Chapter 7 - Membrane Structure and Function Flashcards
Fluid Mosaic Model
the accepted model of cell membrane structure consisting mainly of lipids and proteins drifting in the fluid of phospholipid bilayers showing its dynamic nature
- Fluid because of the gel like consistency(no chemical bonds between molecules) - Mosaic due to distribution of proteins and cholesterol molecules embedded
Amphipathic
it has both hydrophobic and hydrophilic regions
- phospholipids and Membrane proteins are due to residing in the phospholipid bilayer with their hydrophilic regions exposed
Fluid Structure of the Membrane: characteristics
- Not static sheets of molecules held rigidly but held together through hydrophobic interactions which are looser than covalent bonds
- Movement of phospholipids within the membrane are rapid while proteins which are larger, move more slowly○ Phospholipids can move laterally, frequently as well as flip flop(swapping inside to out) extremely rarely.
- This fluidity in the membrane allows the movement of molecules through
How Permeability in membrane is maintained
- Permeability is also maintained by fatty acid chains have unsaturated portions, causing kinks in chains, preventing the phospholipids being tightly packed and creating pores(less common in animal cells)
- Cholesterol is used in animal cells to negate the compactness of saturated fatty acid chains
Effects of Temperature on membrane fluidity
- A membrane can remain fluid as temperature decreases until it reaches a point where the phospholipids pack tightly together and solidify
- The temperature this occurs at depends on the type of lipids it is made of; the more unsaturated hydrocarbons, the lower the temperature it can remain fluid due to the kinks of double bonds in tails
Cholesterol Impact on Membrane Fluidity
has different effects on fluidity at different temperatures by acting as a fluidity buffer, minimizing changes to the membrane
○ At relatively high temperatures, cholesterol makes the membrane less fluid by restraining phospholipid movement ○ At lower temperatures it hinders the close packing of phospholipids and therefore lowers the temperature required for it to solidify
Evolution of Membrane Differences due to Lipid Composition
- Lipid compositions have evolved over time to ensure membrane functionality in different environments
- E.g. fishes in extreme cold environments have membranes with a higher proportion of unsaturated hydrocarbon chains to aid in keeping their membrane fluid at lower temperatures
- In some plants, bacteria and archaea, they have developed the ability to change lipid composition in cell membranes to alter fluidity levels because they live in an environment with varying temperature extremes
Membrane Proteins and Common Function
- More than 50 types of proteins have been found in membranes and they determine the membranes function
- Transport
- Enzymatic activity
- Signal transduction: receptor molecules
- Cell-cell recognition: glycoproteins are identification tags
- Intercellular joining
- Attachment to cytoskeleton or ECM: cytoskeleton help maintain cell shape while ECM helps coordinate extra and intracellular changes
2 Types of Membranous Proteins
- Integral Proteins: a transmembrane protein with hydrophobic regions that extend in, and often completely across, the interior membrane with hydrophilic regions in contact with aqueous solutions on either 1 or both sides
- Peripheral Proteins: a protein loosely bound to the surface of a membrane or to part of an integral protein but not embedded in the phospholipid bilayer
Characteristics of Membrane Carbohydrates
- Usually short, branched chains of fewer than 15 sugar units
- Some are covalently bonded to lipids to form glycolipids however most are bound to proteins to form glycoproteins
- Vary among species, individuals and cell types which enables them to function as markers
Cell-to-cell recognition
a cells ability to distinguish one type of neighboring cell from another and is crucial to the functioning of an organism
- Cells recognize each other by binding to molecules, often containing carbohydrates on the extracellular surface of the plasma membrane
Transport Proteins and their two types
a transmembrane protein which helps specific ions and polar molecules which cant cross on their own
- channel proteins create a hydrophilic channel through the membrane - Carrier proteins binds to their molecule, change shape in order to shuttle them across before releasing them on the other side
Passive Transport
diffusion which requires no energy as molecules are moving down the concentration gradient from an area of high to low concentration
- Due to molecules having thermal energy(from their constant motion), they are able to move so that the concentration of those particles is equal across space.
