Cell — The Unit of Life

The modern cell theory has three core propositions:

1. All living organisms are composed of cells — proposed by Matthias Schleiden (botanist, 1838) for plants, and Theodor Schwann (zoologist, 1839) for animals.

2. The cell is the basic structural and functional unit of life — also from Schleiden & Schwann.

3. All cells arise from pre-existing cells (Omnis cellula e cellula) — added by Rudolf Virchow in 1855.

Exceptions to keep in mind:
• Viruses are not made of cells; they are obligate intracellular parasites with no metabolism of their own outside a host. Whether they are "alive" is debated.
• Mycoplasma is the smallest known cell (~0.3 μm) — smaller than some viruses, but still cellular.

Cell sizes for context:
• Smallest: Mycoplasma (0.3 μm)
• Average bacterium: 1–10 μm
• Average eukaryotic cell: 10–100 μm
• Largest cell: ostrich egg yolk (15 cm)
• Longest cell: nerve cells of giraffe / human (can exceed 1 m)

Cell theory (Schleiden 1838 + Schwann 1839 + Virchow 1855):

1. All living organisms are made of cells.
2. The cell is the basic structural & functional unit of life.
3. All cells arise from pre-existing cells (Omnis cellula e cellula).

Discovery: Robert Hooke first observed cells in cork (1665). Leeuwenhoek saw living cells (1674).

PROKARYOTIC vs EUKARYOTIC

PLASMA MEMBRANE (Fluid Mosaic Model — Singer & Nicolson 1972)

• Phospholipid bilayer with proteins embedded.
• Selectively permeable; cholesterol in animals (stabilizes).
• Transport: passive (diffusion, osmosis, facilitated), active (Na+/K+ pump uses ATP).

CELL WALL (plants/fungi/bacteria)

• Plants: cellulose, hemicellulose, pectin, lignin.
• Provides shape, prevents desiccation, protection.
• Middle lamella (calcium pectate) holds adjacent cells.

ENDOPLASMIC RETICULUM (ER)

• Rough ER (RER): ribosome-studded → protein synthesis (for secretion/membranes).
• Smooth ER (SER): lipid synthesis, detoxification (liver), steroid hormones, Ca²⁺ storage (muscle).

GOLGI APPARATUS (Camillo Golgi, 1898)

• Stack of flattened cisternae (cis face = receiving; trans face = shipping).
• Modifies, sorts, packages proteins from ER into vesicles.
• Sites of glycosylation, lysosome formation.

MITOCHONDRIA ("powerhouse of the cell")

• Double membrane (inner has cristae folds).
• Matrix contains: enzymes for Krebs cycle, mitochondrial DNA (circular), 70S ribosomes.
• Inner membrane has ETC + ATP synthase → oxidative phosphorylation.
• Semi-autonomous — divide by fission. Endosymbiotic origin (Lynn Margulis).

CHLOROPLAST (plant cells)

• Double membrane; stroma + thylakoid stacks (grana).
• Site of photosynthesis. Contains DNA, 70S ribosomes, chlorophyll.
• Light reactions in thylakoid membrane; dark reactions (Calvin cycle) in stroma.

RIBOSOMES

• 70S (prokaryotes, mitochondria, chloroplasts) = 50S + 30S.
• 80S (eukaryote cytoplasm) = 60S + 40S.
• Site of protein synthesis. rRNA + protein.

LYSOSOMES ("suicide bags")

• Membrane-bound sacs with hydrolytic enzymes (active at pH ~5).
• Digestion of worn-out organelles (autophagy), foreign particles (heterophagy).
• Tay-Sachs disease: lysosomal enzyme defect.

PEROXISOMES — H₂O₂ generation/breakdown, β-oxidation of fatty acids. Catalase enzyme.

VACUOLES — large central vacuole in plants (storage, turgor pressure, pigments).

CYTOSKELETON

• Microfilaments (actin) — cell shape, movement, cytokinesis.
• Microtubules (tubulin) — mitotic spindle, cilia, flagella, transport.
• Intermediate filaments — mechanical strength.

NUCLEUS

• Nuclear envelope (double membrane, nuclear pores).
• Nucleolus — rRNA synthesis, ribosome assembly.
• Chromatin = DNA + histones; condenses into chromosomes during cell division.
• Human cells: 46 chromosomes (23 pairs).

