Cell Biology

Higher Cell Biology

Cell Structure

Prokaryotic Cells:

Found in bacteria and archaea. Key features:

• No membrane-bound nucleus; DNA is a single circular chromosome in the nucleoid
• Small ribosomes (70S)
• No membrane-bound organelles
• Cell wall made of peptidoglycan
• Plasmids (small circular DNA molecules)
• Flagella for movement
• Capsule for protection
• Typical size: 1-10 micrometres

Eukaryotic Cells:

Found in animals, plants, fungi, and protists. Key features:

• Membrane-bound nucleus containing linear chromosomes
• Larger ribosomes (80S)
• Membrane-bound organelles (mitochondria, ER, Golgi, etc.)
• Typical size: 10-100 micrometres

Feature	Prokaryotic	Eukaryotic
Nucleus	No	Yes
DNA	Circular, single	Linear, multiple chromosomes
Ribosomes	70S	80S
Membrane-bound organelles	No	Yes
Cell wall	Peptidoglycan	Cellulose (plants) / chitin (fungi)
Size	1-10 $\mu$M	10-100 $\mu$M

Worked Example: Identifying cell types from electron micrographs.

A student is shown an electron micrograph of a cell. The cell has no nucleus, a cell wall, Ribosomes, and a flagellum. The diameter of the cell is approximately 3 $\mu$M.

Since the cell has no membrane-bound nucleus, it is prokaryotic. The presence of a cell wall (ruled Out animal cells), small size (3 $\mu$M is typical for prokaryotes), 70S ribosomes, and flagellum All confirm this. The cell wall is made of peptidoglycan, which is characteristic of bacteria (not Archaea, which have pseudopeptidoglycan or other cell wall compositions).

Cell Ultrastructure

Nucleus: Contains DNA, controls cell activities via gene expression. Surrounded by a double Membrane (nuclear envelope) with nuclear pores.

Mitochondria: Site of aerobic respiration (Krebs cycle and oxidative phosphorylation). Has a Double membrane; inner membrane folded into cristae to increase surface area. Contains its own DNA (supports endosymbiotic theory).

Endoplasmic Reticulum (ER):

• Rough ER: Studded with ribosomes; synthesises proteins for secretion
• Smooth ER: Synthesises lipids and steroids; detoxification

Golgi Apparatus: Modifies, sorts, and packages proteins and lipids. Receives vesicles from the ER and sends them to their destination.

Ribosomes: Site of protein synthesis (translation). Can be free in the cytoplasm or bound to Rough ER.

Lysosomes: Contain digestive enzymes for breaking down waste materials and cellular debris.

Cell Membrane (Plasma Membrane): Phospholipid bilayer with embedded proteins. Controls the Movement of substances in and out of the cell.

The Fluid Mosaic Model

The cell membrane is described by the fluid mosaic model:

• Fluid: The phospholipid bilayer is flexible and constantly moving
• Mosaic: Various proteins are embedded within the bilayer

Components:

• Phospholipids: Form the bilayer; hydrophilic heads face outward, hydrophobic tails face inward
• Intrinsic (integral) proteins: Span the entire membrane; function as channels, carriers, or receptors
• Extrinsic (peripheral) proteins: Attached to the surface; involved in cell signalling and maintaining cytoskeleton
• Cholesterol: Regulates membrane fluidity (in animal cells)
• Glycoproteins and glycolipids: Carbohydrate chains on the surface; involved in cell recognition and signalling

Membrane Transport

Passive Transport (no energy required):

Type	Description	Examples
Simple diffusion	Movement from high to low concentration; small non-polar molecules	\mathrm{O_2, \mathrm{CO_2
Facilitated diffusion	Via channel or carrier proteins; large or polar molecules	Glucose, ions
Osmosis	Movement of water across a semi-permeable membrane	Water in cells

Active Transport (energy required):

Movement against the concentration gradient using ATP and carrier proteins.

