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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.
- Integral and peripheral proteins sit in the membrane. These include:
- Channel proteins
- Glycoproteins - hormone receptors.
- Glycolipid - Used to recognise other cells.
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.
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.
- Solids or fluids are engulfed by membrane.
- Vesicle buds off from membrane and carries molecules into cell.
- Vesicle carrying molecules moves to plasma membrane.
- Vesicle fuses with membrane and contents are released.
How is the mammalian lung adapted for rapid gas exchange?
- Large S.A. to volume ratio:
- Millions of alveoli act as separate gas exchange surfaces. Combined S.A. = rapid diffusion.
- Alveoli and capillaries have thin membranes (one layer of epithelial cells) - short diffusion distance.
- Concentration gradient:
- Maintained by good blood supply that removes O2 from alveoli.
- Ventilation maintains O2 supply.
- Circulation removes O2 from haemoglobin.
- ⇒ Conc. gradient is high.
What are the properties of gas exchange surfaces?
- 1. Large S.A. to volume ratio
- 2. Thin diffusion pathway 3. Steep concentration gradient
Describe the basic structure of amino acid.
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.
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)
- When enzyme and substrate bind it is called an enzyme substrate complex.
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.
- 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)
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.
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.
Explain the nature of the genetic code.
- There are 20 amino acids in the body.
- 4 possible bases:
- 41 = 1 A.A. = 4 in total☒
- 42 = 1 A.A. = 16 in total☒
- 43 = 1 A.A. = 64 in total☑
- Therefore the genetic code consists of triplet codes.
- Each triplet code forms a codon.
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.
Outline the process of protein synthesis.
- DNA double helix is unzipped.
- One of the strands is used as a template to make an RNA copy - complementary base pairing. (mRNA)
- This template is called the antisense strand.
- mRNA is linear.
- mRNA moves out through nuclear pore and is engulfed by ribosome.
- 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.
- 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.
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:
- 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.
Explain the term allele, dominant and recessive.
- A different version of a gene. Have slightly different base sequences which code for different version of same characteristic.
- An allele whose characteristic is always expressed in the phenotype when present.
- An allele whose characteristic only appears in the phenotype is homozygous recessive. Not expressed if paired with a dominant.
Explain the term genotype, phenotype, homozygote and heterozygote.
- The pair of alleles an individual posseses. Not necessarily shown.
- The characteristics the alleles produce. (physical appearance)
- An individual who possesses two copies of the same allele.
- An individual who possesses two different alleles.
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.
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.
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.
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.
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.
- 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.
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.