Osmosis
the diffusion of free water molecules across a selectively permeable membrane from an area of higher free water concentration to an area of lower concentration
Tonicity
the ability of a solution surrounding the cell to cause that cell to gain or lose water
- Depends in part on the concentration of solutes that cant cross the membrane(nonpenetrating solutes) relative to that inside the cell
Measures of Tonicity:what happens if a cell is placed into a __solution
- Isotonic: when there is no net movement due to water diffusing across the membrane at the same rate in both directions
- Hypotonic: the cell gains water due to having a smaller concentration of free water resulting in the cell swelling or lysing
- Hypertonic: the cell loses water due to having a greater concentration of free water
Plant cells in hypotonic Solutions
- When plant cells swell due to increase uptake of water, the cell walls only expand so much before it exerts pressure known as turgor pressure
- The cell is then known as turgid when it opposes further water uptake, a healthy state for plant cells
Importance of plant cells being turgid
- Plants without wood depend on the turgid nature of cells to help develop mechanical support
- if a plant cell is not turgid it wilts and is unable to maintain structure
Reaction of Plant cell in Isotonic Solution
- If a plant cell and surrounding environment is isotonic and no water enters, cells become flaccid and wilts
Reaction of a Plant Cell in hypertonic solutions
Plasmolysis: when a plant cell is in a hypertonic environment, the cell shrivels up and the plasma membrane pulls away from the cell wall at multiple locations causing it to wilt and die
Facilitated Diffusion
the passage of molecules or ions down their electrochemical gradient across the biological membrane with the assistance of specific transmembrane proteins requiring no energy
Channel Proteins in Facilitated Diffusion
- provide corridors to allow specific molecules and ions to cross
- Ions are transported through Ion channels which open or close in response to a stimulus(gated channel) which is either chemical or electrical
Carrier Proteins
- Undergo subtle changes in shape that translocate the solute binding site across the membrane triggered by the binding and release of the transported molecule
- No energy is required as it involves moving down the molecules concentration gradient
Active Transport
the movement of a substance across a membrane against its concentration gradient, through the assistance of transport proteins and requires energy
- Only protein carriers move against solute concentration gradients - ATP hydrolysis supplies energy for most active transport by transferring its terminal phosphate group to a transport protein
Membrane Potential
the difference in electrical charge in volts across a cell plasma membrane due to unequal distribution of ions.
- The cytoplasmic side of the membrane is negative in charge relative to the extracellular side
○ as a result, the membrane potential favors the passive transport of cations into the cell and anions out
- Acts like a battery, an energy source which powers transport
Electrochemical Gradient
the diffusion gradient of an ion, affected by both the concentration differences of an ion across a membrane and the ions tendency to move relative to the membrane potential.
- Ions don’t diffuse down its concentration gradient but down its electrochemical gradient - In cases where the electrical forces due to membrane potential oppose the simple diffusion, active transport is necessary
Electrogenic Pump
a transport protein which generates voltage across a membrane while pumping ions
- E.g. Sodium Potassium pump which translocates them at different rates(3Na+ for 2K+) meaning there is an net transfer of a positive charge
Proton Pump
an active transport protein which uses ATP to transport hydrogen ions out of a cell against their concentration gradient, generating a membrane potential in the process
- Main electrogenic pump in plants, fungi and bacteria
Cotransport
a transport protein which couples the ‘downhill’ diffusion of one substance to the ‘uphill’ diffusion of another against its own concentration gradient.
- E.g. a plant cell uses the gradient of H+ generated by ATP powered proton pumps to drive the active transport of amino acids, sugars and other nutrients into the cell - Sucrose can enter the cell against its concentration gradient if it couples with a hydrogen ion which is entering the cell going down its concentration gradient,
- a gradient maintained by the proton pump pumping H+ back out of the cell
Exocytosis
the cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane
- Used to export products
Process of Exocytosis
- Transport vesicles that have budded from the golgi move along microtubules of the cytoskeleton to reach the plasma membrane
- When it comes into contact with the membrane, specific proteins in both membranes rearrange so that the two membranes fuse
- The contents are then spilled out of the cell and the vesicle becomes part of the plasma membrane
Endocytosis
the cellular uptake of biological molecules and particulate matter via the formation of vesicles from the plasma membrane
- Looks like the reverse of exocytosis
Types of Endocytosis
- Phagocytosis: a cell engulfs a particle by extending pseudopodia around it and packaging it within the membranous sac called a food vacuole
- Pinocytosis: a nonspecific method where the cell continually ‘gulps’ droplets of extracellular fluid into tiny vesicles formed by infoldings of the plasma membrane
- Receptor Mediated Endocytosis: a specialized type of pinocytosis which enables the cell to acquire bulk qualities of specific substances by having receptor sites embedded into the membrane.
○ When solutes bind to them, the receptor proteins then cluster in coated pits, forming a vesicle containing the bound molecules
Hypercholesterolemia
high levels of cholesterol in the blood due to LDLs not being able to enter cells as LDL receptor proteins are defective or missing
- Leads to cholesterol accumulating in the blood
