gcse b2 - topic 1 building blocks of cells.pdf

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Doc Brown's Edexcel GCSE Science-Biology Revision Notes

EDEXCEL GCSE Additional Science BIOLOGY UNIT B2 The components of life STUDY NOTES

BIOLOGY UNIT B2 Topic 1 The building blocks of cells Revision Notes

A cell is the smallest unit of life able to control its own activit ies, but it relies on the rest of the organism (if mult icellular) or thesurroundings (if unicellular) to provide it with raw materials i.e. nutrients and removal

1.1 Be able to describe the function of the components of a bacterial cell including chromosomal

DNA, plasmid DNA, flagella and cell wall (see diagram and notes below).

Bacterial cells are much smaller than plant or animal cells with some quite distinct and different features.

Chromosomal DNA: One long strand of DNA comprises the single chromosome that controls the cells functions and the

cell division of replication.

The chromosomal DNA moves freely around the cytoplasm and is not confined in a distinct nucleus as in plant and

animal cells.

Plasmid DNA: Plasmids are small hoops of extra DNA that are separate from the chromosomal DNA.

Plasmids contain genes that help tolerance against drugs and can be passed from one

bacteria to another.

This is how the dangerous bacteria MSRA have evolved.

Flagella: The flagellum is a long thin tail like structure that projects out of the body of the

cell, and can rotate to move the bacteria along.

Some bacterial cells have more than one flagella (flagellum).

Each flagellum is effectively driven by a tiny biochemical electric motor with moving

parts, mostly made of proteins!

It is quite a remarkable piece of biochemical engineering - bioengineering!

Cell wall: The cell contents i.e. the cytoplasm, DNA etc. are all held together within the cell

wall surface membrane which controls the passage of substances in and out of the cell.

Cytoplasm: Cytoplasm is a jelly like fluid in which most of the cells chemical reactions take

place with the aid of enzyme catalysts.

1.2 Be able to describe the function of the components of a plant cell including chloroplast, large

vacuole, cell wall, cell membrane, mitochondria, cy toplasm and nucleus (see diagram and notesbelow) and know the differences between plant and animal cells.

Plant cells are much larger than bacterial cells, with important differences from animal cells.

Chloroplast: The chloroplasts contain the green chlorophyll molecules which are involved in the energy absorbing processof photosynthesis.

sunlight energy + carbon dioxide + water ==> sugars e.g. glucose + oxygen

Therefore chloroplasts are the site of food production.

Large vacuole: The large vacuole in plant cells contains cell sap, a dilute solution of salts and sugars.

Cell wall: The rigid cell wall in plant cells is made of cellulose and gives the cell membrane and contents extra support

giving plants their rigidity - their stable 3D structure.


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Cell membrane: The cell contents i.e. the cytoplasm, nucleus, vacuole,

mitochondria, chloroplasts etc. are all held together by the cell surface membrane

which controls the passage of substances in and out of the cell.

Mitochondria: The energy releasing chemistry of respiration occurs in the

mitochondria.

e.g. glucose + oxygen ==> carbon dioxide + water + energy

Mitochondria are power house of the cells and provides the chemical energy

for any of the cells functions.

Cytoplasm: Cytoplasm is a jelly like fluid in which most of the cells chemical

reactions take place.

Nucleus: The cell nucleus contains all the DNA of the genes in the chromosomes

which control the cells functions and the cell division of replication. The nucleus

controls the activities of the cell by sending instructions to the cytoplasm.

Starch grains: Stored food for respiration

1.3 Be able to describe the function of the

components of an animal cell including cellmembrane, mitochondria, cy toplasm andnucleus (see diagram and notes below).

Animal cells are much larger than

bacterial cells, with important

differences from plant cells.

Cell membrane: The cell contents i.e.

the cytoplasm, nucleus, vacuoles,

mitochondria etc. are all held together

by the cell membrane which controls

the passage of substances in and out

of the cell.

Mitochondria: The energy releasingchemistry of respiration occurs in themitochondria.

e.g. glucose + oxygen ==> carbon dioxide + water + energy

Mitochondria are power house of the cells and provides the chemical energy for any of the cells functions.

Cytoplasm: Cytoplasm is a jelly like fluid in which most of the cells chemical reactions take place.

Nucleus: The cell nucleus contains all the DNA of the genes in the chromosomes which control the cells functions and the

cell division of replication. The nucleus controls the activities of the cell by sending instructions to the cytoplasm.

Glycogen granules: Stored food for respiration.

Small vacuoles:

1.4 Be able to describe how plant and animal cells can be studied in greater detail with a l ight microscope.

Microscopes enable you to objects (like microorganisms) which you cannot see with the naked eye.

Microscopes using the visible part of the electromagnetic spectrum (visible light) were invented in the late 16th century and

the optical lens systems have been improved through the following centuries even until today.

With these light microscopes you can see individual cells and smaller details such as nuclei and mitochondria in all cells,

and chloroplasts in plant cells.1.5 Be able to demonstrate an understanding of how changes in microscope technology have enabled us to see cells with more

clarity and detail than in the past, including simple magnification calculations.

In the 20th century, with advances in atomic physics, the electron microscope was invented which works off beams of

electrons instead of visible light.

This has enabled the magnification produced by a microscope to be considerable increased to the point where you can see

even smaller structures such as the internal details of mitochondria, chloroplasts and plasmids (hoops of DNA).

magnification = length of image / length of object

1.6 Know that a gene is a section of a molecule of DNA and that it codes for a specific protein.

The genetic code ('blueprint') to make a particular protein is in the form of a sequence of bases attached to the sugar-

phosphate backbone of a DNA molecule.

1.7 Be able to describe a DNA molecule as

a) two strands coiled together to form a double helix

b) strands linked by a series of complementary base pairs joined together by weak hydrogen bonds (base-pairing H bonds

shown here as ):

There are four bases in DNA holding the structure together and the same bases are always paired together.(i) adenine (A) with thymine (T) i.e. A T

(ii) cytosine (C) with guanine (G) i.e. C G

where represents the weak (but crucial) intermolecular attractive force between pairs of bases, called the

hydrogen bond.

The double helix structure diagram below illustrates how the DNA is held together by the hydrogen bonds.


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1.8 Appreciate how to extract DNA from cells

You should have a description of how to ext ract DNA from e.g. onion in your laboratory notebook, exercise book or

textbook.

1.9 Be able to explain how the structure of DNA was discovered, including the roles of the scientists Watson, Crick, Franklin and

Wilkins.

In the 1950s, Rosalind Franklin working for Maurice Wilkins examined strands of DNA using a technique called X-ray

diffraction.

The sample under investigation, e.g. a DNA crystal strands, is bombarded with X-rays and the layers of atoms

behave like a diffraction grating and scatter the X-rays in particular pattern that depends on the 3D arrangement of

atoms in the molecule. The path of the scattered X-rays is recorded on a photographic plate.

Rosalind Franklin died tragically young from cancer, and never received the Nobel Prize she would have undoubtedlyreceived, BUT, in one of the last things she wrote in her laboratory notebook, she speculated that DNA had a helix

structure.

Later Frances Crick and James Watson gathered together this X-ray data (Crick had access to Rosalind Franklin's 'classic'

X-ray photograph of crystallised DNA, characteristic of a helical structure) with other information ...

e.g. the chemical analysis of DNA, particularly the ratios of the four bases (adenine, cytosine, guanine and thymine),

the shape of the four base molecules ...

and then built a model and deduced what we recognise today as the double helix structure of DNA - brilliant insight,

more Nobel Prize winners along with Maurice Wilkins.

The important thing is that the experimental observations from chemical and structural analysis fitted the evidence

based model.

1.10 HT only: Be able to demonstrate an understanding of the implications of sequencing the human genome (Human Genome

Project) and of the collaboration that took place within this project.

The project has mapped the DNA sequence for the ~25,000 genes of the 23 pairs of chromosomes from human cells.

By gett ing many genetic research groups to collaborate and work together on the project simultaneously e.g. sharing

out the genes/chromosomes to be analysed between them, it became much quicker to produce the full human

genome sequence.

What is the point of the Human Genome Project? What can we gain from it?

We are gradually building up a database of which genes ('genetic character') that predispose people to particular

conditions.

Therefore, it may enable us to predict which people are likely to suffer from a particular disease or disorder and

therefore perhaps offer a preventive course of action, which may involve medical treatment or lifestyle changes.

It may be possible, using genetic engineering, to prevent diseases such as cystic fibrosis and sickle cell anaemia.

Could we produce 'designer medicines' based on our own genetic blueprint?

Can we develop more accurate diagnostic techniques for certain conditions which are difficult to diagnose at an early

stage?

Each person has unique and characteristic DNA sequence, and genetic fingerprinting is already being used to identify

bodies, suspects and innocent people by forensic scientists.

It is also used by archaeologists too (see the AQA biology page about the discovery of the bones of

Richard (III, its a good science story!)

Will it be possible in the future to even get a more detailed picture of a person just from a DNA sample? e.g.

hair/skin colour, eye colour and other body characteristics?

So far, all positive possibilities, so is there a downside to the Human Genome Project?

I'm afraid so, although its great science, the social implications of this genetic knowledge raise serious ethical issues

about what is acceptable to society.

If it is known that you may be susceptible to a particular disease or disorder which you may suffer from later

in life, what happens if your employer, medical insurance company or life insurance company has your genetic

profile?

You could be discriminated against, e.g. an insurance company may demand your genetic profile and modify

the premiums you pay according to your 'genetic risk '.

This may not be the only thing that bothers you, if are told that you may suffer from a particular disease or

disorder, you may be worried about or perhaps undertake preventative courses of action which may not be

required?

1.11 Be able to demonstrate an understanding of the process of genetic engineering, including the removal of a gene from the

DNA of one organism and the insertion of that gene into the DNA of another organism

This is exemplified by the production of insulin from bacteria by inserting the human insulin gene into bacteria and growing

the bacteria to produce lots of insulin quickly and economically efficiently.

http://docbrown.info/page20/AQAscibio27.htm#unique_DNA


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1. An appropriate bacteria is selected that will give a good yield of insulin.

2.The bacterial plasmids are extracted f rom the bacteria.

3. A section of the plasmid DNA is cut by enzymes.

4. The human gene responsible for insulin production is cut out f rom the human chromosomal DNA with enzymes.5. Other enzymes are used to insert ('splice') the insulin gene in to the bacterial plasmid DNA.

6. The modified ring plasmids of DNA are put back into the bacteria cells, so human genes have used to produce

genetically modified or GM bacteria.

7. The bacteria rapidly reproduce when grown in a fermenter.

8. The insulin is extracted and the waste bacterial cells destroyed.

1.12 Discuss an understanding of the advantages and disadvantages of genetic engineering to produce GM organisms, including:

a) beta carotene in golden rice to reduce vitamin A deficiency in humans

Beta-carotene is essential for our bodies to make vitamin A.

Vitamin A deficiency is common in many Asian and African countries and can cause blindness.

Golden rice is GM rice whose genetic make-up contains two genes from other organisms which enable this variety of

rice to produce suffic ient quantities of beta-carotene.

With golden rice as part of their diet, the risk of vitamin A deficiency is reduced and less people are likely to go blind.

b) the production of human insulin by genetically modified bacteria

GM produced insulin production has been described in detail in section 1.11

The process overall is one of inserting the human insulin gene into bacteria and growing the bacteria to produce lots

of insulin quickly and economically efficiently (cheaply!).

c) the production of herbicide-resistant crop plants

You can modify the genetic make-up of plants by inserting genes that resistant to certain 'pests' e.g. fungal attack.

You can also make crops resistant to a herbicide being used to kill all weeds in the field of growing crops i.e. only the

crop that you want survives!

Both of these effects will help to increase crop yields.

Again we see three positive examples of the use of genetic engineering, but there are, as ever!, issues and problems to

solve.

1. This is new technology, new 'biotechnology' to be precise, and people quite rightly are concerned about e.g. GM

crops, though curiously enough, I've never heard anybody express worries about GM produced insulin - the latest

versions of which are produced by GM techniques!

2. There are concerns as to whether GM crops e.g. cereals or rice have the same nutrient contents (mineral ions,

vitamins etc.).

3. Are there any long-term effects from consuming GM modified grain or vegetables etc.?

4. Will GM plants spread and affect the local diversity of the farmland and environs e.g. GM plants becoming more

successful than local plants?

