3. membranes, ions, water and proteins

Question 1

describe water. 5

Answer 1

1. very small molecule
2. all life on earth exists in an aqueous environment
3. molecular properties of water influence the structure and behaviour of cells
4. water must be understood in order to understand neurons
5. water is the molecular key to life

Question 2

describe the h2o molecule. 5

Answer 2

1. molecular mass of approx 18
2. polar- this influences life
3. o2 is very electronegative and pulls the other electrons closer that it is supposed to share with H
4. this has implications for water and its interaction with other molecules and itself
5. H(positive dipole)-O(negative dipole)-H(positive dipole)

Question 3

What are H bonds? 6

Answer 3

1. charges allow h2o molecules to interact
2. hydrogen bond - special electrostatic bond betweem oxygen and hydrogen
3. o2 has towo lone electron pairs and there are two H atoms
4. this allows each water molecule to interact with four others
5. more interactions than most molecules of that size, makes water liquid at room temperature instead of gas
6. h bonds lead to anomalously high boiling point which allows water to exist on earth

Question 4

Talk about the states of matter of water. 4

Answer 4

1. liquid water has a randomized tetrahedral arrangement
2. solid ice is a very organized lattice of bonds
3. liquid water bonds make water denser than ice, so ice can float
4. this is important for life on earth

Question 5

Hoe are ice caps related to life on earth? 5

Answer 5

1. without floating ice caps, water wouldn’t be protected from sun’s heat
2. all water would freeze from bottom up
3. severe impact on earth’s ability to support life
4. less h2o available to organisms
5. ice caps above insulate water below and allow it to remain liquid

Question 6

What are ions? 4

Answer 6

1. any atom or molecule that has gained or lost an electron is charged
2. sodium is the most important ion in body
3. sodium atom electron arrangement is 2-8-1
4. can lose one electron (outer), so positively charged

Question 7

what is the importance of ions? 6

Answer 7

1. ions carry signals in the body eg. APs are movement of ions over membranes
2. skeletal muscle, cardiac muscle and many hormone secretors use ions as important signals
3. ions moving across membranes creates a gradient that can power other cell processes like an energy store eg. secondary active transport
4. they interact biochemically with proteins and other molecules
5. ca2+ and troponin c bind in muscle contraction
6. ATP complexes with mg2+

Question 8

what are the different used of ions? 6

Answer 8

1. ions can be classified as physiologically or/and biochemically useful
2. physiologically useful ions -charge carriers
3. they include na+, K+ and cl-
4. biochemically useful ions are required in smaller amounts eg. mg2+, trace metals eg. Fe3+ (Hb), zn2+ (enzymes)
5. Ca2+ is both physiologically and biochemically useful
6. Ions usually exist in aqueous solution, not isolation

Question 9

Describe ions in aqueous solution. 5

Answer 9

1. water has many interactions between the molecules
2. insert a negatively charged ion and it organizes water aorund it
3. it attracts the negative dipole oxygen parts
4. ion becomes coated with water, which stabilises the water
5. this is why ionic compounds eg. nacl dissociate in water

Question 10

what is a hydration shell? 6

Answer 10

1. a ring of water immediately surrounds the ion, then a more loosely associated layer of water
2. the immediate layer of water around the ion is called the primary hydration shell/layer
3. with a positive ion, negative dipole oxygen is attracted
4. also happens with chloride ion. a hydration shell also forms
5. the hydration shell is very important in behaviour of ions in solution and interaction with proteins encountered in membranes.
6. charge density - a small ion attracts more water molecules than a larger one of the same charge, spread over smaller area

Question 11

HOW IS CHARGE DENSITY RELATED TO IONIC CHARGE AND RADIUS? 5

Answer 11

1. EG in the alkaline metals, Li+ has the smallest size, therefore largest charge density, and so attracts the largest hydration shell
2. so, it becomes as if it is larger than the others (na+, K+, Cs+, Rb+)
3. hydration shell affects mobility in solution
4. hydration shell is the effective size of the ion
5. hydration shell affects the interactions with proteins

Question 12

describe biological membranes. 5

Answer 12

1. membranes help cell organization and prevent contents from floating away
2. membranes are made from lipid bilayers
3. phospholipids are amphipathic - hydrophilic heads and hydrophobic tails
4. entropy drives the layers into a bilayer sheet, which organizes itself
5. this allows the membrane to interact with water via h, but also presents challenges

