Comprehensive IB Biology SL Revision Guide Notes
Topic 1: Cell Biology and the Essential Principles of Cell Theory
Cell biology is governed by the cell theory, which consists of three fundamental statements that must be learned exactly. First, all living organisms are composed of one or more cells. Second, the cell is the basic unit of life. Third, all cells come from pre-existing cells, a principle often referred to by the Latin phrase omnis cellula e cellula. Despite its broad application, there are notable exceptions and nuances. For instance, viruses are not considered cells and are not living because they cannot reproduce independently, meaning they do not support cell theory. Additionally, there are biological exceptions such as giant algae, specifically the genus Acetabularia, which exists as one giant cell, and skeletal muscle fibres, which are multinucleate structures rather than single contained cells.
Comparative Analysis of Prokaryotic and Eukaryotic Cells
Cells are categorized into two primary types: prokaryotes and eukaryotes, which differ in complexity and structure. Prokaryotes lack a membrane-bound nucleus and instead contain their genetic material in a nucleoid region. In contrast, eukaryotes possess a true nucleus enclosed by a nuclear envelope. The DNA in prokaryotes is circular and resides freely in the cytoplasm, whereas eukaryotic DNA is linear, located within the nucleus, and associated with histone proteins. Ribosomes also distinguish the two, with prokaryotes having smaller ribosomes and eukaryotes possessing larger ribosomes. Eukaryotes feature membrane-bound organelles such as mitochondria, the endoplasmic reticulum, and the Golgi apparatus, all of which are absent in prokaryotes. The cell walls differ as well; bacteria have peptidoglycan walls, while eukaryotes like plants have cellulose, fungi have chitin, and animals have none. In terms of size, prokaryotes are generally , while eukaryotes are larger at . Despite these differences, both cell types share ribosomes, DNA, a cell membrane, and cytoplasm. The difference between and ribosomes is medically significant as it allows antibiotics to target bacterial cells without damaging human cells.
Membrane Structure and the Fluid Mosaic Model
The cell membrane is described by the fluid mosaic model, which highlights its dynamic nature. It is composed of a phospholipid bilayer where hydrophilic, water-loving heads face outward toward water, and hydrophobic, water-fearing tails face inward. This arrangement is spontaneous and self-sealing. The term fluid refers to the ability of phospholipids and proteins to move laterally across the membrane rather than remaining fixed. The mosaic aspect refers to the scattered pattern of proteins throughout the bilayer. Integral proteins span the full bilayer and function as channel or carrier proteins for transport. Peripheral proteins are attached only to the surface and are involved in cell signalling. Cholesterol sits between phospholipids to stabilize fluidity, preventing the membrane from becoming too rigid in cold environments or too fluid in heat. Glycoproteins, which are proteins with carbohydrate chains, are essential for cell recognition, receptor sites, and the immune response.
Mechanisms of Membrane Transport and Osmosis
Membrane transport occurs through several active and passive mechanisms depending on the molecule and concentration gradient. Simple diffusion is a passive process where small nonpolar molecules like , , and lipids move from high to low concentration without ATP or protein assistance. Facilitated diffusion is also passive, moving polar molecules and ions like glucose, , and down their gradient through protein channels or carriers. Osmosis is specifically the passive movement of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane, sometimes utilizing proteins called aquaporins. Active transport requires and carrier proteins to move ions and molecules against their concentration gradient, as seen in the pump. Large particles are transported via endocytosis, where the membrane engulfs them into the cell, or exocytosis, where vesicles fuse with the membrane to release large molecules like neurotransmitters externally. Both endocytosis and exocytosis require .
The Stages of Mitosis and Cell Division
Mitosis is the process of nuclear division resulting in two genetically identical diploid daughter cells, used for growth, repair, and replacement. It occurs in four main stages known as PMAT. In prophase, chromosomes condense and become visible, consisting of two sister chromatids joined at a centromere; the spindle fibres form from centrioles, and the nuclear envelope breaks down. During metaphase, chromosomes align at the equator or metaphase plate, and spindle fibres attach to the centromeres. This is the optimal stage for chromosome counting. In anaphase, sister chromatids are pulled to opposite poles as spindle fibres shorten, causing the cell to elongate. Finally, in telophase, nuclear envelopes reform around each set of chromosomes, which then decondense, and cytokinesis begins to divide the cytoplasm. Mitosis differs from meiosis, which results in four genetically different haploid cells used exclusively for gamete production.
