B2.1: Membranes And Membrane Transport Flashcards
What are some key features of membranes?
Mainly made of phospholipids
Amphipathic
-> polar hydrophilic head, nonpolar hydrophobic tail
Flexible
-> Able to break/fuse easily
Allow cellular compartments
-> different conditions
How to phospholipids mono/bilayers forms?
When PL placed in water
-> hydrophilic heads oriented towards water
-> hydrophobic tails orient away from water
= PL monolayer
With sufficient concentration
-> two layered structure form
= PL bilayer
What properties of PL bilayers allow it to be a barrier?
Two region:
-> hydrophobic core
-> hydrophilic outer layer
hydrophobic regions → attracted to each other
hydrophilic regions → attracted to water in the cytoplasm/extracellularfluid
Very effective barrier -> able to control which molecule pass through and out of cell
How does the structure of PL bilayers allow it to be a barrier?
Large molecules cannot pass -> hydrophobic region tightly packed -> low permeability to larger molecules
Polar (hydrophilic) molecules/ions -> cannot pass through hydrophobic tails
-> hydrophilic nature -> will not interact with hydrophobic fatty acid tails
What is simple diffusion?
A type of membrane transport that involves particles passing directly between the phospholipids in the plasma membrane
The net movement (as a result of random motion of molecules/ions) of a substance from a region of high concentration -> lower concentration
-> random movement -> kinetic energy of molecules/ions
-> molecules/ions move down a concentration gradient
-> diffusion for long enough -> molecules reach equilibrium -> distributed evenly on both sides of membrane
Examples of simple diffusion
Oxygen:
Oxygen diffuse into cells from surrounding capillaries
Oxygen used in respiration -> low concentraiton in cells -> concentration gradient
Carbon dioxide:
CO2 diffuse out of cell into surrounding capillaries
CO2 produced in respiration -> high concentration inside cells -> concentration gradient
What can effect the rate of diffusion?
Greater difference in concentration -> increased rate
Higher the temperature -> increased rate
-> more kinetic energy -> random movement faster
Greater surface area -> increased rate
Properties of the molecules/ions
-> large molecule -> more energy to move -> decreased rate
-> uncharged molecules -> move directly through BL -> increased rate
-> non-polar molecules -> soluble in the non-polar BL -> increased rate
-> smaller polar -> cannot easily pass through hydrophobic -> decreased rate
What are the two main type of proteins in membranes?
PL bilayer -> main functions of the membrane
Protein -> additional functions
-> protein content of membranes -> varies depending on the function
Integral:
-> amphipathic
-> embedded in the BL
-> can be across both layers or just one
Peripheral:
-> hydrophilic
-> attached to either surface of integral proteins/plasma membrane via hydrocarbon chain
-> inside/outside cell
What are the functions of membrane protein?
JETRAT
Junctions: serve to connect and join two cells together
Enzymes: fixing to membranes localizes metabolic pathways
Transport: responsible for facilitated diffusion/active transport
Recognition: function as markers for cellular identification
Anchorage: attachment point for cytoskeleton/extracellular matrix
Transduction: receptors for peptide hormones
What are some examples of the different categories of membrane proteins?
Receptors: binding of peptide hormones (insulin, neurotransmitters, antibodies)
-> binding -> generate signal -> triggers series of intracellular reactions
Immobilized enzymes:
-> integral proteins
-> active site exposed on the surface of membranes
-> inside or outside of cell
Cell adhesion: allows cells to attach to neighboring cells in tissue
Glycoprotein: cell markers, antigens, cell-to-cell recognition
-> ex: ABO blood groups antigens -> glycolipids/proteins with differing carb chains
What is osmosis?
Diffusion of solvent molecules (the liquid)
The net movement of water molecules from a region of high water potential -> low water potential across a partially permeable membrane
-> random movement
-> move down concentration gradient
What is osmotic concentration?
Amount of dissolved solutes
-> insoluble molecules don’t affect osmotic concentration
Concentrated:
-> high osmolarity
-> low WP
Dilute:
-> low osmolarity
-> High WP
What protein is involved in osmosis?
Aquaporins:
Type of channel proteins
Allow water to move directly in between PL freely
-> abnormal bc water = polar
What happens to plant cells in different solution?
Dilute solution -> water moves into cell via osmosis -> in vacuole -> cell become more turgid
-> increased turgor pressure
-> hypotonic solution (lower concentration than cell)
Concentrated solution -> water moves out of cell via osmosis -> vacuole shrinks -> plasmolysis
-> hypertonic (higher concentration than cell)
Plotting on graph -> osmolarity of cell determined by when cross x-axis (no osmosis)
-> isotonic point
-> isotonic solution (equal concentration as the cell)
How does the properties of a molecule/ion effect how they are transported across a membrane?
