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General/Credit
A
Investigating Living Cells
Cells are the basic units of all living organisms. Although cells vary
in shape they do have some structures in common.
Microscopes and cells
When looking at cells under a microscope they must be
1.
Thin – to let the light pass through them
2.
Flat and not wrinkled so that cells can be seen clearly
3.
Moist - to stop the cells drying out
4.
No air bubbles - these can spoil the viewing of cells
Stains are added to cells so that they can be seen more clearly
and more detail is shown.
Plant cell
cell wall
chloroplast
Cytoplasm
vacuole
nucleus
cell membrane
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Animal cell
Cytoplasm
Part of a Cell
nucleus
Celll membrane
Function
Cytoplasm
Where cell’s activities take place
Chloroplast
Contains green chlorophyll;
involved in photosynthesis
Supports cell and gives it
structure. Made of cellulose.
Boundary of cell; controls entry
and exit of materials
Fluid filled sac; stores and
regulates water.
Controls all cell activities
Cell wall
Cell membrane
Vacuole
Nucleus
The nucleus, cytoplasm and cell membrane are found in both
animal and plant cells.
The vacuole, cell wall and chloroplasts are only found in plant
cells.
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B
Investigating Diffusion
The cell membrane controls the movement of materials into and
out of the cell.
Low concentration
of ammonia
Cotton wool soaked
in ammonia
Litmus
Paper
high
concentration
Of ammonia
The litmus paper is tracking the progress of the ammonia
molecules as they move through the tube from right to left. As the
ammonia molecules move, the litmus turns blue.
The process by which molecules move from a region of high
concentration to a region of low concentration is called diffusion.
The apparatus below simulates the movement of materials in and
out of a cell membrane
The Visking tubing is meant to be the cell
membrane.
Glucose manages to move out of the bag and
into the surrounding water but starch does not.
Solution of glucose and starch
Visking tubing bag
Water
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Not all substances can pass through the cell membrane; some
molecules, like starch are too large. The cell membrane is called
selectively permeable because of this.
What sort of substances can move in and out of cells?
Living organisms are constantly using up substances in the cell
and constantly producing waste products. These substances have
to pass in or out of the cell by diffusion. Only small and soluble
substances can do this.
oxygen
glucose
Concentration outside cell
Glucose high
Carbon dioxide low
Oxygen high
Wastes low
waste
carbon dioxide
Concentration inside cell
Glucose low
Carbon dioxide high
Oxygen low
Wastes high
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The importance of diffusion to animals and plants
1. Gas exchange in animals
Large animals have specialist areas eg lungs to exchange gases.
air sac
High O2
conc.
low 02 conc .
LowCo2
conc.
High CO2 conc.
in the bloodstream
2. Gas exchange in plants
The diagram below shows the passage of oxygen leaving a leaf.
high
O2
Palisade
mesophyll
cell
spongy
mesophyll
cell
.
low O2
Guard cell
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Osmosis - a special case of diffusion
Some substances are made up of molecules that are too big to
pass through the membranes of living cells.
Osmosis is a special case of diffusion that only
involves water moving from a high concentration to a
low concentration.
Selectively permeable membrane
A selectively permeable membrane only allows certain substances
to pass through its pores. Usually this is because some substances
are too big to pass through the pores in the cell membrane.
The arrows show the movement of the small molecules through
the membrane.
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Concentration gradients
Osmosis is the movement of water in response to a concentration
gradient through a selectively permeable membrane.
What this means is that whenever there is a difference in
concentration on either side of a membrane, water will move by
osmosis to balance out the concentrations.
The bigger the difference (or the steeper the gradient) between
the inside and the outside the quicker water will move to balance
the concentrations.
90%
water
The water leaves the cell because
there is a higher water concentration
inside the cell.
10% water
10% sucrose
The water enters the cell because
there is a lower concentration of
water inside the cell.
1% sucrose
Remember, the more substances are dissolved in the water the
lower the water concentration.
A 5% sucrose solution has 95% water. A 10% sucrose solution
has only 90% water.
The definition of osmosis can now be written like this.
Osmosis is the movement of water down a concentration
gradient through a selectively permeable membrane.
