Cell Membrane Flashcards
Why is cell membrane described as ‘fluid mosaic’ model ?
Fluid - ability of the phospholipids and proteins in the cell membrane to move laterally within each layer
Mosaic - patchwork distribution of proteins within the phospholipid bilayer
What kind of molecules make up the phospholipid membrane ?
Phospholipids
Proteins
Cholesterol
Carbs - only extend from exterior surface of cell surface membrane
Are the two sides of the phospholipid bilayer symmetrical ?
No, may differ in lipid and protein composition
What are the two main interactions present to help stabilise the cell surface membrane ?
Hydrophobic interactions
Hydrophilic interactions
Hydrophobic interactions are formed between … in cell membrane
Hydrophobic portion of proteins and hydrocarbon tails of phospholipids
Hydrophilic interaction are formed between … in cell membrane
Hydrophilic portions of proteins and phosphate group of phospholipids, the aqueous medium of the cytoplasm and outside the cell
Carbohydrate antennae and aqueous medium outside the cell
What are the five roles of cell membrane ?
- Act as barrier
- Regulate passage of substances
- Allows cell to communicate with external env
- Maintain structural relations with neighbouring cells
- Allows for compartmentalisation (within cell)
Why does cell membrane acting as a barrier between cell contents and external environment important ?
Allows cell to exist as an entity separated from env
Enables cell to maintain constant env within cell
How does cell membrane allow cell to communicate with external environment ?
Cell membranes contain receptors and enzymes that allow the cells to respond to chemical messengers
Glycoproteins and glycolipids can also act as recognition sites for cell to cell recognition for immune response
What are structural relationships between cells
Cell to cell adhesion for tissue formation
Why is compartmentalisation in cells important ?
Allows formation of organelles, setting up of different conditions to provide optimum conditions for enzymes to work
Membrane bound organelles allow for structural adaptations
How are phospholipids arranged in cell membrane ?
Arranged in bilayer due to amphipathic nature with polar phosphate heads interacting with aqueous medium and hydrophobic hydrocarbon tails facing the inside
Where are cholesterol found in cell membrane ?
Fits between the phospholipid molecules with hydrophobic hydroxyl groups interacting with hydrophilic heads of phospholipids while hydrophobic portion fits between the FA hydrocarbon tails
Which part of cholesterol is hydrophilic ?
The hydroxyl group
What are the two kinds of membrane proteins?
Peripheral/extrinsic proteins
Integral/intrinsic proteins
Where are peripheral proteins found ?
Loosely attached at the polar surfaces of the phospholipids or proteins
Found on both interior and exterior of cell
What are the two types of integral proteins ?
Proteins that either partially penetrate membrane or span the entire membrane
What are proteins that span the entire membrane called ?
Transmembrane proteins
Where are carbohydrates found in cell membrane ?
Attached to proteins to form glycoproteins or attached to lipids to form glycolipids
What are the functions of phospholipid in cell membrane ?
- Constitutes basic bilayer structure - orientation of phosphate head and hydrocarbon tails ensure stability of membrane
- Separates cell contents from surrounding, allows for compartmentalisation
- Allows only some specific molecules to pass through
- Provides fluidity to membrane
What molecules are allowed to pass through phospholipid bilayer and what are not ?
Fat-soluble substances and small molecules
Barrier against most water-soluble molecules and ions due to hydrophobic core
Why is fluidity of phospholipid bilayer important ?
Important for self repair / sealing and fusion of transport vesicles to organelles or cell surface membrane`
What are the three functions of cholesterol ?
Mechanical stability
Maintain membrane fluidity
Reduce uncontrolled leakage
Why does cholesterol give mechanical stability to membrane ?
Membranes without cholesterol tend to break easily
When does cholesterol maintain fluidity of membrane ?
At Low temperatures, cholesterol increase fluidity
At high temperatures, it decreases fluidity
How does cholesterol reduce uncontrolled leakage of the cell membrane ?
