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Intermediate 2
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Section 1
Introduction to cells
Cells are the living units from which all organisms are made.
Simple organisms have only one cell
e.g. bacteria
they are called unicellular
Other organisms have many cells
working together
they are called multicellular
e.g. humans
This is the structure of an animal cell
cell membrane
allows certain substances
into or out of the cell
nucleus
cytoplasm
controls all activities in the cell
chemical reactions take place here
Iodine is used to stain a cheek cell.
Stains make certain cell structures show up more clearly.
This is the structure of a plant cell
fluid filled sac
stores water and minerals
nucleus
vacuole
cell wall
made of cellulose
provides support for cell
cytoplasm
green
chloroplast
cell
membrane
contains chlorophyll
traps light energy in photosynthesis
Functions of parts of the cell
structure
nucleus
cell membrane
cytoplasm
cell wall
vacuole
chloroplast
feature
contains genetic
material
function
controls cell activities
thin layer surrounding controls entry and
cytoplasm
exit of materials
fluid jelly like
material
cellulose outer layer
in plants only
site of chemical
reactions
fluid filled sac in
cytoplasm
stores water and
solutes
contains chlorophyll
traps light energy
supports cell
• cell wall
Only plant cells contain :
• chlorophyll
• vacuole
The cell wall and vacuole provide support to the cell.
Plant cells with many chloroplasts would be found in parts of a
plant which produces food by photosynthesis.
Plant cells are rigid and have a fixed shape.
Animal cells are flexible and can vary their shape.
Comparing plant and animal cells
nucleus
Found in both plant and animal cells.
cytoplasm
Found in both plant and animal cells.
membrane
Found in both plant and animal cells.
cell wall
Only found in plant cells.
vacuole
Only found in plant cells.
chloroplast
Only found in plant cells.
A microbe is a microscopic one-celled organism
Examples of microbes are:
bacteria
amoeba
yeast
• Microbes are found almost anywhere that life exists.
• Some are harmful because they can cause disease and rot
food.
• Others are useful for wine making, recycling nutrients in
nature, and in biotechnology.
bacteria
•Bacteria feed on the milk sugar lactose.
•The lactose is converted into lactic acid during this process.
•The lactic acid changes the milk into yoghurt.
yeast
One yeast cell [Very small. Not much detail]
Yeast is a unicellular fungus.
The cells are not mobile.
Not much detail is seen in the cells.
The cells are smaller than plant or animal cells.
Fermentation
yeast
+
-
Biological process by which yeast changes
sugar into alcohol and carbon dioxide.
sugar
alcohol
+
carbon dioxide
Humans have made use of 3 yeast industries for thousands of years.
•Wine making
•Baking
•Brewing
Uses of Yeast (1)
Wine Making
yeast + fruit sugar
alcohol + carbon dioxide
•Wines, beers, spirits for drinking.
•Used as a fuel substitute for petrol!
Uses of Yeast (2)
Baking
yeast + sugar
carbon dioxide
carbon dioxide
makes dough rise
+
alcohol
alcohol
evaporates when
dough is cooked
Uses of Yeast (3)
Brewing
yeast + malted barley + hops
Starch in barley
changes to sugar
added for
flavour
alcohol + carbon dioxide
Antibiotics
•Alexander Fleming discovered penicillin
•Penicillin is an antibiotic
•Antibiotics come from fungi
•Antibiotics kill bacteria
Antibiotics do not kill viruses (colds & flu are viruses)
Some bacteria are now resistant to antibiotics
Diffusion
This is the natural movement of particles from an area
of high concentration to an area of low concentration
until they are evenly spread.
A purple crystal is added to a beaker of water.
purple dye spreads evenly through water
water
star
t
purple crystal
after 15 mins
molecules of the purple crystal move
from an area of high concentration
(the crystal) to an area of low
concentration (the water)
this is an example of diffusion
The process of diffusion
purple dye
spreading out
•The water molecules are moving all the time
•The movement of the water molecules causes the crystal
molecules to spread out
•Diffusion will stop when the crystal molecules are evenly spread
throughout the water
Diffusion allows useful substances to pass into a cell
high concentration
of oxygen
low concentration
of CO2
high concentration
of glucose
And waste substances to
pass out of a cell
In multicellular organisms e.g. humans,oxygen cannot diffuse into
the cells directly from the air, so there must be a method to take
the oxygen to the cells.