Question 13

describe membrane permeability. 6

Answer 13

1. its hard to move things across membranes
2. membranes are essentially impermeable to ions
3. glucose is less polar so moves through the membrane more easily
4. h2o and ethanol are very small so move through even more easily
5. impermeability gives the advantage of compartmentalization of ion distribution
6. endoplasmic reticulum, sarcoplasmic reticulum and mitochondria contain many ions

Question 14

What are the intra and extra cellular concentrations of the main physiological ions in the body? What is the ratio? 4

Answer 14

1. Na+, E: 150 mM, I: 15, R: 10:1
2. K+, E: 5, I: 100, R1:20
3. Ca2+, E: 2, I: 0.0002, R: 10000:1
4. Cl-, E: 150, I: 13, R: 11.5:1

Question 15

What is the importance of proteins and gradients? 5

Answer 15

1. to create gradients, ions must move across membranes, into/out of cells
2. membranes can contain a variety of proteins
3. all of the proteins have hydrophobic portions which can live in and interact with the bilayer
4. Part of the protein must also be hydrophilic
5. the proteins allow cells to establish ion gradients and use them

Question 16

What are pumps? 6

Answer 16

1. movement of ions against a conc. gradient requires hydrolysis of ATP, so pumps are often couples to ATP - some have enzymes that can be involved
2. special proteins called pumps carry out primary active transport
3. transmembrane proteins
4. fairly slow
5. nearly always move cations
6. eg. sodium-potassium ATPase, most important protein in body

Question 17

What is the sodium potassium pump? 6

Answer 17

1. uses 25-40% of body’s atp
2. largest single energy consuming activity
3. electrogenic- causes electrical gradient as well as chemical
4. 3Na+ out, 2K+ in for every ATP
5. Generates sodium and potassium gradient
6. Na+ decreases in cytoplasm, K+ increases in cytoplasm

Question 18

What is ion gradient energy? 3

Answer 18

1. gradients represent a source of energy
2. can be used to power cellular processes eg. secondary active transport of other ions via carriers
3. can be used to transmit information eg. signalling via ion channels

Question 19

What are secondary active transport carriers? 5

Answer 19

1. don’t use energy from ATP directly - ATP burned to create gradient of downhill ions, then energy comes from this gradient
2. antiporters eg. na+ - ca2+ exchanger couples downhill flow to uphill flow in opposite directions
3. symporters do the same but some direction
4. carriers can be very effective eg. na+-ca2+ exchanger - sodium moves in, energy created pushes calcium out
5. faster than calcium pump, 2000/s rather than 30/s

Question 20

What are ion channels? 5

Answer 20

1. v. important in excitable membranes
2. allow ions to pass through membranes quickly and control flow
3. Found in all organism types, incl. archaea
4. evolved for all major physiological ions
5. incl. na+, ca2+, k+ and cl-

Question 21

what are the properties of ion channels? 5

Answer 21

1. diverse, with same basic properties
2. transmembrane proteins
3. passive transport down gradient
4. selective
5. opening and closing controlled by membrane potential and ligands

Question 22

what are voltage gated channels? 5

Answer 22

1. Open in response to membrane potential changes, usually depolarisation
2. pore lets ions through
3. voltage sensor tells channel to open in response to voltage change
4. coupling mechanism couples channel opening to voltage sensor
5. inactivation mechanisms close channel to prevent depolarisation lasting too long

Question 23

What is the structure of a voltage gated ion channels, using potassium as an example? 5

Answer 23

1. 4 subunits make a protein complex
2. central pore with gate at the top of the channel
3. has a voltage sensor somewhere inside
4. simplest voltage gated channel
5. important in action potentials and other excitable cell processes

Question 24

What are ligand gated ion channels? 6

Answer 24

1. different protein family lead to different structures and behaviours
2. open in response to binding of an activating ligand (agonist) eg. Ach
3. pore lets ions through
4. ligand binding site tells channel to open in response to ligand binding
5. coupling mechanism couples channel opening to ligand binding
6. desensitization mechanisms close channel is ligand binds for too long

Question 25

Describe the nAChR. 5

Answer 25

1. pentameric, 5 units
2. NAR - electron microscopes allowed us to see structural detail
3. combine images to find density of e- in protein
4. alpha subunit involved in ach binding
5. narrow structure with gate