GCSE Biology Edexcel B2 Topic 1

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B2 Topic 1 The building blocks of cells
Plant cells and animal cells
Even though they belong to different organisms, plant and animal cells have some features in common:
o Cell Membrane ­ separates the contents of the cell and its surroundings. It controls the movement of
substances like glucose, oxygen and carbon dioxide in and out of the cell.
o Cytoplasm ­ many of the chemical reactions needed to carry out life processes take place here. It
also contains organelles.
o Nucleus ­ an organelle that contains DNA which is the genetic material. The nucleus controls all
activities of the cell.
o Mitochondria ­ these are organelles where respiration occurs. They are incredibly small and cannot
be seen easily with a light microscope.
Plant cells have other structures:
o Cell Wall ­ made of tough cellulose which supports the cell and helps it keep its shape.
o Large vacuole- this fills up a space in the cytoplasm and is filled with cell sap and keeps the cells rigid,
thus supporting the plant.
o Chloroplasts ­ organelles which contain chlorophyll which is a green substance that absorbs light
energy, used for photosynthesis.
A bacterium
Bacteria are single celled organisms that are much smaller than plant or animal cells. They do not contain a
Bacterial cells contain two types of DNA; chromosomal DNA and Plasmid DNA. The former is a giant loop of
DNA containing most of the genetic material. It is not neatly packaged like plant or human chromosomes.
Plasmid DNA comes in small loops and carries extra information.
Using information from electron microscopes we know that bacterial cells have a cell wall but it is different to
that of plant cell walls. It is more flexible and not made of cellulose, but does a similar job.
Some bacteria also have flagella on the outside of the cell; they are long whip-like structures that bacteria
can use to move themselves along.
Microscope magnification

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Microscopes have improved greatly over the past 350 years. The best modern light microscopes can magnify
specimens more than 1500 times.
As a result, we can observe very small structures inside cells. For example, we can see inside bacterial cells.
In the 1930s, the electron microscope was invented. This casts out a beam of electrons to magnify specimens
up to about 2,000,000 times! As well as this, electron microscopes produce very clear images and show us
more about the detailed structure of cells.…read more

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RNA (transfer) codons are attached to amino acids, each codon has a different amino acid
4. tRNA codons join to the complementary codons (bases) on the mRNA
5. As more amino acids are brought by the tRNA they bond together with peptide bonds.
6. Long chain of amino acids forms a polypeptide
7. Protein formed.
A mutation is a change in the sequence of bases in the genetic code.…read more

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Put the beaker into a water bath at 60C for 15 minutes as this denatures the enzymes that could digest the
DNA and help soften the onion cells.
3. The beaker must be put into ice to cool down quickly which stops the DNA from breaking down.
4. When the mixture is ice-cold, it should be put into a blender for a few seconds to break open the cell walls
and release (but not break up) the DNA.
5.…read more

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Golden rice is a variety of GM rice which contains two genes from other organisms which together
enable the rice to produce beta-carotene.
c. Growing golden rice in these places will mean the people will eat them and fewer people will suffer
from vitamin A deficiency.
2. Producing human insulin
a. The human insulin gene can be inserted into bacteria to produce human insulin (see later).
b.…read more

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This is the division of somatic/body cells. These cells are diploid which means they have the full number of
chromosomes ­ 46.
When a cell divides by mitosis it makes two cells identical to the original cell, each with a nucleus containing
the same 46 chromosomes.
This type of cell division is for growing/replacing cells that have been damaged, or simply growth.
Asexual reproduction also uses mitosis, such as strawberry plants.
The process
1.…read more

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Meiosis only occurs in reproductive organs. Four haploid nuclei are produced whose chromosomes are not
Two divisions occur during this process.
Division 1
As with mitosis, the cell duplicates its DNA.
The chromosome pairs line up in the centre of the cell.
The pairs of chromosomes are pulled apart so each new cell only has one copy of each chromosome. Some of
the father's chromosomes and some of the mother's chromosomes go into each new cell.…read more

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Implantation of embryo into surrogate mammal. (sheep 3)
6. The offspring born by the surrogate mammal is a clone of sheep 1 not sheep 2.
Making plant clones can be relatively easy; you start with a bit of leaf, stem or root of the original plant. The
plant cells divide and produce new cells, which grow into a clone of the original plant. This is an example of
asexual reproduction.
Advantages of cloning
Cloning mammals could help with the shortage of organs for transplants.…read more

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These will survive in the patient as their body would recognise the new white blood cells as
their own cells.
This technique could treat many more problems than adult stem cells could as embryonic stem cells can
differentiate into almost every kind of cell. However, that means that this could be used illegally by people
to produce human clones.
Scientists have also been investigating ways of reprogramming specialised cells into stem cells. This would
avoid the ethical problem with using embryos if it works.…read more

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Therefore, adding
more substrate molecules will make no difference.
Specific enzymes
Enzymes only work with a particular substrate, so we say they have a specificity for their substrate. Enzymes
are therefore named according to the substrate they catalyse:
o Carbohydrases catalyse the breakdown of carbohydrates.
o Proteases catalyse the breakdown of proteins.
All substrate molecules for one particular enzyme have the same 3-D shape in some part of their molecules.
This suggests shape is incredibly important in enzyme reaction.…read more


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