1. Cellular and Molecular Structure and Function Flashcards
Give 4 reasons why membranes are important.
1) Form cells by separating cytoplasm from external medium
2) Act as a permeability membrane to maintain ionic gradients
3) Form internal organelles
4) Enable integration of cell-surface receptor proteins
Give some examples of places where membranes are found.
- Plasma membrane
- ER
- Golgi apparatus
- Mitochondria
- Nuclear envelope
- Lysosomes
- Secretory vesicles
Describe how the composition of a cell membrane may be analysed and why.
- RBCs can be used for a pure fraction of a membrane
- This is because RBCs have few organelles, which may have different membrane compositions
- So RBCs provide a pure membrane fraction
Why are RBCs ideal for studying cell membranes?
- Large number of cells can be obtained easily
- Anucleate and lack intracellular organelles
- Easy to manipulate (so can study both inside and outside of membrane)
Describe how a RBC can be manipulated for membrane study.
- Placed in hypotonic solution to lyse
- Washed and resealed either right-side-out or inside-out vesicles
- Outside can now be studied
Describe the general structure of a membrane.
Fluid mosaic model where proteins are floating freely in a sea of phospholipid.
Describe the structure of a phospholipid.
- 2 fatty acids
- Glycerol
- Phosphate
- Alcohol (e.g. choline, serine, ethanolamine, etc.)
2 fatty acids attach to glycerol at the tail, while phosphate and alochol form a chain and attach to the glycerol at the top
Draw the structure of a phospholipid.
What is the difference between different phospholipids?
The variable species at the top of the phosphate head (e.g. choline, serine, ethanolamine). It is an alcohol.
Which part of a phospholipid is hydrophilic and which part is hydrophobic?
- Hydrophilic - Phosphate head
- Hydrophobic - Fatty acid tail
How many double bonds are there in the fatty acids in a phospholipid?
Only one, on one of the fatty acids.
What does the double bond in one fatty acid tail of a phospholipid cause?
- Causes bend in the chain
- So the phopshlipids can’t pack as tightly
- So membrane fluidity is increased
What are the principle phospholipids?
- Phospatidylcholine (lecithin)
- Phosphatidylserine
- Phosphatidylethanolamine
- Sphingomyelin
In terms of hydrophobicity/hydrophility, what type of molecule is a phospholipid?
Amphipathic - it has both hydrophobic and hydrophilic parts.
Why does a C=C bond in a fatty acid tail in a phospholipid induce a kink?
It shows no rotation, and it is cis.
Are all principle phospholipids charged?
No, only phosphatidylserine, which has a net negaytive charge.
Describe the order of permeability of difference substance across a lipid bilayer.
Most permeable to least permeable:
- Hydrophobic molecules (O2, N2, CO2)
- Small, uncharged, polar (H2O, Urea, Glycerol)
- Large, uncharged, polar (Glucose, Sucrose)
- Ions (Na+, K+)
What is the term for phospholipids being both hydrophilic and hydrophobic?
Amphipathic
What are some pieces of evidence for the bilayer arrangement of phospholipids in cell mambranes?
1) Electron microscopy with osmium tetroxide -> Reacts with hydrophilic heads to give a black precipitate
2) Calculations -> E.g. counting the number of phosphilipids in a RBC and calculating the only way they could possibly be arranged in this surface area
What are the two models of neurotransmitter release at a synpase?
- Full fusion -> Everything released
- Kiss-and-run -> Just touches the membrane and only releases a bit of contents before jumping back
In one word, describe how phospholipids are arranged in a lipid bilayer.
Asymmetrically
Describe the charges on a phospholipid bilayer and how this arises.
- Inside is more negative
- Because all of the phosphatidylserine (PS) is on the inside layer -> This is negatively charged due to its head group
What occurs when a cell membrane loses asymmetry and why?
- Cell death can occur
- Because phosphatidylserine moves to the outside and can be recognised by phagocytes that destroy the cell
How is lipid assymtery maintained?
Phospholipid transfer proteins:
- Scramblases -> Both directions
- Flippase -> Outer to inner
- Floppase -> Inner to outer
The flippase and floppase require ATP to function.
Which way does flippase move phospholipids?
Outer to inner
(REMEMBER: FlIppase = Inwards)
Which way does floppase move phospholipids?
Inner to outer
(REMEMBER: FlOppase = Outwards)
What is the purpose of lipid asymmetry?
- Different proteins function better when surrounded by different phospholipids -> Asymmetric protein distribution could lead to asymmetric lipid distribution
- Localises proteins through protein-lipid binding
- Lipids on the inner side of the plasma membrane are used to derive signalling molecules that function in the cytoplasm
What are the different types of protein in a cell membrane?
Examples:
- Channels
- Carriers
- Receptor proteins
- Cell-adhesion proteins -> Enable cells to attach to ECM
What are the two ways in which a protein may associate with the plasma membrane?
- Integral -> Permanently within the membrane by adding a fatty acid chain or anchor
- Peripheral -> Temporarily bound by non-covalent linkage
Do proteins and lipids move within the membrane or are they static?
They move within the membrane.
Describe an experiment to demonstrate that the plasma membrane is fluid.
- Mouse cell and human cell are fused into a heterokaryon (cell containing genetically different nuclei)
- Different antibodies are prepared for the proteins from each cell
- The heterokaryon is incubated for 40 minutes
- The membrane proteins can be seen to have moved around and mixed with each other
Describe how the movement of a protein in an organism can be studied.
- The gene for the protein can be modified by inserting a sequence that codes for a Green Fluorescent Protein (GFP)
- When the protein is produced, it is tagged with a green fluorescent marker that can be
Give some examples of when cells might not want the proteins in the plasma membrane to move.
- Post-synaptic membrane receptors
- Polarised cells such as epithelial cells of the gut
How can the movement of proteins in the plasma membrane be restricted?
- Scaffolding proteins
- Lipid rafts (controversial existence)
Give an example of scaffolding proteins.
Ankyrin is a scaffolding protein that links integral membrane proteins to the cell cytoskeleton.
Give the clinical relevance of protein localisation within the membrane.
Heart failure:
- In healthy cardiac myocytes, calcium channels are arranged in a regular pattern
- But this structure can be disturbed by heart failure
Describe some examples of the lateral heterogenity of membranes.
- Lipid rafts -> Sections of the membrane with a high concentration of cholesterol and glycosphingolipids -> Examples include small invaginations called caveolae
- Multiprotein complexes -> A form of quaternary structure where several proteins associate non-covalently
- Signalling domains -> Allow communication from outside of the cell to inside it
Give some examples of the functions of integral cell membrane proteins.
- Receptors
- Transporters
- Enzymes
- Adhesion molecules (e.g. for adhesion to ECM and other cells)
What type of cell membrane protein do receptors tend to be?
Transmembrane
Give two examples of adhesion molecules within a cell membrane.
- Integrins -> For attachment to the ECM
- Cadherins -> For attachment to other cells
What type of molecule is cholesterol?
An unsaturated alcohol (sterol).
What is the function of cholesterol within the cell membrane?
- It affects the fluidity of the membrane -> Allowing for association of components
- At high temperatures, it stabilizes the membrane and raises its melting point
- At low temperatures it intercalates between the phospholipids and prevents them from clustering together and stiffening
Give some examples of the functions of peripheral (extrinsic) cell membrane proteins.
- Components of the cytoskeleton that hold membrane components in place
What are some functions of glycoproteins and glycolipids in cell membranes?
They predominantly act as cell markers. They may also have a role in cell membrane stability.
Potassium Sodium Chloride Phosphate Calcium Hydrogencarbonate Protein Magnesium
Describe which cations and anions have higher concentration intracellularly and extracellularly.
Higher intracellularly:
- Potassium
- Phosphate
- Protein
- Magnesium
- Hydrogen
Higher extracellularly:
- Sodium
- Chloride (NOTE: Varies a lot intracellularly)
- Calcium
- Hydrogencarbonate
Give the main ways in which transport across membranes can occur.
- Vesicles
- Through the lipid bilayer
- Proteins -> Channels, Carriers, Pumps
Give the order of the permeability of different species across a lipid bilayer.
From most to least permeable:
- Small, non-polar
- Small, uncharged, polar
- Large, uncharged, polar
- Ions
If a species can move across a cell membrane by diffusing through the cell membrane, can it still be transported by proteins too?
