Transport Across Membranes Flashcards
Explain why a cell membrane may be described as a fluid-mosaic? (2)
- The position of the molecules within the membrane is fluid - they are able to move around within the membrane
- Membrane is made up from a variety of different molecules arranged into a mosaic
Explain the arrangement of phospholipids in a cell-surface membrane. (2)
- Bilayer
- Hydrophobic fatty acid tails point away/repelled from water OR hydrophilic phosphate head point towards water
What is Fick’s Law?
Rate of Diffusion = (surface area x concentration gradient) / diffusion distance
What are 4 factors affecting the rate of diffusion?
- Temperature
- Surface Area
- Concentration Gradient
- Diffusion Distance / Pathway
How does increasing temperature affect the rate of diffusion? (1)
increased kinetic energy so faster rate of diffusion of molecules
How does increasing surface area affect the rate of diffusion? (1)
larger surface area provides for space for molecules so faster rate of diffusion
How does increasing concentration gradient affect the rate of diffusion? (1)
as concentration difference increases, rate of diffusion increases
How does increasing diffusion distance affect the rate of diffusion? (1)
longer diffusion distance, slower rate of diffusion
What happens if the tertiary structure of the carrier protein changes? (1)
Facilitated diffusion / active transport can’t occur (because the binding site has changed shape and is no longer complementary to the molecule, therefore there will not fit/bind)
Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substance can cross a cell surface membrane. (5)
- Simple / Facilitated diffusion from high concentration to low concentration
- Small / non-polar molecules pass through phospholipid bilayer
OR - Large / polar molecules pass through proteins
- Water moves by osmosis / high water potential to low water potential / less to more negative water potential
- Active transport is movement from low to high concentration
- Active transport / facilitated diffusion involves proteins / carriers
- Active transport requires ATP / energy
- Ref to co-transport of Na+ and glucose
The movement of substances across cell membranes is affected by membrane structure. Describe how. (5)
- Phospholipid (bilayer) allows movement/diffusion of non-polar substances
- Phospholipid (bilayer) prevents movement/diffusion of polar/charged substances OR proteins allow polar/charged substances to cross the membrane/bilayer
- Carrier proteins allow active transport
- Channel/carrier proteins allow facilitated diffusion / co-transport
- Shape/charge of channel/ carrier determines which substances move
- Number of channels/carrier determines how much movement
- Membrane surface area determines how much diffusion/movement
- Cholesterol affects fluidity/rigidity/permeability
Describe the relationship between size and surface area to volume ratio of organisms (1)
As the size of an organism increases, it’s surface area to volume ratio decreases
Give two similarities in the movement of substances by diffusion and by osmosis. (2)
- (Movement) down a gradient / from high concentration to low concentration
- Passive / not active processes
OR - Don’t use energy from respiration / ATP
Compare and contrast the processes by which water and inorganic ions enter cells. (3)
- Both move down concentration gradient
- Both move through protein channels in membrane
- Ions can move against a concentration gradient by active
transport
Contrast the processes of facilitated diffusion and active transport. (3)
- Facilitated diffusion involves channel or carrier proteins whereas
active transport only involves carrier proteins - Facilitated diffusion does not use ATP whereas active transport uses ATP
- Facilitated diffusion takes place down a concentration gradient
whereas active transport can occur against a concentration gradient
