2.1.5a Plasma Membrane Flashcards
Appearance of cell surface membrane
Seen using a light microscope, the cell membrane appears as a THIN LINE
but with an electron microscope, it appears as a DOUBLE LINE
Functions of membranes at the surface of cells (plasma membranes)
- They are a barrier between the cell & its env, controlling which substances enter & leave the cell. Theyre partially permeable, letting some molecules through (by diffusion, osmosis, active transport) but not others
- They allow recognition by other cells
- They allow cell signalling by acting as an interface for communication between cells
Functions of membranes within cells
- The membranes around organelles divide the cell into diff compartments - they act as a barrier between the organelle & the cytoplasm. This makes diff functions more efficient
- They can form vesicles to transport substances between diff areas of the cell
- They control which substances enter & leave the organelle. Theyre partially permeable
- You can also get membranes within organelles - these act as barriers between the membrane contents & the rest of the organelle (eg. thylakoid membranes in chloroplasts)
- Membranes within cells can be the site of chemical reactions (eg. inner membrane of mitochondrion contains enzymes needed for respiration)
What is compartmentalisation
Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane
Importance of compartmentalisation
This is vital as it allows different parts of the cell to carry out specific functions under optimum conditions
Example of a membrane-bound organelle & why it needs to be kept compartmentalised
An eg of a membrane-bound organelle is the lysosome (animal cells), each containing many hydrolytic enzymes that can break down many diff kinds of biomolecule.
These enzymes need to be kept compartmentalised otherwise they would breakdown most of the cellular components
see slide 8-10 for structure of a phospholipid dia
10 in particular
Structure of a phospholipid
Has a hydrophilic (phosphate) head
And hydrophobic (lipid) tails
What does hydrophilic mean
Water loving - attracts water
What does hydrophobic mean
Water hating - repels water
Relationship between water and lipids
These two substances do not mix
Why do water & lipids not mix
- Water is a polar molecule (the oxygen end is slightly -ve and the hydrogen end slightly +ve)
- Fats are non-polar & do not form hydrogen bonds w water
- Fats are said to be hydrophobic & lie on the surface of the water to reduce the SA in contact between the fat & the water
What happens when phospholipids are exposed to water
When exposed to water, phospholipids form one of two structures: a micelle or a bilayer
(see slide 12 for dia)
Structure of the head and tails of phospholipids
In both a micelle and a bilayer, the hydrophilic heads face the water & the hydrophobic tails point inwards away from the water
This behaviour is key to the role that phospholipids play in membranes
How do phospholipids form a barrier to dissolved substances
- Phospholipids have a ‘head’ & ‘tail’
- The head is hydrophilic & the tail is hydrophobic
- The molecules automatically arrange themselves into a bilayer - the heads face out towards the water on either side of the membrane (see slide 13)
- The centre of the bilayer is hydrophobic so the membrane doesnt allow water-soluble substances (like ions) through it - it acts as a barrier to these dissolved substances
- (But fat-soluble substances, eg fat-soluble vitamins, can dissolve in the bilayer & pass directly through the membrane)
What is the cell surface membrane
A phospholipid bilayer that contains intrinsic and extrinsic proteins
Who came up with the ‘fluid mosaic model’
Singer and Nicholson, 1972
What does the ‘fluid mosaic model’ help explain
- Passive and active movement between cells & their surroundings
- Cell-to-cell interactions
- Cell signalling
Why does the ‘fluid mosaic model’ describe cell membranes as ‘fluid’
- The phospholipids in the bilayer are constantly moving via diffusion
- The phospholipids mainly move sideways, within their own layers
- The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
Why does the ‘fluid mosaic model’ describe cell membranes as ‘mosaics’
- The scattered pattern produced by the scattered protein molecules within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
What are the 4 main component the ‘fluid mosaic model of membranes’ mention
- Phospholipids
- Cholesterol
- Glycoproteins & glycolipids
- Transport proteins
Structure of the bilayer
The cell surface membrane is a bilayer of phospholipids with embedded proteins
(slide 16-17)
What actually is a bilayer
2 layers of phospholipid molecules
4 points
Functions of the cell surface membrane
- Partially permeable barriers between the cell & its outside env, between organelles and the cytoplasm & within organelles
- Controls which substances enter & leave the cells
- Membranes allow recognition by other cells (eg. cells of the immune system)
- Sites of cell communication (‘cell signalling’)
9 components
Components of the bilayer
- Phospholipid bilayer
- Various protein molecules (channel proteins, carrier proteins)
- Intrinsic proteins (completely embedded in bilayer)
- Extrinsic proteins (partially embedded in bilayer)
- Glycoproteins (carbohydrate attached to protein)
- Glycolipids (carbohydrate attached to phospholipid)
- Cholesterol (between fatty acid tails)
- Receptor sites (for chemical signals)
- Enzymes & coenzymes
see slide 20-22, pg50 for components of the bilayer dia
Structure of phospholipids (part 1)
pg60 dia
Phospholipid molecules have a ‘head’ & a ‘tail’
- The head is hydrophilic - attracts water
- The tail is hydrophobic - repels water
What are the 2 main types of protein in the plasma membrane
INTRINSIC PROTEINS - channel proteins, carrier proteins, glycoproteins
EXTRINSIC PROTEINS (peripheral proteins)
What type of protein is a glycoprotein
An intrinsic protein
(see slide 26 for dia)
What are Glyoclipids
Lipids that have a polysaccharide chain attached
Structure of extrinsic proteins
- Normally have hydrophilic R groups which interact with polar heads of phospholipids or intrinsic proteins
What do extrinsic proteins on the extracellular side of the membrane act as
- Receptors for hormones or neurotransmitters
- Cell recognition
Many are glycoproteins
What are extrinsic proteins on the cytosolic involved in
- Cell signalling
- Chemical reactions
They can dissociate from the membrane & move into the cytoplasm
How does Cholesterol work
- Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely tg. This makes the membrane less fluid & more rigid
- At lower temps, cholesterol prevents phospholipids from packing too close tg & so increases membrane fluidity
Where is Cholesterol found
- In all cell membranes (except bacterial cell membranes)
- Located randomly in the plasma membrane
between the tails of the phospholipid molecules
What is Cholesterol
A type of lipid with the molecular formula C27 H46 O
(slide 30-31)
Function of Cholesterol
Gives the membrane stability
Where are extrinsic proteins found
- Found outside the cell surface membrane or bound to an intrinsic protein
Function of Glycolipids
- Act as receptor for cell signalling
- Act as antigens (cell markers)
- Can be recognised by the cells of the immune system as self (cells of the organism) or non-self (cells from another organism)
(see slide 27)
Functions of Glycoproteins
- Involved in cell adhesion (cells joining tg to form tissues)
- Act as receptors for cell signalling
- Act as binding sites for drugs, hormones * antibodies
- Stabilise the membrane by forming hydrogen bonds w surrounding water molecules
Where are glycoproteins located
- Embedded in the cell surface membrane w attached carbohydrate (sugar) chains of varying length/shape
How can Phospholipids be chemically modified to act as signalling molecules
Phospholipids can be chemically modified to act as signalling molecules by:
- moving within the bilayer to active other molecules (enzymes)
- being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
Purpose of Phospholipids
- Phospholipids form the basic structure of the membrane (the phospholipid bilayer)
- Phospholipids bilayers act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)
- This ensures water-soluble molecules sa sugars, amino acids, proteins cannot leak out of the cell & unwanted water-soluble molecules cannot get it
What would happen to membranes without Cholesterol
Cholesterol increases the mechanical strength & stability of membranes - w/o it, membranes would break down & cells would burst
What type of protein are channel proteins
Intrinsic proteins
What are channel proteins
Channel proteins are water-filled (hydrophilic) pores
(see slide 32 for dia)
Purpose of channel proteins
- They allow charged substances (eg. ions) to diffuse through the cell membrane
- The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
- This allows the channel protein to control the exchange of ions
What type of protein are carrier proteins
Intrinsic proteins
Structure of carrier proteins
- Unlike channel proteins which have a fixed shape, carrier proteins can switch between 2 shapes
- This causes the binding site of the carrier protein to be open to one side of the membrane first, & then open to the other side of the membrane when the carrier protein switches shape
Diffusion in carrier proteins
- The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
- Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
(see slide 33)
see slide 34 for dia of all the proteins in the membrane
Site of chemical reactions
- Like enzymes, proteins in the membranes forming organelle, have to be in particular positions for chemical reactions to take place.
FOR EG. the electron carriers & the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion) for the production of ATP in respiration. - The enzymes of photosynthesis are found on the membrane stacks within the chloroplasts
EXAM Q: Alcohol, caffeine and nicotine are all lipid-soluble molecules. They have an almost instant and widespread effect on the body. Explain why (2)
Lipid-soluble molecules can pass through membranes by simple diffusion so diffuse quickly through the whole Boyd
EXAM Q: Membranes, particularly those present within mitochondria, are often highly folded. Suggest what advantages this folding provides (6)
Processes occur across membranes.
These processes are enzyme controlled
Folding gives increased SA
so more enzymes, increasing rate of reaction
therefore an increased rate of ATP production
EXAM Q: Explain why the protein, phospholipid, cholesterol & carbohydrate composition of different cell types varies
Composition depends on cell function -
- if cells need to transport specific types of molecules across their membrane, they will have more proteins for channels & carriers.
- If cells have enzymes embedded in their membranes, they will have a greater proportion of protein
- If cells have recognition or signalling molecules, they will have more glycoproteins
General structure of all membranes
Composed of lipids (mainly phospholipids), proteins, & carbohydrates
What are glycoproteins
Proteins that have a polysaccharide (carbohydrate) chain attached
see pg50 for dia of bilayer
Function of Phospholipids
Form a barrier to dissolved substances
Structure of Phospholipids (part 2)
(pg50 dia)
- The molecules automatically arrange themselves into a bilayer - the heads face out towards the water on either side of the membrane
- The centre of the bilayer is hydrophobic so the membrane doesnt allow water-soluble substances (like ions) through it - it acts as a barrier to these dissolved substances
(but fat-soluble substances CAN dissolve in the bilayer)
QUICK SUMMARY: What do channel proteins do
Allow small or charged particles to enter/leave the cell
QUICK SUMMARY: What do carrier proteins do
Transport molecules & ions across the membrane by active transport & facilitated diffusion
Why do cells need cell signalling (communication)
To control processes inside the body & to respond to changes in the env
How do cells communicate with eachother
Using messenger molecules:
1. One cell releases a messenger molecule (eg. hormone)
2. This molecule travels (eg. in the blood) to another cell
3. The messenger molecule is detected by the cell bc it binds to a receptor on its cell membrane
What are receptors
Proteins in the cell membrane act as receptors for messenger molecules. These are called ‘membrane-bound receptors’
Structure of receptors
- Have specific shapes - only messenger molecules w a complementary shape can bind to them
- Different cells have different types of receptors - they respond to different messenger molecules
- A cell that responds to a particular messenger molecule is called a target cell (see pg52 dia)
What else binds to cell membrane receptors
DRUGS
- Many drugs work by binding to receptors in cell membranes
- They either trigger a response in the cell, or block the receptor & prevent it from working
see p53 for membrane permeability at each degree celsius
