Mitochondria is called the powerhouse of the cell because it is the site of cellular respiration — the process that generates ATP (adenosine triphosphate), the primary energy currency of the cell. Through oxidative phosphorylation, mitochondria produce most of the cell's ATP from glucose.
A typical cell has hundreds to thousands of mitochondria depending on its energy requirements — muscle cells and liver cells have the most.
Mitochondria are double-membrane bound organelles found in all eukaryotic cells. The outer membrane is smooth and permeable to small molecules. The inner membrane is highly folded into cristae — these folds dramatically increase surface area for ATP synthesis. The space enclosed by the inner membrane is the mitochondrial matrix, which contains the mitochondrial DNA (circular, like prokaryotes), ribosomes (70S), and enzymes of the Krebs cycle.
Outer membrane: smooth, permeable
Inner membrane: folded into cristae → ATP synthase (F₀F₁ particles)
Matrix: Krebs cycle enzymes, mt-DNA (circular), 70S ribosomes
Intermembrane space: electron transport chain components
Cellular respiration occurs in stages. Glycolysis (in cytoplasm): glucose → 2 pyruvate + 2 ATP + 2 NADH. Pyruvate oxidation (matrix): pyruvate → acetyl-CoA + CO₂ + NADH. Krebs cycle (matrix): acetyl-CoA → CO₂ + NADH + FADH₂ + GTP. Oxidative phosphorylation (inner membrane): NADH and FADH₂ donate electrons to ETC; proton gradient drives ATP synthase. Net ATP per glucose: approximately 36–38 ATP molecules.
📌 Glycolysis: 2 ATP net (cytoplasm, anaerobic possible)
📌 Krebs cycle: 2 GTP + 6 NADH + 2 FADH₂ per glucose
📌 Oxidative phosphorylation: ~32–34 ATP (inner mitochondrial membrane)
📌 Each NADH → ~2.5 ATP; each FADH₂ → ~1.5 ATP (P/O ratios)
📌 Total: ~36–38 ATP per glucose molecule
Lynn Margulis proposed that mitochondria were once free-living aerobic bacteria that were engulfed by a proto-eukaryotic cell. Evidence supporting this theory: mitochondria have their own circular DNA (like bacteria), have 70S ribosomes (bacterial, not eukaryotic 80S), divide by binary fission independently of cell division, have double membranes (outer = host membrane, inner = original bacterial membrane), and are sensitive to antibiotics that target bacterial ribosomes.
📌 Nucleus: control centre, contains DNA, site of transcription
📌 Ribosome (80S eukaryote, 70S prokaryote): protein synthesis
📌 Endoplasmic reticulum: RER — protein synthesis/transport; SER — lipid synthesis, detox
📌 Golgi apparatus: packaging, modification, and secretion of proteins (post office of cell)
📌 Lysosome: intracellular digestion (suicidal bags) — contains hydrolytic enzymes
📌 Vacuole: storage; large central vacuole in plant cells maintains turgor
📌 Chloroplast: photosynthesis (only in plant cells and algae)
📌 Centrosome: cell division, forms spindle fibres (absent in higher plants)
📌 Peroxisome: β-oxidation of fatty acids, detoxification of H₂O₂
Prokaryotic cells (bacteria, archaea) lack a true nucleus and membrane-bound organelles. DNA is circular and in the nucleoid region. Cell wall present (peptidoglycan in bacteria). Ribosome: 70S (50S + 30S). No mitochondria — ATP produced at cell membrane. Eukaryotic cells have true nucleus with nuclear envelope, membrane-bound organelles (mitochondria, ER, Golgi, lysosomes), 80S ribosomes (60S + 40S), linear chromosomes, and complex cytoskeleton.
📌 Bacteria: peptidoglycan (murein) — target of penicillin
📌 Fungi: chitin
📌 Plant cells: cellulose (primary wall) + lignin (secondary wall in woody plants)
📌 Algae: cellulose, pectin, or silica (diatoms)
📌 Animal cells: NO cell wall (only cell membrane)
Singer and Nicolson (1972) proposed the Fluid Mosaic Model for plasma membrane. The membrane is a phospholipid bilayer (hydrophilic heads outward, hydrophobic tails inward) in which proteins are embedded — some span the full membrane (integral/intrinsic proteins), others are loosely attached to surface (peripheral/extrinsic proteins). The membrane is fluid — lipids and proteins can move laterally. Cholesterol in animal cell membranes regulates fluidity — prevents crystallisation at low temperature and excessive fluidity at high temperature.
Lysosomes are single-membrane vesicles containing ~50 different hydrolytic enzymes (acid hydrolases) that work at acidic pH (~5). They digest: bacteria/foreign particles (phagocytosis), worn-out organelles (autophagy), and even the cell itself during programmed cell death (apoptosis). They are called "suicidal bags" because rupture of lysosomal membrane releases enzymes into cytoplasm, digesting the cell. Lysosomes are formed by budding from the Golgi apparatus.