Cell Processes - Lecture 1 Flashcards
How thick is a membrane structure
8nm (8x10^-9 m)
general features of a membrane (structure / barrier)
- thin
- flexible
- sturdy
how can we describe a membrane structure
fluid mosaic model
what is a membrane made up of
50% lipid
50% protein
held together by Hydrogen bonds
What is a lipid
barrier to entry or exit of POLAR substances
what are proteins?
“gatekeepers” - regulate traffic
can molecules cross the lipid bilayer?
some can, some cannot
What is the role of cholesterol in the lipid bilayer
it is scattered among a double row of phospholipid molecules
- is a lipid which can insert in phospholipid backbone
- can change composition or properties of lipid bilayer
what is the role of glycolipids in the lipid bilayer
it is scattered among a double row of phospholipid molecules
- is a sugar group
- attached to hydrophobic lipid molecule which can embed in lipid bilayer
NOTE: it is a sugar group in and out of the membrane
Give the properties of phospholipids
- comprises 75% of lipids
- phospholipid bilayer = 2 parallel layers of molecules
- each layer in the bilayer known as a leaflet
- each molecule is amphipathic (has both polar & non-polar regions)
features of membrane fluidity
- membranes are fluid structures
- cholesterol changes fluidity
features of lipids according to membrane fluidity
- lipids can move around within the plane of the membrane leaflet
- lipid rarely flip flop between the leaflets
- due to reduced flip flop, lipid composition of leaflets can be asymmetric
what is membrane fluidity determined by
- lipid tail length –> longer tail = less fluid of the membrane
- number of double bonds –> more double bonds = increased fluidity
- amount of cholesterol –> more cholesterol = decreased fluidity
Two different types of membrane proteins and where they can be found
- Integral proteins
extend into or completely across cell membrane
(also known as a transmembrane protein) - Peripheral proteins
attached to inner or outer surface of cell membrane
can be easily removed from it
features of integral membrane proteins
- amphipathic
- have hydrophobic regions which span hydrophobic core of lipid bilayer
- hydrophobic region consists of non-polar amino acids coiled into helices
- hydrophilic ends / regions interact with aqueous solution
what are the different functions of membrane proteins
Can act as:
- receptor proteins
- cell identity markers
- linkers
- enzymes (can form enzymes)
- ion channels
- transporter proteins
Ion channels & transporter proteins move things across cell membrane
what does permeability mean
ability to cross membrane
what causes selective permeability
the molecular organisation
^^ the membrane allows some substances to cross but excludes others
How can be apply selective permeability to a lipid bilayer
- permeable to non-polar, uncharged molecules (O2, N2, benzene)
- permeable to lipid soluble molecules (steroids, fatty acids, some vitamins)
- permeable to small uncharged polar molecules (water, urea, glycerol, CO2)
- impermeable to large uncharged polar molecules (glucose, amino acids)
- impermeable to ions (Na+, K+, Cl-, Ca2+, H+)
How do large uncharged polar molecules AND ions move across the membrane?
via transport protein
^^ it moves them across the membrane
How do substances which cannot permeate the hydrophobic core of the lipid bilayer get across? and give examples
membrane proteins mediate the transport of the substances across the membrane.
this is:
ion channels
transport protiens
when does diffusion occur
if we have a difference in concentrations & molecules will move across the membrane until they reach equilibrium
How does net diffusion work
net diffusion from a higher dye concentration to the region of lower dye
How do we know equilibrium has been reached?
when the dye (or colour) is spread out evenly / evenly distributed
What are the principles of diffusion
- random mixing of particles in a solution due to a particles kinetic energy
- more molecules move away from an area of high conc to an area of low conc
- greater difference in concentration between 2 sides of the membrane = faster the rate of diffusion
- higher temp = faster rate of diffusion
- larger size of diffusing substance = slower rate of diffusion
- increase in surface area = higher rate of diffusion
- increasing diffusion distance = slower rate of diffusion
what are the physical consequences of diffusion
- rate of diffusion sets a limit on the size of cells of about 20nm
- to increase diffusion a cell can increase membrane area available for exchange (diffusion) of a substance
- thicker membrane = slower rate of diffusion
- small distances = fast diffusion
What are the two gradients across the cell membrane
- concentration gradient
- electrical gradient
Concentration gradient
- non charged molecules will diffuse down their concentration gradients
NOTE: driving force for dilution will solely be conc gradient because they are non-charged so no electrical gradient acting
the selective permeability of the membrane enables:
- a difference in concentration
OR
- concentration gradient across the membrane to be established
electrical gradient
- ions will be influenced by membrane potential in addition to their conc gradient
NOTE: electrochemical gradient drives
cells can maintain a difference in charged ions between the inside & outside of membrane establishing an electrical gradient or membrane potential
what is the movement of ions influenced by
electrochemical gradient
What do membranes mimic
capacitors: can seperate AND store charge
ion gradients across the membrane
- cells use ~30% of resting energy to maintain concentration and electrical gradients
- gradients represent stored energy
what is osmosis
net movement of water through a selectively permeable membrane from an area of high water concentration to an area of low water concentration
NOTE: we want to have hemeostatis
features of osmosis
- only occurs if membrane is permeable to H2O but not to certain solutes
- this is the situation in biological membranes
- if an osmotic gradient exists water will move to eliminate it
osmosis equation and meaning of the variables
Pw = Pd + Pf
Pw = membrane permeability to water
Pd = through lipid bilayer
- small
- mercury insensitive
- temp dependent (lipid fluidity)
Pf = through water channel
- large
- mercury sensitive
- temp independent
NOTE:
- Pf is mediated by the aquaporins (9 isoforms)
- cells have different Pw because they express different aquaporin isoforms
osmotic pressure
is the pressure applied to a solution to prevent inward flow of water across semi-permeable membrane