Topic 2: Molecular Biology and DNA Structure
DNA is a double helix composed of two antiparallel strands made of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The sugar-phosphate backbone is on the exterior, while the bases pair in the center. Adenine pairs with thymine via hydrogen bonds, while guanine pairs with cytosine via hydrogen bonds, making the G-C bond stronger. The strands run in opposite directions, to and to . RNA differs from DNA because it contains ribose sugar instead of deoxyribose, uracil instead of thymine, and is single-stranded. Bases are classified as purines, such as adenine and guanine, which have a double ring structure, and pyrimidines, such as cytosine, thymine, and uracil, which have a single ring structure. Purines always pair with pyrimidines to maintain a constant helix width.
The Semiconservative Mechanism of DNA Replication
DNA replication is semiconservative, meaning each new molecule contains one original strand and one new strand. The process involves several key enzymes. Helicase unwinds and unzips the double helix by breaking hydrogen bonds at the replication fork. Primase then adds a short RNA primer to provide a starting point for DNA polymerase. DNA polymerase reads the template strand from to and builds the new strand in the to direction by adding complementary free nucleotides. Because DNA polymerase only works in one direction, the lagging strand is synthesized in short fragments called Okazaki fragments. Finally, ligase joins these Okazaki fragments together. This precise orchestration ensures that two identical DNA molecules are produced from one original.
Protein Synthesis: Transcription and Translation
Protein synthesis involves transcription in the nucleus and translation at the ribosome. In transcription, RNA polymerase uses a DNA template strand, reading it to to build a complementary strand in the to direction. Following transcription, processing or splicing occurs in the nucleus, where spliceosomes remove non-coding introns and join exons. The resulting moves to the ribosome for translation. Translation involves codons, which are triplets of bases on that code for one specific amino acid. There are possible codons for amino acids. The transfer RNA or carries a specific amino acid and possesses a complementary anticodon. Translation starts at the codon, which codes for methionine, and terminates at stop codons like , , or . The genetic code is universal, meaning the same codons code for the same amino acids in most organisms, and degenerate, meaning multiple codons can code for the same amino acid to reduce the impact of mutations.
Enzyme Kinetics and Inhibitory Mechanisms
Enzymes function according to the induced fit model, where the active site is flexible and changes shape slightly to mould around a substrate, forming an enzyme-substrate complex. Enzyme activity is influenced by temperature, pH, and substrate concentration. Increasing temperature increases the rate due to more kinetic energy and collisions until an optimum temperature is reached, after which denaturation occurs. pH changes affect the ionization of amino acids; for example, pepsin has an optimal pH of , while trypsin thrives at pH . Substrate concentration increases the rate until the enzyme becomes saturated and acts as a limiting factor. Inhibition can be competitive, where an inhibitor of similar shape competes for the active site, or non-competitive, where an inhibitor binds to an allosteric site and changes the active site's shape. Competitive inhibition can be overcome by increasing substrate concentration, whereas non-competitive inhibition generally cannot.
Cellular Respiration: Aerobic and Anaerobic Pathways
Respiration provides energy in the form of . Aerobic respiration uses oxygen to convert glucose into and water, yielding . This process starts with glycolysis in the cytoplasm, followed by the Krebs cycle in the mitochondrial matrix and the electron transport chain on the inner mitochondrial membrane. Anaerobic respiration occurs in the cytoplasm without oxygen. In animals, glucose is converted to lactic acid with a yield of . In yeast, fermentation converts glucose to ethanol and , also yielding . Most in aerobic respiration is produced via chemiosmosis, where synthase uses a ion gradient across the folded inner mitochondrial membrane, known as cristae, to generate energy.
Topic 3: Fundamentals of Genetics and Inheritance
Genetics is defined by several precise terms. A gene is a heritable factor and a DNA sequence that codes for a polypeptide. An allele is a specific form of a gene differing by a few bases at the same locus. Genotype refers to the specific alleles an organism possesses, such as , , or , while the phenotype is the observable characteristic resulting from the genotype and environment. Dominant alleles are expressed in the phenotype even in heterozygotes, while recessive alleles require two copies. Codominance occurs when both alleles are expressed, as seen in blood groups. Meiosis involves two divisions, creating four non-identical haploid gametes. Genetic variation in meiosis is generated through crossing over at chiasmata in Prophase I, independent assortment in Metaphase I, and random fertilisation. Monohybrid inheritance is calculated using Punnett squares to determine genotype and phenotype ratios, often using a test cross with a homozygous recessive to determine an unknown genotype.
Sex-Linked Traits and Genetic Mutations
Sex-linked traits involve genes on the chromosome. Males are more affected by -linked recessive conditions like haemophilia and red-green colour blindness because they have only one chromosome and no second allele to mask a recessive one. Females can be carriers if they possess one recessive allele. Mutations in the genetic sequence can have various effects. Substitution mutations replace one base and may be silent, missense, or nonsense. Insertion or deletion mutations are generally more harmful because they cause a frameshift, altering every codon downstream and typically resulting in a non-functional protein. Down Syndrome, or Trisomy , results from non-disjunction during meiosis when chromosomes fail to separate, leading to three copies of chromosome . The risk of this increases with maternal age.