Lipid soluble/small -> diffuse directly through BL
-> ex: O2 and CO2
Slightly polar/some slightly large -> diffuse through hydrophilic channels
-> ex: H2O
Large/polar/ions -> via facilitated diffusion down their concentration gradient with help of protein
What are two type of transport protein that are used in facilitated diffusion?
Transport protein: highly specific -> only one type of molecule/ion can pass
-> integral
Move specific molecules/ions across membrane
-> Channel: form holes/pores which molecules can travel through
-> Carrier: change shape to transport a substance across membrane
What is facilitated diffusion?
The net diffusion of molecules/ions in/out of a cell through a transport protein from a region of higher concentration -> lower concentration
-> passive
-> down a concentration gradient
What are channel proteins?
Type of transport protein
Pores that allow the passage of a specific substance across a membrane
Allow charged substances (ex: ions)
Some are gated:
Part of the protein can move in order to close/open the pore
-> certain conditions needed to open (chemical or electrical stimuli)
-> can control the exchange of ions (selective permeability)
—> allow certain ion when open, but not when closed
What are carrier proteins?
A type of transport protein
Do not have a fixed shape -> can switch between 2 shapes
Type 1:
Transported substance -> attached to binding site -> shape change in carrier protein
Initially binding site of the protein is open on one side of the membrane
When protein switch shape -> opens to other side of membrane
Type 2:
Pick up molecule of matching shape
Rotates through the membrane -> deposits molecule on other side
Why is active transport needed to maintain concentration gradients?
To maintain concentration gradients -> individual molecules need to be moved from region of low concentration to high concentration
-> cannot occur via passive transport, need active transport
What are the difference between passive transport and active transport?
Passive:
-> no energy
-> random movement
-> high - low concentration
-> diffusion no membrane, osmosis/facilitated diffusion yes membrane
Active:
-> energy (ATP)
-> actively moves molecules to where they are needed
-> low - high concentration
-> need membrane
What is active transport?
The movement of molecules/ions across a cell membrane, from a region of low-high concentration using energy from respiration
-> against/up concentration gradient
-> requires carrier proteins (called pumps)
-> requires energy
—> used to help carrier protein change shape
—> energy provided by ATP
—> ATP hydrolyzed to release energy
What are pump proteins?
A type of membrane transport protein that actively moves particles across the membrane
-> use of energy (ATP)
-> low to high concentration (against gradient)
Function:
Transported particle -> enter pump embedded in membrane -> energy from ATP used to change shape of protein (chemically stable conformation to less stable) -> after change in shape particle is moved+released to opposite side of membrane -> convert back to stable shape w/o added energy
What is selective permeability?
The ability of the membrane to differentiate between different types of molecules
Only allow some molecules through while blocking others
How much do the different methods of transport allow selective permeability?
Facilitated diffusion + active transport -> mechanisms that allow selective permeability
Simple diffusion -> less control as it is only dependent on size/polarity+hydro-liking nature of molecules
-> no control over small, polar molecules
-> allows selective permeability in regards to large/polar molecules (Cannot pass w/o transport protein)
What are glycolipids/glycoproteins?
Only found on the extracellular surface of plasma membrane
-> function in cell adhesion/recognition
Glycolipids -> lipid with a carb chain attached on the extracellular side
Glycoproteins -> cell membrane proteins that have carb chain attached on extracellular side
What is the function of glycoproteins and glycolipids?
Function:
Carb chain -> receptor molecules
-> bind with substance on cell surface
Receptor types:
-> signaling -> bind to hormones/neurotransmitters
-> involved in endocytosis
-> involved in cell adhesion/stabilization
Some act as cell markers/antigens/cell identification:
Ex. allow immune system to determine if a cell belongs in the body or if its a pathogen/foreign invader
Cell adhesion:
Form layer on the outside of plasma membrane called glycocalyx
-> molecules of adjacent glycocalyx can fuse -> adhere cells together -> tissue no fall apart
What is the fluid mosaic model?
Model of membranes
First outlined in 1972 by Singer and Nicholson
Main components: PL, cholesterol, glycolipids/proteins, integral/peripheral proteins
Explains:
How biological molecules arranged to form cell membranes
Passive/active movement between cells + surroundings
Cell-to-cell interactions
Cell signalling
Fluid because:
PL and proteins can move around within their own layers
Mosaic because:
Scattered pattern produced by membrane in PL BL resembles mosaic when view from above
What is the importance of membrane fluidity?
