Gas exchange in small animals

• Why is diffusion over the body surface of an amoeba sufficient enough to meet its gas exchange needs?

  Amoeba are unicellular organisms and need little oxygen. The cell membrane has a large surface area to volume ratio, it is thin (short diffusion pathway), and moist.
• What are the issues for gas exchange in multicellular organisms?

  -the larger the organism, the smaller the surface area to volume ratio

  -substances need to be exchanged between different organs, as well as between the organs and the environment

  -diffusion through the cell surface is insufficient to meet the needs of the organism, as the diffusion pathway is longer
• Why do simple/unicellular organisms not have a specialised gas exchange surface?

  -they have a large surface area to volume ratio

  -it is a short distance to the centre of the cell, so diffusion paths are short

  -so they can gain sufficient oxygen and glucose fast enough by diffusion across their cell membranes to sustain their slow metabolism
• How are flatworms adapted for efficient gas exchange?

  Flattened shape -> increases surface area to volume ratio -> short diffusion pathway
• How are earthworms adapted for efficient gas exchange?

  -elongated shape -> large surface area to volume ratio

  -keeps its skin moist by secreting mucus onto the surface

  -closed circulatory system

  -oxygen diffuses through the body surface into capillaries
• Why do larger multicellular organisms need specialised gas exchange surfaces?

  -they have a higher metabolic rate -> need more energy -> higher oxygen requirement

  -tissues and organs are more dependent on one another

  -they are larger, therefore have a smaller surface area to volume ratio
• Why do earthworms require a closed circulatory system for gas exchange?

  -the diffusion distance from the centre to the surface is too large for diffusion alone to supply oxygen at a sufficient rate to sustain its metabolic requirements
• How do earthworms maintain a concentration gradient?

  -they have a closed circulatory system

  -blood vessels are close to the external surface so that diffusion path from the surface to the blood is short

  -blood contains haemoglobin which has a high affinity for oxygen, and transports it to the tissues