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BIOLOGY TOPIC 4
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1
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Contains organelles – tiny structures that do
specific jobs



The nucleus controls all the activities of the cell
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Converts oxygen and
glucose to produce energy (more later)



Ribosomes – make protein

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Plant cells contain all the above – but they also have some other structures

Mitochondria
Cell membrane
Chloroplasts
Vacuole

Cell wall

Cytoplasm
Nucleus



The cell wall is made out of tough cellulose (gives fruit and vegetables their ‘crunchy
feel’) – which keeps the cell in a rigid shape
...
(much more later)

Bacteria – no nucleus - Prokaryotes

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Cell wall

Plasmid DNA




Cytoplasm

Cell membrane

Plasmid DNA – a small loop of DNA containing extra information
(maybe more than one)
The cell wall is not rigid like that of plant

Cell specialisation
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Some cells are specialised to carry out a particular function
Some specialised animal cells:


Sperm cell

Tail

Mitochondria

Nucleus

Males produce sperm cells:
This is a tiny cell
...

(To join with a female cell to make a baby – see later)


Nerve cell
Receptor

Long and thin – to carry electric nerve impulses


Muscle cell

Can contract – to move bones

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Some specialised plant cells:


Root hair cells

Large surface area – to absorb water and minerals from soil


Xylem and Phloem cells

Plants possess special tissue called xylem
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(so no organelles) These tubes transport water and
dissolved mineral salts and also they have rigid sides which give support to the plant
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These are living cells which also line together to make continuous tubes
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We can see individual cells using light microscopes
Modern electron microscopes have much higher magnification
and resolution - (seeing separate points)
magnification =

𝑖𝑚𝑎𝑔𝑒 𝑠𝑖𝑧𝑒
𝑜𝑏𝑗𝑒𝑐𝑡 𝑠𝑖𝑧𝑒

Image size

Magnification x Object size

Size of objects: 1m – ‘giant step’
1
1cm - 100 m - 1 x 10-2 m

1

1mm - 1000 m - 1 x 10-3 m

1 micrometre - 1 μm - 1 millionth of a metre 1 nanometre

1
1 000 000
1

m - 1 x 10-6 m

1 thousand millionth of a metre 1 000 000 000 m - 1 x 10-9 m

To change metre, m to micrometres, μm  multiply by 1 000 000
To change millimetre, mm to micrometres, μm  multiply by 1 000

Required Practical Activity 1
Use a light microscope to observe, draw and label cells

CELL DIVISION 4
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Chromosomes

The nucleus of a cell contains chromosomes – looking like long threads
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Genes are the instructions as to what our body is like
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Human cells have 46 chromosomes arranged in 23 pairs
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The cells reproduce themselves to produce new identical
cells
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46

46

46

2 identical cells

It works like this: (called the cell cycle)
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Each nucleus contains 46 chromosomes
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Each chromosome makes an identical copy of itself (DNA replication)

2
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The copies and the original chromosomes are pulled to either end of the cell
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And then the cell splits into two identical “daughter” cells

The two ‘daughter’ cells are genetically identical to the parent cell –and therefore each has 46
chromosomes

Stem Cells
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Stem cells in mammals:
Males produce sperm cells:

This is a tiny cell
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Females produce ovum or egg cells:

This is a larger cell because it carries a store of food as well
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Once the sperm and ovum are joined together – fertilisation - it will divide and form a new
baby
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This first
group of identical cells is called an embryo
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It will only be
after about 2 weeks (with humans) that differences start to emerge - the cells start to
differentiate – with some cells forming bone tissue, some forming bones and muscle etc
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fertilized egg

4-cell embryo

8-cell embryo

Stem cells are cells with no particular specialisation – but which can differentiate to form
specialised cells

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A second type of stem cells found in our bone marrow
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They cannot
change into any cell – like embryonic stem cells can
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They can differentiate into different
types of plant cells
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Transplant patients have to wait a long time for a suitable organ to become
available
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(That is why transplanted kidneys are usually removed from a
close relative)
If we could take stem cells from a person who has, say, a damaged liver and use these to form
a new liver, then this could be used to cure the person
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Stem cell research is finding possible uses for stem cells
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They can use the clones
as a source of stem cells
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Some people object to obtaining stem cells this way
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They say that it is wrong to alter or remove embryos
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Others say that an early embryo is only a collection of
identical cell which has not yet started to form into a baby and therefore there’s nothing
wrong with removing or changing them
In January 2008, scientists found a way to extract embryo stem cells without harming the
embryo
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Sometimes the stem cells can become infected by
viral infections – and infect the patient
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g
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We say that the dye has diffused through the water
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Pure
Wate
r

High
concentration

Drop of
Dye

Diffusion

Dye evenly spread
in water

Low
concentration

Diffusion is the net movement of particles from an area of higher concentration to an area of
lower concentration until they are evenly spread out

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Examples of diffusion


In the lungs

CO2 CO2
O2
alveoli

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The kidney removes
the urea from the blood
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Factors Affecting Diffusion


The difference in concentration

Diffuse slowly
– low concentration gradient


Diffuse quickly
– high concentration gradient

Temperature
At high temperature, the particles move faster and diffuse faster



The surface area of the membrane

If a person is cold, she’ll curl herself up

If a person is hot, she’ll spread herself out

Large surface area

Small surface area

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Some maths!
This cube has side 1cm
Surface area = 6
Volume = 1
Surface area to volume ratio = 6:1

This cube has side 2cm
Surface area = 24
Volume = 8
Surface area to volume ratio = 24:8  3:1

A single cell organism has a large surface area to volume ratio
This means that sufficient molecules can diffuse in and out of the cell to meet the needs
of the organism

A multicellular organism has a smaller surface area to volume ratio
...

Instead, these organisms have special cells which allow substances to pass through easily

In order to allow a high rate of diffusion cells should have:


a large surface area



a thin membrane



an efficient blood supply – to carry the molecules to and from the cell



good ventilation (breathing mechanism) – to carry the molecules to and from the cell

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In mammals, the small molecules of digested food enter the body in the small intestine
Large surface area

Thin wall
Blood supply

In mammals, in the lungs, carbon dioxide leaves the body and oxygen

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