Lecture 21: Membrane Structure And Function Flashcards
What is the function of the cell membrane?
To act as a barrier, isolating cells from their external environment
What is a cell?
The functional unit of specialised function in living systems
What is the general structure of a membrane like?
Not static, but should be viewed as a two dimensional fluid
Why should the membrane be viewed as a two dimensional fluid?
Because lipids and proteins are therefore able to move in the plane of the membrane
What is a lipid?
Any class of organic compound that are fatty acids (or their derivatives) and are insoluble in water (but soluble in organic solvents)
What is the model of the membrane called?
The mosaic model
How thin is the cell membrane?
8nm (8x10^9)
What are there main constituents of the cell membrane?
50% lipids (phospholipids)
50% protein
What holds the two main components of the membrane together?
Hydrogen bonds
What does the lipid aspect of the membrane do?
It’s the barrier to entry or exit of polar substances
What are the function of the proteins?
They’re the gate keepers (ie they regulate traffic) -if they can’t pass through lipids, they gave to go through a protein
What is a lipid bilayer?
Two back to back layers of three types of lipid molecule
What are the three types of lipid molecule in the lipid bilayer?
Cholesterol and glycolipids are scattered among a double row of phospholipid molecules
What is the phospholipid bilayer’s association with water?
In water, most phospholipids tend to form a lipid bilayer or structure with polar head group at the surface in contact with water and the fatty acid chain in the core screened from water contact
The phospholipid bilayer comprises what number of lipids?
75%
What is the phospholipid bilayer?
Two parallel layers of molecule
What is amphipathic?
The molecule has both a polar and non polar region- this is a phospholipid
Describe phospholipids in water
In water, most phospholipids tend to form a lipid bilayer or structure with the polar head group at the surface in context with water and the fatty acid chain in the core (screened from water contact)
What part of the lipid bilayer is charged?
The hydrophilic surface (ie by the head)
Which part of the lipid bilayer is polar?
The head
Which part of the lipid bilayer is non-polar?
The tail (hydrophobic core)
What provides a highly impermeable barrier to the passage of ions?
The hydrophobic core
Why do we describe membranes as fluid?
Because lipids can move around within the membrane leaflet
Why can the compositions of leaflets be as symmetrical?
Because lipids rarely flip flop between membrane leaflets (therefore, different compositions of cholesterol, channel ion receptors, polysaturated fatty acid etc)
What three things determine the fluidity of the membrane?
- Lipid tail length
- Number of double bonds
- Amount of cholesterol
How does the lipid tail length affect fluidity?
The longer the tail, the less fluid the membrane
How does the number of double bonds affect the fluidity?
More double bonds increases fluidity
How does the amount of cholesterol affect fluidity?
More decreases fluidity (think restricted arteries)
Which two aspects primarily determine fluidity?
The number of double bonds and the amount of cholesterol
What does the fact that the cell membrane controls the passage of substances into cells allow for?
- The concentration gradient to be maintained
- The spatial organisation of chemical and physical processes within the cell
- The controlled uptake of nutrients and discharge of waste products and the secretion of molecules
- The development of a membrane potential
What are the two types of membrane proteins?
- Integral proteins
2. Peripheral proteins
What integral proteins?
They are proteins that extend into or completely across the membrane (transmembrane)
What are the properties of integral proteins?
They’re amphipathic (is have hydrophobic regions that span hydrophobic core of lipid layer)
Describe the common form of integral proteins?
Usually hydrophobic regions consist of non-polar amino acids coiled into helices and the hydrophilic end of protein interact with aqueous solution
What are peripheral proteins?
Proteins attaches to either inner or outer surface of cell membrane and are easily removed from it
What can membrane proteins act as?
- Receptors (of signal)
- Cell identity markers
- Linkers
- Enzymes (eg glucose is broken down often at membrane surface)
- Channels
- Transporters
What is permeability?
The ability of a particular molecule to cross a cell membrane is known as its permeability and is governed by the laws of diffusion
What is the cell membranes permeability?
Semipermeable (ie, the membrane allows some substances to cross, but excludes others)
What determines the permeability of a molecule?
Size, charge and lipid solubility
What suggests there is more than one mechanism of permeation acting on the cell membrane?
The wide range of membrane permeability
What is the cell membrane permeable to?
- Non polar uncharged molecules
- Lipid soluble molecules
- Small uncharged polar molecules
What are examples of non polar, uncharged molecules?
Oxygen, N2, benzene
What are examples of lipid soluble molecules?
Steroids, fatty acids, some vitamins
What are examples of small, uncharged polar molecules?
Water, glycerol, urea, CO2
What is the cell membrane impermeable to?
- Large, uncharged polar molecules
2. Ions
What are examples of large uncharged polar molecules?
Glucose, amino acids
What are examples of ions impermeable to the cell membrane?
NA+, K+, Cl-, Ca2+, H+
What is diffusion?
The random mixing of particles in a solution as a result of the particle’s kinetic energy
Where do molecules move during diffusion?
They move away from an area of high concentration to an area of low concentration
What increases the rate of diffusion?
- The greater the difference in concentration between the two sides of the membrane
- The higher the temperature
- Increase in surface area
What do membrane proteins mediate?
The transport of substances across the membrane that cannot permeate the hydrophobic core of the lipid bilayer
What enables a difference in concentration, or concentration gradient, across the membrane to be established?
Selective permeability
What establishes an electrical gradient or membrane potential?
How cells maintain a difference in charged ions between the cytosol side and extra cellular side of membrane
What decreases the rate of diffusion?
- The larger the size of the diffusing substance = slower
2. Increase of diffusion distance
Is diffusion faster over small or larger distances?
Small distances
What is the rate of diffusion size limit of a cell?
20 um
What can a cell do to increase diffusion?
A cell can increase the membrane area available for exchange (diffusion) of a substance
What is a concentration gradient?
Non-charged molecules will diffuse down their concentration gradient
What is an electrical gradient?
Ions will be influenced by membrane potential in addition to their concentration gradient (ie, movement of ions will be influenced by their electrochemical gradient)
What percentage of resting energy do cells use to maintain a concentration and electrical gradient?
30%
What do gradients represent?
Stored energy
What is the net movement of ions across a membrane influenced by?
The electrochemical gradient
What is the electrochemical gradient?
The effect of both chemical and electrical gradients
What do membranes mimic and how is it similar?
Mini capacitors, in the sense that they can separate and store energy
How does Na flow?
High on extra cellular side to low inner cellular
How does K+ flow?
Low on extra cellular side, high in intracellular side (so flows out)
How does chloride flow?
High on outside, low on inside, but flows both ways
What is osmosis?
The net diffusion of water across a semi permeable membrane
What is a semipermeable membrane more permeable to- water or solutes?
Water
When solutions of different solute concentrations are separated by a semipermeable membrane, what tends to happen?
Water tends to move from the solution of lesser solute concentration to the solution of greater solute concentration
When does diffusion stop?
When a equilibrium is reached
What prevents osmotic water movement?
An opposing force called “osmotic pressure of the solution”, or hydrostatic pressure
What is colligative property?
Colligative properties are the physical changes that result from adding solute to a solvent.
What is osmotic pressure?
A colligative property
How is osmotic pressure and colligative property?
It depends on the numbers and not the type of particles in solution (CHECK)
What do you need to know to determine a cell’s osmolarity in Osmoles?
The concentration of a solution and knowledge of its ionisation properties
What is the osmolarity for 1m glucose + H20?
= 1m glucose
= 1 Osmol (no association)
What does 0.5 M NaCl+ H2O equal?
0.5M Na+ + 0.5 M Cl-
1 Osmol
What does 0.5M CaCl2 + H2O equal?
0.33 M Ca2+ + 2x (0.33 M Cl-)
= 1 Osmol
What is the solution if it has the same osmolarity when comparing two solutions?
Isosmotic
When comparing two solutions, what do we call it if the solution has lower osmolarity?
Hyposmotic
When comparing two solutions, what do we call it if the solution has higher osmolarity?
Hyperosmotic
What is the osmolarity of the concentration of body fluids?
280 mOsmol
Why does the osmolarity of extra cellular and intracellular fluid have to be equal? (Isosmotic)
So that no net water flow (osmosis) occurs. If osmosis occurs, a change in cell volume will occur
What is tonicity?
The effect a solution has on cell volume
What does the tonicity of a solution depend on?
The membrane permeability of the solute (so the osmolarity of a solution does not always determine the effect it will have on cell volume)
Are osmolarity and tonicity always the same thing?
Not always
What do we term “no change in cell volume”?
Isotonic solution
What do we term a solution that causes cell swelling and eventually cell lysis (haemolysis)?
Hypotonic solution
What do we term a solution that causes cell shrinkage (crenation)?
Hypertonic solution
What types of molecules diffuse down a concentration gradient?
Non-charged molecules
What types of molecules are influenced by both membrane potential (electrical gradient) and their concentration gradient? (Electrochemical)
Ions
What types of molecules diffuse down a concentration gradient?
Non-charged molecules
What types of molecules are influenced by both membrane potential (electrical gradient) and their concentration gradient? (Electrochemical)
Ions
Size of the cell membrane?
8nm (8x10^-9)
What bond holds lipids and proteins together?
Hydrogen bonds
Which lipid comprises 75% of lipids?
Phospholipids
What is amphipathic?
A molecule with both polar and non polar regions
What does the number of double bonds do to fluidity?
It increases it
Describe integral proteins?
Amphipathic
Non-polar, hydrophobic amino acids coiled into helices spanning hydrophobic core, hydrophilic polar ends interacting with aqueous solution
Size limit of diffusing molecule?
20um
How does membrane thickness affect diffusion rate?
It slows it down
