Cell Membranes & Transport Flashcards
What is the physiological role of membranes?
1) Provide a selectively permeable barrier which protects the interior of the cell and associated organelles, etc. while allowing the movement of certain metabolites in/out of the cell.
2) Allow for transport of various things - waste, O2, nutrients, ions, etc.
3) Give cell shape and structure
4) Compartmentalizations of organelles
5) Offer sites for enzyme and hormone binding
6) Allow for electrochemical gradient to form
What is a metabolite?
Small molecules (usually) that are intermediate or end products of metabolism.
What is the general composition of a cell membrane?
1) Lipids, proteins and carbohydrates
Primary component of a cell membrane
Phospholipid
Describe the structure of the membrane
1) Asymmetric bilateral - refers to arrangement of proteins embedded or attached within or to the membrane.
2) Semi-permeable
3) Amphipathic - hydrophilic head and hydrophobic tail (form hydrophobic core, interior)
What are the 3 types of membrane lipids?
1) Phospholipids (PL)
2) Glycolipids
3) Cholesterol
Phospholipid types
-Type is dependent open the backbone of the lipid
1) Glycerophospholipid (alcohol)
- glycerol backbone, 2 fatty acids and phosphate group
- Ex. Phosphatidylcholine, phosphatidylserine
2) Sphingolipids (SL)
- Sphingosine backbone, 1 fatty acid, phosphorylcholine
- Ex. Sphingomyelin (IMPORTANT!!!)
Clinical relevance of sphingomyelin
Niemann-Pick Disease
- Rare autosomal recessive disorder - Results in sphingomyelinase deficiency, preventing breakdown sphingomyelin - Build up of lipid leads to hepatoslenomegaly (liver and spleen enlargement) and neurological damage (retardation, seizures) - Victims die before Age 2.
Structure of Sphingomyelinase
Phosphorylationcholine + ceramics
Describe a glycolipid structure
1) Sphingosine backbone
2) Carbohydrate (oligosaccharide) backbone
- Note: MAKES IT DIFFERENT FROM SPHINGOPHOSPHOLIPID
-Located on outside of lipid membrane (aka outer leaflet)
Describe cholesterol structure and location
1) Steroid
2) Hydroxyl group
3) Hydrocarbon side-chain
-Embedded in bilateral, between PLs
Why are cholesterol microdomains (area of membrane with distinct structure/function important?
- “Lipid rafts”
- Essential for cell signaling
- Because
Location of membrane lipids
Inside (Inner sheet)
- Glycolipids - Sphingomyelin - Phosphatidylcholine
I.e. sphingolipids
Outside (outer sheet)
- Phosphatidylinositol - Phosphatidylserine - Phosphatidylethanol
I.e. glycerophospholipids (except phosphatidylcholine)
Types of Membrane Proteins
1) Integral - embedded
2) Peripheral - attached
3) Lipid-anchored - tethered
Describe an Integral Membrane Protein
1) Protein within (embedded) the bilayer
Ex. Polytopic transmembrane protein
What is a polytopic transmembrane protein
An integral membrane protein that goes across the entire width of the membrane and as such, not only interacts with the interior and exterior of the cell but serves and important transport role
- Includes transport proteins, ion channels and receptors
- Move things in and out of cell and responsible for cell signaling
What is a Peripheral protein
- A protein loosely attached to the membrane
- Attached via electrostatic interactions with either lipids or other proteins.
Tip - when you hear peripheral think outside, doesn’t matter so it is loosely held to the cell which wouldn’t mind losing it.