5. Will GM crops hybridise with other crops or grasses to produce new strains of plant, again, these could affect the

original biodiversity of the local flora (plants) and fauna (animals).

6. Points 4. and 5. have considerable implications e.g. if the genes from GM plants spread to other native plants, we

do not know what genotypes will be formed and what will be the resulting phenotypes (gene expression)? If we

produce a herbicide resistant plant, what happens if a group of herbicide weeds evolves, that are even more herbicide

resistant than the crop! From an agricultural point of view, a bit scary!


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1.13 Be able to describe the division of a cell by mitosis as the production of two daughter cells, each with identical sets of

chromosomes in the nucleus to the parent cell, and that this results in the formation of two genetically identical diploid bodycells.

Cell division by mitosis which occurs during growth, repair and asexual reproduction (diagram above, notes below)

Human body cells are diploid because have two versions of each chromosome, one from the individual's father and one

from the individual's mother (23 pairs of chromosomes in total).

On cell division, two identical cells are formed in mitosis, and both nuclei will contain the same number of chromosomes as

the original cell (i.e. both cells are once again diploid).

How does mitosis take place? What is the 'mechanism'? - refer to diagram where a few genes are graphically exaggerated to

help explain the process.

1. Prior to cell division, the DNA is in long strings within the very thin membrane of the nucleus

2. When the cell gets the signal to divide, the DNA must be copied (duplicated) exactly, and the result is X shaped

chromosomes. Remember, to make two identical cells, you need two lots of identical DNA and both V-sections of the X-shaped chromosome are identical.

3. The nucleus membrane is temporarily removed and the X-shaped chromosomes then line up across the centre of the

cell. Simultaneously, very fine fibres pull each X-shaped chromosome apart into two identical sections (e.g. both V-shaped).

4. The two sets of chromosomes collect together on opposite sides of the cell and two nuclear membranes form around

each set of chromosomes

5. The cytoplasm divides in two with both sections surrounded by a cell membrane to give two identical diploid cells.

1.14 Know that mitosis occurs during growth, repair and asexual reproduction

See 1.13 above for details of this type of cell division.

Mitosis creates new cells for growth, replacing damaged cells or tissue, and many organisms (both plant and animal) use

mitosis for asexual reproduction.

It should be noted that in asexual reproduction, there is no genetic variation.

1.15 Know that, at fertilisation, haploid gametes combine to form a diploid zygote.

via a fertilised egg!

Sex cells are called gametes, the ova/ovum in females and sperm in males.

Gametes are haploid cells because they only have one copy of each chromosome.

When two gametes combine in fertilisation the resulting cell is referred to as being zygote.

Zygote cells are diploid because they have two copies of each chromosome e.g. human cells have 23 pairs of

chromosomes (46 chromosomes in total).

Gamete cells contain one copy of each chromosome (23 in human haploid cells).

In sexual reproduction two gametes (sex cells) combine to form a new individual with the full compliment of

chromosomes (46 in human diploid cells, 23 from mother's egg, 23 from father's sperm) and, because the offspringcells have a mixture of the two sets of male and female chromosomes, each new individual is unique in genetic andphenotype character.


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1.16 Be able to describe the division of a cell by meiosis as the production of four daughter cells, each with half the number of

chromosomes, and that this results in the formation of genetically different haploid gametes.

Cell division by meiosis diagram above, notes below

This double cell division process is called meiosis and only occurs in the reproductive organs.

Meiosis generates cells that have half the normal number of chromosomes (haploid cells).

These haploid gamete cells have different single sets of chromosomes and explains why sexual reproduction produces

genetic variation.

1. The process starts with a diploid cell in which the DNA has been replicated to form X-shaped chromosomes (so has

two copies of each chromosome).

2. The chromosomes pair up, held by very f ine fibres.

3. The pairs of chromosomes are then pulled apart to form two groups, each encased in a nuclear membrane, so

forming two separate nuclei. This process is much more complicated than shown in the diagram and the alleles canget quite mixed up creating considerable variation in the offspring (see 5.).

4. The cytoplasm divides in two, completing the first cell division, noting that some of the male's chromosomes and

some of the female's chromosomes go into each new cell.

