Cell Membrane and Transport Across Membrane Flashcards
Name the functions of membranes at the surface of the cell and within the cell
Definition of cell's boundaries Organisation and localisation of function Regulate cell's content Signal transduction Cell-to-cell communication -
Relate the structure of the cell membrane to how it defines cell’s boundaries
Definition of cell’s boundaries - The cell membrane keeps the interior of the cell physically separated from the surrounding environment. This phospholipid bilayer is selectively permeable and allows for desirable substances to be kept within and undesirable substances kept out of the cell
Relate the structure of the cell membrane to how it organises and localises functions within the cell
Organisation and localisation of function - Molecules or structures with specific functions are embedded in membranes or localised within organelles. (e.g. Many electron carriers are embedded in the thylakoid membranes of chloroplasts and inner mitochondrial membrane) These serve to organise and compartmentalise functions within eukaryotic cells.
Relate the structure of the cell membrane to how it regulates cell’s contents
Regulate cell’s content - Proteins and other components of the membrane help to regulate the transport of substances into and out of the cell and its organelles. This includes:
To take up and accumulate useful substances such as water, ions and small molecular weight metabolites such as glucose into various compartments.
To remove various metabolic waste products.
To confine certain chemicals within specific regions of the cell.
Relate the structure of the cell membrane to signal transduction
Signal transduction - Specific protein receptors on the outer surface of the cell membrane plays a key role in the detection of specific signals (after binding to the receptors), and thus triggering specific responses within the cell, such as drug or hormone induced responses.
Relate the structure of the cell membrane to cell-to-cell communication
Cell-to-cell communication - Depending on the cell’s environment, the cell membrane has membrane proteins that bind the extracellular matrix or cell surface constituents to mediate adhesion and communication between adjacent cells.
Describe the structure of phospholipids
Each phospholipid molecule consists of:
A glycerol backbone (a three-carbon molecule) with three hydroxyl (-OH) groups
Two fatty acid chains - contributing the hydrophobic hydrocarbon “tails”
A negatively charged phosphate group and additional small, charged molecules which may be linked to the phosphate group - Contributes to the hydrophobic head
Describe the properties of phospholipids
Phospholipid molecules are amphipathic as it has a hydrophobic (water-hating) “tail” and a hydrophilic (water-loving) “head”.
Explain how the structure and properties of phospholipids are related to their roles in living organisms
The amphipathic properties of phospholipid molecules result in the formation of phospholipid bilayers in an aqueous environment (i.e. hydrophilic heads are exposed to water, while hydrophobic tails are sheltered from water).
The long hydrocarbon chains of fatty acids 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, which is about 8 nm.] not impt
Membranes are not static sheets of molecules locked rigidly in place. The membrane comprises of phospholipid molecules which are held together primarily by hydrophobic interactions between the hydrophobic fatty acid tails.
State the fluid mosaic model
In the fluid mosaic model, the membrane is viewed as a mosaic or collage of proteins randomly distributed in or loosely attached to a fluid phospholipid bilayer which is free to move above laterally.
Name the constituent biomolecules in cell membranes
Phospholipids, & glycolipids
Cholesterol
Proteins
Carbohydrates
Name the factors affecting membrane fluidity
Temperature
Length of fatty acid chains
Degree of saturation of fatty acid chains
Amount of cholesterol
Explain how temperature generally affects membrane fluidity
As temperature decreases, membrane fluidity decreases, and vice versa
[Membranes remain fluid as temperature decreases, until finally the phospholipids settle into a closely-packed arrangement and the membrane solidifies. This change of state is known as the phase transition and the temperature at which it occurs is known as the phase transition temperature.] - for understanding
Explain the effect of low temperature on membrane fluidity
At low temperature,
- the kinetic energy of the hydrocarbon chains decreases and
- the hydrocarbon chains are tightly packed, resulting in
- increased hydrophobic interactions between phospholipid molecules
- and thus, their motion is restricted.
- the bilayer exists in a semisolid state, i.e. membrane is less fluid
Explain the effect of high temperatures on membrane fluidity
At high temperature,
- the kinetic energy and thus motion of the hydrocarbon chains increases,
- this increase allows for increased lateral movements of individual molecules, flexing of the chains and transverse flipping,
- thus overcoming hydrophobic interactions between phospholipids, resulting in increased space between adjacent phospholipid molecules.
- the bilayer exists in a fluid state, i.e. membrane is more fluid
Explain the general effect of the length of fatty acid chains on membrane fluidity
As length of fatty acid chains increases, membrane fluidity decreases.
In general, the longer the hydrocarbon chains, the higher the melting point (phase transition temperature) due to increased hydrophobic interactions between hydrocarbon chains.
Explain how the degree of saturation of fatty acid chains affect membrane fluidity
As the degree of saturation of fatty acid chains increases, membrane fluidity decrease and vice versa.
Saturated lipids have long, straight hydrocarbon chains, which allows for close packing and thus enhances membrane solidification
Unsaturated lipids have kinks, which prevents the hydrocarbon chains from packing closely together thus enhances membrane fluidity.
Explain the general effect of the amount of cholesterol on membrane fluidity
Cholesterol increases the stability and regulates the fluidity of membranes in
animal cells
Where are cholesterols founds in cell membranes?
Cholesterols are steroids commonly found wedged between phospholipid molecules
Name how can cholesterols affect cell membranes
Membrane stability
Membrane fluidity
Membrane permeablity
Explain cholesterol’s effect on 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, thus enhancing the mechanical stability of the membrane
Explain cholesterol’s effect on membrane fluidity
At high temperature,
- cholesterol restrains the movements of phospholipids by interfering with the motions of the hydrocarbon chains, resulting in decreased membrane fluidity.
At low temperature,
- cholesterol prevents the hydrocarbon chains from packing closely together, thus decreasing the tendency of the membrane to freeze upon,
resulting in increased membrane fluidity.
In short, cholesterol has dual effects on the fluidity of the membrane; resisting changes in membrane fluid caused by changes in temperature, acting as a ‘temperature buffer’ for the membrane.
Explain cholesterol’s effect on membrane permeability
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.
