Diffusion and Osmosis

Diffusion• Solute molecules moving

from an area of high concentration to an area of low concentration

– Random motion drives diffusion

– Movement is based on kinetic energy (speed), charge, and mass of molecules

– Equilibrium is reached when there is an even distribution of solute molecules

2

3

14

(water)

Osmosis• Diffusion of water through a

semi-permeable membrane– Semi-permeable:

permeable to solvents (WATER), but not to large molecules

– High [water] to low [water]

• Dissolved molecules (i.e. glucose, starch) are called solutes

• REMEMBER:Water = solventGlucose, Starch = solutes

Effect of Water on Cells

• Hypertonic Environment – High [solute], low [water]

• Hypotonic Environment– High [water], low [solute]

• Isotonic Environment– [water] = [solute]

Isotonic

HypotonicHypertonic

Osmosis in Living Cells

Cellulose in cell wall

Osmosis in Plant Cells

HypertonicHypotonic

Plasmolysis

Osmosis in Red Blood Cells

Isotonic

Hypotonic

Hypertonic

Tonicity LabTonicity of a Potato Cell

-40

-30

-20

-10

0

10

20

30

40

0 0.2 0.4 0.6 0.8 1

Molar

Per

cen

t C

han

ge

in M

ass

Hypotonic

Hypertonic

Isotonic

Membrane Permeability

• The cell membrane is a selectively permeable membrane; it lets some molecules pass through by passive diffusion, but not others.

• Four factors:

*Lipid solubility

*Molecular size

*Polarity

*Charge

Transportation of Molecules

• Passive Transport

-Movement of molecules across a semi-permeable membrane

- no energy required

• Facilitated Diffusion Movement of molecules across a semi-permeable membrane protein - no energy required

• Active Transport

-Movement of molecules across a semi-permeable membrane against a concentration gradient with a protein

- ENERGY – ATP

•

Passive Transport

Simple diffusion: membrane does not influence the direction of movement (high conc. → low conc).

Facilitated diffusion: a molecule that is too big or too polar to diffuse across the membrane combines with a specific transport carrier protein and is released in the cytoplasm.

No addition of energy (ATP) is required.

Transport by Carrier Proteins

Some biologically useful molecules pass through the plasma membrane because of channel proteins and carrier proteins that span the membrane.

Carrier proteins are specific and combine with only a certain type of molecule.

Facilitated transport and active transport both require carrier proteins.

Facilitated transport

During facilitated transport, substances pass through a carrier protein following their concentration gradients.

Facilitated transport does not require energy.

The carrier protein for glucose has two conformations and switches back and forth between the two, carrying glucose across the membrane.

Facilitated diffusion of glucose

Active transport

During active transport, ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration.

Energy in the form of ATP is required for the carrier protein to combine with the transported molecule.

Active Transport

During exocytosis, vesicles fuse with the plasma membrane for secretion.

Some cells are specialized to produce and release specific molecules.

Examples include release of digestive enzymes from cells of the pancreas, or secretion of the hormone insulin in response to rising blood glucose levels.

Exocytosis and Endocytosis

Exocytosis

Endocytosis

During endocytosis, cells take in substances by invaginating a portion of the plasma membrane, and forming a vesicle around the substance.

Endocytosis occurs as:

Phagocytosis – large particlesPinocytosis – small particlesReceptor-mediated endocytosis – specific particles

Phagocytosis

Pinocytosis

Receptor-mediated endocytosis