WEEK 4 (Plasma membrane) Flashcards
What is the composition of plasma membrane?
- consists of a two-ply sheet of lipid molecules about 5nm thick
- act as selective barriers
- separates a cell from its surroundings enabling the molecular composition of a cell to differ from its environment
Which organelles are enclosed by two membranes?
The nucleus and mitochondria
Which cell activities is the plasma membrane involved in?
- Cell communication
- Import and export of molecules
- Cell growth
Which components of the plasma membrane enable it to do its function?
- RECEPTOR PROTEINS in plasma membrane act as sensors that enable the cell to receive information about changes in its environment and respond to them
- FLEXIBILITY of the membrane and it’s capacity for expansion allow for cell growth and cell movement
- HIGHLY SELECTIVE CHANNELS AND PUMPS-PROTEIN MOLECULES allow specific substances to be imported and others to be exported
What happens in a typical communication between cells?
The signaling cell produces a particular type of extracellular signal molecule that is detected by the target cell
What is the significance of the difference between hydrophilic signal molecules and hydrophobic signal molecules?
Both molecules are extracellular signal molecules however hydrophilic signal molecules bind to the cell surface receptors on the cell surface membrane and hydrophobic signal molecules bind to intracellular receptors
In all eukaryotic cells, a steady stream of vesicles buds from the trans Golgi network fuses with the plasma membrane in the process of _______________
Exocytosis
What is the difference between the Constitutive exocytosis pathways and the Regulated exocytosis pathway?
Constitutive exocytosis pathway supplies the plasma membrane with newly made lipids and proteins
Regulated exocytosis pathway operates only in cells that are specialised for secretion
Insulin production and release is an example of what?
Regulated secretion
Describe the stages of insulin production as an example of regulated secretion
- An increase in blood glucose signals insulin-producing endocrine cells in the pancreas to secrete the hormone
- Secretory vesicles store insulin in a pancreatic B cell in which the insulin in each secretory vesicle is stored in a highly concentrated aggregated form
- After secretion, the insulin aggregates dissolve rapidly in the blood
What are the two main types of endocytosis that occur in eukaryotic cells?
PINOCYTOSIS & PHAGOCYTOSIS
PINOCYTOSIS involves the ingestion of fluid and molecules via small pinocytic vesicles (250nm in diameter)
PHAGOCYTOSIS ingests large particles via phagocytic cells
What is the function behind phagocytosis?
- against infection by ingesting invading microorganisms
- in scavenging dead and damaged cells and cell debris
__________________ in some cases is considered to be a constitutive process while in others it’s receptor-mediated and highly regulated
Pinocytosis
What is pinocytosis mainly carried out by?
Clathrin-coated pits and vesicles
Describe the structure of a typical membrane lipid molecules
A hydrophilic head and two hydrophobic tails
Describe the structure of Phosphatidylcholine
Two hydrocarbon (usually fatty acid) tails are attached to the glycerol molecule via ester linkage; one tail is saturated and is ‘straight’ and the other tail is unsaturated and is ‘bent’. Phosphate molecule is attached to the glycerol molecule via phosphodiester bond and a choline molecule is attached to the phosphate molecule.
All membrane lipids are _______________ and form a bilayer
Amphipathic
What is the reason behind the phospholipid bilayers spontaneously closing in on themselves to form sealed compartments?
Amphipathic molecules such as phospholipids assemble into self-sealing containers that define closed compartments. The closed structure is stable because it avoids exposure of the hydrophobic hydrocarbon tails to water which would be energetically unfavourable.
Pure phospholipids can form closed, spherical liposomes when added to water; what is their size variation?
25nm to 1mm in diameter
What are the two major properties of hydrocarbon tails that affect how tightly they pack in the bilayer?
Length & the number of double bonds they contain
How does length affect how tightly hydrocarbon tails can pack together in the bilayer?
A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another and therefore increases the fluidity of the bilayer
How does the number of double bonds that hydrocarbon tails contain affect how tightly they pack in the bilayer?
Lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid
What are the types of movement of membrane phospholipid molecules?
- “flip-flop” event where phospholipids molecules tumble from one half of the bilayer, or monolayer, to the other rarely occurs
- lipid molecules continuously exchange places with their neighbours in the same monolayer due to random thermal motions
What do the lipid molecule movements allow the bilayer to behave like?
Due to the rapid lateral diffusion of lipid molecules within the plane of each monolayer, the bilayer behaves as a two-dimensional fluid in which the individual lipid molecules are able to move in their own monolayer
What is membrane fluidity modulated by in animal cells?
Sterol cholesterol
How does cholesterol modulate membrane fluidity?
Cholesterol molecules are short and rigid and fill the spaces between neighbouring phospholipid molecules left by kinks in their unsaturated hydrocarbon tails. Cholesterol stiffens the bilayer, making it less flexible and less permeable.
