L8 - The Plasma Membrane Flashcards
What is the cell membrane?
A very thin membrane (7 - 8nm) with a flexible, yet sturdy barrier
What is the bilayer composed of?
Two layers in which hydrophilic heads organise themselves towards the outside and hydrophilic tails point inwards
Why is the cell membrane referred to using the Fluid Mosaic Model?
- Fluid: 2D liquid that restricts lateral diffusion of membrane components
- Mosaic: Regions within the membrane contain lipid rafts, proteins, and/or glycoproteins
What does the basic fabric of the membrane consist of? State the percentage made up by each component.
- Phospholipids (75%)
- Cholesterol (20%)
- Polar Glycolipids in the external layer (5%)
What is cholesterol?
A steroid lipid
Describe the structure of cholesterol.
Ringed carbon structure
- with floppy carbon chain
- and hydrophilic hydroxyl group
What is cholesterol useful for?
- Membranes
- Lipid rafts
Describe how cholesterol affects membrane fluidity.
- Neighbouring lipid molecules swap places 10 million times per second
- Cholesterol immobilises the first few hydrocarbon groups of the phospholipid molecules
- Makes the lipid bilayer less deformable and decreases its permeability to small water-soluble substances
What does fluidity allow?
Movement of membrane components required for:
- Cell movement
- Growth
- Division
- Secretion
- Formation of cellular junctions
What is a glycolipid?
A lipid covalently attached to an oligosaccharide.
What do glycolipids form part of?
- Cell membrane
- Glycocalyx
What do glycolipids determine?
ABO blood group
How is the glycocalyx formed?
From many membrane proteins (glycoproteins with associated oligosaccharides)
What is the glycocalyx required for?
Detection of ‘self’ in immunity
What does the glycocalyx aid?
- Cell to cell adhesion
- Makes RBCs slippery
- Protects the GI from drying out
Describe the 5 functions of the phospholipid bilayer.
- Regulates what enters and exits the cell (altering pH / charge)
- Involved in cell recognition
- Plays major role in cell signalling (e.g. from hormones)
- Has enzymatic functions
- Aids in cell linking and crosstalk
State the 4 different types of membrane proteins.
- Transporters
- Anchors
- Receptors
- Enzymes
Describe the permeability of the cell membrane.
- Cell membrane is selectively permeable
- Permeable to non-polar molecules (such as O2, CO2 and hormones)
- Impermeable to ions and large molecules (such as Na+ and glucose)
- Slightly permeable to small, uncharged, polar molecules (such as water)
What does selective permeability allow a cell to do?
- Establish concentration gradients
- Regulate pH
- Build electrical gradients (e.g. inside of cell is more negative = creates membrane potentials)
State the 5 different types of membrane transport.
- Simple Diffusion
- Channel Mediated Transport (Passive)
- Carrier Mediated Transport (Passive)
- Active Transport
Describe passive transport.
A type of transport in which no cellular energy is used, as substances move down their own concentration gradient
State the 3 types of passive transport.
- Diffusion through lipid bilayer
- Channel mediated facilitated diffusion
- Carrier mediated facilitated diffusion
What is equilibrium potential?
The potential gradient across the membrane to maintain concentration gradient (The electrical potential needed to stop diffusion down chemical gradients)
What are Nerst potentials?
From the Nerst equation, one can calculate voltages that relate to the magnitude of the ionic gradient for each of these ions
In what 3 ways can channel mediated transport be regulated?
- Gated
- Timed
- Signal
What is carrier mediated facilitated transport subject to?
- Transport maximum
- Saturation
How can homeostasis be maintained? Provide an example.
Selective permeability can be regulated to maintain homeostasis.
For example, the hormone insulin (via its receptors) up-regulates glucose transporters.
What can diabetic patients lack?
The ability to up regulate GluT
Define osmosis.
The diffusion of water through a semi-permeable membrane
What does the diffusion of water through the liquid bilayer occur through?
Specific transmembrane protein channels called aquaporins.
How can RBCs for transfusions be destroyed?
By hypertonic and hypotonic solutions.
What are most intravenous solutions?
Most are isotonic and are made up of 0.9% saline / 5% dextrose.
Describe active transport.
A form of transport that involves the expenditure of energy from the hydrolysis of ATP.
What is active transport used for?
To transport essential ions against their concentration gradient (in order to help maintain tonicity, volume and charge)
What 2 types can active transport be classified as?
- Primary active transport (uses energy from the hydrolysis of ATP)
- Secondary active transport (uses energy stored by an ionic concentration gradient)
Describe primary active transport.
- Movement against a concentration gradient (mainly ions)
- ATP changes the shape of the carrier protein, pumping the substance across
How much ATP is used in primary active transport?
Cells typically use 40% of their generated ATP in this way.
What is the most common example of primary active transport in the body?
Sodium-potassium pump
Why must the sodium-potassium pump work non-stop?
As passive leaking may otherwise occur.
Describe the action of the sodium-potassium pump.
1) 3Na+ bind
2) ATP is hydrolysed (Using ATPase)
3) 2K+ bind, P is released
4) 2K+ enter
Describe secondary active transport.
A form of active transport across a biological membrane in which a transporter protein couples the movement of an ion down its electrochemical gradient with the uphill movement of another molecule or ion against a concentration / electrochemical gradient.
Describe the differences between symport and antiport.
- Symport: Molecules travel in the same direction
- Antiport: Molecules travel in the opposite direction
Describe 2 instances when symport is used.
