Download B3 Intervention and Revision Higher B3a Molecules for

B3 Intervention and Revision
Higher
B3a Molecules for life
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Explain why liver and muscle cells have large numbers of mitochondria due to large amounts of
respiration taking place. Some structures in cells, such as ribosomes, are too small to be seen
with the light microscope. Ribosomes are in the cytoplasm and are the site of protein synthesis
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Describe the structure of DNA as two strands coiled to form a double helix, each strand
containing chemicals called bases, of which there are four different types, with cross links
between the strands formed by pairs of bases.
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A chromosomes is a long, coiled molecules of DNA, divided up into regions called genes.
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Each gene contains a different sequence of bases and codes for a particular protein.
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Proteins are made in the cytoplasm, A copy is made from the nucleus because the gene itself
cannot leave the nucleus.
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The four bases of DNA are A, T, C and G. Complementary base pairings: A – T and G – C.
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Protein structure is determined by the DNA base code, to include: the base sequence
determines amino acid sequence and each amino acid is coded for by a sequence of 3 bases.
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Ribosomes are the site of protein synthesis. They are found in the cytoplasm but DNA is found in the nucleus. The genetic
code needed to make a particular protein is carried from the DNA to the ribosomes by a molecule called mRNA. Making:
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mRNA from DNA is called transcription
Proteins from mRNA is called translation
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The code needed to produce a protein is carried from the DNA to the ribosomes by a molecule
called mRNA.
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DNA controls cell function by controlling the production of proteins, some of which are
enzymes.

Watson and Crick used data from other scientists to build a model of DNA, using X-ray data
showing that there were two chains wound in a helix and data indicating that the bases
occurred in pairs.
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Explain why new discoveries, such as Watson and Crick’s, are not accepted or rewarded
immediately, to include:• the importance of other scientists repeating or testing the work.
B3b Proteins and Mutations
Proteins are made of long chains of amino acids. Each protein has its own number and sequence
of amino acids, which results in differently shaped molecules, which have different functions.
The function of proteins includes:
• structural (limited to collagen)
• hormones (limited to insulin)
• carrier molecules (limited to haemoglobin)
• enzymes.
Describe enzymes as:
• biological catalysts
• catalysing chemical reactions occurring in living cells: respiration, photosynthesis, protein
synthesis
• having a high specificity for their substrate.
Explain the specificity of enzymes in terms of the ‘lock and key’ mechanism.
Describe how changing temperature and pH, away from the optimum, will change the rate of
reaction of an enzyme-catalysed reaction.
Explain how enzyme activity is affected by pH and temperature, to include:
• lower collision rates at low temperatures
• denaturing at extremes of pH and high temperatures
• denaturing as an irreversible change inhibiting enzyme function
• denaturing changing the shape of the active site.
Calculate and interpret the Q10 value for a reaction over a 10°C interval, given graphical or numerical
data, using the formula:
Q10 = rate at higher temperature
rate at lower temperature
For example, if rate of a certain reaction is 10 units/min at 20 °C and 24 units/min at 30 °C. What is its Q 10?
Q10 = rate at higher temperature ÷ rate at lower temperature = 24 ÷ 10 = 2.4
Gene mutations may lead to the production of different proteins. Mutation may occur spontaneously
but can be made to occur more often by radiation or chemicals. Mutations are often harmful but may be
beneficial or have no effect.
Only some of the full set of genes are used in any one cell; some genes are switched off. The genes
switched on determine the functions of a cell. For example, only pancreas cells switch on the gene for making the
hormone insulin.
Changes to genes alter, or prevent the production of the protein which is normally made. E.g type 1
diabetes.
B2c – Respiration
The symbol equation for aerobic respiration:
C6H12O6 + 6O2→ 6CO2 + 6H2O
Glucose + oxygen → carbon dioxide + water
The rate of oxygen consumption can be used as an estimate of metabolic rate because aerobic
respiration requires oxygen using the respiratory quotient (RQ) formula:
RQ = carbon dioxide produced / oxygen used
ATP
ATP is a substance that is used as the energy source for many processes in cells. ATP is produced as a result of respiration.
