Membranes Structure and Function
Chapter 5
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Plasma Membrane Consists of:
•
•
* nonpolar lipid bilayer impedes the passage
of water soluble substances
* degree of fluidity is determined by degree
of alignment of phospholipids.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Plasma Membrane Consists of:
1. Lipid bilayer
•Phospholipids have a glycerol backbone with two
attached fatty acid chains.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Plasma Membrane is:
1. Selectively Permeable – meaning that it allows
substances to cross the cell membrane passively
Examples: small non-charged molecules, CO2,
O2, glycerol, and alcohol.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
2. Proteins
•
* allows for passage of substances
Mosaic of proteins float in fluid lipid bilayer.
 Transmembrane Proteins
 Network of Interior Proteins
 Cell Surface markers
Copyright © McGraw-Hill Companies
Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Cell Membrane Proteins
•
Six kinds of Membrane Proteins
 Transporters – channels or carriers
 Enzymes – rxns. occur in interior of cell
 Cell Surface Receptors – detect chem.
messengers
 Cell Surface Identity Markers – id cell
types
 Cell Adhesion Proteins – connect cells
 Cytoskeleton Attachments
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
How are proteins anchored on the cell
membrane?
-
•
•
Proteins contain polar and nonpolar regions
that allow them to be anchored.
Anchoring Proteins - Attached to membrane
surface.
Transmembrane Proteins
 Anchor proteins
 Channels and Carriers
 Pores
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Passive Transport:
•
•
•
Diffusion
Facilitated Diffusion
Osmosis
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Diffusion
•
Random movement of substances from
regions of high concentration to regions of
lower concentration.(with the [gradient])
 Continues until equilibrium reached.
Copyright © McGraw-Hill Companies Permission
required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Facilitated Diffusion
•
•
Each polar molecule transported across the
plasma membrane has its own type of channel
protein.
 Selective permeability
Ions move across membrane in ion channels.
 Direction of movement determined by
relative concentrations and voltage across
membrane.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Facilitated Diffusion
•
Carrier proteins facilitate movement of solutes
across membrane by physically binding to
them on one side of the membrane and
releasing them on the other.
 Driven by concentration gradients.
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Facilitated Diffusion
•
Three Essential Characteristics:
 Specific
 Passive
 Saturates
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Osmosis
•
In an aqueous solution, both water and
solutes diffuse down a concentration
gradient.
 Most solutes are not lipid soluble and thus
cannot cross membrane.
 Water flows through aquaporins in
membrane.
- Net movement of water across a
membrane is termed osmosis.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Osmosis
•
Osmotic Concentration - Concentration of all
solutes in a solution.
 Hyperosmotic (Hypertonic)- Solution with
higher concentration of solutes.
 Hypoosmotic (Hypotonic)- Solution with
lower concentration of solutes.
 Isosmotic (Isotonic)- Osmotic
concentrations of both solutions are the
same.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Osmosis
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Fig. 6.15
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Osmotic Pressure
•
•
•
Hydrostatic Pressure is caused by cytoplasm
pushing out against cell membrane.
Osmotic Pressure is the pressure necessary
to stop osmotic movement of water across a
membrane.
Mechanisms For Maintaining Osmotic Balance
 Extrusion
 Isomotic Solutions
 Turgor
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Fig. 6.16
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Bulk Transport:
1. Endocytosis - Plasma membrane extends
outward and envelops food particles.
•Phagocytosis - “Cell eating” Particulate
form – discrete particles.
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Pinocytosis - “Cell drinking” Liquid form.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Receptor-Mediated Endocytosis - Molecules
first bind to specific receptors on plasma
membrane before being engulfed
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
2 Exocytosis - Discharge of material from
vesicles at cell surface; mechanism for secreting
many hormones, digestive enzymes, etc.
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Active Transport:
•
•
Movement of substances across a membrane
against their concentration gradients.
 Requires input of energy.
 Enables cell to take up additional molecules
of a substance already present in its
cytoplasm in concentrations higher than in
extracellular fluid.
Example: Sodium-Potassium Pump
moves 3 Na+ out and 2 K+ in
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Coupled Transport
•
Active transport that moves molecules
coupled with sodium ions or protons down
their concentration gradients
(Cotransport).
 Establish the down gradient.
- Sodium-potassium or proton pump.
 Trasverse the upgradient.
 Cotransport
- Symport – ex. Na+ and sugar/a.a.
 Countertransport
- Antiport – ex. Na+ inward and Ca++
outward
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Intercellular Adhesion
•
Cell clusters form long-lasting or permanent
connections with each other (cell junctions).
 The nature of the connections determines
what the tissue is like.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Tight Junctions
•
Tight junctions connect plasma membranes
of adjacent cells in a sheet.
 Prevent small molecules from leaking
between cells and through the sheet.
 Effectively segregate proteins on opposite
sides of the sheet.
 Fuse like a zipper
 Ex. Prevent digestive enzymes from
seeping into blood
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Anchoring Junctions
•
Anchoring junctions mechanically attach
cytoskeleton of one cell to cytoskeleton of
other cells, or to the extracellular matrix.
 Desmosomes - Connect cytoskeletons of
adjacent cells; prevent cells subjected to
mechanical stress from pulling apart; they
are buttonlike thickenings of plasma
membrane
 Hemidesmosomes - Anchor epithelial cells
to basement membrane.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Desmosomes
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Anchoring Junctions
Cadherins - Connect actin frameworks of
cells in cadherin-mediated junctions.
 Adherens - Connect actin filaments of one
cell with those of neighboring cells or with
extracellular matrix.
- Integrin proteins

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Communicating Junctions
•
•
•
Communication junctions pass chemical
signals directly from one cell to adjacent cells.
 Establish direct physical connections.
Animals
 Gap Junctions
- Connexons.
Plants
 Plasmodesmata
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
The End
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies