Edexcel IGCSE Biology 复习笔记 2.8.1 The Requirement for Transport Systems

• In order for any organism to function properly, it needs to exchange substances, such as food molecules and waste products, between itself and its environment
• This exchange of substances occurs across the cell membrane
• There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport
• Unicellular (single-celled) organisms like amoeba have very large surface areas (SA) in comparison to their volumes
  • This means that the distance between the surface of the organism to its centre is very small

   

• As a result, unicellular organisms do not need to have specialist exchange surfaces or transport systems; as diffusion, osmosis and active transport through the cell membrane occur at a sufficient rate to meet the organisms needs

Unicellular organisms such as amoeba do not require transport systems due to their large surface area to volume ratio

• Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes
• This means that the distance between the surface of the organism to its centre is relatively long
• As diffusion, osmosis and active transport cannot occur at a sufficient rate to meet the needs of the organism, larger organisms usually have exchange surfaces and transport systems
• Exchange surfaces in animals include:
  • The lungs and alveoli for gas exchange
  • The small intestines and villi for absorption of digested food

   

• Transport systems in animals include
  • The blood and circulatory system carries the necessary substances around the body

   

• Exchange surfaces in plants include:
  • Roots and root hairs where mineral ions and water are absorbed
  • The leaves for gas exchange

   

• Transport systems in plants include:
  • The xylem moves water and mineral ions from roots to shoots
  • The phloem moves sugars and amino acids to where they are needed in the plant

   

Some examples of exchange surfaces in plants and animals

Some examples of transport systems in plants and animals

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