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The effect of osmosis on plant cells
Cylinders of potato were placed in two different solutions for two
days. The results are in the table below:
Test
tube
Potato in
water
Potato in
sugar
soln
Initial
weight
(g)
5g
Final
weight
(g)
7g
Difference Change
in weight in texture
(g)
+2g
Firm
squishy
5g
3.5g
-1.5g
When a plant cell is placed in water, water enters the cell by
osmosis and pushes the membrane against the cell wall.
The potato feels firm to the touch.
Plant cell that are full of water are called turgid.
Placed in
water
The cell takes in water and becomes larger and turgid.
When plant cell is placed in salt solution, water leaves the
cell and the cytoplasm moves away from the cell wall.
The potato feels soft to the touch. Plant cells that are lacking
water are called plasmolysed.
Placed in strong
Sucrose soln.
The cytoplasm shrinks as does the vacuole.
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The effect of osmosis on animal cells
Animal cells are surrounded only by a cell membrane.
If too much water enters the cell they will burst.
placed in a higher
water conc.
Cell bursts
Same concentration
Cell stays the
same
placed in a lower
water conc.
C
Cell shrinks
Investigating Cell Division
Most of the cells of an organism contain a nucleus.
The nucleus controls all of the processes that take place within
each cell.
The information required for this control is carried in a code on the
chromosomes.
When the cell divides the next generation must contain an exact
copy of this information.
For an organism to grow replace cells or repair damage it must
make new cells.
The process by which cells make new copies of themselves is
called cell division. The cell membrane, cytoplasm etc, have to
be divided between the two new cells.
However the information has to be copied before the cell divides
so that each new cell has a full set of information.
Most human cells have 46 chromosomes in their nucleus so each
chromosome has to make a copy of itself before the cell divides.
The process by which the chromosomes are duplicated is called
mitosis.
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46 chromosomes
MOTHER CELL
MITOSIS
92 chromosomes
CELL DIVISION
46 chromosomes
46 chromosomes
DAUGHTER CELL
DAUGHTER CELL
The stages of Mitosis
At the end of mitosis, two daughter cells are produced which have
the identical number of chromosomes to the original mother cell
and carry the same information.
Each of the following descriptions corresponds to a diagram on the
next page.
1.
2.
3.
4.
5.
6.
The cell has grown from the last cell division.
The chromosomes make copies of themselves
The duplicated chromosomes are now visible. The two
copies of each chromosome are still joined by the
centromere and are called a pair of chromatids.
The pairs of chromatids line up along the equator of the cell.
The nuclear membrane has disappeared.
Each pair of chromatids is pulled apart to opposite ends or
poles of the cell.
The nuclear membrane reforms around each set of
chromosomes. The cytoplasm starts to divide.
The original mother cell has now divided to produce two
identical daughter chromosomes.
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1
2
3
4
5
6
12
The importance of mitosis
The most important feature of mitosis is that the new cells that are
produced have the same number of chromosomes as the original
cell.
This is important so that the daughter cells carry the same genetic
information as the mother cells and will be able to function
properly.
D
Investigating Enzymes
Most chemical reactions occur very slowly at room temperature.
eg
Hydrogen
peroxide
water + oxygen
Catalysts are special chemicals that can speed up a chemical
reaction but are not altered in any way and can be reused. Eg
manganese dioxide.
Catalysts in Living Cells
Living cells produce special catalysts. These catalysts speed up
reactions taking place inside living cells.
One easily studied biological catalyst is catalase.
Catalase speeds up the breakdown of hydrogen peroxide.
Catalase is an example of an enzyme.
Hydrogen peroxide
catalase
water + oxygen
Various fresh and boiled substances can be added to catalase and
the presence of the enzyme can be discovered if bubbles of
oxygen are released. Liver produces a lot of catalase but boiled
liver does not. This shows that boiling destroys the enzyme.
13
Degradation and Synthesis
There are two basic kinds of enzyme reactions.
One kind of reaction involves enzymes acting like scissors to ’cut
up’ or break down large molecules into smaller ones. These are
called degradation enzymes. Most of the enzymes you have met
so far have been degradation enzymes.
eg digestive enzymes like amylase and amylase and the enzyme
that breaks down hydrogen peroxide, catalase.
