Science Revision Year 11



Cells and Microscopy

Animal Cell & Plant Cell

  • Nucleus - contains DNA that controls what the cell does
  • Cytoplasm - gel-like substance where most of the chemical reactions happen
  • Cell membrane - holds the cell together and controls what goes in and out
  • Mitochondria - these are where most of the reactions for respiration take place (respiration releases energy that the cell needs to work)

Plant Cell

  • Rigid cell wall - made of cellulose, gives support for the cell
  • Large vacuole - contains cell sap, a weak solution of sugar and salts
  • Chloroplasts - where photosynthesis occurs. They contain a green substance called chlorophyll
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Cells and microscopy 

Bacterial Cells

  • Have NO nucleus
  • 1) Chromosomal DNA - controls the cell's activities and replication. It floats free in the cytoplasm
  • 2) Plasmids - small loops of extra DNA that aren't part of the chromosome. Plasmids contain genes for things like drug resistance and can be passed between bacteria
  • 3) The flagellum - a long, hair-like structure that rotates to make the bacterium move
  • 4) Cell wall - supports the cell


  • Let us see things we can't see with the naked eye
  • Light microscopes - let us see nuclei, chloroplasts and mitochondria
  • Electron microscopes - let us see internal structure of mitochondria and chloroplasts
  • MAGNIFICATION = length of image/length of specimen
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  • Has two strands coiled together in the shape of a double helix 
  • Two strands held together by chemicals called bases
  • Four different bases - adenine (A), cytosine (C), Guanine (G), Thymine (T)
  • The bases are paired; A-T and C-G - This is called base-pairing
  • The base pairs are joined together by weak hydrogen bonds
  • A gene is a section of DNA 
  • The sequence of bases in a gene code for a specific protein
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Protein Synthesis

  • A gene codes for a specific protein
  • A gene contains the instructions to make a specific protein
  • Cells make protein by stringing amino acids together in a particular order
  • Only 20 different amino acids are used to make up thousands of different proteins
  • The order of bases in a gene simply tell cells in what order to put the amino acids together
  • Each set of triplet codes for a particular amino acid
  • DNA also determines which genes are switched on and off; determines which type of cell it is
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Protein Synthesis


  • Made in the cell by organelles called ribosomes
  • DNA is found in the nucleus and cant move out of it because it's really big
  • The cell needs to get information from the DNA to the ribosome in the cytoplasm.
  • Done using molecule called mRNA - very similar to DNA but shorter and single-stranded
  • mRNA is like a messenger between the DNA in the nucleus and the ribosome

1) The two DNA strands unzip. DNA is used as a template to make the mRNA; base pairing ensures it's complementary. This is called TRANSCRIPTION

2) The mRNA molecule moves out of the nucleus and joins with a ribosome. 

3) Amino acids that match the mRNA code are brought to the ribosome by molecules called tRNA

4) The job of the ribosome is to stick amino acids together in a chain to make a polypeptide. This step is called TRANSLATION

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Protein Synthesis


  • A change to an organism's DNA base sequence affecting the sequence of amino acids in the protein
  • HARMFUL - could cause a genetic disorder such as cystic fibrosis
  • BENEFICIAL - could produce a new characteristic that is beneficial to an organism
  • NEUTRAL - neither harmful nor beneficial; don't affect a protein's function
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  • Enzymes are all proteins and are what make chemical reactions work
  • Enzymes are catalysts produced by living things
  • A CATALYST is a substance which INCREASES the speed of a reaction, without being CHANGED or USED UP in the reaction
  • DNA replication - enzymes help copy a cell's DNA before it divides by mitosis/meiosis
  • Protein synthesis - enzymes hold amino acids in place and from bonds between them
  • Digestion - various enzymes are secreted into the gut to digest different food molecules
  • Chemical reactions involve things being split apart of joined together
  • Substrate is the molecule changed in the reaction
  • Every enzyme has an active site; part where it joins on to its substrate to catalyse the reaction
  • Enzymes have a high specificity for their substrate - 'lock and key' mechanism
  • For the enzyme to work, the substrate has to fit into the active site otherwise the reaction won't be catalysed
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More on Enzymes

