1.3 cell membranes and transport Flashcards
1
Q
Width of cell membrane?
A
Doesn’t vary between organisms: 7-8nm
2
Q
How does the cell membrane appear under an electron microscope?
A
A double line
3
Q
Functions of cell membrane?
A
- The plasma membrane is the boundary that separates the living cell from its non-living surroundings.
- Controls what substances pass into and out of the cell.
- Controls the uptake of nutrients.
- Allows waste products to pass out of cell.
- Responsible for secreting substances such as enzymes and glycoproteins, by exocytosis.
- Responsible for cell recognition and provides receptor sites for hormones.
4
Q
Structure of cell membrane
A
Made up of almost entirely phospholipids and proteins.
5
Q
Phospholipids can form…
A
bilayers (with one sheet of phospholipids forming over another)
6
Q
Is the phosphate head of the phospholipid polar or non polar
A
Polar molecule - hydrophilic.
7
Q
What are polar molecules attracted to?
A
To other polar molecules, like water.
8
Q
How many fatty acid tails are there in the phospholipid bilayer?
A
2 fatty acid tails.
9
Q
Are the 2 fatty acid tails in the phospholipid polar or non polar?
A
Non polar (hydrophobic) and repel water.
10
Q
What forms the basis of the membrane structure?
A
The phospholipid bilayer
11
Q
What does the phospholipid component do?
A
Allows lipid-soluble (non polar) molecules to enter and leave the cell but prevents water soluble (polar) molecules from doing so
12
Q
How are proteins arranged in the membrane?
A
Arranged randomly in contrast to more regular patterns of phospholipids.
13
Q
Where do extrinsic proteins occur?
A
on the surface of the bilayer or are partly embedded in it
14
Q
What do extrinsic proteins do?
A
Provide structural support and form recognition sites by identifying cells
15
Q
Where do intrinsic proteins span?
A
Span the bilayer
16
Q
What do intrinsic proteins do?
A
Some act as channels or carriers to facilitate the diffusion of polar (water soluble) molecules, like ions, across the cell membrane.
Others form pumps and carry out active transport against a concentration gradient (low to high concentration which requires ATP)
17
Q
What changes a cell membrane’s properties?
A
Number and type of proteins within the cell membrane
18
Q
What does the fluid mosaic model by Singer and Nicholson propose?
A
- Bimolecular phospholipid layer.
- Extrinsic proteins.
- Intrinsic proteins.
- Movement.
- Fluid mosaic.
19
Q
Description/structure of the Bimolecular Phospholipid Layer?
A
Phospholipids form a bilayer. Polar heads are hydrophilic and associate with water. Non-polar tails turn towards each other since they’re hydrophobic.
20
Q
Function of the Biomolecular phospholipid layer?
A
Forms the basis of the cell membrane and allows lipid soluble molecules to diffuse in-out of the cell.
21
Q
Description and structure of extrinsic proteins
A
Occur on the surface of the cell membrane, closely associated with polar heads.
22
Q
Function of extrinsic proteins
A
Form the cell recognition sites
23
Q
Description and structure of the intrinsic proteins
A
Span the entire phospholipid bilayer
24
Q
Function of intrinsic proteins
A
Allow the transport of polar molecules. Channels and carriers allow transport down a concentration gradient by facilitated diffusion.
Pumps transport charged molecules against a concentration gradient by active transport
25
Description of movement
Phospholipids can move in relation to each other: they're not fixed, but fluid.
26
How can fluidity of the phospholipids be increased?
Increased by an increase in temperature, which increases permeability of the cell membrane.
27
Fluid mosaic description
The proteins can also move in relation to each other due to the fluidity of the phospholipid bilayer. Extrinsic and intrinsic proteins are dotted throughout the bilayer and form a mosaic pattern
28
What is cholesterol
A type of sterol found in animal cells. Fits between the phospholipid molecules, stabilising the cell membrane at high temperatures and maintaining fluidity at low temperatures.
