Biology 2

Edexcel biology unit 2 revision cards.


Leaf Adaptations

  • Leaves are broad, think and flat with lots of internal air space
  • This creates a large surface area and increases the efficiency of photosynthesis. 
  • They are filled with specialised palisade cells, which contain lots of chloropast
  • Chloropast contains chlorophyl: obtains light for photosynthesis
  • On the bottom of the leaves there are many tiny pores called stomata which open in response to light
  • Stomata allow carbon dioxide from the atmosphere to diffuse into the leaf; so can be used during photosynthesis
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  • Norman human cells, 23 pairs of chromosomes
  • DNA - deoxyribonucleic acid
  • A gene is a section of DNA that codes for a particular protein during protein synthesis
  • DNA molecule - 2 strands, double helix
  • Linked by pairs of complementary bases
  • Adenine (A), Cytocine (C), Guanine (G), Thymine (T)
  • A is only ever paired with T
  • C is only ever paired with G
  • Bases held together by weak, hydrogen bonds
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Breeding Herbicide - Resistant Crops (HRC)

  • Herbicides are used to kill weeds, which prevent plants from growing
  • Breeding HRC's enable people to produce more food, and up crop yield


  • Scientists find a naturally occuring plant that is resistant to herbicides
  • They then find the gene that is responsible for that resistance
  • A vector, such as the bacterium Agrobacterium tumejaciends is used to transfer the gene coding for the herbicide resistance to the embryo crop plants DNA
  • The crops are allowed to grow and are then treated with herbicides to kill weeds
  • The new plants are reisistant to the herbicide and are not killed
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Genetic Engineering

  • A gene from one organism is inerted into the DNA of another organism
  • The inserted gene then mamkes its polypeptide in the genetically modified organism

Process -

  • A gene, (eg. insulin) is cut out of an human chromome using enzymes
  • A DNA plasmid is taken out of a bacterium and cut open using enzymes
  • The human insulin gene and the plasmid are combined
  • The combined insulin gene and plasmid are stuck together to make a new plasmid
  • The new plasmid with the human insulin gene is put back into a bacterium. The bacterium has been genetically modified. The bacterium will now make human insulin

Advantages - 

  • Can treat human conditions (diabetes)
  • Golden Rice - more vitamin A to prevent illness

Disadvantages -

  • Few diabetics react badly
  • Cost is high
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The Heart

  • Left and right side of the heart work together
  • 2 sides of the heart sepreated by the septum
  • Vena carva brings blood from the body into the right atrium
  • Atrium is full, muscles wall contracts forcing blood through the valve into the right ventricle
  • Valves are flaps that prevent tissue that stops the blood from flowing backwards
  • When the ventricle is full of blood the muscle of the ventricle wall contracts, forcing blood out through more valves into the pulmonary artery
  • This carries deoxygenated blood to the lungs where it picks up oxygen
  • The oxygenated blood returns from the lungs to the left atrium of the heart in the pulmonary vein.
  • When the atrium is full, it contracts and forces blood through the valves into the left ventricle.
  • Once the ventricle is full of oxygenated bloodthe muscle of the ventricle wall contracts
  • This forces blood out through more valves into the aorta
  • The muscle wall of the left ventricle is thicker than that of the right ventricle because it has to pump all around the body rather thans just to the lungs
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Sexual and Asexual Reproduction

Sexual Reproduction

  • Haploid gametes (sperm) from the father and (ovul) from the mother fuse together to form a diploid zygote
  • It replicates then divides by mitosis and forms and orgamism that has half its genes from both parents, leading to genetic variation
  • It produces an embryo that divides by mitosis

Asexual Reproduction

  • The offspring produced by asexual reproduction get their genes from one parent only
  • They then have identical genes of that parent
  • Plants produce asexually
  • The chlorophylum (spider plant) throws off runners.
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Mitosis and Meiosis


  • Diploid cell with 2 pairs of chromosomes
  • Each chormsome replicates itself
  • The copies seperate and divide
  • Each new diploid daughter cell contains the same number of genes as the mother cell


  • Diploid cell
  • Divides, then divides again to produce 4 haploid cells
  • Each cell has half the number of the original cell
  • This produces cells with genetically different sets of chromosomes and happens in sexual reproduction
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Clones and Ethics of Cloning

  • Clones are genetically identical individuals
  • Using tissue culture technique, it is possible to clone tissues and organs for transplant surgeries


  • The fear of creating the perfect race
  • The possible abnormalities occuring
  • Clones will have no parents
  • Does not allow 'natural' evolution

