Substances pass through the cell
membrane by:
1.DIFFUSION
2.OSMOSIS
3.ACTIVE TRANSPORT

To remain alive, a cell needs:
to take in the gas:
to eliminate the gas:
WHY?
WHY?

To remain alive, a cell needs FOOD:
Name TWO examples
GLUCOSE
AMINO ACIDS
 Name the material which also enters a
cell & is used as a SOLVENT.
water

Salts are also needed
example in plant cells:
Nitrates

By which process do these materials enter
or leave a cell?
Gases by: DIFFUSION
Water by: OSMOSIS
Glucose
Amino acids ACTIVE
Salts
TRANSPORT

Active or Passive transport?
- No energy is needed

The cell membrane is semi-permeable /
selectively permeable. Explain.
Outside a cell
Inside a cell
CELL MEMBRANE

The cell membrane allows only certain
substances to pass through
Outside a cell
Porous membrane
Inside a cell
Only small
molecules pass

Is the movement of molecules from a
region of high concentration to a region
of lower concentration

Liquids & Gases diffuse

Draw arrows to show direction of
molecules:
Concentration
gradient for
red molecules
Concentration
gradient for
blue molecules

Fig. 1 Diffusion in a liquid.

Why does oxygen keep on entering an
amoeba?
Due to a
concentration
gradient.

Examples of ‘where’ diffusion occurs:
 Diffusion of CO2 into stomata of leaves
during photosynthesis.

Diffusion of:
 O2 into the alveoli of the lungs
 CO2 out of capillaries.

The rate of diffusion depends on:
1. Concentration
 the higher the concentration, the faster is
the rate of diffusion

The rate of diffusion depends on:
2. Temperature
 the higher the temperature, the faster is
the rate of diffusion

The rate of diffusion depends on:
3. Size of molecules
 the smaller the size, the faster is the rate
of diffusion

Is the diffusion of water across a semi-
permeable membrane from a weak
solution to a strong solution.
Strong Weak
solution solution

Two conditions for osmosis to occur:
1. A semi-permeable membrane.
Dilute salt Strong salt
or sugar or sugar
solution solution
2. A difference in concentration.

Red blood cells are placed in solutions
of a different sugar concentration.
Explain the result.
Sugar
molecule
Let’s
explain
each:

RBC & and external solution are at the
SAME concentration
1. Water entering RBC =
water leaving.
What
happens?
No change in RBC size.

RBC are placed in a very concentrated
sugar solution
2. RBC are dilute
compared to external
solution & lose water.
What
happens?
RBC shrink.

RBC are placed in a dilute sugar solution
3. RBC take up water .
What
happens?
RBC swell and
finally burst.

Osmosis in plant cells
In a weak solution In a concentrated
water will enter solution the cell
the cell and fill the loses water and
vacuole. The cell goes flaccid
membrane will because the
push against the vacuole becomes
cell wall making flaccid and the
the cell very cytoplasm stops
inflexible. It is said pushing against
that cells in this the cell wall. This
state are turgid. state is called
plasmolysis.
4.9

What happens to PLANT cells?
Cell in dilute Cell in same Cell in Cell in a much
solution concentrated concentrated
becomes: concentration solution solution
of solution. becomes: becomes:
TURGID
FLACCID PLASMOLYSED

Summary: What happens to cells when
placed in a solution which is :

Explain why red blood cells burst in
distilled water but plant cells do not.
Rigid cell wall in plants prevents
bursting.

What happens to the cytoplasm in a
plasmolysed cell?
It is pulled away from the cell wall.
A plant wilts when
cells are plasmolysed.
Is the plant dead in
this wilted sate?

Lose
water
Gains
water
TURGID PLASMOLYSED

TURGID PLASMOLYSED
CELL CELL

Stomata are:
OPEN when CLOSED when
guard cells are: guard cells are:
FLACCID
TURGID

Soil became flooded with seawater. The plant wilted.
However, the plant recovered after it rained. Explain.
Wilted: Plant recovered:
cells lost water cells absorbed water
by osmosis. by osmosis.

