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Medical Physiology > Exam #1: Membrane Transport > Flashcards

Exam #1: Membrane Transport Flashcards

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USMLE® Step 1 flashcards
1
Q

Define osmosis.

A
  • Movement of water across a selectively permeable membrane
  • Water travels from an area of low solute concentration to an area of high solute concentration.

*Rate of osmosis is increased when water channel aquaporins are inserted into the membrane.

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2
Q

Define simple diffusion.

A

Net movement of a substance from an area of high concentration of that substance, to an area of low concentration of that substance.

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3
Q

Define facilitated diffusion. List the characteristics of facilitated diffusion.

A

Movement of a single molecule down a concentration gradient, utilizing a carrier or transporter.

  • Much faster than simple diffusion
  • Shows saturation kinetics/ vmax & has Km
  • Transport direction is reversible (if concentration grad. changes)
  • Transport is specific
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4
Q

What factors control simple diffusion through a membrane? Write Fick’s Law of Diffusion & Membrane Permeability.

A 
Increased membrane SA 
Increased concentration gradient 
Increased membrane permeability 
Increased lipid solubility 
Decreased molecular size 
Decreased membrane thickness

Temperature (increased= more diffusion)

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5
Q

What factors control diffusion through pores or channels in the membrane?

A
  • Permeability= # of channels & % of channels open
  • Selectivity
  • Concentration gradient
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6
Q

What factors control the rate of facilitated diffusion through uniporters?

A
  • Concentration gradient
  • Carrier number= Vmax
  • Km i.e. affinity
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7
Q

What are the differences between carrier-mediated diffusion & active transport?

A

1) Active transport requires energy input

2) Active transport will transport uncharged & charged substances AGAINST their concentration gradients

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8
Q

Define uniport.

A

Facilitated diffusion of a substance down its concentration gradient

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9
Q

Define symport.

A

Secondary active transport of one substance up its concentration gradient, coupled to the movement of another substance down its concentration gradient

  • Direction of movement is the same for both molecules
  • Energy loss from downhill movement is captured to move alternate substance against its concentration gradient
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10
Q

Examples of uniporters & symporters

A 
Uniport= GLUT1
Symport= Na+/ Glucose Symporter (2 Na+ per glucose)
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11
Q

What is an ABC protein?

A

ABC= ATP Binding Cassette

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12
Q

What is the clinical importance of ABC proteins?

A
  • CFTR (cystic fibrosis)
  • MDR-proteins (multi-drug resistance proteins), which are often overexpressed in tumors & cause resistance to cancer drug therapy
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13
Q

Define endocytosis.

A

Plasma membrane invaginates to form a vesicle that brings substances into the cell

  • Phagocytosis= cell eating
  • Pinocytosis= cell drinking
  • Fluid-phase endocytosis= uptake of materials dissolved in the ECF
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14
Q

Define exocytosis.

A

Material inside the cell is packaged into vesicles & excreted into the extracellular medium

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15
Q

What is fluid-phase endocytosis?

A

Uptake of materials dissolved in the ECF

- Note that this is a very INEFFICIENT process

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16
Q

What is receptor-mediated endocytosis?

A

Uptake of material from ECF via receptors concentrated in the plasma membrane
- Much more efficient for the movement of specific substances

17
Q

What is caveolae endocytosis?

A

Similar to clathrin mediated endocytosis, but uses caveolae & caveolin as coat protein.

18
Q

What is clathrin? What shape does clathrin form?

A
  • A coat protein on the inner surface of the plasma membrane that mediates endocytosis
  • Triskelion shape that forms 3x interlocked spirals/ net
19
Q

How does clathrin function in clathrin coated pits?

A

1) Receptor binding
2) Receptor/ adaptin interaction
3) Adaptin/ clathrin interaction
4) Clathrin coated pit formation

20
Q

Compare & contrast passive, active, and vesicular transport.

A

Passive= no energy required

  • Diffusion through membrane
  • Diffusion through channels
  • Facilitated diffusion

Active= energy required but NO vesicles

  • Primary active
  • Secondary active

Vesicular= lipid fusion involved & energy required

  • Endocytosis
  • Exocytosis
21
Q

What molecules can pass through a lipid membrane?

A
  • Gasses

- Small, uncharged, & polar molecules

22
Q

What is Fick’s Law of Diffusion?

A

Rate of diffusion is proportional to surface area x concentration gradient x membrane permeability/ membrane thickness

23
Q

What is Membrane Permeability?

A

Membrane permeability is proportional to lipid solubility/ molecular size

24
Q

What are aquaporins?

A

Proteins that form water channels through the cell membrane.

25
Q

What can a mutation of an aquaporin cause?

A

Nephrogenic Diabetes Insipidus (AQPS-2)

  • Patients produce large amount of dilute urine & complain of polydipsia
  • Unable to properly reabsorb water in renal collecting ducts
26
Q

What is Km? How is Km related to affinity?

A
  • Half the concentration of substrate at which transport is half maximal.
  • Lower Km= Higher affinity
27
Q

Describe the function of the GLUT1 transporter.

A
  • Found in plasma membrane & brings glucose into cell
  • Concentration gradient maintained by phosphorylating glucose upon entry
  • Insulin increases # of GLUT1 in skeletal muscle cells & adipocytes
  • Smaller Km= Higher affinity= Higher rate of transport
28
Q

What is the relationship between Type-1 DM & GLUT1 transporters?

A
  • DM-1= immune mediated destruction of beta cells in pancreas, resulting in vastly decreased insulin secretion
  • Insulin inserts GLUT1 into cell membranes to absorb glucose from blood
  • Thus, DM-1 results in hyperglycemia
29
Q

Describe the function of the Na+/K+ Pump.

A

3x Na+ OUT & 2x K+ IN

30
Q

How do mutations of the CFTR ABC transporter result in cystic fibrosis?

A
  • CFTR is found in the epithelial cells of many organs including the lung.
  • Normally, the channel protein moves chloride out to the covering mucus, increasing the total electrolyte concentration in the mucus, resulting in the movement of water out of cell by osmosis.
  • CFTR mutations prevents Cl- ions movement out of the cell resulting in a buildup of thick mucus in the lungs
31
Q

What is the clinical importance of receptor mediated endocytosis?

A

Familial hypercholestermia

  • Individuals lacking LDL receptors/ defective LDL receptors
  • High LDL in blood
  • Predisposition to atherosclerosis & heart disease

Medical Physiology (74 decks)

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