5. The 2nd cell division is a bit like mitosis, and, produces four haploid gamete cells, each with their own unique set of

chromosomes - the source of genetic variation when gamete cells combine in sexual reproduction.

1.17 Know that cloning is an example of asexual reproduction that produces genetically identical copies

1.18 HT only: Be able to demonstrate an understanding of the s tages in the production of c loned mammals, including:

a) removal of diploid nucleus from a body cell

b) enucleation of egg cell

c) insertion of diploid nucleus into enucleated egg cell

d) stimulation of the diploid nucleus to divide by mitosis

e) implantation into surrogate mammals

Adult cell cloning – the nucleus containing the genetic material is removed from an unfertilised egg cell.

Cloning is a type of asexual reproduction producing cells that are genetically the same as the original starting cell.

A nucleus extracted from an adult body cell, e.g. from a skin cell, is inserted into the egg cell from which the original

nucleus was removed ie the egg cell nucleus has been replaced with a complete set of chromosomes (diploid cell).

An electric shock stimulus then causes the egg cell to begin to divide to form embryo cells, just as a normal embryo

would do.

These embryo cells contain the same genetic information as the adult skin cell.

When the embryo has developed into a ball of cells, it is inserted (implanted) into the womb of an adult female

(surrogate mother) to hopefully continue its development from embryo ==>foetus ==> baby.

1.19 Be able to demonstrate an understanding of the advantages, disadvantages and risks of cloning mammals

Although cloning is a successful technique, it is not without problems and raises social and ethical issues.

Cloning involves retaining the same restricted pool of DNA but it is providing valuable research into embryo developmentand cell aging and age related disorders.

Cloning mammals inevitably produces a reduced gene pool whereas sexual reproduction provides genetic variety.

The limited pool of alleles which make up chromosomes can make the species more susceptible to a contracted disease

and other conditions such as premature aging, organ and immune system failures etc. (Look up the case of 'Dolly theSheep').


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The rate of successful cloning very low, genetic defects are common and those animals which survive the cloning

procedure are often unhealthy and are much more susceptible to disease i.e. they are a 'genetically weak' animal.

If human cloning was attempted, it could lead to babies being born with disabilities, there is only a certain chance that an

embryo would develop into a completely normal healthy baby - an ethical and moral dilemma for potential parents and themedical profession.

Cloning mammals is a means providing organs for transplants e.g. genetically modified pigs could be bred to provide donor

organs for humans and cloning the pigs could meet the ever increasing demand from critically ill patients on the waiting list.

Cloning could be used to reproduce endangered animal species, whose numbers were falling dangerously low i.e. in danger

of extinction.

1.20 Know that stem cells in the embryo can differentiate into all other types of cells, but that cells lose this ability as the animal

matures.

In mature animals, cell division is mainly restricted to repair and replacement.

1.21 Be able to demonstrate an understanding of the advantages, disadvantages and risks arising from adult and embryonic stem

cell research.

Cells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types

of cells, eg nerve cells.

Stem cells are found in early human embryos and have the potential to be converted into any type of cell found in the

human body.

In the early stages, embryonic cells are undifferentiated (all the same) and are called embryonic stem cells.

Later, as the embryo develops, the stem cells divide, producing more stem cells, but also differentiated cells - the processof differentiation in which cells for a specific specialised function are produced e.g. cells for skin, organ tissue, blood cellsetc.

Adults have stem cells in their bone marrow but these can only be converted into a few specific type of cells - you may

have heard the phrase 'bone marrow transplant' - this involves t reating a patient with a supply of healthy stem cells todifferentiate into specific healthy cells to replace damaged or faulty cells e.g. blood cells.

A bone marrow transplant is a gene therapy procedure that involves replacing damaged bone marrow with healthy bone

marrow stem cells.

Stem cells in bone marrow produce three important types of blood cells : red blood cells – which carry oxygen around the

body, white blood cells – which help fight infect ion and platelets – which help stop bleeding.

Bone marrow transplants are used to t reat sufferers of leukaemia, non-Hodgkin's lymphoma and sickle cell anaemia.

Human stem cells have the ability to develop into any kind of human cell.

It is possible to extract stem cells from early human embryos and reproduce them under particular conditions so that

they differentiate into particular types of specialised cells.

