Cell MembranesOsmosis and Diffusion

Functions of Membranes

1. Protect cell

2. Maintain homeostasis

3. Selectively permeable - allows some molecules in, others are kept out

Homeostasis

•Balanced internal condition internal condition of cellsof cells

• Also calledAlso called equilibrium•Maintained by cell

membrane controlling what controlling what enters & leaves the cellenters & leaves the cell

Polar heads are hydrophilichydrophilic “water loving”Nonpolar tails are hydrophobichydrophobic “water fearing”

Phospholipid Bilayer

Makes membrane “Selective” in what crosses

Fluid Mosaic Model:“Proteins floating in a sea of lipids”

Proteins Are Critical to Proteins Are Critical to Membrane FunctionMembrane Function

Blood-Brain Barrier•Allows some substances into

the brain, but screens out toxins and bacteria

•Substances allowed to cross include: water, CO2, Glucose, O2, Amino

Acids, Alcohol, and antihistamines. HIV and bacterial meningitis can cross the barrier.

Solutions

• Solutions are made of solute and a solvent

• Solvent – The liquid part. It does the dissolving.

• Solute – The thing being dissolved. Salt and sugar are solutes.

Methods of Transport Across Membranes

1. Diffusion

2. Osmosis

3. Facilitated Diffusion

4. Active Transport

Methods of Transport Across Membranes

1. Diffusion -passive transport - no energy used

2. Osmosis - Passive transport of water across membrane

3. Facilitated Diffusion – Needs a helper to get things across

4. Active Transport- requires energy to transport molecules against a concentration gradient – energy is in the form of ATP

Diffusion• Movement of molecules from

an area of high concentration to an area of low concentration.

• Movement from one side of a membrane to another, without help.

Diffusion

Diffusion of Liquids

Diffusion through a Diffusion through a MembraneMembrane

Cell membrane

Solute moves DOWN concentration gradient (HIGH to LOW)

Facilitated DiffusionFacilitated Diffusion• Does NOT require energy

• Uses transport proteins to move things from high to low concentration

Examples: Examples: GlucoseGlucose or or amino acidsamino acids moving from moving from blood into a cell.blood into a cell.

Facilitated Diffusion

Molecules will randomly move Molecules will randomly move through the through the porespores in in Channel Channel ProteinsProteins..

Facilitated Diffusion

• This is a helper.

Active TransportActive Transport

• Requires energy (ATP)

• Moves materials from LOW to HIGH concentration AGAINST the concentration gradient.

Osmosis

• Diffusion of water across a membrane.

• Moves from HIGH water amount (low solute) to LOW water amount (high solute).

Diffusion across a membrane

Semipermeable

membrane

You could think of it this way…

• In osmosis, water FOLLOWS salt!! This is sometimes an easier way to remember this concept.

Salt

Where is the water moving?

• The water is moving out of the cell.

• Why?• Remember

wherever salt is water follows!

SALTSALT

Light blue = SaltDark blue = water

There is a lot of salt outside of the cell. What will happen?

Why?

What will happen if there is a lot of salt inside of the cell?

Red Onion Cells• Normal • In Salt Water

• What happened to the cytoplasm and cell membrane?

Diffusion of H2O Across A Membrane

High H2O potential (amount) andLow solute concentration

Low H2O potential (amount) andHigh solute concentration

Osmosis: Draw this

Three Forms of Transport Across the MembraneThree Forms of Transport Across the Membrane

Question• Why does eating

popcorn make you thirsty?

• Popcorn is very salty and may cause water to leave the cells of your mouth and through due to diffusion. This makes a person thirsty.

When a sea urchin egg is removed from the ocean and placed in freshwater, the egg swells and bursts. Which of these

causes water to enter the egg?

A. Coagulation

B. Sodium Pump

C. Active Transport

D. Osmosis

D

The picture shows a cell model and the solutions associated with it. In this

situation the cell model will —

A. Gain mass

B. Shrink

C. Increase in solute content

D. Start to vibrate

B

Which of these best explains why a freshwater aquarium would be a

dangerous habitat for saltwater fish?

A The tissues of the saltwater fish would absorb too much acid.

B The cells of the saltwater fish would lose too much water.

C The organ systems of the saltwater fish would consume too much energy.

D The cells of the saltwater fish would gain too much water.

D

On a hot summer day, a road-crew worker perspires and then feels thirsty as her

body temperature increases. This response is an example of –

A. Releasing enzymesB. Maintaining homeostasisC. Decreasing respirationD. Assimilating proteins

B

Think/Pair/Share

• Work with a partner to create different situations that can be classified as diffusion.

• Do the same for osmosis.

Another Video:http://www.youtube.com/watch?v=ML-SWe5bRaE

Active Transport Video: http://www.iteachbio.com/Life%20Science/LifeFunctionsandTheCell/ActiveTransport.mov

Diffusion Video: http://www.iteachbio.com/Life%20Science/LifeFunctionsandTheCell/Diffusion.mov