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Cell Theory

All cells come from cells — "Omnis cellula e cellula"

3 tenets: all life = cells · cell = basic unit · cells from pre-existing cells

Cell Theory has three core tenets: (1) All living organisms are composed of one or more cells. (2) The cell is the basic unit of life. (3) All cells arise from pre-existing cells (Virchow, 1855). This disproved spontaneous generation. Prokaryotic cells (no nucleus, no membrane-bound organelles) vs Eukaryotic cells (nucleus + organelles).

📖 Full Lesson →Difficulty: Beginner

Prokaryote vs Eukaryote

Prokaryotes: no nucleus, no membrane-bound organelles, 70S ribosomes, usually smaller (1-10μm). Eukaryotes: true nucleus, membrane-bound organelles, 80S ribosomes, larger (10-100μm).

Scientists

Hooke (1665) — coined "cell" looking at cork. Leeuwenhoek — first to observe live cells. Schleiden (plants) + Schwann (animals) — all organisms made of cells. Virchow — cells from cells.

Cell size limits

Surface area to volume ratio limits cell size. As cells grow, volume increases faster than surface area — reduced ability to exchange materials. Cells divide before getting too large.

Smallest vs largest cells

Smallest: Mycoplasma (~0.2μm). Largest: ostrich egg yolk (single cell, ~10cm). Longest: motor neurons can be over 1 meter. Human cells average ~10μm.

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🃏 Cell Theory

Cell theory — the key principle?

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🃏 Answer

All cells come from cells — "Omnis cellula e cellula"

Prokaryote vs Eukaryote — Prokaryotes: no nucleus, no membrane-bound organelles, 70S ribosomes, usually smaller (1-10μm). Eukaryotes: true nucleus, membrane-bound organelles, 80S ribosomes, larger (10-100μm).

Scientists — Hooke (1665) — coined "cell" looking at cork. Leeuwenhoek — first to observe live cells. Schleiden (plants) + Schwann (animals) — all organisms made of cells. Virchow — cells from cells.

Cell size limits — Surface area to volume ratio limits cell size. As cells grow, volume increases faster than surface area — reduced ability to exchange materials. Cells divide before getting too large.

Smallest vs largest cells — Smallest: Mycoplasma (~0.2μm). Largest: ostrich egg yolk (single cell, ~10cm). Longest: motor neurons can be over 1 meter. Human cells average ~10μm.

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Organelles

Mighty Men Can't Run Laps — Mitochondria, Membrane, Chloroplasts, Ribosomes, Lysosome

Key organelles and their functions

Major eukaryotic organelles: Nucleus (DNA storage, control center). Mitochondria (ATP production — "powerhouse"). Ribosomes (protein synthesis — rough ER has them). ER (protein/lipid processing). Golgi apparatus (packaging/shipping). Lysosomes (digestion). Chloroplasts (photosynthesis — plants only). Vacuole (storage — large in plants).

📖 Full Lesson →Difficulty: Beginner

Endomembrane system

Connected system: Nuclear envelope → ER → vesicles → Golgi → vesicles → plasma membrane/lysosomes. Proteins made on rough ER, modified in Golgi, shipped to destination.

Mitochondria features

Double membrane (outer smooth, inner folded into cristae). Own DNA (circular, like bacteria). Own ribosomes (70S). Divide by binary fission. Evidence for endosymbiotic origin.

Rough vs Smooth ER

Rough ER: studded with ribosomes, makes membrane proteins and secretory proteins. Smooth ER: no ribosomes, makes lipids, detoxifies drugs, stores Ca²⁺ in muscle cells.

Golgi apparatus

Cis face (receives from ER) → Trans face (ships to destination). Modifies, sorts, and packages proteins. Adds sugars to glycoproteins. "Post office of the cell."

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🃏 Organelles

Organelles (Mighty Men Can't Run Laps) — what does each do?

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🃏 Answer

Mighty Men Can't Run Laps — Mitochondria, Membrane, Chloroplasts, Ribosomes, Lysosome

Endomembrane system — Connected system: Nuclear envelope → ER → vesicles → Golgi → vesicles → plasma membrane/lysosomes. Proteins made on rough ER, modified in Golgi, shipped to destination.

Mitochondria features — Double membrane (outer smooth, inner folded into cristae). Own DNA (circular, like bacteria). Own ribosomes (70S). Divide by binary fission. Evidence for endosymbiotic origin.

Rough vs Smooth ER — Rough ER: studded with ribosomes, makes membrane proteins and secretory proteins. Smooth ER: no ribosomes, makes lipids, detoxifies drugs, stores Ca²⁺ in muscle cells.

Golgi apparatus — Cis face (receives from ER) → Trans face (ships to destination). Modifies, sorts, and packages proteins. Adds sugars to glycoproteins. "Post office of the cell."

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Cell Membrane

Fluid Mosaic Model — phospholipid bilayer with floating proteins

Hydrophilic heads face out · hydrophobic tails face in · proteins float throughout

The plasma membrane is a fluid mosaic of phospholipids and proteins. Phospholipids: hydrophilic head (faces water) + hydrophobic tails (face inward — repel water). Proteins float in the bilayer (integral) or attach to surface (peripheral). Cholesterol stabilizes fluidity. Carbohydrates on outer surface form glycocalyx — cell recognition and communication.

📖 Full Lesson →Difficulty: Intermediate

Selective permeability

Small nonpolar molecules (O₂, CO₂, lipids) cross freely. Small polar molecules (water — via aquaporins) cross slowly. Ions and large molecules need protein channels or carriers.

Cholesterol role

Inserted between phospholipids. At high temp: reduces fluidity (prevents membrane from becoming too fluid). At low temp: prevents solidification. Acts as fluidity buffer.

Membrane proteins

Transport proteins (channels, carriers, pumps). Receptor proteins (signal transduction). Enzymes. Cell identity markers (MHC (Major Histocompatibility Complex) proteins). Cell adhesion molecules. Each protein has specific function.

Glycocalyx

Sugar coating on outer cell surface. Made of glycoproteins and glycolipids. Functions: cell-cell recognition, immune response, protection. ABO blood types determined by glycocalyx sugars.

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🃏 Cell Membrane

Cell membrane — what is the fluid mosaic model?

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🃏 Answer

Fluid Mosaic Model — phospholipid bilayer with floating proteins

Selective permeability — Small nonpolar molecules (O₂, CO₂, lipids) cross freely. Small polar molecules (water — via aquaporins) cross slowly. Ions and large molecules need protein channels or carriers.

Cholesterol role — Inserted between phospholipids. At high temp: reduces fluidity (prevents membrane from becoming too fluid). At low temp: prevents solidification. Acts as fluidity buffer.

Membrane proteins — Transport proteins (channels, carriers, pumps). Receptor proteins (signal transduction). Enzymes. Cell identity markers (MHC (Major Histocompatibility Complex) proteins). Cell adhesion molecules. Each protein has specific function.

Glycocalyx — Sugar coating on outer cell surface. Made of glycoproteins and glycolipids. Functions: cell-cell recognition, immune response, protection. ABO blood types determined by glycocalyx sugars.

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Membrane Transport

PAID — Passive, Active, Inward (endocytosis), Discharge (exocytosis)

No energy needed for passive · ATP required for active · vesicles for bulk transport

Transport types: Passive (no ATP) — simple diffusion (down concentration gradient), facilitated diffusion (via channels/carriers), osmosis (water). Active transport (ATP required) — pumps move against gradient (Na⁺/K⁺ pump). Bulk transport: endocytosis (phagocytosis, pinocytosis, receptor-mediated) brings material IN; exocytosis releases material OUT.

📖 Full Lesson →Difficulty: Intermediate

Osmosis

Water moves from low solute (high water) to high solute (low water) via osmosis. Hypotonic solution → cell swells (may lyse). Hypertonic → cell shrinks (crenation in RBCs, plasmolysis in plants). Isotonic → no net movement.

