Biological Membranes Flashcards
1
Q
What are the two components of a phospholipid?
A
- Hydrophobic tail
- Hydrophilic head
2
Q
What do membranes cover/surround?
A
The surface of every cell and most organelles
3
Q
Functions of membranes
A
- Keeping all cellular components inside the cell
- Allowing selected molecules to move in and out of the cell
- Isolating organelles from the rest of the cytoplasm, which allows cellular processes to occur separately
- Site for biochemical reactions
- Allows the cell to change shape
4
Q
What was further evidence against the Dawson-Danielli model?
A
Freeze-fracture images of cell membranes
5
Q
What did freeze-fracture images of cell membranes lead to?
A
The development of the fluid mosaic model
6
Q
Who development the fluid mosaic model?
A
Singer and Nicholson
7
Q
When did Singer and Nicholson development the fluid mosaic model?
A
1972
8
Q
What did the fluid mosaic model suggest?
A
Proteins are within, not outside, the phospholipid bilayer
9
Q
What is one of the main components of membranes?
A
Phospholipids
10
Q
Why do phospholipids form the shape of the structure that they do?
A
Their polar nature and the way they interact with water
11
Q
When exposed to water, what structures do phospholipids form?
A
- Micelle
- Bilayer
12
Q
What way do the hydrophilic heads face?
A
Towards water
13
Q
What way do the hydrophobic tails face?
A
Away from water (inwards)
14
Q
What behaviour of phospholipids is key to the role that they play in membranes?
A
How they react when they are in water
15
Q
Explain why phospholipids form a bilayer in plasma membranes
A
- Phospholipids have a polar phosphate group which is hydrophilic and will face the aqueous environment
- The fatty acids are non-polar and will move away from an aqueous environment
- As both tissue fluid and cytoplasm are aqueous, phospholipids form two layers with the hydrophobic tails facing inwards and the phosphate groups face outwards, interacting with the aqueous environment
16
Q
How much of cells do proteins make up
A
Between 25-75% depending on the cell type
17
Q
Where are intrinsic/transmembrane proteins?
A
Proteins that span the whole width of the membrane
18
Q
Where are extrinsic proteins?
A
Confined to the inner or outer surface of the membrane
19
Q
Many of the proteins in a membrane are…?
A
Glycoproteins
20
Q
What are glycoproteins?
A
Proteins with attached carbohydrate chains
21
Q
What are intrinsic/integral proteins?
A
Transmembrane proteins that are embedded through both layers of a membrane
22
Q
What do intrinsic proteins have on their external surfaces?
A
Amino acids with hydrophobic R-groups
23
Q
What do the hydrophobic R-groups on intrinsic proteins interact with and why?
A
The hydrophobic core of the membrane as it keeps them in place
24
Q
What type of protein are channel and carrier proteins?
A
Intrinsic
25
What are channel and carrier proteins involved in?
Transport across the membrane
26
What do channel proteins do?
Provides a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes
27
What holds channel proteins in position?
By interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins
28
What do carrier proteins have an important role in?
Passive transport (down a concentration gradient) and active transport (against a concentration gradient) into cells
29
What does passive and active transport into cells via carrier proteins often involve?
Changing the shape of the protein
30
What can the carrier proteins often be?
- Receptor for hormones
- Receptor for neurotransmitters
- Enzymes for catalyst reactions
31
What type of proteins are glycoproteins?
Intrinsic
32
Where are glycoproteins?
Embedded in the cell-surface membrane with attached carbohydrate chains of varying lengths and shapes
33
What roles do glycoproteins have?
- Cell adhesion
| - Receptors for chemical signals
34
What is cell adhesion?
When cells join together to form tight junctions in certain tissues
35
Describe the process of cell communication/cell signalling
- When the 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
36
What are examples of cell signalling?
- Receptors for neurotransmitters such as acetylcholine at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone
- Receptors for peptide hormones, including insulin and glucagon, which affects the uptake and storage or glucose by cells
- Some drugs act by binding to cell receptors. For instance, beta blockers are used to reduce the hearts response to stress
37
What are glycolipids similar to?