Process of cloning mammals

  • The diploid nucleus from a body cell of a mammal is from the doner organism
  • An egg cell is enucleated
  • The diploid nucleus containing all of the doners genetic information is inserted into the empty eggcell
  • This is nuclear transfer
  • The egg containing the diploid nucleus is stimulated by an electric shock and begins dividing by mitosis cell devision
  • The resulting embryo is implanted into the endometrium of the surrogate mother
  • The embryo develops into a feotus and the birth is normal
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Discovery of DNA

  • 1953, Cambridge Universiy, James Watson and Francis Crick discovered the 3D model of the DNA double helix
  • Several years before Rosalind Franklin and Maurice Wilkins at Kings College University Hospital took an X-ray diffraction image of DNA
  • Watson, Crick, Wilkins and recieved a nobel prize shortly after Franklin died, in 1962
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Stem Cells

  • Most cells are specialised which allow them to perform a particular job
  • The process of which these cells become specialised is differentiation
  • Plant cells can differentiate at any time, where as animal cells tend to differentiate soon after they are made
  • Animal stem cells can differentiate into all other types of animal cells yet lose the ability to mature as the animal matures
  • Stem cells are undifferentiated. This means they could theoretically, differentiate into any type of cell.
  • Research has shown stem cells have the potential to replace damaged tissue to help in the treatment of disease and injury
  • When different chemicals, known as growth factors, are added to stem cells, they are made to develop into:
  • Insulin producing cells in the pancreas (islet cells)
  • heart muscle cells
  • blood cells
  • neurones
  • bone marrow cells 
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Stem Cells and Parkinsons disease

  • When the neurones in the brain stop producing dopamine, a person develops Parkinsons Disease
  • Dopamine is a chemical that the neurones use to comminicate with eachother
  • When a person is unable to produce dopamine, they can no longer coordinate movements of the body.
  • If the stem cells could be made into brain neurones and made to produce dopamine this could potentially provide a cure for Parkinsons
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  • Plants make their own foods by the process of photosynthesis
  • Photosynthesis means making food through light
  • It occurs in the chloropast of the cells of green plants that are exposed to light; mainly in the leaves

carbon dioxide + water + light --> glucose + oxygen

  • After the glucose has been made in photosythesis it is:
  • used in respiration
  • stored as starch
  • made into celluose cell alls
  • mode into polypeptides

Glucose is transported around the plant in specialised cells called phloem cells

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Factors Affecting Photosynthesis

  • Temperature, carbon dioxide concentration and light intensity affect the rate of photosynthesis
  • Any one of them, at a particular time, may be a limiting factor

Effect of temperature

  • As the temperature rises so deos the rate of photosynthesis. This means temperature is limiting the rate of photosynthesis
  • As the rare exceeds about 45'c, the enzymes controlling photosynthesis strart to be denatured and the rate of photosynthesis drops to zero

Effects of carbon dioxide concerntration

  • As the rise in carbon dioxide concerntration so does the rate of photosynthesis
  • Carbon dioxide is limiting the rate of photosynthesis
  • The rise in carbon dioxide levels now has no effect. Carbon dioxide is no longer the limiting factor. Light or temperate must now be the limiting factor

Effect of Light Intensity 

  • As the light intensity increases so does the rate of photosynthesis. This means light intensity is limiting the rate of photosynthesis
  • The rise in light intensity now has no effect. Light is no longer the limiting factor. Carbon dioxide or temperature must now be the limiting factor
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Plant Roots, Osmosis and Active Transport

  • Water enters the plant through the roots
  • Most of the water is absorbed by the root hair cells by a process called osmosis
  • Osmosis is diffusion of water molecules from an area of higher concerntration of water to an area of lower concerntration through a partially permeable membrane
  • The root hair cell membranes are partially permeable because they allow water to across them but do not allow the solute to move from inside the cell 
  • Plants are adapted to take up water by having many root hair cells. The hair like structure in root hair cells provide a large sufrace area to take up water
  • Mineral salts are also taken in by the plants roos. As the concerntration of mineral salts is often higher in the plant than in the soil, energy is needed to do this. This is called active transport, and it works against the concerntration gradient
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Xylem and Transpiration

  • Water enters plants through their roots is transported to the cells in long tube like structures called xylem
  • Water moved up through these tubes against gravity.
  • The plant does not use any energy to do this. The water moves because of a process called transpiration.

Process -

  • Water evaporated from the internal leaf cells through the stomata
  • Water passes from the xylem vessels to leaf cells due to osmosis 
  • This pulls the entire threat of water in that vessl upwards by a very small amount
  • Water enters xylem from root hair tissue to replace water that has moved upwards 
  • Water enters root hair cells by osmosis to replace water that has entered the xylem 
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