The other kind of reaction is where enzymes act like glue and build
up large molecules from smaller ones. These are called synthesis
enzymes.
eg processes like photosynthesis and the building of new muscles
involve synthesis enzymes.
A synthesis enzyme
When green plants photosynthesise they produce glucose. Some
of this glucose is not needed right away and so the plant store this
glucose as starch. Starchy food plants such as potato have an
enzyme called phosphorylase that synthesises starch.
Glucose - 1 - phosphate
phosphorylase
starch
The following experiment was carried out:
0min
5min
10min
15min
Gl –1-P
+ enzyme
Gl-1-P
+ water
Enzyme
+ water
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Iodine was added at 5 min intervals to detect the presence of
starch.
No starch was made in the second and third rows because a vital
substance was missing – these two rows are called controls. Only
the first row made starch because both the enzymes and the
substance it works on is present.
Phosphorylase can only synthesise starch from glucose-1phosphate because glucose-1-phosphate is its substrate, the
substance that the enzyme works on.
You have now met most of the enzymes that appear in the
Standard grade course.
The table below shows the enzymes and their substrates.
Enzyme
Substrate
Lipase
Produced
in
Salivary
glands &
pancreas
Pancreas
Fat
Fatty acids
& glycerol
Degradation
Protease
(pepsin)
Stomach
cells
Protein
Peptides
Degradation
catalase
Most living
cells
Hydrogen
peroxide
Water &
oxygen
Degradation
Phosphorylase
Potato cells Glucose-1phosphate
Starch
Synthesis
Amylase
Starch
Product(s) Synthesis/
Degradation
Maltose
Degradation
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What kind of molecules are enzymes?
Enzymes are made of protein. We know this because they react to
heat just like other proteins eg egg white.
Egg white is mostly a protein called albumen. What happens when
egg white is heated?
heated
clear
liquid
white
solid
The change in the protein cannot be reversed.
When proteins become too hot they change shape and this change
is permanent. We say that the protein has become denatured.
For enzymes this means that they will stop working permanently.
The graph below shows the change in an enzymes activity when
the temperature increases.
optimum
temperature
Rate
Of
Enzyme
Reaction
0
10
20
30
40
50
60
Temperature (oC)
16
Above 50 the graph drops steeply because the enzyme has
become denatured.
Enzymes work best at a certain temperature. They work slowly
when the temperature is too cold . If the temperature is too
hot the enzyme becomes denatured an stops working altogether.
This change cannot be reversed.
There is a temperature at which the enzyme works best. This is
called its optimum temperature.
The effect of pH on enzyme activity
Pepsin is an enzyme produced in the stomach. It speeds up the
breakdown of protein.
The table below show the results of an experiment where the
activity of pepsin was investigated by changing the pH.
pH
1.0
2.5
3.7
4.9
7.0
8.4
9.0
Activity of enzyme
9
14
8
6
3
1
0
A similar graph to temperature is obtained but the rate of reaction
does not drop steeply after the optimum pH as it does with
temperature. This is because the enzyme is not denatured.
Optimum pH
Rate
Of
Enzyme
reaction
1
2
3
4
5
6
7
8
9
pH
17
Like temperature, the pH at which an enzyme works best is
called its optimum pH.
Not all enzymes have the same optimum temperature or pH.
The optimum temperature for enzymes working in your body is 37
0
C
Each enzyme has a pH range over which it can function. Outside
this range it does not function.
The table below shows the ranges and optimum pH for 3 enzymes.
Graph
Enzyme name
A
Amylase
Working pH
range
5 to 9
Optimum pH
B
Pepsin
1 to 5
2.8
C
Catalase
6 to 12
9
7
Specificity
Each enzyme acts on only one specific substrate and does not
affect other substances.
Amylase breaks down starch to maltose only. Another enzyme,
phosphorylase, builds up starch from glucose. Neither enzyme can
do the other’s job; amylase cannot synthesise starch and
phosphorylase cannot break it down. Why is this?
Enzymes and their substrates have specific shapes and the
substrate fits into the enzyme a bit like a key fits into a lock.
The enzyme has a special area called the active site into which the
substrate fits. This enables the enzyme to be specific. Only one
substrate will fit into this spot, no other substrates will have the
exact shape and fit.
Once in place the enzyme allows the substrate to react and then
the products leave the enzyme leaving the active site to be reused.
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The diagram below show how a degradation reaction might take
place.
Enzyme
Active site
Substrate
Enzyme-substrate
complex
Enzyme unchanged
Product
product
19
E
Investigating Aerobic Respiration
Why do cells need energy?
Cell divison
Cell growth
Muscular
contraction
Cells
Need
Energy
For
Uptake
of chemicals
synthesis of
large molecules
Animals and plants need energy to grow, to make
heat to keep warm and to move to catch their prey.
Energy from food
Animals get the energy they need from their food
It is becoming very common on food labels to see the energy
content of food and how much fat, carbohydrate and protein are
in the food.
20
Three foods were analysed for their nutritional content.
Food
%
% fat %
carbohydrate
protein
Olive oil (FAT)
0
Gelatin
0
(PROTEIN)
Sugar
100
(CARBOHYDRATE)
100
0
0
100
Energy
content
kJ/g
39
19
0
0
19
It can be seen from the table that olive oil had the highest energy
content, in fact twice as much energy as proteins or
carbohydrates.
Metabolism
All the chemical reactions that occur in all the cells of a living
organism, are known as the metabolism of the cell.
All these reactions are controlled by enzymes.
Reactions which release energy eg aerobic respiration are
important because the energy they release is used by synthesis
reactions to makes new compounds and cells.
The aerobic respiration equation
The sequence of chemical reactions within a cell that release
energy from food is known as aerobic respiration. Each stage of
the process is controlled by enzymes.
Aerobic respiration requires oxygen.
Glucose + oxygen
energy + carbon dioxide + water
The raw materials are glucose and oxygen
The useful product is energy
The waste products are carbon dioxide and water.
We can demonstrate various parts of the aerobic respiration as
experiments.
21
The uptake of oxygen
The apparatus below is called a respirometer. It can measure how
quickly an organism uses up oxygen and hence how fast it is
respiring aerobically.
respiring
peas
Co2
Absorbing
liquid
1.
As the organism respires, oxygen is used up and the
coloured liquid moves up the tube to fill the vacant space.
Carbon dioxide does not interfere with this experiment
because it is absorbed by soda lime.
2.
The faster the liquid moves up the tube, the faster the
organism is doing aerobic respiration .
3.
A suitable control for this experiment would be exactly the
same experimental set up but with either no peas or peas
that had been boiled.
4.
It is important to do this experiment at a steady temperature
because temperature fluctuations would cause the gases to
expand or contract and interfere with the results.
22
5.
A warm blooded animal would be unsuitable for this
experiment because it s body would give out heat and
interferes with the experiment so normally this experiment is
done with plant material or an invertebrate like woodlice or a
snail.
The release of carbon dioxide
leaf
Black paper
Bicarbonate indicator is a useful substance for showing relative
quantities of carbon dioxide.
A red colour indicates that there is the same level of CO2 as in the
air.
Yellow means that there is a higher concentration of CO2 than in
the air; organisms must have being doing respiration.
Purple means that there is less CO2 than in the air; organisms
must have been doing photosynthesis.
Test Contents
tube
Indicator CO2
colour at Concentration
end
A woodlouse Yellow
Higher than
air
B Germinating Yellow
Higher than
peas
air
C mushroom yellow
Higher than
air
D Leaf in dark purple
Lower then
air
Process going
on
respiration
respiration
respiration
photosynthesis
23
Where does the CO2 come from?
Some pupils designed an investigation to find the source of the
carbon dioxide given off by organisms when they respire
aerobically. They used the following apparatus:
Burning food
After the food was burned, lime water was poured into the jar.
The lime water turned milky. This experiment shows that the
carbon dioxide released in respiration has come from food.
The release of energy as heat
Energy is the useful product from aerobic respiration.
Some of this energy is released as heat.
Plants release energy when they germinate and grow.
24
The control flask has boiled seeds because they are dead and will
not ne respiring.
A typical set of results would be:
_____________________________________________________
Flask
Temp at start
Temp at end
Temp rise 0C
0
0
C
C
A
21
32
11
B
21
21
0
From this experiment we can conclude that living cells release heat
energy during respiration.
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