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Chapter 7: Membrane Structure and Function. Important Point:. If you are having trouble understanding lecture material: Try reading your text before attending lectures. And take the time to read it well!.

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Presentation Transcript
important point
Important Point:

If you are having trouble

understanding lecture material:

Try reading your text

before attending lectures.

And take the time to read it well!

lipid bilayer

The “ability of the cell to discriminate in its chemical exchanges with the environment is fundamental to life, and it is the plasma membrane that makes this selectivity possible.”

Lipid Bilayer
history of fluid mosaic model
History of Fluid Mosaic Model

For more on this history, see:

cholesterol temp buffer
Cholesterol: Temp. Buffer

“Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting regular packing of phospholipids.”Campbell & Reece (2005) p. 126

lateral movement of proteins
Lateral Movement of Proteins

Protein diffusion is not as fast as phospholipid diffusion

not all proteins move
Not all Proteins Move

Proteins attached to cytoskeleton or ECM don’t move

transmembrane protein
Transmembrane Protein

Know definition of transmembrane protein

Note distinct orientation

membrane protein functions
Membrane Protein Functions

Membrane asymmetry allows cells to automatically differ their intracellular environment from that existing extracellularly

membrane protein trafficking

Note the equivalent orientation of endomembrane lumen and extracelullar environment

Membrane Protein Trafficking

Note the orientation of carbohydrate (always lumen or outside of cell)

transport across membranes
Transport Across Membranes

Active transport is pumping against concentration gradients

primer on diffusion

Diffusion is movement from area of high concentration to low

  • Diffusion is Passive Transport: no energy is required
Primer on Diffusion
simple diffusion across bilayers
Simple Diffusion Across Bilayers
  • Unaided by transport proteins
  • No metabolic energy expended
  • Movement is down chemical concentration gradient
  • Diffusion rate is proportional to concentration gradient and hydrophobicity
  • Rate limiting step is movement across hydrophobic portion of membrane
  • The greater the hydrophobicity of a water-soluble molecule, the faster it diffuses across phospholipid bilayer

Selective permeability


More water

Less water

Down water’s concentration gradient


More solute in than out

facilitated diffusion1
Facilitated Diffusion

The parallels between the properties of transport proteins and enzymes are fairly extensive, to the point where one may consider a transport protein simply as an enzyme-like protein that “catalyzes” the physical process of movement from one side of a membrane to another

group translocation
Group Translocation

Note that ATP-mediated phosphorylation of glucose inside of cell drives this transport, making it an example Group Translocation

sodium potassium pump1

The sodium-potassium pump is a specific (and important) example ATP-powered active transport

  • In addition to their intrinsic relevance, studying membrane-transport proteins allows us to appreciate mechanisms of protein-mediated catalysis without getting bogged down in the details of chemical reactions!
  • As we shall see, there are other means of powering active transport that don’t involve a direct hydrolysis of ATP
  • First, though, we will take another look at the sodium-potassium pump
Sodium-Potassium Pump
sodium potassium pump2
Sodium-Potassium Pump

Why is the sodium-potassium pump considered to be an eletrongenic pump?

electrochemical gradient

An Electrochemical Gradient is a Concentration Gradient with Ions:

  • These ions want to move down their concentration gradient
  • These ions (particularly) also want to move towards the opposite charge found on the other side of the membrane
  • This attraction for the other side of membranes (membrane potential) can be harnessed to do work
  • Electrochemical gradients essentially are batteries, i.e., means of physically storing electrical energy
  • Proton pumps are used by plants, bacteria, and fungi to create electrochemical gradients
  • Sodium-potassium pumps are employed by animals for the same purpose
Electrochemical Gradient
movement across membranes1


  • Phagocytosis
  • Pinocytosis
  • Receptor mediated endocytosis
  • Exocytosis
  • These are mechanisms that involve movement into and out of the lumen of the endomembrane system
  • Not movement directly across membrane
  • That is, substances enter the Endomembrane System but not the Cytoplasm
Movement “Across” Membranes

Allows digestion in confined space

Taking up solids


Taking up fluids

Increases absorptive membrane area

receptor mediated endocytosis
Receptor-Mediated Endocytosis

A way of taking up specific substances