Membranes 2.1.5 Flashcards
1
Q
What is compartmentalisation?
A
The formation of seperate membrane bound areas in a cell
2
Q
What are the roles of membranes?
A
- Partially permeable barriers between the cell and its environment, between organelles and the cyptoplasm and within organelles
- Sites of celll communication
3
Q
Membrane structure
A
- The cell surface membrane which seperates the cell from its external environment is known as the plasma membrane.
- Phospholipid bilayer = Hydrophillic (phosphate) head facing outward and hydrophobic tails facing inwards
4
Q
Fluid mosaic model
A
- Proposed by Singer and Nicolson
- Known as fluid mosaic molecule because phospholipids are free to move within the layer relative to eachother, giving the membrane flexibility. And because the proteins embedded in the bilayer vary in shape, size and position
5
Q
Intrinsic proteins
A
- Transmembrane proteins which are embedded through both layers of 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.
- Channel and carrier proteins are both intrinsic
- Channel proteins = provide a hydrophillic channel which allows for passive movement of polar molecules and ions down a concentration gradient through membranes. They are held in position by interactions between the hydrophobic core and hydrophobic R groups on outside
- Carrier proteins have an important role in both passive transport (down concentration gradient) and active transport (against gradient) into cells which often involves shape of the protein changing.
6
Q
Glycolipids
A
- Lipids with carbohydrate chain attatched
- These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self or non self
7
Q
Extrinsic proteins
A
- AKA peripheral proteins
- Normally have hydrophillic R groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins.
- Can be presdent in either layer and some move between them
8
Q
Glycoproteins
A
- Are intrinsic proteins
- Embedded in cell surface membrane with carbohydrate chain attatched of varying lengths and sizes.
- They play a role in cell adhesion and as receptors for chemical signals
- When chemicals bind to receptors it elicits a response from the cell, this is cell signaling, examples include: neurotransmitters, peptide hormones
- Some drugs act by binding to cell receptors e.g. beta blockers are used are used to reduce the response of the heart to stress.
9
Q
Cholesterol
A
- A lipid with a hydrophillic end and a hydrophobic end
- Positioned between phospholipids in the membrane bilayer
- Regualtes membrane fluidity and stability
10
Q
Cell signalling in membranes
A
- The protein receptors are glycoproteins that have a complementary binding site to a specific hormone on the extra celular side of the membrane
- The hormone does not enter the cell but the message is relayed to the inside of the cell by the second messenger.
11
Q
Receptors as drug binding sites (e.g. antihistamine)
A
- Agonist + receptor = drug action/effect
- antagonist + receptor = no drug action/effect
- Antihistamine has a similiar shape to the signalling molecule, can bind to the complementary receptor and block or enhance its action. The antihistamine blocks the receptor so less histamine binds to receptor and inflammation is reduced.
12
Q
Beta blockers
A
- Beta blocker block access of adrenaline to target tissue
13
Q
Site attachment for the cytoskeleton in the cell membrane
A
- Anchoring point for cytoskeleton
-Microfilaments attach to move the plasma membrane
14
Q
cell to cell recognition and cell to cell adhesion
A
- Glycoproteins act as antigens on the surface of the cell
15
Q
Diffusion
A
The net movement of a substance down its concentration gradient until equilibrium is reached. The molecules have their own kinetic energy, there is no need for an input of energy.
16
Q
Simple diffusion
A
- Diffusion of molecules through the phospholipid bilayer
- This includes small nonpolar molecules (CO2, O2), H2O, lipid soluble hormones
17
Q
Facilitated diffusion
A
- Diffusion of molecules through transmembrane proteins
- There are 2 types of facilitated diffusion requiring 2 types of proteins:
- Channel proteins/ion channels = contain hydrophillic channels, allow diffusion of charged particles/ions across a membrane
- Carrier proteins = Diffusion of small organic molecules, e.g. glucose or amino acids, where the conformation (shape) of protein changes
18
Q
Active transport
A
Movement of molecules against their concentration gradient, using carriers as proteins (often called pumps) and input of energy.
19
Q
Factors affecting membrane structure - temperature
A
- When temperature increases the phospholipids will have more energy and will move more. This makes the membrane more fluid and it begins to lose its structure.
