National Year 12 Biology
Chapters: 4
1. 3.1 Biological molecules
3.1.1 Monomers and polymers · 3.1.2 Carbohydrates · 3.1.3 Lipids · 3.1.4 Proteins · 3.1.5 Nucleic acids · 3.1.6 ATP · 3.1.7 Water · 3.1.8 Inorganic ions
- Biomolecules – A cell is mostly water, plus four big families of carbon compounds: proteins, carbohydrates, lipids and nucleic acids. Grinding tissue in acid separates small molecules (acid-soluble pool) from big ones – proteins, polysaccharides and nucleic acids (acid-insoluble pool). Proteins are chains of amino acids folded into four levels of structure. Polysaccharides are chains of sugars; lipids are fatty acids on glycerol; nucleic acids are chains of nucleotides. Enzymes are protein catalysts that bind a substrate at the active site, lower the activation energy, and are affected by temperature, pH, substrate level and inhibitors.
- Molecular Basis of Inheritance – DNA is the genetic material in most living things (some viruses use RNA). It is a double helix: two antiparallel strands of nucleotides, A pairs with T by 2 hydrogen bonds and G with C by 3. Long DNA is packed on histones into nucleosomes and then chromatin. DNA copies itself semi-conservatively (Meselson–Stahl). The central dogma says DNA → RNA → protein. Transcription makes RNA from one strand; in eukaryotes the hnRNA is capped, tailed and spliced. The genetic code is a triplet, has 64 codons (61 for amino acids, 3 stops), starts with AUG, is nearly universal and degenerate. Ribosomes translate mRNA into protein with tRNA adaptors. Genes are switched on and off; the lac operon is the classic example. The Human and Rice Genome Projects read whole genomes, and DNA fingerprinting uses repeat DNA (VNTRs) to identify people.
- ATP: The Energy Currency of the Cell – ATP (adenosine triphosphate) is a nucleotide made of the base adenine, the sugar ribose and three phosphate groups. The enzyme ATP hydrolase splits off the last phosphate using water (hydrolysis): ATP + H₂O → ADP + Pi, releasing about 30 kJ per mole in small, usable amounts. ATP synthase joins ADP and Pi back together by a condensation reaction during respiration and photosynthesis. Cells make and use ATP constantly instead of storing it. One glucose gives roughly 30–32 ATP in aerobic respiration but only 2 in anaerobic respiration.
- Water: The Small Molecule That Makes Life Possible – A water molecule (H₂O) is bent and polar: oxygen is slightly negative and the hydrogens slightly positive. Because of this, water molecules stick together with hydrogen bonds. These bonds explain why water dissolves many substances, stores a lot of heat, takes a lot of energy to evaporate, sticks together (cohesion) and to surfaces (adhesion), and why ice floats. Living things also use water as a reactant, for transport and to keep cells firm. Mineral salts dissolved in water supply ions like Na⁺, K⁺, Ca²⁺, Fe²⁺ and phosphate.
- Inorganic Ions in Living Things – Inorganic ions are charged particles without carbon chains, dissolved in the water of cells and body fluids. They are needed in small amounts but each has a key job. Iron ions (Fe²⁺) sit in the haem groups of haemoglobin and bind oxygen. Sodium ions (Na⁺) drive the co-transport of glucose and amino acids into cells and are vital for nerve impulses. Phosphate ions (PO₄³⁻) build ATP, DNA, RNA and phospholipids. Hydrogen ions (H⁺) set the pH: more H⁺ means lower pH, which can denature enzymes and helps haemoglobin release oxygen.
2. 3.2 Cells
3.2.1 Cell structure · 3.2.2 Cell division (all cells arise from other cells) · 3.2.3 Transport across cell membranes · 3.2.4 Cell recognition and the immune system
- Cell: The Unit of Life – Every living thing is made of cells, and every new cell comes from an old cell. Prokaryotic cells (bacteria) have no nuclear envelope and no membrane-bound organelles; eukaryotic cells have a true nucleus and many organelles. The membrane is a fluid mosaic of lipids and proteins; plants add a cellulose wall. ER, Golgi, lysosomes and vacuoles form the endomembrane system. Mitochondria and plastids make energy and food, ribosomes make proteins, the cytoskeleton, cilia, flagella and centrioles give shape and movement, and the nucleus holds the DNA.
