Biology SL - IB Academy

Cell Biology

Cell Theory

• All organisms are composed of cells.
• Cell is the basic unit of life.
• All cells originate from a pre-existing cell.
• Functions of life: Nutrition, Metabolism, Excretion, Response, Homeostasis, Growth, Reproduction.

Sizes of Cells

• Surface area is related to the rate of exchange of materials.
• Volume is related to the rate of metabolic reactions.
• Cell growth is limited by surface area to volume ratio.
• $SA= 6 × a^2$ and $V = a^3$
• $Real cell size = \frac{Image of the cell}{magnification}$
• Electron microscope has greater resolving power than a light microscope.

Cell Properties

• Emergent properties emerge from the interaction of individual cell components creating new functions.
• Cell differentiation is the process whereby different genes are put on lockdown to reach a very specific cell type.
• Stem cells are undifferentiated cells that can divide and differentiate into any cell type.
• Therapeutic sources of stem cells include umbilical cord blood, bone marrow, and human embryonic stem cells.

Cells and Membrane Transport

• Eukaryotic cells have membrane-bound organelles; prokaryotic cells do not.
• Prokaryotic cell structures: plasma membrane, cell wall, nucleoid (naked DNA), pili, flagella, plasmid, cytoplasm, 70S ribosomes.
• Eukaryotic cell structures: plasma membrane, ribosome, nuclear envelope, rough ER, smooth ER, Golgi apparatus, cytoplasm, nucleolus, nucleus, lysosome, mitochondria, centrioles.
• Cell membrane is composed of phospholipids, cholesterol, proteins, and lipoproteins.
• Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails.
• Membrane proteins: channel, carrier, recognition, receptor, enzymatic.
• Membrane transport: passive (simple and facilitated diffusion, osmosis) and active transport (primary, secondary, exocytosis, endocytosis).
• Osmosis: movement of water from low solute concentration to high solute concentration.
• Osmolarity is a measure of solute concentration.
• Hypotonic: low osmolarity, system loses water.
• Hypertonic: high osmolarity, system gains water.

Origin of Cells

• Pasteur's experiment refuted spontaneous generation.
• Miller-Urey experiment: water vapor, ammonia, and methane could form amino acids and carbon compounds with electricity.
• Endosymbiotic theory: eukaryotic cells evolved from prokaryotic cells through symbiosis.
• Mitochondria and chloroplasts have double membranes and circular DNA.

Cell Division

• Mitosis is the division of the cell’s nucleus into two identical daughter nuclei.
• Cytokinesis is the division of the cell’s cytoplasm and organelles.
• Cell cycle phases: G1 (growth), S (DNA synthesis), G2 (preparation for mitosis).
• Cyclins regulate the cell cycle.
• CDKs (Cyclin dependent kinases) allow progression through a stage of the cycle via phosphorylation specific molecules.
• Cancer is the result of uncontrollable cell division.
• Mutagens cause mutations in the DNA.
• Oncogenes are responsible for normal cell division; mutations can lead to cancer.
• Proto-oncogenes in their mutated state become overactivated and promote cell division leading to tumour formation.
• Tumour suppressor genes negatively regulate the cell cycle; when mutated, they fail to prevent uncontrollable cell divisions.
• Metastasis refers to the movement of the primary cancerous cells to a new formation where they continue to form tumours.
• Mitosis phases: prophase, metaphase, anaphase, telophase.
• $Mitotic index = \frac{number of cells undergoing mitosis}{total number of cells}$

Molecular Biology

Molecules to Metabolism

• Carbon is the core of organic compounds, forming four covalent bonds.
• Metabolism: enzyme-catalyzed reactions in a system.
• Anabolism: synthesis of complex molecules from simpler ones.
• Catabolism: breakdown of complex molecules into simpler ones.

Water

• Water is polar molecule consisting of 2 hydrogen atoms bound by covalent bonds to an oxygen atom.
• Water is essential for life, with thermal, cohesive, adhesive, and solvent properties.
• Hydrophilic substances dissolve in water; hydrophobic substances do not.

Carbohydrates and Lipids

• Carbohydrates are composed of hydrogen, oxygen, and carbon atoms.
• Carbohydrate monomers: monosaccharides.
• Important carbohydrates: glucose, fructose, galactose, maltose, sucrose, lactose, cellulose, glycogen, starch.
• Lipids are hydrophobic compounds for energy storage, insulation, and shock absorption.
• Lipid monomers: fatty acids (saturated, monounsaturated, polyunsaturated, cis, trans).
• Lipid classes: phospholipids, steroids, triglycerides.
• $BMI = \frac{mass in kg}{(height in meters)^2}$

Proteins

• Proteins consist of amino acids linked by peptide bonds.
• Amino acids contain a carboxyl, an ammine and an R group.
• Protein structure: primary, secondary, tertiary, quaternary.
• Protein functions: structural, transport, movement, defense.
• Proteome: the entire set of proteins expressed by a genome at a given time.
• The Central Dogma relates to a sequential transfer of information where DNA is transcribed into RNA, which in turn is translated into proteins.

Enzymes

• Enzymes are biological catalysts that speed up chemical reactions.
• Substrates bind to the enzyme's active site.
• Factors influencing enzyme activity: temperature, pH, substrate concentration.

Structure of DNA and RNA

• Nucleic acids (DNA and RNA) are responsible for information storage.
• Nucleotides consist of a 5-carbon sugar, a phosphate group, and a nitrogenous base.
• DNA contains deoxyribose; RNA contains ribose.
• DNA: double-stranded, bases A, T, C, G.
• RNA: single-stranded, bases A, U, C, G.
• Base pairing: A-T (DNA), A-U (RNA), C-G.

DNA Replication, Transcription, and Translation

• DNA replication: one double-stranded DNA molecule gives rise two daughter DNA molecules, Helicase unwinds the double helix DNA polymerase, semi-conservative.
• Transcription: mRNA is synthesized from a DNA template.
• Translation: polypeptides are synthesized by ribosomes from mRNA sequences.
• Genetic code: codons (three bases) code for specific amino acids.
• Universality of genetic code: the same codon codes for the same amino acid in all organisms.
• PCR (polymerase chain reaction) is used to produce many copies of DNA

Cell Respiration

• Cellular respiration: controlled release of energy (ATP) from organic compounds.
• Aerobic (with oxygen) and anaerobic (without oxygen) cell respiration.
• Anaerobic: glycolysis (glucose to pyruvate). In yeast cells, pyruvate is converted into ethanol and carbon dioxide . In mammalian cells, pyruvate molecules are converted into lactate molecules.
• Aerobic: pyruvate broken down in mitochondria via Krebs cycle and electron transport chain.

Photosynthesis

• Photosynthesis: plants produce organic substances using sunlight, water, and carbon dioxide.
• Chlorophyll absorbs red and blue light, reflects green light.
• Light-dependent reactions: ATP, oxygen, and hydrogen yield.
• Calvin cycle (light-independent reactions): ATP and carbon dioxide are used, carbon fixation to form carbohydrates.
• Factors limiting photosynthesis: light intensity, carbon dioxide concentration, temperature.