Week 1

Question 1

Cell Membrane

is composed of
what diffuses through easily but what cannot cross without help
is impermeable to
what does permeability depend on

Answer 1

1. phospholipid bilayer
2. lipid soluable molecules and gases diffuse through easily but water soluable molecules cannot cross without help
3. organic anions (proteins)
4. premeability depends on molecular size, lipid solubility and charge

Question 2

Simple diffusion

1. definition and what passes through
2. movement
3. rate of diffusion is porportional to
4. passive meaning

Answer 2

1. Small, lipid-soluble molecules and gases (e.g. O2, CO2, ethanol, urea etc…) pass either directly through the phospholipid bilayer or through pores
2. Movement of substance is down its concen gradient
3. The relative rate of diffusion is roughly proportional to the concen gradient across the membrane. Greater CG means faster rate
   * Passive: No energy input required from ATP

Question 3

Faciliated Diffusion

1. definition
2. carrier proteins do what
3. movement
4. where does energy come from
   passive

Answer 3

1. Facilitated Diffusion is a process of diffusion, where molecules diffuse
   across membrane, with the assistance
   of carrier protein
2. Carrier protein aid the movement of
   polar molecules (e.g. sugars and amino acids) across cell membrane
3. Movement of substance is down its [ ]
   gradient
4. The energy comes from the [ ]
   gradient of the solute
   * Passive: No energy input required
   from ATP

Question 4

Transporters

Answer 4

• type of faciliated diffusion
• conformational change where one side is always closed
• finate amount and meaning that too many molecules results in saturation

Question 5

Active Transport

1. movement
2. how does it work
3. energy

Answer 5

1. against CG
2. Substance binds to protein carrier that changes conformation to move substance across membrane
3. requires energy from ATP hydrolysis

ex: ATPases (Na/K pump)

Question 6

Secondary Active Transport

1. definition
2. what powers it
3. what induces conformation change

Answer 6

1.When a substance is carried up its concentration gradient without ATP
catabolism, this is known as Secondary Active Transport
2. Kinetic energy of movement of one substance down its concentration gradient powers the simultaneous transport of another up its concentration gradient. Secondary Active Transport is powered by the chemical energy in the substance
diffusing down its CG and this energy is used to ‘push’ some other
substance against its CG
3.Sequential binding of a substance and ions to specific sites in the transporter
protein induces a conformational change in the protein

energy derived from substance down CG to move molecule against CG

Question 7

Channels

1. what is it
2. what is a pore loop and what does it go
3. what are the pores called

Answer 7

1. membrane spanning proteins that forms pore through membrane
2. 4-5 protein subunits fit together such
   that a central pore is created
   through membrane, through which
   specific ions can diffuse through called ‘Pore loops’ of the protein molecules which dangle inside the channel and create a selectivity filte
3. pores are called membrane channels

Question 8

gated channels

2 types

Answer 8

binding of chemical agent
voltage across membrane

Question 9

liagnd gated channels

Answer 9

1. Cell membrane receptors are part of
   the body’s chemical signaling system
2. The binding of a receptor with its
   ligand usually triggers events at the
   membrane, such as activation of an
   enzyme

Question 10

voltage gated channels

1. what is the voltage sensing mechanism and its natural position
2. what happens when the membrane is polarized
3. what happens during depolarization

Answer 10

1. The voltage sensing mechanism is in the 4th transmembrane domain of
   the protein, the S4 segment. S4 sticks out to the side of the protein (like a wing). The natural position of the S4 ‘wing’ is up towards the outer surface of
   the cell membrane.
2. when the membrane is polarized, the positively charged wing is attracted downwards to the negatively charged inner surface of the membrane
3. Depolarization of the membrane to
   about -50 mV no longer provides
   sufficient electrical attraction to hold
   the S4 wing downwards, so it migrates back-up. In the up position, S4 removes a structural occlusion from the pore
   such that ions can now diffuse
   through i

opens when membrane polarity changes

Question 11

endocytosis

Answer 11

inward ‘pinching’ of membrane to create a vesicle; usually receptor-
mediated to capture proteins, from outside to inside

