plasma membranes Flashcards
1
Q
What is the plasma
membrane?
A
All the membranes of cells, which have the same basic structure described by the fluid-mosaic model The cell-surface membrane which separates the cell from its external environment
2
Q
What is the phospholipid
bilayer?
A
Arrangement of phospholipids found in cell membranes; the hydrophilic phosphate heads form both the inner and outer surface of a membrane, sandwiching the fatty acid tails to form a hydrophobic core
3
Q
Why are phospholipid
bilayers suited as
membranes?
A
• Cells normally exist in aqueous environments • The inside of cells and organelles are also usually aqueous environments • Phospholipid bilayers are suited because the outer surfaces of the hydrophilic phosphate heads can interact with water
4
Q
What is the fluid-mosaic
model?
A
Model of the structure of a cell membrane in which phospholipids within the phospholipid bilayer are free to move and proteins of various shapes and sizes are embedded in various positions • A model proposed by American scientists Singer and Nicolson in 1972 • Explains how cell membranes are dynamic and interact with the cells environment
5
Q
Describe cell membrane
components
A
a. Glycoprotein - branching carbohydrate portion of a protein which acts as a recognition site for chemicals e.g. hormones b. Glycolipid - acts as a recognition site e.g. for cholera toxins c. Cholesterol - for stability/flexibility d. Hydrophilic heads of phospholipid molecules - point outwards e. Protein molecule lying on the surface (extrinsic protein) f. Pore g. Protein molecule spanning the phospholipid layer (intrinsic protein) h. Hydrophobic tails of phospholipid molecules - point inwards i. Protein molecule partly embedded (extrinsic protein
6
Q
What a membrane proteins?
A
Protein components of cell-surface
membranes
7
Q
What are intrinsic proteins?
A
Also known as integral proteins • Transmembrane proteins that are embedded through both layers of a membrane • Have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane, keeping them in place • Channel and carrier proteins (both involved in transport across the membrane
8
Q
What are channel proteins?
A
Membrane proteins that provide a hydrophilic channel through a membrane, which allows passive movement of polar molecules and ions down a concentration gradient through membranes • Held in position by interactions between the hydrophobic core of the membrane, and the hydrophobic R-groups on the outside of the proteins
9
Q
What are carrier proteins?
A
Membrane proteins that have in important role in both passive transport and active transport into cells • This often involves the shape the protein changing
10
Q
What are glycoproteins?
A
Extrinsic membrane proteins with attach carbohydrate molecules of varying lengths and shapes that are embedded in the cell-surface membrane • Play a role in cell adhesion and as receptors for chemical signals
11
Q
What is cell signalling?
A
A complex system of intercellular communication • 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
12
Q
Give examples of cell
signalling
A
Receptors for neurotransmitters e.g. acetylcholine at nerve cell synapses. Binding of the neurotransmitters triggers or prevents an impulse in the next neurone • Receptors for peptide hormones inc. insulin and glucagon, which affect the uptake and storage of glucose by cells • ß-blockers are used to reduce the response of the heart to stress
13
Q
What are glycolipids?
A
Cell-surface membrane lipids with attached carbohydrate molecules of varying lengths and shapes • Called cell markers or antigens, and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism)
14
Q
What are extrinsic proteins?
A
Also called peripheral proteins • Present in one side of the bilayer • Normally have hydrophilic Rgroups on their outer surface and interact with the polar heads of the phospholipids or with intrinsic proteins • Can be present in either layer, and some move between layers
15
Q
What is cholesterol?
A
A lipid with a hydrophilic end and a
hydrophobic end, like a
phospholipid
16
Q
What is the role of
cholesterol molecules in a
membrane?
A
• Regulates the fluidity of membranes • Positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads, and the hydrophobic end interacting with the tails, pulling them together • Adds stability to membranes without making them too rigid • Prevent the membranes becoming too solid by stopping the phospholipid molecules rom grouping too closely and crystallising (at low temps)
17
Q
What is the role of
membranes within cells?
A
Cristae in mitochondria give a large SA for the some of the reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur • Thylakoid membranes in chloroplasts have chlorophyll, and one these membranes some of the reaction of photosynthesis occur • Digestive enzymes on the plasma membranes of epithelial cells that line the small intestine, and these enzymes catalyse some of the final stages in the breakdown of certain types of sugars
18
Q
What factors affect
membrane structure?