29
What are glycolipids
Lipids which have combined with short polysaccharide chains. Glycoproteins are also attached to chains of polysaccharide, reaching out from the upper membrane surface
30
What is the glycocalyx
Outer layer of carbohydrates surrounding animal cells
31
Function of glycocalyx
Molecules within the glycocalyx provide hormone receptor sites and take part in cell to cell recognition
32
The membrane as a barrier
Cell surface membrane is selectively permeable to water and some solutes.
Lipid soluble (non polar) substances can move through the membrane more easily than water soluble (polar) substances
33
How do small uncharged molecules diffuse (non polar)
Small uncharged molecules like oxygen and carbon dioxide freely pass through the membrane by simple diffusion
34
How do lipid soluble molecules diffuse (non polar)
Lipid soluble molecules like glycerol can also pass through the membrane through the phospholipid bilayer
35
What does the hydrophobic core of the membrane do to ions and polar molecules?
The hydrophobic core of the membrane impedes the transport of ions and polar molecules.
36
Why can’t charged particles (ions) and relatively large charged molecules (glucose) diffuse across the non polar centre of the phospholipid bilayer
Can’t diffuse across the non pilar hydrophobic centre of the phospholipid bilayer as they’re insoluble in lipid
37
What are intrinsic proteins
Proteins which extend across both phospholipid layers
38
How do these ions and pilar molecules cross the membrane
Intrinsic proteins allow them to cross the membrane. Channels and carriers allow facilitated diffusion (Diffusion helped by an intrinsic protein). Pumps carry out active transport.
39
What factors affect the permeability of membranes
- increasing temperature
- increasing ethanol concentration.
- increasing sodium chloride concentration
- increasing detergent concentration
40
Explain how increasing temperature affects permeability of a membrane
Increased heat energy increases kinetic energy. The phospholipids vibrate further apart which increases membrane permeability.
41
What happens to membrane after 40°C
Membrane is stable up to 40°C.
Proteins in membrane denature at high temps which allow betalains to diffuse out of the cells more readily.
42
Explain how increasing ethanol concentration affects permeability of a membrane
Organic solvents like ethanol dissolve phospholipids. The greater the concentration of ethanol, the more permeable the membranes become.
43
Explain how increasing sodium chloride concentration affects permeability of a membrane
Sodium irons attached to the oxygen atoms on the hydrophilic heads of the phospholipid bilayer. This reduces mobility of the phospholipid molecules so less betalain is released. As sodium chloride concentration increases the permeability will decrease.
44
Explain how increasing detergent concentration affects permeability of a membrane
Detergents reduce surface tension of phospholipids and disperse the membrane. As the concentration of detergents increases, the permeability of the membrane increase.
45
What do beetroot cells have?
Beetroot cells have both of cellulose cell wall, cell membrane, tonoplast and a large permanent vacuole
46
What are betalains
Red pigments inside the vacuole
47
Testing cell membrane and tonoplast permeability in beetroot cells
Any changes in the tonoplast on cell membrane will result in some of the cell contents being released, and the betalain will cover any surrounding bathing medium
48
What is diffusion?
Diffusion is the movement of molecules or ions from a region of high concentration to lower concentration, down a concentration gradient, until they're equally distributed
49
What is equilibrium?
Concentration equal on both sides of cell membrane. No net movement of water.
50
What kind of process is diffusion?
Passive process - no ATP needed.
51
Factors which affect the rate of diffusion
- Concentration gradient.
- Distance of travel.
- The surface area of the membrane.
- The thickness of the membrane
- An increase in temperature.
- Particle size.
52
Explain the effect of concentration gradient on rate of diffusion
Steeper = faster.
The greater concentration gradient, the greater the rate of diffusion.
53
Explain the effect of distance of travel on rate of diffusion
Shorter = faster.
The shorter the distance of travel, the greater the rate of diffusion.
54
Explain the effect of surface area of membrane on rate of diffusion
Larger = faster.
