B5 - Plasma membranes Flashcards
1
Q
What is the role of membranes at the surface of cells?
A
- they are partially permeable barriers between the cell and its environment (limits what enters/leaves the cell)
- allow cell recognition (cells of immune system)
- site of cell communication (cell signalling)
2
Q
What is the role of membranes within cells?
A
- acts as a barrier between organelles and the cytoplasm/within organelles
- can form vesicles to transport substances around the cell
- compartmentalise the cell (separate processes can occur in specialised areas of the cell)
- can form conc. gradients
- site of chemical reactions (e.g. mitochondria’s inner membrane contains enzymes for respiration)
3
Q
What are the components of the fluid mosaic model membrane?
A
- phospholipid bilayer
- cholesterol
- intrinsic proteins (channel/carrier/glycoproteins/glycolipids)
- extrinsic proteins
4
Q
What is the role of the phospholipid bilayer?
A
- forms a barrier that limits the movement of some substances into/out of the cell and organelles
- only small, fat-soluble substances can dissolve into the bilayer/diffuse across the membrane
- water-soluble substances/ions cannot pass through as the centre of the membrane is hydrophobic/non-polar
5
Q
What is the role of cholesterol?
A
- fits between the tails of the phospholipid bilayer and holds them together (mechanical stability)
- it inhibits the movement of the phospholipids, regulating the membrane’s fluidity
- makes it less permeable to water/ions
6
Q
What is the role of intrinsic proteins (channel/carrier)?
A
- channel :
- provides a hydrophilic channel
- allows passive movement of polar molecules down conc. gradient
- held in position by interactions between the hydrophobic core and hydrophobic R-groups outside of proteins
- carrier :
- helps with both passive/active transport (against conc. gradient) into cells
- shape of protein changes during this
7
Q
What is the role of glycoproteins?
A
- intrinsic proteins
- embedded in cell-surface membrane with attached carbohydrate chain
- they play a role in cell adhesion/as receptors for chemical signals
- the binding of the receptor and the chemical signal elicits a cell response
8
Q
What is the process of cell signalling?
A
- one cell releases a messenger molecule (e.g. hormone)
- this travels (e.g. in the blood) to another cell
- the molecule is then detected by the cell and binds to the receptor on the membrane
- receptor proteins have specific shapes (complementary to the messenger molecules)
- a target cell responds to the particular messenger molecule
- drugs can also bind to receptors to trigger a response/to block the receptor to prevent it from working
9
Q
What is the role of glycolipids?
A
- similar to glycoproteins
- they are lipids with attached carbohydrate chains
- called ‘antigens’/cell markers and can be recognised by cells of immune system as self or non-self
10
Q
What is the role of extrinsic proteins?
A
- present on one side of bilayer
- consist of hydrophilic R-groups on outer surface (interacts with polar phosphate heads/intrinsic proteins)
11
Q
What are the factors affecting membrane structure?
A
- temperature
- solvents
12
Q
How does temperature below 0°C affect membrane structure?
A
- phospholipids are packed closely together (rigid membrane)
- channel/carrier proteins lose their structure/function, which increases membrane permeability
- ice crystals also may pierce the membrane
13
Q
How do temperatures between 0°C and 45°C affect membrane structure?
A
- phospholipids gain kinetic energy and are able to move around (not as tightly packed)
- membrane is partially permeable
- as they move about, the phospholipids leave temporary gaps (allows small molecules to enter the membrane)
14
Q
How do temperatures above 45°C affect membrane structure?
A
- phospholipid bilayer may melt and lose its mechanical stability
- this further increases membrane permeability
- carrier/channel proteins denature so they cannot control what goes into/out of the cell
15
Q
How do solvents affect membrane structure?
A
- organic solvents (e.g. alcohol/ethanol) can increase membrane permeability
- they are able to dissolve the lipids in the bilayer causing the membrane to lose its structure
- as conc. of alcohol increases, permeability of membrane increases
16
Q
What is passive transport?
A
- the movement of particles that does not require energy from respiration
- makes use of the natural motion of the particles
17
Q
What are the passive methods of transport?
A
- diffusion
- facilitated diffusion
18
Q
What is diffusion?
A
- the net movement of particles from a region of higher conc. to a region of lower conc.
- occurs when a membrane is fat-soluble/when the molecules are able to fit between the phospholipids
19
Q
Factors affecting the rate of diffusion
A
- temperature
- as temp. increases, kinetic energy of particles also increases so there is a higher rate of diffusion
- conc. difference
- greater the difference = faster rate of diffusion
- overall movement from high conc. to low conc. is larger
20
Q
What factors affect diffusion across (partially permeable) membranes?