CILIA AND FLAGELLA

• 9+2 microtubule arrangement.
• Cilia: short, many, beat in coordinated waves (trachea, fallopian tube).
• Flagella: long, few (sperm).

EXAM HOOKS:

• "Powerhouse" = mitochondria; "suicide bag" = lysosome; "protein factory" = ribosome.
• DNA in nucleus, mitochondria, chloroplasts (also plasmids in bacteria).
• Plant cell vs animal cell: cell wall + large vacuole + chloroplast = plant only.
• Centrosome/centrioles: animals only.

Quick mnemonic: PROK = Primitive, Round (DNA), Outside (no nucleus), Kompact (smaller). The opposite for eukaryotes.

Endosymbiotic theory explains why mitochondria and chloroplasts have 70S ribosomes (like prokaryotes): they descended from engulfed bacteria.

Cell theory (Schleiden 1838 + Schwann 1839 + Virchow 1855):

1. All living organisms are made of cells.
2. The cell is the basic structural & functional unit of life.
3. All cells arise from pre-existing cells (Omnis cellula e cellula).

Discovery: Robert Hooke first observed cells in cork (1665). Leeuwenhoek saw living cells (1674).

PROKARYOTIC vs EUKARYOTIC

PLASMA MEMBRANE (Fluid Mosaic Model — Singer & Nicolson 1972)

• Phospholipid bilayer with proteins embedded.
• Selectively permeable; cholesterol in animals (stabilizes).
• Transport: passive (diffusion, osmosis, facilitated), active (Na+/K+ pump uses ATP).

CELL WALL (plants/fungi/bacteria)

• Plants: cellulose, hemicellulose, pectin, lignin.
• Provides shape, prevents desiccation, protection.
• Middle lamella (calcium pectate) holds adjacent cells.

ENDOPLASMIC RETICULUM (ER)

• Rough ER (RER): ribosome-studded → protein synthesis (for secretion/membranes).
• Smooth ER (SER): lipid synthesis, detoxification (liver), steroid hormones, Ca²⁺ storage (muscle).

GOLGI APPARATUS (Camillo Golgi, 1898)

• Stack of flattened cisternae (cis face = receiving; trans face = shipping).
• Modifies, sorts, packages proteins from ER into vesicles.
• Sites of glycosylation, lysosome formation.

MITOCHONDRIA ("powerhouse of the cell")

• Double membrane (inner has cristae folds).
• Matrix contains: enzymes for Krebs cycle, mitochondrial DNA (circular), 70S ribosomes.
• Inner membrane has ETC + ATP synthase → oxidative phosphorylation.
• Semi-autonomous — divide by fission. Endosymbiotic origin (Lynn Margulis).

CHLOROPLAST (plant cells)

• Double membrane; stroma + thylakoid stacks (grana).
• Site of photosynthesis. Contains DNA, 70S ribosomes, chlorophyll.
• Light reactions in thylakoid membrane; dark reactions (Calvin cycle) in stroma.

RIBOSOMES

• 70S (prokaryotes, mitochondria, chloroplasts) = 50S + 30S.
• 80S (eukaryote cytoplasm) = 60S + 40S.
• Site of protein synthesis. rRNA + protein.

LYSOSOMES ("suicide bags")

• Membrane-bound sacs with hydrolytic enzymes (active at pH ~5).
• Digestion of worn-out organelles (autophagy), foreign particles (heterophagy).
• Tay-Sachs disease: lysosomal enzyme defect.

PEROXISOMES — H₂O₂ generation/breakdown, β-oxidation of fatty acids. Catalase enzyme.

VACUOLES — large central vacuole in plants (storage, turgor pressure, pigments).

CYTOSKELETON

• Microfilaments (actin) — cell shape, movement, cytokinesis.
• Microtubules (tubulin) — mitotic spindle, cilia, flagella, transport.
• Intermediate filaments — mechanical strength.

NUCLEUS

• Nuclear envelope (double membrane, nuclear pores).
• Nucleolus — rRNA synthesis, ribosome assembly.
• Chromatin = DNA + histones; condenses into chromosomes during cell division.
• Human cells: 46 chromosomes (23 pairs).