Divert certain polar molecules and ions to proper channels where their movement can be properly regulated instead of diffusing through the membrane
What is the function of transport proteins ?
Allow water-soluble ions, glucose, AA and proteins to be transported into or out of the cells as these molecules cannot diffuse through the hydrophobic core of the cell membrane
What are some types of transport proteins ?
Channel proteins
Carrier proteins
Protein pump
What is the function of an energy transducer (protein), with reference to specific examples ?
Electron carriers and ATP synthase allow the transport of electrons and protons respectively during respiration and photosynthesis for ATP synthesis
What is the function of cell surface receptor ?
Act as receptor, binding with chemicals allowing cells to respond to external stimuli
What are the 5 functions of membrane proteins ?
- Transport
- Energy transducer
- Cell surface receptor
- Enzymes
- Structural support
What are the 3 functions of carbohydrates in cell membrane ?
- Orientation of the membrane structures
- Cell communication
- Structural relationships
How do carbohydrates help with orientation of membrane structures ?
Carbs are highly hydrophilic and helo orientate glycoproteins and glycolipids to face the exterior
It also forms H bond with water and thereby stabilising the membrane structures
How do carbs allow cell communication ?
Carbs are involved in a variety of physiological phenomenon like recognition of the same cell type for immune response - cell to cell recognition
How do carbs allow for structural relationships between cells ?
Carbs act as glue for adhesion of cells to neighbouring cells for tissue formation - cell to cell adhesion
Why is fluidity of membrane important ?
Allows membrane to fuse which is important for self repair / sealing and fusion of transport vesicles to organelles or cell surface membrane
Allows for ease of binding of metabolites and hormones to surface receptors
Allows for activity of membrane-bound enzymes and transport enzymes
What will decrease in membrane fluidity result in ?
Decreased ability of membrane to fuse
Inactivation of membrane-bound enzymes and transport proteins resulting in reduction in membrane permeability
What factors affect fluidity of membrane ?
Temperature
Length of hydrocarbon tails of phospholipids
Degree of saturation of hydrocarbon chains of phospholipid
Presence of cholesterol
How does temperature affect fluidity of membrane ?
As temperature increases membrane becomes more fluid as kinetic energy of phospholipids increase. This results in breaking of the intermolecular interaction and bonds between its components
How does length of hydrocarbon tail of phospholipids affect fluidity of membrane ?
As length increases, fluidity decreases as there is more intermolecular interaction between te hydrocarbon tails and vice versa
How does degree of saturation of hydrocarbon chains of phospholipids affect fluidity of membrane ?
The more unsaturated (C=C leading to kinks in the straight chains) the hydrocarbon chains are, the more fluid the membrane is. This is because the unsaturated hydrocarbon chains have kinks which prevent the packing of the phospholipids resulting in weaker intermolecular interaction between the chains
How does the presence of cholesterol affect fluidity ?
Cholesterol acts as buffer against resisting changes caused by changes in temperature
At Low temperatures, cholesterol increases fluidity by disrupting the close packaging of phospholipids thus lowering the temperature at which the membrane solidifies
At high temperatures, cholesterol decreases fluidity as it hinders movement of phospholipids by interacting with the hydrocarbon tails
What are the five methods used to transport substances across the cell surface membrane ?
Simple diffusion
Facilitated diffusion
Active transport
Bulk transport
Osmosis
What is simple diffusion?
Net movement of molecules of any substance from a region of higher concentration to a region of lower concentration, down a concentration gradient until equilibrium is achieved
Does simple diffusion require energy ?
No it does not require expenditure of energy by cell thus will occur readily in living and non-living systems
What type of substances enter cell through simple diffusion ?
Small, non-polar or hydrophobic substances
What is facilitated diffusion ?
Net movement of particles down a concentration gradient through protein molecules
Is energy required for facilitated diffusion ?
No
Compare simple and facilitated diffusion
Both :
Net movement of particles down concentration gradient
Does not require energy from cell
Differs :
Simple does not require membrane proteins, facilitated requires
Why does facilitated diffusion require transport proteins ?