Question 12

exocytosis

Answer 12

partial or complete fusion of vesicles with cell membrane for bulk
trans-membrane transport of specific molecules, from inside to outside.
- Intracellular materials, packaged in vesicles, are either secreted or delivered to
the plasma membrane by exocytosis

Question 13

Exocytosis 1
what is it used for
Specific locations

kiss and run

Answer 13

The secretory vesicles dock and fuse with the plasma
membrane at specific locations called ‘fusion pores’
* Vesicle can connect and disconnect several times before contents are emptied
* Since only part of the contents are emptied in one ‘Kiss’, the process can be repeated several times before the vesicle is depleted
* Generally only part of vesicle contents diffuse into the interstitial fluid, used for low rate of signaling

Question 14

exocytosis 2
what is it necessary for
how and why must it be counterbalanced

full exocytosis

Answer 14

involves complete fusion of the vesicle with the membrane, leading to total
release of vesicle contents at once
* Necessary for delivery of membrane
proteins and high levels of signaling
* Must be counterbalanced (because you cant keep adding to the membrane because it becomes too loose) by
endocytosis to stabilize membrane
surface area.

Question 15

Membrane Potential

1. Defintion
2. short term goal
3. 2 conditions
4. what happens when MP is 0

Answer 15

1. Potentilal difference across membrane of cell
2. to generate electrical impulse/action potential to use as electrical signal
3. 1. Create a concentration gradient: an enzyme ion pump (functions as an ATPase) must actively transport certain ion species across the membrane to create a concentration gradient
4. Semi-permeable membrane: allows one ion species to diffuse across the membrane more than others. Diffusion of that ion species down its conc. gradient creates an electrical gradient
5. Cell is dead

Question 16

Na/K Pump

1. importance
2. ATPase
3. how much Na and how much K and where are they pumped
4. energy consumption
5. energy inequality

Answer 16

1. All cell membrane is loaded with Na+/K+ pumps, this is the staple of all living cells
2. Na+/K+ dependent ATPase is enzyme that moves Na+ out of cell, and K+ into cell by breaking down ATP
3. For each ATP molecule broken down, 3 Na+ ions are pumped out and 2 K+
   pumped in (creates a concentration gradient)
4. Consumes 1/3 of energy needs of body (in neurons it’s 2/3, i.e. huge consumer of energy)
5. Na/K inequality > potential difference of -10 mV. 3 positives out but 2 positives pump in. Inside of cell is negative

Question 17

what is the resting MP in neurons and why

Answer 17

-70V due to diffusion of K+ ions outwards

Question 18

Resting Membrane Potential

1. the resting membrane is most permeable to what
2. where does K+ diffuse
3. net charge inside membrane
4. what causes the equilbirium

Answer 18

1. most permeable to K+ ions
2. K+ diffuses out of cell, down concentration gradient, via K+ channels that are always open during rest
3. Cations accumulate on the outside of the membrane, leaving a net negativity
   inside membrane
4. l there is such a build up of “+” charge on the outside of the membrane that further diffusion of K+ is repelled by the electromagnetic force

Question 19

Equilibrium Potential

1. what happens at equilbirum
2. Membrane potential at equilibrium is determined by
3. How can it be calculated

Answer 19

1. At equilibrium, electrical work to repel outward cation diffusion equals chemical work of diffusion down conc. gradient
2. Membrane potential at equilibrium is determined by the concentration gradient
3. Can be calculated using the Nernst Equation

Question 20

Nernst Equation

1. what does it describe
2. what does it give
3. when is it valid

Answer 20

1. In the ideal situation, the Nernst equation, describe the balance between the chemical work of diffusion with electrical work of repulsion
2. The equation gives the potential difference across the membrane, inside with respect to outside, at equilibrium
3. The result is valid if and only if one ion species (K+ in this case) is diffusing across the membrane

Question 21

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K+ Equilibrium Potential

Answer 21

-90 mV