4.1 - structure of the cell surface membrane Flashcards
What are the membranes around and within cells known as
Plasma membranes
What is the cell-surface membrane
The name given to: the membrane that surrounds the cell, forming a boundary between the cell cytoplasm and the environment
What are the 2 basic functions of the cell-surface membrane
- allows different conditions to be established inside and outside the cell
- controls movement of substances in and out the cell
What are the different molecules that form the structure of the cell-surface membrane
- Phospholipids
- proteins
- cholesterol
- glycolipids
- glycoproteins
Describe how the phospholipids form the phospholipid bilayer
- The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides
- The hydrophobic tails of both phospholipid layers point to the centre of the cell-surface membrane repelled by water on both sides
What type of material moves through the membrane via the phospholipid portion, and how does this relate to the functions of phospholipids in the membrane
- lipid-soluble material
FUNCTIONS
—> allow lipid-soluble substances to enter and leave the cell
—> prevent water-soluble substances entering and leaving the cell
—> make the membrane flexible and self-sealing
How are proteins embedded in the phospholipid bilayer
Proteins are dispersed throughout the membrane: there’s 2 types
- Extrinsic/peripheral proteins
- Intrinsic/integral proteins
What are extrinsic proteins
- occur in the surface of the bilayer and never extend completely across it
- Either: give mechanical support to membrane, or act as cell receptors for peptide hormones (role in cell recognition)
What are intrinsic proteins
- completely span the phospholipid bilayer from one side to the other
- act as: protein channels = form water-filled tubes to allow water-soluble ions to diffuse across the membrane
- act as: carrier proteins = bind to ions or molecules (like glucose and amino acids), then change shape in order to move these molecules across the membrane
What are the functions of the proteins in the membrane
- structural support
- act as channels, transporting water-soluble substances across the membrane
- allow active transport across the membrane through carrier proteins
- form cell-surface receptors for identifying cells
- help cells adhere together
- act as receptors, e.g. for hormones
Where do cholesterol molecules occur
- present in all cell membranes (except bacterial)
- fit between the phospholipids = bind to the hydrophobic tails of phospholipids = causing them to pack closer together = restricting movement of phospholipids = making membranes less fluid and more rigid
What are the functions of cholesterol molecules in the membrane
- add strength to the membranes, maintaining the shape (important for cells that aren’t supported by other cells, e.g. red blood cells)
- very hydrophobic = play role in preventing loss of water and dissolved ions from the cell
- fits between the phospholipids = bind to the hydrophobic tails of phospholipids = causing them to pack closer together = restricting movement of phospholipids = making membranes less fluid and more rigid —> reduce lateral movement
- make the membrane less fluid at high temps
How do the phospholipids molecules form a bilayer
They automatically arrange themselves into a bilayer
Why can’t water-soluble substances pass through the phospholipid bilayer
- the centre of the bilayer is where hydrophobic tails meets = centre is hydrophobic
- act as as a barrier to these dissolved substances
What are glycolipids made up of
A carbohydrate covalently bonded with a lipid
Describes the structure of glycolipids
- the carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals
- e.g. the human ABO blood system operates as a result of glycolipids on the cell-surface membrane
What are the functions of glycolipids in the membrane
- act as recognition sites
- help maintain the stability of the membrane
- help cells to attach to one another and so form tissues (cell adhesion)
What are glycoproteins
- Carbohydrate chains are attached to many extrinsic proteins on the outer surface of cell membrane
- act as cell-surface receptors, more specifically for hormones and neurotransmitters
What are the functions of glycoproteins in the membrane
- act as recognition sites
- help cells to attach to one another and so form tissues
- allows cells to recognise each one another, e.g. lymphocytes can recognise an organism’s own cells
What is the functions of membranes within cells
- control entry and exit of materials in discrete organelles such as mitochondria and chloroplasts
- separate organelles from cytoplasm so that specific metabolic reaction can take place within them
- provide an internal transport system, e.g. endoplasmic reticulum
- isolate enzymes that might damage the cell, e.g. lysosomes
- provide surfaces on which reactions can occur, e.g. protein synthesis using ribosomes on rough endoplasmic reticulum
Why might most molecules not be able to freely diffuse across the cell-surface membrane
- they’re not soluble in lipids: therefore can’t pass through the phospholipid layer
- too large to pass through the channels in the membrane
- they’re the same charge as the charge on the protein channels = even if they’re small enough, they’re repelled anyway
- they’re electrically charged (polar) = have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
What is the fluid-mosaic model
The model that shows the arrangement of all the molecules that create the cell-surface membrane
Why is it called the Fluid-Mosaic model
- fluid: individual phospholipid molecules can move relative to one another = gives membrane a flexible structure that’s constantly changing
- mosaic: the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way that the stones/tiles in a mosaic do
What’s the function of the phospholipid bilayer
- allow movement of lipid soluble molecules, e.g. steroid hormones
- prevent water soluble molecules leaving the cell
- make membrane flexible and self-sealing
- forms a barrier to dissolved substances
What’s the function of the fatty acid tails
- forms (water) impermeable barrier to water-soluble substances, and allows non-polar molecules to pass through
- allows cell to maintain different conc. either side
- makes membrane self-sealing/able to fuse other membranes/flexible
Fill in the blanks
Sketch a CSM diagram
Describe what the phospholipid bilayer is formed from
- the molecules automatically arrange themselves into a bilayer
- phospholipid molecules have a hydrophilic ‘head’ that points out towards the water either side of the membrane
- and a hydrophilic ‘tail’ that points inwards. This makes the centre hydrophobic = so doesn’t allow water-soluble substances (like ions) through it - acting like a barrier
What is cholesterol and where is it
- A type of lipid
- present in all cell membranes (except bacterial cell membranes)