Na⁺/K⁺ pump

Uses 1 ATP to pump 3 Na⁺ OUT and 2 K⁺ IN. Maintains electrochemical gradient. Uses 30% of cell's ATP. Essential for nerve impulse transmission and muscle contraction.

Phagocytosis vs pinocytosis

Phagocytosis ("cell eating"): engulfs large particles (bacteria, dead cells). Pinocytosis ("cell drinking"): takes in extracellular fluid in small vesicles. Receptor-mediated: specific molecules bind receptors, then internalized.

Cotransport

One substance moved actively creates gradient that drives another substance passively. Na⁺/glucose cotransporter: Na⁺ gradient (from Na⁺/K⁺ pump) drives glucose into intestinal cells against its gradient. Secondary active transport.

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🃏 Membrane Transport

PAID

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🃏 Answer

PAID — Passive, Active, Inward (endocytosis), Discharge (exocytosis)

Osmosis — Water moves from low solute (high water) to high solute (low water) via osmosis. Hypotonic solution → cell swells (may lyse). Hypertonic → cell shrinks (crenation in RBCs, plasmolysis in plants). Isotonic → no net movement.

Na⁺/K⁺ pump — Uses 1 ATP to pump 3 Na⁺ OUT and 2 K⁺ IN. Maintains electrochemical gradient. Uses 30% of cell's ATP. Essential for nerve impulse transmission and muscle contraction.

Phagocytosis vs pinocytosis — Phagocytosis ("cell eating"): engulfs large particles (bacteria, dead cells). Pinocytosis ("cell drinking"): takes in extracellular fluid in small vesicles. Receptor-mediated: specific molecules bind receptors, then internalized.

Cotransport — One substance moved actively creates gradient that drives another substance passively. Na⁺/glucose cotransporter: Na⁺ gradient (from Na⁺/K⁺ pump) drives glucose into intestinal cells against its gradient. Secondary active transport.

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Mitosis

PMAT — Prophase, Metaphase, Anaphase, Telophase

Prophase: condense · Metaphase: middle · Anaphase: apart · Telophase: two cells

Mitosis produces 2 identical diploid daughter cells for growth and repair. PMAT: Prophase — chromosomes condense, spindle forms, nuclear envelope breaks down. Metaphase — chromosomes align at cell equator (metaphase plate). Anaphase — sister chromatids pulled to opposite poles. Telophase — nuclear envelopes reform, chromosomes decondense. Cytokinesis divides the cytoplasm.

📖 Full Lesson →Difficulty: Intermediate

Interphase

G1 (cell growth, normal functions) → S phase (DNA replication — each chromosome duplicated into 2 sister chromatids) → G2 (preparation for division, protein synthesis). G0 = non-dividing state.

Spindle apparatus

Made of microtubules from centrosomes. Kinetochore microtubules attach to centromere of each chromosome. Motor proteins walk chromosomes along microtubules. Colchicine disrupts spindle — stops mitosis at metaphase.

Cytokinesis

Animal cells: cleavage furrow (actin-myosin ring pinches cell in two). Plant cells: cell plate forms from Golgi vesicles in the middle, builds new cell wall between daughter cells.

Cancer connection

Cancer = uncontrolled cell division. Tumor suppressor genes (p53, Rb) normally halt division. Proto-oncogenes (ras) promote division. Mutations converting them to oncogenes drive cancer. Checkpoints (G1, G2, M) normally catch errors.

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🃏 Mitosis

PMAT

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🃏 Answer

PMAT — Prophase, Metaphase, Anaphase, Telophase

Interphase — G1 (cell growth, normal functions) → S phase (DNA replication — each chromosome duplicated into 2 sister chromatids) → G2 (preparation for division, protein synthesis). G0 = non-dividing state.

Spindle apparatus — Made of microtubules from centrosomes. Kinetochore microtubules attach to centromere of each chromosome. Motor proteins walk chromosomes along microtubules. Colchicine disrupts spindle — stops mitosis at metaphase.

