ELM4: Water, membranes and ions

Question 1

What is meant by the polarity of water?

Answer 1

The molecule has two different charges

Oxygen more electronegative so has more electrons so more negative

Question 2

What is an ion?

Answer 2

Any atom or molecule that has gained or lost an electron which gives them charge

Question 3

What are three uses of ions?

Answer 3

Carry signals in body
Act as energy store
Interact biochemically with proteins and other molecules

Question 4

What are the two ways that ions can be biologically important?

Answer 4

Physiologically
Biochemically

Question 5

What happens when a positive ion is placed in water?

Answer 5

Attracts oxygen from water molecules
Forms shell of water molecules around the ion

Question 6

What happens when you put a negative ion in water?

Answer 6

Attracts all the hydrogen from water molecules

Question 7

What is the relationship between the size of an ion and the hydration shell?

Answer 7

Small ions have larger hydration shells

Question 8

What is the structure of membranes?

Answer 8

Lipid bilayers
Hydrophilic head and hydrophobic tail drives the formation

Question 9

What is active transport?

Answer 9

Movement against concentration gradient using energy

Question 10

What is passive transport?

Answer 10

Movement with the concentration gradient not using energy

Question 11

How do pumps work?

Answer 11

Primary active transport using energy from ATP

Question 12

What are some features of pumps?

Answer 12

Couple to ATP
Slow
Usually move cations

Question 13

What is an example of a pump?

Answer 13

Sodium potassium ATPase
Generates calcium and potassium gradient

Question 14

How do cotransporters work?

Answer 14

Secondary active transport
Use movement of ions to power transporters

Question 15

How do ion gradients act as a source of energy?

Answer 15

Used to transmit information
Used to power cellular processes

Question 16

What is a carrier/transporter protein?

Answer 16

Bind substrate to one side of the membrane
Conformational change to move it inside

Question 17

What are some features of carrier/transporter proteins?

Answer 17

Maximum rate is 10000/s
Active or passive

Question 18

What is an ion channel?

Answer 18

Continuous pathway to allow ions to pass through membrane

Question 19

What are some features of an ion channel?

Answer 19

Rate of 1000000/s
Passive transport
All transmembrane proteins
Selectively permeable using selectivity filter
Controlled opening using gating

Question 20

What are the two ways used to characterise and name an ion channel?

Answer 20

Gating
Ion selectivity

Question 21

What are the two main groups of ligand gated ion channels?

Answer 21

Cys loop receptors
Ionotropic glutamate receptors

Question 22

What are cys loop receptors?

Answer 22

All have the cys loop structure
Protein loop in extracellular domain linked by bond between two cysteine
S-S

Question 23

What are some common features of cys loop receptors?

Answer 23

Pore to let ions through
Ligand binding side
Coupling mechanism
Desensitisation mechanism

Question 24

How did Ca and Na channels evolve?

Answer 24

Originally there was one gene for K channels which replicated
Created copies of the gene strung together

Question 25

What is the pore forming subunit in Ca and Na channels?

Answer 25

The alpha subunit

Question 26

What is the structure of alpha 2 delta subunits in Ca and Na channels

Answer 26

1. Gene makes a single polypeptide 2. Disulphide bond formed in extracellular domain 3. Extracellular domain cleaved to yield two linked peptides

Question 27

What is the structure of a nicotinic acetylcholine receptors nAChR?

Answer 27

Pentamer 2 alpha subunits 1 beta 1 delta 1 epsilon Arranged around central ion channels

Question 28

How were studies performed to see which part of nAChR did different things?

Answer 28

1. Subunits put into a plasmid expression vector 2. Mutate single sections 3. Introduce plasmid into mammalian cell to compare mutant receptor to wild type receptor

Question 29

What is the synaptic role of acetylcholine binding protein AChBP?

Answer 29

1. ACh released into synapse and binds to nAChRs and glial cells 2. Glial cells release AChBP into synapse 3. Acts as molecular sponge for acetylcholine and lowers levels in synapse

Question 30

How is AChBP used as a model of other receptors (Homology modelling)?

Answer 30

1. Amino acid sequences aligned to give best match 2. Alignment used to map unknown protein onto 3D structure of known protein 3. Gives structure of unknown protein

Question 31

Why can AChBP be used as a model of other receptors?

Answer 31

Strong relationship with real nicotinic receptors Acts as a template for homology modelling

Question 32

What are the two types of ligand gated ion channels in prokaryotes?

Answer 32

ELIC and GLIC Homopentamers Unknown function No cys loop structure

Question 33

What is ELIC?

Answer 33

Cation selective channel Activated by amines like GABA

Question 34

What is GLIC?

Answer 34

Cation channel Gated by protons