Glycoproteins
38
What are glycolipids?
Lipids with attached carbohydrate chains
39
What are glycolipids also called?
Cell markers or antigens
40
What can recognise glycolipids and what do they recognise glycolipids as?
```
Cells of the immune system
As self (of the organism) or non-self (of cells belonging to another organism)
```
41
Where can extrinsic proteins be found?
Free on the cell membrane or bound to an intrinsic protein. Can only be found on one side of the bilayer, but can move in between layers
42
If an extrinsic protein is on the extracellular side then it?
- Acts as receptors for hormones or neurotransmitters
| - Involved in cell recognition (many are glycoproteins)
43
If an extrinsic protein is on the cytosolic side then it?
- Involved in cell signalling
| - Involved in chemical reactions
44
What can extrinsic proteins on the cytosolic side do?
Can dissociate from the membrane and move into the cytoplasm
45
What do extrinsic proteins normally have and do?
Have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins
46
What type of biological molecule is cholesterol
Lipids
47
What are the roles of cholesterol in membranes?
- Controlling membrane fluidity
| - Keeps membranes stable at normal body temperature, without it, cells would burst
48
The membrane is this when there is more cholesterol
Less fluid and less permeable
49
Without cholesterol, what would happen to cells?
Cells would burst
50
What does cholesterol have (in terms of water)?
Hydrophilic end
| Hydrophobic end
51
What is the molecular formula of cholesterol?
C27H46O
52
Where is cholesterol located in cell membranes?
Positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the tails, pulling them together
53
What gives membranes stability without making them too rigid?
Cholesterol
54
How does cholesterol give membranes stability?
The hydrophilic end interacting with the tails, pulling them together
55
How does cholesterol not make membranes too rigid?
By stopping the phospholipids from grouping too closely and crystallising
56
The smaller and less polar a molecule the...
Easier and faster it will diffuse across a cell membrane
57
How fast do small, non-polar molecules such as oxygen and carbon dioxide diffuse across a membrane?
Rapidly
58
How fast do small, polar molecules, such as water and urea, diffuse across a membrane?
More slowly than small, non-polar molecules
59
How fast do charged particles (ions) diffuse across a membrane?
They are unlikely to diffuse across, even if they are very small
60
Why is it important for a cell membrane to maintain its fluidity?
The cell would not be able to function
61
A fluid membrane is needed for?
- Diffusion of substances across the membrane
- Membranes to fuse e.g. a vesicle fusing with the cell membrane during exocytosis
- Cells to move and change shape e.g. macrophages during phagocytosis
62
What is the effect of temperature on membranes?
An increase in the kinetic energy of phospholipids disrupts the structure of the membrane, creating gaps between the phospholipids making it more permeable as the molecules can pass through the gaps
63
What is the effect of solvents on membranes?
- Water is a polar molecule and it is important in creating membrane stability with the phospholipids
- Other solvents, like alcohols, are less polar, or benzene, which is not polar
- This can move into the bilayer, disrupting the
structures
64
Explain a graph of the permeability of membranes
- Phospholipids don't have much energy, so can't move much, so they are packed close together, which forms a rigid layer.
Channel and carrier proteins denature
Ice crystals may pierce membrane, which increases permeability significantly
- Phospholipids can move as they aren't packed closely together.
The membrane is partially permeable
The temperature gives the phospholipids more energy, which increases the permeability
- Phospholipid bilayer starts to melt and become more permeable
The water on the inside of the cell puts pressure on the membranes
Channel and carrier proteins denature, increasing permeability
65
All cells are surrounded by a?
Partially-permeable membrane
66
How does the cell get what it needs and get rid of what it does not want?
"Imports" substances it needs
| "Exports" waste and substances needed outside of the cell
67
Define diffusion
The net movement of molecules down a concentration gradient
68
What type of transport is diffusion and why?
Passive, as no metabolic energy is used
69
Give an example of where diffusion is used in humans
Gas exchange across respiratory systems i.e. lungs and gills
70
What is Fick's law used to summarise?
