Surface area

• The need for transport systems (part 1):

  • In order for any organism to function properly, it needs to exchange substances between itself and the 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.
• The need for transport systems (part 2):

  • Single-celled organisms (like amoeba) have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion.
  • The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products.
  • The small volume means the diffusion distance to all areas is short.
• The need for transport systems (part 3):

  • As organisms increase in size their SA:V ratio decreases.
  • There is less surface area for the absorption of nutrients and gases and secretion of waste products.
  • The greater volume results in a longer diffusion distance to the cells and tissues of the organism.
  • Large multicellular animals and plants have evolved adaptations to facilitate the exchange of substances between their environment.
  • They have a large variety of specialised cells, tissues, organs and systems.
  • Eg. gas exchange system, circulatory system, urinary system, xylem and phloem.
• As the size of an organism increases, it’s surface area : volume ratio decreases. Notice for this particular shape the distance between the surface and the centre increases with size:

  
• Multicellular organisms (part 1):

  • For larger, multicellular organisms the distance between the surface of the organism to its centre is relatively long.
  • This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise.
• Multicellular organisms (part 2):

  • Transport systems in animals include:
  • The blood and circulatory system - carries the necessary substances around the body.
  • 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 transport systems in plants and animals:

  
• The need for exchange surfaces (part 1):

  • Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes.
  • This is why larger organisms need exchange surfaces within their transport systems to carry out diffusion, osmosis and active transport at a sufficient rate.
• The need for exchange surfaces (part 2):

  • Exchange surfaces in animals include:
  • The lungs and alveoli for gas exchange.
  • The small intestines and villi for absorption of digested food.
  • Exchange surfaces in plants include:
  • Roots and root hairs where mineral ions and water are absorbed.
  • The leaves for gas exchange.
• Some examples of exchange surfaces in plants and animals:

  
• Properties of exchange surfaces (part 1):

  • Multicellular organisms have surfaces and organ systems that maximise the exchange of materials by increasing the efficiency of exchange in a number of ways:
  • Having a large surface area to increase the rate of transport.
  • A barrier that is as thin as possible to separate two regions, to provide as short a diffusion path as possible for substances to move across.
• Properties of exchange surfaces (part 2):

  • In addition, animals have:
  • A large network of blood vessels throughout the body:
  • To reduce the distance of exchange of materials between cells and the bloodstream.
  • To move substances towards or away from exchange surfaces to maintain concentration gradients.
  • Gas exchange surfaces that are well ventilated to maintain concentration gradients.
• Calculating the surface area to volume ratio:

  
• Surface area:

  • The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface.
  • Many cells which are adapted for diffusion have increased surface area in some way - e.g. root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion).
• The highly folded surface of the small intestine increases its surface area:

  
• Diffusion distance:

  • The smaller the distance molecules have to travel the faster transport will occur.
  • This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible.
• Concentration gradient:

  • The greater the difference in concentration on either side of the membrane, the faster movement across it will occur.
  • This is because on the side with the higher concentration, more random collisions against the membrane will occur.
• Temperature:

  • The higher the temperature, the faster molecules move as they have more energy.
  • This results in more collisions against the cell membrane and therefore a faster rate of movement across them.
• Summary of Diffusion Factors Table: