Investigating Living Cells
State that cells are the basic units of living things
All living things (plants and animals) are made up of at least one cell.
Cells are called the basic unit of living things, in the same way that bricks are the basic
unit of a building like this school.
Microscopes and cells
Most cells are very small, and can only be seen properly by looking at them under a
microscope.
When looking at cells under a microscope they must be:
a.
thin – to let the light pass through them
b.
flat and not folded- to let us see the cells clearly
c.
moist - to stop the cells drying up
d.
clear of air bubbles - these can spoil the clear view of the
cells
Explain the purpose of staining animal and plant cells
Stains are used to show up internal structures which would not otherwise be seen.
Unstained specimen
specimen stained with iodine solution
Describe the structural similarities of and differences between animal and plant cells
Cells can be a variety of different shapes and sizes, but they all have structures in
common. When we look closely at plant and animal cells, we can also notice that the
plant cells have structures that are not found in animal cells – plant cells are different
from animal cells.
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Structure
Nucleus
Cytoplasm
Cell
membrane
Large vacuole
Chloroplast
Cell wall
Similarities
Function
Controls cell activities
Site of reactions
Controls the entry and exit of materials into and out of
the cell
Contains cell sap
Site of photosynthesis
Rigid to give the cell shape
Differences
All cells have:
1.
nucleus
2.
cell membrane
3.
cytoplasm
In addition plant cells have:
1.
vacuole
2.
chloroplast
3.
cell wall
Investigating Diffusion
State that a substance can diffuse from a high concentration to a low one
Any substances that need to move into or
out of cells must move through the cell
membrane. To do this the substance must
be dissolved in a liquid like water.
The process that allows substances to move
into and out of cells is called DIFFUSION.
Diffusion is the movement of a substance
from an area of high concentration to an
area of low concentration.
the cell membrane controls the passage of substances into and out of the cell
Cell membranes control what can enter or leave the cell because they have tiny pores in
them that only allow small substances to pass through by diffusion.
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Explain the importance of diffusion to organisms
Cell membranes have tiny pores in them that allow only small substances to pass
through them by diffusion, as long as the small substances are soluble (dissolved in a
liquid).
Many substances need to be transported
Useful
Waste
into cells – these are needed to allow the
substances
substances
cell to function properly.
Many substances need to be transported
out of cells – these include waste products
that would poison the cells.
Give examples of substances which enter and leave the cell by diffusion, e.g. dissolved
food, oxygen, carbon dioxide and water
Substance
Glucose
Carbon dioxide
Oxygen
Waste
Concentration OUTSIDE cell
High
Low
High
Low
oxygen
Concentration INSIDE cell
Low
High
Low
High
Carbon dioxide
glucose
waste
cell
Diffusion is a very important process for all living cells because it ensures that all the
substances a cell needs move into the cell, and all the ones the cell does not need move
out of the cell. Remember . . . substances always move from a high concentration to a
low concentration.
Identify osmosis as a ‘special case’ of the diffusion of water
Water is a very important substance to all living cells, water will move into cells if they
have too little and move out of cells if they have too much.
Water moves in or out of cells by a special type of diffusion called osmosis. Osmosis
only ever involves the movement of water.
Osmosis is a special case of diffusion that only involves water moving from an area of
high water concentration to a region of low water concentration.
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Explain osmosis in terms of a selectively permeable membrane and of a concentration
gradient
If substances can pass through a membrane we say
that the membrane is permeable. Since cell membranes
can choose or select which substances can pass through
them (small ones only), we say that the membrane is
selectively permeable.
The bigger the concentration gradient between the inside
and outside of the cell the quicker the water will move by
osmosis.
Osmosis is the movement of water down a concentration gradient through a selectively
permeable membrane.
Explain observed osmotic effects in plants and in animal cells in terms of the
concentration of water in the solutions involved
There are three different situations a cell can be in when investigating osmosis.
1
The water concentration outside the cell is the same as inside.
2
The water concentration outside the cell is higher than inside.
3
The water concentration outside the cell is lower than outside.
In the first situation, water moves in and out at the same rate and the cell stays the
same – it neither gains nor loses water.
In situation 2 the water moves from a high concentration outside the cell to a lower
concentration inside.
In situation 3 the water moves from a high concentration inside the cell to a lower
concentration outside the cell.
Animal cells
e.g. red blood cells
Cells placed in pure/distilled water
Water enters the cell, the cell swells and
bursts
Cells placed in a solution of
equal water concentration
There is no net movement of water so the cell
stays the same
cells placed in a strong salt/sucrose solution
(low water concentration)
Water moves out of the cell. The cell shrinks
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Plant cells
e.g. onion cells
Cell placed in pure/distilled water
Water moves in
Cell is turgid
Cell placed in a solution of equal water
There is no net movement of
water so the cell is unchanged
concentration
cell placed in strong sucrose/salt solution .
Water moves out of the cell by osmosis
and the cell is plasmolysed
External
solution
Appearance of
Cell membrane
Shape of
vacuole
Appearance of
Cell wall
Turgid cells
Pure water
Plasmolysed cells
Strong sucrose
Pushed up against the cell
wall
Full
Pulled away from the cell
wall
Shrunk
Stretched slightly
Normal
Plant cells do not burst when placed in distilled water as the cell wall stops them from
doing so.
Investigating Cell Division
The nucleus of every cell contains all the
information that is needed for that cell to carry
out all of its necessary functions.
This information is present in thread-like
structures called chromosomes. Chromosomes
are made up of genes. Chromosomes are only
seen using a very powerful microscope because
they are very small.
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State that cell division is a means of increasing the number of cells in an organism
and occurs when a cell divides (splits) to form two new cells.
Cells divide when there is a need to increase the number of cells in the organism i.e.
when:
1
The organism is growing (remember we all began life as a single cell).
2
Bone cells divide to repair a break (or a fracture).
3
Skin cells divide to heal a wound (or a cut).
4
A zygote (fertilised egg cell) divides to form an embryo. This is called
development.
State that the nucleus of the cell controls cell activities including division
State that each of the two cells produced by cell division has a complete set of
chromosomes and the same information
Two cells are made when a cell divides.
These new cells are exactly the same as each other and the cell that they came from
(the mother cell).
They are identical to each other because they have the same information – the same
number and type of chromosome in their nucleus.
The chromosomes in the nucleus of the mother cell are copied before division so
that each new cell has a full set.
Describe the stages of mitosis
1
2
6
3
Stage 1 each chromosome in the nucleus of
the mother cell makes a copy of itself.
Stage 2 Chromosomes are now made up of 2
chromatids joined at a point called the
centromere. The membrane around the nucleus
disappears.
Stage 3 the chromosomes (made up of 2
chromatids) line up along the equator of
the cell
Stage 4 The 2 chromatids of each chromosome
are pulled apart (separated) by long, thin
structures called spindle fibres.
The chromatids of each chromosome are pulled
to opposite ends of the cell (the poles). The
chromatids are now called chromosomes again.
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4
Stage5 A membrane forms around each of the two groups of chromosomes. The cell
now has 2 identical nuclei. The nucleus has now divided. The mother cell starts to be
pulled in around its middle.
Stage6 The cytoplasm divides into 2 identical halves. The cytoplasm has now divided.
The original mother cell has now divided into two identical daughter Cells. Mitosis is
now complete.
Explain why it is important that the chromosome complement of daughter cells in
multicellular organisms is maintained
Cells made by mitosis are identical to each other and the mother cell they came from.
They have the same chromosome complement – same type and number of chromosomes.
This is important as it allows new cells to carry out an identical job as the mother cell.
Identify the correct sequence of stages of mitosis
Investigating Enzymes
The cytoplasm is the site of chemical reactions.
Explain why enzymes are required for the functioning of living cells
Enzymes are proteins that speed up the rate of chemical reactions. Without enzymes,
chemical reactions would take place too slowly to allow life to continue.
Explain the meaning of the term ‘catalyst’
Catalysts are special chemicals that speed up the rate of a chemical reaction but
remain unchanged themselves. Catalysts can be used again.
Enzymes are called biological catalysts since they are produced by living cells.
Hydrogen peroxide

water + oxygen
All living cells contain the enzyme catalase which speeds up the breakdown of hydrogen
peroxide.
There are 2 types of chemical reaction: synthesis
degradation
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(build up)
(break down)
Give an example of an enzyme involved in the chemical breakdown of a substance
amylase
starch

maltose
Give an example of an enzyme involved in synthesis
Green plants make glucose by photosynthesis. Extra glucose is stored as starch.
The synthesis enzyme that builds up glucose into starch is called phosphorylase.
Phosphorylase is found in large quantities in potato tubers.
Potato phosphorylase
Glucose – 1 – phosphate
 starch
Explain the term ‘specific’ as applied to enzymes and their substrates
All enzymes are specific because they work on only one type of substrate.
Enzyme
Type of reaction
catalysed
Amylase
Degradation
Catalase
Degradation
Phosphorylase Synthesis
Substrate
Starch
Hydrogen peroxide
Glucose-1-phosphate
Product(s)
Maltose
Water + oxygen
Starch
Enzymes recognise their substrates by their
shapes.
Enzymes are like keys because they: match exactly
into the shape of one substrate this is the LOCK
and KEY theory
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State that enzymes are proteins
Describe the effect of temperature on enzyme activity
Enzymes work best at certain temperatures, they
work slower if the temperature is too cold. If the
temperature is too hot, the enzyme stops working
altogether, this change cannot be reversed. The
enzyme changes shape and stop working and this
change is permanent.
When this occurs (temperatures above 50 C) we say
that enzymes are denatured.
Describe the effect of a range of pH on the activity of pepsin and catalase
Like temperature, pH affects how fast an enzyme works.
The activity of an enzyme is affected by
how acid or alkali it is, this is called the
pH of an enzyme. Most enzymes work best
at neutral pH of number 7. The enzyme
pepsin works in a pH range of pH 1 to pH 5
but works best at pH 3. If the pH is too
high or too low the enzyme does not work
as fast as it could do. The enzyme
catalase is active in the pH range of pH 6
to pH 12 but is most active at pH 9
Explain the term optimum as applied to the range of conditions in which enzymes
operate
The
The
The
The
The
The
conditions that allow an enzyme to work best are called optimum conditions.
temperature an enzyme works best at is called its optimum temperature.
optimum temperature for human enzymes is 37oC.
optimum temperature for plant enzymes is between 20oC and 25oC
pH an enzyme works best at is called its optimum pH.
optimum pH for: catalase is pH 9; pepsin is pH 3
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Investigating Aerobic Respiration
State three reasons why living cells need energy
All cells need energy to
a
b
c
d
e
carry out important functions like:
making more of themselves (Cell division)
increasing their size (Growth)
building up or breaking down substances in
the cell (Chemical reactions)
allowing the cell to change position (Movement)
keeping the temperature of the cell constant
(Heat production)
All cells get their energy from food and how they get their food depends on the type
of cell:
i
Animal cells get their food when the animal Eats
ii
Plant cells can Make their own food by a process called PHOTOSYNTHESIS
State that fats and oils contain more chemical energy per gram than carbohydrates or
proteins
The energy in our food is used in respiration to provide our cells with energy in a form
that they can use.
Fats contain twice as much energy per gram as carbohydrates or protein.
Carbohydrate 19KJ/g
Fat 38KJ/g
Protein 19KJ/g
State that cells need oxygen to release energy from food during aerobic respiration
Living cells use the gas Oxygen when they carry out Aerobic respiration.
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Describe aerobic respiration in terms of a word equation
GLUCOSE + OXYGEN
raw materials
 ENERGY
useful product
+ CARBON DIOXIDE +WATER
waste products
State that carbon dioxide is given off by cells during tissue respiration and is derived
from food
All living cells produce the gas carbon dioxide when they carry out aerobic respiration.
The carbon dioxide comes from the food. Remember glucose contains carbon,
hydrogen and oxygen.
State that heat energy may be released from cells during respiration
Some of this energy is released as heat. This is important to warm blooded animals who
need to maintain their body temperature at a constant level.
Explain the importance of energy released from food during respiration to the
metabolism of cells
All the chemical reactions that occur in the cytoplasm of a cell are, together known as
the metabolism of the cell.
Every one of these reactions is controlled by an enzyme.
There are 2 different types of chemical reaction that make up the metabolism of a
cell: Synthesis and Degradation
Build up reactions usually use energy and breakdown reactions usually release energy.
Respiration is important because it provides the energy to allow all the other reactions
in the cell to occur.
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