3.1.4 SA

Ratio & Metabolic Rate

Key Concepts:

1. Surface Area to Volume Ratio (SA
   ):

• Small organisms have a large SA
  ratio, allowing efficient exchange of gases and nutrients across their surface.
• Large organisms have a small SA
  ratio, making diffusion alone insufficient to meet their metabolic demands.

2. Metabolic Rate:

• Refers to the rate of energy expenditure in an organism.
• Organisms with higher metabolic rates require more oxygen for respiration and produce more carbon dioxide, necessitating efficient gas exchange systems.

SA

Ratio and Size:

1. Small Organisms:

• High SA
  ratio means substances like oxygen and nutrients can diffuse quickly to meet the organism's needs.
• Example: Single-celled organisms rely entirely on diffusion for gas exchange.

2. Large Organisms:

• Low SA
  ratio makes diffusion inefficient over long distances.
• They develop specialised exchange surfaces and transport systems to meet their metabolic demands.
• Example: Mammals have lungs and a circulatory system to deliver oxygen to cells efficiently.

SA

Ratio and Heat Exchange:

1. Heat Loss:

• A high SA
  ratio increases heat loss, making small organisms more susceptible to temperature fluctuations.
• Larger organisms lose heat more slowly due to their smaller SA
  ratio, helping them retain body heat.

2. Adaptations:

• Small animals in cold climates (e.g., shrews) have high metabolic rates to generate enough heat.
• Large animals in hot climates (e.g., elephants) have adaptations like large ears to increase surface area for heat loss.

Metabolic Rate and Gas Exchange:

1. High Metabolic Rate:

• Organisms with high metabolic rates need efficient gas exchange systems to supply oxygen and remove carbon dioxide quickly.
• Example: Birds and mammals have high metabolic rates and require large lungs with extensive surface areas (e.g., alveoli) to support their high energy demands.

2. Low Metabolic Rate:

• Organisms with lower metabolic rates have less oxygen demand and produce less waste, reducing their reliance on specialised exchange systems.
• Example: Reptiles and amphibians have simpler lungs or rely partially on their skin for gas exchange.

Examples of Adaptations:

1. Elephants:

• Large ears increase surface area to lose heat efficiently.

2. Polar Bears:

• Small surface area relative to volume reduces heat loss.

3. Desert Animals:

• High SA
  ratio allows faster heat dissipation, preventing overheating.

4. Small Mammals (e.g., Mice):

• High SA
  ratio increases heat loss, necessitating a higher metabolic rate to generate heat.

SA

Ratio and Water Loss:

• Organisms with a high SA
  ratio lose water quickly through evaporation.
• Small desert animals often have adaptations to conserve water, such as producing concentrated urine.

Key Adaptations to Balance SA

and Metabolic Rate:

1. Exchange Surfaces:

• Large surface area (e.g., alveoli in lungs).
• Thin barriers for short diffusion distances.

2. Transport Systems:

• Blood circulation in mammals ensures efficient oxygen and nutrient delivery.

3. Behavioural Adaptations:

• Small animals may burrow during the day to minimise heat loss or gain.