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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
vacuole
Cytoplasm nucleus cell membrane
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Animal cell
Cytoplasm nucleus
Celll membrane
Part of a Cell Function
Cytoplasm
Chloroplast
Cell wall
Where cell’s activities take place
Contains green chlorophyll; involved in photosynthesis
Supports cell and gives it structure. Made of cellulose.
Cell membrane
Vacuole
Boundary of cell; controls entry and exit of materials
Fluid filled sac; stores and regulates water.
Controls all cell activities 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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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 waste carbon dioxide
Concentration outside cell Concentration inside cell
Glucose high
Carbon dioxide low
Glucose low
Carbon dioxide high
Oxygen high
Wastes low
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 O
2
LowCo
2 conc. conc. high
O
2 low 0
2
conc . High CO
2
conc. in the bloodstream
2. Gas exchange in plants
The diagram below shows the passage of oxygen leaving a leaf.
Palisade mesophyll cell spongy mesophyll cell
. low O
2
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
Initial weight
(g)
5g
Final weight
(g)
7g
Difference in weight
(g)
+2g
Change in texture
Firm
Potato in sugar sol n
5g 3.5g -1.5g squishy
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.
Cell bursts
water conc.
placed in a higher
Same concentration
placed in a lower
water conc.
Cell stays the
same
Cell shrinks
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.
The cell has grown from the last cell division.
The chromosomes make copies of themselves
2.
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.
3. The pairs of chromatids line up along the equator of the cell.
The nuclear membrane has disappeared.
4. Each pair of chromatids is pulled apart to opposite ends or poles of the cell.
5. 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
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1
3
5
2
6
4
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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.
Most chemical reactions occur very slowly at room temperature. eg Hydrogen water + oxygen peroxide
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.
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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.
phosphorylase
Glucose - 1 - phosphate starch
The following experiment was carried out:
0min 5min 10min
Gl –1-P
+ enzyme
Gl-1-P
+ water
Enzyme
+ water
15min
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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-1- phosphate 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 Produced in
Amylase Salivary glands &
Lipase
Substrate Product(s) Synthesis/
Starch pancreas
Pancreas Fat
Maltose
Degradation
Degradation
Degradation Fatty acids
& glycerol
Protease
(pepsin) catalase
Stomach cells
Most living cells
Protein
Hydrogen peroxide
Glucose-1phosphate
Peptides
Water & oxygen
Starch
Degradation
Degradation
Synthesis Phos- phorylase
Potato cells
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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? clear liquid
heated 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 ( o C)
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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 Activity of enzyme
1.0 9
2.5 14
3.7 8
4.9 6
7.0 3
8.4 1
9.0 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
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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.
0
The optimum temperature for enzymes working in your body is 37
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 Working pH range
Optimum pH
Amylase 5 to 9 7 A
B Pepsin 1 to 5 2.8
C Catalase 6 to 12 9
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
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Why do cells need energy?
Cell divison
Cell growth
Cells
Need
Energy
For
Muscular
contraction
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.
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Three foods were analysed for their nutritional content.
Food
Olive oil (FAT)
Gelatin
(PROTEIN)
% carbohydrate
0
0
% fat %
100
0 protein
0
100
Energy content kJ/g
39
19
Sugar
(CARBOHYDRATE)
100 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.
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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
Co
2
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.
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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 CO
2
as in the air.
Yellow means that there is a higher concentration of CO
2
than in the air; organisms must have being doing respiration.
Purple means that there is less CO
2
than in the air; organisms must have been doing photosynthesis.
Test tube
Contents Indicator colour at end
CO2
Concentration
Process going on respiration A woodlouse Yellow
B Germinating peas
Higher than air
Yellow Higher than air
C
D mushroom yellow
Leaf in dark purple
Higher than air
Lower then air respiration respiration photosynthesis
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Where does the CO
2
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.
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The control flask has boiled seeds because they are dead and will not ne respiring.
A typical set of results would be:
_____________________________________________________
Flask
A
B
0
Temp at start
C
21
21
0
Temp at end
C
32
21
Temp rise
11
0
0 C
From this experiment we can conclude that living cells release heat energy during respiration.
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