These cells could be used to replace diseased damaged tissue or tissue damage from injury e.g. new nerve

connections, cardiac tissue for people suffering from heart disease.

Quite simply, there is huge potential from stem cell research and application to alleviate many medical conditions,

which up to now, have been very difficult to treat.

e.g. treatment with stem cells may be able to help conditions such as paralysis and it is hoped to be able to grow

nerve cells for people disabled by a spinal injury.

Stem cell research is a very controversial area despite the obvious great medical benefit to individual patients.

The ethical issue of using embryos for medical purposes is abhorrent to some people.

This is the argument of 'potential life' versus help for seriously ill 'living people' i.e. each embryo has the

potential to develop into a human being, but equally potently, using embryonic stem cells might save a life.

It is possible to use unwanted embryos from fertility clinics because there is no other source of universal stem

cells and these unwanted embryos would be destroyed.

Stem cell research is allowed in some countries like the UK, but there are very strict guideline as to how it can

be carried out.

1.22 Be able to describe how the order of bases in a section of DNA decides the order of amino acids in the protein.

DNA is shorthand for deoxyribonucleic acid.

The genetic code ('blueprint') to make a particular protein is in the form of a sequence of bases attached to the sugar-

phosphate backbone of a DNA molecule.

All the different proteins are built up f rom only 20 amino acids and each amino acid is indicated by a 3 base code sequence

in a gene, which is part of a DNA molecule.The sequence of these triplet base codes in the gene on the DNA molecule determines the order in which the amino acids

are put together to synthesise the protein molecule.

So, the order of the triplet base codes on the DNA molecule can be eventually translated into an amino acid

sequence to form a specific protein molecule.

HT only: The base code forms the template for the production of RNA which is made first from DNA in order to then

synthesize the protein molecules (all of this is described in the next section).


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1.23 HT only: Be able to demonstrate an understanding of the s tages of protein synthesis, including transcription and translation:

Proteins are made in the cell by organelles called ribosomes (an organelle is a differentiated structure in a cell, with its own

membrane, and performs are particular function in the cell's chemistry e.g. chloroplast, vacuole, mitochondria, ribosome

etc.)

(a) the production of complementary mRNA strand in the nucleus (diagram and notes below).

RNA is shorthand for ribonucleic acid.

The large DNA molecules cannot move out of the nucleus because of their size, so the information must be transferred byother means to the ribosomes in the cytoplasm.

This is done by messenger RNA (mRNA) which is shorter than DNA and only a single strand (as opposed to the double

strands of the DNA helix).

mRNA conveys the genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm which will construct

the proteins from the base code information.

The diagram shows that the DNA double helix strands unwind and by base pairing (A-T and C-G) the complimentary single

strand of mRNA is built up.

Therefore a single DNA strand acts as a template for the formation of the complimentary mRNA and this process is called

transcription i.e. the copying of the triplet base codes into another form.

Points (b) to (e) are illustrated above, and this process of translation is explained below.

(b) the attachment of the mRNA to the ribosome

The mRNA exits from the nucleus and docks into a ribosome

(c) the coding by triplets of bases (codons) in the mRNA for specific amino acids

The triplet base codes for particular amino acids and their joining up sequence can now be read from the mRNA

molecules.

(d) the transfer of amino acids to the ribosome by tRNA

After the mRNA joins onto a ribosome, molecules of transfer RNA (tRNA) bring the amino acid that matches the code

on the mRNA, the complimentary base codes of the mRNA and tRNA ensure that all proteins are synthesised with

their specific protein sequence, so all proteins are completely reproducible.

(e) the linking of amino acids to form polypeptides

The ribosome then acts as the catalytic site for linking the amino acids together to synthesise a specific protein.

The second process is called translation i.e. the triplet base code sequence is read and translated into the amino

acid sequence of a protein.

A sequence of amino acids joined together in a chain is called a polypeptide (a natural polymer or macromolecule).

All of these reaction are catalysed by enzymes.

1.24 Be able to describe each protein as having its own specific number and sequence of amino acids, resulting in different-shaped

molecules that have different functions, including enzymes.


