Cell Processes Lecture 21 Flashcards
Functions of the Cell Membrane
acts as an impermeable (to charged ions), selective barrier to the movement of substances in and out of the cell
- isolates cells from external environment
The control of substances in/out of cells allows:
- concentration gradients to be maintained
- the spatial organisation of chemical and physical processes within cell
- the controlled uptake of nutrients and discharge of waste products and the secretion of molecules
- the development of membrane potential
The structure of the cell membrane can be described as a _____ _____ _____ made up of _____ and _____ which surrounds the _______ of the cell
Fluid mosaic model; phospholipids; proteins; cytoplasm
What is the width of the average cell membrane
8nm
The phospholipids in the bilayer contain: and are
Hydrophobic tails (which make up the hydrophobic core) and Hydrophilic heads. Amphipathic
What substances are scattered among the double row of phospholipid molecules
Cholesterol and glycolipids
Membrane Fluidity is determined by:
- number of double bonds (the more the higher the fluidity)
- amount of cholesterol (the more the lower the fluidity)
- lipid tail length (the longer the tail the less fluid the membrane)
Two types of membrane proteins
Integral proteins and Peripheral Proteins
Integral Proteins
extend into or completely across cell membrane (transmembrane), are aphipathic and consists of non-polar amino acids coiled into alpha helices and hydrophilic ends which interact with aqueous solution
Peripheral Proteins
Attached to either the inner or outer surface of the cell membrane and are easily removed from it
Functions of membrane proteins
- Receptor Proteins
- Cell Identity Markers
- Linkers
- Enzymes
- Ion Channels
- Transporter Proteins
Permeability is governed by….
…. the laws of diffusion
Permeability depends on molecules’:
- size
- charge
- lipid solubility
Membrane proteins function in permeability
Mediate the transport of substances across the membrane which can’t permeate the hydrophobic core of the lipid bilayer
The membrane is permeable to:
- non polar, uncharged molecules e.g. glucose, amino acids
- lipid-soluble molecules e.g. steroids, fatty acids and some vitamins
- small uncharged polar molecules e.g. water, urea, glycerol and CO2
The membrane is impermeable to:
- large uncharged polar molecules e.g. glucose, amino acids
- ions e.g. Na+, K+, Cl-, Ca2+ and H+
Principle of Diffusion
random mixing of particles in a solution as a result of the particles Ek from an area of high to low concentation
Factors that increase rate of diffusion
- greater difference between concentrations
- higher temperature
- smaller size of diffusing substance
- increasing surface area
- smaller diffusion distance
Concentration Gradient
non charged molecules will diffuse down their concentration gradients
Electrical Gradient
ions will be influenced by membrane potential in addition to their concentration gradient
The combination of these two which ultimately influences movement of ions
Electrochemical gradient
What is the size limit that the rate of diffusion sets
20 micrometres
What percentage of resting energy is used to maintain the gradients
~ 30%
These gradients represent ____ ____
stored energy
Osmosis is
the net movement of water through a selectively permeable membrane from an area of higher water concentration or low solute concentration to an area of low water concentration or high solute concentration
Osmotic pressures is
the pressure applied by a solution to prevent inward flow of water across a semi permeable membrane
Pw =
Pd + Pf where Pd is permeability through lipid bilayer and Pf is permeability through water channel
Properties: through bilayer
- small
- mercury insensitive
- temperature dependent
Properties: through water channel
- large
- mercury insensitive
- temperature independent
Pf is mediated by
aquaporins (isoforms)
When comparing a solution to a reference solution it is isosmotic if
the osmolarity is the same
When comparing a solution to a reference solution it is hyposmotic if
the solution has a lower osmolarity
When comparing a solution to a reference solution it is hyperosmotic if
the solution has a higher osmolarity
The concentration of body fluids has an osmolarity of
280 Osmols
Tonicity is
the effect a solution has on cell volume
Tonicity depends
on the membrane permeability of the solute
A solution is isotonic if
there is no change in cell volume
A solution is hypotonic if
it causes cell swelling and eventually cell lysis
A solution is hypertonic if
it causes cell shrinkage
