3. Cell membranes Flashcards
(29 cards)
Phospholipids are amphiphilic
Hydrophilic head
Hydrophobic tail
Lipid bilayers are permeable to…
Water molecules
Small, uncharged molecules e.g. O2 and CO2
Lipid bilayers are not permeable to…
Cations Anions Small hydrophilic molecules e.g. glucose Macromolecules e.g. Proteins Large polar molecules
Diffusion
Net movement of particles from an area of higher concentration to an area of lower concentration
Down a concentration gradient
Osmosis
Diffusion of water molecules across a partially permeable membrane down a water potential gradient
Facilitated diffusion
Movement of hydrophilic molecules down their concentration gradient through protein pores that hide ionic charges from the hydrophobic core of the lipid bilayer.
Proteins provide a water-filled channel which can be ‘gated’
Active transport
Movement of molecules against their concentration gradient from a region of lower concentration to an area of higher concentration
Using energy in the form of ATP
Pinocytosis
engulfment by the membrane of extracellular solute and small molecules which end up in small intracellular membrane-bound vesicles
Phagocytosis
engulfment by the membrane of extracellular objects e.g. bacteria, cell debris, other cells, specifically bound to the cell membrane by receptors.
Exocytosis
Movement of proteins (e.g. hormones, blood clotting factors) and other molecules from intracellular vesicles into the extracellular space by fusion with the cell membrane.
Cholesterol is a steroid – abundant in plasma membrane
Binds to hydrophobic tails of phospholipids- causes them to pack closer together
Decreases permeability
Modulates membrane stiffness
Affects interactions with cytoskeleton
Symporters
sugars and amino acids can be dragged into the cell with Na+, as it moves down its concentration gradient
Antiporters
other molecules can move in the opposite direction to Na+
Protein regions in the core of the lipid bilayer have a predominantly alpha helical conformation
Hydrophobic AA side chains facing hydrophobic core of membrane
What is the role of protein regions in the lipid bilayer?
Increase membrane fluidity.
Involved in transport and transmission of signals
What prevents dissipation of ionic gradients across cell membranes?
Na+ - K+ pumps
What is the consequence of a high concentration of fixed anions inside cells?
Water is drawn into cells by the resulting osmotic gradient
What is the consequence of a high concentration of Na+ and Cl- in the extracellular space?
There is an opposing osmotic gradient
Na+ will tend to move down its concentration gradient into the cell.
The Na+-K+ ATPase maintains osmotic balance and stabilises cell volume by exporting Na+
Na+ gradient is thus maintained
Where are sodium-potassium pumps found?
In plasma membrane of all cells
Consist of 2 polypeptide chains, alpha and beta
What are the roles of alpha and beta chains in sodium-potassium pumps?
Alpha: forms hydrophilic pore for cations to move through
Beta: is the controller
How is a negative intracellular potential created?
Na+-K+ pump exchanges 3 Na+ ions from inside cell for 2 K+ ions outside
mediated by successive conformational transitions of the pump molecule
driven by phosphorylation of an aspartyl residue using ATP
followed by hydrolysis of aspartylphosphate
What are the consequences of transporting 3 Na+ out and 2K+ in?
Ionic gradients are created: less Na+ & more K+ inside the cell than outside.
A charge gradient is created: as more positive charges are pushed out than are coming in.
Inside of cell at a more negative potential than outside
Resting membrane potential
-70mV
Describe the event of an action potential
Na+ ions enter cell: cause depolarisation from –70 mV to +50 mV.
Na+ channels become inactivated locally because of the voltage reversal, preventing further Na+ entry.
Voltage-gated K+ channels open, resulting in K+ efflux and help to restore resting membrane potential.
The process propagates down the nerve/muscle