Cytokinesis — Animal cells: cleavage furrow (actin-myosin ring pinches cell in two). Plant cells: cell plate forms from Golgi vesicles in the middle, builds new cell wall between daughter cells.

Cancer connection — Cancer = uncontrolled cell division. Tumor suppressor genes (p53, Rb) normally halt division. Proto-oncogenes (ras) promote division. Mutations converting them to oncogenes drive cancer. Checkpoints (G1, G2, M) normally catch errors.

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Cellular Respiration

GLICK — Glycolysis, Link reaction, Intermediate, Citric acid, Krebs

Glucose → Pyruvate → Acetyl-CoA → Krebs → ETC (Electron Transport Chain) → 36-38 ATP

Cellular respiration converts glucose to ATP in 3 stages. Glycolysis (cytoplasm): glucose → 2 pyruvate, net 2 ATP, 2 NADH. Pyruvate oxidation (mitochondrial matrix): pyruvate → acetyl-CoA + CO₂. Krebs cycle (matrix): 2 ATP, 6 NADH, 2 FADH₂ per glucose. Electron Transport Chain (ETC) (inner mitochondrial membrane): 32-34 ATP via oxidative phosphorylation. Total: ~36-38 ATP.

📖 Full Lesson →Difficulty: Intermediate

Glycolysis details

10 enzyme-catalyzed steps. Glucose (6C) → 2 pyruvate (3C). Invests 2 ATP, produces 4 ATP (net 2). Produces 2 NADH. Occurs in cytoplasm — no oxygen required.

Krebs cycle

Runs twice per glucose (one per pyruvate). Each turn: 3 NADH, 1 FADH₂, 1 ATP, 2 CO₂. NADH and FADH₂ carry electrons to ETC. CO₂ released — this is why you exhale CO₂.

Electron Transport Chain

NADH and FADH₂ donate electrons to protein complexes I-IV in inner mitochondrial membrane. Electron flow pumps H⁺ into intermembrane space → H⁺ flows back through ATP synthase → makes ATP (chemiosmosis). O₂ is final electron acceptor → becomes H₂O.

Anaerobic respiration

Without O₂: glycolysis only (2 ATP). Pyruvate converted to lactate (animals/bacteria) or ethanol + CO₂ (yeast — fermentation). Regenerates NAD⁺ to keep glycolysis running. Much less efficient than aerobic.

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🃏 Cellular Respiration

GLICK

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🃏 Answer

GLICK — Glycolysis, Link reaction, Intermediate, Citric acid, Krebs

Glycolysis details — 10 enzyme-catalyzed steps. Glucose (6C) → 2 pyruvate (3C). Invests 2 ATP, produces 4 ATP (net 2). Produces 2 NADH. Occurs in cytoplasm — no oxygen required.

Krebs cycle — Runs twice per glucose (one per pyruvate). Each turn: 3 NADH, 1 FADH₂, 1 ATP, 2 CO₂. NADH and FADH₂ carry electrons to ETC. CO₂ released — this is why you exhale CO₂.

Electron Transport Chain — NADH and FADH₂ donate electrons to protein complexes I-IV in inner mitochondrial membrane. Electron flow pumps H⁺ into intermembrane space → H⁺ flows back through ATP synthase → makes ATP (chemiosmosis). O₂ is final electron acceptor → becomes H₂O.

Anaerobic respiration — Without O₂: glycolysis only (2 ATP). Pyruvate converted to lactate (animals/bacteria) or ethanol + CO₂ (yeast — fermentation). Regenerates NAD⁺ to keep glycolysis running. Much less efficient than aerobic.

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Photosynthesis

Light reactions make ATP + NADPH · Calvin cycle makes sugar

Light-dependent (thylakoid) + Light-independent/Calvin cycle (stroma)

Photosynthesis: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Two stages: Light reactions (thylakoid membranes) — light splits water, produces ATP, NADPH, and O₂. Calvin cycle (stroma) — uses ATP and NADPH to fix CO₂ into G3P (used to make glucose). 3 CO₂ fixed per turn; 3 turns needed per pyruvate equivalent.