PL can move around/switch positions within BL (rotate/drift laterally)
Important because:
-> Enable molecules to diffuse towards areas of cell they are needed
-> Facilitated protein interaction - cell signaling
-> Enable membranes to fuse during vesicle formation/endo+exocytosis
-> ensure even distribution of membrane molecules between daughter cells
How do fatty acids effect membrane fluidity?
PL -> fatty acid tail
Two things vary: length and saturation
Length:
Variation in hydrocarbon chain
Longer -> more interactions possible -> less fluid
Saturation:
Saturated tail:
Straight tail -> can be pressed closely together -> viscous + dense membrane -> decreased fluidity
Desaturated tail:
Kink in tail -> cannot pack tightly -> increased fluidity/permeability
How does temperature effect membrane fluidity?
Dependent on saturation of fatty acid:
Saturated:
Pack closely -> stronger intermolecular forces (high mp) -> more solid at increased temps than unsaturated
Unsaturated:
Pack loosely/not neat -> intermolecular attraction weaker (lower mp) -> more fluid at decreasing temp than saturated
Generally, decreased temp more solid, increased temp more fluid
How is fluidity regulated? Why is it important to regulate fluidity of membranes?
Ratio of saturated:
Unsaturated will depend on the temp of the habitat
-> fluidity important in cold environment - cold will cause compacting -> less fluid -> more susceptible to rupturing
Some organisms can modify membrane saturation of seasonal changes
-> low temp -> + unsat
-> high temp -> + sat
-> Can also vary in the body of a single organism
What are examples of membrane fluidity regulation?
Seasonal changes:
Chickpeas -> 31% increase in unsat when cold
Variation within a single body:
Hooves vs upper leg
Bacteria cannot regulate internal temp = subject to temp changes:
Requires mechanism to overcome
Some produce enzymes (fatty acid desaturase) -> increases number of double bonds in fatty acid -> helps maintain membrane fluidity in colder temps
Deep-sea marine organisms = extreme temperatures:
Correlation found between sea temp and membrane fluidizing lipid components (ex: polysaturated fatty acids)
Ex: plant -> arabidopsis thaliana:
Have show fatty acid unsaturation pathways have key role in acclimatization of membrane to high temps
What is the structure of cholesterol?
Lipid
Hydroxyl group at one end
4 linked hydrocarbon rings
HC tail
Amphipathic molecules:
Polar/hydrophilic hydroxyl group:
H bond with phospholipid head
Exposed to water
Nonpolar/hydrophobic HC rings and tail:
Near HC chains of phospholipid tail
In hydrophobic core
What is the function of cholesterol?
Modulator/adjustor of membrane fluidity and permeability
High temp: restrains movement of phospholipid fatty acids
Stabilize the membrane -> less fluid, reduce permeability to small molecules
Low temps: prevent stiffening of the membrane
Prevent tight packing of fatty acid chains -> maintain membrane fluidity
Barrier between phospholipids -> prevents water soluble substances from diffusing across the membrane
What is bulk transport?
Diffusion/osmosis/active transport -> transport of individual moelcules/ions
Bulk transport of larger quantities possible:
-> into cell = endocytosis
-> out of cell = exocytosis
Requires energy -> form of active transport
Require formation fo vesicles -> dependent on fluidity of membranes
-> vesicle can fuse with membrane (membrane need to be able to flex/bend -> fluidity important)
-> vesicle formation: active process , involves a small region of plasma membrane being pinched off
What is endocytosis?
Transport of materials INTO cells
Plasma membrane engulfs material -> form sac around it
Two types:
Phagocytosis:
Bulk intake of solid material by a cell
Cell specialized for this -> phagocytes
Vacuoles formed -> phagocytes vacuoles
Ex: white blood cells
Pinocytosis:
Bulk intake of liquids
Examples:
Macrophages engulf pathogens
Single cell organisms (amoeba) engulf other organisms as food source
Protein taken from mothers blood -> placenta -> fetus
Endo symbiotic theory
What is exocytosis?
Process by which materials are removed from/transported out of cells
Substance to be released -> secretory vesicle
-> travel to cell surface membrane
-> fuse with cell membrane
-> released contents to outside of cell
Examples:
Neurotransmitters from a presynaptic membrane
Secretion of hormones from endocrine glands (insulin) and pancreas (glucagon)
Excess water out of contractile vacuole (unicellular organism)
Release of cortical granules from egg cell during fertilization
Excretion of digestive enzymes from pancreatic cells
What are two type of gated ion channel protein?
