Chapter Review

The Cell

Plant and Animal Cells · Prokaryotic and Eukaryotic Cells · Cell Organelles and Chromosomes

Eukaryotic Cell Basics

Eukaryotic cells possess a membrane-bound nucleus and specialised organelles. They range from 10–100 μm in diameter and include plants, animals, fungi, and protists.

Key Points

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    Every cell has three basic components: plasma membrane, cytoplasm, and nucleus
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    Eukaryotes have a membrane-bound nucleus; prokaryotes lack this feature
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    Cell sizes are measured in micrometres (1 μm = 10⁻⁶ m)
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    Cell fractionation isolates cellular components by density gradient centrifugation
Formula

Plasma Membrane and Fluid Mosaic Model

The plasma membrane is the selectively permeable outer boundary composed of a phospholipid bilayer with embedded proteins, described by the Fluid Mosaic Model.

Key Points

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    Composition: 60–80% proteins, 20–40% lipids, small amount of carbohydrates
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    Fluid Mosaic Model: proteins float in a fluid lipid bilayer in a mosaic pattern (not the outdated protein-lipid-protein sandwich)
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    Peripheral proteins sit on the surface; integral proteins span the bilayer
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    Cholesterol stabilises membrane fluidity; carbohydrates form glycoproteins and glycolipids on the extracellular surface
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    Transport: passive (down gradient, no ATP) vs active (against gradient, requires ATP)
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    Endocytosis (phagocytosis + pinocytosis) is an animal cell feature — blocked by plant cell walls

Cell Wall Structure and Composition

The cell wall is a non-living secretion providing shape and rigidity. It is fully permeable and differs in composition across organisms.

Key Points

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    Three layers in plants: middle lamella (cements cells), primary wall (cellulose + pectin + hemicellulose), secondary wall (lignin, cutin, waxes)
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    Plant cell wall: cellulose — Prokaryotic cell wall: peptidoglycan (murein) — Fungal cell wall: chitin
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    Penicillin targets peptidoglycan synthesis (bacterial-specific)
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    The cell wall is NOT selectively permeable — that is the plasma membrane's role
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    Plant cells are rectangular due to the wall; animal cells are irregular/round

Cytoplasm, ER, and Ribosomes

Cytoplasm (cytosol + organelles) is the site of glycolysis. The ER is a network of membrane channels continuous with the plasma and nuclear membranes, existing as rough (with ribosomes) and smooth (without) forms.

Key Points

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    Cytosol is ~90% water; exists as sol (non-viscous) or gel (viscous); peripheral regions are gel-like
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    RER synthesises proteins (ribosomes attached); SER handles lipids, detoxification, and impulse transmission
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    Both types provide mechanical support to maintain cell shape
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    ER cisternae separate ER contents from the cytoplasm
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    Free ribosomes make proteins for intracellular use; bound ribosomes make proteins for export
Formula

Golgi Apparatus and Secretory Pathway

The Golgi apparatus modifies, sorts, and packages proteins and lipids. Materials enter at the forming face (cis) and exit at the maturing face (trans) as secretory vesicles.

Key Points

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    Discovered by Camillo Golgi (1898); consists of stacked cisternae with vesicles
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    Forming face (cis, convex): receives transport vesicles from SER
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    Maturing face (trans, concave): secretory vesicles bud off for export
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    Adds carbohydrates to proteins (forming glycoproteins) and lipids (forming glycolipids)
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    Pathway: ribosomes → RER → SER vesicles → Golgi forming face → Golgi maturing face → plasma membrane
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    Plant cells often have many smaller dictyosomes rather than one large complex

Mitochondria — Structure and Function

Mitochondria are double-membrane organelles that produce ATP through aerobic respiration. They contain their own DNA and ribosomes, making them semi-autonomous.

