Cell Structure and Functions

🟢 Lite — Quick Review (1h–1d)

Rapid summary of cell structure and functions for NABTEB biology.

The Cell is the basic unit of life. All living organisms are composed of cells.

Two Main Cell Types:

1. Prokaryotic Cells (e.g., bacteria):

• No membrane-bound nucleus (DNA is free in cytoplasm)
• No membrane-bound organelles
• Smaller (1–5 μm)
• Examples: Bacteria, blue-green algae

2. Eukaryotic Cells:

• Membrane-bound nucleus containing DNA
• Membrane-bound organelles
• Larger (10–100 μm)
• Examples: Animals, plants, fungi, protoctists

Key Organelles and Their Functions:

Organelle	Function	Found In
Cell membrane	Controls what enters/exits; boundary	All cells
Nucleus	Contains DNA; controls cell activities	All eukaryotes
Mitochondria	Produces ATP (cellular respiration)	All eukaryotes
Ribosomes	Protein synthesis	All cells
Endoplasmic reticulum (rough)	Protein modification and transport	Eukaryotes
Endoplasmic reticulum (smooth)	Lipid synthesis	Eukaryotes
Golgi apparatus	Packages and secretes substances	Eukaryotes
Chloroplasts	Photosynthesis	Plant cells
Cell wall	Provides structure and support	Plant cells, bacteria
Vacuole	Storage; maintains turgidity	Plant cells (large, central)
Lysosome	Contains digestive enzymes	Animal cells
Centrioles	Cell division	Animal cells

The Cell Membrane:

The fluid mosaic model describes the cell membrane:

• Phospholipid bilayer (fluid)
• Proteins embedded within or spanning the bilayer (mosaic)
• Some proteins have carbohydrate chains (glycoproteins) — for cell recognition

Functions of the Cell Membrane:

• Selective permeability (controls what passes through)
• Cell recognition (glycoproteins act as “name tags”)
• Transport (proteins act as channels or carriers)

⚡ NABTEB Exam Tip: Plant cells have a cell wall (made of cellulose), a large central vacuole, and chloroplasts — animal cells have none of these. Animal cells have centrioles and lysosomes — plant cells do not. Both have mitochondria.

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🟡 Standard — Regular Study (2d–2mo)

For NABTEB students who want thorough understanding of cell biology.

Detailed Structure — Plant vs Animal Cells:

Plant Cell Unique Features:

• Cell wall: Rigid outer layer of cellulose; provides support and prevents excessive water uptake
• Central vacuole: Large, fluid-filled sac; maintains turgor pressure, stores nutrients and waste
• Chloroplasts: Contain chlorophyll; site of photosynthesis (in light)
• Plasmodesmata: Channels connecting adjacent plant cells for communication

Animal Cell Unique Features:

• Centrioles: Form spindle fibres during cell division (also in some plant cells like ferns)
• Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes; break down old organelles and engulfed pathogens

Nucleus Details:

The nucleus contains:

• Nuclear envelope: Double membrane with nuclear pores
• Nucleoplasm: Gel-like matrix
• Nucleolus: Produces ribosomal RNA (rRNA)
• Chromatin: DNA wrapped around histone proteins; becomes chromosomes during cell division

Mitochondria Structure:

The “powerhouse of the cell”:

• Double membrane (outer smooth, inner folded into cristae)
• Cristae increase surface area for ATP production
• Contains its own DNA and ribosomes (supports endosymbiotic theory)

Endosymbiotic Theory: Mitochondria and chloroplasts evolved from free-living bacteria that were engulfed by ancestral eukaryotic cells. Evidence:

• Both have double membranes
• Both have their own DNA and ribosomes
• Both divide by binary fission (like bacteria)

Cell Membrane Transport:

Transport Type	Mechanism	Requires Energy?	Example
Diffusion	Particles move from high to low concentration	No	Oxygen into cells
Osmosis	Water moves across semi-permeable membrane	No	Water absorption by roots
Active transport	Particles move from low to high concentration	Yes (ATP)	Ion pumps, glucose absorption
Bulk transport	Large particles moved in vesicles	Yes	Phagocytosis, exocytosis

Osmosis:

• Isotonic: Equal concentration; no net water movement
• Hypotonic: Lower solute concentration outside; water moves INTO cell (lysis in animal cells, turgid plant cells)
• Hypertonic: Higher solute concentration outside; water moves OUT of cell (crenation in animal cells, plasmolysis in plant cells)

⚡ NABTEB Exam Tip: In osmosis questions, always identify the direction of water movement. Water moves towards the solution with HIGHER solute concentration. Plant cells become turgid in hypotonic solutions and plasmolysed in hypertonic solutions.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage of cell structure for thorough NABTEB preparation.

