Cell Structure and Functions

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your NECO exam.

The Cell Theory (established by Schleiden, Schwann, and Virchow):

1. All living organisms are composed of cells
2. The cell is the basic unit of life
3. All cells arise from pre-existing cells

Two Types of Cells:

Feature	Prokaryotic	Eukaryotic
Example	Bacteria, blue-green algae	Plants, animals, fungi, protoctists
Nucleus	No (nucleoid region)	Yes (bounded by nuclear membrane)
DNA	Circular, naked	Linear, associated with histones
Membrane-bound organelles	No	Yes
Size	0.1–10 μm	10–100 μm
Ribosomes	70S (50S + 30S)	80S (60S + 40S)

Key Organelles and Their Functions:

• Nucleus: Contains DNA (genetic material), controls cell activities, site of ribosome assembly (nucleolus)
• Mitochondria: Site of aerobic respiration — produces ATP. Has its own DNA and ribosomes (semiautonomous)
• Chloroplasts: Site of photosynthesis in plants. Contains thylakoids (grana) and stroma
• Ribosomes: Site of protein synthesis (70S in prokaryotes, 80S in eukaryotes)
• Endoplasmic reticulum (rough): Protein synthesis and transport
• Endoplasmic reticulum (smooth): Lipid synthesis and detoxification
• Golgi apparatus: Packaging and secretion of proteins
• Lysosomes: Contain hydrolytic enzymes for cell digestion
• Cell wall (plants): Cellulose, provides structural support
• Cell membrane: Phospholipid bilayer with embedded proteins, controls what enters/exits the cell

⚡ NECO Tip: Plant cells have chloroplasts, a large central vacuole, and a cell wall made of cellulose — animal cells have none of these. Animal cells have centrioles and lysosomes which plant cells lack.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for NECO Biology students with a few days to months.

The Fluid Mosaic Model of the Cell Membrane:

The cell membrane is described as a “fluid mosaic” because:

• Fluid: The phospholipid bilayer is not rigid — individual phospholipids move laterally within their own layer
• Mosaic: The pattern created by the scattered protein molecules resembles a mosaic

Structure of the Plasma Membrane:

• Phospholipid bilayer: hydrophilic heads face outward, hydrophobic tails face inward
• Cholesterol molecules (in animal cells) embedded within the bilayer — provides stability
• Proteins: integral proteins span the membrane; peripheral proteins are attached to the surface
• Glycoproteins and glycolipids: carbohydrate chains on outer surface for cell recognition

Functions of the Cell Membrane:

• Selective permeability: controls what enters and exits
• Cell recognition: glycoproteins act as identity markers (e.g., blood groups)
• Communication: receptor proteins bind to hormones
• Transport: carrier proteins facilitate active and facilitated diffusion

Transport Mechanisms:

Mechanism	Requires ATP?	Direction	Example
Simple diffusion	No	High → low concentration	O₂, CO₂ across membrane
Facilitated diffusion	No	High → low, via protein channels	Glucose, ion channels
Osmosis	No	Water, across semipermeable membrane	Water balance in cells
Active transport	Yes	Low → high (against gradient)	Na⁺/K⁺ pump
Bulk transport (endocytosis/exocytosis)	Yes	Into or out of cell	Phagocytosis, secretion

Osmosis:

• Isotonic: Equal concentration inside and outside cell — no net movement
• Hypotonic: Lower solute concentration outside — water enters cell (lysis in animal cells, turgor in plant cells)
• Hypertonic: Higher solute concentration outside — water leaves cell (crenation in animal cells, plasmolysis in plant cells)

⚡ NECO Common Mistakes:

• Confusing prokaryotic and eukaryotic cell features
• Forgetting that the cell membrane is selectively permeable
• Mixing up diffusion and osmosis — diffusion moves solutes, osmosis moves water only
• Thinking plant cells can’t undergo lysis — they can if their cell wall is damaged
• Not knowing that mitochondria have their own DNA and ribosomes (endosymbiont theory)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for NECO and JAMB Biology preparation.

Nucleus and Chromatin:

The nucleus is bounded by a double nuclear membrane (nuclear envelope) with nuclear pores. The nucleolus (inside the nucleus) is the site of ribosomal RNA (rRNA) synthesis.

DNA is packaged with histone proteins to form chromatin. During cell division, chromatin condenses to form visible chromosomes (46 in humans, 40 in Nigerian dwarf goat — wait, that’s wrong, humans have 46, fruit flies have 8).

The Endomembrane System:

This system includes: nuclear envelope → endoplasmic reticulum → Golgi apparatus → vesicles → cell membrane (and lysosomes).

Proteins synthesised on the rough ER are folded, glycosylated, and packaged into vesicles that bud off and fuse with the Golgi apparatus. They are then modified further in the Golgi and dispatched to the cell membrane (for secretion) or to lysosomes.

Semiautonomous Organelles:

Both mitochondria and chloroplasts:

• Have their own DNA (circular, like bacteria)
• Have their own 70S ribosomes (prokaryotic type)
• Divide by binary fission (like bacteria)
• This supports the endosymbiont theory (they were once free-living bacteria)

The Cytoskeleton:

• Microfilaments (actin): 7 nm diameter, involved in cell movement, muscle contraction, cytokinesis
• Intermediate filaments: 10 nm diameter, provide mechanical strength (e.g., keratin in skin cells)
• Microtubules: 25 nm diameter, made of tubulin, form spindle fibres during mitosis, cilia, flagella

Cell Wall (Plant Cells):

Primary cell wall: cellulose microfibrils embedded in pectin. Formed during cell growth.

Secondary cell wall: formed after cell growth stops, contains lignin (in xylem vessels) for extra rigidity and waterproofing.

Plasmodesmata: Channels through the cell wall connecting adjacent plant cells, allowing communication and transport.

The Cell Membrane and the Phospholipid Bilayer:

Each phospholipid has a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails. The bilayer self-seals because the hydrophobic interior excludes water.

Membrane proteins perform various functions:

• Channel proteins: form pores for specific molecules
• Carrier proteins: undergo conformational change to transport molecules
• Receptor proteins: bind to specific molecules (hormones, neurotransmitters)
• Enzymatic proteins: catalyse reactions at the membrane surface
• Glycoproteins: cell surface markers / antigens

Specialised Cells:

• Red blood cells (erythrocytes): biconcave disc shape, no nucleus in mammals, haemoglobin for oxygen transport
• Nerve cells (neurons): long axon for transmission, dendrites for receiving signals, myelin sheath for insulation
• Muscle cells (myocytes): contain actin and myosin filaments for contraction, many mitochondria
• Root hair cells: long projection increases surface area for water and mineral uptake
• Palisade mesophyll cells: contain many chloroplasts for photosynthesis

NECO/JAMB Patterns:

• NECO often asks: label diagrams of animal and plant cells; state functions of organelles; distinguish between prokaryotes and eukaryotes; explain the fluid mosaic model; describe what happens to plant and animal cells in isotonic, hypotonic, and hypertonic solutions
• Be able to draw and label the fluid mosaic model
• Know adaptations of specific cell types (root hair cell, red blood cell, nerve cell)

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