B2.1: Membranes And Membrane Transport

Question 1

What are some key features of membranes?

Answer 1

Mainly made of phospholipids

Amphipathic
-> polar hydrophilic head, nonpolar hydrophobic tail

Flexible
-> Able to break/fuse easily

Allow cellular compartments
-> different conditions

Question 2

How to phospholipids mono/bilayers forms?

Answer 2

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

Question 3

What properties of PL bilayers allow it to be a barrier?

Answer 3

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

Question 4

How does the structure of PL bilayers allow it to be a barrier?

Answer 4

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

Question 5

What is simple diffusion?

Answer 5

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

Question 6

Examples of simple diffusion

Answer 6

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

Question 7

What can effect the rate of diffusion?

Answer 7

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

Question 8

What are the two main type of proteins in membranes?

Answer 8

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

Question 9

What are the functions of membrane protein?

Answer 9

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

Question 10

What are some examples of the different categories of membrane proteins?

Answer 10

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

Question 11

What is osmosis?

Answer 11

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

Question 12

What is osmotic concentration?

Answer 12

Amount of dissolved solutes
-> insoluble molecules don’t affect osmotic concentration

Concentrated:
-> high osmolarity
-> low WP

Dilute:
-> low osmolarity
-> High WP

Question 13

What protein is involved in osmosis?

Answer 13

Aquaporins:
Type of channel proteins
Allow water to move directly in between PL freely
-> abnormal bc water = polar

Question 14

What happens to plant cells in different solution?

Answer 14

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)

Question 15

How does the properties of a molecule/ion effect how they are transported across a membrane?

Answer 15

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

Question 16

What are two type of transport protein that are used in facilitated diffusion?

Answer 16

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

Question 17

What is facilitated diffusion?

Answer 17

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

Question 18

What are channel proteins?

Answer 18

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

Question 19

What are carrier proteins?

Answer 19

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

Question 20

Why is active transport needed to maintain concentration gradients?

Answer 20

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

Question 21

What are the difference between passive transport and active transport?

Answer 21

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

Question 22

What is active transport?

Answer 22

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

Question 23

What are pump proteins?

Answer 23

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

Question 24

What is selective permeability?

Answer 24

The ability of the membrane to differentiate between different types of molecules
Only allow some molecules through while blocking others

Question 25

How much do the different methods of transport allow selective permeability?

Answer 25

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)

Question 26

What are glycolipids/glycoproteins?

Answer 26

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

Question 27

What is the function of glycoproteins and glycolipids?

Answer 27

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

Question 28

What is the fluid mosaic model?

Answer 28

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

Question 29

What is the importance of membrane fluidity?

Answer 29

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

Question 30

How do fatty acids effect membrane fluidity?

Answer 30

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

Question 31

How does temperature effect membrane fluidity?

Answer 31

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

Question 32

How is fluidity regulated? Why is it important to regulate fluidity of membranes?

Answer 32

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

Question 33

What are examples of membrane fluidity regulation?

Answer 33

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

Question 34

What is the structure of cholesterol?

Answer 34

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

Question 35

What is the function of cholesterol?

Answer 35

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

Question 36

What is bulk transport?

Answer 36

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

Question 37

What is endocytosis?

Answer 37

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

Question 38

What is exocytosis?

Answer 38

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

Question 39

What are two type of gated ion channel protein?

Answer 39

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

Question 40

What is an example of a gated ion channel protein?

Answer 40

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

Question 41

What is an example of an exchange transporter?

Answer 41

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

Question 42

What is co-transport and indirect active transport?

Answer 42

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

Question 43

What are the steps of indirect active transport?

Answer 43

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

Question 44

What is an example of indirect active transport?

Answer 44

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 1. Sodium-potassium pump active transports Na ions into blood -> reduce concentration of Na ion in cell 2. Na ions move down gradient into cell via cotransporter protein 3. Glucose drawn into cell with Na ions via cotransporter protein (glucose against gradient) 4. 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

Question 45

What allows cell adhesion?

Answer 45

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

Question 46

Explain the loss of cell adhesion in cancer cells

Answer 46

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