What is a lipid-anchored protein
Protein tethered to lipids in the membrane via covalent bonds (“hook”)
-Ex. G proteins
Structure and location of carbohydrate in membrane
1) Covalently attached to some membrane lipids and proteins
2) Faces the extracellular environment/space
- Glycocalyx is common shell
What is glycocalyx and describe its function
- A carbohydrate shell covering many membranes
- Consists of (1) Glycolipids and (2) glycosylated proteins
Function:
1) Protection/barrier - prevents membrane mechanical breakdown and premature degradation
2) Cell connection/adhesion - allows cell to have better contact with their cells (essential during fertilization and tissue creation/formation)
3) Cellular identification - allows body to differentiate its cells from foreigners (EXAMPLE: glycocalyx on RBC allows for A, B, AB blood type differentiation due to differences in oligosaccharide residues)
Why is membrane fluidity important?
1) Proteins and lipid ability to move freely (fluid mosaic) allows for them to go to specific membrane locations to carry out designated functions.
***But, too fluid or too rigid is also not good
Factors that impact membrane fluidity
1) Temperature
2) Lipid make up/composition
3) Cholesterol
What is Tm
Melting temperature
The temperature at which a membrane goes from a fluid to stiff/rigid state.
too rigid
»>Tm -> too fluid
>Tm -> Juustttt riiighhtt
Lipid composition w/ respect to membrane fluidity
Lower membrane fluidity
-Lipids with long, saturated fatty acids
Higher membrane fluidity
-Lipids with short, unsaturated fatty acids (due to kinks in the chain, which prevent dense packing and make MORE FLEXIBLE)
Saturation and fluidity are inversely proportional
Cholesterol w/ respect to membrane fluidity
Depending on other factors, membrane fluidity can increase or decrease.
Rigid membrane (high saturated fat) + cholesterol = increase in fluidity Fluid membrane (high unsaturated) + cholesterol = decrease in fluidity
Buffers a potentially large impact on fluidity from temperature
Describe membrane permeability (i.e. types of molecules)
Membrane is permeable to lipophilic molecules (diffusion) while being impermeable to hydrophilic molecules
Hydrophilic molecules need transport protein
What is a transport protein
A type of transmembrane protein that allows molecules, ions, etc. to move in and out of the cell by either passive or active means
What are the important ion gradients for cells? Describe the concentrations of each ion in and out of a typical neuronal cell (In mM).
Na+ / K+ / Ca2+ / Cl-
E. 145 / 4 / 2 / 50
————————————
In. 15. /. 150/. 10. /. 10^-8 (Cl- has 10,000 fold gradient!)
Describe Passive Transport
- Does not depend on energy to move molecules across the membrane
- Molecules move across membrane by moving from high to low concentration (I.e down their concentration gradient)
Describe Active Transport
-Molecules are moving against their concentration gradient (low to high concentration) and thus require transport that NEEDS ENERGY to move them across membrane.
ATP hydrolysis provides energy (-12kcal/mol)
Describe the types of passive transport
1) Simple Diffusion
- No assistance required
- Only for small, non polar and polar, uncharged molecules
- Greater concentration difference (steeper gradient)=faster diffusion
2) Facilitated Diffusion
- Assistance from Transmembrane Proteins required
- Function as (a) ion channels or (b) transporters
- For large, charged molecules
- Really increase speed of transport (similar to enzymes and rxns)
- Examples. Aquaporin (for water transfer), Voltage-gated Na+, Glucose transporter
Explain the types of facilitated diffusion
1) Ion channels - openings or “gates” in the bilayer that allows charged and polar molecules to pass through (again, down concentration gradient) - very common, extremely present throughout
2) Ligand-gated channel - a gate that undergoes a conformational change when attached by a ligand (neurotransmitter or hormone). The change opens the gate allowing ions to pass through (again, down concentration gradient).
- Once ligand is release, gate closes (Ex. Glutamate receptor - antagonist is Mimantine/Namenda)
3) Voltage-gated Ion Channels - open in response to change in membrane electrical potential (large negative charge inside cell)
Explain the three conformational states of a Ligand-gated Ion Channel
1) Resting - gate closed
2) Open - ions travel through freely
3) Desensitized - channel doesn’t respond to ligand - remains closed
Explain depolarization in regards to Voltage-gated channels
Occurs when a positively charged ions come into the cell, creating a large increase in membrane potential - this is what triggers the opening of the gated channels, allowing SPECIFIC ions to come across membrane.
Example. Sodium channel and potassium channel. Common in neurons
Stages of Votage-gated ion channel
1) Closed
2) Open
3) Inactivated - brief period following depolarization where new signal does not open channel.
Consequences of blocking ion channels?
Pufferfish toxin - blocks sodium pump - can be fatal!
Topical aesthetics - block sodium channel, inhibiting neurotransmission for pain!! Cool!!!
What is Active Transport? Types?
Transport requiring energy input to move molecules across against concentration gradient. Directed by Integral Proteins (Polytopic transmembrane protein transporters)
1) Primary - requires direct input of energy
2) Secondary - use of energy secondhand/indirectly (stored in concentration gradient) - thus it is connected to a primary transport system.
What are the two types of Primary Active Transporters
1) P-type ATPases - When ATP is hydrolyzed, the protein becomes phosphorylated
2) ABC Transporters - the above phosphorylation does not occur.
Explain P-type ATPases
- Uses ATP hydrolysis to transport against gradient
- After hydrolysis, covalent bond between the free phosphate is formed with the protein - phosphorylation of aspartame residue.
- HUGE conformational changes occur, allowing for transport
Examples: Na+/K+-ATPase (Sodium/Potassium pump -> 3Na out, 2K+ in) and Ca2+-ATPase (Ca2+ out)
Uniporter vs. Antiporter vs Symporter
Uniporter is a transport protein which moves one ion in one direction. (Mitochondrial calcium transporter)
Antiporter - moves molecules in opposite directions. (Sodium calcium exchanger (NCX))
Symporter - moves two molecules in same direction (Sodium-glucose transporter, Lactose permease)
Explain ABC Transporters and how it works
- Moves small molecules (large diversity) against concentration gradient, out of the cell.
- ATP is energy source
Example. Glycoproteins (has carbohydrate group attached)
How it works:
1) Substrate binds
2) Transporter undergoes conformational change
3) Increase want (affinity) for ATP
4) ATP binding
5) Substate expelled following ATP binding conformational change
6) Substrate expelled out of cell
7) ATP hydrolysis resets and protein goes back to resting state.
Importance of ABC transporters in drug resistance?
Some pathogens increase the expression of ABC transporters, which in turn expel more drugs out of the cell. Therefore, the cells aren’t affected by the drug, resulting in “drug resistance.”
Explain Secondary Active Transport
- ATP not used directly - coupled with primary transport mechanism
- Rather, molecules going against gradient are transported along with a molecule that is going with gradient due to the difference in concentration created by a primary transporter (I.e. Sodium-glucose transport works by glucose coupling with Na - glucose is going against gradient while sodium is going down gradient due to Na/K-ATPase)
How does NCX move molecules?
NCX is a secondary transporter, moving Ca2+ against it’s gradient by utilizing the energy from the Na+ gradient set by NA/K-ATPase. NCX is an antiporter.
Movement -> 3Na moves down gradient, 1 Ca2+ move upgradient
Explain how Dietary Monosaccharides are moved from intestine to blood stream
Monosaccharides from polysaccharides (starch) and disaccharides (sucrose and lactose) are moved from the lumen to the blood via: 1) Active Transport and 2) Facilitated Diffusion
1) D-Glucose and D-galactose enter intestinal epithelial cells via secondary active transport permitted by sodium-glucose transporter 1 (SGLT1)
2) Glucose Transporter (GLUT2) moves glucose across enterocyte (cell of intestinal lining) via Facilitated Diffusion and into bloodstream
Fructose -> facilitated diffusion -> move down gradient using GLUT5 (apical side) and GLUT2 (basal side) transporters into blood stream
See page 95 in Panini