A
- surface area = larger the area, the higher the rate of diffusion
- thickness of membrane (thinner)
21
Q
What is facilitated difffusion?
A
- phospholipid bilayers are barriers to polar molecules/ions
- diffusion across protein channels allows for them to be transported
- these membranes are selectively permeable
- these membranes are selectively permeable
- facilitated diffusion can also include carrier proteins
- they change shape when a specific molecule binds
- does not require external energy as it is the movement down a conc. gradient
22
Q
What factors affect facilitated diffusion?
A
- temp.
- conc. gradient
- membrane surface area/thickness
- no. of proteins channels present (more = higher rates of diffusion)
23
Q
What is active transport?
A
- movement of molecules/ions into/out of a cell from a region of lower conc. to a higher conc.
- this requires energy and carrier proteins
- metabolic energy is supplied by ATP
- the molecule/ion binds to receptors in the channel of carrier proteins
- ATP binds to the carrier proteins and is hydrolysed (ADP and Pi)
- the Pi molecule binds to the protein causing a change in shape
- molecule/ion is released into the cell
- Pi is released and carrier protein returns to its original shape
24
Q
What is bulk transport?
A
- another form of active transport
- larger molecules are unable to move through channel/carrier proteins
25
Q
What is endocytosis?
A
- transport of materials into cells
- phagocytosis (solids)
- pinocytosis (liquids)
- cell-surface membrane bends inwards and enfolds the material until it forms a vesicle
- it then pinches off and moves into the cytoplasm to transfer the material
26
Q
What is exocytosis?
A
- the vesicle is formed by the Golgi apparatus
- fuses with the cell surface membrane
- its contents are released outside of the cell
- energy (ATP) is required to move the vesicles along the cytoskeleton
27
Q
What is water potential?
A
- pressure exerted by water molecules as the collide with a membrane/container
- measured in Pa/kPa
- pure water has a water potential of 0 kPa
- highest possible value
- presence of solute in water lowers the w.p. below 0
- solutions of different w.ps. are separated by a partially permeable membrane
- net movement = higher w.p. to a lower w.p.
- this continues until equilibrium is reached
28
Q
What is hydrostatic pressure?
A
- pressure created by water in an enclosed system
- cellular level = relatively large and potentially damaging
29
Q
How does osmosis affect animal cells?
A
- higher water potential outside:
- water moves in by osmosis
- thin cell membranes cannot withstand the high pressure
-
cytolysis will occur as cell bursts
- lower water potential inside:
- loses water by osmosis
- causes crenation as the cell-surface membrane ‘puckers’
- multicellular animals have control mechanisms to ensure surrounding solutions are constantly at equal w.p.
30
Q
How does osmosis affect plant cells?
A
- unlike animals, plants are unable to control the w.p. of the fluid around them
- higher water potential outside:
- water enters cell by osmosis
- due to strong cellulose cell walls, the membrane pushes against it (turgor = pressure against it)
- lower water potential outside:
- water is lost by osmosis
- cell-surface membrane is pulled away from the cell wall
- cell is plasmolysed
31
Q
PAGs: Dialysis
A
- cells are too small and cell membranes too thin to be used in practicals so a dialysis tube is used instead
- is partially permeable like a membrane and small molecules like glucose and water can pass through but larger molecules like starch and sucrose can’t
- to test for glucose, use benedict’s solution and estimate the concentration
- starch will not cross and use iodine can be used to test for this
- rate of diffusion can be tested for by placing model cell in a water bath and changing the temperature
- difference between real membrane and dialysis is the real membrane is more complex and has other components e.g. carrier/ channel proteins which impact diffusion with complex features like hydrophobic cores (channel protein)
32
Q
PAGs: membrane permeability
A
- beetroot contains betalain, when membranes are disrupted, this is released and the surrounding solution is coloured, more disruption = more pigment released
- 5 small pieces of beetroot are cut into small sizes (using a cork borer)
- pieces are thoroughly washed (to remove pigement released from damaged cells) and dried, then placed in 100ml of distilled water in a water bath and temp increased by 10°C
- samples taken 5 minutes after each temp was reached ( to allow mixture to equilibrate)
- zero a cOlOrimeter by using distilled water to act as standard to compare other solutions to
- absorbance of each piece is measured with a blue filter
- repeat with fresh beetroot pieces and calculate the mean
- plot a calibration curve of absorbance against temperature and use graph to read off change in betalain concentration
33
Q
A
