5.3 diffusion (d) Flashcards
What is passive transport?
A process that does not require energy.
Describe channel proteins
They provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes.
Describe carrier proteins
Role in passive + active transport into cells, involving the shape of the protein changing.
What is diffusion?
The net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient. Passive process.
Why does diffusion occur and when does it occur until?
Because liquid and gas particles have kinetic energy. It will continue until the 2 areas are in equilibrium - when the random and unequal distribution becomes equal in both directions.
Why are cells microscopic?
Although particles move at high speeds, constant collisions slow them down, thus short distances mean faster diffusion since less collisions occur. Thus cells are microscopic for quick diffusion or ATP supply.
What is simple diffusion?
The diffusion in the absence of a barrier or membrane.
How does temperature affect the rate of diffusion?
As temperature increases, particles have more kinetic energy and travel at higher speeds, thus increasing rate of diffusion.
How does concentration affect rate of diffusion?
The greater the concentration difference between the 2 regions, the faster the rate since net movement is larger.
What can move across membranes?
Since particles must move through the phospholipid bilayer, diffusion across a membrane only occurs if the membrane is permeable to the particles, so non-polar molecules like oxygen diffuse freely down a concentration gradient.
Why can’t ions pass through a membrane?
The hydrophobic interior of the membrane repels charged substances - ions - so they cannot easily pass through.
Can polar molecules pass through membranes?
Polar molecules such as water with partial (ẟ) charges can diffuse through membranes, but very slowly. Smaller polar molecules pass through easier than larger ones. Membranes are partially permeable.
How does surface area affect diffusion?
The larger the surface area of the membrane, the higher the rate of diffusion.
How does membrane thickness affect diffusion?
The thinner the membrane the faster the rate of diffusion.
How can polar molecules ands ions move through membranes?
Polar molecules and ions can pass through channel proteins.
What is facilitated diffusion?
Diffusion across a plasma membrane through protein channels. Passive process.
How does facilitated diffusion work?
Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion. Can involve carrier proteins which change shape when a specific molecule binds.
How does the number of protein channel affect diffusion?
As the number of protein channels increases, the rate of diffusion also increases.
Practical investigation: rate of diffusion with surface area
- Use different sized agar blocks full of phenolphthalein indicator which is pink in alkali and colourless in acid.
- Immerse them in NaOH (aq) for 10 minutes then remove them and measure the volume diffuses
- rate of diffusion can be measure by distance travelled/time or volume filled/time
Practical investigations: diffusion rates in model cells
- Model cell = dialysis tubing - partially permeable with similar sized pores to a real membrane
- It is a barrier to larger molecules
- Concentration inside and out of tubing can be changed
- Glucose cannot cross - can test with Benedict’s test
- Starch will not cross - can test with iodine solution
- Water will cross
- To change temperature the model cell can be placed in different water baths
