NOTES FOR BIOLOGY 1201

Section 001

Spring 2005

DR. STEVEN POMARICO

Membranes: Basic Structure - 3.1.4

>>>>>Membrane structural models have evolved as new data is acquired.

Earliest models were deduced from indirect evidence

1. Membranes are made of lipid

2. Phospholipids are amphipathic molecules that can form membranes

---amphipathic

3. Cell membranes are phospholipid bilayers

4. Biological membranes contain proteins

5. Biological membranes are coated with proteins that generally absorb water.

Davson-Danieli model

         -phospholipid (PL) bilayer

         -globular protein coating

         -Hydrophilic zones = polar head groups + globular proteins

         -Hydrophobic zone = 2 x hydrophobic tails

         -about 8 mm thick

Confirmed in 1950's

Problems with this model:

         -all membranes are not the same

         -How do the proteins stay attached?

Singer and Nicolson: FLUID-MOSAIC MODEL

         -proteins embedded in the lipid bilayer

         -Hydrophilic zones = polar head groups + hydrophilic portion of proteins

         -Hydrophobic zone = 2 x hydrophobic tails + hydrophobic portion of proteins

Confirmed by freeze-fracture and freeze-etch micrographs

---Fluid

---Mosaic

The Plasma Membrane: The Fluid Mosaic Model - 3.4.2

>>>>>A membrane is a fluid mosaic of lipids, proteins, and carbohydrates.

Membranes as a fluid

Fluidity is two-dimensional (not three)

Factors which influence fluidity of membranes

                   -lipid composition

                   -temperature

Proteins as the Mosaic of the Cell Membrane - 3.4.3

Membranes as a Mosaic

A mosaic of different proteins are embedded in the lipid bilayer. These proteins have two types of spacial arrangements.

         1. integral proteins

                   Some functions of integral membrane proteins

                             -transport proteins

                             -enzymes

                             -receptors - signal transduction

                             -cell-to-cell junctions

                             -cell-to-cell recognition

                             -attachment of cytoskeleton to extracellular matrix (ECM)

         2. peripheral proteins

Membranes have a bifacial orientation (i.e. sidedness)

         -different lipid composition

         -different proteins or protein orientation

         -carbohydrate (always outside)

                             What’s outside on the inside of the cell?

Simple and Facilitated Diffusion - 3.5.1

>>>>>Membranes are built to be selectively permeable.

---Selective permeability

         Two factors which influence permeability

                   -solubility characteristics of the substance crossing the membrane

                   -presence of transport proteins

Permeability of the bilayer

         Nonpolar (hydrophobic) molecules

         Polar (hydrophilic) molecules

---Transport proteins

>>>>>Passive transport (i.e., diffusion) across a membrane

---Concentration gradient

---Net directional movement

---Diffusion

         -caused by thermal motion

         -movement of molecules is random, but net movement is directional

         -net movement continues until the system reaches a dynamic equilibrium

         -spontaneous (-ΔG)

Much of the movement of substances across membranes occurs by diffusion and therefore is a form of passive transport.

---Passive transport

Passive Transport: Osmosis - 3.5.2

>>>>>>Osmosis is the diffusion of water

---Hypertonic solutions

Hypertonic (a.k.a. Hyper-osmotic) solutions have greater solute concentration and therefore a lower water concentration.

---Hypotonic solutions

Hypotonic (a.k.a. Hypo-osmotic) solutions have lower solute concentration and therefore a higher water concentration.

---Isotonic solutions

Isotonic (a.k.a. Iso-osmotic) solutions have the same solute concentration and equal water concentrations.

---Osmosis

         -Osmosis is direction determined by total solute concentration

         -Influenced by factors which govern diffusion

>>>>>Cellular survival depends on balancing water movement

Cells without walls

         Cells without cell walls are not tolerant to excessive water movements.

Three water balancing scenarios

         1. Hypertonic environment

                   Water moves out of the cell and the cell crenates or shrivels

         2. Isotonic environment

                   No net movement of water, and the cell volume remains stable.

         3. Hypotonic environment

                   Water moves into the cell. the cell swells and eventually lyses or bursts.

---Osmoregulation

Cells with walls

         Cells with cell walls are more tolerant to excessive water movements, but still exhibit cellular changes.

Three water balancing scenarios

         1. Hypertonic environment

                   Water moves out of the cell and the cell undergoes plasmolysis.

---Plasmolysis

         2. Isotonic environment

                   No net movement of water, and the cell becomes flaccid or limp.

---flaccid

         3. Hypotonic environment

Water moves into the cell. the cell swells until the internal pressure equals

                             the osmotic pressure and the cell becomes turgid.

---Turgid

>>>>>>Specific proteins facilitate diffusion of selected solutes

---Facilitated diffusion (

Still diffusion but with a twist.

         -Involves transport proteins

                   Three types

                             -Bind-and-release

                             -Selective channel

                             -Gated channel

Active Transport: Ion Pumps and Cotransport - 3.5.3

Active Transport: The Sodium-Potassium Pump - 3.5.4

>>>>>>Active transport of solutes against a concentration gradient.

---Active transport

         -Energy requiring

         -Used to maintain ion gradients (e.g., sodium-potassium pump) 8.15)

>>>>>Ion pumps can generate voltage across membranes.

Cell that have an unequal distribution of ions across their plasma membrane have a membrane potential.

---Membrane potential

May act to drive diffusion of ions across membrane by creating an electrochemical gradient.

---Electrochemical gradient

The sodium-potassium pump (Na⁺/K⁺ ATPase pump) in animals is an electrogenic pump that translocates 3 Na⁺ ions out of the cell, for every 2 K⁺ ions into the cell.

---Electrogenic pump

The proton pump is another of the main electrogenic pumps in biological systems (bacteria, fungi, plants, chloroplasts, mitochondria).

>>>>>In cotransport, a membrane protein couples the transport of one solute to the transport of a different solute

---Cotransport

May be a symporter (both solutes heading in the same direction) or an antiporter (solutes heading in opposite directions)

Energy-Requiring Transport: Endocytosis and Exocytosis - 3.5.5

>>>>>Exocytosis and endocytosis, the transport of large molecules

---Exocytosis

---Endocytosis

Three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis

---Phagocytosis

---Pinocytosis

---Receptor-mediated endocytosis

         -Occurs in progressive stages

         -Involves ligand-receptor interaction.

---Ligand