Exchanging Substances

Question 1

What do organisms exchange substances with, give examples

Answer 1

• organisms exchange substances with their environment
• cells use diffusion to take in substances they need and get rid of waste products
  E.g.
• Oxygen and carbon dioxide are transferred between cells and the environment during gas exchange
• IN humans, urea (waste product from breakdown of proteins) diffuses from cells into the blood plasma for removal from the body by the kidneys

Question 2

What controls how easy it is for an organism to exchange substances with it’s environment

Answer 2

The organism’s surface are to volume ratio

Question 3

How do gases and dissolved substances diffuse in single-called organisms

Answer 3

• They diffuse directly into (or out of) the cell across the cell membrane.
• they have a large SA compared to their volume
• so enough substances can be exchanged across the membrane to supply the volume of the cell

Question 4

How do gases and dissolved substances diffuse in multicellular organisms

Answer 4

• multicellular organisms have a smaller SA compared to their volume
• so not enough substances can diffuse from their outside surface to supply their entire volume
• This means they need some sort of exchange surface for efficient diffusion
• the exchange surface structures have to allow enough of the necessary substances to pass through

Question 5

How are exchange surfaces adapted to maximise effectiveness

Answer 5

• thin membrane —> substances only have a short distance to diffuse
• large SA —> lots of a substance can diffuse at once
• Exchange surfaces in animals have lots of blood vessels —> get stuff into and out of the blood quickly

Question 6

Explain how gas exchange happens in the lungs

Answer 6

• the job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it
• the lungs contain millions of air sacs called alveoli where gas exchange takes place

Question 7

How are the alveoli specialised to maximise the diffusion of oxygen and carbon dioxide

Answer 7

• A large SA (about 75m^2 in humans)
• A moist lining for dissolving gases
• Very thin walls
• A good blood supply

Question 8

How do the Villi provide a really big SA

Answer 8

• they’re found in the small intestine
• They increase the SA in a big way so that’s digested food is absorbed much more quickly into the blood
• They have a single layer of surface cells
• they have a very good blood supply to assist quick absorption

Question 9

How does the structure of leaves let gases diffuse in and out cells

Answer 9

• CO2 diffuses into the air spaces within the leaf, then is diffuses into the cells where photosynthesis happens.
• The underneath of the leaf is an exchange surface. It’s covered in little holes called stomata where CO2 diffuses through
• Oxygen and water vapour also diffuse through the stomata
• The size of the stomata is controlled by the guard cells. These close the stomata if the plant is losing water faster than it is being replaced by the roots. without these guard cells the plant would soon wilt
• The flattened shape of the leaf increases the area of this exchange surface so that it’s more effective
• The walls of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase the area of this surface so there’s more chance for CO2 to get into the cells
• The water vapour evaporates from the cells inside the leaf. Then it escapes by diffusion because there’s a lot of it inside the leaf and less of it in the air outside

Question 10

How do fish do gas exchange

Answer 10

• Their gills are the gas exchange surface in fish
• Water (containing oxygen) 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 CO2 diffuses from the blood into the water
• Each gill is made of lots of thin plates called gill filaments, which give a big SA for exchange of gases
• The gill filaments are covered in lots of tiny structures called lamellae, which increase the SA even more
• The lamellae have lots of blood capillaries to speed up diffusion
• they 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 over 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