Chapter 4: Cell Membranes Flashcards
What is osmosis
Net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of random motion (diffusion).
What is water potential equation
Water p= Solute p + Pressure p
Symbol psi
What is water potential
Tendency of water to move out of a solution
Water potential depend on 2 factors :
Solute potential
Pressure potential
What is water potential of pure water at atmospheric pressure
0
What is solute potential
Contribution of concentration of solute to water potential. It is extent to which solute decreases water potential of solution.
The more the solute the lower the tendency of water to move out
Solute potential for pure water
0, and has negative value for a solution
Greater the solute concentration the more the negative value of solute potential
What is pressure potential
Contribution of pressure to water potential. Value always positive
Effect of isotonic solution on animal cell
- amount of solute in sol=solute in cell
- water potential in sol=water potential in cell
- net movement of water is 0
- no effect seen in cell
Effect of hypertonic solution on animal cell
- amount of solute in sol> in cell
- water potential of sol less than cell’s
- net movement from cell to outside
- cell shrinks
Effect of hypotonic solution on animal cell
- amount of solute in sol less than that of cell
- water potential of sol more than cell’s
- net movement from sol to cell
- cell swells and bursts
Effect of isotonic solution on a plant cell
- solute concentration of solution= solute concentration of cell cytoplasm
- water potential in and out is same
- net movement of water=0
- no effect on cell
Effect of hypotonic solution on plant cell
- solute concentration of solution < solute conc in cell cytoplasm
- water potential in sol > water potential in cell
- net movement of water from sol to cell
- cell becomes turgid, as cytoplasm increases in volume. Does not burst due to elastic cell wall. As cell wall swells exerts equal opposite wall pressure, little water needed to achieve this
Effect on hypertonic solution on plant cell
- solute concentration of solution > solute conc in cell cytoplasm
- water potential in sol < cells
- net movement of water from plant cell to sol
- cytoplasm shrinks and detaches from cell wall
Continued effect of hypertonic solution on plant cell
Cytoplasm shrinks and detaches from cell wall. Point cell membrane fully detached pressure potential (of cytoplasm of cell wall) is 0
W=Ws+Wp, Wp=0, Wn=Ws. WaterP=SoluteP
Water and solute from solution in cell both move through permeable cell wall. Hence external solution remains with shrinking protoplasm. As it shrinks pulls away from cell wall. Hypertonic sol occupies spaces
Called plasmolysis
What is protoplasm
Cell membrane+Interior
What is incipient plasmolysis
Point at which pressure potential has just reached 0 and plasmolysis is about to occur
What is diffusion
Net movement of molecules or ions from a region of higher concentration to region of lower concentration down a gradient as a result of random movement of particles
Factors affecting diffusion :
Steepness of conc gradient Temperature Surface area Distance Nature of molecule/ion
Steepness of conc gradient
Difference in conc os substance on 2 sides of the surface
Greater the difference in conc greater no of molecules passing in the 2 directions, hence faster rate of diffusion
Temperature affecting diffusion
Higher temp, molecules have more kinetic energy than at low temp. Move around faster and diffusion is faster
Surface area affecting diffusion (and to volume ratio)
Greater surface area faster diffusion
Larger the cell smaller the surface area in relation to volume
Example of structures that increase surface area
Microvilli are foldings in small intestine increase surface area
Distance affecting diffusion
Diffusion is inversely proportional to square of distance travelled
D α 1/d^2
Nature of molecule affecting diffusion
- large ones need more energy to diffuse than small ones
- non polar ones(glyercol,alcohol,steroids)diffuse more easily than polar through hydrophobic part of membrane
- uncharged and nonpolar mols eg respiratory gases(O2, CO2)diffuse through membrane
- water, being polar, rapidly diffuses across phospholipid bilayer because it’s small
What is active transport
Movement of substances from lower conc to higher conc using energy of ATP, using carrier proteins. It is specific
What is the Sodium-potassium ATP pump
Na+–K+ pump uses energy from ATP to pump 3Na from inside to outside and 2K from outside to inside
3 instances of active transport
- reabsorbtion of certain substances and ions in kidney into blood
- loading sucrose from mesophyll cells to phloem
- inroganic ions loaded into root hairs from soil
What is facilitated diffusion
Diffusion of substances from higher to lower conc facilitated by transport proteins ie channel and carrier proteins
What are channel proteins
Water filled pores that help in movement of ions and water
What is non gated channel protein and eg
Open all the time
Aquaporin/porin
What is gated channel protein and eg
Can move to open or close on the inside surface of membrane. Remain closed until receive chemical/electrical signal
Na+, K+ channel proteins found in nerve cell membrane helping in nerve impulse transmission
Channel protein shape changes or is fixed?
