cell membrane and transport Flashcards
what are the functions of the cell surface membrane?
- definition of the cell’s boundaries
- organisation and localisation of function
- regulation of cell’s contents
- signal transduction
- cell-to-cell communication
what structural features allow for the definition of the cell’s boundaries?
the phospholipid bilayer is selectively permeable and allows for desirable substances to be kept within and undesirable substances kept out of the cell. as such, the cell surface membrane keeps the interior of the cell physically separated from the surrounding environment
what structural features allow for the organisation and localisation of function in the cell?
molecules or structures with specific functions are embedded in membranes or localised within organelles. for example, many electron carriers are embedded in the thylakoid membranes or chloroplasts and inner mitochondrial membrane. these structures serve to organise and compartmentalise functions within eukaryotic cells
what structural features allow for the regulation of cell’s contents?
proteins and other compounds of the membrane help to regulate the transport of substances into and out of the cell and its organelles. these mechanisms can include: accumulation of useful substances, removing various metabolic waste products, and confining certain chemicals within specific regions of the cell
what structural features allow for signal transduction within the cell?
specific protein receptors on the outer surface of the cell membrane play a key role in the detection of specific signals, and thus triggering specific responses within the cell, such as drug or hormone induced responses
what structural features allow for cell-to-cell communication?
depending on the cell’s environment, the cell membrane has membrane proteins that bind the extracellular matrix of cell surface constituents to mediate adhesion and communication between adjacent cells
what are some characteristics of the fluid mosaic model?
- the fluid layer is asymmetrical - the lipid bilayer may differ in composition and arrangement of proteins and lipids. the three major types of membrane lipids are phospholipids, cholesterol and glycolipids
- the phospholipid bilayer is fluid or mobile, and lateral movement of phospholipids is possible
- the unit membrane is a dynamic structure, where the embedded proteins can float, some moving freely while others are fixed in positions by microfilaments on the cytoplasmic face
- membranes are amphipthic - the hydrophilic phosphate head of the phospholipids face outwards into the aqueous environment both inside and outside of the cell. the hydrophobic hydrocarbon tails face inwards and create a hydrophobic core
what is the evidence for the ‘mosaic’ structure?
definitive evidence for the mosaic nature of the cell membrane came from the use of electron microscopy and freeze fracturing. a cell is frozen and fractured with a knife. the fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. the membrane proteins go wholly with one of the layers. the SEMs show membrane proteins like bumps in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer
what are the components of the cell membrane?
- phospholipids
- cholesterol
- proteins
- carbohydrates
what is the definition of the fluid mosaic model?
the membrane is viewed as a ‘mosiac’ of collage of proteins randomly distributed in or loosely attached to a fluid phospholipid bilayer which is free to move about laterally
how does the structure of phospholipids contribute to its function in the cell membrane?
- the two fatty acid chains which are hydrophobic form an effective hydrophobic barrier against polar and charged solutes. as the fatty acid chains are much longer than the glycerol head, their length dictates the thickness of the membrane
- having a hydrophobic tail and a hydrophilic head, a phospholipid molecule is amphiphatic, and results in the formation of phospholipid bilayers in an aqueous environment
how is membrane fluidity maintained in the cell membrane?
the membrane comprises of phospholipid molecules which are held together primarily by weak hydrophobic interactions between the hydrophobic fatty acid tails. as such, phospholipid molecules are free to move about laterally. it is rare for phospholipids to flip transversely across the membrane, because the hydrophilic part of the molecule must cross the hydrophobic core of the membrane to do so.
how does temperature affect membrane fluidity?
at low temperatures, the KE of the hydrocarbon chains decreases and the hydrocarbon chains are tightly packed, resulting in increased hydrophobic interactions between phospholipid molecules, restricting their motion. the bilayer exists in a semi solid state and is less fluid. at high temperatures, the KE of the hydrocarbon chains increases and this increases the motion of them. this allows for increased lateral movements and transverse flipping of individual molecules. this overcomes hydrophobic interactions between phospholipids, resulting in increased space between adjacent phospholipid molecules, increasing membrane fluidity
how does length of fatty acid chains affect membrane fluidity?
as the length of fatty acid chains increases, membrane fluidity decreases. in general, the longer the hydrocarbon chains, the higher the melting point due to increased hydrophobic interactions between hydrocarbon chains.
how does the degree of saturation of fatty acid chains affect membrane fluidity?
as the degree of saturation of fatty acid chains increases, membrane fluidity decreases. saturated lipids have long straight hydrocarbon chains, which allows for close packing and thus enhances membrane solidification. unsaturated lipids have kinks, which prevent the hydrocarbon chains from packing closely together, thus enhancing membrane fluidity.
how does cholesterol affect membrane stability?
cholesterol molecules are usually found in both layers of the cell membrane, intercalated into the lipid monolayers. its rigid steroid ring interferes with the motions of the hydrocarbon chains of phospholipids, enhancing the mechanical stability of the membrane
how does cholesterol affect membrane fluidity?
at high temperatures, cholesterol restrains the movements of phospholipids by interfering with the motions of the hydrocarbon chains, resulting in decreased membrane fluidity. at low temperatures, cholesterol prevents the hydrocarbon chains from packing closely together, thus decreasing the tendency of the membrane to freeze upon, resulting in increased membrane fluidity.
how does cholesterol affect membrane permeability in the cell membrane?
presence of cholesterol molecules decreases the permeability of a lipid bilayer to ions and small polar molecules. it does so by filling in spaces between hydrocarbon chains of phospholipids, thereby plugging transient gaps through which ions and small molecules might otherwise pass
what is the location of integral proteins?
integral proteins are deeply embedded in the hydrophobic interior of the lipid bilayer. lateral proteins constitute of unilateral and transmembrane proteins
what is the location of peripheral proteins?
peripheral proteins are loosely bound to membrane surface, often to exposed parts of integral proteins. peripheral proteins are found on the cytoplasmic side, held by network proteins, or on the exterior side, attached to fibres of extracellular matrix
what is the structure of integral proteins?
integral proteins contain both hydrophilic and hydrophobic regions, and is amphipathic. the proteins are held in place by extensive hydrophobic interactions with the hydrocarbon portions of phospholipids. they are released only through use of detergents or non-polar solvents
what is the structure of peripheral proteins?
peripheral proteins are rich in hydrophilic amino acids so as to allow for interaction with surrounding water and polar surface of the phospholipid bilayer. they are easily released by relatively mild treatment such as adjustment of ionic strength of pH of the suspending medium
how do membrane proteins provide anchorage for the cell?
anchoring proteins attach the cell membrane other substances, stabilise the position of the cell membrane, and help maintain cell shape. anchoring proteins attached to the extracellular matrix can coordinate extracellular and intracellular changes.
how do membrane proteins aid in cellular transport?
carrier proteins bind solutes and transport them across the membrane. this process involves a conformation change of the protein when solute binding occurs, and a return to its original form when the solute is released. ATP may or may not be required, depending on if the protein participates in FD or active transport
channel proteins contain a hydrophilic channel that forms a passageway to permit the movement of molecules through FD. leak channels permit movement of water and ions at all times, while gated channels open or close to regulate ion passage
how do membrane proteins aid in enzymatic activity?
enzymes embedded on the cell surface membrane catalyse reactions in the extracellular fluid or within the cytosol, depending on the location of the active site. several enzymes can be grouped together to carry out sequential steps in a metabolic pathway
how do membrane proteins engage in signal transduction?
chemical signalling works by the binding of a ligand to the specific 3D conformation of the receptor protein which triggers changes in the cell. cell membranes differ in the type and number of receptor proteins they contain, and this accounts for the differing sensitivities to hormones and neurotransmitters
how do membrane proteins engage in cell-to-cell recognition?
recognition proteins are usually glycoproteins and there is a wide array of possible shapes to the carbohydrate side chains, hence each cell type has its own specific markers. this enables cells to recognise other cells, and provides a means for foreign markers to be recognised and attacked by the immune system
how do membrane proteins engage in intercellular joining?
membrane proteins of adjacent cells may adhere together in various kinds of intercellular junctions, such as gap junctions and tight junctions
what functions are glycolipids and glycoproteins involved in?
- sorting of cells into tissues and organs in animal embryos
- binding extracellular signal molecules in antibody-antigen reactions
- intercellular adhesion to form tissues
- cell-to-cell recognition
carbohydrate groups aid in maintaining the orientation of the glycoproteins and glycolipids, as carbohydrates are highly hydrophilic, and thus the glycoproteins and glycolipids are kept in contact with the external aqueous environment and are unlikely to rotate towards the interior
what is the significance of transport across membranes?