Key Points

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    Outer membrane is smooth; inner membrane folds into cristae, increasing surface area
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    F₁ particles (ATP synthase) on cristae catalyse ATP synthesis
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    Matrix contains enzymes for Kreb's cycle, aerobic respiration, and fatty acid metabolism
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    Own circular DNA and ribosomes (55S) support the endosymbiotic theory
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    Present in BOTH plant and animal cells — not exclusive to either
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    Number correlates with energy demand: muscle and liver cells have more mitochondria

Nucleus and Nuclear Envelope

The nucleus controls all cell activities and houses genetic material. It is enclosed by a double nuclear envelope with pores that regulate molecular traffic between nucleus and cytoplasm.

Key Points

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    Discovered by Robert Brown (1831); central in animal cells, peripheral in plant cells (pushed by vacuole)
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    Nuclear envelope: outer membrane continuous with ER, inner membrane encloses contents; fused at nuclear pores
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    Pore numbers vary: ~30,000 in egg cells (undifferentiated) vs 3–4 in erythrocytes (differentiated)
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    Nucleolus: non-membrane-bound; synthesises rRNA and assembles ribosomes (ribosome factory)
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    Nucleolus has two regions: peripheral granular (ribosomal precursors) and central fibrillar (rRNA, rDNA)
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    Nucleoplasm + cytoplasm = protoplasm

Chromosomes and Chromosome Numbers

Chromosomes are condensed DNA-protein structures formed during cell division. Each consists of two identical chromatids joined at a centromere. Chromosome number is species-specific and constant.

Key Points

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    Chromatin (loose, interphase) condenses into chromosomes (tightly coiled, cell division)
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    Each chromosome has two sister chromatids held at the centromere; spindle fibres attach here
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    Diploid (2n) in somatic cells; haploid (n) in gametes
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    Human: 2n = 46, n = 23; Drosophila: 2n = 8, n = 4; Onion: 2n = 16
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    Genes are DNA segments on chromosomes carrying hereditary instructions
Formula

Prokaryotic vs Eukaryotic Cells

The fundamental divide in cellular biology: prokaryotes (bacteria, cyanobacteria) lack a membrane-bound nucleus and organelles, while eukaryotes (animals, plants, fungi, protists) possess both.

Key Points

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    Prokaryotes: no true nucleus — DNA in a nucleoid (no membrane); single circular DNA without histones
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    Eukaryotes: double nuclear membrane; linear chromosomes with histones; multiple origins of replication
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    Prokaryotic ribosomes: 70S (50S + 30S); Eukaryotic ribosomes: 80S (60S + 40S)
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    Antibiotics like tetracycline target 70S ribosomes selectively
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    Prokaryotes divide by binary fission (no spindle, no phases); eukaryotes divide by mitosis
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    Prokaryotes may carry plasmids (extrachromosomal circular DNA with advantageous genes)
Formula

Plant vs Animal Cell Differences

Plant and animal cells share most organelles but differ in key features: plant cells have a cell wall, chloroplasts, large central vacuole, and glyoxysomes; animal cells have centrioles and abundant lysosomes.

Key Points

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    Plant-exclusive: cell wall (cellulose), large central vacuole (turgor), chloroplasts/chromoplasts/leucoplasts, glyoxysomes
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    Animal-exclusive/prominent: centrioles (9+0, absent in higher plants), abundant lysosomes (absent in most plants), endocytosis
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    Chloroplasts: double membrane, stroma (Calvin cycle), thylakoids stacked as grana (light reactions); chlorophyll has Mg²⁺ centre
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    Glyoxysomes: plant-only; convert fatty acids to carbohydrates via glyoxylate cycle during germination
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    Lysosomes: hydrolytic enzymes from RER/Golgi; perform phagocytosis and autophagy; defects cause Tay-Sachs and glycogenosis type II
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    Cytoskeleton: microtubules (spindle, cilia, centrioles), microfilaments (actin, cyclosis), intermediate filaments (cell shape)

Formulas

Micrometre Conversion

Standard unit for measuring cell dimensions

Eukaryotic Ribosome

80S ribosome formed from 40S + 60S subunits; Svedberg units are not additive

Prokaryotic Ribosome

70S ribosome formed from 30S + 50S subunits; target of many antibiotics

Diploid to Haploid

Somatic cells are diploid (2n); gametes are haploid (n). Reduction occurs during meiosis.