Water TransportOsmosis, osmolarity, and osmotic pressure

• Semipermeable membrane – allow some molecules to cross while restricting others

• Osmotic pressure – force associated with the movement of water

• Osmolarity – ability of solution to induce water to cross a membrane

Figure 2.13

Osmolarity is reflective of the concentration of solute particles in a solution.

Relative Osmolarity

• Hypotonic solutionshave a lower concentration of solutes than cytoplasm (ICF)swelling or even lysis

• Hypertonic solutionshave a higher concentration of solutes than cytoplasmcrenation

• Isotonic solutionsconcentration of solutes is the same as cytoplasm

Tonicity• Hypotonic solutions

have a lower concentration of solutes than cytoplasm

• Hypertonic solutionshave a higher concentration of solutes than cytoplasmcrenation

lysis

• Isotonic solutionsconcentration of solutes is the same as cytoplasm

Bulk Transport• Endocytosis

1. phagocytosis

Bulk Transport• Endocytosis

2. pinocytosis

Bulk Transport• Endocytosis

can remove and store membrane proteins

Bulk Transport• Exocytosis

used to install membrane proteinssecretion of products

Epithelial Tissue-Protection, Secretion, & Absorption

A. Membranes (coverings and linings)1. sheets of attached individual epithelial cells

2. transport of substances across an epithelium can be active or passive

3. epithelial cells packed tightly together by cell junctions and attached to underlying tissue at basement membrane

B. Gap junction (nexus) 1. circular intercellular connection

2. electrical or metabolic couplings- ions or chemical signals

C. Tight junctions1. often located nearest apical surface

C. Tight junctions2. seal intercellular space3. continuous band around cell

Tight junctions tend to be leaky in areas where large volumes are absorbed across the epithelium (leaky epithelium).

Epithelial Transport

A. Transcellular pathway may use active transport.1. substances crossing an epithelium must cross cell membrane at least twice

Epithelial Transport

2. differentiated membranesbasal and apical membranes can have different functional properties

Epithelial Transport

B. Water transport across an epithelial membrane- can be passive or can be “uphill”- there is no such thing as active transport of water- uphill movement of water is done via osmosis- is a secondary consequence of solute transport

Epithelial Transport

standing-gradient hypothesis (solute-coupled water transport)a. tight junctions near apical surfaceb. leaves intercellular cleftsc. active transport of salt out of cellsd. water follows salt