Transcript Chapter 5

Chapter 5
Homeostasis and
Cell Transport
Section 1: Passive Transport
• Objectives:
• Explain how an equilibrium is established as a result of diffusion.
• Distinguish between diffusion and osmosis.
• Explain how substances cross the cell membrane through facilitated
diffusion.
• Explain how ion channels assist the diffusion of ions across the cell
membrane.
Diffusion
• Passive transport involves the movement of
molecules across the cell membrane without an
input of energy by the cell.
• Simplest type of passive transport is
diffusion.
• Diffusion is the movement of molecules from
an area of higher concentration to an area of
lower concentration, driven by the molecules’
kinetic energy until equilibrium is reached.
Diffusion
Diffusion
• Diffusion is driven by the molecules’
kinetic energy.
• Molecules are in constant motion because
they have kinetic energy.
• High Concentration
Low Concentration
Visual concept
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Diffusion Across Membranes
 Molecules can diffuse across a cell membrane by
dissolving in the phospholipid bilayer or by
passing through pores in the membrane.
 Cell membranes allow some molecules to pass
through, but not others.
 If a molecule can pass through a membrane, it will
diffuse from an area of high concentration to an
area of low concentration.
Simple Diffusion
• Diffusion across a membrane is also called
simple diffusion.
• The simple diffusion of a molecule across a
cell membrane depends on the size and
the type of the molecule.
Osmosis
• Is the diffusion
of water across
a membrane.
• Osmosis doesn’t
require cells to
expand energy.
• Osmosis is the
passive transport
0f water.
Osmosis
Solutions
• Hypertonic Solution: A hypertonic
solution contains a greater
concentration of solutes than the
solution on the other side of the
membrane.
• Hypotonic Solution: A hypotonic
solution contains a lesser concentration
of solutes than the solution on the other
side of the membrane.
Solutions
• When the concentrations of solutes
outside and inside are equal, the outside
solution is said to be isotonic to the
cytosol.
• Hypo, hyper and iso- refer to the relative
solute concentrations of two solutions.
Direction of Osmosis
• The direction of osmosis depends on the
relative concentration of solutes on the
two sides of the membrane.
• Water tends to diffuse from a hypotonic
solution to a hypertonic solution.
Direction of Osmosis
o When the solute concentration outside the
cell is higher than that in the cytosol, the
solution outside is hypertonic to the
cytosol, and water will diffuse out of the
cell.
When the solute concentrations outside
and inside the cell are equal, the solution
outside is isotonic, and there will be no
net movement of water.
Visual concept, osmosis
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Hypertonic, Hypotonic,
Isotonic Solutions
Visual concept, Osmosis
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How cells deal with Osmosis?
– To remain alive, cells must compensate for the
water that enters the cell in hypotonic
environments and leaves the cell in hypertonic
environments.
– Contractile vacuoles are organelles that
regulate water levels in paramecia. They
collect the excess water and then contract,
pumping the water out of the cell.
How cells deal with Osmosis?
• Cells of multicellular organisms
respond to hypotonic environment by
pumping solutes out of the cytosol.
• This lowers the solute concentration
in the cytosol brining it to the solute
concentration in the environment;
thus water molecules are less likely to
diffuse into the cell.
How cells deal with Osmosis?
• Some plant cells may be surrounded
by water that moves into the cell by
osmosis.
• Cells become turgid.
• The cell wall is strong to resist the
pressure exerted by water called
turgor pressure.
How cells deal with Osmosis?
• In a hypertonic environment, the cell
shrinks and turgor pressure is lost.
• This condition is called plasmolysis; it is
the reason that plants wilt if they don’t
receive enough water.
How cells deal with Osmosis?
• Some cells can’t compensate for
changes in the solute concentration of
their environment.
• Human RBCs lack contractile vacuoles,
solute pumps, and cell walls.
• RBCs loose their normal shape when
they are exposed to an environment that
isn’t isotonic to their cytosol.
How cells deal with Osmosis?
• Hypertonic environment leads to cell
shrinkage.
