3.2.3 Transport Across Cell Membranes Flashcards
Structure of a phospholipid
2 fatty acids and a glycerol along with one phosphate group
Explain the key function of phospholipids in cell surface membranes
- Form bilayers in water
- Hydrophilic head attracts water(faces towards water) (makes contact with extra-cellular fluid or cytoplasm) - Hydrophobic tail repels water (faces away from water)
Which model does the cell surface membrane have?
Fluid mosaic model
Explain why cell surface membranes have a fluid structure (2)
- Phospholipids form a bilayer
- Molecules (that the cell surface membrane consists of) are constantly moving around (relative to one another)
Explain why cell surface membranes have a mosaic structure (1)
Protein and phospholipid molecules unevenly distributed
Explain why cell surface membranes are semi permeable (1)
Because of the type and distribution of transport proteins and phospholipid molecules present in the membrane
Draw and label the structure of a cell surface membrane
Should include:
1. Phospholipid bilayer
2. Channel protein
3. Cholesterol
4. Carrier protein
5. Receptor
6. Enzyme
7. Glycoprotein and glycolipid
Channel proteins
- Specific tertiary structure that spans membrane and makes hydrophilic tunnels across it
- Selective channels (only accepts one type of molecule or few closely related) for transports
- Ions and polar molecules pass through (facilitated diffusion)
- Aquaporins allow water to cross CSM via osmosis
- Some open all the time, some ‘gated’ (channel opens or closes) in response to particular signal
Carrier proteins
- Specific tertiary structure - allow transport of ions and polar molecules (facilitated diffusion)
- Changes shape to move target molecule from one side of membrane to other
- Selective
- Often change shape for binding of their target molecule, moving it to opposite side of membrane
Enzymes in cell surface membranes
- Specific, complementary active site
- Maltese found in CSM of small intestine where it hydrolyses maltose into glucose
Receptors in cell surface membranes
- Other protein molecules act as specific receptors for hormones with a complementary shape
- Attach to binding site and allow cell to respond
Glycolipids in cell surface membranes
Phospholipid attached to carbohydrates - important in cell recognition
Glycoprotein
Carbohydrates and proteins - on outer surface of membrane and important in cell recognition, sometimes antigens
Where are glycoproteins and glycolipids produced
Golgi apparatus (body)
Cholesterol in cell surface membranes
- Decreases permeability and increases stability by restricting movement of other molecules
- Causes fatty acid chains to become compact
How is Fick’s law used?
It explains what factors influence the rate of diffusion across a cell surface membrane
State the equation for Fick’s law
Diffusion rate = (surface area x concentration gradient) ÷ diffusion distance or pathway
How is rate of diffusion affected when the surface area increases
Rate increases
How is rate of diffusion affected when the concentration gradient increases
Rate increases
How is rate of diffusion affected when the diffusion distance decreases
Rate increases
How is rate of diffusion affected when the temperature increases
Rate increases
How is rate of diffusion affected when the surface area deceases but concentration gradient stays the same
Rate stays the same
What is meant by a ‘passive process’?
Uses no ATP
Which type of process is simple diffusion?
Passive (process)
State the definition of simple diffusion (2)
The net movement of molecules from a higher concentration to a lower concentration until evenly distributed
What is meant by equilibrium in simple diffusion?
Equal number of that type of molecule on either side of the membrane - no more net movement of that molecule
Which type of molecules diffuse down the concentration gradient and crosses the phospholipid bilayer
Non polar, lipid soluble molecules
During simple diffusion, molecules move ____________ the concentration gradient
Down
How does temperature affect the rate of diffusion?
- Increases kinetic energy
- Faster movement of molecules
- Faster rate of diffusion
How does surface area affect the rate of diffusion?
- More cell surface membrane for molecules to pass through
- Faster rate of diffusion
How does concentration gradient affect the rate of diffusion?
Increase leads to faster rate of diffusion
How does thinner diffusion distance affect the rate of diffusion?
The thinner the distance, the faster the rate of diffusion
Facilitated diffusion helps which type of molecules pass through the bilayer?
Polar molecules
Which proteins does facillitated diffusion use?
Channel and carrier proteins
Definition of facillitated diffusion
Channel and carrier proteins allow polar molecules to pass through the bilayer, DOWN the concentration gradient
Why can’t polar molecules pass between the hydrophobic tails of the bilayer
Fatty acid tails are non-polar and repel polar molecules
Facilitated diffusion is which type of process?
Passive
Explain how channel and carrier proteins bind to specific molecules
They have specific tertiary structures and only transport molecules that have a complementary shape to the binding site
True or false
Channel and carrier proteins are limiting factors in reactions
True
Explain the effect of having more channel/carrier proteins
Faster rate of diffusion
A graph shows facilitated diffusion, explain why it levels off at a certain point (1)
All channel or carrier proteins have filled binding sites, so are transporting at faster rate
Explain why on a graph, simple diffusion has a higher rate of diffusion longer than facilitated diffusion (2)
- Simple diffusion has the whole phospholipid bilayer or larger surface area
- In facilitated diffusion, the channel or carrier proteins are less, so takes longer time to diffusion through
Definition of osmosis
Net movement of water molecules from higher water potential to a solution with lower water potential through a partially permeable membrane
Which channel protein does water move through
Aquaporin
Explain what is meant by ‘water potential’
Free water molecules collide with membranes which experts pressure on them
Water potential is measured in…
kPa
How is water potential affected when there are more free water molecules?
- More collisions
- Greater the pressure
- Increase in WP
Pure water has a water potential of…
0kPa
Why does water have a high water potential?
