CELL TRANSPORTCELL TRANSPORT

How is a cell like .. a How is a cell like .. a cell?cell?

Both kinds of cells are designed to:

•Create compartments.

•Keep certain things inside the cell.

•Keep certain things from entering the cell.

•Control what gets in and out.

Let’s review Let’s review some some

terminology terminology from general from general

biology.biology.

Terms

• Solvent Solvent

• Solute Solute

• GradientGradient

• HydrophobicHydrophobic

• Diffusion Diffusion

• Osmosis Osmosis

• Solute – what is dissolved

• Solvent – what does the dissolving.

• What is the solute in saline (NaCl) solution?

• What is the solvent in a sugar water solution?

Gradient means difference.

• A concentration gradient is a difference in concentration of solutes across a membrane.

• If a substance can cross a membrane, it crosses from an area of high concentration to an area of low concentration. We say that the substance travels “down its concentration gradient.”

One of the functions of the cell and membrane-bound organelles is to create and

maintain gradientsgradients.

For example, mitochondria create

and maintain a concentration

difference in H+ (acidity) between the matrixmatrix and the intermembrane intermembrane

spacespace.

In fact, the HH++ gradient created inside mitochondria is used in making ATP.

Take AP biology is you want the details…

Organelles form and maintain concentration gradients concentration gradients

(differences) of many substances.

The cell membrane acts as a barrier. The cell membrane acts as a barrier. Membrane phospholipids form a Membrane phospholipids form a

hydrophobichydrophobic “wall.” “wall.”

Some substances that can’t cross the membrane. Some substances that can’t cross the membrane.

• Ions (Na+, K+, Cl-)

• Amino acids

• Sugars

• Large peptides and proteins, including peptide hormones like insulin.

Think. Think. If the cell membrane is hydrophobic how can water diffuse across it so easily?

A special note about A special note about waterwater

•The cell membrane poses a “hydrophobic” barrier to water.

•But water can cross cell membranes easily by diffusion.

•This is because special water “channels” called aquaporins aquaporins enable water to cross an otherwise impermeable barrier.

AquaporinsAquaporins are water channels. They allow water to freely enter and leave the

cell. One aquaporin aquaporin protein

molecule. Each one has four pores that

allow water to pass – but not ions and most

other solutes.

AquaporinsAquaporins are tiny proteins imbedded in the cell membrane – they are integral proteins. These pores allow waterwater to

diffuse down its concentration gradient.

DiffusionDiffusion• Diffusion is the effect of random motion random motion of

molecules in a fluid (liquid or gas.)

• It is a spontaneousspontaneous process. It is not “guided” or controlled. It happens on its own. It is energizedenergized by molecular motion.

• A short video showing diffusion of a dye in water.

http://video.google.com/videoplay?docid=-5242394503257451479

More diffusion stuffMore diffusion stuff

• The end-result of complete diffusion is a uniform composition uniform composition in the fluid.

• The net movement of diffusing particles is from a higher concentration to a lower a higher concentration to a lower concentration concentration until equilibriumequilibrium is reached. At equilibrium, the concentration of diffusing particles will be the same throughout the container.

Perfume diffuses from an area of high concentration (in the bottle) into the surrounding air. In a closed room, diffusion would continue until equilibriumequilibrium is reached – the distribution of perfume molecules is uniformuniform throughout the entire room.

DiffusionDiffusion occurs in fluid fluid matter – gases and

liquids.

A model of some of the possible paths a single sugar molecule in water might

take.

Diffusion is powered by the kinetic energy of molecules in a fluid --

random molecular motion.

ImpermeableImpermeable

A membrane is impermeable impermeable to a substance if that substance cannot

cross the membrane. Membranes may be permeable to some substances and

impermeable to others.

The French word for

raincoat is impermeablimpermeabl

ee. Why does this

make sense?

Boat Boat – impermeable to water?

HuntersHunters – impermeable to reason?

Selectively permeable membranesSelectively permeable membranes

• Allow some substances to cross more easily than others.

• This membrane is permeable to but impermeable to .

Sorry, folks. I detest the word semipermeable. The better term is

selectively permeable.

Why?

1. “Semi” means half. Which half?

2. Membranes are selectively permeable; they allow certain substances to pass, not others.

… and all membranes are permeable to

semis!

Think.• Do you have to have a

membrane to have diffusion?

• Do you have to have a membrane to have osmosis?

• Can Na+ undergo osmosis?

Cells compartmentalize water and solutes and move them around.

The two basic types of transport:

• 1. passive transportpassive transport – does not require special energy input from the cell membrane. Relies on diffusion.

• 2. active transportactive transport – requires energy input from the cell membrane. A “pump.”

Diffusion across a membrane

• NOTE: DiffusionDiffusion is random molecular motion powered by the kinetic energy in molecules. You do not have to have a membrane to have diffusion.

• OsmosisOsmosis, on the other hand, is diffusion of water across a selectively permeable membrane. By definition, you have to have a membrane in order to have diffusion.

Active transport

• Movement of molecule X against (“up”) against (“up”) its concentration gradientits concentration gradient – from low concentration of X to a higher concentration of X.

• This requires an input of energy. That energy is usually derived from ATP → ADP + Pi.

• We call the proteins on the cell membrane that carry out active transport pumps. pumps.

Why “pump”?Why “pump”?

A bicycle pump moves air from an area of low air concentration to an area of higher air concentration. It requires

energy input to make the pump work.

Cell pumps do the same.Cell pumps do the same.This happens in the kidney.

Active transporter pumps are proteins embedded in cell membranes. They are highly specific. There are Na+ pumps, H+

pumps, and Cl- pumps.

In kidney tubules, water diffuses from high water concentration to lower water concentration. Na+ ion is pumped from low [Na+] to higher [Na+].

• This lady from This lady from Nigeria is using a Nigeria is using a hand pump to obtain hand pump to obtain clear water. But in clear water. But in order for the pump order for the pump to work, she must to work, she must work! work!

• The energy provided The energy provided by her arms moves by her arms moves water “uphill.” water “uphill.”

An example: the Hthe H++ pump pump

Your stomach is very acidic. It has a high [H+]. The cells in the lining of your

stomach that make the acid have low [H+].

The HH++ pump pump in the stomach lining

The cells that secrete H+ into your stomach are working “up” a concentration gradient – from

low [H+] in the cell to high [H+] in the stomach.

The HH++ pump pump in the stomach lining

Since we are pumping H+ “up” a concentration gradient, this requires cell work. So the pump in

the membrane is an ATPaseATPase. It breaks down ATP into ADP. This is the energy source for the pump.

Nexium® – the “little purple pill” is a PROTON PUMP INHIBITOR. It inhibits

secretion of H+ into the stomach.

ThinkThink The interior (cytoplasm) of the cell has a high concentration of KK++. . The exterior (intercellular fluid) of the cell has a much lower concentration of KK++ ion. KK++ exitsexits the cell by:

A. active transport

B. diffusion

C. chance

ThinkThink The interior (cytoplasm) of the cell has a high concentration of KK++. . The exterior (intercellular fluid) of the cell has a much lower concentration of KK++ ion. KK++ entersenters the cell by:

A. active transport

B. diffusion

C. chance

Which of these diagrams shows activeactive transport?

Which show passive passive transport?

The NaNa++--KK++ pump pump will be important when we talk about transmission of nerve impulses.

• Na+-K+ ATPase pump actively transports K+ into the cell and Na+ out of the cell.

• The pump protein also functions as an ATPase enzyme that breaks down ATP to release energy to run the pump.

• Na+-K+ ATPase pump actively transports K+ into the cell and Na+ out of the cell.

• But notice that it pumps in only two K+ for every three Na+ that it expels from the cell. This creates a permanent negative polarity to the inner membrane of the cell.

An animation showing the action of the NaNa++--KK+ + ATPase pump ATPase pump protein in the cell membrane.

Cell transport can be classified as either uniport, symport, or

antiport.

OxygenOxygen isn’t pumped in cells. isn’t pumped in cells. OO22 diffuses “down diffuses “down

its concentration gradient” – from an area of high its concentration gradient” – from an area of high OO22 concentration to an area of lower O concentration to an area of lower O22

concentration. concentration.

Facilitated diffusion

We said that ions and large compounds can’t cross the membrane. They have to use a protein gate or channel to enter the cell. If a substance is traveling “down” its

concentration gradient, no special cell work is needed. It just diffuses – through

the gate.

Glucose is polar. It cannot diffuse through the membrane.

Glucose requires a special transporter protein to enter the cell. Therefore, glucose enters

the cell by facilitated diffusion. The hormone insulininsulin is part of the glucose transporter.

This is an example of facilitated diffusion.

