2.1.5b Diffusion Flashcards
see slide 2 for dia of outside/inside the cell membrane
5 different ways substances enter/leave cells
- Simple diffusion (passive process)
- Facilitated diffusion (passive)
- Osmosis (passive)
- Active transport (active, requires ATP energy)
- Bulk transport (endocytosis & exocytosis) (active)
What is a concentration gradient
The difference in concentration of particles between 2 areas. Either steep or shallow
Concentration gradient in relation to rate of diffusion
- If concentration gradient is steep, rate of diffusion is faster
- If concentration gradient is shallow, rate of diffusion is slower
What is simple diffusion
The net (overall) random movement of particles (atoms, molecules, ions) from a region of higher concentration to lower concentration, down a concentration gradient
How are particles able to move in simple diffusion
Due to the random movement & collisions of particles (which have their own kinetic energy)
What type of process is simple diffusion
PASSIVE - meaning no ATP energy required
When does simple diffusion stop
Continues until there is a concentration equilibrium between both sides
Eg of simple diffusion
Oxygen molecules diffuse into cells by simple diffusion
What type of molecules can move via simple diffusion
- Small, non polar molecules (eg. oxygen & CO2) can diffuse easily through the cell surface membrane
- This is bc they are very small & can pass through spaces between phospholipids
- Water is also small enough to fit between phospholipids, so it’s able to diffuse across plasma membranes even though it’s polar. This is called osmosis
What is facilitated diffusion
The passive movement of molecules down a concentration gradient (higher to lower concentration) across a membrane, & involves a carrier proteins & channel proteins in the membrane
What molecules can move in facilitated diffusion
Some larger polar molecules (eg. amino acids, glucose, or ions sa sodium ions & chloride ions) cannot simply diffuse directly through the phospholipid layer of the cell surface membrane
Instead, they diffuse through carrier proteins or channel proteins in the cell membrane
What is needed in facilitated diffusion
Substances can only cross the phospholipid bilayer w the help of certain proteins:
- Channel proteins
- Carrier proteins
They are highly specific (they only allow one type of molecule or ion to pass through)
see slide12 for dia
What are channel proteins
-
Water-filled pores. They allow charged substances (eg. ions) to diffuse through the cell membrane.
(see slide13)
How do channel proteins work
- The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
- This allows the channel proteins to control the exchange of ions
What are carrier proteins
- Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
- This causes the binding site of the carrier protein to be open to one side of the membrane first, & then open to the other side of the membrane when the carrier protein switches shape
(see slide 14 for dia)
What does the direction of movement of molecules depend on
Direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
Net diffusion of molecules or ions into or out of a cell will occur…
…down a concentration gradient (higher to lower)
What does facilitated diffusion depend on
Concentration gradient - the higher the concent gradient, the faster the rate of facilitated diffusion (up to a point). As equilibrium is reached, rate of facilitated diffusion levels off
No. of channel or carrier proteins - once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concent gradient. So the grater the no. of channel or carrier proteins in cell membrane, the faster the rate of facilitated diffusion
Why can water diffuse directly through the membrane
Even though its polar, it can diffuse directly through due to its relatively small size
see slide 17 for comparison between carrier & channel proteins
Where does the energy come from in simple & facilitated diffusion
Kinetic energy of the molecules
see slide 21 for comparison between simple & facilitated diffusion
Factors affecting the rate of diffusion
- Concentration gradient (steep/shallow)
- Temperature
- SA of membrane
- Thickness of exchange surface/membrane
- SA:V
- Properties of molecules/ions
Factors affecting the rate of diffusion: Concentration gradient
- If there are more molecules on one side of a membrane than on the other, at any one moment more molecules will randomly move across membrane from that side than from the other.