• Hypotonic environment leads to cell
swelling and bursting.
• Cell bursting is called cytolysis.
Facilitated Diffusion
• Facilitated Diffusion is another type of
passive transport.
• This process is used for molecules that
can’t readily diffuse through the cell
membrane:
o may not be soluble in the lipid bilayer
o Too large to pass in the pores of the membrane.
Facilitated Diffusion
• In facilitated diffusion, a molecule
binds to a carrier protein on one
side of the cell membrane.
• The carrier protein transports the
molecule from an area of high
concentration to an area of low
concentration
no energy
expenditure.
Facilitated Diffusion
• A molecule binds to a specific carrier
protein that transports it.
• The carrier protein then changes its
shape (may shield the molecule from
the hydrophobic interior of the lipid
bilayer).
• It transports the molecule down its
concentration gradient to the other side
of the membrane.
Facilitated Diffusion
Diffusion Through Ion Channels
• Ion channels are proteins that
provide small passageways across the
cell membrane through which specific
ions can diffuse.
• Ion channels transport ions such as
calcium, sodium, potassium, and
chloride. (These ions aren’t soluble in
lipids).
• Each type of ion channel is specific for
one type of ion.
Diffusion Through Ion
Channels
• Some ion channels are always open.
• Others have gates that open or close.
• The gates may open or close in response to three
kind of stimuli:
 Stretching of the cell membrane.
 Electric signals.
 Chemicals in the cytosol or external environment.
• These stimuli control the ability of specific ions
to cross the membrane.
Ion Channels
Passive Transport
Passive
Diffusion
Osmosis
Facilitated
Diffusion
Section 2: Active Transport
• Objectives:
• Distinguish between passive transport and
active transport.
• Explain how the sodium-potassium pump
operates.
• Compare endocytosis and exocytosis.
Types of Transport
Passive
No energy imput required
Active
Energy imput required
Active Transport
• Movement of materials
from low concentration to
high concentration using a
protein carrier that
requires energy
(costs ATP)
• High Concentration
Low Concentration
Active Transport
• Active
Pumps
Membrane
Movements
Endocytosis
Exyocytosis
Cell Membrane Pumps
• Ion channels & carrier proteins not only
assist in passive transport but also help
with active transport.
• The carrier proteins that serve in active
transport are called cell membrane
pumps.
Cell Membrane Pumps
• Carrier proteins involved in
facilitated diffusion and
those involved in active
transport are very similar.
Cell Membrane Pumps
• In both, the molecule first binds to a
carrier protein, the protein changes shape,
the protein then transports the molecule.
• However, cell membrane pumps require
energy, which is supplied by ATP.
Cell Membrane Pumps
• Sodium-Potassium Pump
– The sodium-potassium pump moves three Na+
ions into the cell’s external environment for every two
K+ ions it moves into the cytosol.
– ATP supplies the energy that drives the pump.
Sodium-Potassium Pump
Sodium-Potassium Pump
• The exchange of 3 sodium ions
for 2 potassium ions creates an
electric gradient across the
cell membrane.
Sodium-Potassium Pump
• The outside of the membrane becomes
positively charged relative to the inside
which becomes negatively charged.
• This is important for conduction of
electrical impulses.
Movement in Vesicles
• Endocytosis
– In endocytosis, cells ingest external materials by
folding around them and forming a pouch.
– The pouch then pinches off and becomes a
membrane-bound organelle called a vesicle.
Movement in Vesicles
• Endocytosis
– Endocytosis includes pinocytosis, in which the
vesicle contains solutes or fluids, and phagocytosis,
in which the vesicle contains large particles or cells.
– Cells known as phagocytes ingest bacteria and
viruses by phagocytosis. The vesicles then fuse with
lysosomes, where lysosomal enzymes destroy the
bacteria and viruses.
Endocytosis
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Movement in Vesicles
• Exocytosis
– In exocytosis, vesicles made by the cell fuse with the
cell membrane, releasing their contents into the
external environment.
Exocytosis
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Endocytosis & Exocytosis