Biological membranes Flashcards
(122 cards)
1
Q
What do intracellular membranes do?
A
form compartments within the cell, such as organelles (including the nucleus, mitochondria and RER) and vacuoles
2
Q
Why are membranes partially permeable?
A
because they control the exchange of materials passing through them
3
Q
do membranes play roles in cell signalling by acting as an interface for communication between cells?
A
yes
4
Q
What different factors affect cell membrane permeability?
A
temperature and organic colvent concentration
5
Q
what does the fluid mosaic model help to explain?
A
-Passive and active movement between cells and their surroundings
-Cell-to-cell interactions
-Cell signalling
6
Q
how can substances cross memebranes?
A
by diffusion, facilitated diffusion, osmosis and active transport
7
Q
Why does the fluid mosaic model describe cell membranes as mosaics?
A
because the scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
8
Q
Why does the fluid mosaic model describe cell membranes as fluids?
A
because…
-The phospholipids and proteins can move around via diffusion
-The phospholipids mainly move sideways, within their own layers
-The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
9
Q
What are the 4 main components of the fluid mosaic model?
A
-Phospholipids
-Cholesterol
-Glycoproteins and glycolipids
-Transport proteins
10
Q
are the heads of phospholipids hydrophilic or hydrophobic?
A
hydrophillic
11
Q
are the tails of phospholipids hydrophilic or hydrophobic?
A
hydrophobic
12
Q
what molecules do phospholipids act as a barrier to?
A
to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane).This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
13
Q
how can phospholipids be chemically modified to act as signalling molecules?
A
by….
-Moving within the bilayer to activate other molecules (eg. enzymes)
-Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
14
Q
what are phospholipids made up of?
A
2 fatty acid molecules bonded to a glycerol molecule. The glycerol is also bonded to a highly charged phosphate group
15
Q
What are the functions of membranes?
A
-acting as barriers between the internal contents of the cell and the external environment
-barriers between the contents of an organelle and the cytoplasm
-separation of one part of an an organelle from another
-location for chemical reactions e.g some stages of respiration
-cell signalling
16
Q
why can hydrophobic non-polar molecules pass through the cell membrane?
A
because the phospholipid bilayer has a hydrophobic centre
17
Q
why can water molecules pass through the cell membrane despite being polar?
A
because water molecules are extremely small
18
Q
does cholesterol have a polar hydrophilic group at one end?
A
yes and this group can attract the polar head groups of the phospholipid molecules
19
Q
what can the non-polar hydrophobic part of a cholesterol molecule attract?
A
it can attract the non-polar fatty acid tails in the phospholipid
20
Q
How does cholesterol control the fluidity of the cell membrane?
A
-it can prevent the cell surface membrane from becoming too fluid at high temperatures and stop the cell membrane from becoming too rigid at low temperatures.
-This occurs because cholesterol can make the phospholipid tails pack closer together or less close together
21
Q
what are the 2 categories of proteins in the cell membrane?
A
intrinsic/integral and extrinsic/peripheral
22
Q
describe intrinsic proteins
A
they’re fully embedded in the membrane from one side to the other. Because they pass right through the bilayer, intrinsic proteins have hydrophobic amino acids on the outside surface of the protein. These can interact with the hydrophobic fatty acid tails in the phospholipid bilayer.
23
Q
what are 2 types of intrinsic protein?
A
carrier proteins and protein channels
24
Q
describe protein channels
A
they run through the centre of the phospholipid bilayer. This channel is lined with hydrophilic amino acids and is filled with water molecules. They allow water -soluble molecules and ions to diffuse through (hydrophilic substances)
25
describe carrier proteins
they have a binding site for certain chemicals . When that chemical binds, it causes the tertiary structure of the carrier protein to change. This change in tertiary structure brings the chemical across the membrane where the chemical is now released
26
describe extrinsic proteins
they don't span the membrane and are found on one side of the membrane or the other. They can sometimes be attached to intrinsic proteins
27
describe the functions of extrinsic proteins
- some play a structural role within the membrane
- some act as enzymes
- some are receptors for other molecules, such as hormones
-
28
what are glycoproteins?
membrane proteins attached to a carbohydrate molecule
29
what are the functions of glycoproteins?
