Biology - Unit 1 Topic 2

  1. Describe the fluid mosaic model.
    • The fluid mosaic model shows a phospholipid bilayer, which consists of a phosphate head (hydrophilic) and a lipid tail (hydrophobic).
    • It is called 'fluid' due to the cholesterol - makes membrane fluid.
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    • Integral and peripheral proteins sit in the membrane. These include:
    • Channel proteins
    • Glycoproteins - hormone receptors.
    • Glycolipid - Used to recognise other cells.
  2. What is osmosis?
    Osmosis is the movement of water across a partially permeable membrane, from a region of high concentration, to a region of low concentration. Down a water potential gradient.
  3. What is passive transport, facilitated diffusion, active transport, endocytosis and exocytosis?
    • Passive transport:
    • No energy is used.
    • Molecules of liquids of gases move down a conc. gradient.
    • Small molecules (O2 + CO2) diffuse freely.
    • Facilitated diffusion:
    • No energy used.
    • Involves proteins in the membrane allowing molecules to move through. e.g. channel proteins.
    • Channel proteins are specific so only open under certain circumstances.
    • Molecules move down a conc. gradient.
    • Active Transport:
    • Movement is against conc. gradient.
    • ATP energy is required.
    • e.g. Ion pumps - move ions such as sodium and potassium across the membrane. ATP changes the shape of the protein to allow molecules through.
    • Endocytosis:
    • Solids or fluids are engulfed by membrane.
    • Vesicle buds off from membrane and carries molecules into cell.
    • Exocytosis:
    • Vesicle carrying molecules moves to plasma membrane.
    • Vesicle fuses with membrane and contents are released.
  4. How is the mammalian lung adapted for rapid gas exchange?
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    • Large S.A. to volume ratio:
    • Millions of alveoli act as separate gas exchange surfaces. Combined S.A. = rapid diffusion.
    • Distance:
    • Alveoli and capillaries have thin membranes (one layer of epithelial cells) - short diffusion distance.
    • Concentration gradient:
    • Maintained by good blood supply that removes Ofrom alveoli. 
    • Ventilation maintains Osupply.
    • Circulation removes Ofrom haemoglobin.
    • ⇒ Conc. gradient is high.
  5. What are the properties of gas exchange surfaces?
    • 1. Large S.A. to volume ratio
    • 2. Thin diffusion pathway 
    • 3. Steep concentration gradient
  6. Describe the basic structure of amino acid.
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  7. How do proteins form?
    • Primary Structure:
    • Amino acids join by condensation reactions to form peptide bonds. 
    • 2+ = polypeptide chain.
    • Sequence of amino acids in a polypeptide chain is the primary structure of a protein. It will determine how the protein folds.
    • Secondary structure:
    • Hydrogen bonds form.
    • a helix or β pleated sheets form.
    • Tertiary structure:
    • Further folding creates a 3D shape.
    • This shape is held together by hydrogen bonds, ionic bonds and disulphide bridges.
    • Quaternary structure:
    • 2+ polypeptide chains are held together by hydrogen bonds. e.g. haemoglobin.
  8. How so the structure of enzymes relate to their role? What do the do?
    • Enzymes are globular proteins.
    • They are biological catalysts that reduce activation (energy required to start a reaction).
    • They are specific to their substrate. (active site)
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    • When enzyme and substrate bind it is called an enzyme substrate complex.
  9. Describe the basic structure of a mono nucleotide and the structures of DNA and RNA. How does the DNA double helix form?
    • Mono nucleotides contain a phosphate group, a pentose sugar and a nitrogenous base.
    • The N base can be:
    • Thymine (Uracil in RNA), Adenine, Guanine, Cytosine.
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    • In DNA ⇒ sugar is deoxyribose