- For Ca2+ regulation
- For H+ regulation
Describe an example of secondary active transport.
Co-transport of sodium ions:
1) Sodium ions (Na+) are actively transported out of the ileum epithelial cells into the blood by the sodium-potassium pump (this creates a concentration gradient)
2) The high sodium ions in the blood cause sodium ions from the ileum lumen to move into the ileum epithelial cells, down a concentration gradient (via the sodium-glucose co-transporter proteins)
3) Co-transporter proteins carry glucose from the cells with sodium, so the concentration of glucose in the ileum epithelial cells increases
4) This causes glucose to diffuse out of the ileum epithelial cells into the blood, down a concentration gradient (via facilitated diffusion)
What is vesicular transport used for?
- Endocytosis (bringing substances into the cell)
- Exocytosis (expelling substances from the cell)
- Inter-organelle transport
Does vesicular transport use ATP?
Yes
What 3 subdivisions can endocytosis be further classified into?
- Receptor-mediated endocytosis
- Phagocytosis
- Pinocytosis
What is receptor-mediated endocytosis / clathrin-mediated endocytosis used for?
- Uptake of LDLs
- Uptake of some vitamins, proteins and hormones
What is clathrin?
A protein for enriching membrane domains.
Describe the steps of receptor-mediated endocytosis / clathrin-mediated endocytosis.
1) Binding
2) Vesicle formation
3) Unseating
4) Fusion with endosome
5) Recycling of receptor
6) Degradation in lysosomes
What is phagocytosis?
A process by which certain living cells called phagocytes ingest / engulf other cells / particles.
What does the term ‘phagocytosis’ literally mean?
‘To eat’
What is the purpose of phagocytosis?
To engulf large particles, such as worn out cells and bacteria
What is phagocytosis carried out by?
- Macrophages
- Neutrophils
Describe the steps of phagocytosis.
1) Chemotaxis and adherence of microbe to phagocyte
2) Ingestion of microbe by phagocyte
3) Formation of a phagosome
4) Fusion of the phagosome with a lysosome to form a phagolysosome
5) Digestion of ingested microbe by enzymes
6) Formation of residual body containing indigestible material
7) Discharge of waste materials
What does the term ‘pinocytosis’ literally mean?
‘To drink’
How else can pinocytosis be described?
Bulk phase endocytosis
What happens during pinocytosis?
Small droplets of extracellular fluid are taken in
Are receptors needed for pinocytosis?
No
What is significant about pinocytosis being non-selective?
All solutes in the extracellular fluid are brought in
Where does pinocytosis occur?
In most cells, but especially absorptive cells in the intestines and kidneys.
What is exocytosis used for?
To transport material out of the cell.
Where does exocytosis occur?
Exhibited by all cells, but most important in:
- Secretory cells (digestive enzymes and hormones)
- Nerve cells (neurotransmitters)
Describe the steps of exocytosis in nerve cells.
1) An action potential depolarises the axon terminal
2) The depolarisation opens voltage-gated Ca2+ channels and Ca2+ enters the cell
3) Calcium entry triggers exocytosis of synaptic vesicle contents
4) Neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell
5) Neurotransmitter binding initiates a response in the postsynaptic cell
What is trancytosis?
A combination of endocytosis and exocytosis
What happens during trancytosis?
Substances are passed through a cell
Where in the body is trancytosis common?
In endothelial cells which line the blood vessels
State the 5 different types of cell junctions.
- Tight Junctions
- Adherens Junctions
- Desmosomes
- Hemidesmosomes
- Gap Junctions
What is the role of Tight Junctions?
Prevent movement of substances
What is the role of Adherens Junctions?
Maintain cellular position
What is the role of Desmosomes?
Distribute tension and prevent tearing
What is the role of Hemidesmosomes?
Distribute tension and prevent tearing
What is the role of Gap Junctions?
Allow movement of substances
How are Tight Junctions formed?
By the fusion of integral proteins of adjacent cells
What do Tight Junctions prevent?
- Prevent anything passing through the extracellular space between them
- Bar the movement of dissolved materials through the space between epithelial cells
What is absent where there is a Tight Junction?
Intercellular space
What forms complex meshwork?
Long rows of Tight Junction proteins
What do Adherens Junctions usually form?
Adhesion belts
What do Adherens Junctions resist?
Resists separation in contractile activities such as peristalsis
What does the plaque on an Adherens Junction attach to?
- Membrane proteins
- Microfilaments of the cytoskeleton
How are the anchoring junctions: Desmosomes and Hemidesmosomes held together?
By linker protein filaments called cadherins or integrins that extend from button-like thickenings called plaques.
Where are Gap Junctions found?
In electrically excitable tissues for synchronisation.
How are adjacent cells featuring Gap Junctions connected?
By hollow cylinders called connexions made of transmembrane proteins.
What are the 2 important roles of intercalated discs?
- Act to firmly bind adjacent cells together (mechanical coupling)
- But also, to allow electrical coupling between adjacent cells
What is mechanical coupling?
A small gap (0.02 μm) between membranes of adjacent cells filled with connective tissue.
What does mechanical coupling do?
Firmly binds 2 adjacent cells together.
What are cells bound together more strongly at certain points called?
Desmosomes
What is electrical coupling?
Longitudinal segments containing specialised regions where the membrane of adjacent cells come very close together.
What do regular arrays of proteins (called connexins), in the nexus (Gap Junction) form?
Large channels which allow the passage of ions and other small molecules between one cell and another.