To demonstrate osmosis in non-living
material.
Visking tubing / dialysis
tubing / cellulose tubing

To demonstrate osmosis in living tissue.

What do you expect to happen?
Strong sucrose Distilled water
solution in bag. in beaker.
Bag membrane
Water enters bag by osmosis.

Explain the result of this experiment:
1. Starch is placed inside a bag.
2. The bag is put in a tube
containing iodine solution and
left for one hour.
Iodine molecules diffused into
the bag.
Bag is selectively permeable.

Explain the results of this experiment
Salty water
Distilled water
1. Slice your potato into
5 mm slices. 2. Place one slice in distilled
water and one in salty water.
Distilled
3. After 30 minutes, you
water
can bend the slice of
potato in salty water
easily.
Salty
water

Two pieces of celery were for left for 4
hours as shown:
Distilled Salty
water water
Explain the
result.

What has happened?
Pins mark level of water.
Set up apparatus. Result after 1 hour.

Why does water enter an amoeba?
An amoeba is more concentrated than the
surrounding water. Water enters by osmosis.
H2O

Why doesn’t an amoeba burst like a RBC?
Contractile vacuole releases excess water.
2 Excess water enters
1 Water enters contractile vacuole.
due to osmosis.
7 The cycle is
repeated. 3
Contractile
vacuole swells.
6 Contractile vacuole
bursts and expels water. 4 Contractile vacuole
moves to edge of cell.

Salting: Is a way to
preserve meat.

Explain why there is no need to place jam
in a refrigerator.
Bacteria DIE
Dilute
Bacteria die as they
lose water by osmosis. Concentrated

Have a sore throat? Use a salt gargle.
WHY is this effective against
bacteria?
Bacteria die as they
lose water by osmosis.

Name the process:
 Oxygen moves out from a
stoma: diffusion
 Water enters a root hair /
amoeba: osmosis

Name the process:
 Glucose absorbed by small
active transport
intestine:

Is when substances cross the cell membrane
against a concentration gradient, using
energy and carriers in the membrane.

Large molecules and ions pass through
proteins in the cell membrane
proteins

ATP: Adenosine Triphosphate
ATP is the molecule that supplies
ENERGY

Fig. 4 Active transport.
AGAINST
a concentration
gradient

How do Osmosis & Diffusion occur?
DOWN
a concentration gradient
Low
High conc.
conc.
AGAINST
a concentration gradient

Question: [MAY, 2009] Paper 2
Paramecium lives in stagnant water. During an
investigation about the average time taken by a
contractile vacuole to fill up and empty, it was
found that the contractile vacuoles fill up and
empty every 60 seconds when placed in water
compared to 180 seconds when placed in 0.3% salt
concentration. Explain. (4)

Question: [MAY, 2009] Paper 2
More water enters a Paramecium living in
water than when is a salt solution. Water
enters the cell by osmosis from a dilute to a
more concentrated salt solution. The more
concentrated the external solution is, the less
water that enters.

Question: [APRIL, 2010] Paper 2
Explain ONE benefit of each of the following
features in the parasitic mode of life of a
parasite living in the gut of its host:
a) body surface covered with microvilli; (1)
To increase the surface area for absorption of
food

Question: [APRIL, 2010] Paper 2
b) long and thin body; (1)
For a large surface area for absorption of food
(Accept allows food to flow past so that the
parasite is not washed away/decrease in
diffusion distance)
c) body wall has active transport systems. (1)
For uptake of food molecules even against a
concentration gradient.

Question: [APRIL, 2010] Paper 1
Fresh cherries are fleshy and juicy. Candied
cherries are prepared by placing fresh cherries in a
very strong sugar solution. Explain why candied
cherries lose their fleshy juicy texture. (2)

Question: [APRIL, 2010] Paper 1
Water moves out of the cherries by osmosis.
Cherry cells are dilute when compared to the
surrounding solution. Water follows its
concentration gradient. Cells become plasmolysed.