Biology 107Biology 107Cellular MembranesCellular Membranes

September 20, 2004September 20, 2004

Cells are Cells are Microscopic Microscopic StructuresStructures

All cells are delimited by a All cells are delimited by a membranemembrane

The size of cells is limited by The size of cells is limited by the surface area of the surface area of plasma plasma membranemembrane available for available for transport of materials into transport of materials into and out of the celland out of the cell

Phospholipids are Amphipathic Phospholipids are Amphipathic MoleculesMolecules

The Nature of The Nature of Phospholipid Phospholipid

Molecules Directs Molecules Directs the Spontaneous the Spontaneous

Assembly of Assembly of Bilayers in Bilayers in Aqueous Aqueous

EnvironmentsEnvironments

Polar (hydrophilic) head Polar (hydrophilic) head regionregion

Non-polar (hydrophobic) Non-polar (hydrophobic) tail regiontail region

Lipid BilayerLipid Bilayer

The Cellular Membrane Bilayer The Cellular Membrane Bilayer is Associated with Proteinsis Associated with Proteins

Computer Model of Lipid BilayerComputer Model of Lipid Bilayer

Functions of Functions of Membrane Membrane

ProteinsProteins

Transport – Active or passiveTransport – Active or passive

EnzymaticEnzymatic

Signal transductionSignal transduction

Cell-cell recognitionCell-cell recognition

Cell-cell adhesionCell-cell adhesion

Cell-matrix adhesionCell-matrix adhesion

Transmembrane ProteinTransmembrane Protein

Example of Transmembrane Example of Transmembrane ProteinProtein

Movement Across MembranesMovement Across Membranes

PassivePassive – driving force is diffusion gradient. May – driving force is diffusion gradient. May use membrane protein(s) to facilitate movement use membrane protein(s) to facilitate movement through bilayer, but some molecules (e.g., through bilayer, but some molecules (e.g., water)water) cross membranes unaided by proteins.cross membranes unaided by proteins.

ActiveActive – driving force is cellular energy supply (e.g., – driving force is cellular energy supply (e.g., ATP). Uses membrane proteins.ATP). Uses membrane proteins.

Transport Across MembranesTransport Across Membranes

Osmosis Across Cell MembranesOsmosis Across Cell Membranes

Experiment that Shows the Experiment that Shows the Fluid Nature of MembranesFluid Nature of Membranes

Fluid Nature of Fluid Nature of MembranesMembranes

Phospholipids not covalently Phospholipids not covalently linked togetherlinked together

Lipid composition (e.g., Lipid composition (e.g., cholesterol amount/location; cholesterol amount/location; degree of saturation of fatty degree of saturation of fatty acids) affects the degree of acids) affects the degree of fluidity and “flexibility” of fluidity and “flexibility” of membranesmembranes

Lipid “sea” forms the Lipid “sea” forms the environment in which environment in which membrane proteins may be membrane proteins may be embeddedembedded

Fluid Mosaic Model of Fluid Mosaic Model of MembranesMembranes

Cellular MembranesCellular MembranesStudent Objectives:Student Objectives: As a result of this lecture and the As a result of this lecture and the

assigned reading, you should understand the following:assigned reading, you should understand the following:

1.1. Membranes confine the contents of cells.  Membranes confine the contents of cells.  

2.2. Membranes provide the structural basis for metabolic order in Membranes provide the structural basis for metabolic order in organisms.  They partition and compartmentalize functions, such organisms.  They partition and compartmentalize functions, such as different enzyme systems.  Membranes also form the physical as different enzyme systems.  Membranes also form the physical matrix on which enzymes may be positioned.matrix on which enzymes may be positioned.

3.3. Membranes are Membranes are selectively permeableselectively permeable. Control of exchanges necessary to: 1) protect the cell's integrity; 2) maintain the conditions at which optimal metabolic activities take place; and 3) coordinate the activities of different cells in a multicellular association.

Cellular MembranesCellular Membranes

4.4. The main structural components of membranes are lipids, mainly The main structural components of membranes are lipids, mainly phospholipidsphospholipids containing a hydrophilic polar head region and a containing a hydrophilic polar head region and a hydrophobic non-polar tail region, that spontaneously form stable hydrophobic non-polar tail region, that spontaneously form stable bilayersbilayers..

5. Much of the selective permeability of membranes depends upon the proteins present.

6. The two mechanisms of crossing a membrane are: 1) passive transport (e.g., diffusion) or 2) active transport.

Cellular MembranesCellular Membranes

7. Membrane proteins may be peripheral proteins or integral proteins depending upon the degree of interaction with the membrane. Integral membrane proteins often span the thickness of the bilayer.

8. The model that best explains the organization of membranes is the fluid mosaic model.

Fluid Mosaic Plasma Membrane as Fluid Mosaic Plasma Membrane as an Interface Between Intracellular an Interface Between Intracellular and Extracellular Environmentsand Extracellular Environments