Chapter 6 Membranes

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Membranes

Chapter 6



Outline

• Phospholipid Bilayer

• Fluid Mosaic Model

• Cell Membrane Proteins

• Diffusion Facilitated Diffusion

• Osmosis

• Bulk Transport

• Active Transport

• Life of a Cell

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Phospholipid Bilayer

• Phospholipids have a glycerol backbone withtwo attached fatty acid chains.

One end is nonpolar (water-insoluble)while other end is polar (water-soluble).

- When placed in water, polar ends seekpartners for hydrogen bonding, andnonpolar tails pack together.

Lipid bilayer formed. Degree of fluidity determined by

degree of alignment.



Phospholipid Bilayer

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Structure of Cell Membrane

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Fluid Mosaic Model

• Mosaic of proteins float in fluid lipid bilayer.

Transmembrane Proteins

Network of Supporting Fibers

Exterior Proteins and Glycolipids

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Cell Membrane Proteins

Six Major Classes

Figure

6.8

p. 111



Structure of Membrane Proteins

• Anchoring Proteins - Attached to membrane

surface.

• Transmembrane Proteins

Anchors Channels and Carriers

Pores



Membrane Transport Processes

• Passive Processes

Require no input of cellular Energy Move molecules with/down a concentration gradient

- Diffusion

- Facilitated diffusion

-

Osmosis- Bulk Flow

• Active Processes

Require input of cellular Energy

Move molecules against/up or with/down a concentration

gradient faster - Active Transport

- Endocytosis

- Exocytosis



Diffusion

• Random motion causing net movement of

substances from regions of high concentration toregions of lower concentration.

Continues until equilibrium reached.

Diffusion

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Facilitated Diffusion

• Each polar molecule transported across the

plasma membrane has its own type of

channel.

Selective permeability

• Ions move across membrane in ion channels.

Direction of movement determined by

relative concentrations and voltage across

membrane.




• Carriers 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.




• Three Essential Characteristics:

Specific

Passive

Saturates






Osmosis

• In an aqueous solution, both water andsolutes diffuse down a concentrationgradient.

Most solutes are not lipid soluble and thus

cannot cross membrane. Water flows through aquaporins in

membrane.

- Net movement of water across amembrane is termed osmosis.

- Osmosis





Osmosis

• Osmotic Concentration - Concentration of all

solutes in a solution.

Hyperosmotic - Solution with higher

concentration.

Hypoosmotic - Solution with lower

concentration.

Isomotic - Osmotic concentrations of both

solutions are the same.



Osmosis

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Cell Transport





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



Bulk Passage

• Endocytosis - Plasma membrane extendsoutward and envelops food particles.

Phagocytosis - Particulate form.

Pinocytosis - Liquid form.

Receptor-Mediated Endocytosis - Moleculesfirst bind to specific receptors on plasmamembrane.

- Receptor Mediated • Excoytosis - Discharge of material from

vesicles at cell surface.

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Exocytosis

Endo / Exocytosis - McGraw Hill








Active Transport

• Proton Pump

• Moving 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.

• Sodium-Potassium Pump

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Coupled Transport

• Active transport that moves molecules with sodium

ions or protons moving down their concentrationgradients (Cotransport).

Establish the down gradient.

- Sodium-potassium or proton pump.

Transverse the upgradient.

Cotransport

- Symport

Countertransport- Antiport

CoTransport







Review

• Phospholipid Bilayer

• Fluid Mosaic Model

• Cell Membrane Proteins

•

Diffusion Facilitated Diffusion

• Osmosis

• Bulk Transport• Active Transport



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Chapter 6 Membranes