Cell MembraneIn This Lesson:

Cell Membranes and Transport

https://www.youtube.com/watch?v=dPKvHrD1eS4&safe=active

Cell Membrane Function Overview

• Cells must take in and release substances:– Food in, products and waste out.

• They can do it with one of two general modes:– Passive Transport (does not require energy)

• Diffusion• Facilitated Diffusion• Osmosis

– Active Transport (requires energy)• Endocytosis• Exocytosis• Molecular Transport

• To fully understand these, we need to understand concentration gradient.

Concentration Gradient

• Concentration refers to the amount of a substance in a certain area.

• Particles diffuse down their concentration gradient.– What does that mean?

• In passive transport, particles always go from an area of high concentration to an area of low concentration.

Concentration Gradient

HighConcentration

LowConcentration

Concentration Gradient

In Passive Transport, particles move from areas of high concentration to areas of low concentration.Substance

5

Simple Diffusion

• Requires NO energy

• Molecules move from area of HIGH to LOW concentration

6

Diffusion of Liquids

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Diffusion through a Membrane

Cell membrane

Solute moves DOWN concentratio (HIGH to LOW)

What can diffuse?

• Can diffuse:– Lipids– CO2

– O2

• Can’t diffuse:– H2O and other polar molecules– Ions and other charged particles– Large molecules (like starches and proteins)

Diffusionnot all substances can pass through a cell membrane.

Facilitated Diffusion

• Simply put, it’s diffusion with help.

• Those particles that can’t diffuse can get through channel proteins.

• No energy needed.• This leads to semi-

permeability for molecules that can’t otherwise diffuse.– There are specific channels

for specific molecules, too.

HIGH

LOW

inside cell

outside cell

sugaraaH2O

saltNH3

Summary of Passive Transport

Osmosis

• Osmosis is basically the same thing as diffusion, only with water molecules and some form of a barrier.– Osmosis is another form of passive transport.

• Just like in diffusion, in osmosis, water moves from areas of high water concentration to low water concentration.

• Or, water moves from areas of low solute concentration to areas of high solute concentration.

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Diffusion of H2O Across A Membrane

High H2O potentialLow solute concentration

Low H2O potentialHigh solute concentration

• At the beginning of this PowerPoint I said that polar substances like H2O can’t diffuse into the cell through the membrane, and now osmosis is like water diffusion. How could that be?”

• “For a while scientists noticed the same thing. Water clearly efficiently enters a cell, but how?”

Aquaporins

• Aquaporins are channel proteins that move water rapidly into the cell through facilitated diffusion.– They were discovered by these two in 1991.– They shared the 2003 Nobel Prize in

Chemistry.

Peter AgreJohns Hopkins

Roderick MacKinnon

Rockefeller

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Aquaporins• Water Channels• Protein pores used during

OSMOSISWATERMOLECULES

Tonicity• Hypertonic solution– Relatively more solute outside the cell than inside.– Relatively more free water inside the cell.

• Hypotonic solution– Relatively less solute outside the cell.– Relatively more free water outside the cell.

• Isotonic solution– The same amount of solute inside and outside of

the cell.• No net water change.

Osmosis in Plant Cells

• As we have learned, plant cells are good at holding water.

• If they’re placed in a hypertonic solution, however, they lose water and wilt.– Their cells undergo plasmolysis.

• Place them in a hypotonic solution and they will swell slightly, like a garden hose with water.– Their cells become turgid.– In animal cells, without a cell wall, the cell may burst

in a process called cytolysis.

Osmosis – The Big Idea

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Cytolysis & Crenation

Cytolysis Crenation

Managing Water Balance

• Animals:– Kidneys.– Methods to either remove

salt or pump in water.• Unicellular Organisms:– Contractile Vacuoles

• Pump water out at a cost of ATP (energy).

• Maintaining water balance is just another aspect of homeostasis.

Osmosis Supplements

• Woman Dies After Water Drinking Contest

Equilibrium

• For things like diffusion and osmosis, eventually the solutes reach a point where there is no net change in molecule movement.– This is equilibrium.

• We call it “dynamic equilibrium” because the molecules are still moving, but there is no net change in concentration or movement.

Equilibrium

• When dynamic equilibrium is reached, diffusion and osmosis stop.– Molecular motion continues, though.

1.0% Sugar

0.50%Sugar

0.75%Sugar

0.75%Sugar

WATER

WATERWATER

WATER

Net Water Flow Inward No Net Water Flow

Osmosis Practice Problem SAMPLE

• Suppose a human blood cell (saline concentration 0.9%) is sitting in a beaker of 2% NaCl. Will it shrink, expand, or remain unchanged?– Make a sketch!

0.9%2%

Hyper

Hypo

The blood cell will shrink.

Osmosis Practice Problem #1

• If you soak your hands in dishwater, you may notice that your skin absorbs water and swells into wrinkles. This is because your skin cells are _______________ to the _______________ dishwater.

A. hypotonic…hypertonicB. hypertonic…hypotonicC. hypotonic…hypotonicD. isotonic…hypotonicE. hypertonic…isotonic

Osmosis Practice Problem #1

• If you soak your hands in dishwater, you may notice that your skin absorbs water and swells into wrinkles. This is because your skin cells are _______________ to the _______________ dishwater.

A. hypotonic…hypertonicB. hypertonic…hypotonicC. hypotonic…hypotonicD. isotonic…hypotonicE. hypertonic…isotonic

Osmosis Practice Problem #2

• You decide to buy a new fish for your freshwater aquarium. When you introduce the fish into its new tank, the fish swells up and dies. You later learn that it was a fish from the ocean.

