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 TransportCELL 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. Isotonic2. Hypertonic3. Hypotonic** The next few slides will illustrate how

these situations affect the cell.

Isotonic

HypertonicSolute 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 REQUIRED1.Osmosis2.Facilitated Diffusion3.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

gatedThe gates respond to three stimuli:

Stretching of the cell membraneElectrical signalsChemicals in the cytosol or external

environmentNo energy is used, so it is still passive transport

Active Transport – (cells actively work to move some substances

in/out) – CELL ENERGY IS REQUIRED1. 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.