Cell Biology Comprehensive Notes Summary & Study Notes

🧬 Overview

Cells are the basic units of life, organized into prokaryotic and eukaryotic types. All living organisms rely on carbon-based chemistry and macromolecules built from monomers. Understanding cellular structure and metabolism requires tracing from atomic bonds to organelle function and organismal physiology.

🧪 Biomolecules: Carbon and Organic Chemistry

• Carbon is the backbone of life because it forms four covalent bonds, enabling infinite molecular diversity. Covalent bonds yield stable, functionally diverse molecules.
• Organic molecules can be formed abiotically early in Earth's history, giving rise to proteins, DNA, lipids, and carbohydrates.
• Isomerism matters for function: cis-trans isomers, structural isomers, and enantiomers (mirror images) can drastically alter biological activity.
• Simple carbohydrate units (CH$_2$O) assemble into complex polymers via dehydration synthesis.

🧪 Lipids

• There are three main lipid groups: fats, phospholipids, and steroids.
• Fats store energy; they are formed by glycerol linked to three fatty acids via ester bonds to form a triacylglycerol.
• Phospholipids have two fatty acids, glycerol, phosphate, and a head group; they form membranes due to amphipathic properties.
• Steroids have four fused rings and are components of membranes and signaling molecules (e.g., hormones).

🧪 Macromolecules and Monomers

• Monomers are building blocks for polymers; macromolecules are polymers formed by covalent bonds.
• In summary: carbohydrates, lipids, proteins, and nucleic acids are the major macromolecules of life.

🧫 Carbohydrates

• Monosaccharides are simple sugars (3–7 carbons) with a carbonyl group that defines aldose or ketose. They serve as energy sources and raw materials for biosynthesis. Glucose is a central monosaccharide.
• Disaccharides (e.g., maltose, sucrose) are formed by dehydration reactions and serve transport or storage roles.
• Polysaccharides: Plants store energy as starch; animals store as glycogen; cellulose provides plant cell wall structure; chitin forms arthropod exoskeletons and fungal cell walls (beta-1,4 linkages).
• Structural polysaccharides are generally unbranched (cellulose) or branched (glycogen in animals).

🧬 Proteins

• Amino acids are the building blocks linked by peptide bonds into polypeptides. Functional proteins arise from specific sequences and folding.
• The primary structure is the linear amino acid sequence; secondary structures include helices and beta sheets stabilized by hydrogen bonds.
• Tertiary and quaternary structures depend on side chains and multiple polypeptides, respectively. Proteins can denature and lose function when conditions change.

🧬 Nucleic Acids

• Nucleic acids store, transmit, and express genetic information. DNA is transcribed to RNA, which is translated into proteins.
• DNA replication passes genetic information to daughter cells.
• Building blocks: nucleotides (base + sugar + phosphate).
• Nitrogenous bases include pyrimidines (C, T, U) and purines (A, G).

🧬 Genomics & Proteomics

• Milestones: 1953 — structure of DNA; 1970 — DNA sequencing; 2001 — first draft of the human genome. These advances lowered costs and accelerated discovery in biology.

🧫 Cells: Units of Life

• All life is composed of cells. Prokaryotes include bacteria and archaea; Eukaryotes include protists and all higher organisms.
• Eukaryotic cells are organized into organelles bound by membranes, enabling compartmentalization and complex metabolism.

🧪 Organelles and Subcellular Architecture

• Nucleus houses genetic material and mediates transcription.
• Ribosomes synthesize proteins; they can be free in the cytosol or bound to the ER.
• Endomembrane system: rough ER synthesizes proteins for membranes/lumen; smooth ER performs lipid synthesis and detoxification; Golgi apparatus modifies, sorts, and ships proteins.
• Lysosomes contain hydrolytic enzymes for digestion; peroxisomes generate and detoxify reactive oxygen species; mitochondria generate ATP via respiration; chloroplasts conduct photosynthesis in plants and algae.
• Vacuoles store substances; plant cells have a central vacuole; animal cells have smaller vesicular vacuoles.
• Mitochondria & chloroplasts possess their own DNA and ribosomes, and are double-membrane bound.
• The cytoskeleton (microtubules, actin filaments, intermediate filaments) provides structure, aids movement, and organizes organelles.

🧬 Cytoskeleton and Cell Reach

• Microtubules (alpha-beta tubulin dimers) form hollow tubes; they enable movement of cilia/flagella and vesicles and act as tracks for motor proteins.
• Actin filaments balance cell shape and movement; they are dynamic and essential for motility.
• Intermediate filaments offer mechanical stability and are more permanent than microtubules or actin.