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The shape of the protein, especially if an enzyme, is vitally important for it to function properly. If the 'active site' on the

enzyme is not the perfect shape for the substrate molecule to dock into, then the enzyme cannot perform that specific

chemical change.

In a cell the DNA also controls which genes are switched on and off to make particular proteins in a cell.

These proteins may end up in muscle cells, brain cells, enzyme catalysts, haemoglobin molecules etc.

The proteins, particularly enzymes, are involved in building up non-protein molecules e.g. fats, cell walls, glycogen etc.

1.25 Be able to demonstrate an understanding of how gene mutations change the DNA base sequence and that mutations can be:

(i) harmful - causing genetic disorders like cyst ic f ibrosis, Downe syndrome, haemophilia and colour blindness.

(ii) beneficial - the gene expression produces an enhanced feature that makes that organism more able to survive, this is

partly responsible for driving the evolution of more successful species, but not always to our benefit! e.g. bacteria genes are

quite susceptible to mutations and some are becoming very resistant to antibiotics as their DNA subtly changes!

(iii) or neither ('neutral') - any faults from DNA mutations do not affect the organisms existence i.e. protein functions are not

affected, no advantage is gained and no disadvantage either.

1.26 Be able to describe enzymes as biological catalysts.

A catalys t is substance that speeds up a chemical reaction by lowering the activation energy of the reaction, that is the

threshold energy the reactants must have before they can change to products on collision.

A true catalys t is not used up in the reaction, but may temporarily change in the course of the reaction, and subsequently

be regenerated to act again (you should include this 2nd point when defining the action of ANY true catalys t).

Most chemical reactions ('biochemistry') in living organisms are catalysed by enzymes, hence their descriptions as

'biological catalysts'.

The chemical reactions in living organisms must be carefully controlled in order for the organism to survive successfully and

these reactions are enabled by enzymes without the need for high temperatures i.e. the enzymes lower the activation

energy required.

This makes enzyme controlled reactions very ef ficient even at the relative low temperature of the human body temperature

i.e. ~37oC.

Enzymes have a special shape, and within this 3D shape, a particular site called the 'active site' where the molecule to

changed 'docks into', hence the term 'key and lock mechanism' (illustrated below).

The enzyme will only work with specific substrate molecules which fit 'snugly' into the active site and are held in place

temporarily by chemical bonds, or more likely, the weaker intermolecular bonds/forces.

1.27 Be able to demonstrate an understanding that enzymes catalyse chemical reactions occurring inside and outside living cells,

including:

a) DNA replication (see also see sect ion 1.23)

The diagram above gives an extremely simplified summary of how DNA can self-replicate, with the help of several

enzymes.

Essentially the original DNA double helix is unwound ('unzipped') by one enzyme E1 to form two template strands.

Other enzymes e.g. E2, can then select the matching base component (A T or C G), pairing them up and

then zipping up to form the two new replica double helix st rands of DNA completing the replication process.

So, enzymes help the DNA copying process.

b) protein synthesis (see section 1.23)

Proteins (polypeptides) are synthesised in structures called ribosomes and the constituent amino acids are knitted

together' to form the polypeptide by 'polymerising enzymes'.

The enzymes help join the amino acids together to synthesise the protein.

c) digestion

The 'key and lock' mechanism diagram shown above illustrates typical function and regeneration of biochemical

catalysts we call enzymes (which often have a complex protein structure).

The catalyst structure is the same at the start and the end of the reaction, but the permanent chemical change is the


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reactant substrate molecule changing into the two new molecules - the reaction products.

The above diagram illustrates the breaking down of a larger molecule into smaller ones e.g. typical of a digestion

enzyme catalysed reaction.

The enzymes are secreted in saliva and gut, each enzyme performing a particular digestion breakdown reaction.

1.28 Be able to describe the factors affecting enzyme action, including:

a) temperature

The first graph diagram is typical, and the 2nd temperature graph shows what happens to the speed of the enzyme

catalysed process of photosynthesis as the temperature is increased.

As the temperature increase the rate of catalysis increases (normal effect on the speed of reaction as the average

kinetic energy of the molecules increases), but at high temperatures the protein structure of the enzyme isdestroyed, so the act ive site on the enzyme is damaged and won't work correctly because the shape of the proteinmolecule has changed and the substrate molecule can't 'dock in'.

