Principles - Membranes Flashcards by Natasha Hadfield

What is a plasma membrane?

Fluid lipid bilayer embedded with proteins

Mostly phospholipids and cholesterol

Proteins

Small amount of carbohydrate

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What is the structure of a phospholipid molecule?

Negatively charged, polar, hydrophilic head

Uncharged, nonpolar, hydrophobic tail

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What effect does cholesterol have on the membrane?

It contributes to fluidity and stability

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How can proteins in the bilayer be classified?

Integral - embedded in the bilyar (receptors)

Transmembrane - extend through the membrane (transporters, channels)

Peripheral - most common intracellularly, don’t penetrate membrane

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What is the glycocalyx?

Small amount of carbohydrate located on outer surface of cells

Often bound to membrane protein, or to a lesser extent, lipids forming glycoproteins or glycolipids

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What are docking-marker acceptors?

Proteins located on inner membrane surface

They interact with secretory vesicles leading to exocytosis of the vesicle contents

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What are desmosomes?

Adhering junctions anchoring cells together.

Particularly common in tissues that can stretch.

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What are tight junctions?

Join the lateral edges of epithelial cells near their lumenal (apical) membranes

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What are gap junctions?

Communicating junctions allowing movement of charge carrying ions between two adjacent cells

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What properties influence whether a particle can permeate the plasma membrane unassisted?

Solubility of particle in lipid

Size of particle

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How can molecules and ions passively cross the membrane?

Diffusion down concentration gradient

Movement along electrical gradient

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Which factors influence the rate of net diffusion (Fick’s Law)?

Magnitude of concentration gradient

Surface area of membrane

Lipid solubility of substance

Molecular weight of substance

Thickness of membrane

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What is osmosis?

Net diffusion of water down its own concentration gradient

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Why can water molecules permeate the plasma membrane more easily than would be expected?

Because of aquaporins - water channels

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What is osmolarity?

Concentration of osmotically active particles present in solution

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What is tonicity?

The effect a solution has on cell volume

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What happens in hypotonic conditions?

Cells swell

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What happens in hypertonic conditions?

Cells shrink

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What three characteristics define carrier mediated transport?

Specificity

Saturation

Competition

How does the transport rate between simple diffusion and facilitated diffusion compare?

Simple diffusion - straight line

Facilitated diffusion - hyperbolic

What are the two types of carrier-mediated transport?

Facilitated diffusion - passive

Active transport - requires energy

What are the two types of active transport?

Primary active transport - ATP is used as energy source

Secondary active transport - secondhand energy stored in the form of an ion concentration gradient is used

Na⁺K⁺ pump (Na⁺-K⁺ ATPase) transports how many ions in each direction?

3 Na⁺ out of the cell

2 K⁺ in to the cell

What are the three important roles of the Na⁺-K⁺ pump?

Helps establish Na⁺ and K⁺ concentration gradients

Helps regulate cell volume by controlling solute concentration

Energy used to drive pump serves indirectly as energy source for secondary active transport

What is symport?

Co-transport of a solute and ion (usually Na⁺) across the membrane in the same direction (e.g. glucose absorption at apical membrane of enterocytes)

What is antiport?

Solute and ion (usually Na⁺) move in opposite directions across the cell membrane

What are the functions of exocytosis?

Secretion of enzymes, protein hormones Method of adding carriers, channels or receptors to plasma membrane, e.g. GLUT4

What is the membrane potential?

Potential difference between extracellular and intracellular fluid

What causes the membrane potential?

Difference in concentration and permeability of key ions

What direction is the concentration gradient for K⁺?

Outwards

What is the direction of concentration gradient for Na⁺?

Inwards

What direction is the electrical gradient for both Na⁺ and K⁺?

Towards the negatively charged side of the membrane

What is the difference in permeability of the membrane to Na⁺ and K⁺?

The membrane is about 100x more permeable to K⁺ than Na⁺

What is the equilibrium potential of K⁺?

-90mV

What is the equilibrium potential for Na⁺?

+61mV

What is the resting membrane potential?

-70mV

What is the effect of the Na⁺-K⁺ pump on the membrane potential?

Generates a hyperpolarizing current

How do nerve and muscle cells rapidly change their membrane potentials?

Altering their membrane permeabilities in response to stimulation

What external factor is linked to a change of membrane potential in nerve and muscle cells?

Release of insulin by β cells of pancreas

What do the terms hyperpolarization and depolarization mean?

Hyperpolarization - membrane potential becomes more negative Depolarization - membrane potential becomes less negative or positive

What sort of stimuli control ion channels?

voltage-gated ligand-gated mechanical thermal

What type of ion channels are responsible for the action potential in neurones?

Voltage-gated Na⁺ channels - depolarization Voltage-gated K⁺ channels - hyperpolarization

How do voltage-gated Na+ channels exhibit positive feedback?

Opening of a few channels causes further channels to open causing further depolorization

How do voltage-gated K+ channels exhibit negative feedback?

Outward movement of K+ causes repolarization, therefore turning off the stimulus for channel opening.

What causes the refractory period?

Na+ channels enter a non-conducting, inactivated state during maintained depolarization This allows the channel to enter the closed state again, ready for a further action potential

How does the myelin sheath of a neuronal axon contribute to the conduction of the action potential?

Action potentials just along the nodes of Ranvier