H3 Biology 2025 Notes

!!! DISCLAIMER !!! I took A Levels in 2025, so they were created for the 9816 syllabus. Additionally, this set of notes are INCOMPLETE and were personally curated for my own revision, so some topics may be explained in greater detail than others while some sections may be missing or less developed. The explanations are also written in a way that made sense to me, so they may not always match how your school or teacher teaches them. Please use these notes at your own discretion!! If there are any differences between these notes and your teacher’s notes, lectures, or official materials, listen to them. Since this was done as a google docs, I have included the link so you can also view the comments and annotations, which may provide additional context. Plus, just a tip, for H3, content is not that important in comparison to your application and evaluative skills, so it's really helpful to work on SBQ/writing practice essays then memorising content! PLEASE PLEASE do not copy, redistribute, sell, or monetise these notes. All the best for your A levels! You got this :D Contents Link to docs CORE IDEA 1 – The Cell & Biomolecules of Life CORE IDEA 2 – Genetics & Inheritance CORE IDEA 3 – Energy & Equilibrium CORE IDEA 4 – Biological Evolution EXTENSION TOPIC 1 – Infectious Diseases EXTENSION TOPIC 2 – Climate Change ChatGPT DUMP

CORE IDEA 1

Table Of Contents THE FLUID MOSAIC MODEL.................................................................................................3 DEVELOPMENT OF THE MODEL....................................................................................3 PRIONS...................................................................................................................................5 PROTISTS...............................................................................................................................5 FUNGI......................................................................................................................................6 YEAST................................................................................................................................6 FILAMENTOUS FUNGI......................................................................................................7 CELL DIFFERENTIATION.......................................................................................................7 CELL THEORY........................................................................................................................8 ACELLULARITY.................................................................................................................8 VIRUSES......................................................................................................................8 PRIONS........................................................................................................................8 MULTINUCLEATION..........................................................................................................9 FUNGI HYPHAE..........................................................................................................9 ENDOSYMBIOSIS.............................................................................................................9 EUKARYOTES ENDOSYMBIOTIC ORIGINS..............................................................9 PROTEINS.............................................................................................................................10 PROTEIN BINDING SITES & SUBUNITS.......................................................................10 HAEMOGLOBIN.........................................................................................................11 IMMUNOLGLOBIN.....................................................................................................11 PROKARYOTIC RNA POLYMERASE.......................................................................12 PROTEIN MODIFICATIONS............................................................................................14 GLYCOSYLATION......................................................................................................14 PHOSPHORYLATION................................................................................................16 PROTEOLYTIC CLEAVAGE......................................................................................17 ENZYME REGULATION........................................................................................................19 ENZYME AMOUNT..........................................................................................................19 ENZYME ACTIVITY.........................................................................................................19 LOCALISATION...............................................................................................................20 ANNEX...................................................................................................................................21 1

★ ‘fluid’: lipids & proteins can move laterally & some even transversely ★ ‘mosaic’: there are proteins randomly embedded in the phospholipid bilayer DEVELOPMENT OF THE MODEL TIME EVIDENCE CONCLUSION mid to late 1800s ● cells were viewed as bags of protoplasm – ‘protoplasmic theory’ ● the boundary between cell & env. was thought to be just the outer layer of the cytoplasm ⇒ thought membranes were witchcraft Pfeffer (1877) ● studied plant cells and osmotic pressure using semi-permeable membranes ● he showed that cell membranes allowed water passage but restricted solute movement ● selective nature of membranes – cell membranes are semi-permeable barriers that regulate exchange between the cell and its environment Overton (1895) ● found that lipid-soluble molecules entered cells more easily than water-soluble molecules ● suggested that the cell membrane is made of lipid-like substances, likely a thin layer of fats late 1800s - early 1900s ● starting to accept existence of a thin lipid boundary outside protoplasm ● evidence from osmosis & solubility experiments convinced most scientists that the membrane is (1) semi-permeable & (2) have lipophilic character Chambers (1922) ● used micro-injection techniques and mechanical manipulation of sea urchin eggs ● he showed membranes behaved like physical barriers that could be stretched and deformed ● membranes are physical structures with flexible but resistant properties, not just invisible boundaries Gorter & Grendel (1925) ● extracted lipids from red blood cells and spread them as a monolayer on a water surface - langmuir trough experiment ● ⇒ the area covered was twice the surface area of the red blood cells ● proposed that membranes are a lipid bilayer, w hydrophilic heads facing outwards and hydrophobic tails facing inwards 1920s ● G&G’s 1925 study gave quantitative evidence of a lipid bilayer ⇒ one of the first structural models of the membrane ● but the role of proteins were unclear 2 THE FLUID MOSAIC MODEL

Danielli & Davson (1935) ● proposed a model to explain how membranes could be both thin and selectively permeable ● suggested proteins must play a role in stability and permeability ● introduced the ‘protein–lipid sandwich’ model → a lipid bilayer coated on both sides with protein layers ● explained selective permeability better than a lipid-only model 1930s - 1950s ● widespread acceptance of D&D’s ‘protein-lipid sandwich’ model ● but many viewed the membrane to be ‘static’ → just coating the cell Robertson (1959) ● used electron microscopy with improved staining to visualise membranes ● found they appeared as a trilaminar structure ○ two dark lines separated by a light line ● proposed the unit membrane hypothesis → all cell membranes share a common trilaminar st