S1-L6: The Plasma Membrane Flashcards
Define a phospholipid
-lipids with phosphate group (PO4 2-) covalently bonded to glycerol backbone instead of fatty acid
Describe the following groups as part of phospholipid:
(figure 1)
1-Phosphate
2-Fatty acid
1-polar/ soluble in water (hydrophilic)/ head
2-non-polar/ insoluble in water (hydrophilic)/ tail
How are cell membranes organised in water?
-phospholipids organise themselves to keep hydrophilic “heads wet” AND hydrophobic “tails dry”
Describe the “bilayer” (figure 2)
- composed of 2 layers of lipids organised as sheet
- v. thin membrane (7-8 nm) with flexible yet sturdy barrier
- ->fluid mosaic model best explains it’s structure
Describe the fluid mosaic model? (figure 3)
- 2D liquid which restricts lateral diffusion of membrane components
- regions with membrane that contain lipid rafts/ proteins AND glycoproteins
What is the basic composition of cell membranes?
- phospholipids- 75%
- cholesterol- 20%
- Polar glycolipids in external layer- 5%
Outline and describe “cholesterol” (figure 4)
- steroid lipid
- ->made up of ringed C structure with floppy C-chain AND hydrophilic hydroxyl (OH) group-useful structural lipid for membranes- lipid rafts
What is the effect of “fluidity” in terms of cell membranes?
-allows movement of membrane components needed for cell movement/ growth/ division/ secretion AND cellular junction formation
How do cholesterol molecules affect fluidity?
- neighbouring lipid molecules swap places 10 million times per/sec
- cholesterol immobilises first few hydrocarbon groups of phospholipid molecules
- ->makes lipid bilayer less deformable AND decreases it’s permeability to small water-soluble molecules
State what glycolipids are and their use
- lipid covalently attached to oligosaccharide
- forms part of cell membrane AND glycocalyx
- determines ABO blood group
What is a “glycocalyx”? (figure 7)
- many membrane proteins are glycoproteins containing associated oligosaccharide
- ->form glycocalyx
Outline the uses of the glycocalyx
- required for detection of “self” in immunity
- aids in cell-cell adhesion (stick to each other)
- makes red blood cell’s slippery AND protects GI from drying out
List the functions of the phospholipid bilayer
- regulates what enters AND exits cell
- altering pH AND charge
- involved in cell recognition
- major role in cell signalling-like from hormones
- has enzymatic functions
- aids in cell linking AND cross-talk
Different types of cell membrane proteins (figure 8)
- transporters
- anchors
- receptors
- enzymes
Explain how the cell membrane is selectively permeable:
1-Permeable
2-Impermeable
3-Slightly permeable
(figure 9)
1-lipid bilayer permeable to non-polar molecules like O2/ CO2 AND hormones
2-impermable to ions AND large molecules like Na+ & glucose
–>need trans-membrane channel AND carrier proteins
3-to small permeable uncharged polar molecules
–>like water
What is the link between gradients and membrane permeability and it’s consequent effect?
- selective permeability allows cell to build conc gradient
- ->exmple: O2 and Na+ more conc in extracellular fluid
- ->opposite true for K+ AND CO2
- pH can be regulated
- electrical gradients built–>inside of cell more (-)- this creates membrane potentials
Outline the different types of membrane transport (refer to figure 10)
- transport can be classed in to 3 types:
- ->passive: diffusion/ osmosis/ facilitated diffusion
- ->active: primary/ secondary
- ->vesicular: endocytosis/ phagocytosis/ pinocytosis/ transcytosis
What is passive transport?
-no cellular energy used as substances move down their own conc. gradient
State the different types of passive transport
- diffusion via lipid bilayer
- channel mediated facilitated diffusion
- carrier mediated facilitated diffusion
Explain how ionic gradients (K+) can create potential difference across a membrane
- only K+ able to leave cell–>diffuses out & down chemical gradient
- as K+ leaves cell inside becomes more (-)
- separation of charges occurs
- ->creates potential difference across membrane
What is “equilibrium potential”?
- potential gradient across membrane to maintain conc gradient
- ->i.e: electrical potential needed to stop on diffusion down chemical gradients
How may channel mediated facilitated diffusion occur?
- can be gated
- may be timed/signal regulated
- refer to figure 11
What is carrier mediated facilitated diffusion dependent on?
- subject to transport maximum AND saturation
- refer to figure 12
How may facilitated diffusion be regulated (in relation to diabetes)?
- selective permeability can be regulated to maintain homeostasis
- ->Example: hormone insulin- via it’s receptor up-regulates glucose transporters
- diabetic patients may lack ability to up-regulate GluT
Outline what osmosis is and its relation to facilitated transport (figure 13)
- diffusion of water through semi-permeable membrane
- diffusion through lipid bilayer occurs through specific transmembrane protein channels- aquaporins
Define the following types of solution:
1-Hypertonic
2-Hyopotonic
1-Higher conc of solution outside–> water leaves
2-lower conc of solution outside–>water comes in
How may red blood cells be destroyed? (refer to figure 14)
-RBC’s for transfusion destroyed by hypertonic AND hypotonic solutions
What are most intravenous (administering in to veins) solutions like?
- most intravenous solution isotonic
- ->0.9% saline OR 5% dextrose
What is active transport and it’s importance in the body?
- involves expenditure (use) of energy from hydrolysis of ATP
- ->used to transport essential ions against their conc gradient
- ->helps maintain tonicity/volume AND charge
Outline the two types of active transport
- primary–> uses energy stored from hydrolysis of ATP
- secondary–> uses energy stored by ionic conc. gradient
Describe primary active transport
- movement against conc. gradient (mainly ions)
- ATP changes shape of carrier proteins–>pumps substances across
- ->cells typically use 40% of their generated ATP this way
What is a common example of primary active transport AND why does it have to work non-stop?
- most common example is sodium-potassium pump (figure 15)
- must work non-stop due to passive leaking
How does the Na+-K+ pump work?
- 3Na+ bind/ ATP hydrolysed (via ATPase)
- 2K+ bind/ P released
- 2K+ enters
Briefly explain what secondary active transport (Co-transport) is
- form of a. transport across biological membrane in which transporter protein couples movement of ion (typically Na+ OR H+) down it’s electrochemical gradient
- ->to uphill movement of another molecule/ion against a conc/electrochemical gradient
- ->this gradient made by primary a. transport
Define the terms “symport” and “antiport”
- symport: where molecules travel in same direction –>used for Ca2+ regulation & H+ regulation
- antiport: where molecules go in opposite directions
Secondary active transport example (refer to figure 17)
- Na used to drive glucose transport across membrane
- primary active transport then recycles Na via Na-K pump
What is Vesicular transport and it’s use?
- used for endocytosis-bringing substances in to cell &
- ->exocytosis- expelling from cell
- used for inter-organelle transport
- both use ATP
Outline what endocytosis can further be classified in to
-receptor mediated endocytosis/ phagocytosis/ pinocytosis
What is receptor mediated endocytosis used for?
-for LDL/ some vitamins/ proteins and hormone uptake
Define the term “Cathrin”
-protein for enriching membrane domains
State the process of Receptor Mediated endocytosis (figure 18)
1-Binding: receptor-ligand complex forms
2-Lateral diffusion- clathrin coated pit binds to it
3-Invagination- process of being turned inside out
–>molecule moves from outside to inside
4-Vesicle formation- vesicle is coated (2)
5-Uncoating
6-Unocated vesicle fuses with early endosome AND ligand released in to endosome
7- vesicle transport to late endosome for digestion
–>receptors transported to cell surface membrane for recycling
–>transcytosis occurs- ejection of certain macromolecules other side of cell
Outline what phagocytosis is
-engulf large particles like worn out cells AND bacteria –>carried out by macrophages AND neutrophils
State the process of phagocytosis (figure 19)
1-microbe becoming phagocyte
-cell movement in direction to decreasing in conc
2-ingestion of microbe to phagocyte
3-phagosome formation
4-phagosome fusion with lysosome to form a phagolysome
5-digestion of ingested microbe by enzyme
6-residual body containing indigestible material formation
7-waste material discharge
Briefly explain what Bulk Phase endocytosis is (figure 20)
- small droplets of extracellular fluid taken in
- no receptors needed
- all solutes in extracellular fluid brought in (non-selective)
- occurs in most cells BUT especially absorptive cells in intestines AND kidneys
In appropriate detail outline what exocytosis is
- used to transport material out of cell
- is exhibited by all cells BUT v. important in:
- ->secretory cells- digestive enzymes AND hormones
- ->nerve cells- neurotransmitter
What is transcytosis?
- combination of endo AND exocytosis
- substances passed through a cell
- common in endothelial cells which line blood vessels
Cell-to-Cell: What happens at the synapse (including exocytosis)?
1-action potential depolarises axon terminal
2-depolarisation opens voltage-gated Ca2+ channels AND Ca2+ enters cell
3-Calcium entry triggers exocytosis of synaptic vesicle content
4-neurotransmitter diffuses across synaptic cleft AND binds with receptors on postsynaptic cell
5-neurotransmitter binding initiates response in postsynaptic cell
What are the different types of cell junctions?
-tight junctions/ adherens junctions/ desmosomes/ hemidesmosomes/ gap junctions
Outline the functions of the following junctions:
1-Tight junctions
2-Adhering junctions
3-Gap junctions
1-prevent substance movement
2-maintain cellular respiration
3-allows movement of substances
Explain what tight junctions are and how they work with reference to figure 22
- formed by fusion of integral proteins of adjacent cells
- ->prevent anything passing through extracellular space between them
- ->E.G: cells lining digestive tract
- bar movement of dissolved materials through space between epithelial cells
- no intercellular space where tight junction
- long tight junction protein rows form complex meshwork
Outline and describe Adherens junctions (figure 23)
- plaque which attaches to membrane proteins & microfilaments of cytoskeleton
- often form adhesion belts
- resist separation in contractile activities like peristalsis (involuntary constriction & intestine muscle relaxation)
- ->creates wave-like movements which push canal content forward
In appropriate detail outline what desmosomes (figure 24) AND hemidesmosomes (figure 25) are
- anchoring junctions held together by linker protein filaments- called cadherins
- ->OR integrins extending from button like thickenings called plaques
- distribute tension AND prevent tearing
- ->like skin/ heart muscle
Describe gap junctions
- adjacent cells connected by hollow to cylinders (connexons) made of trans membrane proteins
- found in electrically excitable tissues for synchronisation –> like heart AND smooth muscle
Cell to Cell junction- Intercalated disk example used:
1-What is an Intercalated disk?
1-Junction between two cardiomyocytes (cells composing heart muscle)
2-Using the example, outline what mechanical coupling is
- small gap (0.2 um) between membranes of adjacent cells which filled with connective tissue
- ->this firmly binds 2 adjacent cells together
- ->cells bound together more strongly at certain points-desmosomes
3-Explain what electrical coupling is
- longitudinal (length wise up-down) segments contain specialised regions where membranes of adjacent cells come v. close together
- ->in nexus (gap junction) regular arrays of proteins-connexins
- ->these form large channels which allow passage of ions + other small molecules between cells
What are the two important roles of intercalated discs?
- act firmly to bind adjacent cells together (mechanical coupling)
- also allow electrical coupling between adjacent cells
