2.1.5 - Biological membranes Flashcards
1
Q
What is compartmentalisation ?
A
Compartmentalisation is the formation of separate membrane bound areas in a cell
2
Q
Why is compartmentalisation useful in cells ?
A
- Protects cell components
- Allows reaction conditions to be maintained
- Isolates reactions making them more efficient
3
Q
State 4 functions of plasma membranes
A
- Recognition of other cells
- Cell communication
- Separates cell contents from extracellular space
- Controls the passage of materials in and out of the cells
4
Q
State 4 functions of membranes inside cells
A
- Isolates DNA
- Compartmentalises reactions
- Form vesicles
- Acts as an intracellular transport system
5
Q
Describe the membrane structure
A
Membranes are formed from a phospholipid bilayer with intrinsic and extrinsic proteins
6
Q
What is the phospholipid bilayer ?
A
- It is the arrangement of phospholipids found in cell membranes
- Hydrophilic phosphate heads on the outside
- Hydrophobic fatty acid tails on the inside
- It has a hydrophobic core
7
Q
Why are membranes referred to as having a fluid mosaic structure ?
A
- They are called fluid as they are able to move freely relative to each other. It gives the membrane flexibility
- They are referred to as mosaic structures due to the fact that they have different proteins embedded in the bilayer which vary in shape size and position
8
Q
What is a phospholipid ?
A
A phospholipid is a lipid made of glycerol, two fatty acid tails and a phosphate head
9
Q
What is the function of a phospholipid ?
A
Phospholipids create a selectively permeable barrier to the movement of ions and molecules important for cellular function
10
Q
What are extrinsic proteins ?
A
- They are present in one side of the bilayer
- They normally have hydrophilic R - groups on their outer surfaces and interact with the polar heads of the phospholipids or with the intrinsic proteins
11
Q
What are the functions of extrinsic proteins in cell surface membranes ?
A
Can attach to the cytoskeleton
12
Q
What are intrinsic proteins ?
A
- Intrinsic proteins are transmembrane proteins that are embedded through both layers of the membrane
- They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place
13
Q
What are channel proteins ?
A
They are intrinsic proteins which are held in position by interactions between the hydrophobic core of the membrane and the hydrophilic R - groups on the outside of the proteins
14
Q
What is the function of a channel proteins ?
A
They provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes
15
Q
What are carrier proteins ?
A
Carrier proteins are intrinsic proteins whose shape can change during active transport
16
Q
What is the function of a carrier protein ?
A
- They have an important role in both passive and active transport into the cell
- It often involves the shape of the protein changing
17
Q
What is a glycoprotein ?
A
- Glycoproteins are extrinsic proteins with attached carbohydrate chains
- They are embedded in the cell surface membrane
18
Q
What is the function of a glycoprotein ?
A
- Receptors for chemical signals
- Cell adhesion
19
Q
What is chemical signalling ?
A
- When a chemical binds to the receptor, it elicits a response from the cell. This may cause a direct response or set off a cascade of events inside the cell.
- Used for intracellular communication
20
Q
What is cell adhesion ?
A
- When cells join together and move as one piece
21
Q
What is a glycolipid ?
A
- They are lipids with attached carbohydrate chains.
- These molecules are called cell markers or antigens.
22
Q
What is the function of a glycolipid ?
A
- They can recognise cells of the immune system as self or non self
- Acts as an antigen that can be recognised by the immune system
- Can form hydrogen bonds with water molecules to stabilise membrane
23
Q
What is cholesterol ?
A
- A lipid with a hydrophilic and hydrophobic end. The hydrophilic end interacts with the heads and the hydrophobic end interacts with the tails.
- It is made up of four carbon rings with a hydroxyl groups
24
Q
What are the functions of cholesterol ?
A
- Regulates fluidity of membranes
- Adds stability
- Prevents membrane becoming too solid by stopping phospholipids from crystallising
- Pull phospholipids together
25
What do membranes control ?
- The passage of substances in and out of cells
| - If membranes lose their structure, cell processes will be disrupted
26
What 3 factors affect membrane structure?
- Temperature
- Solvents
- Cholesterol
27
How does an increase in temperature affect cell membranes ?
- Increases membrane fluidity
- Increases membrane permeability
- Membrane loses its structure
- Carrier and channel proteins are denatured at higher temperatures
28
Why does an increase in temperature increase membrane fluidity?
- The phospholipids will have more kinetic energy and will move more
- Increases fluidity and it begins to lose its structure
29
Why does an increase in temperature increase membrane permeability?
