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1.4 Cell membrane and cellular transport (Role of cell membranes (cell…
1.4 Cell membrane and cellular transport
Fluid mosaic model
fluid - phospholipids and proteins constantly move rapidly and laterally in the plane of the membrane
mosaic - scattered arrangement of protein molecules among phospholipid molecules when viewed from the surface of the membrane
Membrane phospholipids
phospholipid bilayer
hydrophobic, non-polar, fatty acid hydrocarbon 'tails' facing inwards
hydrophilic, polar 'heads facing outwards
Properties
amphiphatic molecules (both hydrophobic and hydrophilic regions)
dynamic (constantly in motion) hence showing membrane fluidity
properties allow cell membranes to fuse together and to reseal when disrupted
Roles
hydrophobic layer orientate away from surrounding aqueous medium to form a hydrophobis core that acts as a barrier to seperate content of cell from external environment
phospholipids in bilayer regulate membrane fluidity (permeability)
proportion of saturated to unsaturated fatty acids that make up hydrophobic hydrocarbon 'tail'
saturated fatty acids - more linear, more readily and closely packed together, increases hydrophobic interactions between saturated hydrocarbon 'tails', membrane more rigid
unsaturated fatty acids - c=c (formation of kinks), phospholipids less packed together, increases membrane fluidity and permeability
length of hydrocarbon 'tails'
as length increases, increase in number of hydrophobic interactions between neighbouring chains, decrease in membrane fluidity
Cholesterol
Structure
lipids characterised by c skeleton of 4 fused rings
Occurrence
common component of animal cell membranes, precursor from which steroids are synthesised
Property
amphipathic thus they can be wedged neatly between hydrophobic regions of phospholipids
Roles
regulate membrane fluidity to help membrane stability
Membrane proteins
peripheral proteins
hydrophilic globular proteins found at either side of the phospholipid bilayer, exposed to the aqueous medium. Occur at surfaces not embedded withn phospholipid layer
charged amino acid residue loosely bound by electrostatic FOA to charged portion of phospholipids thus can be easily dissociated from membrane
exist as
external peripheral proteins - found on the extracellular face of the membrane, attached to fibres of the extracellular matrix. keep cell in place
internal peripheral proteins - found on the intracellular face of the membrane, attatched to the cytoskeleton. maintain cell shape and mobility
integral proteins
embedded within hydrophobic interior they may be
unilateral - partly embeded within one phospholipid layer)
transmembrane - spanning across entire phospholipid bilayer)
transmembrane proteins may have a single phospholipid-spanning region or several
contain both hydrophobic and hydrophilic regions.
hydrophobic region - inner region with hydrophobic amino acid residues that interact with the fatty acid 'tails' of phospholipids
hydrophilic reigion - outer region with hydrophilic amino acid residues facing and interacting with the aqueous medium or hydrophilic amino acid residues
Strong hydrophobic and hydrophilic interactions hold the integral proteins in place hence cannot be removed without disrupting membrane
Structure
linked to the membrane by a glycolipid or phospholipid anchor
Roles
Transport proteins
Channel proteins
hydrophilic channel / hydrophilic pore across membrane for selective movement of certain large, hydrophilic, charged ions or polar molecules
Carrier proteins
specific solute molecules bind then undergo conformational change involving expenditure of energy (hydrolysing ATP) to transport substance against their concentration gradient across the membrane
other carrier proteins that do not require energy allow for transport go substances down their concentration gradient across the membrane
Membrane-bound enzymes
protein built into the membrane that functions as an enzyme with its active site exposed to substrates needed to carry out sequential steps of a metabolic pathway
Receptor proteins - attachment for signal molecules for cell signalling
-membrane protein has an attatchment site complementary to a specific chemical messanger ehich upon binding causes conformational change in the protein. brings about a series of changed taht relays the message to the inside of the cell (ligand receptors)
Surface identity markers
glycoproteins (proteins with short chains of sugars) cans serve as identification tags that are recognised by other cells
Cell adhesion
membrane proteins of adjacent cells may be hooked in various junctions
Anchor proteins - for attachment to cytoskeleton and extracellular matrix
membrane proteins are attatched to micro filament or other elements of the cytoskeleton to help maintain cell shape. attatchment fixes location of these membrane proteins
membrane proteins attatch to extracellular matrix proteins
glycoproteins and glycolipids
structure
membrane proteins, phospholipids covalently linked to short, branching carbohydrate chains forming glycoproteins and glycolipids
membrane carbohydrate chains with 15> sugar residues are always oriented away from the CSM into the exterior of the cell
roles
recognition sites for cell-to-cell recognition - act as identification tags of a particular cell type to enable distinguishing from one cell type to another
receptor sites for attatchhment of chemical signals
attatchment sites for cell adhesion which is involved in binding of correct cells together to form tissues maintaining structural relationship between neighbouring cells
Role of cell membranes
cell boundary
to separate and prevent indiscriminate mixing of cytoplasmic content with external environment. allows specialisation of cell function by maintaining concentration of specific substances required for specific functions
Selective permeable barrier
regulate and control the exchange of substances in and out of the cell across the membrane
prevents free interchange of materials from one side of the membrane to the other due to the close packing of the phospholipids which prevents larger molecules like glucose from passing through.
hydrophobic, non-polr hydrocarbon fatty acid 'tails' of the phopholoipds also repel polar substances and charged ions. Hence the membrane is impermeable to ions and this is significant for the maintanence of ionic concentration gradients for nervous transmission.
membrane allows the passage of hydrophobic and non-polar molecules
Surface for materials exchange
CSM may be thrown into folds to increase SA for exchange of substances
CSM contains transport proteins that transport substances such as charged ions and larger polar molecules do not readily pass through allows the buildup of certain substances.
sites for receptor proteins involved in recognising and detecting external stimuli or signal molecules
carbohydrates attatched to the membrane proteins or phospholipids play a part in the way specific signal molecules such as hormones and neurotransmitters can recognise particular cell types and subsequent attatch or associate with them.
this is important for cell signalling and signal transduction, where the chemical information information of these signal molecules can subsequently bring about changes within the cell to generate appropriate specific cellular responses.
sites for cell-to-cell adhesion
tight junctions - regions where membranes of two adjacent cells are fused together, forming zone of fusion that completely encirles each cell
serves as continuous barrier, preventing leakage of substances via intercellular spaces
gap junction - regions consist of protein-lined channels or pores that pass through the cell membrane of two adjacent cells
serve as communication junctions to allow movement and exchange of material from one cell to the next
sites for surface identity marker for cell-to-cell recognition
surface carbohydrates attached to the membrane molecules enable cells to recognise each other so that the same type of cells can be sorted into tissue and organs
compartmentalisation by intercellular membranes
enable separate compartments to be formed within cell by isolating internal environments of different organelles from the rest of the cytoplasm
prevents intermediates of one pathway from mixing with others, maintaining a high concentration of intermediates at specific sites that can be immediately passed on incresaing overall efficiency of the biochemical reactions
sites for biochemical processes
help to concentrate specific enzymes and necessary transport or carrier molecules needed for biochemical processes
endomembranal systems act as transport systems
diffusion