A
- Temperature
* Presence of solvents
19
Q
What happens to a
membrane when
temperature drops?
A
Saturated fatty acids become compressed • Many unsaturated fatty acids in phospholipid bilayer, and as they become compressed, kinks in their tails push adjacent phospholipids molecules away, maintaining membrane fluidity • Proportions of different types of fatty acids determine the membrane’s fluidity at low temps • Cholesterol prevents reduction in the membrane’s fluidity
20
Q
What happens to a
membrane when
temperature increases?
A
Phospholipids get more KE, move around more, in a random way, increasing the membrane’s fluidity • Permeability increases • Affects the way membraneembedded proteins are positioned and may function • If some of the proteins that act as enzyme drift sideways, it could alter the ROR they catalyse • Increase in membrane fluidity may affect the infolding of the plasma membrane during phagocytosis • Cholesterol molecules buffer to some extent the effects of increasing heat, as they reduce the increase in membrane fluidity
21
Q
What happens to proteins
when temperature
increases?
A
Atoms within their large molecule vibrate, reading the hydrogen and ionic bonds that hold their structure together • They unfold • Tertiary structure changes and cannot change back again when they cool - they are denatured • If both the membrane-embedded proteins and the cytoskeleton threads become denatured, the plasma membrane will begin to fall apart, and it will be more permeable there will be holes in it
22
Q
What type of solvents affect
membranes?
A
Organic solvents (e.g. alcohols)and non-polar solvents (e.g. benzene
23
Q
What effect do organic
solvents have on
membranes?
A
• They will dissolve membranes, disrupting cells • Pure or very strong alcohol solutions are toxic as they destroy cells in the body • Less concentrated solutions e.g. alcoholic drinks, will not dissolve membranes, but still cause damage • Non-polar alcohol molecules enter the cell membrane, and the presence of these molecules between the phospholipids disrupts the membrane • Disrupted membrane = more fluid and more permeable
24
Q
Give an example of a cell
that needs an intact cell
membrane for a specific
function
A
• Nerve cells • Need intact cell membranes for the transmission of nerve impulses • When neuronal membranes are disrupted, nerve impulses are no longer transmitted as normal • Also happens in neurones in the brain, explaining by behaviour changes after the consumption of alcoholic drinks
25
What is diffusion?
```
The net movement of particles from
a region of higher concentration to a
region of lower concentration.
• Passive process i.e. only uses the
KE of the molecules, NOT ATP
• Takes place down a concentration
gradient
• Will continue until there is a
concentration equilibrium between
the two areas
```
26
What is simple diffusion?
Diffusion in the absence of a barrier
| or membrane
27
What are the factors that
affect the rate of simple
diffusion?
```
Temperature
• As temp increases, molecules
have more KE, so rate of diffusion
will increase
Concentration gradient
• The steeper the gradient, the
faster the diffusion to the side
where there are fewer molecules,
down the gradient
```
28
What type of molecules can
| diffuse across membranes?
```
• Membranes are partially
permeable
• Non-polar molecules
• Ions cannot easily pass through
because they are repelled by the
hydrophobic interior of the
membrane
• Polar molecules can diffuse
through membranes but only at a
very slow rate
• Small polar molecules pass
through more easily than larger
ones
```
29
What are the factors that
affect the rate of diffusion
across a membrane?
```
Surface area
• The larger the area of an exchange
surface, the high the rate of
diffusion
Thickness of membrane
• The thinner the exchange surface,
the higher the rate of diffusion
```
30
What is facilitated diffusion?
```
Diffusion across a plasma
membrane through protein channels
• Down a concentration gradient
and doesn’t require external
energy
• Membranes contain channel
proteins through which polar
molecules and ions can pass
• Membranes with protein channels
are selectively permeable as most
protein channels are specific to
one molecule or ion
• Can also involve carrier proteins
which change shape when a
specific molecules binds
```
31
What is the rate of facilitated
| diffusion dependent on?
```
• Temperature
• Concentration gradient
• Membrane surface area and
thickness
• Number of channel proteins
present
```
32
What is active transport?