For example, one glucose molecule can release enough energy during respiration for the production of:
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38 ATP molecules by aerobic respiration
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2 ATP molecules by anaerobic respiration
Explain why the rate of respiration is influenced by changes in temperature and pH. The rate of respiration
is influenced by changes in temperature and pH. This is because enzymes are involved in respiration, and their activity
varies with temperature and pH.
Anaerobic respiration which releases energy:
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Glucose
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Takes place when there is insufficient oxygen available due to hard exercise
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Results in the incomplete breakdown of glucose
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Lactic acid (oxygen debt) accumulates in muscles causing pain and fatigue
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Releases much less energy per glucose molecule that aerobic respiration
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Recovery of oxygen debt
lactic acid
o
continued panting replacing oxygen allowing aerobic respiration
o
increased heart rate ensuring that blood carries lactic acid away to the liver.
B3d – Cell Division
Advantages of being multicellular:
• allows organism to be larger
• allows for cell differentiation
• allows organism to be more complex.
Becoming multi-cellular requires the development of specialised organ systems, limited to:
• communication between cells (nervous system)
• supplying the cells with nutrients (digestive system)
• controlling exchanges with the environment (respiratory and excretory system)
New cells for growth are produced by mitosis, new cells are genetically identical because they contain
the same genetic information – it is a copied cell. Body cells are DIPLOID (contain two copies of each
chromosome).
DNA replication must take place before cells divide, by:
• ‘unzipping’ of the chromosome to form single strands
• new double strands forming by complementary base pairing.
Describe how in mitosis the chromosomes:
• line up along the centre of the cell
• they then divide
• the copies move to opposite poles of the cell.
Gametes are produced by meiosis. Gametes are haploid (contain one chromosome from each pair) In
meiosis, the chromosome number is halved and each cell is genetically different because:
• one chromosome from each pair separate to opposite poles of the cell in the first division
• chromosomes divide and the copies move to opposite poles of the cell in the second division.
Fertilisation results in genetic variation because:
• gametes (1 sperm and 1 egg) combine to form a diploid zygote
• genes on the chromosomes combine to control the characteristics of the zygote.
Structure of a sperm cell is adapted to its function, to include:
• many mitochondria to provide energy
• an acrosome that releases enzymes to digest the egg membrane.
B3e – The Circulatory system
Red blood cell is adapted to its function: small size, round flattened concave disc shape, contains
haemoglobin, lack of nucleus to increase the surface area. Large surface to volume ratio.
Feature
Reason
Small size
Lets red blood cells pass through narrow capillaries
Flattened disc shape
Provides a large surface area, allowing rapid diffusion of oxygen
Containshaemoglobin
Haemoglobin absorbs oxygen in the lungs and releases oxygen in the rest of the body
Does not contain a nucleus Increases amount of space inside the cell for haemoglobin
Plasma transports dissolved substances around the body, including:
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Hormones
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Antibodies
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Nutrients, such as water, glucose, amino acids, minerals and vitamins
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Waste substances, such as carbon dioxide and urea
Haemoglobin in red blood cells reacts with oxygen in the lungs to form oxyhaemoglobin. In tissues the
reverse of this reaction happens.
How the circulatory system works:
• arteries transporting blood away from the heart - thick muscular and elastic wall in arteries
• veins transporting blood to the heart - large lumen (inside diameter) and presence of valves in
veins
• capillaries exchanging materials with tissues - permeability of capillaries
Identify the names and positions of the parts of the heart and describe their functions, to
include:
• left and right ventricles to pump blood
• left and right atria to receive blood
• semilunar, tricuspid and bicuspid valves to prevent back flow
• four main blood vessels of the heart.
The left ventricle has a thicker muscle wall than the right ventricle. This is because the left ventricle has
to pump blood all the way around the body, but the right ventricle only has to pump it to the lungs.
Mammals have a double circulatory system, which means that:
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One circuit links the heart and lungs
The other circuit links the heart with the rest of the body
The circulatory system
The advantage of the double circulatory system in mammals, means it can withstand higher pressures
and therefore greater rate of flow to the tissues.
B3f – Growth and development
Plant cell
Animal cell
Bacterial cell
Cell wall
chromosomes.
presence of a nucleus
NO cell wall
chromosomes.
presence of a nucleus
Sometimes has a cell wall
single circular strand of DNA
Absence of a nucleus, mitochondria
and chloroplasts
Describe the main phases of a typical growth curve.
