Chapter 5
Homeostasis and Cell Transport
PASSIVE TRANSPORT – the movement of substances across a cell membrane without the use of
energy by the cell.
Example: it’s like riding your bike downhill, you can coast the entire way and never use
any energy.
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There are 4 types of passive transport
1. Diffusion- the movement of molecules from an area of higher concentration to
an area of lower concentration. The difference in concentration of molecules
across a space is called a concentration gradient.
 It is driven entirely by the molecules kinetic energy.
 They will continue to move in a straight line until they collide with
another molecule.
 Their constant motion allows the molecules to evenly spread out across a
space.
 Example: spray perfume in the middle of a room; eventually you
will smell the perfume anywhere in the room.
 When the concentration of molecules is the same throughout a space it is
at equilibrium
 Once at equilibrium the molecules continue their random
movement throughout the space they occupy.
 Simple diffusion is the diffusion of molecules from an area of higher
concentration to an area of lower concentration across a membrane.
 This is regulated in part by whether the molecules are non-polar,
size of the molecule, and type of molecule
2. Osmosis- the process by which water molecules diffuse across a cell membrane
from an area of higher concentration to an area of lower concentration
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Three conditions that control the direction of osmosis- In each of the
following conditions we are comparing the solute concentration outside
the cell to inside the cell.
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Hypotonic condition- the concentration of solute molecules
outside the cell is lower than the concentration in the cytosol.
This will cause water to flow into the cell until equilibrium is
reached.
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Hypertonic condition- the concentration of solute molecules
outside the cell is higher than the concentration in the cytosol.
This will cause water to flow out of the cell until equilibrium is
reached.
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
Isotonic condition- the concentration of solutes outside the and
inside the cell are equal so water will flow in and out of the cell at
equal rates.
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How cells deal with osmosis
 Many cells, especially unicellular fresh water organisms, have to
deal with living in an extreme hypotonic environment.
o In this situation water continually rushes into the cell
forcing the organism to constantly rid itself of excess
water.
o Parameciums are unicellular freshwater organisms with
this problem.
o They have contractile vacuoles which are organelles that
remove excess water. They collect water and then
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contract, pumping the water out of the cell. This action
requires energy.
Multi-cellular organisms respond to hypotonic environments by
pumping solutes out. This helps control the flow of water into the
cell.
Plants in a hypotonic environment can stand upright. The water
fills the cell pressing the cell membrane up against the cell wall
which is strong enough to resist breaking. The pressure water
molecules exert against the cell wall is called turgor pressure. In
hypertonic environments water leaves the cell through osmosis,
the cell membrane then shrinks away from the cell wall causing
the plant to wilt. This process is called plasmolysis.
Animal cells like red blood cells have no cell walls to stop them
from expanding and when in a hypotonic environment will swell
until they burst. The bursting of cells is called cytolysis.
3. Facilitated Diffusion- Carrier proteins help transport molecules not soluble in
lipids or too large to pass through the cell membrane across the membrane.
 The proteins move molecules from an area of higher concentration to an
area of lower concentration
 Each carrier protein is specific for a certain molecule
 As soon as the molecule binds to its carrier protein, the carrier
protein changes shape
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The change in shape shields the molecule from the hydrophobic
interior of the lipid bi-layer
The molecule can then be delivered either into or out of the cell
o Example- glucose is too large to diffuse across the
membrane but essential for the cell
4. Diffusion Through Ion Channels- allow for the passage of ions through the cell
membrane
 Each ion channel is specific for one kind of ion
 Some ion channels are always open
 Some ion channels have gates that open and close as needed
 They open and close in response to 3 different stimuli
o Stretching of the cell membrane
o Electrical signals
o Chemicals in their environment
Active Transport- The movement of materials from an area of lower concentration to an
area of higher concentration or up their concentration gradient. This requires energy.
1. Cell Membrane Pumps- Ion channels and carrier proteins can also assist with
active transport.
a. Carrier proteins that serve in active transport are called cell
membrane pumps.
b. They work in the same way as they do for passive transport by
changing shape and being specific for certain molecules except they
require energy in the form of ATP
i. Example- Sodium Potassium Pump
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1. Removes 450 sodium and brings in 300 potassium ions
in one second
2. Movement in Vesicles- Macromolecules and nutrients are too large to pass
through the cell membrane so they need a special way to enter or leave the
cell.
a. Endocytosis- process by which cells ingest external fluids,
macromolecules, and large particles, including other cells.
i. The cell membrane folds into itself and forms a pouch
ii. The pouch then pinches off from the cell membrane and
becomes a membrane bound organelle called a vesicle
iii. Some vesicles fuse with lysosomes and the contents get
digested
1. 2 Types of endocytosis
a. Pinocytosis- the transport of solutes and
liquids, often called cell drinking
b. Phagocytosis- the transport of large particles or
whole cells, often called cell eating
b. Exocytosis- the removal of a substance from a cell through a vesicle.
i. The vesicle fuses with the cell membrane to release the
substance
ii. Cells will do this to get rid of large molecules, waste products,
or toxins
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