Transcript CHAPTER 5 – HOMEOSTASIS + TRANSPORT

CHAPTER 5 – HOMEOSTASIS +
TRANSPORT
5-1: PASSIVE TRANSPORT
 5-2: ACTIVE TRANSPORT

5-1: PASSIVE TRANSPORT
Cell membranes help organisms maintain
homeostasis by controlling what
substances go in or out of cells.
 Some substances can cross the cell
membrane without any input of
energy – known as passive transport
 Let’s look at some types of passive
transport…

Diffusion

Diffusion – movement of molecules from
high concentration to low concentration

Difference in the concentration of
molecules across a space is called the
concentration gradient
– Ex. Sugar in Water
Look at the Example
Diffusion (cont.)
Diffusion is driven by kinetic energy – nrg
of motion – molecules in constant,
involuntary motion
 Molecules move down their
concentration gradient (High to Low)

Equilibrium

When there are no other influences,
diffusion will inevitably cause the
concentration of molecules to even
out the space - - known as
EQUILIBRIUM
 At equilibrium, there is no concentration
gradient - - the molecules can move in
any direction (Randomly)
Simple Diffusion
Cell membrane controls what goes in and
out – Selectively Permeable
 Molecules will go from area of greater
concentration to an area of lower
concentration

What types of molecules can cross
a C.M.?

Depends on 3 characteristics:
– Size of molecule
– Type of molecule
– Chemical nature of C.M.
– Examples:
 Amino Acids
 CO2
 O2
 H2O
Osmosis
Solution = Solute dissolved in a
solvent
 Osmosis – the process by which water
molecules move from High to Low through
a cell membrane

– *No nrg – Passive Transport
Direction of Osmosis

The net direction of osmosis depends on
the relative concentration of solutes on
the two sides of the membrane
Types of Osmotic Environments

Hypotonic – when solute concentration
outside the cell is lower than the solute
concentration in the cytosol
– Water moves into cell
– Cell swells until equilibrium is reached
Types of Osmotic Environments

Hypertonic – when the solute
concentration outside cell is higher than
solute concentration in cytosol
– Water leaves the cell
– Cell shrinks until equilibrium is reached
Types of Osmotic Environments

Isotonic – when solute concentration
inside = solute concentration outside
– Water moves in + out at same rate = no net
movement of water
– Cell stays same size
RBCs in Solutions
How Cells deal w/ Osmosis

Contractile Vacuoles – organelles that
remove excess water
– Used by unicellular organisms (protists) that
live in hypotonic environments
– Collects water and “pumps” it out into the
environment
– “pumping” requires nrg
How Cells deal w/ Osmosis

Turgor Pressure – pressure water
molecules exert against the cell wall
– If turgor pressure decreases, plasmolysis
may occur
– If turgor pressure increases, cytolysis may
occur
Plasmolysis
Cytolysis
Facilitated Diffusion
Used for molecules that cannot readily
diffuse through cell membranes
 Uses Carrier Proteins to assist the
movement of molecules down their
concentration gradient


Example molecule - GLUCOSE
How F.D. works
1.
2.
3.
4.
Carrier protein binds to molecule on
outside of C.M.
Carrier protein changes shape + shields
molecule from interior of C.M.
Carrier protein releases molecule inside
the cell
Carrier protein returns to normal shape
and is available to transport another
molecule
Two Properties of FD
1.
Helps substances move in or out
depending on concentration gradient
2.
Carrier proteins are specific to types of
molecules they move
Diffusion through Ion
Channels

Ion channels move ions from High to
Low
– Important ions: Ca+, Na+, K+, Cl– Channels are specific for each ion they
transport
Open vs. Gated Ion Channels
Open – always open
 Gated – may open or close due to:

– Stretching of C.M.
– Electrical signals
– Chemicals in environment
Passive Transport Video
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Video Recap