cell membranes osmosis and diffusion n.
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Cell Membranes Osmosis and Diffusion. Functions of Membranes. Protect cell Maintain homeostasis 3. Selectively permeable - allows some molecules in, others are kept out. Homeostasis. Balanced internal condition of cells Also called equilibrium

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functions of membranes
Functions of Membranes
  • Protect cell
  • Maintain homeostasis

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

  • Balancedinternal condition of cells
  • Also calledequilibrium
  • Maintained by cell membranecontrolling what enters & leaves the cell
phospholipid bilayer
Phospholipid Bilayer

Polar heads arehydrophilic“water loving”

Nonpolar tails arehydrophobic“water fearing”

Makes membrane“Selective”in what crosses


Fluid Mosaic Model:

“Proteins floating in a sea of lipids”

blood brain barrier
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 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
Methods of Transport Across Membranes

1. Diffusion

2. Osmosis

3. Facilitated Diffusion

4. Active Transport

methods of transport across membranes1
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

  • 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 through a membrane
Diffusion through a Membrane

Solute moves DOWNconcentration gradient (HIGH to LOW)

Cell membrane


Facilitated Diffusion

  • Does NOT require energy
  • Uses transport proteins to move things from high to low concentration
  • Examples: Glucose or amino acids moving from blood into a cell.
facilitated diffusion
Facilitated Diffusion

Molecules will randomly move through the pores in Channel Proteins.

facilitated diffusion1
Facilitated Diffusion
  • This is a helper.

Active Transport

  • Requires energy (ATP)
  • Moves materials from LOW to HIGH concentration AGAINST the concentration gradient.
  • 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
You could think of it this way…
  • In osmosis, water FOLLOWS salt!! This is sometimes an easier way to remember this concept.
where is the water moving
The water is moving out of the cell.


Remember wherever salt is water follows!

Where is the water moving?




red onion cells

In Salt Water

What happened to the cytoplasm and cell membrane?

Red Onion Cells
diffusion of h 2 o across a membrane
Diffusion of H2O Across A Membrane

High H2O potential (amount) andLow solute concentration

Low H2O potential (amount) andHigh solute concentration

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?

  • Coagulation
  • Sodium Pump
  • Active Transport
  • Osmosis


The picture shows a cell model and the solutions associated with it. In this situation the cell model will —
  • Gain mass
  • Shrink
  • Increase in solute content
  • Start to vibrate


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.



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 –

  • Releasing enzymes
  • Maintaining homeostasis
  • Decreasing respiration
  • Assimilating proteins


think pair share
  • Work with a partner to create different situations that can be classified as diffusion.
  • Do the same for osmosis.

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

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

Another Video: