A-Level Biology (UK)
UK A-Level Biology (Years 12-13, ages 16-18) covering AQA/OCR/Edexcel core content: biological molecules, cells, exchange and transport, genetic information and variation, energy transfers (photosynthesis and respiration), organisms responding to change, genetics and ecosystems, and control of gene expression, with required practicals throughout.
Ämne: Biologi · Nivå: Gymnasium (16–19) · 426 kort
Innehåll
- Monomers are smaller units from which larger molecules (polymers) are made. Examples: monosaccharides, amino acids, nucleotides.
- A hydrolysis reaction breaks a chemical bond between two molecules and involves the use of a water molecule.
- Monosaccharides are the monomers of carbohydrates. Examples include glucose, galactose and fructose (all hexoses, C₆H₁₂O₆).
- Glucose has two isomers: α-glucose and β-glucose. They differ in the orientation of the hydroxyl (–OH) group on carbon 1 (below the ring in α, above in β).
- A glycosidic bond forms by condensation between two monosaccharides. A 1,4-glycosidic bond links carbon 1 to carbon 4.
- Disaccharides: maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose).
- Starch is a polymer of α-glucose, made of amylose (unbranched, 1,4-linked, helical) and amylopectin (branched, with 1,6 bonds). It is the storage carbohydrate in plants.
- Glycogen is the storage carbohydrate in animals and fungi. It is a polymer of α-glucose, like amylopectin but more highly branched, giving rapid glucose release.
- Cellulose is a polymer of β-glucose. Alternate β-glucose molecules are inverted, producing straight chains that hydrogen-bond into microfibrils — strong fibres for plant cell walls.
- Reducing sugars (all monosaccharides and some disaccharides like maltose) give a brick-red precipitate with Benedict's solution on heating.
- Triglycerides are formed from one glycerol molecule and three fatty acids, joined by three ester bonds in condensation reactions.
- A saturated fatty acid has no carbon-carbon double bonds. An unsaturated fatty acid has one (mono-) or more (poly-) C=C double bonds, causing kinks that lower the melting point.
- In a phospholipid, one fatty acid of a triglyceride is replaced by a phosphate group. This gives a hydrophilic (phosphate) head and two hydrophobic (fatty acid) tails.
- Amino acids are the monomers of proteins. The general structure: a central carbon bonded to an amine group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group.
- A peptide bond forms by condensation between the amine group of one amino acid and the carboxyl group of another, releasing water. The product is a dipeptide.
- Primary structure is the sequence of amino acids in a polypeptide chain, held by peptide bonds. It is coded for by a gene.
- Secondary structure: hydrogen bonds between the amino acids cause the chain to coil into an α-helix or fold into a β-pleated sheet.
- Tertiary structure is the overall 3D shape of a polypeptide, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between R groups.
- Quaternary structure arises when two or more polypeptide chains associate into a functional protein. Example: haemoglobin has four polypeptide subunits.
- The biuret test detects proteins: add biuret reagent (sodium hydroxide + copper(II) sulfate). A colour change from blue to purple/lilac indicates peptide bonds.
- Enzymes are biological catalysts. They lower the activation energy of a reaction, speeding it up without being used up themselves.
- The induced-fit model: the active site is not an exact complementary shape initially. As the substrate binds, the active site changes shape to mould around it, straining bonds.
- Raising temperature increases enzyme activity (more kinetic energy, more collisions) up to an optimum. Above the optimum, the enzyme denatures as hydrogen bonds break.
- Each enzyme has an optimum pH. Above or below it, H⁺ or OH⁻ ions disrupt ionic and hydrogen bonds in the tertiary structure, altering the active site and denaturing the enzyme.
- A competitive inhibitor has a similar shape to the substrate and binds to the active site, blocking it. Its effect is reduced by increasing substrate concentration.
- A non-competitive inhibitor binds to an allosteric site (away from the active site), changing the active site's shape so the substrate can no longer bind. Increasing substrate does not overcome it.