🧫 Extracellular Matrix and Cell Junctions

• In plants, the cell wall (primary, middle lamella, secondary) provides structure and limits water uptake; cellulose is a major component.
• The ECM in animals is rich in collagen and proteoglycans, connecting exterior and interior signaling.
• Animal cell junctions include tight junctions, desmosomes, and gap junctions that coordinate tissue integrity and communication.
• In plants, plasmodesmata create cytoplasmic channels between neighboring cells for transport and signaling.

🧪 Membranes and Transport

• Membranes comprise a lipid bilayer with embedded proteins. The bilayer is fluid mosaic and amphipathic: hydrophobic tails and hydrophilic heads.
• Membrane proteins include integral and peripheral proteins, enabling transport, signaling, and enzymatic activity.
• Selective permeability governs what crosses the membrane via passive diffusion, channels, or transporters; active transport uses energy to move solutes against gradients.
• Passive transport includes simple diffusion and osmosis; tonicity influences cell volume.
• Passive channels (e.g., aquaporins, ion channels) and carriers (e.g., glucose transporter) facilitate movement without energy input.
• Active transport uses energy (e.g., Na$^+$–K$^+$ pump, proton pumps) to maintain gradients essential for cellular function.

🧪 Metabolism and Energy

• Metabolism comprises anabolic (biosynthetic; energy input) and catabolic (degradative; energy release) pathways.
• Energy forms include kinetic, thermal, and chemical energy. Cells are energy transformers that convert energy from one form to another.
• Thermodynamics: living organisms are open systems exchanging matter and energy with their surroundings. The first law states energy is conserved; the second law relates to increasing entropy in energy transformations.
• Gibbs free energy ($G$) determines spontaneity: if $G$ decreases, the process is favorable; otherwise, energy input is required.
• ATP couples exergonic and endergonic reactions to power cellular work.

🧪 Enzymes and Regulation

• Enzymes speed up reactions by lowering the activation energy without being consumed themselves.
• Mechanisms include optimal orientation, substrate positioning, and microenvironment effects at the active site.
• Enzyme activity is influenced by temperature, pH, and regulation by gene expression, cofactors, and localization.
• Regulation also occurs via allosteric control, cooperativity, and feedback inhibition.
• Inhibitors can be reversible (competitive at the active site; non-competitive at allosteric sites) or irreversible (covalent bonding).

🔬 Respiration and Fermentation

• Cellular respiration includes glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation; oxygen is the final electron acceptor.
• The Electron Transport Chain (ETC) pumps protons to create a gradient used by ATP synthase to produce ATP.
• In the absence of oxygen, cells may perform anaerobic respiration or fermentation, generating ATP via substrate-level phosphorylation and regenerating NAD$^+$.

🌱 Plant Structure, Organs, and Tissues

• Plant life is organized into four organs: roots, stems, leaves, and reproductive organs.
• The plant body consists of two main systems: the root system and the shoot system.
• Monocots often have a fibrous root system; dicots typically have a taproot.
• Plant tissues include dermal (protective), vascular (transport), and ground (photosynthesis, storage, support).
• The endodermis in roots acts as a selective barrier for water and solutes.

🌱 Plant Cells and Tissues: Cell Types

• Plant cells come in three major types: parenchyma, collenchyma, and sclerenchyma.
• Parenchyma cells perform photosynthesis and storage and are alive at maturity; they have thin primary walls.
• Collenchyma provides flexible support with unevenly thickened primary walls.
• Sclerenchyma contains thick secondary walls with lignin and provides rigid support; they are usually dead at maturity.
• Complex tissues combine these cell types to form xylem (water transport) and phloem (sugar transport) through living and dead elements.

🧭 Xylem, Phloem, and Transport

• Xylem includes tracheids and vessel elements that conduct water and minerals; water movement follows transpiration-driven pull.
• Phloem transports organic molecules from sources to sinks via sieve elements and companion cells; sieve elements are alive but lack nuclei and some organelles.
• Vascular tissue is supported by sclerenchyma and reinforced by the overall plant anatomy.

🧭 Growth, Transport, and Tall Growth in Plants

• Tall growth is driven by apical meristems at the tips of roots and shoots, enabling primary growth and elongation.
• Secondary growth (in woody plants) results from cambia, increasing girth and wood formation.
• Efficient transport relies on coordinated uptake, loading/unloading of solutes, and the cohesion-tension mechanism driving water movement through xylem.

📚 Key Connections and Concepts

• The same themes recur: compartmentalization by membranes, energy flow through metabolism, info storage and expression via nucleic acids, and structure-function relationships across molecules, organelles, and tissues.
• Understanding these notes provides a foundation for topics in physiology, genetics, microbiology, and systems biology.