The larger the surface area, the greater the rate of diffusion.
55
Explain the effect of thickness of membrane on rate of diffusion
Thinner = faster.
The thinner the membrane the greater the rate of diffusion (short diffusion pathway)
56
Explain the effect of increase in temperature on rate of diffusion
Higher temp = faster.
Increase in temperature increases molecular kinetic energy and therefore increases rate of diffusion
57
Explain the effect of particle size on rate of diffusion
Smaller = faster.
Small particles diffuse faster than larger molecules.
58
Simple diffusion on graph
Linear graph. Rate of uptake directly proportional to the concentration difference across membrane
59
Relationship between rate of uptake and concentration difference across membrane on simple diffusion graph
As the concentration gradient increases, the rate of diffusion across the membrane will also increase.
60
Effect of stopping respiration or using toxin on simple diffusion?
Stopping respiration or killing the cell with a toxin like cyanide won't stop diffusion as it needs no ATP from the cell.
61
What molecules are transported directly across the phospholipid bilayer of the cell membrane by simple diffusion
Carbon dioxide and oxygen
62
What is facilitated diffusion
Diffusion which is helped/facilitated by intrinsic proteins.
63
Why do charged particles, ions and large molecules need facilitated diffusion
They can't readily pass through the phospholipid bilayer (because they're insoluble in lipid). So intrinsic proteins allow these to diffuse in or out of cells
64
Why can't charged particles, ions and large molecules readily pass through the phospholipid bilayer?
Because they're insoluble in lipid
65
What are the two types of intrinsic proteins?
Channels and carriers
66
What are channel proteins
An intrinsic protein consisting of pores lined with polar, hydrophilic groups, which allows charged ions to pass through.
Each channel is specific for one type of ion.
67
What are gated channels
Proteins which can open and close depending on the needs of the cell - like a channel protein
68
Structure of channel protein
Within the phospholipid bilayer. Channel and the tunnel has a polar lining/has charge, so anything with charges can go through
69
How do channel proteins work?
Charged ions can go through the tunnel of the channel, since the tunnel is polar so anything with a charge can go through.
70
What are carrier proteins
Proteins that allow facilitated diffusion across the membrane of larger polar molecules like sugars and amino acids.
71
How do carrier proteins work?
A molecule attaches to a carrier protein at the binding site and causes the carrier protein to change shape/rotate within the membrane. This releases the molecule on the other side of the membrane.
72
Structure of carrier protein
Within the phospholipid bilayer. Has a binding site and either flips, rocks, or spins.
73
Effect of carrier and channel proteins on rate of diffusion
Increase the rate of diffusion down the concentration gradient, without the need for energy in the form of ATP from respiration
74
Why is there an initial increased rate of diffusion on a graph of facilitated diffusion?
Because the channel and carrier proteins are speeding up the process
75
Why does the rate of diffusion level off on a graph of facilitated diffusion?
All the channel or carrier proteins are occupied (not enough proteins), so rate of diffusion is limited
76
What happens to facilitated diffusion if respiratory inhibitors are added
Not affected by respiratory inhibitors like cyanide, because they stop ATP but ATP isn't required.
77
What is co-transport
A type of facilitated diffusion that transports molecules and ions together through the same carrier protein
78
Why is sodium-glucose co transport important?
Important in absorbing glucose and sodium ions across cell membranes and into the blood in the ileum and kidney nephrons
79
Difference between channel and carrier
Channel doesn't have a binding site but carriers do.
80
Steps of sodium-glucose co-transport
1. Sodium ions and glucose molecule bind to specific attachment sites on a carrier protein in the cell membrane.
2. Carrier protein changes shape and deposits the sodium and glucose into the cell
3. Sodium and glucose diffuse separately across the cytoplasm to the opposite membrane.
4. Sodium ions pumped out of epithelial cells by active transport. This lowers sodium concentration inside the cell, maintaining the concentration gradient needed for the diffusion of sodium ions from the gut lumen into the cell
5. Glucose leaves the epithelial cells by facilitated diffusion and enters blood in capillaries
81
What process pumps sodium ions out of epithelial cells?