CILIA AND FLAGELLA

• 9+2 microtubule arrangement.
• Cilia: short, many, beat in coordinated waves (trachea, fallopian tube).
• Flagella: long, few (sperm).

EXAM HOOKS:

• "Powerhouse" = mitochondria; "suicide bag" = lysosome; "protein factory" = ribosome.
• DNA in nucleus, mitochondria, chloroplasts (also plasmids in bacteria).
• Plant cell vs animal cell: cell wall + large vacuole + chloroplast = plant only.
• Centrosome/centrioles: animals only.

Cell theory (Schleiden 1838 + Schwann 1839 + Virchow 1855):

1. All living organisms are made of cells.
2. The cell is the basic structural & functional unit of life.
3. All cells arise from pre-existing cells (Omnis cellula e cellula).

Discovery: Robert Hooke first observed cells in cork (1665). Leeuwenhoek saw living cells (1674).

PROKARYOTIC vs EUKARYOTIC

PLASMA MEMBRANE (Fluid Mosaic Model — Singer & Nicolson 1972)

• Phospholipid bilayer with proteins embedded.
• Selectively permeable; cholesterol in animals (stabilizes).
• Transport: passive (diffusion, osmosis, facilitated), active (Na+/K+ pump uses ATP).

CELL WALL (plants/fungi/bacteria)

• Plants: cellulose, hemicellulose, pectin, lignin.
• Provides shape, prevents desiccation, protection.
• Middle lamella (calcium pectate) holds adjacent cells.

ENDOPLASMIC RETICULUM (ER)

• Rough ER (RER): ribosome-studded → protein synthesis (for secretion/membranes).
• Smooth ER (SER): lipid synthesis, detoxification (liver), steroid hormones, Ca²⁺ storage (muscle).

GOLGI APPARATUS (Camillo Golgi, 1898)

• Stack of flattened cisternae (cis face = receiving; trans face = shipping).
• Modifies, sorts, packages proteins from ER into vesicles.
• Sites of glycosylation, lysosome formation.

MITOCHONDRIA ("powerhouse of the cell")

• Double membrane (inner has cristae folds).
• Matrix contains: enzymes for Krebs cycle, mitochondrial DNA (circular), 70S ribosomes.
• Inner membrane has ETC + ATP synthase → oxidative phosphorylation.
• Semi-autonomous — divide by fission. Endosymbiotic origin (Lynn Margulis).

CHLOROPLAST (plant cells)

• Double membrane; stroma + thylakoid stacks (grana).
• Site of photosynthesis. Contains DNA, 70S ribosomes, chlorophyll.
• Light reactions in thylakoid membrane; dark reactions (Calvin cycle) in stroma.

RIBOSOMES

• 70S (prokaryotes, mitochondria, chloroplasts) = 50S + 30S.
• 80S (eukaryote cytoplasm) = 60S + 40S.
• Site of protein synthesis. rRNA + protein.

LYSOSOMES ("suicide bags")

• Membrane-bound sacs with hydrolytic enzymes (active at pH ~5).
• Digestion of worn-out organelles (autophagy), foreign particles (heterophagy).
• Tay-Sachs disease: lysosomal enzyme defect.

PEROXISOMES — H₂O₂ generation/breakdown, β-oxidation of fatty acids. Catalase enzyme.

VACUOLES — large central vacuole in plants (storage, turgor pressure, pigments).

CYTOSKELETON

• Microfilaments (actin) — cell shape, movement, cytokinesis.
• Microtubules (tubulin) — mitotic spindle, cilia, flagella, transport.
• Intermediate filaments — mechanical strength.

NUCLEUS

• Nuclear envelope (double membrane, nuclear pores).
• Nucleolus — rRNA synthesis, ribosome assembly.
• Chromatin = DNA + histones; condenses into chromosomes during cell division.
• Human cells: 46 chromosomes (23 pairs).

CILIA AND FLAGELLA

• 9+2 microtubule arrangement.
• Cilia: short, many, beat in coordinated waves (trachea, fallopian tube).
• Flagella: long, few (sperm).

