Chapter 5Homeostasis and Transport

Section 5.1

Passive Transport

The movement of substances across a cell membrane without any input of energy from the cell

Diffusion

Simplest type of passive transport

Molecules move from an area of higher concentration to an area of lower concentration

Driven by kinetic energy

Equilibrium

When the concentration of the molecules of a substance are the same throughout a space

Diffusion Across Membranes

Cell membranes allow some molecules to pass through, but not others

Depends on size and type of molecule

Also depends on the chemical nature of the membrane

Osmosis

Water molecules diffuse across a cell membrane from an area of higher concentration of H2O to an area of lower concentration

Direction of Osmosis

Hypotonic- the concentration of solute molecules outside the cell is LOWER than the inside of the cell

Water moves INTO the cell

Hypertonic- the concentration of solute molecules outside the cell is HIGHER than the inside of the cell

Water moves OUT OF the cell

Isotonic- the concentrations of solutes outside and inside the cell are equal

No movement of water occurs

How Cells Deal with Osmosis

Contractile vacuole- organelles to remove water

Turgor pressure- pressure of water against the cell wall

Plasmolysis- cells shrink away from cell walls

Facilitated Diffusion

The movement of molecules across a cell membrane through the use of carrier proteins

First, a carrier protein binds to a molecule on one side of the cell membrane

Second, the carrier protein changes shape shielding the molecule from the interior of the membrane

Finally, the molecule is released on the other side of the membrane

Diffusion Through Ion Channels

Ion channels provide small passages for ions to diffuse across the cell membrane

Channels may have “gates” that respond to stretching of the cell membrane, electrical signals, or chemicals in the cytosol

Ions such as Na+, K+, Ca2+, and Cl- are important for a variety of cell functions