Great words to live by…

Osmosis is the diffusion of water

across a selectively-permeable membrane.

Osmosis

Water diffuses from an area of high water concentration to an area of lower water concentration.

Osmosis – diffusion of water

• This membrane is not permeable to glucose; glucose can’t cross.

• The membrane is permeable to water.

• Which direction will the water flow?

Osmosis can generate a lot of force! (force/area = pressure)

But the source of energy for osmosis is the kinetic energy generated by random molecular motion in a fluid.

Osmotic pressure in roots can

be so great that it can break up

sidewalks!

If you need more review, try this animation.

http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Osmosis – flow of water across a selectively permeable membrane.

… and things always diffuse from an area of higher concentration to an area of lower concentration.

What is the source of energy that powers diffusion of gases and

liquids?

TermsTerms

•HypertonicHypertonic

• Isotonic Isotonic

•HypotonicHypotonic

What’s the concentration of water in a 0.9% saline (NaCl) solution?

If it’s 0.9% salt, it must be 99.1% water!

Think: Think: On the left side of a membrane permeable only to water, we have pure water. On the right side of the membrane, there is a 0.9% saline (salt) solution.

• A. Which side has the highest concentration of salt?

• B. Which side has the highest concentration of water?

• C. Which way will the water diffuse?

Pure water 0.9% salt solution

Hypertonic, isotonic, or hypotonic?

They’re relative terms.

• If two solutions are isotonic to one another, the concentration of water and non-diffusible solutes in both are the same.

• If solution A is hypertonic to solution B, solution A has a higher concentration of non-diffusible solute (and a lower concentration of water) than B.

• If solution A is hypotonic to solution B, solution A has a lower concentration of non-diffusible solute (and a higher concentration of water) than B.

Hypertonic or hypotonic?• Solution A: 100% water

• Solution B: 95% water, 5% glucose

• Solution C: 90% water, 10% glucose

Solution AA is __ to solution BB. Solution BB is __ to solution AA.

Solution AA is __ to solution CC. Solution CC is __ to solution AA.

Solution BB is __ to solution CC. Solution CC is __ to solution BB.

Net movement of water in and out of the cell by osmosis depends on the

concentration of solution it is in.

A nurse errs by giving a patient pure water in an IV. The patient has to undergo kidney dialysis because:

• A. the patient’s RBCs shrunk.

• B. the patient’s RBCs ruptured.

• C. the patient developed an allergy to water.

Osmosis: a quick-and-dirty summaryOsmosis: a quick-and-dirty summary

• Hypertonic solutions “draw” water out of hypotonic environments by osmosis.

• Cells take on water and swell when placed in a hypotonic environment (unless they have active transport “pump” mechanisms to counteract this effect).

• We are assuming that the solute (Na+, glucose, etc.) is unable to diffuse across the membrane, but that water can cross the membrane.

“Water, water everywhere, but not a drop to drink.” Why?

Actually, Coleridge says in the Rime of the Ancient Mariner “Water, water everywhere, nor any drop to drink.”

Exocytosis and endocytosis

ExocytosisExocytosis of a cell product (usually a protein like a hormone or enzyme). The product is enclosed in a membrane-bound vesicle. The vesicle fuses with the cell membrane and the product is expelled.

EndocytosisEndocytosis

• PhagocytosisPhagocytosis – engulfing of particulate matter (bacteria, fungal spores, dust particles, etc.) into a cell. The engulfed material is often degraded by lytic enzymes in lysosomes. “Cell eating”

• PinocytosisPinocytosis – consumption of liquids by the cell. “Cell drinking”

Phagocytosis of a bacterium by a WBC – a form of

endocytosis.

Phagocytosis – engulfing of a particulate solid particulate solid into the cell

Pinocytosis – engulfing of a liquid into the cell.

REVIEW

• Osmosis is the ____ of ____ across a ______ ______ membrane.

• What is the energy source for diffusion?

• How does facilitated diffusion differ from “simple” diffusion?

• Another word for gradient is ____.

• Diagram A shows simple ________, a passive process. K+ is flowing “down” its gradient from high [K+] in the cell to low [K+] in the extracellular fluid.

• Diagram B shows an example of _____ transport – the Na+-K+ ATPase “pump.”

Is this facilitated diffusion or active transport? How do you know?

What What bulk transport bulk transport process is shown process is shown here? here? (The purple spheres represent water.)

Artist’s conception of aquaporin aquaporin proteins in a cell membrane.