So, a graph of rate versus temperature rises to a maximum (optimum temperature) and then falls away as the

enzyme becomes thermally denatured and destroyed and ceases to function at high temperatures (see diagramabove on right).

b) substrate concentration

The higher the enzyme concentration, the faster the reaction, and likewise, the higher the substrate concentration,

the faster the reaction.

For a given enzyme concentration, when the concentration of substrate is high, all the active sites on the enzymes

are occupied and the rate of reaction reaches a maximum and stays constant no matter how much more

concentrated the substrate concentration is..

c) pH

Different enzymes have different optimum pHs (diagrams above).

The pH, ie how acid or how alkaline the aqueous medium is, affects the protein structure of the enzyme, so like the

temperature graph, the graph rises to a maximum for the optimum pH.

Increase in acidity or alkalinity creating a pH well away from the optimum, can affect the protein structure of the

enzyme and so affecting the active site, and, the substrate molecule can no longer readily lock into place into theactive site and cannot be transformed into the product molecules.

The first diagram is typical of many enzymes operating in near neutral solutions (~pH 7)

The second diagram shows the wide range of pH that different enzymes can operate in

e.g pepsin breaks down proteins in the very acid conditions of the stomach.

Blood has a pH of ~7.4 and carbonic anhydrase (optimum pH ~7) is found in red blood cells. This enzyme enables

the efficient conversion of carbon dioxide and water into the carbonic acid and the hydrogen carbonate ion

('bicarbonate ion') and operates in near neutral conditions.

Trypsin is a protease enzyme from the pancreas that breaks down proteins (peptides) in the alkaline conditions (~pH

8.5) of the smaller intestine, so its optimum rate of reaction is around that value.

1.29 Know that enzymes are highly specific for their substrate


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1.30 Be able to demonstrate an understanding of the action of enzymes in terms of the ‘lock-and-key’ hypothesis

-

1.31 Be able to describe how enzymes can be denatured due to changes in the shape of the act ive si te

-

1.32 Revise any investigations you did on the factors that affect enzyme activity

i.e. measuring how long an enzyme reaction takes (measuring speed/rate of reaction).

e.g. using the enzyme amylase to breakdown starch to sugar.

Starch gives a blue black colour with iodine, which sugar does not.

As the starch is hydrolysed to sugar in test tubes, testing with iodine solution, shows the blue-black colour gradually

fades until the iodine solution no longer gives a positive test for starch.

You can modify the experiment to put test tubes of starch solution in water baths of increasingly higher temperatures.

You can make up starch solutions in different buffers to investigate the effect of pH on the rate of hydrolysis (breakdown) of

starch, all at the same room temperature.

Also at the same temperature, you can do more quantitative experiments with different concentrations of the starch

substrate or different concentrations of the enzyme amylase.

Edexcel GCSE Additional Science BIOLOGY

When revising, these pages provide you with a summary of what you need to know and be able to do.

BUT remember, your primary source of revision are your class notes, investigations and Edexcel GCSE science textbooks as well

as examination practice with past papers.

EDEXCEL GCSE Additional Science BIOLOGY 2 UNIT 2 B2 The components of life INDEX

Biology Unit B2 Topic 1 The building blocks of cells

Biology Unit B2 Topic 2 Organisms and energy

Biology Unit B2 Topic 3 Common systems

ALL Edexcel GCSE Science Units: Edexcel GCSE Science Biology Unit B1 Influences on li fe

Edexcel GCSE Science Biology Unit B2 The components of life * Edexcel GCSE Science Unit B3 Using biology

Edexcel GCSE Science Unit C1 Chemistry in our world * Edexcel GCSE Science Unit C2 Discovering Chemistry

Edexcel GCSE Science Unit C3 Chemistry in action * Edexcel GCSE Science Unit P1 Universal Physics

Edexcel GCSE Science Unit P2 Physics for your future * Edexcel GCSE Science Unit P3 Applications of Physics

GCSE Science-Biology courses AQA GCSE Science A BIOLOGY * EDEXCEL GCSE Science BIOLOGY

OCR GCSE 21st Century Science A BIOLOGY * OCR GCSE Gateway Science A BIOLOGY

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