The rate of diffusion in a given direction across an exchange surface
71
What is Fick's law?
Rate of diffusion is proportionate to: surface area x difference in concentration / length of diffusion path (thickness of membrane)
72
What are the factors that affect diffusion?
- Surface area
- Thickness
- Concentration gradient
- Temperature
- Length of diffusion path (diffusion distance)
73
What acts as a barrier to most substances in membranes?
The non-polar, hydrophobic tails of the phospholipid molecules
74
What is simple diffusion?
Molecules go straight through the bilayer
75
What type of molecules can use simple diffusion?
Lipid soluble
76
What is facilitated diffusion?
Large and/or hydrophilic particles can use facilitated diffusion
77
What type of molecules cannot pass straight through the bilayer?
Polar/large molecules
78
What lets polar/large molecules through the bilayer?
Carrier/Channel proteins
79
What type of transport is facilitated diffusion?
Passive
80
Define osmosis
The net movement of water molecules from a region of high water concentration to a region of low water concentration, through a partially permeable membrane
81
What is osmosis used for?
The process by which cells exchange water with their environment, such as in the mammalian kidney
82
What is osmosis determined by?
The difference in water potential between two solutions connected by a partially-permeable membrane
83
What is water potential?
The tendency of water molecules in a system to move
84
What symbol denotes water potential?
Ψ (psi)
85
What is water potential measured in?
kiloPascals (kPa)
86
What is the water potential of pure water?
0 kPa
87
What water has the highest water potential?
Pure water
88
Solutions have a ... water potential than pure water
Lower, negative
89
What is true of water molecules in terms of what areas they move to and from based on their water potential?
Water molecules always move from a region of high water potential to a region of low water potential
90
What is the water potential of a solution affected by?
The amount of solute it contains
91
What happens to the water potential of a solution when there is a high amount of solute?
It is lower
92
Why does an increase in the amount of solute decrease the water potential?
The water molecules bind to the solute molecules, reducing the number of water molecules that are free to diffuse
93
Is the solute potential positive or negative?
Negative
94
What is the symbol for solute potential?
Ψs (psi)
95
Other than the amount of solute in a solution, what also affects the water potential of a solution?
The pressure applied to it
96
The greater the pressure...?
The higher the water potential
97
What is the symbol for pressure potential?
Ψp
98
Is the pressure potential positive or negative?
Positive
99
In plant cells, what is the pressure potential a result of?
The cell wall exerting pressure on the cytoplasm
100
What is the calculation for water potential?
Ψ = Ψs + Ψp
| Water potential = solute potential + pressure potential
101
What does osmotic concentration relate to?
The amount of dissolved solutes in a solution
102
What does not affect osmotic concentration?
Insoluble molecules
103
What do partially permeable membranes only let through?
Water and very small molecules
104
What does a partially permeable membrane result in?
An uneven distribution across the membrane
105
What osmotic concentration do isotonic solutions have?
The same as the cytoplasm of the cell
106
What happens to the cell when it is placed in an isotonic solution (both plant and animal)?
The cell does not change (both animal and plant)
In a plant cell, incipient plasmolysis (the protoplast is just pulled away from the cell wall) (plant only)
The net movement of water into and out of the cell
107
What osmotic concentration do hypertonic solutions have?
A higher osmotic concentration than the cytoplasm of the cell
108
What happens to the cell when it is placed in a hypertonic solution (both plant and animal)?
The animal cell shrinks
Cell wall exerts an inward pressure. Cell is turgor
The net movement of water out of the cell
109
What do hypertonic solutions contain more of, water or solute, compared to the cell?
Solute
110
What do isotonic solutions contain more of, water or solute, compared to the cell?
The same amount of each
111
What do hypotonic solutions contain more of, water or solute, compared to the cell?
Water
112
What osmotic concentration do hypotonic solutions have?
A lower osmotic concentration than the cytoplasm of the cell
113
What happens to the cell when it is placed in a hypotonic solution (both plant and animal)
Can cause animal cells to burst
Plant - full plasmolysis; causes a plant to wilt
The net movement of water into the cell