Fixed
Eg of a signalling molecule coming through gated channel protein
Acetyl choline (opens after binding)
How carrier proteins work
Flip between 2 shapes
Do not have fixed shapes
Binding site is alternatively open to one side of membrane
Does not use ATP
Rate of facilitated diffusion depends on which 2 factors :
No of channel/carrier proteins
In case of gated channel, whether it is open or closed
Which ions from active transport can also move by facilitated diffusion
Na+
K+
2 types of bulk transport
Endocytosis - engulfing material by cell surface membrane
Exocytosis - removal of material by cell membrane
2 types of endocytosis
Phagocytosis - bulk uptake of solid material by cell into phagocytic vacuole
Pinocytosis - bulk uptake of liquid material by cell into vacuole/vesicle. If small vesicles called micropinocytosis
Step by step process for phagocytosis
1) bacterium engulfed by cytoplasm fingers
2) bact engulfed by phagocytic vacuole
3) lysosomes with digestive hydrolytic enzymes fuse with vacuole
4) bact digested by enzyme
5) undigested remains removed by exocytosis
Example if exocytosis
Secretory pathway
RER–>Protein–>Golgi complex (collect modify sort)–>*enzymes–>secretory vesicles–>cell membrane
Structure of cell membrane
Phospholipid bilayer
With:
Hydrophilic heads outside with glycolipid attached, and hydrophobic tails inside
And within:
Smaller intrinsic/integral proteins attached with glycoprotein, and larger intrinsic protein(type) transmembrane protein spanning entire membrane (made of 1 or more α helical shape)
Outer:
Extrinsic protein
Glycolipid is _ and glycoprotein is _
GL - carb chain attached to phospholipid
GP - carb chain attached to protein
Function of proteins in cell membrane
- Acts as tranport proteins for molecules and ions
- Extrinsic proteins on the inside attached to cytoskeleton help maintain shape
- in mitochondrial (respiration) and chloroplast (photosynthetic) membrane
Function of glycolipid and glycoprotein in cell membrane
- acts as receptor - nature:gl/gp - molecules eg cell signalling, endocytosis, cell adhesion
- acts as cell mark/antigen(ABO blood grouping) help in cell wall recognition
- helping in membrane stability by forming H bonds with H2O molecules
Functions of cholesterol in cell membrane
- mechanical stability prevent cell from break and burst
- hydrophobic part prevents polar molecule passage
- acts as buffer
Where is it significant that cholesterol in membrane has hydrophobic part
ie in myelin sheet around nerve cell helps in fast impulse transmission (jump, lipid insulator)
How does cholesterol in membrane act as buffer
- at low temp it prevents close packing of phospholipid tails which prevents bilayer becoming rigid which restores fluidity to normal by decreasing it
- at high temp cholesterol interacts/melts tails, stabilising bilayer, increasing fluidity and restoring it to normal
Cholesterol amounts in organisms
Animal>plant>prokaryotes=0
Function of phospholipids in cell membrane
- act as signalling (extracellular) molecules that can move about in bilayers actigtaing other molecules such as enzymes
- make cell membrane selectively permeable. Only uncharged non polar molecules can pass.
