extracellular fluid (outside)

carbohydrate

phospholipidcholesterol

binding site

phospholipid bilayer

recognition protein

receptorprotein transport

proteinprotein filaments

cytoplasm (inside)

Membrane Proteins

Fig. 3-1

Phospholipid bilayer blocks passage to most molecules

• Polar H20 soluble: Salts, AA, sugars• Pass freely: Small molecules: H20, uncharged

lipid soluble molecules

3.3 How Does The Plasma Membrane Play Its Gatekeeper

Role?• Membrane proteins– Transport proteins: allow the movement of water-

soluble molecules through the plasma membrane by forming channels or by carrying them across

– Receptor proteins: possess a binding site on the outer surface for binding specific chemicals that may alter overall cell function

– Recognition proteins: with sugar groups attached to the exterior of the cell, are used by the immune system to identify cells as belonging to “self”

3.4 What Is Diffusion?• Characteristics of molecules in fluids

– Concentration: the number of molecules in a given unit of volume

– Gradient: a physical difference (concentration, pressure, electrical charge) between two regions of space that causes molecules to move from one region to another

3.4 What Is Diffusion?• Molecules in fluids move in response to

gradients.– Diffusion: the movement of molecules from regions

of high molecular concentration to low molecular concentration

• A drop of dye in water illustrates diffusion.

3.4 What Is Diffusion?• Diffusion of a dye in water

Fig. 3-4

watermolecule

drop of dye

A drop of dye isplaced in water

The dye moleculesdiffuse into the water;the water moleculesdiffuse into the dye

Both dye moleculesand water molecules areevenly dispersed

23

1

3.4 What Is Diffusion?• Summing up: the principles of diffusion

– Diffusion is the movement of molecules down a gradient from high concentration to low concentration.

– The greater the concentration gradient, the faster the rate of diffusion.

– If no other processes intervene, diffusion will continue until the concentration gradient is eliminated.

3.5 What Is Osmosis?• Osmosis: the diffusion of water molecules from

a high water concentration to a low water concentration across a biological membrane

• Pure water has the highest water concentration.• The addition of dissolved solutes to pure water

reduces the number of water molecules and thus lowers the water concentration.

3.5 What Is Osmosis?• Osmotic water flow across a membrane takes

place across selectively permeable membranes that allow water to pass, but not certain small impermeable molecules, such as sugars.

selectivelypermeablemembrane

pore

water

sugarWater molecule:can fit throughthe pore

Sugar with water moleculesclustered around it: cannotfit through the pore

3.5 What Is Osmosis?• A selectively permeable membrane

Fig. 3-5

3.5 What Is Osmosis?• Example of osmosis: a water-permeable bag

that is impermeable to sugar and has sugar inside of it– Water flows into the bag, down a water

concentration gradient.– The bag swells and eventually bursts from the

additional water.

3.5 What Is Osmosis?• Osmosis

Fig. 3-6

Bagbursts

selectivelypermeablemembrane

sugarmolecule

watermolecule

Waterflows in

3.5 What Is Osmosis?Animation—OsmosisPLAY

3.5 What Is Osmosis?• Summing up: the principles of osmosis

– Osmosis is the diffusion of water across a selectively permeable membrane.

– Dissolved substances reduce the concentration of water molecules in a solution.

– Water moves across a membrane down its concentration gradient from a high concentration of water molecules to a low concentration of water molecules.

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?

• There are two kinds of transport across cell membranes:– Passive transport– Energy-requiring transport

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?

• Passive transport: movement of molecules across a membrane, down a concentration gradient, without the use of energy– The phospholipid bilayer and transport proteins

regulate which molecules can cross the membrane down concentration gradients.

– Membranes are selectively permeable, and only allow some molecules to cross and not others.

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?

• Examples of passive transport– Simple diffusion: the transfer of gases, water, and

lipid-soluble substances—such as ethyl alcohol and vitamin A—across the phospholipid bilayer