chapter 7: membrane structure & function i. membrane structure & function ii. selective...

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