Biology - Unit 1 Topic 2

• 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.

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.

• 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.

• 
• 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 O₂ from alveoli. 
• Ventilation maintains O₂ supply.
• Circulation removes O₂ from haemoglobin.
• ⇒ Conc. gradient is high.

• 1. Large S.A. to volume ratio
• 2. Thin diffusion pathway 
• 3. Steep concentration gradient

• 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.

• 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.

• 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)
• 

• 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.

• 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.
• 

• There are 20 amino acids in the body.
• 4 possible bases:
• Adenine 
• Thymine
• Cytosine
• Guanine
• 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.

A gene is a sequence of bases on a DNA molecule that code for a sequence of amino acids in a polypeptide chain.

• Transcription:
• 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.
• 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.
• 
• 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.

• 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.

• 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.

• 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.

• 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.
• 

• 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.

• 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.

• 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.

• 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.

• 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.