Homeostasis, Osmosis, Transport
Unit 6 – Chapter 5
Diffusion Through Cell Boundaries
All living cells need a watery environment to survive!
The cell membrane helps organisms maintain Homeostasis (Equilibrium) by controlling what substances enter or leave the cell
To remain alive, cells must maintain biological balance.
Cells maintain this balance (homeostasis) in response to their immediate environment
Types of Cellular Transport Passive Transport
CELL DOES NOT USE ENERGY Diffusion Osmosis Facilitated Diffusion
Active Transport
CELL DOES USE ENERGY Protein Pumps Endocytosis Exocytosis
high
low
This is gonna
be hard
work!!
high
low
Weeee!!!
3 Types of Passive Transport
Diffusion – constant motion of molecules that causes them to spread out from high to low concentrations
Osmosis – diffusion of water
Facilitated Diffusion – diffusion with the help of transport proteins in the cell membrane
concentration (concentration gradient)
Equilibrium occurs when the concentration of solute (particles) is the same throughout (the particles still move!)
Because diffusion depends upon random particle movements (kinetic energy), substances diffusion across membranes without cellular energy
Diffusion
The Dye = SoluteWater= Solvent (In cells, water is
always the Solvent).
Law of DiffusionSubstances ALWAYS diffuse
from HIGH to LOW concentrations. This fact is key to understanding much of this chapter.
This is called moving DOWN the Concentration Gradient.
OSMOSISOsmosis is the name for an important
type of diffusion. It is the diffusion of water across the cell membrane. Since cells are usually bathed in a watery environment, they have to deal with water moving in/out of them. Too much water in or out of the cell can become a problem.
Osmosis – water moves from high to low concentration100% pure water 90% water
10% salt
membrane
More water passes fromPure water to salt solution...
...until water concentrationsbecome equal
level riseslevel falls
4
Water passes easily across membranes
Osmosis is the diffusion of water across a selectively permeable membrane
Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane
Osmosis between cells
If the concentration of the cell sap is greater in one cell than in its neighbour, water will pass by osmosis from the less concentrated to the more concentrated.
cell sap more concentrated
cell sap less concentrated
20
There is a greater concentration of free water molecules outside the cell than inside
so water diffuses into the cellby osmosis
and the cell swells up
Osmosis in animal cells
cell wall cytoplasm and cell membrane
vacuole
The cell absorbs waterby osmosis ....
....but the cell wall stops the cell expanding any more
Plant cells
SolutionsThe relative concentrations of solutions to
one another inside/outside of the cell can lead to 3 different situations. These situations are known as:
1. Isotonic
2. Hypertonic
3. Hypotonic
** The next few slides will illustrate how these situations affect the cell.
Isotonic
Hypertonic
Solute concentration is greater outside the cell, so water moves OUT of the cell
Remember, hypertonic,
the cell shrinks
The shrinking of cells is called Plasmolysis
HypotonicSolution concentration is
greater inside the cell, so water moves INTO the cell
Remember, hypotonic, the
cell POPS!!!
• The bursting of cells is called
Cytolysis
How Single Celled Critters Deal with Osmosis
Unicellular organisms in hypotonic environments need to get rid of the excess water that diffuses into them
Contractile vacuoles are organelles that collect water and pump it out of the cell (uses energy)
How Multi-celled Critters Deal with Osmosis
Other cells (especially in multicellular organisms) respond to a hypotonic environment by pumping solutes out of the cytoplasm
Water molecules are less likely to diffuse into the cell
Types of Passive Transport(How cells transport materials in/out of
themselves) – NO CELL ENERGY REQUIRED
1.Osmosis
2.Facilitated Diffusion
3.Ion Channels
**Refer to the next 2 slides.
Facilitated DiffusionSome molecules cannot diffuse through
the cell membrane because they are:Not soluble in lipids Or are too large to pass through the
pores in the membrane (I.E. Glucose)These molecules are helped across the
membrane by carrier proteinsThe carrier proteins change shape after
the molecule binds to themThe molecule is passed through the
membrane (either in or out)No cellular energy is used, so it is still
passive transport
Diffusion Through Ion Channels
Ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) are important for cell functionsSince they are not soluble in lipids
they will not pass through the cell membrane on their own
Diffusion Through Ion Channels…
Ion channels provide small tunnels across the cell membraneEach type of ion channel is usually specific for
one type of ionSome channels are always open, some are
gated
The gates respond to three stimuli:Stretching of the cell membraneElectrical signalsChemicals in the cytosol or external
environment
No energy is used, so it is still passive transport
Active Transport – (cells actively work to move some substances
in/out) – CELL ENERGY IS REQUIRED
1. Pumps in the cell membrane – proteins in the cell membrane use cell energy to change their shape to actively pump molecules in/out of cell. Ex.) Sodium/Potassium Pump.
2. Endocytosis – moving very large molecules INTO the cell. Cell wraps its membrane around the large molecule. This requires the cell to spend energy.
3. Exocytosis – moving large OUT OF the cell. Cell membrane changes its shape to push molecule out of cell. This requires cell energy.
***See pages 101 to 104 in book.
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