Structure and function of cell membrane Flashcards
What is the function of a cell membrane? What does this allow?
Acts as a barrier isolating cells from their external environment Allows concentration gradients to be maintained, spatial organisation of chemical and physical processess within the cell
How should a cell membrane be viewed?
As a two dimensional fluid
What are the main constituents of a cell membrane?
Phospholipids and proteins
What do phospholipids tend to form in water? Explain the organisation
A bilayer structure with the polar head groups at the surface in contact with water and fatty acid chains in the cor separated from water contact
What is an important features ate hydrophobic core?
It provides a highly impermeable barrier to the passage of charged ions
Phospholipids have two regions on them, what are they? What does this make it?
Has both polar and non polar regions Makes it amphipathic
What is the model used to describe the structure of the cell membrane?
Fluid mosaic model
What is selective permeability?
Not all molecules are equal in terms of how they cross cell membranes
What affects the permeability of a molecules across a cell membrane?
Properties of molecules, interactions with cell membrane and transport proteins that are used
What does selective permeability allow between cells? What does this create?
Fluid compartments Dividing one side from the other stores different fluid gradients therefore stores energy
Describe the cell membrane in general structural terms
A sea of lipids which membrane proteins are imbedded
What happens, due to the integral membrane proteins, when the lipid bilayer is pulled apart ?
The membrane proteins stay in one of the lipid bilayers and it leaves a hole in the other one that it did not attach to
What is the thickness of the cell membrane?
8nm
Describe the cell membrane in general behavioural terms. What does it cover?
A thin flexible and study barrier the surrounding the cytoplasm of a cell
What is the compositional percentage of lipids and proteins in the cell membrane?
50/50
How is the lipid layer held together?
By hydrogen bonds
What is the lipid controlling?
The entry and exit of polar substances
What are the three general classes of lipids in the phospholipid bilayer?
Cholesterol, glycolipids and phospholipids
Where are cholesterol and glycolipids located relative to phospholipid? How are the phospholipids arranged?
They are scattered among the double row of phospholipids molecules Glycolipids are imbedded in and protrude from the lipid bilayer Cholesterols are imbedded within the fatty acid tails
What do phospholipids do within the cell membrane?
Make up the back bone of the cell membrane, the lipid bilayer
What is the structure of the glycolipid?
A lipid tail with a sugar coating
What can cholesterol do to the phospholipid backbone?
it can modify it’s properties
What can impact the phospholipids fluidity? How does increasing amounts affect fluidity?
-Cholesterol, more decreases fluidity -Number of double bonds between the bilayers, more double bonds make it more fluid as there are less kinks in the fatty acid chains to decreases mobility -Lipid tail length, the longer the tail the less fluid the membrane
How much of the total lipids in a cell is used in the cell membrane?
75% of all lipids used in cell membrane
Can the composition of the lipid leaflets in the phospholipid bilayer be asymmetrical? Why
Yes it can be because the lipids don’t ‘flip flop’ between membranes
How is a phospholipid bilayer formed? Why does it form this arrangement?
It is formed automatically as the hydrophilic head are attracted to the polar charged water which they are immersed in and the hydrophobic tails orientate themselves to exclude the water It is the lowest energy state
What are the types of membrane proteins?
Integral membrane proteins and peripheral membrane proteins
How are integral membrane proteins connected to the cell membrane?
Extend into or completely across the cell membrane
What type of membrane protein is a transmembrane protein?
A integral membrane protein that extends across the entire cell membrane
How do you get integral membrane proteins out of the bilayer? What does this indicate about the layers relationship with the cell membrane? How do you get integral membrane proteins out?
You have to break down the bilayer Shows they have a strong interaction with the bilayer You have to use a detergent (it breaks down the phospholipid bilayer by interacting with the hydrophobic layer FYI)
Where are peripheral membrane proteins attached to the cell membrane?
Either the inside or outer surface of the cell membrane
How are peripheral membrane proteins attached to the cell membrane?
Attached to an integral membrane protein and other membrane proteins
Which is harder to remove, peripheral or integral membrane proteins? Why?
Integral because they are imbedded in the phospholipid bilayer and have a strong interaction, peripheral are only attached onto other proteins no the lipid bilayer, so are easily removed
What kind of bonds are used by peripheral membrane proteins to hold onto integral membrane proteins? What can break these bonds?
Ionic Changes in salt concentration
What property do integral membrane proteins require to be able to imbed themselves into the cell membrane?
They need to be amphipathic
Describe the distribution of an integral membrane protein’s charge, where are they located relative to the cell membrane?
Has non-polar regions that span the hydrophobic core of the bilayer (this makes these regions hydrophobic as well) and hydrophilic ends (polar regions) which interact with the aqueous solution outside of the bilayer
What is the structure of the hydrophobic region of the integral membrane proteins? What kind of molecule are they?
Tend to be non-polar amino acids that are coiled into a helix
What are the hydrophilic regions of the integral membrane proteins made of?
Polar amino acids
What can membrane proteins do? Briefly outline what each of these functions do inside a cell
Receptor proteins (receive neurotransmitter and hormones and transduce into the cell)
cell identify markers (prevents immune response from killing cell)
linkers (link onto other cell membranes/cytoskeletal elements)
enzymes (allow chemical reactions to occur on cell membrane)
ion channels (allow ion movement into and out of cell)
transporters (move molecules in and out of cell)
What is about a membrane that creates selective permeability?
The molecular organisation of the membrane
Label the diagram
Label the diagram
What do protein channels do to allow impermeable molecules through the cell membrane?
Mediate substances across the hydrophobic core of the bilayer
In the lipid bilayer, what kind of molecules are permeable? What makes them permeable through this bilayer?
Small molecules (both non-polar and slightly polar) (e.g. O2, CO2 , H2O, N2) - can easily dissolve into the bilayer Non-polar uncharged molecules and lipid soluble molecules - can pass through fatty acid tails
In the lipid bilayer, what kind of molecules are impermeable? What makes them impermeable through this bilayer?
Large molecules (e.g. glucose, amino acids) - too large to move through gaps of membrane Highly charge ions (Na+, K+, Cl-, Ca2+ etc.) - can’t move across due to electrical gradients across membranes
How do molecules that are impermeable through the cell membrane get into the cell membrane?
Through protein channels (ion channel or transporter)
What process allows permeable molecules to come into the cell?
Diffusion
What is diffusion?
The random mixing of particles in a solution as a result of the particles kinetic energy
What factors increase the rate of diffusion?
The rate of diffusion is increased by: the greater the difference in concentration between the two side of the membrane, the higher the temperatures, the smaller the size of the diffusing substance, the larger the surface area, the smaller the distances needed to travel
What happens once the concentration of substance is equally distributed?
There is no net movement, solution reaches equilibrium
Because cells depend on diffusion, what are the physical consequences of this on the cells?
-The rate of diffusion sets a limit on the size of the cell to 20µm -cells require a high surface area -the membrane must be thin as thicker membrane slow diffusion
How is the electrical gradient established?
A cell has a charge of -80µV and 0V outside so positive ions are electrically attracted into the cell and negative ion are repelled from the cell
How will a molecule such a O2 be attracted to a cell differently from Na+?
O2 is uncharged so it will only be attracted to the cell by the concentration gradient, however Na+ will be affected by a combination of the concentration gradient and electrical gradient
Are concentration and electrical gradients working in the same direction?
Sometimes, depends on the condition in the cell
What is the movement of ions influenced by? What is this a combination of?
The electrochemical gradient Combination of the affects of the concentration and electrical gradient
What kind of electrical device do cells mimic? Why is this?
Capacitors They can seperate and store charge
How much of a cells resting energy is used to maintain concentration and electrical gradients?
30%
What do the concentration and electrical gradients represent?
Stored energy
What are the relative concentrations of Na+, K+ and Cl- inside the cell?
Low Na+, high K+, low Cl-
What is the movement of Na+, K+ and Cl- across the cell membrane? Explain
Na+ want to go into the cell with the electrical gradient (going into (-) cell environment) and with its concentration gradient (from high outside cell to low inside cell) K+ wants to leave cell due to concentration gradient (high inside, low outside), however electrical gradient pulls K+ back, however in this case the concentration gradient is stronger than the electrical gradient Cl- can be considered at electrochemical equilibrium as concentration gradient takes it into the cell (high outside, low inside) but electrical gradient takes it out of the cell (- charge inside repels).
What is osmosis?
The net diffusion of water across a semi-permeable membrane
What is more permeable through a semi-permeable membrane, water or solutes?
Water
What causes osmosis? What direction does the water flow?
When solutions of different solute concentrations are separated by a semi-permeable membrane The water flows from the solution of less solute (and more water) to the solution of more solutes and (less water)
What can prevent osmosis from occurring between a semi-permeable membrane? What is this defined as?
A hydrostatic pressure Defined as the osmotic pressure of the solution
What is the osmotic pressure determined by?
It depends on the number of particles in solution (not the type of particles FYI)