Topic 4: Ecological Principles and Population Dynamics
Ecology studies the interactions between organisms and their environments. A species is a group that can interbreed to produce fertile offspring, while a population is a group of the same species in the same area. A community includes all populations in an area, and an ecosystem includes the community and its abiotic environment. Autotrophs or producers create organic molecules from inorganic sources, whereas heterotrophs consume other organisms. Detritivores like earthworms ingest dead matter and digest it internally, while saprotrophs like fungi secrete enzymes for external digestion and absorption. In energy flow, only approximately of energy is transferred between trophic levels, with the remaining lost as heat, faeces, or used for growth. While pyramids of numbers and biomass can be inverted, pyramids of energy are always upright as energy always decreases up a food chain.
Biogeochemical Cycles and Population Growth Models
The nitrogen cycle involves five essential processes. Nitrogen fixation by bacteria like Rhizobium or Azotobacter converts to . Nitrification converts to and then . Assimilation is the absorption of nitrates by plants for protein synthesis. Ammonification involves decomposers releasing from dead matter. Denitrification by bacteria in anaerobic conditions converts back to . The carbon cycle involves photosynthesis removing and respiration, combustion, and decomposition returning it. Population growth follows either a J-curve in ideal conditions or an S-curve, which reaches a carrying capacity due to limiting factors. These factors can be density-dependent, like disease and competition, or density-independent, such as natural disasters.
Topic 5: Evolution, Natural Selection, and Biodiversity
Evolution is supported by five types of evidence: the fossil record, selective breeding, homologous structures like the pentadactyl limb, comparative biochemistry, and real-time examples like antibiotic resistance. Natural selection follows four steps: overproduction leads to competition, heritable variation exists in populations, individuals with advantageous traits have higher differential survival or fitness, and this leads to a change in allele frequency over generations. In taxonomical classification, the hierarchy follows Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. Binomial nomenclature uses the Genus species format. Speciation, specifically allopatric speciation, occurs when geographic barriers lead to genetic divergence and eventual reproductive isolation. Cladistics classifies organisms based on shared derived characteristics, often using molecular DNA evidence to construct cladograms.
Topic 6: Human Physiology and Digestive Processes
Human physiology is a critical topic covering systems like digestion and circulation. Digestion involves various enzymes with specific optimal pH levels. Salivary amylase breaks starch into maltose at pH . In the stomach, chief cells produce pepsin to break proteins into polypeptides at an acidic pH of . The pancreas produces amylase, lipase, and trypsin, which work in the small intestine at a pH of to . Trypsin is secreted as trypsinogen and processes polypeptides into shorter peptides. Absorption occurs in the small intestine villi, where glucose and amino acids enter blood capillaries, and fatty acids and glycerol are absorbed into lacteals. Villi adaptations include being one-cell thick and having microvilli to maximize surface area.
The Cardiovascular, Immune, and Respiratory Systems
The blood system comprises arteries, which carry blood away from the heart under high pressure with thick walls; veins, which return blood to the heart at low pressure and contain valves; and capillaries, which are one-cell thick for exchange. The heart's left ventricle has a thicker wall than the right to pump blood through the systemic circuit. The immune system has three lines of defense: physical barriers like skin and mucus, non-specific phagocytes that engulf pathogens, and specific lymphocytes. B lymphocytes produce antibodies for humoral immunity, while T lymphocytes handle cell-mediated immunity. Vaccines work by stimulating a primary response to create memory cells, allowing for a faster and stronger secondary response. In the respiratory system, alveoli are adapted with thin walls, a moist surface, and a rich capillary network for gas exchange. Ventilation is driven by pressure changes caused by the diaphragm and intercostal muscles.
Nervous Coordination and Endocrine Glucose Regulation
Nervous signals travel via sensory, relay, and motor neurons. Synaptic transmission is a chemical process where an action potential triggers the entry of into the pre-synaptic knob, causing vesicles to release neurotransmitters like acetylcholine via exocytosis into the synaptic cleft. These diffuse and bind to receptors on the post-synaptic membrane, generating a new impulse before being broken down by enzymes. Blood glucose is regulated by the pancreas: insulin is secreted by beta cells when glucose is high to promote glycogenesis, while glucagon is secreted by alpha cells when glucose is low to promote glycogenolysis. Type diabetes is an autoimmune destruction of beta cells, while Type is characterized by insulin resistance linked to obesity and lifestyle factors.
Questions and Discussion
Q: State two features shared by both prokaryotic and eukaryotic cells. [2] ANSWER: Ribosomes, DNA, cell membrane, or cytoplasm. Both cell types can carry out protein synthesis and cellular respiration.