  • Changing the temperature changes the rate of an enzyme-catalysed reaction
  • More heat means the enzymes & substrate particles have more energy therefore more likely to react
  • If it gets too hot, some of the bonds holding the enzymes together break
  • The enzymes loses its shape so its active site no longer fits the shape of the substrate so it cannot catalyse the reaction - the enzyme is DENATURED; it cannot go back to its normal shape if things cool down
  • Each enzyme has an optimum temperature when the reaction goes fastest; temperature before it gets too hot and starts to denature
  • Human enzymes optimum temperature is about 37*C -same as body temperature
  • Changing pH also interferes with the bonds; changes shape of active site and denatures enzyme
  • All enzymes have an optimum pH that they work best at - often pH 7
  • The higher the substrate concentration, the faster the reaction
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The Human Genome Project

  • Idea was to find every single human gene
  • The collaboration of lots of scientists meant that all genes were found more quickly


  • Predict and prevent diseases
  • Develop new and better medicines
  • Accurate diagnoses
  • Improve forensic science


  • Increased stress
  • Gene-ism
  • Discrimination by employers and insurers 
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Genetic Engineering

1) A useful gene is "cut" out from one organism's chromosome using enzymes

2) Enzymes are then used to cut another organism's chromosome and then to insert the useful gene

3) This technique produces genetically modified organisms


  • Reducing vitamin A deficiency 
  • Producing human insulin
  • Increasing crop yield


  • GM crops may not be safe
  • Transplanted genes may get out into the natural environment
  • Reducing farmland biodiversity
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  • Human body cells are diploid - have two versions of each chromosome (23 pairs)
  • When a cell divides it makes two cells identical to the original cell - each with a nucleus containing the same number of chromosomes as the original cell
  • Type of cell division is called mitosis - used when humans, animals or plants want to grow or replace cells that have been damaged

1) Cell duplicates its DNA so there's one copy for each new cell - forms X-shaped chromosomes

2) Chromosomes then line up at centre of cell and cell fibres pull them apart - two arms of each chromosome go to the opposite ends of the cell

3) Membranes form around each of the sets of chromosomes - become nuclei of the two new cells

4) Cytoplasm divides - now have two new diploid cells containing exactly same DNA - they're genetically identical

  • Asexual reproduction also uses mitosis
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  • Gametes are sex cells- ova in females and sperm in males; only happens in the reproductive organs
  • During sexual reproduction, two gametes combine to form a new cell
  • Gametes are haploid - only have one copy of each chromosome; this is because when two gametes combine at fertilisation, the resulting cell will have two copies of each chromosome
  • Meiosis is when a cell divides to produce four haploid nuclei whose chromosomes are NOT identical

1) Cell duplicates its DNA and chromosomes pairs line up in centre of the cell

2) They're then pulled apart so each new cell only has one copy of each chromosome

3) Each new cell will have a mixture of the mother's and father's chromosomes - HUGE advantage of sexual reproduction over asexual reproduction

4) Chromosomes line up again in the centre of the cell and arms of the chromosomes are pulled apart 

5) You get four haploid gametes, each with only a single set of chromosomes in it

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Cloning Mammals

  • Cloning is a type of asexual reproduction
  • Produces cells that are genetically identical to an original cell

1) Adult cell cloning involves taking an unfertilised egg cell and removing nucleus (now enucleated)

2) Nucleus is taken from an adult body cell - diploid nucleus containing full number of chromosomes

3) Diploid nucleus is inserted into the 'empty' egg cell

4) Egg cell is then stimulated by an electric shock - makes it divide by mitosis

5) When the embryo is a ball of cells, its implanted into an adult female to grow into a genetically identical copy of the original adult body cell

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Cloning Mammals


  • Could help with the shortage of organs for transplants
  • Study of animal clones could lead to greater understanding of development of embryo.
  • Cloning could also be used to help preserve endangered species


  • If population are all closely related and new disease appears - all could be wiped out as there may be no allele giving resistance to the disease
  • Cloned mammals mightn't live as long 
  • Process often fails - hundreds of attempts
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