- to maintain a suitable pH and ionic concentration within the cell for enzyme activity
- to obtain food supplies for energy and raw materials
- to excrete toxic substances
- to secrete useful substances
- to generate the ionic gradients essential for nervous and muscular activity
when does simple diffusion occur?
simple diffusion occurs for molecules that are able to cross the phospholipid bilayer directly, which have a smaller molecular weight or are readily soluble in the lipid bilayer. it can occur directly across the plasma membrane without any need for the aid of membrane proteins, and can occur in either direction
when does facilitated diffusion occur?
FD occurs for molecules that are larger and hydrophilic, and occurs down a concentration gradient without the use of ATP, with the help of membrane proteins.
what are the factors that affect the rate of diffusion?
- concentration gradient - the steeper the concentration gradient, the faster the diffusion process
- distance over which diffusion occurs - the shorter the distance over which diffusion occurs, the faster the diffusion process
- area across which diffusion occurs - the larger the surface area, the greater the number of molecules that can diffuse across per unit time, and the faster the diffusion process
- structure through which diffusion occurs - the presence of transient gaps in the cell membrane may enhance diffusion. the type and number of transport proteins present per unit surface area of membrane will also affect the diffusion rate
- size and type of molecule - the smaller the molecule, the faster they can diffuse across the cell membrane
- the higher the temperature, the faster the diffusion process
what is the formula for water potential?
water potential = solute potential + pressure potential
pure water has a water potential of 0, which is the highest possible value. the presence of solutes will make the water potential more negative
what are the factors that affect the water potential in plant cells?
- solute concentration, expressed as solute potential which is negative
- pressure exerted by cell wall on its contents, which is expressed as the pressure potential which is positive
what is the definition of solute potential?
solute potential is the measure of the ability of a solute to make the water potential more negative. dissolving solute molecules in pure water reduces the number of free water molecules as the solute molecules bind to water molecules, since water molecules clustered around solute molecules are not free to move across a membrane. this makes the water potential of the solution more negative.
what is the definition of pressure potential?
pressure potential is the measure of the pressure exerted by the cell wall on its contents. the pressure potential of a plant cell increases as water molecules move into the cell. more water molecules move into the cell and exert a greater pressure on the cell wall. the cell wall exerts the same pressure back on its cell contents. hence, as the plant cell becomes turgid, the water potential of the plant cell gradually becomes less negative as water tends to moves out of the cell
what is the definition of incipient plasmolysis?
at incipient plasmolysis, the protoplast has just ceased to exert any pressure against the cell wall, as the cell membrane starts to pull away from the cell wall. the cell is said to be in a flaccid state. water continues to leave until the contents in the protoplast of of the same water potential as the external environment. incipient plasmolysis is defined as the point where 50% of the plant cells are plasmolysed
what is the step-by-step process of active transport?
- the process behinds when solutes on the cytoplasmic side of the plasma membrane bind to a specific binding site on the transport protein.
- ATP then transfers a phosphate group to the transport protein
- this causes the protein to change its conformation in such a way that the solute is released on the other side of the membrane
- the phosphate group detaches and the transport protein returns to its original conformation.
active transport enables a cell to maintain internal concentrations of molecules that are much higher or lower than those in the extracellular environment
what are the three types of active transport carriers?
- uniport
- symport (2 substances are transported across the membrane together in the same direction)
- antiport (2 substances are transported across the membrane together in opposite directions)
describe the 3 types of endocytosis
- phagocytosis - large solid particles such as food particles and bacteria are taken into the cell. pseudopodia extend outwards from the cell to enclose the solid particle. the vesicle formed usually fuse with lysosomes which contain hydrolytic enzymes to digest these macromolecules
- pinocytosis - droplets of extracellular fluid are incorporated into small vesicles. this process is non-specific and the cell takes in all solutes dissolved in the droplet
- receptor-mediated endocytosis - coated pits form vesicles when specific molecules bind to receptor proteins on the cell surface. coated pits are reinforced on their cytoplasmic side by clathrin. this allows the cell to take up only specific substances from the extracellular fluid
what is the definition of exocytosis?
exocytosis is a process where the cell secretes macromolecules by fusion of vesicles. the vesicles usually bud off from the ER or GA, migrate to and fuse with the plasma membrane. this process is used by secretory cells to export products, or for the removal of waste materials
