Lecture 1 - plasma membrane and transport across membranes Flashcards
Describe the structure of the plasma membrane
- 3 layers - inner, middle and outer
- contains a phospholipid bilayer ( hydrophobic tail and hydrophilic head)
- contains integral and peripheral membrane proteins
- Contains cholesterol between each phospholipid molecule
What are the functions of the plasma membrane?
- encloses contents of the cell
- separates cell from external environment
- cell communication
- import and export of molecules
How are proteins organized in the plasma membrane?
- Integral proteins - go across membrane that serve as membrane transporters
- peripheral proteins are loosely attached to membrane, act as enzymes shaping the cell
- lipid anchored proteins on surface of cell membrane, the lipd anchors the protein on membrane
What are the 4 mechanisms of transport across plasma membrane?
- passive diffusion
- facilitated diffusion
- active transport
- bulk transport
what does the rate at which a molecule passes the membrane depend on?
- size of molecule
- solubility of molecule
What is passive diffusion?
- does not require energy to move molecules across the membrane
- it relies on the concentration gradient across the membrane
- eg 02 c02 & small molecules pass freely
How do ions travel across the membrane?
ions travel across the membrane via ion channels, pumps, exchangers or co-transporters (eg sec active transport)
How is osmosis facilitated in the membrane ?
- by a family of membrane proteins called aquaporins that form a channel for water to flow through
What are the 4 types of ion channels present in the membrane?
- gated channels (open or closed)
- voltage gated channels (conformational change depends on difference of charge on both sides of membrane)
- Ligand gated channels ( depends on binding of ligand molecule)
- mechanically gated (physically stretching the membrane may cause it to open eg)
What is facilitated diffusion?
- concentration gradient dependent
- requires gated channels /transmembrane integral proteins
- eg glucose
What is active transport?
- Movement of molecules across a membrane from low- high conc against the concentration gradient
- requires energy - ATP
- occurs through transport protein pumps
- Eg) Na-K+ - ATPase pump
How does Na-K+ ATPase pump work?
- A transmembrane protein that pumps k+ into cells while also pumping Na+ out of cells
- Against both of their conc gradients
- ATP converted to ADP
- 3 Na3+ out and 2 K+ in
Describe bulk transport
- Transport of large molecules or small particles into cell, requires energy
- Phagocytosis
- endocytosis
- exocytosis/secretion out of cell
- Pinocytosis
What are gap junctions?
Sites between cells that are specialised for intercellular communication
Give examples of gap junctions
- Tight junctions
- Adherens junctions
- desmosomes
what are the different types of ‘channels’ on the membrane?
- ion channels that are selective for specific ions eg Na+, K+, Ca2+
- mixed cation channels - permeable to more than 1 ion - eg Na+ and K+
- membrane transporters eg ATPase pumps, ion exchangers (egNa+Ca2+ exchanger) & co-transporters - NB THESE ARE ACTIVE - REQUIRE ENERGY
What do the ion exchangers & cotransporters rely on?
they rely on ion gradients that are set up by ATPase pumps
what are the specialized functions of ion channels?
- control the release of neurotransmitters & hormones
- initiate muscle contraction
- mediate the generation, conduction & transmission of electrical signals in the nervous system
what are ion channels made up of?
- made up of subunits
- if they have different subunits - hetero-oligomers
- if they have the same subunits - homo-oligomers
- also auxiliary subunits eg in some voltage gated ion channels
what are the 3 basic functional properties of ion channels?
- selection - ie certain channels only allow one type of ion to pass eg Na+ or K+
- gating - what regulates ion channel opening/closing eg stretch or binding of neurotransmitter or change in membrane potential
- conduction - the channels allow ions to flow down their electro-chemical gradients
what are the 3 main families of ‘ion channels’?
- ligand gated ion channels
- gap junction ion channel
- voltage gated ion channels
What did the patch clamp method discover when studying Na+ channels?
- Na+ has a specific threshold for opening to allow Na+ to enter the cell
- only opens when the membrane depolarizes to approx. -50mv
- the channel opens and closes rapidly
What did the patch clamp method discover about K+ channels?
- K+ channels open as the membrane depolarises and they stay open while the membrane is depolarised
- they close when the when the membrane potential returns to a more negative value than at rest
- K+ channels have delayed closing
what is myotonia congenita?
- an inherited neuromuscular disorder - difficulty in mobility
- caused by a mutant voltage gated chloride channel CLCN
- adrenaline makes the condition worse
what types of stimuli control gating of ion channels?
- ligand gated - eg GABA or glutamate neurotransmitters present
- protein phosphorylation
- changes in membrane voltage
- stretch or pressure changes
what are examples of voltage gated channels?
- Na+ channels
- Ca2+ channels
- K+ channels
what are examples of ligand gated ion channels?
- neurotransmitter receptors
- Ca2+ activated K+ channel
what is an ‘inward rectifying’ channel?
- a channel that passes positive charge more easily into the cell (inward direction) than in the outward direction (ie out of the cell)