- may be hydrolysed to release small water soluble glycerol related molecules that move through cytoplasm to bind to receptors (intracellular signalling)
5 properties of phospholipids
1) Hphilic head, Hphobic tail making membrane have nonpolar interior (interacts cytoplasm) and polar exterior (aqueous surrounding)
2) tail can be saturated or unsaturated fatty acid
3) forms basis of membrane fluidity
4) micelles
5) liposomes
Fluidity depends on 2 factors:
•Length of phospholipid tail
Longer fatty acid, more interaction, less fluid
•Proportion of unsaturated fatty acid chain
More proportion, more fluidity
What are micelles
Single layer of phospholipid.
Hphilic head interacts with aq surrounding, Hphobic makes interior non polar.
What are liposomes
Made up of phospholipid bilayer
Hphilic heads inside AND outside
(recall diagram)
Types of receptors
Extracellular - on surface of cell
Intracellular - inside cell: 1)cytoplasmic 2)nuclear
Types of signalling molecules
- hydrophobic:bind to intracellular receptors, can cross cell membrane
- hydrophilic:bind to extracellular recepto, can’t cross cell membrane
Basic signal transduction pathway
Conversion of signal into transmitted message
stimulus/signal—>receptor—transmission—>target(effector)—>response
Cell signalling mediated by membrane bound enzyme
Insulin binds outside cell to extracellular domain to it’s receptor and induces structural change that’s propagated across membrane to intracellular kinases domain inside cell that is responsible for activation of cell signalling cascade
Cell signalling mediated by membrane bound protein
Signalling molecule eg acetyl choline neurotransmitter (transmit nerve impulse between neurons)
Binds to binding site of channel protein
Channel protein for Na+ opens
Causes Na+ to move inside cell
Depolarization- change in membrane potential
Cell signalling involving a second messenger steps:
1) hydrophilic signal binds with specific shaped extracellular receptor which recognises it
2) signal brings change in shape of receptor that spans membrane so message is in effect passed to inside of cell (signal transduction). Changing receptor shape allows it to interact with next component so msg is transmitted.
3) activates G protein which releases second messenger. (Inactive G protein bound to GDP-> active G protein bound with GTP)
4) second messenger activates enzymes which further activates more enzymes eg adenylyl cyclase.
Amplification at each stage. Until an enzyme is produced which brings about required change in cell metabolism
5)Response towards original signal bound to extracellular receptor: secretion, trancsiption, movement, metabolic change/reaction
About the second messenger
-Second messenger-small, double signalling molecules spread through cell greatly amplifying signal.
Intracellular and released by cell in response to exposure to extracellular signalling molecule
eg CAMP, IP3, CGMP
-Signal amplification: many second msngers made in response to 1 receptor being stimulated, ie original signal amplified.
Intrinsic proteins stay in memvrane because
Hydrophobic region made of hydrophobic amino acids are next to hydrophobic fatty acid tails and repelled by watery environment either side of membrane
Fluid mosaic model:
Described as such because phospholipids and proteins (layers) can move about by diffusion.
Mosaic is pattern produced by scattered protein in bilayers when membrane surface is viewed from above
Phospholipids move sideways mainly in own layers
Some protein molecules move about within phospholipid bilayer, others remain fixed to their structures inside/outside cell
Why can polar molecules move through transport proteins
Their interiors are hydrophilic
What forms a glycolipid or glycoprotein
Branched carbohydrate attached to lipid or protein
Altered gene expression :
- hydrophobic signal ie steroid hormones crosses cell membrane, binds to cytoplasmic receptor
- this complex crosses nuclear membrane and binds with DNA, and alters it’s gene expression
- DNA (bound with cytoplasmic receptor with hydrophobic signal) undergoes transcription to form altered mRNA and hence altered protein from translation which causes change in process
How can phospholipids be modified to act as signalling molecules
Chemically modified
Move about in bilayers activating other molecules eg enzymes
Or
Be hydrolysed to release small water soluble glycerol related molecules, which diffuse through cytoplasm and bind to specific receptors