- allowing cells to attach to each other to form tissues such as nervous tissues
- presenting antigens to T cells
- acting as receptors for hormones
-
30
what are glycolipids?
carbohydrates attached to phospholipid molecules
31
what are glycolipids used for?
-contacting other cells. the glycolipids of one cell can be recognised by another cell and this can determine whether cells come into contact .
- they act as antigens e.g in determining blood groups
32
what 2 factors affect the permeability of the cell membrane?
temperature and solvent concentration
33
what happens to the lipids when the temperature of the phospholipid bilayer increases?
-The lipids become more fluid and so the cell membrane becomes a less effective barrier to polar molecules and so they can easily pass through.
- Diffusion takes place at a higher speed due to increased kinetic energy. So, more molecules pass through
34
what happens to the proteins in the cell membrane at really high temperatures (temperatures exceeding 40 degrees)?
the proteins in the cell membrane denature, which means that it can no longer form an effective barrier and so substances can freely pass through the disrupted membrane
35
why do organic solvents increase membrane permeability
because they dissolve the lipids in the membrane, causing the membrane to lose its structure
36
what can be used to investigate how different factors affect membrane structure and permeability?
beetroot
37
the higher the permeability of the beetroot cell membrane...
the more pigment leaks out of the cell
38
summarise the practical that investigates the affects of temperature on membrane permeability
Test tubes containing beetroot are put into water baths of different temperatures. A colorimeter will consequently be used to measure the absorbance of light. The higher the absorbance of light, the greater the membrane permeability- as it means more pigment was released. The general trend would've been the higher the temperature, the greater the increase in membrane permeability. This is due to the increased fluidity and increased diffusion rate at higher temperatures.
39
why does membrane permeability also increase below 0 degrees Celsius?
-Increased permeability can be caused by channel or carrier proteins deforming at these low temperatures
-Ice crystals that form can also pierce the cell membrane, making it highly permeable
40
what are some limitations to the practical that investigates the affects of temperature on membrane permeability
-Cuvettes may differ in thickness (very slightly). A thicker (or scratched) cuvette will absorb slightly more light than a thinner unscratched cuvette
-The beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others
-Some parts of beetroot tissue have more pigment in their cells than others
41
what is a solution to the fact that cuvettes may differ in thickness ?
use the same cuvette for every reading, or repeat the investigation many times and find a mean
42
what is a solution the fact that the beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others?
cut the discs as accurately as possible using a scalpel and ruler, and repeat each investigation several times to find a mean
43
what is a solution the fact that some parts of beetroot tissue have more pigment in their cells than others?
conduct several repeats, using different parts of the beetroot and find a mean
44
what is diffusion?
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.
45
what factors is the rate of diffusion dependant on?
concentration gradient, surface area, the distance over which diffusion occurs
46
how does the steepness of the concentration gradient affect the rate of diffusion?
The greater the difference in concentration between 2 areas, the greater the rate of diffusion
47
how does the surface area affect the rate of diffusion?
The greater the surface area, the greater the rate of diffusion
48
how does the distance over which diffusion occurs affect the rate of diffusion ?
The shorter the diffusion path, the greater the rate of diffusion
49
what does Fick's Law state?
'the rate of diffusion is proportional to both the surface area and concentration difference and the rate of diffusion is inversely proportional to the thickness of the membrane'.
50
what is the formula for the rate of diffusion?
rate of diffusion = surface area X difference in concentration / length of the diffusion path
51
what can increase the surface area of cell membranes?
folding e.g microvilli in the intestines or cristae in mitochondria
52
what molecules can't diffuse through the phospholipid bilayer of cell membranes.?
-Large polar molecules such as glucose and amino acids
-Ions such as sodium ions (Na+) and chloride ions (Cl-)
so they need the help of certain prtoeins
53
what 2 proteins enable facilitated diffusion?
Channel proteins
Carrier proteins
54
do channel proteins have a fixed shape?
yes
55
can carrier proteins can switch between two shapes?
yes
56
is facilitates diffusion a passive process?
yes
57
what is equilibrium in terms of diffusion?
when there's no net movement of particles so diffusion has stopped
58
what is meant when diffusion is said to be a passive process?
that it doesn't require metabolic energy to take place
59
what is metabolic energy?
energy released by respiration
60
why can't charged particles diffuse through the cell membrane?
because of the hydrophobic, uncharged tails in the middle of the phospholipid bilayer
61
Can uncharged particles, such as oxygen, diffuse rapidly across the cell membrane?