- Cell Cycle and Cell Division – A cell grows, copies its DNA and splits in a fixed order called the cell cycle: interphase (G1, S, G2) and M phase. In S phase the DNA doubles (2C → 4C) but the chromosome number stays the same. Mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis give two cells identical to the parent – it is an equational division used for growth and repair. Meiosis has two divisions; in meiosis I homologous chromosomes pair, cross over and separate, halving the chromosome number (2n → n). It makes four haploid cells for gametes and creates variation.
- Membrane Transport: How Things Get In and Out of a Cell – The cell membrane is a thin double layer of fat-like molecules with proteins in it. It lets some substances through and stops others (selectively permeable). Diffusion moves particles from high to low concentration with no energy. Facilitated diffusion does the same through protein channels. Osmosis is the diffusion of water through a partially permeable membrane. Active transport uses ATP and a protein pump to move particles from low to high concentration. Very large things enter and leave in bubbles (endocytosis and exocytosis).
- Human Health and Disease – Health means the body, mind and social life all work well. Diseases can be caused by germs (pathogens) like bacteria, viruses, protozoa, worms and fungi. Our body fights them with two kinds of immunity: innate (we are born with it) and acquired (it learns and remembers). Vaccines use this memory. HIV destroys helper T cells and causes AIDS. Cancer is cells dividing without control. Drugs and alcohol harm the body and mind, especially in teenagers.
3. 3.3 Organisms exchange substances with their environment
3.3.1 Surface area to volume ratio · 3.3.2 Gas exchange · 3.3.3 Digestion and absorption · 3.3.4 Mass transport
- Surface Area to Volume Ratio – Surface area (SA) is the outside area of an object; volume (V) is the space inside. SA : V tells how much surface there is for each unit of volume. For a cube of side L, SA = 6L², V = L³ and SA : V = 6 ÷ L, so the ratio falls as size rises. Living things exchange oxygen, food, waste and heat across their surface, while the whole volume needs them. Small organisms have a big ratio and short diffusion distances, so diffusion is enough. Large organisms have a small ratio, so they use flattened shapes, folded exchange surfaces (alveoli, villi, gills, root hairs) and transport systems. Small mammals lose heat fast and need a high metabolic rate per gram.
- Breathing and Exchange of Gases – Breathing moves air in and out of the lungs. Air travels nose → trachea → bronchi → bronchioles → alveoli. We breathe in when the diaphragm and outer rib muscles contract and make the chest bigger, so pressure inside falls. In the alveoli, oxygen moves into blood and carbon dioxide moves out, always from higher partial pressure to lower. Blood carries O2 mostly on haemoglobin and CO2 mostly as bicarbonate. A centre in the brain sets the rhythm. Lung volumes like tidal volume and vital capacity can be added and measured.
- Digestion and Absorption: Enzymes, Micelles and Co-transport – Big food molecules are cut into small ones by enzymes: carbohydrases (amylase, then membrane-bound disaccharidases) make monosaccharides, proteases (endopeptidases, exopeptidases, membrane-bound dipeptidases) make amino acids, and lipase makes monoglycerides and fatty acids after bile salts emulsify fat. Glucose and amino acids enter ileum cells by co-transport with sodium ions. Lipid products travel in micelles, diffuse into the cell, are rebuilt into triglycerides, packed into chylomicrons and leave into the lacteal.
- Body Fluids and Circulation – Blood and lymph are the body's transport fluids. Blood is plasma (about 55%) plus formed elements: RBCs, WBCs and platelets. Blood groups (ABO, Rh) depend on antigens on RBCs. A clot forms when fibrinogen turns into fibrin threads. The human heart has four chambers and pumps blood through two loops: to the lungs and to the body (double circulation). The SA node starts each beat; one beat is a cardiac cycle of about 0.8 s, recorded as an ECG with P, QRS and T waves. Cardiac output = stroke volume × heart rate ≈ 5 L/min.
