Exchanging Substances

• Small organisms can use diffusion through their surface to exchange substances with enviroment
• Large multicellular organisms need specialised exchange systems to exchange substances with the environment. They also need specialised transport systems to transport substances around the body.
• • Single-celled organisms have to carry out chemical reactions (building up and breaking down molecules) continuously to survive.
  • To do this it needs to absorb resources, like oxygen, glucose, and amino acids
  • It also needs to get rid of waste, like carbon dioxide.
  • How well it can do this depends on its surface area to volume ratio
• As organisms get larger, their surface area to volume ratio decreases
• humans have a low surface area to volume ratio
• Lungs and intestines increase our surface area to volume ratio by giving us extra surface on the inside
• Lungs have millions of alveoli which have a huge surface area, these are used to absorb oxygen and get rid of carbon dioxide.
• Intestines have villi which increase the surface area for absorption of nutrients
• as organisms get larger the distance that molecules would have to diffuse to get from the outside of their body to the inside of their body also gets larger
• a larger diffusion distance means diffusion will be slower
• larger organisms have transport systems that transport molecules around the body to whatever cells need them. This means the molecules only have to diffuse a every short distance to get into the cell.
• Plants have roots and leaves to exchange substances withe environment, and phloem and xylem tissues to transport substances around the plants.
• The alveoli are small air sacs in the lungs that allow for gas exchange between the air and the blood.
• The alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide, they have:

  • An enormous surface area (75m^2)
  • A moist lining for dissolving gasses
  • Very thin walls
  • A good bloody supply
• Label the diagram

  A) red blood cells in the capillary

  B) deoxygenated blood in

  C) air out

  D) air in

  E) bronchiole

  F) oxygenated blood out

  G) alveoli

  H) oxygen in

  I) carbon dioxide out

  
• Label the diagram

  A) Air breathed in and out

  B) blood from pulmonary artery

  C) Network of capillaries

  D) cluster of alveoli at each end of the bronchiole

  E) blood to pulmonary vein

  F) bronchiole

  
• • Inside the small intestine it is covered in millions of tiny projections called villi
  • They increase the surface area so that digested food is absorbed faster into the blood
• Villi have a single layer of surface cells and a very good blood supply to assist quick absorption
• Label the diagram

  A) villus

  B) another villus

  C) Network of capillaries

  D) circular muscle

  E) longitudinal muscle

  F) gland cells

  
• good blood flow maintains a steep concentration gradient between the oxygen (and carbon dioxide) in the alveoli and the blood so that the rate of diffusion is faster.
• • Carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leafs structure is adapted so this can happen easily
• • The underneath of the leaf is an exchange surface. It's covered in little holes called stomata which the carbon dioxide diffuses through.
  • Oxygen and water vapour also diffuse out through the stomata. Water vapour is lost from all over the leaf's surface, but most is lost through the stomata.
• • The size of the stomata is controlled by guard cells
  • These close the stomata if the plant is losing water faster than it's being replaced by the roots.
  • Without guard cells, the plant would wilt.
• • The flattened shape of the leaf increases the area of the exchange surface so that it is more effective
• • The walls of the cells inside a lead forms another exchange surface.
  • The air spaces inside the lead increase the area of this surface so there's no more chance for carbon dioxide to get into the cells
• The water vapour evaporates from the cells inside the leaf. Then it escapes by diffusion because there is a lot of it inside the leaf and less of it on the outside.
• Label the diagram

  A) upper epidermis

  B) palisade mesophyll

  C) spongy mesophyll

  D) guard cells with chloroplasts

  E) lower epidermis

  F) exchange of gasses through stomata

  G) guard cells with chloroplasts

  H) phloem

  I) air space

  J) xylem

  K) waxy cuticle

  
• • Gills are the gas exchange surface in a fish
  • Water enters the fish through its mouth and passes out through the gills.
  • As this happens, oxygen diffuses from the water into the blood in the gills and carbon dioxide diffuses from the blood into the water.
• Each gill is made of lots of thin plates called gill filaments, which give a big surface area for exchange of gasses.
• The gill filaments are covered in lots of tiny structures called lamellae, which increases the surface area even more.
• The lamellae have lots of blood capillaries to speed up diffusion.
• Lamellae also have a thin surface layer of cells to minimise the distance that the gases have to diffuse.
• Blood flows through the lamellae in one direction and water flows in the opposite direction. This maintains a large concentration gradient between the water and the blood.
• The concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood.
• Label the diagram

  A) part of gill filament

  B) water

  C) gill filaments

  D) deoxygenated blood

  E) oxygenated blood

  
• Root hair cells absorb water and mineral ions from the soil.
• Common features in exchange surfaces:

  • Large surface area, Lots of molecules can diffuse across at the same time
  • Surfaces are very thin, Substances only have to diffuse a short distance
  • Surfaces are permeable,  Substances are able to pass through the surface
  • Good blood supply (good supply of 'internal medium'), Maintains a strong concentration gradient as blood is constantly replaced
  • Good supply of 'external medium', Maintains a strong concentration gradient as the air or food etc is constantly replaced