Cell Membrane and Transport Across Membrane Flashcards
Functions of a biological membrane (5)
1) Defines cell’s boundaries
2) Organisation and localisation of function
3) Regulates cell contents
4) Signal Transduction
5) Mediates cell-to-cell communication and adhesion
How does the membrane’s selective permeability help in its function?
It allows desirable substances to be kept within and undesirable substances to be kept out of the cell
S->F: Definition of cell’s boundaries
The hydrophobic core of the membrane keeps cell interior physically separated from the surrounding environment
S->F: organisation and localisation of function
Molecules or structures with specific functions are embedded in membranes or localised within organelles
S->F: Regulation of cell’s contents
Proteins and other components like cholesterol embedded in the membrane help to regulate transport of substances into and out of the cell and organelles
S->F: Signal Transduction
Specific protein receptors on the outer surface of the cell membrane are crucial in detecting specific signals and triggering specific responses within the cell
S->F: Cell-to-cell communication and adhesion
Membrane proteins bind to the extracellular matrix or cell surface constituents to mediate adhesion and communication between adjacent cells
‘Fluid’ in Fluid Mosaic Model (FMM) (3)
1) * the membrane is composed of two layers, consisting of mainly phospholipids
2) Phospholipids, cholesterol and some proteins are free to move about laterally within the membrane, within (laterally) or between (flip transversely) layers
‘Mosaic’ in Fluid Mosaic Model (FMM)
proteins are randomly distributed and attached to the phospholipid bilayer, which is asymmetrical on the two lipid layers
In the FMM, the membrane is viewed as _____
a collage of proteins randomly distributed and attached to the fluid phospholipid bilayer which is free to move about laterally
Characteristics of FMM (4)
1) bilayer is asymmetrical
2) bilayer is fluid
3) unit membrane is dynamic
4) membranes are amphipathic
Asymmetry of phospholipid bilayer: Two lipid layers may differ in
(__________ __ _____ & ____)
composition/arrangement of proteins and lipids
Dynamism of membranes: Embedded proteins can float, some (________) while others are
(_________) by (_____________)
some moving freely; fixed in positions; microfilaments on the cytoplasmic face
Amphipathicity of membranes
Phospholipids have a hydrophilic phosphate head and hydrophobic hydrocarbon tail
Where do the head and tail of phospholipids face?
1) p. head face outwards into aqueous environment both inside and outside the cell
3) Hydrocarbon tails face inwards and create a hydrophobic core
Three types of lipids
1) Phospholipids
2) *Cholesterol
3) Glycolipids
Why can phospholipids move about laterally?
Hydrophobic interactions between hydrophobic fatty acid tails are weak
Why is it rare for phospholipids to flip transversely across the membrane?
Hydrophilic head must cross the hydrophobic core of the membrane to do so
Factors affecting membrane fluidity (and their relationship) (4)
1) Temperature (Temp. ↓ , MF ▼)
2) Length of fatty acid chains (length 🠕, MF ▼)
3) Degree of saturation of fatty acid chains (degree of saturation 🠕, MF ▼)
4) Amt. of cholesterol (effect on MF depends on temp.)
Effect of low temp. on MF (3)
1) KE of hydrocarbon chains decreases
2) Hydrocarbon chains are more tightly packed => stronger hydrophobic interactions between phospholipids molecules => restricted motion
3) bilayer exists in semi-solid state (less fluid)
Effect of increased fatty acid chain length on MF
Melting point increases due to increased hydrophobic interactions
Effect of saturated lipids on MF (3)
1) they have long. straight hydrocarbon chains 2) -> allows for close packing ( 🠕 hydrophobic interactions)
3) enhances membrane solidification
Effect of unsaturated lipids on MF
have kinks -> prevent hydrocarbon chains from packing closely tgt => enhances membrane fluidity
Effect of amt of cholesterol on MF (2)
it increases stability and regulates membrane fluidity in ANIMAL cells
Effect of high temp. on MF (4)
1) KE 🠕, motion of hydrocarbon chains 🠕
2) allows for 🠕 lateral movements of individual molecules => overcoming hydrophobic interactions between phospholipids
3) -> increased space between adjacent phospholipids
4) bilayer exists in fluid state (more fluid)
Cholesterol [definition]
steroids commonly found wedged between phospholipid molecules in cell membranes of ANIMAL cells
How does cholesterol decrease membrane fluidity at high temperatures?
restrains mvmt of phospholipids by interfering with motions of HC chains
How does cholesterol increase membrane fluidity at low temperatures?
prevents HC chains from packing closely together => ▼ tendency of membrane to freeze
How does cholesterol decrease membrane permeability?
fills in gaps between HC chains (plugs transient gaps thru which molecules might pass)
Molecules that are prevent from passing thru the membrane by cholesterol
Ions and polar molecules
2 broad categories of membrane proteins
integral and peripheral proteins
Location of integral proteins
deeply embedded in hydrophobic interior of lipid bilayer
Two sub-categories of integral proteins
Unilateral and Transmembrane
Diff. in location between 2 sub-categories of integral proteins
unilateral: reaches only a monolayer
transmembrane: spans entire bilayer
Location of peripheral proteins
loosely bound to membrane surface, often to exposed parts of integral proteins
Why can’t peripheral proteins on cytoplasmic side move far?
they are held by microfilaments of cytoskeleton
Peripheral proteins on exterior side are attached to ________
fibres of extracellular matrix
Structure of integral proteins
contain both hydrophilic and hydrophobic regions (i.e. amphipathic)
What contributes to the hydrophilicity and hydrophobicity of integral proteins?
charged and polar aa; non-polar aa
What interactions hold intrinsic proteins in place?
EXTENSIVE hydrophobic interactions with HC portions of phospholipids
Structure of peripheral proteins
rich in hydrophilic aa (mainly hydrophilic)
Purpose of hydrophilic aa in extrinsic proteins
allows for interaction with surrounding water and polar surface of lipid bilayer
Solubility of integral VS peripheral proteins in aqueous media
insoluble (int.) ; soluble (peripheral)
What interactions hold extrinsic proteins in place?
ionic interactions between hydrophilic portions of integral proteins or polar phosphate heads and the peripheral proteins
How integral proteins are released from membranes
Use of detergents/non-polar solvents
How peripheral proteins are released from membranes
adjusting ionic strength/pH of suspending medium
Functions of membrane proteins (6)
1) Anchorage
2) Transport
3) Enzymatic activity
4) Signal Transduction
5) Cell-to-cell recognition
6) Intercellular joining
Structure of carbohydrates
short, branched chains of fewer than 15 sugar units
Structure of glycolipids
carbohydrates covalently bonded to glycerol backbone
Bonds in glycolipids
glycosidic, ester
Structure of glycoproteins
carbohydrates covalently bonded to membrane proteins
Bonds in glycoproteins
glycosidic, ionic, hydrogen, peptide,
S->F: Maintenance of orientation of glycoproteins and glipids
As carbohydrates highly hydrophilic, carbohydrate chain kept in contact with external aqueous environment and unlikely to flip transversely to face cell interior
Functions of glipids and gproteins
1) Sorting of cells into tissues and organs in animal embryos
2) Binding extracellular signal molecules in antibody-antigen reactions
3) Intercellular adhesion to form tissues
4) Cell-to-cell recognition (usually glycoproteins)
Add. function of glycolipids
fluidity and gas diffusion (lipids)
Function of anchoring proteins : attach (______), stabilise (______) , help maintain (_______), coordinate (____________) changes
cell membrane to other substances; position of cell membrane; cell shape; extracellular and intracellular (proteins attached to extracellular matrix)
Anchoring proteins are bound to _____ on cytoplasmic site
Microfilaments of cytoskeleton
Anchoring proteins are attached to _____ on exterior side
Fibres of extracellular matrix
Disruptions in cell-cell adhesion can contribute to _____ _____ __ ______.
Metastasis stage of cancer
Two types of transport proteins
Carrier and channel
How do carrier proteins transport solutes?
They bind solutes and transport them across the membrane
Differences between carrier and channel proteins (2)
1) Binding site for solutes VS central hydrophilic pore
2) Undergoes conformational change VS does not undergo CC to transport substances across membrane
When does conformational change of the transport protein happen?
when solute binding occurs
When does the carrier protein return to its original form?
