chapter 3 b understanding movement across pm

7/27/2019 Chapter 3 b Understanding Movement Across Pm

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Understanding the Movement of Substances across the

Plasma Membrane in Everyday Life

1. Hypotonic, hypertonic and isotonic solution

• Concentration: proportion of solute in solution at certain

temperature

• Can be stated in g/litre or molarity or percentage

• When 2 solutions of different concentrations are adjacent

to one another, they can be compared relatively :

o If concentration of A is higher than B:

A is hypertonic to B

B is hypotonic to A

o If concentration of A and B are the same:

A and B are isotonic

• Isotonic solutions produce the same osmotic pressure.

• The higher the concentration, higher the osmotic pressure.

• The osmotic pressure depends on the total number of

solvent molecules.

Concentratio

n Statement

Osmotic

pressure of

solutionSolutionA B

High Low A is hypertonic

to B

B is hypotonic to

A

A>B

B<A

Low High A is hypotonic to

B

B is hypertonic

to A

A<B

B>A

Same Same A and B are

isotonic

A=B

2. Effects of heterotonic solutions on cells

• When animal cells and plant cells are put in a salt or sugarsolution, osmosis will take place. The cell concentration

relative to the external solution concentration determines

the direction of movement of water molecules.

• There are three types of relative concentration of cells and

the external solutions:

a) Solution is isotonic to the cell sap.

b) Solution is hypotonic to the cell sap.



c) Solution is hypertonic to the cell sap.

3. Animal cells

• When an animal cell is put in a solution that is isotonic to

its cytoplasm:

• The osmotic pressure of the outer solution is equal to the

osmotic pressure of the cytoplasm.

• This enables the rate of water diffusion into the cell to

equal the rate of water diffusion out from the cell.• The rate of water than diffuses in and out achieves a

dynamic equilibrium.

• When an animal cells is put in a solution that is hypotonic

to its cytoplasm:

o The osmotic pressure of the outer solution is lower than

the osmotic pressure of the cytoplasm.

o The rate at which water diffuses into the cell is higher

than the rate it diffuses out of the cell.

o The cell protoplasm expands and eventually the cell

membrane burst.

o Lysis is the term used for the situation where the cell

membrane bursts and allows the cytoplasm to flow out

from the cell.

o Lysis in red blood cells is known as haemolysis.

o When an animal cell is put in a solution than is

hypertonic to its cytoplasm:

o The external solution has a higher osmotic pressure

than the cytoplasm.

o

So, the cell loses water and the cell shrivels.o This phenomenon is known as crenation.

4. Plant cells

• When a plant cell is put into an isotonic solution.

o The osmotic pressure of the external solution is equal to

the osmotic pressure of the cell sap.

o The rate that water diffuses into the cell is equal to the

rate than water diffuses out of the cell.o The rate of water entering and exiting the cell achieves

as equilibrium and the net movement of water is zero.

o As a result, the plant cell does not change its shape.

• When a plant cell is put into a hypotonic solution:

o The osmotic pressure of the external solution is lower

than the osmotic pressure of the cell sap.

o There is a net movement of water molecules from the

external solution to the vacuole through the cell wall

and cell membrane.

o The cell protoplasm expands but the cellulose wall limits

the expansion and prevents the cell from bursting.

o The protoplast exerts a pressure against the cell wall

and this is known as the turgor pressure. The cell

becomes turgid.

• When a plant cell is put in a hypertonic solution :

o Water than diffuses out from the cell through osmosis is

more than water than the water than diffuses into the

cell.

o The protoplasm shrivels upon a net loss of water.

o Reduction in the amount of water in vacuole causes the

shriveling, resulting in the protoplasm separating from

the cell wall.

o This process is continuous until the vacuole disappears

and the protoplasm appears isolated in the middle of

the cell. The cell becomes flaccid (soft). This process is

known as plasmolysis.



5. Make an inference on the concentration of cell sap in plant

tissues

• Cell sap is the liquid that fills the vacuole of a plant cell.

• Since cell sap contains various soluble nutrients, wastes,

and salts, it is able to exert an osmotic pressure.

• By putting plant cells into a series of heterotonic solutions

(hypertonic, isotonic, and hypertonic) it is possible to

determine the concentration of the external solution that isisotonic to the cell sap.

• This is done by extrapolating from the graph drawn below,

the concentration of the external solution that did not

cause any change in the potato cylinder.

• This value is also taken as the concentration of cell sap in

the potato cells.

ResultsFlaccid + decrease

in length

No change in

physical condition

Turgid + increase in

lengthInference

The concentration of

the cell sap to the

external solution


the cell sap is

isotonic to the

external solution


the cell sap is

hypertonic to the

external solution

Explanation There is a net

movement of water

out from the

vacuole to the

exterior. There is no

There is no net

movement of water

from the cell and

the cell maintains its

physical condition

There is a net

movement of water

into the vacuole

from the exterior ,

there is an increase

pressure exerted

against the cell wall

in the pressure

exerted against the

cell wall

6. Relate the movement of substances across plasma membrane

with concentration gradient

• When two solutions are separated by a semi permeable

membrane:o Water molecules move from a less concentrated

solution to higher concentrated solution

o Low osmotic pressure to high osmotic pressure

Concentration

of external

solution and

concentration

of cell sap

Explanation Diagram

Hypertonicsolution:

-concentration of

external solution

higher than the

concentration of

cell sap

-Osmosis occurswith a net

movement of water

molecules from the

cell sap to the

exterior of the cell.

-Results in

plasmolysis

-Protoplast detaches

from cell wall

Isotonic:-concentration of

external solution

is equal to

concentration of

cell sap

-No osmosis aswater molecules

moves from the cell

sap to the external

solution since the

rate of water

movement through

the plasma



membrane is same

-Cell maintains

initial shapeHypotonic:

-concentration of

external solution

less than

concentration of cell sap

-osmosis occurs and

water molecules

moves from external

solution into the cell

sap faster thanwater molecules

move from cell sap

out to the external

solution.

-protoplast expands

and pushes against

the cellulose wall

-cell becomes turgid(A)The wilting in the plants

a. Happens when the plant lose excessive amounts of waterthrough transpiration in the leaves.

b. Plasmolysis occurs and plant cell becomes flaccid

c. The leaves droop and eventually become dries up.

d. Excessive use of fertilizer

i. Fertilizer contains organic and inorganic salts.

ii. It increases osmotic concentration of soil solution

iii. If excessive – water in the soil becomes hypertonic to

cell sap of root cells

- water diffuse out of the roots through osmosis and

excessive water losses occurs

- The plant cells are plasmolysed and become flaccid

resulting in plants to wilt and die.

iv. Can be prevented by diluting the fertilizer before adding

to the soil.

(B)The preservation of food

a. Preservation of food prolongs its shelf life

b. Food can be preserved by adding high concentration of salt

or sugar on the food’s surface

c. High concentration of sugar or salt is hypertonic to the

cytoplasm of the microorganism that enters the food.

d. So, the microorganisms will lose water and plasmolysed

thus dies.

7. Appreciating the Movement of Substances across the PlasmaMembrane

• For the survival of the cell

• Through passive transport:

oCell obtains oxygen and nutrients and removes carbon

dioxide and other excretory products

• Through osmosis:

o Allows the movement of water inside and outside of the

cell

• Diffusion and facilitated diffusion

o Allows the movement of gas molecules and other small

solute molecules

• Active transport:

o Ensures cell to obtain any necessary substance that is

required for its survival.

• Structure of plasma membrane

oCholesterol molecules and phospholipids enables the

plasma membrane to the controls the movement of

substances.

o

Semi permeable characteristics and only allowing watermolecules rise to osmosis

o Phospholipids bilayer:

o Lipid soluble substances and very small nonpolar

molecules can pass through the membrane easily.

• Damages to the plasma membrane:

o Jeopardize its normal function and those other function it

controls

chapter 3 b understanding movement across pm

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