chapter 7: membrane structure & function i. membrane structure & function ii. selective...
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Chapter 7: Membrane Structure & Function
I. Membrane Structure & Function
II. Selective Permeability of Membranes
I. Plasma Membrane Structure
Plasma membrane
– boundary that ________ living cells
from their nonliving surroundings.
- apprx. 8 nm thick
- composed chiefly of ________ and ______
- surrounds the cell and controls chemical traffic in/out of cell
- is selectively ________!
Enables cells to maintain internal environment different from external environment
Phospholipid bilayer
- composed of ________ phospholipids
Membrane Structure (________ Model)
• amphipathic membrane proteins embedded in phospholipid bilayer
• held together by weak _______________
• give membrane ‘fluidity’ similar to salad oil
• phospholipids & proteins can _____ laterally (2 um / sec)
- solidification causes changes in permeability and enzyme deactivation
Membranes must be fluid to work properly !
How do cells control membrane fluidity ?
1. ___________ fatty acid tails
- enhance fluidity
- kinks at C=C hinder close packing
Decreases fluidity at ______ temps by restraining phospholipid movement
Increases fluidity at _____ temps by preventing close packing of phospholipids
2. Adding _______ makes membrane:
Proteins in Plasma Membrane
- mosaic of proteins ‘_____’ in a fluid lipid bilayer
- proteins determine a membrane’s specific function:
Two types
1. ______ proteins (‘transmembrane’, or embedded)
2. ________ proteins (bound to surface of membrane)
Some Functions of Membrane Proteins
Synthesis of membrane and it components
1. Synthesis of membrane proteins and lipids in the ___
2. Carbohydrate modification in the _________
3. Transport of membrane components to the plasma membrane
4. Fusion of ______ with the membrane
Passive Transport vs. Active Transport
Selective Permeability of Membranes
I. Passive Transport: diffusion and facilitated diffusion
1.1. Diffusion : net movement of a
substance down
a __________________.
• Solutes diffuse from ____ to ____ concentration.
• Continues until a dynamic _________ is reached.
• ____ requirement for ________ expense (passive)
• Examples:
uptake of O2 by cell performing respiration
elimination of CO2 from cell
Diffusion of solutes across a membraneEach dye diffuses down its own ___________ gradient.
Osmosis• Diffusion (passive transport) of
_____ across a selectively permeable membrane, (the membrane are __________ for solute but for water).
• Direction of water movement is determined by the difference in total _____ concentration, regardless of type or diversity of solutes.
• Water moves always from _____ concentration solution to ______concentration solution.
Water balance of living cells
• Tonicity : the ability of a solution to cause a cell to gain or lose water Isotonic : __________ of water across the membrane (same). Hypertonic : the cell ______ water to its environment (more). Hypotonic : the cell ______ water from its environment (less).
QuestionsAn artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to sucrose.
1. Glucose?2. Fructose?3. Hypotonic/
Hypertonic?4. Water?
1.2. Facilitated diffusion
a) Channel protein : aquaporins, ion channels
b) Carrier protein
• _______ transport• Transport ______ speed the movement of molecules
across the plasma membrane.• Channel protein and Carrier protein
2. Active Transport
• Requires the cell to expend _______
• Transport proteins pump molecules across a membrane _________ their concentration gradient.
• “uphill” transport
• Maintain steep ionic gradients across the cell membrane (Na+ , K+ , Ca++ , Mg++ , Cl-)
How is it done?
- using transport proteins and ____!
An Example of Active Transport: The Sodium-Potassium Pump
Passive Transport vs. Active Transport