📖 Full Lesson →Difficulty: Intermediate

Photosystems I and II

PSII (P680) absorbs light first — splits water (photolysis), releases O₂, energizes electrons. PSI (P700) re-energizes electrons to make NADPH. "II before I" — PSII goes first despite the number.

Calvin cycle (C3)

3 stages: Carbon fixation (CO₂ + RuBP → 2 molecules of 3-PGA via RuBisCO), Reduction (ATP + NADPH reduce 3-PGA to G3P), Regeneration (ATP used to regenerate RuBP). 3 turns fix 3 CO₂ and produce 1 G3P net.

C4 vs CAM plants

C3 (most plants): CO₂ fixed directly by RuBisCO — problem in hot conditions (photorespiration). C4 (corn, sugarcane): pre-fix CO₂ in mesophyll, concentrate it in bundle sheath. CAM (cacti): fix CO₂ at night, release during day — minimizes water loss.

Chloroplast structure

Outer membrane → inner membrane → stroma (fluid) → thylakoids (membrane sacs) → grana (stacks of thylakoids). Light reactions on thylakoid membranes; Calvin cycle in stroma.

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🃏 Photosynthesis

Photosynthesis — what do the light reactions and Calvin cycle make?

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🃏 Answer

Light reactions make ATP + NADPH · Calvin cycle makes sugar

Photosystems I and II — PSII (P680) absorbs light first — splits water (photolysis), releases O₂, energizes electrons. PSI (P700) re-energizes electrons to make NADPH. "II before I" — PSII goes first despite the number.

Calvin cycle (C3) — 3 stages: Carbon fixation (CO₂ + RuBP → 2 molecules of 3-PGA via RuBisCO), Reduction (ATP + NADPH reduce 3-PGA to G3P), Regeneration (ATP used to regenerate RuBP). 3 turns fix 3 CO₂ and produce 1 G3P net.

C4 vs CAM plants — C3 (most plants): CO₂ fixed directly by RuBisCO — problem in hot conditions (photorespiration). C4 (corn, sugarcane): pre-fix CO₂ in mesophyll, concentrate it in bundle sheath. CAM (cacti): fix CO₂ at night, release during day — minimizes water loss.

Chloroplast structure — Outer membrane → inner membrane → stroma (fluid) → thylakoids (membrane sacs) → grana (stacks of thylakoids). Light reactions on thylakoid membranes; Calvin cycle in stroma.

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Cell Communication

Reception → Transduction → Response — "RTR: Receive the Signal, Transform it, React"

Signal molecule → receptor → cascade → cellular response

Cell signaling has three stages: Reception (signal molecule binds receptor — often on cell surface). Transduction (signal converted to intracellular message — often via protein cascade and second messengers like cAMP). Response (gene expression change, enzyme activation, or cytoskeletal change). Allows cells to coordinate activities without direct contact.

📖 Full Lesson →Difficulty: Advanced

Types of signaling

Endocrine: hormones travel through blood (long range). Paracrine: signals to nearby cells (local). Autocrine: cell signals itself. Synaptic: neurotransmitters cross synapse. Direct contact: gap junctions, plasmodesmata.

Second messengers

cAMP: made by adenylyl cyclase when G-protein activated. Activates protein kinase A → phosphorylation cascade. Ca²⁺: released from ER by IP₃ → activates calmodulin and other proteins.

Receptor types

G-protein coupled receptors (GPCRs): most common, activate G-proteins. Receptor tyrosine kinases (RTKs): dimerize when ligand binds, phosphorylate each other → RAS pathway. Ion channel receptors: ligand opens channel directly (nicotinic acetylcholine receptor).

Apoptosis

Programmed cell death — essential for development (finger separation, tadpole tail loss) and immune function. Triggered by internal/external signals. Caspases execute cell death. Failure = cancer; excess = neurodegenerative disease.

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🃏 Cell Communication

Cell signaling — the three stages?