Recall that in human growth there are two phases of rapid growth, one just after birth and the other in
adolescence.
Explain the advantages and disadvantages of measuring growth by:
• length
• wet mass
• dry mass.
Height and wet mass can be measured when an organism is alive, but dry mass can only be measured when an organism
has had all its water removed and is dead. However,dry mass is the best measure of growth.
Explain why the growth of parts of an organism may differ from the growth rate of the whole organism
e.g the head of a human foetus in the womb grows faster than the rest of the body for the first two months.
Stem Cells
Undifferentiated cells called stem cells can develop into different cells, tissues and organs.
Stem cells can be obtained from embryonic tissue and could potentially be used to treat medical
conditions including Parkinsons disease and paralysis.
Discuss issues arising from stem cell research in animals.
Explain the difference between adult and embryonic stem cells.
Human embryos (embryonic stem cells)
Adult bone marrow (adult stem cells)
Explain why plant growth differs from animal growth, to include:
• Animals tend to grow to a finite size but many plants can grow continuously.
• Plant cell division is mainly restricted to areas called meristems.
• Cell enlargement is the main method by which plants gain height.
• Many plant cells retain the ability to differentiate but most animal cells lose it at an early stage.
Feature
Plants
Animals
Pattern of growth
Often can grow continuously
Tend to grow to a maximum size
How growth happens
Mainly by cell enlargement (increase in cell
size)
Increasing the number of cells
Where cell division
happens
Mainly at meristems – found at the tips of
shoots and roots
In most tissues
Cell differentiation
Many cells can differentiate
Most cells lose the ability to differentiate at an
early stage
B3g – New genes for old
A selective breeding programme may lead to inbreeding, which can cause health problems within the
species.
Some examples of genetic engineering:
• taking the genes from carrots that control betacarotene production and putting them into
rice. Humans can then convert the beta-carotene from rice into Vitamin A (solving the problem
of parts of the world relying on rice but lacking vitamin A)
• the production of human insulin by genetically engineered bacteria
• transferring resistance to herbicides, frost damage or disease to crop plants.
A selective breeding programme may reduce the gene pool leading to problems of inbreeding, including
accumulation of harmful recessive characteristics, reduction in variation,
Potential advantages and risks of genetic engineering:
• advantage – organisms with desired features are produced rapidly
• risks – inserted genes may have unexpected harmful effects.
Discuss the ethical issues involved in genetic modification.
There are ethical issues involved in genetic modification. For example, some people are concerned about the health risk of
genetically modified food. Others think it is wrong to create new life forms, or to move genes between different species,
especially if this causes harm.
Understand the principles of genetic engineering, to include:
• selection of desired characteristics
• isolation of genes responsible
• insertion of the genes into other organisms
• replication of these organisms.
Changing a person’s genes in an attempt to cure disorders is called gene therapy. Gene therapy could
involve body cells or gametes. Explain why gene therapy involving gametes is controversial (designer
babies).
B3f - Cloning
Describe in outline the cloning technique used to produce Dolly, to include:
• nucleus removed from an egg cell
• egg cell nucleus replaced with the nucleus from an udder cell
• egg cell given an electric shock to make it divide
• embryo implanted into a surrogate mother sheep
• embryo grows into a clone of the sheep from which the udder cell came.
Benefits and risks of using cloning technology:
Possible implications of using genetically modified animals to supply replacement organs for humans:
Some possible uses of cloning include:
• Mass producing animals with desirable characteristics
• Producing animals that have been genetically engineered to provide human products
• Producing human embryos to supply stem cells for therapy.
Advantages of cloning plants
Can be sure of the characteristics
of the plant since all plants will be genetically
identical
Disadvantages of cloning plants
If plants become susceptible to
disease or to change in environmental conditions
then all plants will be affected
It is possible to mass produce plants
that may be difficult to grow from seed
The main steps in cloning plants are:
Lack of genetic variation.
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Choosing a plant that has the desired characteristics
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Removing a large number of small pieces of plant tissue
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Using aseptic technique – keeping everything sterile
Using a suitable growth medium and warm, moist conditions
Cloning plants is easier than cloning animals as many plant cells retain ability to differentiate unlike
animal cells which usually lose this ability at an early stage.