Charged particles and polar molecules do not readily pass through the plasma membrane as they are insoluble in lipids, cannot pass through the hydrophobic core of phospholipid bilayer
Transport proteins allow passage into cell, shielding them from the hydrophobic core of the membrane
What are the two types of transport proteins ?
Channel protein
Carrier protein
How does facilitated diffusion occur through channel proteins ?
Channel proteins forms a water-filled pore in the membrane. Unlike the hydrophobic core of the plasma membrane, the lining of the channel protein is hydrophilic so water-soluble substances can pass through
When do gated channel proteins open ?
When they receive appropriate signal
How does carrier protein work ?
Each type of carrier protein has one or more binding sites for specific solute
Solute is transported across the membrane through conformational change of the protein
What are the two forms that a carrier protein exists in ?
One where the binding site for the solute is exposed to one side of the cell and another where the same binding site is exposed to the other side of the cell
What are the characteristics of transport proteins ?
Are specific - can only transport one type of substance
Can be saturated - only a certain number of each type present
Can be inhibited - competitive inhibition of structurally similar molecules
When is the max rate of facilitated diffusion reached ?
When all transport proteins are being utilised for the transport of substances
What is osmosis ?
Net movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane
What are the two routes which water takes to enter the cell ?
Moving though lipid bilayer
Through water channels called aquaporins
Can water enter cell through simple diffusion?
Yes due to its small size but is limited due to polar nature of water
What are aquaporins ?
Integral membrane proteins that form pores in the membrane of biological cells
Why are aquaporins important ?
Important for the bulk flow of water across the cell membrane
What is active transport ?
Movement of ions or molecules across a membrane from a region of lower concentration to a regions of higher concentration, against concentration gradient, by means of specific transport proteins with the expenditure of energy by the cell
Why is active transport important ?
Allow cells to take up nutrients even when their concentration outside cells are lower than those inside cell
Enables cell to get rid of waste products when their concentration outside cells are higher than those inside cell
How does active transport differ from facilitated diffusion ?
Carrier proteins for active transport can bind with their molecules on one side of the membrane only which carrier proteins in facilitated diffusion can bind to specific molecules or ions on both sides
Conformation changes in carrier protein for active transport requires input of energy
What substances does bulk transport transport across membrane ?
Substances that are too large to cross the membrane via transport proteins
What is bulk transport ?
The transport of material into or out of the cell by enclosing the material within a fluid-filled membrane-bound sac called a vesicle
What are the two types of bulk transport ?
Exocytosis
Endocytosis
What is Exocytosis ?
Release of materials to the outside of the cell
What is endocytosis ?
Materials are taken into cells
What occurs during Exocytosis ?
Vesicles pinched off from the GA moves towards the cell surface membrane and fuses with it. The vesicle then opens to the exterior and its contents leave the cell
What ate the three types of endocytosis ?
Pinocytosis
Phagocytosis
Receptor-mediated endocytosis
What occurs during pinocytosis ?
Cell takes up droplets of extracellular fluid in tiny vesicles
Is pinocytosis selective ?
No
Any solutes dissolved in the droplets are taken into cell
How does phagocytosis occur ?
Cell engulfs large particles by sending out pseudopodia to surround the particle
The ends of the pseudopodia then join, resulting in formation of food vacuole, phagosome, within cell
Lysosome then fuses with phagosome and hydrolytic enzymes will digest the large particles
What occurs in receptor-mediated endocytosis ?
Extracellular macromolecules bind to cell surface receptor proteins embedded in the cell surface membrane
Proteins (clathrin) form a coated pit at teh cytoplasmic side which helps in the formation of a coated vesicle within the cytoplasm
Once vesicle is formed within the cytoplasm, clathrin will disassemble and be recycled
What substances is taken into cell through receptor-mediated endocytosis ?
Allows cell to take up large quantities of specific substances even though the substances may not be very concentrated in the extracellular environment
What is pseudopodia ?
Extensions of the cytoplasm
