Exam #1: Membrane Transport Flashcards
Define osmosis.
- 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.
Define simple diffusion.
Net movement of a substance from an area of high concentration of that substance, to an area of low concentration of that substance.
Define facilitated diffusion. List the characteristics of facilitated diffusion.
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
What factors control simple diffusion through a membrane? Write Fick’s Law of Diffusion & Membrane Permeability.
Increased membrane SA Increased concentration gradient Increased membrane permeability Increased lipid solubility Decreased molecular size Decreased membrane thickness
Temperature (increased= more diffusion)
What factors control diffusion through pores or channels in the membrane?
- Permeability= # of channels & % of channels open
- Selectivity
- Concentration gradient
What factors control the rate of facilitated diffusion through uniporters?
- Concentration gradient
- Carrier number= Vmax
- Km i.e. affinity
What are the differences between carrier-mediated diffusion & active transport?
1) Active transport requires energy input
2) Active transport will transport uncharged & charged substances AGAINST their concentration gradients
Define uniport.
Facilitated diffusion of a substance down its concentration gradient
Define symport.
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
Examples of uniporters & symporters
Uniport= GLUT1 Symport= Na+/ Glucose Symporter (2 Na+ per glucose)
What is an ABC protein?
ABC= ATP Binding Cassette
What is the clinical importance of ABC proteins?
- CFTR (cystic fibrosis)
- MDR-proteins (multi-drug resistance proteins), which are often overexpressed in tumors & cause resistance to cancer drug therapy
Define endocytosis.
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
Define exocytosis.
Material inside the cell is packaged into vesicles & excreted into the extracellular medium
What is fluid-phase endocytosis?
Uptake of materials dissolved in the ECF
- Note that this is a very INEFFICIENT process
What is receptor-mediated endocytosis?
Uptake of material from ECF via receptors concentrated in the plasma membrane
- Much more efficient for the movement of specific substances
What is caveolae endocytosis?
Similar to clathrin mediated endocytosis, but uses caveolae & caveolin as coat protein.
What is clathrin? What shape does clathrin form?
- A coat protein on the inner surface of the plasma membrane that mediates endocytosis
- Triskelion shape that forms 3x interlocked spirals/ net
How does clathrin function in clathrin coated pits?
1) Receptor binding
2) Receptor/ adaptin interaction
3) Adaptin/ clathrin interaction
4) Clathrin coated pit formation
Compare & contrast passive, active, and vesicular transport.
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
What molecules can pass through a lipid membrane?
- Gasses
- Small, uncharged, & polar molecules
What is Fick’s Law of Diffusion?
Rate of diffusion is proportional to surface area x concentration gradient x membrane permeability/ membrane thickness
What is Membrane Permeability?
Membrane permeability is proportional to lipid solubility/ molecular size
What are aquaporins?
Proteins that form water channels through the cell membrane.
What can a mutation of an aquaporin cause?
Nephrogenic Diabetes Insipidus (AQPS-2)
- Patients produce large amount of dilute urine & complain of polydipsia
- Unable to properly reabsorb water in renal collecting ducts
What is Km? How is Km related to affinity?
- Half the concentration of substrate at which transport is half maximal.
- Lower Km= Higher affinity
Describe the function of the GLUT1 transporter.
- 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
What is the relationship between Type-1 DM & GLUT1 transporters?
- 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
Describe the function of the Na+/K+ Pump.
3x Na+ OUT & 2x K+ IN
How do mutations of the CFTR ABC transporter result in cystic fibrosis?
- 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
What is the clinical importance of receptor mediated endocytosis?
Familial hypercholestermia
- Individuals lacking LDL receptors/ defective LDL receptors
- High LDL in blood
- Predisposition to atherosclerosis & heart disease