    • In RNA ⇒ sugar is ribose
    • Mono nucleotides join through condensation reactions to form polynucleotides.
    • RNA is single stranded.
    • DNA forms a double helix by forming hydrogen bonds between complementary bases. (A-T, C-G)
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  10. How does DNA replicate?
    • Double helix in unzipped by DNA helicase.
    • Exposed bases attract free nucleotides.
    • Mono nucleotides line up along both strands.
    • Complementary base pairing (A-T,C-G) Hydrogen bonds are formed.
    • Phosphodiester bonds are formed.
    • DNA polymerase joins nucleotides to backbone.
    • DNA replication follows the semi-conservative method.
  11. How was the semi-conservative method proven?
    • Meselson & Stahl proved the semi-conservative method.
    • They grew several generations of E.coli bacteria in different densities of nitrogen.
    • The first generation was cultured in 15N (heavy DNA).
    • They then transferred the cells to 14N (light DNA)
    • After cells had divided again they found that DNA had the same density.
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  12. Explain the nature of the genetic code.
    • There are 20 amino acids in the body.
    • 4 possible bases:
    • Adenine 
    • Thymine
    • Cytosine
    • Guanine
    • 4= 1 A.A. = 4 in total☒
    • 4= 1 A.A. = 16 in total☒
    • 4= 1 A.A. = 64 in total☑
    • Therefore the genetic code consists of triplet codes.
    • Each triplet code forms a codon.
  13. What is a gene?
    A gene is a sequence of bases on a DNA molecule that code for a sequence of amino acids in a polypeptide chain.
  14. Outline the process of protein synthesis.
    • Transcription:
    • DNA double helix is unzipped.
    • One of the strands is used as a template to make an RNA copy - complementary base pairing. (mRNA)
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    • This template is called the antisense strand.
    • mRNA is linear.
    • mRNA moves out through nuclear pore and is engulfed by ribosome.
    • Translation:
    • tRNA picks up free amino acids and carries them to ribosome.
    • tRNA is a folded molecule of a single polynucleotide strand. They each have an A.A. binding site and an anticodon.
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    • tRNA with complementary anticodon lines up to mRNA codon. (base pairing)
    • Amino acids join together by a peptide bond and form a polypeptide chain.
    • The process is repeated until a stop codon is reached.
  15. How do mutations occur? How does cystic fibrous occur?
    • Mutations are changes in the base sequence of DNA. This happens during DNA replication.
    • Mutations occur when bases are:
    • Substituted
    • Deleted
    • Inserted
    • Duplicated
    • Inverted
    • If bases are changed, the primary structure will be altered, leading to a change in the 3D structure. (doesn't work properly)
    • A mutation in the sequence coding for CFTR proteins can lead to cystic fibrosis.
  16. Explain the term allele, dominant and recessive.
    • Allele:
    • A different version of a gene. Have slightly different base sequences which code for different version of same characteristic.
    • Dominant:
    • An allele whose characteristic is always expressed in the phenotype when present.
    • Recessive:
    • An allele whose characteristic only appears in the phenotype is homozygous recessive. Not expressed if paired with a dominant.
  17. Explain the term genotype, phenotype, homozygote and heterozygote.
    • Genotype:
    • The pair of alleles an individual posseses. Not necessarily shown.
    • Phenotype:
    • The characteristics the alleles produce. (physical appearance)
    • Homozygote:
    • An individual who possesses two copies of the same allele.
    • Heterozygote:
    • An individual who possesses two different alleles.
  18. Explain monohybrid inheritance in CF, albinism, thalassaemia, garden pea height and seed morphology.
    • CF is caused by a recessive allele.
    • Albinism is caused by a recessive allele. - a mutation is the gene that codes for melanin. (no pigmentation in skin)
    • Thalassaemia is caused by a recessive allele - a mutation in gene for alpha haemoglobin. (slow haemoglobin production)
    • Seed morphology:
    • Allele for smooth pea is dominant.
    • Allele for wrinkled pea is recessive.
    • Pea Height:
    • Allele for tall plant is dominant.
    • Allele for dwarf plant is recessive.
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  19. How does the CFTR protein work?
    • The CFTR protein is a channel protein that transports chloride ions out of cells and inhibits sodium ions to enter the cell.
    • Sodium chloride therefore forms outside the cell.
    • This causes water to move out of the cell by osmosis, causing mucus to be runny.
  20. How does a mutation in CFTR proteins impair bodily functions?
    • A mutated CFTR protein causes mucus to be very sticky, due to less NaCl outside the cell and therefore less water in the mucus.
    • Respiratory system:
    • Sticky mucus cannot be moved by cilia.
    • Bacteria remains in lungs, leads to infections.
    • Mucus reduces S.A. for gas exchange = breathlessness, tiredness.
    • Digestive system:
    • Thick sticky mucus lines pancreatic duct.
    • Blocks the release of enzymes therefore food is not digested properly ⇒ malnutrition.

    • Trapped enzymes may digest cells (if insulin affected = diabetes)
    • Thick mucus lines villi - reduced absorption.
    • Reproductive system:
    • Mucus blocks vas deferens in males ⇒ sperm cannot  leave.
    • Blocks fallopian tubes and cervix in females.
    • Overall causes infertility.
  21. Describe gene therapy and the difference between somatic and germ line therapy.
    • Involves altering alleles inside cells to cure genetic disorders.
    • If caused by a recessive - add working dominant.
    • If caused by a dominant - 'silence' the dominant 
    • 1. Working copy of gene is cut out from normal DNA using restriction enzyme.
    • 2. The gene is added to a vector, which will insert new DNA into cells.
    • 3. Vector introduced in patients cells.
    • Vectors can include:
    • Viruses - efficient but risky
    • Liposomes (spheres of lipid) - less efficient than viruses.
    • Plasmid DNA (rings of bacterial DNA) - inefficient
    • Somatic cell therapy:
    • Changing alleles in body cells. Gametes not affected = offspring can inherit.
    • Germ line therapy:
    • Changing alleles in gametes. Offspring wont inherit. Illegal in humans.
  22. Explain the uses of genetic screening. What different methods are there?
    • Identification of carriers:
    • Testing offered to individuals with family history of genetic disorder. 
    • DNA is analysed to see if it contains alleles for genetic disorder.
    • Preimplantation genetic diagnose:
    • Carried out on embryos produced for IVF.
    • Embryo is screened for genetic disorder before implantation.
    • Amniocentis:
    • Prenatal screening.
    • Long needle inserted into uterus and amniotic  fluid obtained.
    • Fetal cells from fluid are analysed.
    • Chorionic villus sampling:
    • Prenatal screening.
    • Sample of cells from chorionic villi in placenta are taken using a long needle and analysed.
  23. What are the social and ethical implications with genetic screening?
    • Can decide to terminate.
    • Can get counselling.
    • Can prepare by buying special medical equipment.
    • Can decide not to have children (if parents are tested).
    • Abortion is unethical.
    • Tests can be inaccurate.
    • Chance of miscarriage.
    • Unnatural procedure.
    • Embryos cannot give informed consent, right to life.
Card Set
Biology - Unit 1 Topic 2
biology edexcel