Download Chapter 5 Homeostasis and Transport

Chapter 5
Homeostasis 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