Voltage gated:
Open/close in response to change in electrical potential of a cell membrane
Responsible of cellular functions that are mediated by electricity
-> muscle cell contraction, neuronal signaling
Ligand gated:
Open/close in response to a chemical messenger/ligand binding to the channel (hormone, neurotransmitter, etc)
Binding causes opening of channel -> + ions diffuse
Process transforms an extracellular chemical signal -> intracellularelectrical signal
What is an example of a gated ion channel protein?
Nicotinic acetylcholine receptors
-> neurotransmitter-gated ion channel
-> NA receptors found specifically at neuromuscular junction (nerve cell connect to muscle)
Neurotransmitter acetylcholine bind to nicotinic acetylcholine receptors -> trigger channel to open -> certain ions through (Ca2+, Na+, etc)
Influx of ions -> membrane potential changes -> generate action potential in neurons
At synapse (junction between neurons+neurons/ neuron+effector cell):
Ach released from presynaptic neuron -> diffuse across synaptic space -> bind to transmembrane ach receptor on post synaptic cell
Essential for muscle contraction
-> involved also in memory, motivation, attention, learning, arousal, REM sleep
What is an example of an exchange transporter?
Sodium-potassium pump proteins
-> integral
-> generate electrochemical gradient between inside+outside of nerve cell
-> moves three sodium ion out of cell, two potassium ion into cell using one ATP molecule
-> move against concentration gradient (active transport)
Pumping process:
1. Three Na ion inside of axon bind to pump
2. ATP attached to pump -> transfers a phosphate to pump (phosphorylation) -> change in shape -> pump opens to the outside of axon
3. three Na ions -> released to outside
4. two K ions from outside enter + bind to their sites
5. Attached phosphate -> released -> alter shape of pump again
6. Change in shape -> K ions released inside of axon
Essential for function of nerve cells:
Transport more positively charge Na ions to out than K ions in -> inside of cell is negatively charge (relatively)
Nerve cell stimulated -> sodium channel open -> sodium rush down electrochemical gradient -> reverse charge across membrane -> generation of nerve impulse
What is co-transport and indirect active transport?
Co-transport:
Coupled movement of substance across a cell membrane via a carrier protein (same time)
-> type of membrane transport protein that moves 2 molecules/ions across the cell membrane at the same time
-> one particle moved with concentration gradient, other against gradient
Involved combination of facilitated diffusion and indirect active transport:
Indirect active transport uses energy released by movement of 1 molecule down gradient to move another molecule against gradient
ATP used to set up initial gradient
What are the steps of indirect active transport?
ATP energy spent actively pump ion1 across the membrane though pump protein
Active pumping -> establishes/maintains concentration gradient of ion1
Ion1 moves back through the membrane via cotransporter protein -> now moving with gradient
Different molecule2 ‘hitches a ride’ with the movement of the ion1 through cotransporter protein
Molecule2 -> against concentration gradient
-> done by using energy released with movement of the ion1 with its gradient
What is an example of indirect active transport?
Sodium-dependent glucose cotransporter
-> on cell surface membrane of epithelial cells lining mammalian ileum/kidneys/small intestine
-> involved in the absorption of glucose from blood
- Sodium-potassium pump active transports Na ions into blood -> reduce concentration of Na ion in cell
- Na ions move down gradient into cell via cotransporter protein
- Glucose drawn into cell with Na ions via cotransporter protein (glucose against gradient)
- Glucose moves down concentration into cell
Active transport in the generation of initial sodium ion gradient
Transport of glucose doesn’t need energy -> why its indirect
Antiport -> molecules going to different places
Symport -> molecules hoping to same place
What allows cell adhesion?
To be multicellular -> cells need to adhere to form tissue
Plasma membrane responsible for adhesion (permanent or temporary)
Cell adhesion molecules (CAMs) -> required to carry out cell adhesion
-> type of cell surface protein
-> work by binding cells with other cells/extracellular matrix
—> extracellular matrix contains supporting structures (collagen proteins, support for the cells)
Different CAMs -> present in different types of cell-cell junctions (tight, adherens, despises, gap, etc)
-> needed for adhesion between neighboring cells
-> anchoring of cells to extracellular matrix
Explain the loss of cell adhesion in cancer cells
Cancer metastasis -> spread of cancer cell to tissues and organs beyond the primary tumor, formation of new secondary tumors
Loss of cell adhesion -> causes malignant tumor cells to dissociate from primary tumor during metastasis
Disease progression:
1. Non-cancerous cell use CAMs to adhere together/to extracellular matrix
2. Cancer cell inactive/functionally block CAMs -> lose ability to adhere together/to extracellular matrix
3. During metastasis -> cancer cells without functional CAMs break free -> cell spread to other parts of the body