Describe the stages of membrane synthesis in the endoplasmic reticulum in eukaryotic cells
- New phospholipids are manufactured by enzymes bound to the cytosolic surface of the endoplasmic reticulum
- Using free fatty acids as substrates, the enzymes deposit newly made phospholipids exclusively in the cytosolic half of the bilayer
- Newly synthesised phospholipids are added to the cytosolic side of the ER membrane
- Scramblase enzyme redistributes and catalyses transfer of random phospholipids from one monolayer to another
- Symmetric growth of both halves of bilayer
The Golgi membrane contains another family of phospholipid-handling enzymes, what is it called and what is its function?
Flippases
These enzymes remove specific phospholipids from the side of the bilayer facing the exterior space and flip them into the monolayer that faces the cytosol
Which phospholipids are concentrated in the noncytosolic monolayer?
Phosphatidylcholine and Sphingomyelin
Which phospholipids are concentrated in the cytosolic monolayer?
Phosphatidylserine and Phosphatidylethanolamine
____________ is distributed almost equally in both monolayers
Cholesterol
Where are phosphatidylinositols found?
In the cytosolic monolayer where they participate in cell signalling
In membranes, proteins contain about 50% of the mass of the plasma membranes but why is there 50 times more lipid molecules than protein molecules?
Since lipid molecules are much smaller than proteins
What are the functions of membrane proteins?
- Transport nutrients, metabolites and ions across the lipid bilayer
- Anchor the membrane to macromolecules on either side
- Receptors that detect chemical signals in the cell’s environment and relay them into the cell interior
- Work as enzymes to catalyse specific reactions at the membrane
What are the different plasma membrane proteins found in the cell membrane?
Transporters
Ion channels
Anchors
Receptors
Enzymes
What is an example of a transporter protein and what is its specific function?
Sodium-potassium pump
actively pumps Na+ out of cells and K+ in
What is an example of an ion channel protein and what is its specific function?
K+ leak channel
allows K+ ions to leave cells having a major influence on cell excitability
What is an example of an anchor protein and what is its specific function?
Intergrins
link intracellular actin filaments to extracellular matrix proteins
What is an example of a receptor protein and what is its specific function?
Platelet-derived growth factor (PDGF) receptor
binds extracellular PDGF which generates intracellular signals that cause the cell to grow and divide
What is an example of an enzyme protein and what is its specific function?
Adenylyl cyclase
Catalyses the production of the small intracellular signalling molecule cyclic AMP in response to extracellular signals
What are the two main classes of membrane proteins?
Integral membrane proteins & Peripheral membrane proteins
What is the difference between integral membrane proteins and peripheral membrane proteins?
Integral membrane proteins are directly attached to the lipid bilayer and can only be removed by disrupting the bilayer with detergents
Peripheral membrane proteins can be released from the membrane by more gentle extraction procedures that interfere with the protein itself, these interactions leave the bilayer intact
Which membrane proteins extend throughout the bilayer?
Transmembrane proteins that are amphipathic
What is the other configuration of proteins that aren’t transmembrane proteins?
- membrane proteins are located almost entirely in the cytosol and are associated with the cytosolic half of the lipid bilayer by an amphipathic α helix exposed on the surface of the protein
- proteins lie entirely outside the bilayer, on one side or the other, attached to the membrane only by one or more covalently attached lipid
- proteins are bound indirectly to one or the other face of the membrane, held in place only by their interactions with other membrane proteins
How is the plasma membrane of animal cells stabilised?
by a meshwork of fibrous proteins called the “cell cortex” that is attached to the underside of the membrane
A _______________ forms the cell cortex in human red blood
spectrin meshwork
Describe the formation of spectrin meshwork
Spectrin dimers are linked end-to-end to form longer tetramers. The spectrin tetramers, together with a smaller number of actin molecules, are linked together into a mesh. This network is attached to the plasma membrane by the binding of at least two types of attachment proteins
What forms the carbohydrate layer or “glycocalyx” of the membrane?
All of the carbohydrates of the glycoproteins, proteoglycans and glycolipids that are located on the outside of the plasma membrane
What is the difference between Glycoproteins and Proteoglycans?
Glycoproteins contain short chains of sugars (Oligosaccharides) whereas Proteoglycans contain one or more long polysaccharide chains
What is the function of the carbohydrate layer or “Glycocalyx”?
- helps protect the cell surface from mechanical damage
- carbohydrates absorbs H2O which gives the cell a slimy surface
- slimy surface helps white blood cells squeeze through narrow spaces and prevents blood cells from sticking to one another or to the walls of blood vessels
Specialised transmembrane proteins (called lectins) are made by the endothelial cells lining the blood vessel in response to what?
In response to chemical signals emanating from a site of infection
What is distinguishable about lectins?
These proteins recognize particular sugar groups carried by glycolipids and glycoproteins on the surface of neutrophils (a type of white blood cell) circulating in the blood. The neutrophils consequently stick to the endothelial cells that line the blood vessel wall. This association is not very strong, but it leads to another, much stronger protein–protein interaction (not shown) that helps the neutrophil slip between the endothelial cells, so it can migrate out of the bloodstream and into the tissue at the site of infection.
What is the name of the group of proteins in the peroxisomal membrane?
Peroxides
What enzyme found in the peroxisome degrades hydrogen peroxide?
Catalase
Who was the fluid mosaic model of the cell membrane proposed by?
Singer and Nicolson
What are ribosomes composed of?
Ribonucleic acid and proteins
FACT: all prokaryotes have 70S (S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol
Where is Ribosomal RNA actively synthesised?
Nucleolus
If a cell has twice as much DNA as in a normal functional cell, it means that the cell ______________
is preparing to divide
The mobility of integral proteins can be measured by the physical state of the ___________________
membrane phospholipids
Two cells can be operated in such a way that leads to a common continuous plasma membrane of both (TRUE or FALSE)
True
The erythrocyte has a ________________ shape
Bi-concave
Which of the following diseases is caused by mutations in a membrane protein?
a) Alzheimer’s disease
b) Parkinson’s disease
c) Anemia
d) Haemolytic anemia
Haemolytic anemia
Haemolytic anemia is caused by the abnormal shapes of erythrocytes which have been traced to the mutations in ankyrin or spectrin which are plasma membrane proteins found in erythrocytes
What attribute of the membrane is most affected at the transition temperature?
Fluidity
The __________ the degree of unsaturation of the fatty acids of the bilayer, the ___________ the temperature before the bilayer gels
greater
lower
What molecules affect the mobility of fatty acyl chains in the plasma membrane?
Cholesterol
What occurs in the membrane when the temperature is lowered?
Desaturation
When the temperature of the surrounding is lowered, the cells respond metabolically. Membrane remodelling takes place to make the cell more cold resistant. This takes place by desaturation of fatty acyl chains to form double bonds.
Cell fusion can be performed on two cells from different species (TRUE or FALSE)
True
Cell fusion is a technique to fuse two cells from different species to form one cytoplasm and one single continuous plasma membrane
In the experiments to demonstrate the mobility of membrane proteins, which cells were fused with human cells?
Tumour cells
The sperm cell is covered by a ___________________
Continuous membrane
Hemolysis is associated with which type of cells?
Red blood cells
Myelin sheath is composed mainly of ___________________
Lipids
Which ion has the highest permeability in a resting nerve cell?
K+
The lipids present in cell membrane are _______________
Ampipathic
In animals, the amount of cholesterol present in cell membrane is ______ %
50%
What is an example of an ectoplasmic phospholipid?
Phosphatidylcholine
What phospholipid promotes curvature of cell membrane?
Phosphatidylethanolamine
Carbohydrates present in cell membrane are generally ____________________
Oligosaccharides
Which of the following is covalently bonded to a lipid molecule in cell membrane?
Lipid-anchored protein
Integral proteins are also known as _________________
Transmembrane proteins
What is the major difference between a fat (triacylglycerol) and a phospholipid (diacylglycerol)?
Fatty acid chain
A bacterium is suddenly expelled from a warm human intestine into the cold world outside. What adjustments would the bacterium make to maintain the same level of membrane fluidity?
Produce lipids with shorter hydrocarbon tails that have more double bonds
_____________ membrane proteins are linked to the membrane through non covalent interactions with other membrane bound proteins
Peripheral
The _______________ makes up about half of the total cell volume of a typical eukaryotic cell
Cytosol
Ingested materials within the cell will pass through a series of compartments called _________________ on their way to ______________ which contain digestive enzymes and will ultimately degrade the particles and macromolecules taken into the cell and will also degrade worn-out organelles
Endosomes
Lysosomes
The _____________ has a cis and trans face and receives proteins and lipids from the ____________________, a system of interconnected sacs and tubes of membranes that typically extends throughout the cell
Endoplasmic reticulum
Golgi apparatus
Proteins enter the nucleus in their ___________ form. Proteins that remain in the cytosol do not contain a _________ __________.
Folded
Sorting signal
Proteins are transported into the Golgi apparatus via ___________ ____________. Proteins transported into the endoplasmic reticulum by ____________ are in their ______________ form
Protein translocators
Transport vesicles
Unfolded
What type of lipid membrane movement does not occur spontaneously in biological membranes?
Switching between lipid layers
Most proteins destined to enter the endoplasmic reticulum _______________________________________
begin to cross the membrane whilst still being synthesised
A drug that blocks the ability of Ran to exchange GDP for GTP. What is the most likely effect of this drug on nuclear transport?
Nuclear transport receptors would be unable to release their cargo in the nucleus
What is the order through which a protein destined for the plasma membrane travels?
ER -> Golgi -> Plasma membrane