Q: Explain why facilitated diffusion does not require ATP. [2] ANSWER: Facilitated diffusion moves molecules down the concentration gradient, from high to low. This is the direction molecules naturally move, so no energy input is required. is only needed for active transport against the gradient.
Q: Describe what happens to a plant cell placed in a hypertonic solution. [3] ANSWER: Water moves out of the cell by osmosis because there is a lower water potential outside. The cell membrane pulls away from the cell wall in a process called plasmolysis, causing the cell to become flaccid and lose turgor pressure.
Q: State the role of helicase in DNA replication. [1] ANSWER: Helicase unwinds the double helix and breaks the hydrogen bonds between complementary base pairs to separate the two strands and expose them as templates.
Q: Explain the difference between competitive and non-competitive inhibition. [4] ANSWER: Competitive inhibitors have a similar shape to the substrate and bind to the active site, which can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, changing the active site's shape so the substrate cannot bind, regardless of substrate concentration.
Q: Outline the process of translation. [4] ANSWER: attaches to a ribosome which reads it in triplet codons. with a complementary anticodon brings a specific amino acid. Peptide bonds form between amino acids as the ribosome moves along the . Upon reaching a stop codon, the polypeptide is released.
Q: Explain why males are more likely to be affected by X-linked recessive conditions than females. [2] ANSWER: Males possess only one chromosome () and thus only need one copy of the recessive allele to express the trait. Females have two () and need two copies to show the condition; females with one copy are non-symptomatic carriers.
Q: State two ways that meiosis produces genetic variation. [2] ANSWER: Crossing over between homologous chromosomes during prophase I and the independent assortment of homologous chromosomes during metaphase I.
Q: Explain why a deletion mutation is usually more harmful than a substitution mutation. [2] ANSWER: Deletion causes a frameshift, shifting the reading frame for all downstream codons and altering every amino acid from that point on. Substitution only affects one codon and may be silent due to the degenerate genetic code.
Q: Explain why energy transfer between trophic levels is inefficient. [3] ANSWER: Only about of energy is transferred. Energy is lost as heat through cellular respiration, in undigested material passed as faeces, and through movement or growth. Also, not all organisms at one level are consumed by the next.
Q: State the role of Rhizobium bacteria in the nitrogen cycle. [2] ANSWER: Rhizobium lives in the root nodules of legumes and performs nitrogen fixation, converting atmospheric nitrogen () into ammonium () for the plant's use in protein synthesis.
Q: Distinguish between a detritivore and a saprotroph. [2] ANSWER: A detritivore, such as an earthworm, ingests dead organic matter and digests it internally. A saprotroph, such as a fungus, secretes enzymes externally and absorbs the products of that digestion.
Q: Outline how natural selection can lead to antibiotic resistance in bacteria. [4] ANSWER: A random mutation provides resistance to some bacteria. When an antibiotic is applied, non-resistant bacteria die while resistant ones survive and reproduce, passing the resistance allele to offspring. Over generations, the frequency of this allele increases in the population.
Q: Explain how allopatric speciation occurs. [4] ANSWER: A geographic barrier isolates two groups of a population. These groups face different environmental pressures and accumulate different mutations. Natural selection acts differently on each, leading to genetic divergence until the groups can no longer interbreed to produce fertile offspring.
Q: Explain how a secondary immune response differs from a primary immune response. [3] ANSWER: Memory cells produced during the primary response recognize the antigen immediately upon second exposure. The secondary response is faster, produces more antibodies at higher concentrations, and often clears the infection before symptoms develop.
Q: Explain why the left ventricle has a thicker wall than the right ventricle. [2] ANSWER: The left ventricle must pump blood through the entire systemic circuit to the whole body, requiring higher pressure. The right ventricle only pumps to the lungs, a shorter distance requiring less force.
Q: Outline what happens at a synapse when a nerve impulse arrives. [4] ANSWER: The action potential triggers the entry of calcium ions, causing vesicles to fuse with the pre-synaptic membrane. Neurotransmitters are released via exocytosis, diffuse across the cleft, and bind to receptors on the post-synaptic membrane to generate a new impulse. Enzymes kemudian break down the neurotransmitter.
Q: Distinguish between Type 1 and Type 2 diabetes. [2] ANSWER: Type is an autoimmune condition where no insulin is produced due to destroyed beta cells. Type involves body cells becoming resistant to insulin, often due to lifestyle factors, even though insulin is produced.
Q: Outline the roles of insulin and glucagon in regulating blood glucose concentration. [4] ANSWER: Insulin is secreted by beta cells when glucose is high to stimulate glucose uptake and glycogenesis in the liver. Glucagon is secreted by alpha cells when glucose is low to promote glycogenolysis in the liver, releasing glucose into the blood.