaf adaptations include a large surface area and a thin structure to facilitate gas exchange.

Mammalian Gas Exchange

• Alveoli in the lungs provide a large surface area for gas exchange, with thin walls for efficient diffusion.
• Ventilation mechanisms (inhalation and exhalation) maintain concentration gradients.

Digestion and Absorption

• Digestive enzymes hydrolyze carbohydrates, lipids, and proteins into absorbable units.
• The ileum and villi are adapted for absorption, featuring a large surface area and a rich blood supply.
• Co-transport mechanisms facilitate the absorption of glucose and amino acids.

Haemoglobin and Oxygen Transport

• Haemoglobin's quaternary structure allows it to bind oxygen efficiently.
• Oxygen dissociation curves illustrate how haemoglobin's affinity for oxygen changes with pH and CO2 levels (Bohr effect).

Circulatory System in Mammals

• The heart's structure supports its function in pumping blood, with valves ensuring unidirectional flow.
• Blood vessels (arteries, veins, capillaries) are adapted to their roles in circulation.

Plant Transport Mechanisms

• Xylem transports water and minerals via cohesion-tension theory, while phloem transports assimilates through mass flow principles.
• Evidence for these mechanisms includes experiments and observations of plant behavior under different conditions.