Yes
What is solute partitioning?
Partitioning is the distribution of a solute between two immiscible solvents (such as aqueous and organic phases).
Give the equation that describes the process of partitioning during diffusion across a lipid bilayer in a cell membrane.
[solute]lipid = [solute]water x alipid solubility coefficient
What does Fick’s Law describe?
The rate at which a solute will dissolve across a cell lipid bilayer.
State Fick’s Law.
J = D x A x (ΔC/Δx)
Where:
- J = Diffusion flux
- D = Diffusion coefficient -> Depends on molecular weight and temperature
- A = Surface area
- ΔC = Concentration difference
- Δx = Membrane thickness
What is electrodiffusion?
The diffusion of ions due to the charge gradient across a membrane.
What is the tonicity of a solution?
The osmotic force exerted by the solution.
What is the normal body fluid osmotic potential?
300mOsm/L
What increases the water permeability of membranes?
Aquaporins
What are channels?
- Aqeuous-filled pores
- Allow the (often selective) transport of ions and some osmolytes across the membrane by passive diffusion
Name the different types of channel.
Always open:
- Leak
Gated:
- Voltage-gated
- Ligand-gated
- G-protein-coupled
- Stretch-activated
What makes transport proteins (channels and carriers) specific?
There is an interaction between the solute and the transport protein.
Give some examples of voltage-gated channels.
- Voltage-gated sodium channel
- Voltage-gated potassium channel
- Voltage-gated calcium channel
Give some examples of ligand-gated channels.
- ACh receptor
Describe simply the functioning of G-protein coupled receptors.
- Ligand binds to receptor
- This causes the release of a G-protein
- This either directly or via an effector protein causes a change in the channel
How does transport via carriers differ from channels?
Transport via carriers is:
- Slower
- More sensitive to temperature
- Saturable (with Vmax and Km)
What are gap junctions and what is their structure?
- Non-selective channels between cells that allow intercellular electrical and chemical communication
- Formed of two aligned connexons (each made of 6 connexins)
What is active transport?
The movement of substances against their electrochemical gradients.
Describe the change that occurs when a carrier moves a substance across the membrane.
The binding of the ligand causes a cyclical conformational change that releases the ligand on the other side.
What are the types of carrier?
- Primary active -> Use energy from ATP to move substances against their electrochemical gradient
- Secondary active -> Use the energy from an electrochemical gradient set up by another primary active transporter to move substances against their electrochemical gradient
- Facilitated diffusion -> Speed up the equilibriation of substances, but cannot move them against electrochemical gradients
Do membrane transporter include channels?
NO, transporters do not include channels.
Give an example of primary active transporters.
- Na+/K+-ATPase
- Ca2+-ATPase
- H+/K+-ATPase (used to acidify the stomach)
Give an example of secondary active transporters.
- Na+/Ca2+ exchanger
- Sodium-glucose symporter
Draw the action of a sodium-potassium pump.
Give an example of a facilitated diffusion passive carrier.
- GLUT transporter
- AE1 (‘chloride shift’)
What are the different types of secondary active transporter? Give an example of each.
- Symporter -> Move substances in the same direction (e.g. Sodium-glucose and sodium-amino acid)
- Antiporter -> Move substances in opposite directions (e.g. Sodium-calcium and sodium-hydrogen)
Describe the arrangement of transporters in epithelial cells.
- Epithelial cells are polarised.
- In the small intestine, the membrane on each side contains different transporters.
State the extracellular and intracellular concentrations for sodium ions.
- Intracellular = 140mM
- Extracellular = 5-15mM
State the extracellular and intracellular concentrations for potassium ions.
- Extracellular = 4mM
- Intracellular = 140mM
State the extracellular and intracellular concentrations for calcium ions.
- Extracellular = 2.4mM
- Intracellular = 0.1 micromolar
State the extracellular and intracellular concentrations for chloride ions.
- Extracellular = 100mM
- Inracellular = Less than extracellular, but varies a lot
State the extracellular and intracellular concentrations for bicarbonate ions.
- Extracellular = 25mM
- Intracellular = 10-20mM
State the extracellular and intracellular concentrations for hydrogen ions.
- Extracellular = 40nM
- Intracellular = 50-100nM