EXAM HOOKS:

• "Powerhouse" = mitochondria; "suicide bag" = lysosome; "protein factory" = ribosome.
• DNA in nucleus, mitochondria, chloroplasts (also plasmids in bacteria).
• Plant cell vs animal cell: cell wall + large vacuole + chloroplast = plant only.
• Centrosome/centrioles: animals only.

Cell theory (Schleiden 1838 + Schwann 1839 + Virchow 1855):

1. All living organisms are made of cells.
2. The cell is the basic structural & functional unit of life.
3. All cells arise from pre-existing cells (Omnis cellula e cellula).

Discovery: Robert Hooke first observed cells in cork (1665). Leeuwenhoek saw living cells (1674).

PROKARYOTIC vs EUKARYOTIC

PLASMA MEMBRANE (Fluid Mosaic Model — Singer & Nicolson 1972)

• Phospholipid bilayer with proteins embedded.
• Selectively permeable; cholesterol in animals (stabilizes).
• Transport: passive (diffusion, osmosis, facilitated), active (Na+/K+ pump uses ATP).

CELL WALL (plants/fungi/bacteria)

• Plants: cellulose, hemicellulose, pectin, lignin.
• Provides shape, prevents desiccation, protection.
• Middle lamella (calcium pectate) holds adjacent cells.

ENDOPLASMIC RETICULUM (ER)

• Rough ER (RER): ribosome-studded → protein synthesis (for secretion/membranes).
• Smooth ER (SER): lipid synthesis, detoxification (liver), steroid hormones, Ca²⁺ storage (muscle).

GOLGI APPARATUS (Camillo Golgi, 1898)

• Stack of flattened cisternae (cis face = receiving; trans face = shipping).
• Modifies, sorts, packages proteins from ER into vesicles.
• Sites of glycosylation, lysosome formation.

MITOCHONDRIA ("powerhouse of the cell")

• Double membrane (inner has cristae folds).
• Matrix contains: enzymes for Krebs cycle, mitochondrial DNA (circular), 70S ribosomes.
• Inner membrane has ETC + ATP synthase → oxidative phosphorylation.
• Semi-autonomous — divide by fission. Endosymbiotic origin (Lynn Margulis).

CHLOROPLAST (plant cells)

• Double membrane; stroma + thylakoid stacks (grana).
• Site of photosynthesis. Contains DNA, 70S ribosomes, chlorophyll.
• Light reactions in thylakoid membrane; dark reactions (Calvin cycle) in stroma.

RIBOSOMES

• 70S (prokaryotes, mitochondria, chloroplasts) = 50S + 30S.
• 80S (eukaryote cytoplasm) = 60S + 40S.
• Site of protein synthesis. rRNA + protein.

LYSOSOMES ("suicide bags")

• Membrane-bound sacs with hydrolytic enzymes (active at pH ~5).
• Digestion of worn-out organelles (autophagy), foreign particles (heterophagy).
• Tay-Sachs disease: lysosomal enzyme defect.

PEROXISOMES — H₂O₂ generation/breakdown, β-oxidation of fatty acids. Catalase enzyme.

VACUOLES — large central vacuole in plants (storage, turgor pressure, pigments).

CYTOSKELETON

• Microfilaments (actin) — cell shape, movement, cytokinesis.
• Microtubules (tubulin) — mitotic spindle, cilia, flagella, transport.
• Intermediate filaments — mechanical strength.

NUCLEUS

• Nuclear envelope (double membrane, nuclear pores).
• Nucleolus — rRNA synthesis, ribosome assembly.
• Chromatin = DNA + histones; condenses into chromosomes during cell division.
• Human cells: 46 chromosomes (23 pairs).

CILIA AND FLAGELLA

• 9+2 microtubule arrangement.
• Cilia: short, many, beat in coordinated waves (trachea, fallopian tube).
• Flagella: long, few (sperm).

EXAM HOOKS:

• "Powerhouse" = mitochondria; "suicide bag" = lysosome; "protein factory" = ribosome.
• DNA in nucleus, mitochondria, chloroplasts (also plasmids in bacteria).
• Plant cell vs animal cell: cell wall + large vacuole + chloroplast = plant only.
• Centrosome/centrioles: animals only.