Yes
62
which diffuse faster across the membrane, smaller particles or larger ones?
smaller particles
63
why do particles diffuse quicker at higher temperatures?
because at higher temperatures, particles have greater kinetic energy
64
what's an example of a hydrophobic substance?
steroid hormones
65
what does it mean by the fact that protein channels are selective?
they only allow certain chemicals to pass through
66
some protein channels are always open, whereas some ...
... can only open in response to a certain trigger e.g. a chemical binding to the protein channel
67
what is osmosis?
the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
68
does osmosis require metabolic energy?
no
69
What is water potential?
The pressure water molecules apply to a membrane when they collide with it
70
what is the unit of water potential?
kilopascals
71
what type of water has the highest water potential?
pure water
72
why does pure water have the highest possible concentration of water molecules?
because pure water has the highest possible concentration of water molecules and so a very high water potential
73
what is the water potential of pure water?
0kPa
74
What is the water potential of solutions besides pure water?
less than 0
75
do more concentrated solutions have higher or lower water potential than a less concentrated solution?
lower water potential
76
isotonic meaning?
the concentration of solutes is the same inside and outside of the cell, meaning the water potential is equal. water will diffuse equally between locations
77
hypertonic meaning?
the solution has a higher solute concentration than the cell, meaning the water potential is lower outside the cell. Water will diffuse out of the cell
78
hypotonic meaning
the solution has a lower solute concentration than the cell, meaning the water potential is lower inside the cell. water will diffuse into the cell
79
describe the visking tubing diffusion practical
- Fill a section of Visking tubing with a mixture of starch and glucose solutions
-Suspend the tubing in a boiling tube of water for a set period of time
-Test the water outside of the visking tube at regular intervals for the presence of starch and glucose to monitor whether the diffusion of either substance out of the tubing has occurred
-the results should indicate that glucose, but not starch, diffuses out of the tubing
80
what is visking tubing?
a non-living partially permeable membrane made from cellulose
81
what is another name for visking tubing?
dialysis tubing
82
what large molecules can't go through the pores in membranes?
starch and sucrose
83
what small molecules can go through the pores in membranes?
glucose and water, and they can pass through diffusion
84
how can the effect of the concentration gradient on the rate of diffusion be investiagted more quantitatively?
-Estimating the concentration of glucose that has diffused into the water surrounding the Visking tubing at each time interval (separate boiling tubes are set up for each time interval) using the semi-quantitative Benedict’s test
-Comparisons of the glucose concentration between the time intervals can be made using a set of colour standards (produced by known glucose concentrations) or a colorimeter to give a more quantitative set of results
-A graph could be drawn showing how the rate of diffusion changes with the concentration gradient between the inside and outside of the tubing
85
describe the practical that uses agar to investigate the rate of diffusion
-The effect of surface area to volume ratio on the rate of diffusion can be investigated by timing the diffusion of ions through different sized cubes of agar
-Coloured agar is cut into cubes of the required dimensions (eg. 0.5cm x 0.5cm x 0.5cm, 1cm x 1cm x 1cm and 2cm x 2cm x 2cm)
-The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
-Measurements can be taken of either
-The time taken for the acid to completely change the colour of the indicator in the agar blocks
-The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time period (eg. 5 minutes)
-These times can be converted to rates (1 ÷ time taken)
-A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area to volume ratio of the agar cubes
86
with an agar cube with a greater surface area to volume ratio, the greater the rate of....
diffusion
87
animal cells can lose and gain water as a result of what?
osmosis
88
what happens if an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution)?
Water will leave the cell through its partially permeable cell surface membrane by osmosis .The volume of the cell will decrease. The cell will consequently shrink/shrivel up.
89
in what environment does crenation occur?
in a hypertonic environment
90
what happens when an animal cell is placed in a solution with a higher water potential?
water will enter the animal cell through its partially permeable cell membrane by osmosis. The volume of the cell will increase. The animal cell has no cell wall to withstand the increased pressure created. The cell membrane will stretch out too far and will eventually burst.
91
in what envrionment does lysis occur?
in a hypotonic environment and is why a constant water potential must be maintained inside the bodies of animals
92
what happens to animal cells in an isotonic environment?
The movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells
93
why is the loss and gain of water less severe in plant cells?
because plant cells have a supporting cell wall
94
what happens if a plant cell is put in a solution with a lower water potential than the plant cell?
water leaves the cell through the partially permeable membrane by osmosis. The volume of the plant will decrease. The protoplast will shrink and pull away from the cell wall. The cell is now plasmolysed
95
what happens if a plant cell is put in a solution with a higher water potential than the plant cell?
water will enter the plant cell through the partially permeable membrane by osmosis. The volume of the plant cell will increase. The protoplast will expand and push against the cell wall. The cell wall withstands the increased pressure created. The pressure increases until the cell is rigid and firm (turgid). The cell becomes fully inflated with water and no more can enter
96
what is it called when a plant cell is fully inflated with water and has become rigid and firm?
it is called turgid
97
why is turgidity important for plants?
as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight
98
what happens if plants do not receive enough water?
the cells cannot remain rigid and firm (turgid) and the plant wilts
99
what cells does plasmolysis only occur in?
plant cells
100
what 2 ways can osmosis in plant cells be analysed?
Using potato cells and using onion cells
101
what does the most common osmosis practial involve?
cutting cylinders of potato and placing them into solutions with a range of different water potentials (usually sucrose solutions of increasing concentration – at least 5 different concentrations are usually required).
102
what is the equation for the percentage change in mass?
(final mass - initial mass/initial mass) X 100
103
what does a positive change in mass in the potato osmosis practical suggest?
that the potato has gained water by osmosis. meaning the solution had a higher water potential than the potato
104
what does a negative change in mass in the potato osmosis practical suggest?
the solution had a lower water potential than the potato.
105
Are calcium ions transported outside of cells via active transport?
Yes
106
What is active transport?
The movement of molecules/ions from a region of lower concentration to a region of higher concentration. Active transport requires energy in the form of ATP
107
Does active transport require the help of carrier proteins?
Yes
108
Describe the steps of active transport
1)Ions bind to receptors on the carrier protein.
2) A molecule of ATP binds to the carrier protein.
3) The ATP undergoes hydrolysis, producing phosphate and a molecule of ADP.
4) The phosphate now attaches to the carrier protein and causes it to change shape.
5)This shape change causes the carrier protein to transport the molecule or ion to the other side of the membrane where it is released.
6)The phosphate now unbinds from the carrier protein, causing the carrier protein to return to its original shape.
7) The ADP and phosphate will later reform ATP during respiration.
109
Because active transport requires a lot of energy, what is there a lot of in cells that carry out a lot of active transport?
Mitochondria
110
What is meant by the fact that the carrier proteins used in active transport are specific?
That each carrier protein will only transport one type of molecule or ion
111
What is active transport important in?
- The reabsorption of useful molecules or ions into the blood after filtration into the kidney tubules.
-The absorption of some products or digestion from the digestive tract
- The loading of sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant.
- The loading of inorganic ions from the soil into root hairs
112
What are the 2 forms of bulk transport?
Endocytosis and exocytosis
113
What is bulk transport?
The use of active transport to move larger molecules and whole cells that cannot move through channel proteins
114
What is endocytosis?
The process by which the cell surface membrane engulfs material, forming an endocytic vacuole around it.
115
What are the 2 types of endocytosis?
Phagocytosis and pinocytosis
116
What is phagocytosis?
When material is taken into a cell in solid form
117
What is pinocytosis?
When material is taken into a cell in liquid form
118
What is exocytosis?
When materials are removed from or transported out of the cell in vesicles, which fuse with the cell membrane and release the materials out of the cell out of the cell
119
What is the role of the ileum in the small intestine?
To absorb the molecules produced by digestion. This includes glucose
120
What does the sodium -potassium pump do?
It actively transports sodium ions outside of the epithelial cells and into the bloodstream. At the same time, it actively transports potassium ions into the epithelial cells
121
What does the sodium glucose cotransporter do?
It allows sodium ions do diffuse down itself and down the concentration gradient into the epithelial cell. The sodium glucose cotransporter also actively transports glucose molecules into the cell
122
How does the fact that the membrane of the microvilli is folded into a large number of microvilli help with the rate of diffusion and the rate of active transport?
This increases the surface area of the membrane, providing more space for the membranes needed for facilitated diffusion and active transport. So, both of their rates are increased.