Osmosis Practice Problem #2

• Based on what you know of tonicity, the most likely explanation is that the unfortunate fish went from a(n) _______________ solution into a(n) _______________ solution.A. isotonic…hypotonicB. hypertonic…isotonicC. hypotonic…hypertonicD. hypotonic…isotonicE. isotonic…hypertonic

Osmosis Practice Problem #2

• Based on what you know of tonicity, the most likely explanation is that the unfortunate fish went from a(n) _______________ solution into a(n) _______________ solution.A. isotonic…hypotonicB. hypertonic…isotonicC. hypotonic…hypertonicD. hypotonic…isotonicE. isotonic…hypertonic

Osmosis Practice Problem #3

• In osmosis, water always moves toward the ____ solution: that is, toward the solution with the ____ solute concentration.A. isotonic…greaterB. hypertonic…greaterC. hypertonic…lesserD. hypotonic…greaterE. hypotonic…lesser

Osmosis Practice Problem #3

• In osmosis, water always moves toward the ____ solution: that is, toward the solution with the ____ solute concentration.A. isotonic…greaterB. hypertonic…greaterC. hypertonic…lesserD. hypotonic…greaterE. hypotonic…lesser

Osmosis Practice Problem #4

• The concentration of solutes in a red blood cell is about 2%. Sucrose cannot pass through the membrane, but water and urea can. Osmosis would cause red blood cells to shrink the most when immersed in which of the following solutions?A. a hypertonic sucrose solutionB. a hypotonic sucrose solutionC. a hypertonic urea solutionD. a hypotonic urea solutionE. pure water

Osmosis Practice Problem #4

• The concentration of solutes in a red blood cell is about 2%. Sucrose cannot pass through the membrane, but water and urea can. Osmosis would cause red blood cells to shrink the most when immersed in which of the following solutions?A. a hypertonic sucrose solutionB. a hypotonic sucrose solutionC. a hypertonic urea solutionD. a hypotonic urea solutionE. pure water

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Three Forms of Transport Across the Membrane

copyright cmassengale

Active Transport

• What happens when a cell gets greedy?– What I mean is, what happens when a cell has

within it a higher concentration of a certain molecule than is present outside the cell, yet still wants more?

• This is where active transport comes in – we’re going to need to expend a little energy to get what we want.

Concentration Gradient

HighConcentration

LowConcentration Concentra

tion Gradient

Active Transport

Sub

stan

ce ENERGYNEEDED!

Quick Note: Transport Proteins

• I’ve been mentioning transport proteins quite loosely this whole lesson. Here’s something concrete about them:

• Channel proteins are basically just tunnels for polar stuff to diffuse in/out. They’re simple.

• Carrier proteins are a bit slower, but they allow for active transport and the movement of nonpolar stuff.– They also typically undergo shape changes to do their

work.– They’re usually glycoproteins.

Channel vs. Carrier

Channel ProteinCarrier Protein

Facilitated Diffusion

Back to Active Transport

• Active transport costs ATP to move molecules against their concentration gradient.

• Proteins in the membrane that do this undergo a conformational change in the process:

Types of Active Transport

1. Protein Pumps -transport proteins that require energy to do work• Example: Sodium /

Potassium Pumps are important in nerve responses.

Sodium Potassium Pumps (Active Transport using proteins)

Protein changes shape to move molecules: this requires energy!

Active Transport: Pumps

Active Transport: Three Forms#toomanynotes

• Exocytosis – Removing stuff from the cell.• Endocytosis – Bringing stuff into the cell.– Phagocytosis – “Cell Eating” – when a cell engulfs a large

particle/other cell. The vesicle fuses with the lysosome.• Amoeba Eats Two Paramecia video

– Pinocytosis – “Cell Drinking” – a continuous intake of small dissolved particles in the nearby solution.

– Receptor-Mediated Endocytosis – Pinocytosis except the cell is bringing in particles that have bonded to receptors on the outside of the membrane.

• Molecular Transport – a general term for using protein pump-like structures embedded in the membrane.

Exocytosis

• Easy one:

Endocytosis

PhagocytosisWill fuse with lysosome for

digestion.

Pinocytosis Non-specific process.

Receptor-Mediated

Endocytosis

Triggered by receptors outside the cell.Helps for low-concentration

“targets.”

Cell Transport Summary

Closure

• So what’s the point of the cell membrane?– By now you should have lots of answers.

• Perhaps one we haven’t discussed enough is homeostasis.

• All these membrane functions, all these pumps and structures…they all serve to help regulate the cell’s “chemical balance.”

• As you know, homeostasis is “maintaining internal balance,” and you should now be able to see how well the cell membrane can do that.

Active Transport Classic Example• The Sodium-Potassium Pump (neurons)

Sodium-Potassium Pump

• We’re going to look at this one in greater detail in the nervous system unit as it’s very important to know.– Fun fact: The mere fact that you’re reading these

words means many of your cells are using this pump right now.

Sodium-Potassium Pump1. Three sodium ions inside the cell bind to a carrier

protein.2. ATP causes a conformational change in the protein,

releasing the Na+ ions to the ECM.3. Two potassium ions then bind to the carrier, causing

another conformational change that releases the K+ ions to the cytoplasm.

4. Repeat.• Key: Both ions are moving against their gradients.• Key: Not only are these concentration gradients, we can

also call them electrochemical gradients because the particles are ions and thus electrically charged.

What do they do?

Transport Enzymes Cell Surface Receptor

Cell-Cell Recognition

Cell Cohesion

Attachment to cytoskeleton

Exam

ple:

Ch

anne

l Pr

otei

nEx

ampl

e:

Antig

enSignal

Transduction Protein