E3: Intro to cell membranes Flashcards
what does amphipathic mean
comprising hydrophilic and hydrophobic regions
what are the three major types of membrane lipids
-glycerophospholipids
-sphingolipids
-sterols
Describe glycerophospholipids (see structure on one note)
- Phosphorylated head group, three-carbon glycerol backbone and 2
hydrocarbon fatty acid chains - Polar group includes, choline or serine (other variations)
- Amphipathic, various derivatives present in varying amounts in
membranes of all cell - Fatty acid tail can be saturated or unsaturated (one or more double
bonds – creates ‘kinks’ in the tail)
Describe sphingolipids (see structure on one note)
- Phosphorylated head group, sphingosine backbone and 2
hydrocarbon fatty acid chains, one of the fatty acid chains is
contributed by sphingosine - Polar group includes choline or can be a sugar
- Amphipathic, present in most cells, but most abundant in myelin
sheath surrounding nerve cells
Describe sterols (see structure on one note)
- Cholesterol in animals (ergosterol in fungi, hoponoids in bacteria)
- Present in varying amounts and influences membrane
fluidity/rigidity - Unsaturated, double bonds in fatty acid tails create space for
sterols to sit in the membrane
Cell membrane is selectively permeable and maintains what?
- Maintenance of pH and ionic composition
- Regulation of cell volume
- Concentration of metabolites and extrusion of waste substances and toxins
- Generation of ion gradients for excitable tissues (muscle and nerve)
how do channels/ pores/ transporters allow compounds to enter or leave the cell?
-along their concentration gradients- passive or facilitated diffusion
-active transport
-membrane proteins allow movement of molecules by opening and closing of subunits
function of receptors and adhesion molecules
bind extracellular molecules without necessarily transporting them across the membrane and allow cells to sense their environment and to
adhere to their tissue
what small molecules can pass through the cell membrane unaided?
Water, gases (e.g., O2, CO2, NO), urea
what cannot pass through the membrane unaided?
Ions, sugars, amino acids
transport by diffusion can be…
-chemical
-electrical
Diffusion is influenced by…
- Steepness of the concentration gradient
- Temperature
- Size or mass of the diffusing substance
- Surface area
- Diffusion distance
- Numbers of channels / transporters on the surface of the cell
Describe membrane transport via active transport
-against a concentration gradient
* Usually highly specific for one molecule e.g., ions or class of molecules e.g., sugars, amino acids
* Require expenditure of energy and there are several key sources e.g.,
* ATP hydrolysis
* Dissipation of proton /sodium gradient
Describe facilitated transport proteins
Gated ion channels are highly selective for specific ions and open and close in response to a particular signal e.g ligand-gated, voltage-gated, mechanosensitive/gated
what are gap junctions
channels that connect directly with neighbouring cells
where can ATP hydrolysis take place
- Sodium-potassium pump, calcium pump
- Mitochondrial ATP synthase
- ATP- Binding Cassette (ABC transporters)
what do active transport proteins use ATP for
establish a proton or sodium gradient
* e.g., Na+ – K+ ATPase or Na+ pump is a membrane transporter that maintains gradients of Na+ and
K+ across the membrane.
* For each ATP hydrolyzed 3 Na+
ions are removed from the cell and 2 K+ are brought in; by keeping
this Na+ gradient, it creates a secondary ‘energy’ source to drive secondary active transport
-Proton gradients are also used to drive activity of mitochondrial ATP synthase and generation
of ATP
what is a symport?
co-transporter that transfers two molecules in the same directions (e.g glucose and Na+ pump)
what is secondary active transport
a process that moves molecules across a cell membrane by using energy stored in electrochemical gradient
Describe the secondary active transport of glucose
-glucose is pumped from a low conc to a high conc using ATP stored in Na+ gradient
-glucose and Na+ is pumped together inside cell, conformational change
-Na+ is transported back outside the cell through sodium-potassium pump (primary active transport)
(process on one note)
Describe what happens in the sodium potassium pump (electrogenic pump)
-3 Na+ ions bind to pump
-ATP undergoes hydrolysis, the phosphate transfers to carrier. Phosphorylation changes conformation and Na+ released
-2 K+ bind to pump
-de-phosphorylation occurs and conformation changes again so K+ enters cell
what are antiporters?
co-transporters that transfer molecules in opposite directions
(look at one note for example)
Describe the ATP-Binding Cassette (ABC) transporter
-present in pro- and eukaryotic cells
-transport best studied in gram negative bacteria
-requires energy generated by hydrolysis of ATP- active transport
(look at one note for structure)
How do receptors work?
most don’t move across the membrane, but cause a change in the cytoplasmic domain of the receptor protein, either through
clustering of the receptors or through inducing a
conformational change of the receptor protein. usually initiate cascade of signalling molecules
Describe exocytosis
-secretion of proteins out
-Packaged into secretory vesicles by Golgi apparatus
-Vesicles fuse with cytoplasmic membrane and release their
contents extracellularly; may involve clathrin-coated pits
* Constitutive and active processes
Describe endocytosis
Uptake of macromolecules from extra-cellular space, across the cytoplasmic membrane
what is pinocytosis
a constitutive and continuous process involving uptake of
extra-cellular fluid via small membrane vesicles
what is phagocytosis
a specialized form of endocytosis in macrophages and neutrophils to ingest bacteria and cell debris in a
phagosome/phagolysosome. Enzymes and reactive oxygen species like superoxide and nitric oxide are pumped in / added to help kill/destroy the phagocytosed bacteria or debris
what is receptor-mediated endocytosis
involves the protein clathrin forming
clathrin-coated pits and vesicles; protein has a distinctive 3-legged
structure called a triskelion, which assemble to form a basket-like structure to stabilize the endocytic vesicle
What are the different mechanisms of cell signalling?
-junction between cells (direct)
-adhesion molecules (direct)
-using intercellular chemical messenger and specific receptor molecule (indirect)
look on one note for diagram
what drugs are used as receptor/ channel binding molecules
agonists and antagonists
what are agonists
drugs that activates receptor and produces biological effect e.g insulin
what are antagonists
drugs that block the activation of the target by ligand e.g., Beta blockers
block the adrenaline receptor (treating hypertension and arrhythmia)
Describe a signal transduction blocker
Kinase inhibitors are being developed as cancer treatments