Solute is released
Two processes carrier protein can participate in
Facilitated diffusion (no ATP) and Active transport (ATP required)
Structure of channel proteins [type of protein, property]
integral proteins that contain a water-filled central pore/hydrophilic channel
S->F of channel proteins:
The hydrophilic channel forms a passage way to permit the mvmt of water, ions and small hydrophilic solutes across the cell membrane
Two types of channels
1) Leak
2) Gated
Leak channels permit the mvmt of _____/_____ at all times
Water;ions
Gated channels can _____/_____ to ____________
Open or close to regulate ion passage
Voltage-gated Na+/K+ channels
Examples of gated channels
Where do the enzymes embedded in the membrane catalyse reactions?(2)
In the extracellular fluid/cytosol, depending on location of active site
several enzymes can be (______ ________) to carry out (________ ____) in a (_______) pathway.
Grouped tgt; sequential steps; metabolic pathway
S->F of membrane proteins: Signal transduction
Proteins have very specific 3D conformation, thus are ideal as receptor molecules for chemical signalling between cells
Mechanism of chemical signalling
A ligand binds to the receptor protein, triggering changes in the cell
Why do different cell types have diff sensitivities to hormones and neurotransmitters?
Cell membranes have different type and number of receptor proteins
Recognition proteins are usually
Glycoproteins
Each cell type has its own (______ ______s), with (__________ ____ ____) of a unique (______).
Specific markers ; carbohydrate side chains; shape
S->F: recognition proteins as specific markers
This allows cells to recognise other cells, and hence allow foreign markers to be recognised and attacked by the immune system
Examples of intercellular junctions
Gap junctions and tight junctions
Main reason why transport across membrane is vital
To maintain a suitable pH and ionic conc. within the cell for enzyme activity
Intercellular joining: membrane proteins of adjacent cells may (_____) tgt in various kinds of (_________ _______)
Adhere, intercellular junctions
Differences between passive and active processes (2)
Occurs down VS against conc. gradient
No ATP VS ATP required
Examples of active transport processes (3)
Active transport, endocytosis, exocytosis
Diffusion [defintion]
Net mvmt of a substance from a region of higher conc. to a region of lower conc., down a conc. gradient, (until dynamic equilibrium is reached)
Dynamic equilibrium is reached when
conc. of the substance is equal on both sides of membrane
Simple diffusion occurs for what molecules?
molecules w small molecular weight/hydrophobic molecules (i.e. can cross the phospholipid bilayer directly)
When dynamic equilibrium is reached,
No net mvmt of substances occur
Facilitated diffusion occurs for what molecules?
Larger, hydrophilic substances
“Facilitated” in FD
Transport proteins are used to enhance/increase rate of transport of substance across membrane (channel/carrier-mediated)
Comparison between simple and facilitated diffusion (molecules, mediated, energy)
1) Occurs for molecules with simple molecular weight/hydrophobic VS larger, hydrophilic substances
2) Occurs directly across plasma membrane without aid VS mediated by transport (carrier/channel) proteins
3) Both passive processes, no ATP required
Comparison between facilitated diffusion and osmosis (molecules, mvmt thru what, driving force)
1) mvmt of water molecules VS larger molecules e.g. glucose, aa, ions
2) mvmt of molecules across selectively permeable membrane VS thru transport protein
3) occurs due to water potential gradient VS conc. Gradient
Factors affecting rate of diffusion (6)
1) Conc. Gradient
2) Distance over which diffusion occurs
3) SA across which diffusion occurs
4) Structure through which diffusion occurs (gaps, proteins)
5) Size of molecule
6) Temperature
Effect of thickness of membrane on no. of molecules that can diffuse per unit time
Thinner the cell membrane, the greater the no. of molecules that can diffuse per unit time
Effect of gaps in the membrane and transport proteins on diffusion rate
1) presence of transient gaps in membrane enhance diffusion
2) type and number of transport proteins present per unit surface area of membrane affects diffusion rate
Effect of surface area on the rate of diffusion
Larger the SA, the greater the no. of molecules that can diffuse across per unit time, hence the faster the diffusion process
Water potential ψ [definition]
A measure of the tendency for water to move from one region to another
Sign of Solute potential Ψ s
negative
Sign of Pressure potential
Positive
Water potential in plant cell is ________________
The sum of its solute potential and pressure potential
The water potential of animal cell is determined primarily by _______
Its solute potential
Solute potential [definition]
The measure of the ability of a solute to make the WP more negative
How does dissolving of solute molecules in pure water lower WP?
It reduces the number of free water molecules as they bind to water molecules.
The more solute molecules present, _______________, and hence ____________.
The more neg. the solute potential (greater ability to make WP more negative), the more neg. the WP
Pressure potential Ψ p [definition]
The measure of the pressure exerted by the cell wall on its contents
Why is Ψ p a positive value?