Active tranport
82
Why are sodium ions pumped out of epithelial cells in co transport?
Lowers sodium concentration inside the cell, maintaining the concentration gradient needed for the diffusion of sodium ions from the gut lumen into the cell
83
What process does glucose use to leave the epithelial cells
Facilitated diffusion
84
What is osmosis
the movement of water from a region of higher water potential to a region of lower water potential, across/through a selectively permeable membrane, down a water potential gradient
85
Most cell membranes are permeable to....
Water and certain solutes only
86
What is water potential?
The tendency of water molecules to move from a high to low concentration of water, down a water potential gradient
87
What has the highest water potential
Pure water - water potential of 0
88
What happens to water potential as you add solutes
water potential decreases
89
What does it mean if water potential decreases
Fewer water molecules able to move about freely
90
The more concentrated a solution, the more... (negative/positive)... the water potential?
Negative.
Fewer free water molecules there are.
91
A higher water potential implies a...
greater tendency of water to leave a system by osmosis.
Water will diffuse from a region of higher to lower water potential
92
How to calculate water potential
Water potential = solute potential + pressure potential
93
What is solute potential?
The concentration of dissolved substances inside the cell vacuole. Always a negative value
94
What is hydrostatic pressure
When water enters a plant cell vacuole by osmosis, the vacuole swells and the cell contents are pushed against the cell wall which generates a hydrostatic pressure
95
What is pressure potential?
The opposing force which develops as the outward pressure builds up the cell wall. Usually a positibe value
96
What does the pressure potential/opposing force do
Resists the movement of more water into the cell.
97
How to determine the net direction of water movement
Calculate the water potential of each cell.
E.g. -360kPa ---> -380kPa ---> -400kPa
Goes from highest (closest to 0) to lowest.
98
What is hypotonic
Lots of water - dilute - high water potential
If cell is hypertonic the outside is hypotonic
99
What will happen if the water potential of the external solution is hypotonic (higher than solution in cell)
Water will move into the cell by osmosis, causing the cell to swell
100
What is lysis
Cell bursting
101
Why does lysis happen in animal cells?
No cell wall to prevent bursting
102
What happens to animal cells in a hypotonic environment
Water will move into the cell by osmosis, causing the cell to swell.
May burst because they have no cell walls to prevent it
103
What happens to plant cells in a hypotonic environment
Water will move into the cell by osmosis, causing the cell to swell.
Cell will become turgid as the cell content pushes against cell wall
104
What is good about plant cells being turgid
Helps support plant tissues
105
What will happen if the cell wall is damaged in a hypotonic environment
Cell will swell then burst (lysis) because cell wall doesn't work properly to prevent it from bursting
106
What is a hypertonic environment
Concentrated.
Has a lower water potential than the cell
107
What will happen to animal cells in a hypertonic environment
Water will move out of the cell by osmosis, causing the cell to shrink.
108
What will happen to plant cells in a hypertonic environment
Water will move out of the cell by osmosis.
Plasmolysis occurs (vacuole and cytoplasm shrink, so cell membrane pulls away from cell wall), causing the cell to be flaccid and the whole plant to wilt
109
What is plasmolysis?
In a plant cell, the vacuole and cytoplasm volume shrink, causing the cell membrane to pull away from the cell wall.
Usually fatal.
Water leaves the plant cell by osmosis.
110
What is an isotonic medium
If the cell has the same solute concentration as the surrounding solution, the cell is in an isotonic medium.
111
What does homeostasis do
Maintains a cells external environment to prevent water loss or gain
112
What kind of external medium do plant cells need to maintain turgor pressure
Need a hypotonic external medium
113
How can the internal concentration of a cell be calculated?
Using potato cylinders
114
What is the point of incipient plasmolysis (on a graph)
Point where everything is equal.
