Transport of Substances

What is a lipid bilayer?

A barrier for movement of water and water-soluble molecules

• lipid-soluble substances CAN pass through

What are the types of protein molecules in the lipid bilayer? (4)

• Transport Proteins

  • Ion Channel

  • Carrier Proteins

• Other

  • Receptors

  • Enzymes

Ion Channel

Pores that allow ions to move in/out of cells

Carrier Proteins

Proteins that bind to one side and move to the other

• Some require energy others do not

Diffusion

Random movement of molecules where no energy is required

Simple Diffusion

Random movement through membrane or through channel protein or pore

Facilitated Diffusion

Movement through a membrane that requires interaction of a carrier protein to aide passage

In simple diffusion, how do lipid-soluble molecules cross the membrane?

They move readily across the membrane (no pore/carrier needed)

What are examples of lipid-soluble molecules? (4)

• Oxygen

• Carbon Dioxide

• Alcohol

• Steroid Hormones

In simple diffusion, how do water-soluble ions diffuse across the membrane?

Via water filled channels or pores

Aquaporin

Water channel where water continually diffuses

How do ion channels work?

They are selectively permeable to certain ions based on hydration, size, and charge

Ungated Channels

Allows ions to diffuse either way all the time

• aka “leak channels”

• ALWAYS open

Why are some channels “gated”?

It controls the permeability of the membrane to an ion

Voltage-Gated Channels

Open or close in response to a voltage change across the membrane

True or False: All voltage-gated channels are only voltage dependent.

False. They are also ion specific

What is the resting membrane potential of all cardiac and skeletal muscle?

-90mV

Chemical (Ligand) Gated Channel

Open or close in response to binding of a chemical

True or False: All chemical gated channels are ion and chemical specific.

TRUE

Give an example of a chemical gated channel and explain how it works.

• Nicotinic Acetylcholine Receptor Channels

When no acetylcholine is in the extracellular fluid, the channel is closed. When acetylcholine is available and binds to a nicotinic receptor, it allows the channel to open and sodium to pass through into the cell.

True or False: Ions move through channels through simple diffusion.

TRUE

Facilitated Diffusion

Required interaction of transported molecule with a carrier protein which aides passage

• NO energy required

Net Diffusion

Average direction of diffusion

What factors affect the net rate of diffusion?

• Concentration Gradient

When will the net diffusion of positively charged molecules stop?

When the electrical potential across the membrane is equal in magnitude but opposite in direction to the force of the concentration gradient.

• So positive in the cell that they start to repel each other

What does the Nernst equation tell us?

The voltage needed for net diffusion to stop

Osmosis

Net diffusion of water

What is the most abundant substance to diffuse across the cell membrane?

Water

Osmotic Pressure

The amount of pressure required to counter osmosis

What causes osmotic pressure?

Concentration of impermeable particles in a solution

• higher the concentration = higher the osmotic pressure

True or False: Each particle in a solution does not exert the same amount of pressure against the membrane.

False. They exert the same amount of pressure regardless of its mass.

Osmolality

Number of osmotic particles/volume

True or False: In ionic compounds, 1mM = 2mOsmoles/L

TRUE.

• EX: NaCl dissociates in water to Na+ and Cl- (slide 27)

What is the extracellular and intracellular osmolarity?

300 mOsmoles/L

True or False: Normal Saline has the same osmolarity as the extracellular and intracellular fluid?

TRUE

What will happen to a RBC in Normal Saline?

Nothing. No osmosis.

What will happen to an RBC in water?

Fluid will rush in and burst the cell. (Water = 0mOs/L)

WHat will happen to a RBC in NaCl (500mOs/L)?

Fluid will diffuse out and the cell will shrivel.

• Same concept if plasma is dehydrated.

Primary Active Transport

Molecules are “pumped” against a concentration gradient at the expense of energy (ATP)

Secondary Active Transport

Transport is driven by the energy stores in the concentration gradient of another molecule

What are examples of primary active transport? (3)

1. Na+-K+ATPase pumps sodium out of cells and potassium into the cell against their concentration gradient. (3Na+ out and 2K+ in)

2. Calcium Pumps - cell membrane pumps calcium out of cell into Sarcoplasmic reticulum and mitochondria

3. Hydrogen Ion Pumps - maintains pH in GI by gastric glands (parietal cells) secreting H+

Secondary Active Transport

Uses energy set up by Na-K-ATPase to transport another molecule

Examples of Secondary Active Transport (2)

1. Co-transport: substance (glucose) is transported in the same direction as the “driver” ion (Na+)

2. Counter-Transport: substance (Ca2+) is transported in the opposite direction of the “driver” ion (Na+)