Lecture 21 C1 Flashcards
Structure and Function of Cell membrane
Describe the fluid mosaic model
Sea of lipids where proteins float like icebergs.
What is membrane fluidity
Lipids can move within the plane of the membrane leaflet (1/2). The lipid composition of the leaflet is asymmetric because lipids can rarely move between leaflets.
How is membrane fluidity determined
Lipid tail length: the longer the tail, the less fluid.
Number of double bonds in tail: unsaturated lipid makes kinks, more fluid . Amount of cholesterol - more decreases fluidity.
what does increased fluidity mean?
Allows more things to pass the lipid barrier to entry or exit of polar substances
Describe the structure of the lipid bilayer and the 3 lipids associated with it
8nm thick, phospholipid molecules with non polar lipid facing inwards and polar heads facing outwards. Cholesterol inside the lipid section, and glycolipids on top of the polar part giving cell identity.
What is the ratio of lipids to proteins in the cell membrane
1:1
What the two classes of membrane proteins
Integral membrane proteins extend into or across the cell membrane so amphipathic. Peripheral protein: attach to inner or outer surface and are easily removed
Give 6 examples of the uses of membrane proteins
cell identity markers, channels, linkers (to the intermediate filaments) , enzymes, transporters, receptors.
Describe the structure of integral proteins.
The hydrophobic regions consist of non polar amino acids coiled into helices. Hydrophilic ends of the protein interact with the aqueous solution
How does selective permeability of the membrane come about
The lipid bilayer barrier to the movement of molecules allows some substances to cross but excludes others. Membrane proteins mediate the transport of substances across the membrane that cannot permeate the bilayer
What is the lipid bilayer permeable to
- non polar, charged molecules: O2, N2, benzene
- lipid soluble molecules: steroids, fatty acids, some vit.
- small uncharged polar molecules: water, urea, glycerol, CO2
What is the is the lipid bilayer impermeable to
- large uncharged polar molecules : glucose, amino acids
2. ions : K+, Na+ , Cl-, Ca2+, H+
What is the principle of diffusion
Random mixing of particles in a solution as a result of the particles kinetic energy.
What factors speed up the rate of diffusion
greater difference in concentration, higher temperature, increase in SA, decrease in diffusion distance, smaller molecule being diffused.
What are the physical consequences of diffusion on the cell
size is limited to 20um. The membrane available for diffusion can be increased, membrane thickness can vary.
What are the main consequences of having a selectively permeable membrane
concentration gradients,
spatial organisation of chemical and physical processes in the cell,
controlled uptake of nutrients, discharge of waste product and secretion, and development of membrane potential
Non charged molecules will diffuse down
their concentration gradient only
The net movement of ions across a membrane is influence by what
electrochemical gradient which is the effect of both concentration and electrical gradient.
How does the cell make an electrical gradient
Cells use 30% of resting energy to maintain conc and electrical gradients. They mimic capacitors and can separate and store charge. This energy is released when ions can flow
what are the three main ion gradients across the cell membrane
Outside the cell is high Na, Low K and High Cl.
Cl- at equilibrium but can go in or out depending on mp.
K+ in electrochemical equilibrium because it goes from high to low but its attracted to negative charge in side
What is the charge on the cell membrane
+ charge outside, - charge inside .
What is osmosis
net movement of water through a selectively permeable membrane from area of high conc to are of low conc
What is the membrane permeability to water (Pw)
Pw= Pd + Pf
What is Pd and its properties
movement of water through the lipid bilayer. Small, insensitive to mercury and temperature dependent (lipid fluidity)
What is Pf and what is properties
Pf mediated by 9 isoforms of aquaporins- movement of water through channel.
Large, mercury senstive and temp independent.
What is osmotic pressure
the pressure applied by a solution to prevent the inward flow of water across a semi permeable membrane
What is the difference between osmolarity and tonicity
Osmolarity is the measure of number of dissolved ions/solutes on either side of the semi permeable membrane while tonicity is the effect of intracellular and extracellular osmolarity (having on water movement that effects ) cell volume .
A cell in hypertonic solution causes water to move what way
Hypertonic solution -> has more solutes outside cell-> hyperosmotic -> less water outside cell-> water moves out of cell-> crenation
A cell in hypotonic solution causes water to move what way
Hypotonic solution-> less solutes outside cell-> hypoosmotic-> more water outside cell-> water moves into cell-> lysis