Organelle Functions in Detail:

Ribosomes:

• Two subunits (large and small)
• Made of rRNA and proteins
• Can be free in cytoplasm or attached to rough ER
• Site of protein synthesis (translation of mRNA)

Endoplasmic Reticulum (ER):

Rough ER:

• Ribosomes attached to outer surface
• Synthesises and processes proteins destined for secretion or membrane insertion

Smooth ER:

• No ribosomes
• Synthesises lipids (including steroid hormones)
• Detoxification (liver cells)
• Calcium storage (muscle cells)

Golgi Apparatus (Golgi Body):

• Receives proteins from ER in vesicles
• Modifies proteins (adding sugars = glycosylation)
• Packages proteins into secretory vesicles
• Packages proteins into lysosomes

Chloroplasts:

• Double membrane (outer, inner)
• Stroma: fluid-filled interior; site of Calvin cycle (CO₂ fixation)
• Thylakoids: stacked into grana; site of light reactions
• Contains chlorophyll (green pigment) and its own DNA

The Cytoskeleton:

A network of protein filaments:

• Microfilaments (actin): Cell movement, muscle contraction, cell division
• Intermediate filaments: Mechanical strength, cell integrity
• Microtubules: Cell shape, intracellular transport, cilia and flagella, cell division spindle

Cell Division:

Mitosis (Somatic Cells):

• One division producing two genetically identical daughter cells
• Used for growth, repair, asexual reproduction
• Stages: Prophase, Metaphase, Anaphase, Telophase, Cytokinesis

Meiosis (Germ Cells):

• Two divisions producing four genetically different haploid cells
• Used for gamete production (sex cells)
• Introduces genetic variation through crossing over and independent assortment
• Stages: Meiosis I (prophase I, metaphase I, anaphase I, telophase I) + Meiosis II

Key Differences:

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Chromosome number	Same as parent (diploid)	Half parent (haploid)
Genetic variation	None (clones)	Yes
Used for	Growth, repair	Gamete production

Specialised Cells:

Cells become specialised through differentiation — they develop specific structures for specific functions.

Examples:

• Red blood cells: Biconcave shape, no nucleus, packed with haemoglobin — maximises oxygen carrying
• Nerve cells (neurons): Long axon for transmitting impulses, dendrites for receiving signals
• Muscle cells: Packed with actin and myosin filaments for contraction; many mitochondria
• Root hair cells: Long projection increases surface area for water and mineral absorption
• Palisade cells: Many chloroplasts for photosynthesis; elongated shape

Cell Membrane Receptors:

Receptor proteins in the membrane allow cells to respond to signals:

• Hormone receptors: Bind specific hormones (e.g., insulin receptor)
• Neurotransmitter receptors: Bind chemical messengers from nerve cells
• Antibody receptors: On immune cells for recognising pathogens

⚡ NABTEB Quick Reference:

• Prokaryote: no nucleus, no membrane organelles (bacteria)
• Eukaryote: nucleus, membrane organelles (plants, animals, fungi)
• Mitochondria: ATP production (respiration)
• Chloroplasts: photosynthesis (plant cells only)
• Cell wall: cellulose (plants); peptidoglycan (bacteria)
• Ribosomes: protein synthesis
• ER + Golgi: protein/lipid modification and transport
• Lysosomes: digestive enzymes (animal cells)
• Central vacuole: turgor pressure (plant cells)
• Mitosis: 2 identical diploid cells; Meiosis: 4 non-identical haploid cells
• Osmosis: water from low → high solute concentration