E3: Intro to cell membranes

Question 1

what does amphipathic mean

Answer 1

comprising hydrophilic and hydrophobic regions

Question 2

what are the three major types of membrane lipids

Answer 2

-glycerophospholipids
-sphingolipids
-sterols

Question 3

Describe glycerophospholipids (see structure on one note)

Answer 3

• 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)

Question 4

Describe sphingolipids (see structure on one note)

Answer 4

• 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

Question 5

Describe sterols (see structure on one note)

Answer 5

• 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

Question 6

Cell membrane is selectively permeable and maintains what?

Answer 6

• 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)

Question 7

how do channels/ pores/ transporters allow compounds to enter or leave the cell?

Answer 7

-along their concentration gradients- passive or facilitated diffusion
-active transport
-membrane proteins allow movement of molecules by opening and closing of subunits

Question 8

function of receptors and adhesion molecules

Answer 8

bind extracellular molecules without necessarily transporting them across the membrane and allow cells to sense their environment and to
adhere to their tissue

Question 9

what small molecules can pass through the cell membrane unaided?

Answer 9

Water, gases (e.g., O2, CO2, NO), urea

Question 10

what cannot pass through the membrane unaided?

Answer 10

Ions, sugars, amino acids

Question 11

transport by diffusion can be…

Answer 11

-chemical
-electrical

Question 12

Diffusion is influenced by…

Answer 12

• 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

Question 13

Describe membrane transport via active transport

Answer 13

-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

Question 14

Describe facilitated transport proteins

Answer 14

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

Question 15

what are gap junctions

Answer 15

channels that connect directly with neighbouring cells

Question 16

where can ATP hydrolysis take place

Answer 16

• Sodium-potassium pump, calcium pump
• Mitochondrial ATP synthase
• ATP- Binding Cassette (ABC transporters)

Question 17

what do active transport proteins use ATP for

Answer 17

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

Question 18

what is a symport?

Answer 18

co-transporter that transfers two molecules in the same directions (e.g glucose and Na+ pump)

Question 19

what is secondary active transport

Answer 19

a process that moves molecules across a cell membrane by using energy stored in electrochemical gradient

Question 20

Describe the secondary active transport of glucose

Answer 20

-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)

Question 21

Describe what happens in the sodium potassium pump (electrogenic pump)

Answer 21

-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

Question 22

what are antiporters?

Answer 22

co-transporters that transfer molecules in opposite directions
(look at one note for example)

Question 23

Describe the ATP-Binding Cassette (ABC) transporter

Answer 23

-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)

Question 24

How do receptors work?

Answer 24

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

Question 25

Describe exocytosis

Answer 25

-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

Question 26

Describe endocytosis

Answer 26

Uptake of macromolecules from extra-cellular space, across the cytoplasmic membrane

Question 27

what is pinocytosis

Answer 27

a constitutive and continuous process involving uptake of
extra-cellular fluid via small membrane vesicles

Question 28

what is phagocytosis

Answer 28

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

Question 29

what is receptor-mediated endocytosis

Answer 29

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

Question 30

What are the different mechanisms of cell signalling?

Answer 30

-junction between cells (direct)
-adhesion molecules (direct)
-using intercellular chemical messenger and specific receptor molecule (indirect)
look on one note for diagram

Question 31

what drugs are used as receptor/ channel binding molecules

Answer 31

agonists and antagonists

Question 32

what are agonists

Answer 32

drugs that activates receptor and produces biological effect e.g insulin

Question 33

what are antagonists

Answer 33

drugs that block the activation of the target by ligand e.g., Beta blockers
block the adrenaline receptor (treating hypertension and arrhythmia)

Question 34

Describe a signal transduction blocker

Answer 34

Kinase inhibitors are being developed as cancer treatments