•We breathe in air which has 20% oxygen in it
•The oxygen diffuses into the blood vessels in the lungs
•The blood carries the oxygen to all body cells
•Oxygen diffuses into the cells from the blood
Cells use the oxygen and glucose to produce energy.
glucose
+
oxygen
energy
+
CO2
+
water
This process is called respiration.
energy
This is used up by the cell.
CO2
This diffuses out of the cell into the blood and
is taken to the lungs to be removed.
water
This diffuses into the blood and is taken to
the kidneys to be removed.
•Substances enter and leave cells by diffusion
•They diffuse through the cell membrane
•Substances diffuse from a high concentration to
a low concentration
•Diffusion stops when molecules are evenly
concentrated
Osmosis is a special type of diffusion
osmosis
Diffusion of water from an area of high water molecule
concentration to an area of low water molecule concentration
across a selectively permeable membrane.
A selectively permeable membrane only allows
small molecules to pass through it.It does not
allow large molecules to pass through.
The difference in concentration between 2 solutions is called a
concentration gradient.
membrane
sugar molecule
The water diffuses down the
concentration gradient
water molecule
area of high water
molecule concentration
area of low water
molecule concentration
water molecules diffuse by osmosis from the area of high water molecule
concentration to the area of low water molecule concentration.
This shows a visking tubing experiment with sugar solutions of different concentrations.
(99.5% water)
0.1% sugar
solution
H2O
0.5% sugar
solution
H2O
H2O
(99.9% water)
H2O
visking tubing
Water will move out of the bag by osmosis and the bag will lose mass.
• Water enters and leaves cells by osmosis
• A selectively permeable membrane lets some substances through
but not others
• A concentration gradient exists between 2 solutions of different
concentrations of the same substance
• Osmosis is special diffusion of water from a high concentration
to a low concentration through a selectively permeable membrane
• A 5% sugar solution contains 5% sugar and 95% water
Osmosis can be explained in terms of water concentrations
A hypotonic solution has a higher water concentration
than that inside the plant cell.
area of higher
water
concentration
hypotonic solution
plant cell
area of lower
water
concentration
water molecules
enter the cell by
osmosis
An isotonic solution has the same water concentration
as that inside the plant cell.
water
concentrations
are the same
inside and
outside the cell
isotonic solution
plant cell
the same number
of water molecules
enter the cell as
leave the cell
A hypertonic solution has a lower water concentration
than that inside the plant cell.
area of higher
water
concentration
area of lower
water
concentration
hypertonic solution
plant cell
water molecules
leave the cell by
osmosis
Osmosis in plant cells
hypotonic solution
(H2O)
potato cylinder
hypertonic solution
(sugar solution)
•Each potato was weighed and the mass recorded
•Potatoes were left soaking for 15 minutes
•After 15 minutes potatoes were blotted dry and weighed
•The texture of each potato was felt
hypotonic solution
(H2O)
H2 O
potato cylinder
H2O
H2O
•water moves into cell
•cells become turgid
•mass of cell increases
hypertonic solution
(sugar solution)
H2O
H2O
H2O
•water leaves cell
•cells become flaccid
•mass of cell increases
Potato cells and osmosis
nucleus
cytoplasm
cell membrane
cell wall
Potato cell in normal healthy condition is described as turgid.
Potato cells in hypotonic solution
vacuole is swollen due to uptake of water by osmosis
cytoplasm is forced against cell wall by pressure
from vacuole
cell wall is stretched to its maximum
Cell is swollen to its maximum size and is said to be fully turgid.
Potato cells in hypertonic solution
vacuole shrinks due to loss of water by osmosis
cytoplasm is pulled away from cell wall because of
reduced pressure from vacuole
cell wall is unstretched
The process of extreme water loss resulting in this situation is called
plasmolysis.
Plasmolysed cells are soft and limp. They are said to be flaccid.
Osmosis in animal cells
water enters cell
Animal cells in hypotonic solution
•water enters by osmosis
•cell membrane stretches and bursts
cell membrane bursts
water leaves cell
Animal cells in hypertonic solution
•water leaves by osmosis
•cell loses water and shrinks
cell membrane shrinks
This is the end of
Section 1
Introduction to cells