-no net movement of water
-no change in mass
-equilibrium
-isotonic
115
Explain the changes in mass of a cell at 0.2M sucrose solutions (dilute)
-HYPOTONIC external environment so water enters cell.
-water moves by osmosis from a high water potential outside the cell to a lower water potential inside the cell, down a water potential gradient across a selectively permeable membrane.
-mass of the cell increases due to water entering.
-cell appears TURGID.
116
Explain the changes in mass of a cell at 0.3M sucrose solutions
-ISOTONIC external environment, so equilibrium
-water potential is equal both inside and outside of the cell, so there is no net movement in water.
-so no change in mass since water doesn't enter or exit.
-cell appears FLACCID
117
Explain the changes in mass of a cell at 1.0M sucrose solutions (concentrated)
-HYPERTONIC external environment, so water leaves cell.
-water moves from a high water potential inside the cell to a lower water potential outside the cell, down a water potential gradient through a semi permeable membrane.
-mass of cell decreases because of water exiting.
-cell appears PLASMOLYSED.
118
what is incipient plasmolysis
the point at which the cell membrane just begins to move away from the cell wall.
119
How can the external concentration where incipient plasmolysis occurs be calculated?
Using red onion epidermal cells
120
What percentage of cells show signs of plasmolysis at the point of incipient plasmolysis?
50% of the cells
121
What is the water potential at the point of incipient plasmolysis
0kPa
122
Net transport of water by osmosis in a HYPOTONIC external medium
water moves into the cell by osmosis across a selectively permeable membrane.
Animal cell - lysis.
Plant cell - turgid.
123
Net transport of water by osmosis in a HYPERTONIC external medium
water moves out of the cell by osmosis across a secretively permeable membrane
Animal cell - shrinks
Plant cell - plasmolysed
124
Net transport of water by osmosis in a ISOTONIC external medium
No net movement of water. Equilibrium (water potential equal in and out of cell).
Animal cell - equilibrium.
Plant cells - flaccid. Incipient plasmolysis
125
What is endocytosis
Taking large particles into cells by endocytosis.
Cell membrane folds and engulfs particles or liquid, forming a vesicle which enters the cytoplasm. Needs ATP.
126
Steps of endocytosis
1. plasma membrane folds inwards (folding requires ATP).
2. plasma membrane continues to fold, engulfing the material.
3. plasma membrane fuses/closes on itself to enclose the material.
4. forms a vesicle (with material inside)
127
Two types of endocytosis
Phagocytosis (cell eating/engulfing the material)
Pinocytosis (entry of liquid into the cell)
128
What is exocytosis
Substances leave the cell by secretion.
129
Steps of secretion
1. Transport vesicles transport proteins from the RER to the Golgi Body for modification into functional proteins.
2. Secretory vesicles form to transport the functional proteins to the cell surface membrane.
3. Secretory vesicles merge with the cell membrane, releasing the proteins to the external medium
130
What is active transport
A process where ions and molecules are moved across membranes, AGAINST a concentration gradient; from a region of low concentration to a region of higher concentration.
Requires ATP. Ions and molecules move in the opposite direction to diffusion.
131
What will happen to active transport if a respiratory inhibitor like cyanide is used
Active transport won't happen.
Because ATP is needed, anything which affects the respiratory process will affect active transport
132
How is active transport carried out/with what?
Carried out by specialised carrier proteins called pumps
133
Why is active transport important for plants
Allows plants to take up substances which are at very low external concentration. E.g. nitrate from soil water into root hair cells
134
Steps of active transport
1. molecule or ion that needs to be transported attaches to a binding site on a specific pump.
2. ATP transfers a phosphate group to the pump on the inside of the membrane.
3. This causes the pump to change shape, which in turn transports the ion or molecule across the membrane.
4. Molecule or ion is released into or out the cell
135
What is a pump
A kind of intrinsic protein for active transport.
Changed shape to transport ions/molecules across a membrane