- Steeper the gradient, faster the rate of diffusion
Factors affecting the rate of diffusion: Temperature
- Molecules & ions have more kinetic energy at higher temps
- They move faster, resulting in more collisions, therefore resulting in higher rates of diffusion
Factors affecting the rate of diffusion: SA of membrane
- The greater the SA across which diffusion is taking place, the greater the no. of molecules or ions that can cross it at any one moment, therefore the faster diffusion occurs
- SA of membranes can be increased by folding (eg. microvilli in the intestine or cristae in mitochondria)
Factors affecting the rate of diffusion: SA:V
- As a cell increases in size, the SA:V decreases which slows the rate of diffusion through a cell as the distance required becomes too great
Factors affecting the rate of diffusion: Properties of molecules/ions
- Large molecules diffuse more slowly than smaller ones as they require more energy to move
- Uncharged & non-polar molecules diffuse directly across the phospholipid bilayer
- Non-polar molecules diffuse more quickly than polar ones as they are soluble in the non-polar phospholipid bilayer
How does SA:V affect diffusion
- As the size of a cell increases, so does the SA & volume
- BUT the volume of cell increases faster than the SA (bc vol is cubed, whereas SA is squared)
- When volume is too large relative to the SA of cell, diffusion cannot occur at a high enough rate to supply raw materials to the entire volume of the cell
What is surface area
Refers to the outside are of an object
unit: cm^2
What is volume
Refers to the amount of space inside the object
unit: cm^3
What is rate
A measure of how much smth is changing over time
How to calculate rate on a linear diffusion graph
Gradient = change in y/change in x
How to calculate rate on a curved (non-linear) diffusion graph
Draw a tangent
Find gradient of the line
do SA:V questions on slide 40
How are organs specialised to ensure diffusion is efficient
- Have a large SA
- Membrane is thin - shot diffusion pathway
- Efficient blood supply surrounding exchange surface
- Large SA:V ratio - there is more SA relative to the volume inside
Adaptations of the intestines
- The inside of the small intestine is covered in millions & millions of these tiny little projections called villi
- They increase the SA so that digested food is absorbed much more quickly into blood
- They also have:
- a single layer of surface cells
- a very good blood supply to assist quick absorption
(see slide 46 for dia)
Adaptations of the alveoli (lungs)
- They are very thin - walls are only one cell thick
- There is a very efficient blood supply network of capillaries around the alveoli
- The lungs & alveoli have a large combined SA
- The walls are moist, encouraging gas molecules to easily dissolve
(see slide 47 for dia)
Adaptations for gas exchange in fish
- Fish have gills which are made up of many gill filaments - large SA
- The gills are covered in tiny structures called lamellae - increases SA
- Lamellae are surrounded by lots of blood capillaries
- Countercurrent exchange system
-
Thin surface layer of cells - short diffusion pathway
(see slide 48-51)
Adaptations of roots of a plant
- Plant roots anchor the plant into the soil & absorb water & mineral ions from the soil
- Have extensions called root hair cells - increases SA to absorb MORE water/mineral ions
- Root hair cells are thin - short diffusion pathway for water/mineral ions to travel into root
(see slide 52)
Gas exchange at the leaf
Leaves absorb CO2 from the atmosphere & release oxygen & water vapour in the process of photosynthesis
Adaptations of leaves
- Very thin - short diffusion pathway
- Broad & flat - gives larger SA so more gases can diffuse into/out of leaf
- Have tiny pores called stomata - this is where gases are exchanged
What is a synapse
A junction between two neurones across which electrical signals must pass
(see slide 54 for dia)
Diffusion across the synapse of nerve cells
Neurotransmitter molecules diffuse from vesicles towards the neurotransmitter receptors moving from an area of high to low concentration
Practical to investigate diffusion in cells
Phenolphthalein is a pH indicator - it’s pink in alkaline solutions & colourless in acidic solutions.
You can use it to investigate diffusion in agar jelly
Method of agar jelly diffusion experiment
- Make up some agar jelly with phenolphthalein & dilute sodium hydroxide. This will make the jelly pink
- Fill a beaker w some dilute hydrochloric acid. Using a scalpel, cut out a few cubes from the jelly & put them in the beaker
- Leave the cubes for a while - eventually, they’ll turn colourless as the acid diffuses into the agar jelly & neutralises the sodium hydroxide
Using this exp, you can investigate factors that affect rate of diffusion (SA, concent gradient, temp)
Agar jelly diffusion experiment (SA)
- Cut the agar jelly into different sized (side lengths: 0.5cm, 1cm, 2cm) cubes & work out their SA:V ratio
- Time how long it takes each cube to turn colourless when placed in the same concentration of hydrochloric acid.
You would expect the cubes with the largest SA:V ratio to go colourless fastest
Agar jelly diffusion experiment (Concentration gradient)
- Prepare the test tubes containing different concentrations of hydrochloric acid.
- Put an equal sized cube of agar jelly in each test tube & time how long it takes each one to turn colourless
You would expect the cubes in the highest concentration of hydrochloric acid to turn colourless fastest
Agar jelly diffusion experiment (Temperature)
- Prepare several boiling tubes containing the same concentration of hydrochloric acid & put the tubes into water baths of varying temps
- Put an equal sized cube in each boiling tube & time how long it takes each cube to turn colourless
You would expect the cubes in the highest temperature to turn colourless fastest
What do Carrier proteins do
(pg56)
- Move large molecules into or out of the cell, down their concentration gradient
- Different carrier proteins facilitate the diffusion of different molecules
Step by step of carrier proteins in action
(pg56)
- First, a large molecule attaches to a carrier protein in the membrane
- Then, the protein changes shape
- This releases the molecule on the opposite side of the membrane
What do Channel proteins do
(pg56)
- Form pores in the membrane for charged particles to diffuse through, down their concentration gradient
- Different channel proteins facilitate the diffusion of different charged particles