- The loss of structure increases the permeability of the membrane, making it easier for particles to cross it.
- Carrier and channel proteins in the membrane will be denatured at higher temperatures. These proteins are involved in transport across the membrane so as they denature membrane permeability will be affected.
20
Q
Factors affecting membrane structure - solvents
A
- Water (a polar solvent) is essential in the formation of the phospholipid bilayer.
- Organic solvents which are less polar than water, e.g. alcohols, can dissolve into membranes and disrupt cells.
- Pure or very strong alcohols are toxic as they destroy cells in the body. Less concentrated solutions of alcohols will not destroy but will damage cells.
- The non polar alcohol molecules can enter the cell membrane and the presence of these molecules between phospholipids disrupts the membrane.
- When the membrane is disrupted it becomes more fluid and more permeable.
21
Q
What is water potential?
A
The ability of a solution to donate water to another solution (measured in kPa)
22
Q
Define osmosis
A
The net movement of water from a higher water potential to a lower water potential across a partially permeable membrane (until equilibrium is reached)
23
Q
Dilute / less concentrated solutions
A
Hypotonic solution, higher water potential
24
Q
Less dilute / more concentrated solution
A
Hypertonic solution, lower water potential
25
Pure water potential
- Pure water has a potential of 0kPa
- The more concentrated the solution the more negative the water potential
26
How can the rate of osmosis be increased?
- The use of protein channels called aquaporins. These are inserted into the membrane by the exocytosis of vesicles.
- The active transport of ions, to lower the water potential and create a gradient across the membrane
27
Hydrostatic pressure
The diffusion of water into a solution leads to an increase in the volume of this solution. If the solution is in a closed system this leads to an increase in pressure.
28
Effect of osmosis on animal cells
- If a animal cell is placed in a solution with higher water potential than the cyptoplasm then water will move out of the cell via osmosis.
Osmosis in a red blood cell:
- High (less negative) water potential of external solution will cause water to enter the cell, the cell will then swell and burst
- Equal water potential between cell and solution will cause water to constantly enter and leave but at a constant rate. There is no change to the cell.
- Lower (more negative) water potential of external solution causes water to leave the cell, the cell then shrinks.
29
Osmosis in plant cells
- When water enters the cell via osmosis, the increases hydrostatic pressure pushes the membrane against the rigid cell wall, this pressure againts the cell wall is called turgor.
- When water is lost from the cellls via osmosis it leads to a reduction in volume of the cyptoplasm, which eventually pulls the cell surface membrane away from the cell wall, the cell is then said to be plasmolysed.
- When water potential of external solution is higher than cell, water enters the cell so it swells and becomes turgid.
- When the water potential is equal water leaves at enters at constant rate so there is no change.
- When water potential of external solution is lower water leaves the cell and plasmoylsis occurs cell shrinks.
30
Exocytosis
- Out of the cyptoplasm
- Intracellular vessicles made by the golgi are moved towards the plasma membrane by the action of motor proteins which carry them along the microtubules of the cytoskeleton using ATP. When the vessicle reaches the membrane it fuses with it and empties its contents outside.
31
Endocytosis
- The reverse of exocytosis
- Substance is carried into the cell and is progressively enclosed by a small portion of the plasma membrane which first inavigates then pinches off to form an intracellular vessicle.
32
Rate of diffusion is inversely proportional to
Diffusion distance and particle size
33
The approximate thickness of the plasma membrane is
7.5nm
34
Which term describes a protein that sits in a cell membrane in a way that it spans both of the layers of phospholipids and so has a surface on both sides of the membrane?
Intrinsic protein
35
Which term describes a protein that allows polar or charged molecules to cross a cell membrane by binding to them and then changing shape to move the molecules from one side of the membrane to the other?
carrier protein
36
Which term describes a protein that has a continuous passage in its centre from one side of a cell membrane to the other? It allows facilitated diffusion across the membrane.
channel protein
37
What is the name for the process that involves the formation of vesicles by the Golgi apparatus, the fusion of these vesicles with the plasma membrane and the release of the vesicle’s contents out of the cell?
exocytosis
38
What is the name of the location on a carrier protein where the molecule being transported across the membrane attaches before the protein changes shape?
binding site