4. 3.4 Genetic information, variation and relationships
3.4.1 DNA, genes and chromosomes · 3.4.2 DNA and protein synthesis · 3.4.3 Genetic diversity from mutation and meiosis · 3.4.4 Genetic diversity and adaptation · 3.4.5 Species and taxonomy · 3.4.6 Biodiversity within a community · 3.4.7 Investigating diversity
- Molecular Basis of Inheritance – DNA is the genetic material in most living things (some viruses use RNA). It is a double helix: two antiparallel strands of nucleotides, A pairs with T by 2 hydrogen bonds and G with C by 3. Long DNA is packed on histones into nucleosomes and then chromatin. DNA copies itself semi-conservatively (Meselson–Stahl). The central dogma says DNA → RNA → protein. Transcription makes RNA from one strand; in eukaryotes the hnRNA is capped, tailed and spliced. The genetic code is a triplet, has 64 codons (61 for amino acids, 3 stops), starts with AUG, is nearly universal and degenerate. Ribosomes translate mRNA into protein with tRNA adaptors. Genes are switched on and off; the lac operon is the classic example. The Human and Rice Genome Projects read whole genomes, and DNA fingerprinting uses repeat DNA (VNTRs) to identify people.
- Cell Cycle and Cell Division – A cell grows, copies its DNA and splits in a fixed order called the cell cycle: interphase (G1, S, G2) and M phase. In S phase the DNA doubles (2C → 4C) but the chromosome number stays the same. Mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis give two cells identical to the parent – it is an equational division used for growth and repair. Meiosis has two divisions; in meiosis I homologous chromosomes pair, cross over and separate, halving the chromosome number (2n → n). It makes four haploid cells for gametes and creates variation.
- Evolution: Origin of Life, Mechanisms and Human Evolution – Life began on the early Earth from simple chemicals: Oparin and Haldane proposed it and Miller made amino acids in a flask. Evidence of evolution comes from fossils, homologous and analogous organs, embryos, molecules and changes we can watch (industrial melanism, drug resistance). Darwin explained it by natural selection acting on variation in populations; the modern synthetic theory adds genes: mutation, recombination, gene flow, genetic drift and natural selection change allele frequencies. If none of these act, frequencies stay constant: Hardy–Weinberg, p² + 2pq + q² = 1. Selection can be stabilising, directional or disruptive. One ancestor spreading into many habitats gives adaptive radiation (Darwin’s finches, Australian marsupials). Humans evolved from Dryopithecus-like apes through Australopithecus, Homo habilis, Homo erectus and Neanderthals to Homo sapiens.
- Biological Classification – Whittaker (1969) sorted living things into five kingdoms, Monera, Protista, Fungi, Plantae and Animalia, using cell type, cell wall, body plan, mode of nutrition, reproduction and relationships. Monera are prokaryotes (bacteria, archaebacteria, cyanobacteria, mycoplasma). Protista are one-celled eukaryotes in five groups. Fungi are heterotrophs with chitin walls in four classes. Lichens are an alga and a fungus living together. Viruses, viroids and prions are not placed in any kingdom.
- Biodiversity and its Conservation – Biodiversity is the variety of life at three levels: genetic, species and ecological. It is highest near the equator and grows with area (log S = log C + Z log A). Every species matters, like rivets on a plane. We are losing species fast because of the 'evil quartet': habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinction. The IUCN Red List (Red Data Book) ranks species from Least Concern to Extinct. We protect life in situ (hotspots, national parks, sanctuaries, biosphere reserves, sacred groves, Ramsar wetlands) and ex situ (zoos, botanical gardens, seed and gene banks).
- Investigating Diversity: Measuring Genetic Diversity, Sampling and Standard Deviation – Genetic diversity within or between species can be measured by comparing observable characteristics, DNA base sequences, mRNA base sequences or amino acid sequences: the fewer the differences, the closer the relationship. Since we cannot measure every organism, we take a random sample large enough to be representative, then describe it with a mean and a standard deviation (SD). SD shows the spread around the mean; about 68% of normally distributed values lie within ±1 SD. Overlapping ±SD ranges suggest a difference may not be real.