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🃏 Answer

Reception → Transduction → Response — "RTR: Receive the Signal, Transform it, React"

Types of signaling — Endocrine: hormones travel through blood (long range). Paracrine: signals to nearby cells (local). Autocrine: cell signals itself. Synaptic: neurotransmitters cross synapse. Direct contact: gap junctions, plasmodesmata.

Second messengers — cAMP: made by adenylyl cyclase when G-protein activated. Activates protein kinase A → phosphorylation cascade. Ca²⁺: released from ER by IP₃ → activates calmodulin and other proteins.

Receptor types — G-protein coupled receptors (GPCRs): most common, activate G-proteins. Receptor tyrosine kinases (RTKs): dimerize when ligand binds, phosphorylate each other → RAS pathway. Ion channel receptors: ligand opens channel directly (nicotinic acetylcholine receptor).

Apoptosis — Programmed cell death — essential for development (finger separation, tadpole tail loss) and immune function. Triggered by internal/external signals. Caspases execute cell death. Failure = cancer; excess = neurodegenerative disease.

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Cell Cycle Control

Checkpoints: G1, G2, M — "Good Grade Makes the Cut"

Three checkpoints verify cell is ready to proceed to next phase

Cell cycle checkpoints prevent damaged or incomplete cells from dividing. G1 checkpoint: is the cell large enough? Is DNA undamaged? Sufficient nutrients? G2 checkpoint: was DNA replicated correctly? M checkpoint (spindle assembly): are all chromosomes attached to spindle? Cyclin-CDK (cyclin-dependent kinase) complexes drive the cycle forward. p53 tumor suppressor halts cycle if DNA damaged — "guardian of the genome."

📖 Full Lesson →Difficulty: Advanced

Cyclins and CDKs

Cyclin levels rise and fall through the cell cycle. Cyclin binds CDK (cyclin-dependent kinase) → active complex drives cycle forward. Different cyclins active at different phases. MPF (maturation promoting factor) = cyclin B + CDK1 — triggers mitosis.

p53 — guardian of genome

Most commonly mutated gene in human cancers. Activated by DNA damage → stops cell cycle at G1 → allows repair → if unrepairable, triggers apoptosis. Loss of p53 = cell divides despite damage = cancer risk.

Proto-oncogenes vs tumor suppressors

Proto-oncogenes (ras, myc): normally promote growth — mutation → oncogene → uncontrolled growth (one mutated copy sufficient — dominant). Tumor suppressors (p53, Rb): normally inhibit growth — both copies must be mutated to lose function (recessive).

Contact inhibition

Normal cells stop dividing when touching neighboring cells — density-dependent inhibition. Cancer cells lose this — continue dividing, pile up, form tumors. Also lose anchorage dependence — can grow without attachment.

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🃏 Cell Cycle Control

Cell cycle checkpoints — where are they?

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🃏 Answer

Checkpoints: G1, G2, M — "Good Grade Makes the Cut"

Cyclins and CDKs — Cyclin levels rise and fall through the cell cycle. Cyclin binds CDK (cyclin-dependent kinase) → active complex drives cycle forward. Different cyclins active at different phases. MPF (maturation promoting factor) = cyclin B + CDK1 — triggers mitosis.

p53 — guardian of genome — Most commonly mutated gene in human cancers. Activated by DNA damage → stops cell cycle at G1 → allows repair → if unrepairable, triggers apoptosis. Loss of p53 = cell divides despite damage = cancer risk.

Proto-oncogenes vs tumor suppressors — Proto-oncogenes (ras, myc): normally promote growth — mutation → oncogene → uncontrolled growth (one mutated copy sufficient — dominant). Tumor suppressors (p53, Rb): normally inhibit growth — both copies must be mutated to lose function (recessive).

Contact inhibition — Normal cells stop dividing when touching neighboring cells — density-dependent inhibition. Cancer cells lose this — continue dividing, pile up, form tumors. Also lose anchorage dependence — can grow without attachment.

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