- Water is a polar molecule: oxygen is slightly negative and hydrogens slightly positive. Hydrogen bonds form between water molecules, giving it many of its properties.
- Inorganic ions in solution have specific roles: iron (Fe²⁺) in haemoglobin, sodium (Na⁺) in co-transport and nerve impulses, phosphate (PO₄³⁻) in DNA/ATP, hydrogen (H⁺) in pH.
- A nucleotide is the monomer of nucleic acids. It consists of a pentose sugar, a phosphate group, and a nitrogen-containing organic base.
- DNA bases: adenine and guanine are purines (double ring); cytosine and thymine are pyrimidines (single ring). In RNA, uracil replaces thymine.
- Nucleotides join by condensation to form a phosphodiester bond between the phosphate of one and the sugar (carbon 3) of the next, forming a sugar-phosphate backbone.
- DNA is a double helix of two antiparallel polynucleotide strands. Complementary base pairing: adenine-thymine (2 hydrogen bonds), cytosine-guanine (3 hydrogen bonds).
- ATP (adenosine triphosphate) is a nucleotide derivative made of adenine, ribose, and three phosphate groups. It is the universal immediate energy currency of cells.
- ATP is hydrolysed to ADP + inorganic phosphate (Pi) by ATP hydrolase, releasing a small, manageable amount of energy. It is resynthesised from ADP + Pi by ATP synthase.
- DNA replication is semi-conservative: each new DNA molecule contains one original (parental) strand and one newly synthesised strand.
- In DNA replication, DNA helicase breaks the hydrogen bonds to unwind the helix, and DNA polymerase joins free nucleotides to the exposed bases by complementary base pairing.
- Meselson and Stahl confirmed semi-conservative replication using heavy (¹⁵N) and light (¹⁴N) nitrogen isotopes and density-gradient centrifugation. After one generation, all DNA was of intermediate density.
- The nucleus contains chromatin (DNA + histone proteins), a nucleolus (makes ribosomes), and is surrounded by a double membrane (nuclear envelope) with nuclear pores.
- Mitochondria have a double membrane; the inner membrane is folded into cristae, surrounding a matrix. They are the site of aerobic respiration and ATP synthesis.
- Chloroplasts have a double membrane and contain stacks of thylakoids (grana) in a fluid stroma. They are the site of photosynthesis and contain chlorophyll.
- The rough endoplasmic reticulum (RER) is studded with ribosomes and folds/transports proteins. The smooth endoplasmic reticulum (SER) synthesises and transports lipids.
- The Golgi apparatus modifies, processes and packages proteins and lipids into Golgi vesicles. It also makes lysosomes.
- Lysosomes are vesicles containing hydrolytic (digestive) enzymes (lysozymes). They break down worn-out organelles, pathogens, and digest material in phagocytes.
- Ribosomes (80S in eukaryotes, 70S in prokaryotes and in mitochondria/chloroplasts) are made of rRNA and protein, and are the site of protein synthesis (translation).
- The cell-wall of plants is made of cellulose; fungi have a cell wall of chitin. Walls provide mechanical strength and prevent the cell bursting under turgor pressure.
- The protein-secretion pathway: DNA in nucleus → mRNA → ribosomes on RER make protein → vesicles to Golgi for modification → vesicles to cell-surface membrane → exocytosis.
- Prokaryotic cells (e.g. bacteria) lack a nucleus and membrane-bound organelles. Their DNA is circular and free in the cytoplasm, with 70S ribosomes and a cell wall of murein (peptidoglycan).
- Some prokaryotes have extra features: plasmids (small DNA rings, often carrying antibiotic-resistance genes), a capsule (slime layer for protection), and flagella for movement.
- Viruses are acellular, non-living particles. They consist of genetic material (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope, plus attachment proteins.
- Magnification is how many times larger an image is than the real object. Resolution is the minimum distance between two points that can still be seen as separate.