2.2.1 Surface Area: Volume Ratio & Transport

• 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
• 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
• 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

• 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
• 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

• 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
• 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

• 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
• 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 surface area to volume ratios

• You should be able to calculate and compare surface area to volume ratios
• You can model the effect of how increasing size affects surface area to volume ratio using simple cubes:

Calculating the surface area to volume ratio

• The rate of diffusion can be described using Fick's Law:

Rate of diffusion ∝ (surface area x concentration gradient) ÷ diffusion distance

• "∝" means proportional to
• According to the law, if the surface area or concentration gradient doubles, or the diffusion distance halves, then the rate of diffusion will double
• Fick's Law governs the evolution of transport systems so that they maximise the rate of diffusion

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