Every water molecule is free moving
Adding solute to water makes the WP…
Negative
Explain why adding solute to water makes the water potential negative, and hence, lower?
Water is dipole, it is attracted to charges on raise ions, so less free water molecules collide
Hypertonic
- Solution has low WP than cell
- Water leaves cell via osmosis
- Cell may shrivel
Hypotonic
- Solution has higher WP than cell
- Water enters cell via osmosis
- Cell may burst
Isotonic
- Solution has same WP as cell via osmosis
- No net movement of water in or out of cell via osmosis
A negative number means a ______________ water potential
Low
Definition of active transport
Transports polar molecules across the membrane against the concentration gradient - low concentration to high concentration using carrier proteins
Active transport is which type of process?
Active, requires ATP
Describe the process of active transport (3)
- Molecule (for transport) binds to binding sites (of) specific carrier protein
- Hydrolysis of ATP (into ADP and Pi) provides small amount of energy (to carrier protein)
- (Causes) protein (to) change shape, pushing molecule through membrane/bilayer
AND - (Where) concentration of molecule is high
A biologist isolated a sample of intestinal tissue and viewed it under an optical microscope.
He found that the sample of tissue contained lots of mitochondria.
Explain why (2)
- Mitochondria produces ATP
- Active transport is active process requiring ATP
Name and describe four ways substances can move across the cell-surface membrane into a cell (4)
- (Simple) diffusion of small/non-polar molecules down a concentration gradient;
If no reference to ‘small/ non-polar’ for 1 accept this idea from ‘large/charged’ given in description of 2
- Facilitated diffusion down a concentration gradient via protein carrier/channel;
*Reject if active rather than passive*
- Osmosis of water down a water potential gradient;
- Active transport against a concentration gradient via protein carrier using ATP;
The action of the carrier protein SGLT1 is linked to a membrane-bound ATP hydrolase enzyme.
Explain the function of ATP hydrolase (2)
- (ATP to ADP + Pi ) Releases energy;
*Reject ‘produces/makes/creates energy’*
- (energy) allows ions to be moved against a concentration gradient
OR
- (energy) allows active transport of ions;
The movement of substances across cell membranes is affected by membrane structure.
Describe how (5)
- Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
- and 2. Accept correct named examples
- and 2. Ignore water
*Accept phospholipid (bilayer) allows movement/diffusion of O2/CO2* *Accept water-insoluble* - Phospholipid (bilayer) prevents movement/diffusion of polar/charged/lipid-insoluble substances
OR
- (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
Accept water-soluble - Carrier proteins allow active transport;
- Channel/carrier proteins allow facilitated diffusion/co-transport;
*Accept aquaporins allow osmosis*
- Shape/charge of channel / carrier determines which substances move;
- Number of channels/carriers determines how much movement;
- Membrane surface area determines how much diffusion/movement;
- and 7. Accept correct reference to faster/slower/rate for ‘how much movement’
*Accept microvilli / Golgi (apparatus) / ER / rER* *Accept surface area to volume for ‘surface area’* - Cholesterol affects fluidity/rigidity/permeability;
*Accept cholesterol affects vesicle formation/endocytosis/exocytosis/phagocytosis;*
The cells of beetroot contain a red pigment. A student investigated the effect of temperature on the loss of red pigment from beetroot. He put discs cut from beetroot into tubes containing water. He maintained each tube at a different temperature. After 25 minutes, he measured the percentage of light passing through the water in each tube.
The student put the same volume of water in each tube.
Explain why it was important that he controlled this experimental variable (2)
- (If) too much water the concentration of pigment (in solution) will be lower / solution will appear lighter more light passes through (than expected);
OR
- (If) too little water the concentration of pigment (in solution) will be greater / solution will appear darker / less light passes through (than expected); So results (from different temperatures) are comparable;
*Ignore reference to too much water so red pigment / solution too weak to measure*
The cells of beetroot contain a red pigment. A student investigated the effect of temperature on the loss of red pigment from beetroot. He put discs cut from beetroot into tubes containing water. He maintained each tube at a different temperature. After 25 minutes, he measured the percentage of light passing through the water in each tube.
Describe a method the student could have used to monitor the temperature of the water in each tube (1)
(Take) readings (during the experiment) using a (digital) thermometer / temperature sensor;
In mammals, in the early stages of pregnancy, a developing embryo exchanges substances with its mother via cells in the lining of the uterus. At this stage, there is a high concentration of glycogen in cells lining the uterus.
Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients (2)
- Membrane folded so increased / large surface area;
OR
Membrane has increased / large surface area for (fast) diffusion / facilitated diffusion / active transport / co-transport; - Large number of protein channels / carriers (in membrane) for facilitated diffusion;
- Large number of protein carriers (in membrane) for active transport;
- Large number of protein (channels / carriers in membrane) for co-transport;
- Accept ‘microvilli to increase surface area’
- Reject reference to villi.
Note feature and function required for each marking
point and reference to large / many / more.
List rule applies.
Water and inorganic ions have important biological functions within a cell.
Compare and contrast the processes by which water and inorganic ions enter cells (3)
- Comparison: both move down concentration gradient;
- Comparison: both move through (protein) channels in membrane;
*Accept aquaporins *(for water)* and ion channels*
- Contrast: ions can move against a concentration gradient by active transport
A scientist investigated the water loss of strawberries.
In this investigation, the scientists cut the strawberries into slices. Explain the advantage of this (2)
- Increases surface area;
- Allowing faster rate of osmosis so more water loss
Why does osmosis happen faster when the temperature is increased?
More kinetic energy
