Active transport across cell membranes

Question 1

What are the main points of active transport?

Answer 1

• Cells exchange molecules with the environment to live and grow
• Plasma membrane: barrier, hydrophobic interior blocks passages of water soluble substances
• Specialised membrane transport proteins span lipid bilayer
• Main types:
- transporters: use energy to move specific substrates across membrane (usually up conc grad)
- channels: allow passive transport of specific substances across membrane (down conc grad)

Question 2

What is active transport?

Answer 2

The moving of solutes up the conc grad using energy in the form of atp

Question 3

Give examples of transport proteins that moves solutes up the concentration gradient

Answer 3

• coupled pump (multiple substances (eg Na+ & K+))
• atp driven pump (ATP)
• light driven pump (Light)

Question 4

How does active transport via ATPase pump work and why is it important

Answer 4

Na + K + ATPase moves Na+ and K+ ions against their conc gradients.

• energy is derived from hydrolysis of ATP to ADP durin metabolism
• P-type ATPase found/available to all cells of the body
• essential for maintenance of osmotic balance and resting potential
• drives couples transport of many solutes

Question 5

What is the Na+ K+ pump cycle?

Answer 5

Na+ pump used ATP to expel Na+ and bring in K+

1) Na+ binds to pump
2) Pump phosphorylates itself
(ATP->ADP) so lone phosphate attached to protein in high energy linkage
3) Phosphorylation triggers conformational change. Na+ ejected
4) K+ binds
5) Pump dephosphorylated (lone phosphate attached earlier is removed)
6) pump returns to original conformation, K+ ejected

Question 6

What are other important ATPase membrane transporters other than Na+ and K+ and what are they used for?

Answer 6

• Calcium ions (Ca+): important intracellular signalling molecules
• H+ ATPase: responsible for secretion of HcL into lumen of stomach for digestion of food

Question 7

How does Na+ K+ ATPase do for the cell?

Answer 7

Creates gradients

Question 8

How do coupled transport via symports and antiports work?

Answer 8

• energy used by Na+K+ATPase is captured in Na+ and K+ gradients
• the dissipation of these gradients can be used to drive other solutes up their conc grads (as Na+ flows into a cell it can be used to drive other solutes in/out the cell)
• This can only happen when Na+ flows into cells via specific transporter proteins (ie not ion channels)

• symports carry solutes in the same direction as Na+
• Antiports carry solutes in the opposite direction

Question 9

Active glucose transport is done via sodium-glucose transporters. What are the two symports that carry glucose into cells?

Answer 9

• SGLT-1: in intestinal mucosa (glucose absorption)
• SGLT-2: in kidney (proximal convoluted tubule for glucose re absorption)

Question 10

How does the Co transporter Serotonin reuptake transporter (SERT) work?

Answer 10

• serotonin: important neurotransmitter in brain
• uses a Na+ gradient to drive serotonin uptake from synaptic clefts
• inhibited by serotonin- specific re uptake inhibitors (SSRIs) eg Prozac (Fluoxetine)

Question 11

How does the co transporter Na+I+ co transporter work?

Answer 11

• Drives iodine uptake in the thyroid gland
• Important for thyroid hormone production

Question 12

How does the exchanger Na+Ca2+ exchanger work?

Answer 12

• Ca2+ influx into the cytoplasm of cardiac muscle drives contraction
• this protein is important for removal of Ca2+ ions from after muscle contaction

Question 13

How does the exchanger Cl-HCO3- co transporter work?

Answer 13

• important for exchange of CO2 from blood to the exhaled air in the lungs
• important for thyroid hormone production

Question 14

How is resting potential formed?

Answer 14

The concentration gradient and electrochemical gradient combine to form the resting potential

Question 15

How is resting potential formed?

Answer 15

The concentration gradient and electrochemical gradient combine to form the resting potential

Question 16

What is membrane potential?

Answer 16

The charge difference across membrane due to the difference in the conc of ions inside and outside the cell

Question 17

How do most cells maintain a potential difference between -20 and -200mv? (interior more negative)

Answer 17

• NaK pump creates K+ conc gradient between inside and outside of the cell
• K+ ions can flow down this via passive K leak channels
• cells contain fixed anions (negatively charged organic molecules confined within the cell)
• this creates an electrochemical gradient that wants to retain cations

Question 18

What membrane potential has no net movement of K+ ions due to the electrochemical force holding K+ ions matches the conc gradient pushing those ions out?

Answer 18

70 mV

Question 19

What is the nernst equation?

Answer 19

V = 62 log10(Co/Ci)

V: membrane potential mV
Co: outside conc of ion
Ci: inside conc of ion

assumes ion carries single positive charged organic molecules and temp is 37°C

Question 20

Why is the resting potential important?

Answer 20

• The cells ability to fire an action potential relies on the existence of a resting potential
- the basic signalling properties of neurons are determines by changes in the resting potential
• Changes in membrane potential trigger intracellular functions (eg secretion of hormones from endocrine cells)