```
Movement of particles across a
plasma membrane against a
concentration gradient. Requires
energy and carrier proteins
• Metabolic energy is supplied by
ATP
• Carrier proteins span the
membranes and act as ‘pump
```
33
Describe the process of
active transport (e.g. outside
to inside a cell)
```
1. The molecule or ions to be
transported binds to receptors in
the channel the carrier protein on
the outside of the cell
2. On the inside of the cell ATP
bind s to the carrier proteins is
hydrolysed into ADP and
phosphate
3. Binding of the phosphate
molecule to the carrier protein
causes the protein to change
shape - opening up to the inside
of the cell
4. The molecule or ion is released
to inside of the cell
5. The phosphate molecule is
released from the carrier protein
and recombines with ADP to
form ATP
6. The carrier protein returns to its
original shape
```
34
What is bulk transport?
```
A form of active transport where
large molecules or whole bacterial
cells are moved into or out of a cell
by endocytosis or exocytosis
Energy in the form of ATP is required
for:
• Movement of vesicles along the
cytoskeleton
• Changing the shape of cells to
engulf materials
• The fusion of cell membranes as
vesicles form or as the meet the
cell-cell-surface membrane
```
35
What is endocytosis?
```
The bulk transport of materials into
cells via invagination of the cell
surface membrane forming a
vesicle. Phagocytosis (solids) and
Pinocytosis (liquids)
• Cell membrane invaginate when it
comes into contact with the
material to be transported
• The membrane enfolds the
material until eventually the
membrane fuses, forming a vesicle
• Vesicle pinches off and moves into
the cytoplasm to transfer the
material for further processing
• e.g. vesicles containing bacteria
are moved towards lysosomes,
where the bacteria are digested by
enzymes
```
36
What is exocytosis?
```
The bulk transport of materials out
of cells. Vesicles (usually formed by
the Golgi apparatus) containing the
material fuse with the cell-surface
membrane and the contents are
released to the outside of the cell
```
37
What is osmosis?
Diffusion of water through a partially
permeable membrane down a water
potential gradient. A passive
process and energy is not required
38
What is water potential (Ψ)?
Measure of the quantity of water
compared to solutes, measured as
the pressure created by the water
molecules in kilopascals (kPa)
39
What is the water potential
| of pure water?
```
• 0 kPa (at standard temperature
and atmospheric pressure : 25˚C
and 100kPa)
• Highest possible value of water
potentia
```
40
Describe the water
| potentials of solutions
```
• Presence of a solute in water
lowers the water potential below
zero
• All solution have negative water
potentials
• More concentrated solution =
more negative the water potential
```
41
What happens when solutions
of different concentrations are
separated by a partially
permeable membrane?
```
There will be a net movement of
water from the solution with higher
water potential (less concentrated)
to the solution with lower water
potential (more concentrated). This
will continue until the water potential
is equal on both sides (equilibrium)
```
42
What is hydrostatic
| pressure?
```
The pressure created by water in an
enclosed system e.g. by the
diffusion of water into a solution
leading to an increase in pressure
• Units: kPa
```
43
What happens if an animal cell
is placed in a solution with a
higher water potential than
that of the cytoplasm?
```
• Water will move into the cell by
osmosis
• Hydrostatic pressure inside the cell
will increase
• Cell surface membrane is thin
(7nm), cannot stretch much and
cannot withstand the increased
pressure
• The cell membrane will break and
the cell will burst in an event called
cytolysis
```
44
What happens if an animal cell
is placed in a solution with a
lower water potential than that
of the cytoplasm?
```
• The cell will lose water to the
solution by osmosis down the
water potential gradient
• This will cause a reduction in the
volume of the cell
• The cell membrane will ‘pucker’
which is referred to as crenation
```
45
How do multicellular
animals prevent cytolysis or
crenation?
```
They usually have control
mechanisms to make sure their cells
are continuously surrounded by
aqueous solutions with an equal
water potential (isotonic)
• e.g. in blood the aqueous solution
is blood plasma
```
46
What happens if a plant cell is
placed in a solution with a
higher water potential than
their own?
```
• Water enters the cell by osmosis
• The increased hydrostatic pressure
pushes the membrane against the
rigid cell walls
• This pressure against the cel wall
is called turgor
• As the turgor pressure increases, it
resists the entry more water, and
the cell is said to be turgid
```
47
What happens if a plant cell is
placed in a solution with a
lower water potential than their
own?
```
• Water is lost from the cells by
osmosis
• This leads to a reduction in the
volume the cytoplasm, which
eventually pulls the cell-surface
membrane away from the cell wall
• The cell is said to be plasmolysed
```