It tends to move water out of the cell
Why does WP become less negative if pressure if applied to water/solution?
It forces water to move from one place to another
Ψ p increases as__________________.
Plant cell absorbs water and increases in water
Implication of less negative water potential: water is (____ ______) to enter the plant cell
Less likely
Plant cell in solution of less neg. WP (WP of cell, mvmt of water, change to cell)
1) WP in cell is more negative than that of the solution
2) Water enters cell by osmosis from the solution
3) Cell swells and become turgid
Plant cell in solution of equal WP (WP of cell, mvmt of water, change to cell)
1) WP in cell is equal to that of solution
2) No net mvmt of water
3) no change in cell volume
Plant cell in solution of more neg. WP (WP of cell, mvmt of water, change to protoplast)
1) WP in cell is less negative than that of solution
2) Water leaves cell via osmosis.
3) protoplasm shrinks and eventually pulls away from cell wall
Order of water leaving the plant cell
First: water is lost from cytoplasm thru the cell membrane
Second: from the vacuole thru the tonoplast
Plasmolysis [definition]
Process where Protoplast shrinks and pulls away from the cell wall
Incipient plasmolysis [definition]
Point at which plasmolysis begins (when 50% of plant cells are plasmolysed)
At incipient plasmolysis: the protoplast had just ______ to _______ any ______ against the ____ ____
Ceased; exert; pressure; cell wall
State of cell at incipient plasmolysis
Flaccid
What happens to animal cell in hypotonic solution
Cell swells and lyses due to lack of cell wall
What happens to animal cell in isotonic solution?
No change in cell vol.
What happens to animal cell in hypertonic solution?
Cell becomes crenated/shriveled
Active transport [definition]
Mvmt of substances from region of lower conc. To a region of higher conc., against the conc. Gradient
Active transport occurs for
Ions and small hydrophilic molecules
Which type of transport proteins is used for active transport?
Carrier proteins
Why is conformational change necessary for active transport?
To ensure that solutes do not leak back across the membrane down the concentration gradient
Step 1 of AT: solutes on the (_________ ____) of the plasma membrane bind to (_______ _______ _____) on the transport protein.
Cytoplasmic side, specific binding sites
Step 2: ATP transfers a (________ _____) to the transport protein.
Phosphate group
Step 3: Protein changes its (___________), causing the solute to be (________) on the (_____ ____) of the membrane.
Conformation; released; other side
Step 4: the phosphate group (______) and the transport proteins returns to its (______ __________).
Detaches; original conformation
Primary active transport involves
The direct use of ATP by a carrier protein to transport solutes
Uniport VS symport VS antiport (transport of molecules across membrane)
Only 1 substance VS 2 substances tgt in the same direction VS 2 substances tgt in opposite directions
Example of antiport
Sodium-potassium pump
Endocytosis and exocytosis occur for what molecules?
Macromolecules such as proteins, polysaccharides, enzyme complexes
Exocytosis [definition]
A process where the cell secretes macromolecules (into extracellular fluid) by fusion of vesicles with the plasma membrane
Where do vesicles for exocytosis come from?
Usually bud off from ER or Golgi apparatus
Function of exocytosis: used by (_________ ____) for the (_____) of products or the (_________) of waste materials.
Secretory cells; export; removal
Endocytosis [definition]
A process where the cell takes in macromolecules by the folding in of the plasma membrane to form vesicles
How do vesicles in endocytosis form?
They form from a localised region of the plasma membrane that folds inwards and then pinches off into the cytoplasm
Function of endocytosis
Used by cells to incorporate extracellular substances
Types of endocytosis (refer to Google doc for more)
1) Phagocytosis
2) Pinocytosis
3) Receptor mediated endocytosis
How are voltage gated channels activated?
Changes in voltage (charge) across membranes
Explain the importance of active transport in a cell.
Enables a cell to maintain internal concentrations of molecules that are much higher or lower in concentration than that in the extracellular environment.
Facilitated Diffusion [definition]
Transport of solutes down a concentration gradient, with the aid of a transport protein embedded in the membrane
Effect of high temp. on membrane permeability (4)
*describe effect on both phospholipids and membrane proteins
1) KE of HC chains of phospholipids increases
2) allows for increased lateral movements of individual molecules => overcoming hydrophobic interactions between phospholipids
3) -> more transient pores between adjacent phospholipids
4) membrane proteins denature at high temperatures
