OSMOSIS & DIFFUSION

SB1.d – Explain the impact of water on life processes (i.e.

osmosis and diffusion).

Methods of Cell Transport• Substances move into and out of cells by

several methods.– One method of movement is diffusion.

Methods of Cell Transport

• Diffusion is the net movement of a substance (liquid or gas) from an area of high concentration to one of lower concentration

• The majority move from higher to lower concentration

• If no energy is put into the system, the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.

The Cell Membrane• All cells, in all types of organisms, are surrounded

by a cell membrane– a thin layer of lipid and protein that separates the

cell's contents from the world around it.– functions like a gate, controlling what enters and

leaves the cell.– controls the ease with which substances pass into

and out of the cell-some substances easily cross the membrane, while others cannot cross at all.• selectively permeable

 Cells and Diffusion• Water, carbon dioxide, and oxygen are among

the few simple molecules that can cross the cell membrane by diffusion. –Water moves through membranes by a type of

diffusion known as osmosis.–Diffusion is one principle method of

movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane.

 Cells and Diffusion• Osmosis is the diffusion of water across a semi-

permeable (or differentially permeable or selectively permeable) membrane.

• The presence of a solute decreases the water potential of a substance.

 Cells and Diffusion• Hypertonic solutions are those in which more

solute (and hence lower water potential) is present.

• Hypotonic solutions are those with less solute (again read as higher water potential).

• Isotonic solutions have equal (iso-) concentrations of substances. Water potentials are thus equal; although there will still be equal amounts of water movement in and out of the cell, the net flow is zero.

 Cells and Diffusion

Cells placed in distilled water (hypotonic solution) take on water, swell and burst.In a salt solution (hypertonic solution), cells lose water and shrink.

Active and Passive Transport

• Two additional methods by which substances may move through cell membranes include:– Passive transport requires no energy from the

cell.• Examples include the diffusion of oxygen and

carbon dioxide, osmosis of water, and facilitated diffusion.

Active and Passive Transport– Active transport requires the cell to spend

energy, usually in the form of ATP.• Examples include transport of large molecules

(non-lipid soluble) and the sodium-potassium pump.

Active and Passive Transport

Vesicle-mediated transport• Vesicles and vacuoles that fuse with the cell

membrane may be utilized to release or transport chemicals out of the cell or to allow them to enter a cell.

• Exocytosis is the term applied when transport is out of the cell.

Active and Passive Transport• Endocytosis is the case when a molecule causes the

cell membrane to bulge inward, forming a vesicle.• Phagocytosis is the type of endocytosis where an

entire cell is engulfed.• Pinocytosis is when the external fluid is engulfed.• Receptor-mediated endocytosis occurs when the

material to be transported binds to certain specific molecules in the membrane. – Examples include the transport of insulin and

cholesterol into animal cells.

Active and Passive Transport