- Higher temperature means the cell membrane loses its structure
- This makes it easier for things to pass through as its fluidity is higher
30
What happens if the temperature of the cell membrane decreases ?
- The fatty acid tails of the phospholipids move less and become more rigid, stiffening the membrane
- This decreases the overall fluidity of the membrane, also decreasing its permeability and potentially restricting entry of important molecules into the cell
- Low temperature can also slow cell growth by preventing the cell's increase in size.
31
What happens in extremely low temperatures ?
In extreme situations, liquid in the cell can begin to freeze, forming crystals that pierce the membrane and might ultimately kill the cell
32
Outline how the effect of temperature on membrane permeability can be investigated
- Beetroot cells contain betalain
- Pieces of equal size beetroot are placed in water and heated in waterbaths at different temperatures for the same amount of time
- The solutions are then placed in a colorimeter
- When membranes are disrupted the red pigment is released
33
How can a colorimeter show the permeability of membranes?
- The more betalain leaks out, the more permeable the membrane
- More betalain leaking means more colour is absorbed by the colorimeter
34
Suggest 3 improvements of the design of the experiment
- Cut pieces from same place in beetroot
- Blot surface of beetroot before experiment
- Use same volume of water
35
What causes membranes to dissolve ?
Organic solvents
36
Why do organic solvents cause membranes to dissolve ?
They are less polar than water and will therefore dissolve membranes, disrupting cells
37
How do solvents affect cell membranes ?
- The membrane loses its structure
| - This increases its permeability
38
Why do solvents increase membrane permeability?
This is because they dissolve phospholipids
39
Give an example of a solvent that can dissolve phospholipids
Ethanol, Methanol etc.
40
Define diffusion
The net movement of particles from a region of high concentration to a region of low concentration
41
Is diffusion active or passive ?
Passive
42
Define passive transport
Movement of substances that does not require energy
43
Which 2 factors affect the rate of diffusion
- Temperature
| - Concentration gradient
44
What effect does an increase in temperature have on the rate of diffusion ?
- It increases the rate of diffusion
45
Why does a higher temperature increase the rate of diffusion ?
- Higher temperature means particles have more kinetic energy and move at higher speeds.
- Therefore, they can diffuse quicker too
46
What effect does a greater difference in concentration of two regions have on the rate of diffusion ?
It increases the rate of diffusion
47
What 4 factors affect the rate of diffusion across a membrane ?
- Temperature
- Concentration gradient
- Surface area of membrane
- Thickness of membrane
48
An increase in surface area of an exchange surface has what effect on rate of diffusion ?
Increases the rate of diffusion
49
An increase in membrane thickness has what effect on the rate of diffusion ?
Decreases the rate of diffusion
50
What sort of molecules can move via diffusion across a membrane ?
- Small molecules
- Lipid soluble molecules
- Non-polar molecules
51
Why can steroid hormones easily cross a membrane by simple diffusion ?
- They are lipid soluble
| - They are non polar
52
Why is it easier for oxygen to diffuse across a membrane than water ?
- Oxygen molecules are small and non-polar whereas water molecules are polar
- Water is only partially charged so it is still able to diffuse through the membrane, though only slowly
53
Define facilitated diffusion
Diffusion across a plasma membrane through protein channels
54
Is facilitated diffusion passive or active ?
It is passive
55
What does facilitated diffusion allow to move across a membrane ?
- large molecules
- lipid insoluble molecules
- charged particles
56
What are the 5 factors which affect the rate of facilitated diffusion ?
- Temperature
- Concentration gradient
- Surface area
- Membrane thickness
- Number of channel proteins
57
Why are membranes containing protein channels selectively permeable ?
- Most protein channels are specific to one molecule or ion
| - Therefore only these specific molecules/ions are able to move across the membrane
58
How do protein channels allow charged particles to move through the membrane ?
They provide a hydrophilic channel for charged particles
59
What happens to charged molecules when they try to cross the membrane ?
The hydrophobic interior of the phospholipid bilayer repels the charged molecules
60
Which types of movements of particles require ATP ?
- Active Transport
| - Bulk Transport
61
What does active transport require to occur ?
- ATP
| - Carrier proteins
62
Why is active transport considered to be selective ?
Specific substances are transported by specific carrier proteins
63
Describe how active transport occurs with aid of a carrier protein
- Molecules or ions bind to receptors in the channel of the carrier protein
- ATP binds to the opposite side of the carrier protein
- ATP is hydrolysed to form ADP and phosphate
- The phosphate molecule binds to the carrier protein which causes the protein to change shape
- The carrier protein opens and allows the molecules through
- Phosphate molecule is released and recombines with ADP to form ATP
- The carrier protein returns to its original shape
64
Define endocytosis
The bulk transport of materials into cells
65
Outline how endocytosis occurs
- In endocytosis, the cell takes in biological molecules by forming new vesicles from the plasma membrane
- A small area of the plasma membrane invaginates (sinks inwards) to form a pocket
- As the pocket deepens, it pinches inwards forming a vesicle containing substances that had been outside the cell
66
Define pinocytosis
The bulk transport of liquids into cells
67
Outline how pinocytosis occurs
- In pinocytosis, a cell gulps droplets of extracellular fluid into tiny vesicles, encapsulating molecules dissolved in the fluid
- Pinocytosis is non specific, any and all included solutes are taken into the cell
68
Define phagocytosis
- The bulk transport of solids into cells
- Cellular eating
- Most commonly the process by which white blood cells engulf pathogens
69
Outline how phagocytosis occurs
- The cell surface membrane invaginates when it comes into contact with a bacterium
- The membrane then enfolds around the bacterium forming a vesicle
- The vesicle then pinches off and moves into the cytoplasm towards lysosomes
70
Define receptor-mediated endocytosis
It enables cells to acquire bulk quantities of specific substances from the extracellular fluid
71
Outline how receptor-mediated endocytosis occurs
- Receptor proteins in the membrane bind to specific substances (ligands), then cluster to form coated pits.
- Coated pits then form a vesicle that contains ligand molecules and is coated in proteins
72
Define exocytosis
The bulk transport of materials out of a cell
73
Outline how exocytosis occurs
- Vesicles are usually formed by the golgi apparatus
- Vesicles move towards and fuse with the cell surface membrane
- Contents of the vesicle are then released outside of a cell and the vesicle membrane becomes part of the cell membrane
74
Name an example of exocytosis
Excretion
75
Does bulk transport require ATP ?
Yes
76
Why would a molecule be moved by bulk transport ?
It is too big for channel or carrier proteins
77
Name 3 things which are moved by bulk transport
- Enzymes
- Hormones
- Bacteria
78
What is osmosis ?
Osmosis is the diffusion water molecules, across a partially permeable membrane, from an area of high water potential to an area of low water potential ?
79
What type of process is osmosis ?
Osmosis is a passive process
80
What is water potential ?
- Osmosis refers to the concentration of free water molecules
- It is the pressure exerted by water molecules as they collide with a membrane or container
81
What are the units for water potential ?
kPa
82
What is the water potential of pure water ?
0
83
Why does water potential decrease as solute is added ?
- When a substance is dissolved in water the kinetic energy of the water is lowered
- Because water molecules cluster around the solute
- Movement of water molecules is partially impeded
- So a lower pressure is exerted
84
What is a hypertonic solution ?
- Having a higher concentration of solute relative to another solution
- Low water potential
85
What is an isotonic solution ?
Having the same concentration of solute relative to another solution
86
What is a hypotonic solution ?
- Having a lower concentration of solute relative to another solution
- High water potential
87
What happens when an animal cell is placed in a hypotonic solution ?
- Water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell
- Causes the cell to burst
- Cytolysis
88
What happens when an animal cell is placed in a hypertonic solution ?
- Water will be lost to the solution by osmosis, down the water potential gradient
- This reduces the volume of the cell
- Crenation
89
What would be the effect of placing a red blood cell in a hypotonic solution?
- Net movement of water into the cell
- Cell swells and bursts
- Causing Haemolysis
- Cell contents lost, haemoglobin released
90
What would be the effect of placing a red blood cell in a hypertonic solution?
- Net movement of water out of the cell
- Cell shrinks and shrivels
- Becomes darker in colour as haemoglobin is more concentrated
- Crenation
91
What would be the effect of placing a plant cell in a hypotonic solution?
- Net movement of water into cell
- Protoplast pushed against cell wall
- Cell becomes turgid
92
Why do plant cell walls not burst from osmosis?
- They have strong cellulose cell walls
| - Cell walls are able to resist the hydrostatic pressure
93
What would be the effect of placing a plant cell in a hypertonic solution?
- Net movement of water out of cell
- Contents shrink
- Protoplast completely pulled away
- Plasmolysis occurs