Example: Sodium-potassium pump: moves 3 \mathrm{ Na^+ out and 2 \mathrm{ K^+ in per ATP Hydrolysed.

Bulk Transport:

• Endocytosis: Cell takes in substances by engulfing them (phagocytosis for solids, pinocytosis for liquids)
• Exocytosis: Cell expels substances via vesicles fusing with the membrane

Osmosis and Water Potential

Water potential ($\psi$): The tendency of water to move from one area to another. Pure water has $\psi = 0$.

$\psi = \psi_s + \psi_p$

Where $\psi_s$ is the solute potential (always negative or zero) and $\psi_p$ is the pressure Potential.

Isotonic: Same water potential inside and outside the cell (no net movement of water).

Hypertonic: Lower water potential outside (water leaves the cell; cell crenates in animal cells, Plasmolyses in plant cells).

Hypotonic: Higher water potential outside (water enters the cell; cell lyses in animal cells, Becomes turgid in plant cells).

Worked Example: Calculating water potential and predicting osmosis.

A plant cell has a solute potential of $-500$ kPa and a pressure potential of $200$ kPa. The cell is Placed in a solution with a water potential of $-400$ kPa.

Cell water potential: $\psi = -500 + 200 = -300$ kPa.

Since the cell water potential ($-300$ kPa) is higher than the solution water potential ($-400$ KPa), water will move from the cell into the solution (from higher to lower water potential). The Cell will lose water and may become plasmolysed.

Worked Example: Determining solute potential from molarity.

The solute potential $\psi_s$ can be calculated using the formula:

$\psi_s = -iCRT$

Where $i$ is the ionisation constant (1 for non-ionic solutes), $C$ is the molar concentration, $R$ Is the gas constant (0.0083 kPa L mol$^{-1}$ K$^{-1}$), and $T$ is the temperature in Kelvin.

For a 0.2 M sucrose solution at 20$\degree$C (293 K):

$\psi_s = -(1)(0.2)(0.0083)(293) = -0.486$ kPa.

---

Cell Division

Mitosis

Mitosis produces two genetically identical daughter cells with the same chromosome number as the Parent cell. It is used for growth, repair, and asexual reproduction.

Stages:

1. Prophase: Chromosomes condense, nuclear envelope breaks down, spindle fibres form
2. Metaphase: Chromosomes align at the cell equator, attached to spindle fibres at the centromere
3. Anaphase: Sister chromatids separate and move to opposite poles
4. Telophase: Chromosomes decondense, nuclear envelope reforms, cytokinesis begins

Significance of mitosis:

• Growth of multicellular organisms
• Repair of damaged tissue
• Asexual reproduction
• Maintains chromosome number (diploid)

Worked Example: Calculating the duration of mitosis stages.

A student observes 100 cells under a microscope and counts the number in each stage:

• Interphase: 80
• Prophase: 10
• Metaphase: 5
• Anaphase: 3
• Telophase: 2

Total time for the cell cycle: 24 hours.

Time in prophase: $(10/100) \times 24 = 2.4$ hours. Time in metaphase: $(5/100) \times 24 = 1.2$ Hours. Time in anaphase: $(3/100) \times 24 = 0.72$ hours. Time in telophase: $(2/100) \times 24 = 0.48$ hours.

This shows that most of the cell cycle is spent in interphase (80%), and mitosis is a relatively Short phase.

Meiosis

Meiosis produces four genetically distinct daughter cells, each with half the chromosome number (haploid). It is essential for sexual reproduction.

Meiosis I (reduction division):

1. Prophase I: Homologous chromosomes pair up (synapsis); crossing over occurs at chiasmata
2. Metaphase I: Homologous pairs align at the equator
3. Anaphase I: Homologous chromosomes separate
4. Telophase I: Two haploid cells form

Meiosis II (similar to mitosis):

1. Prophase II: Chromosomes condense
2. Metaphase II: Chromosomes align at the equator
3. Anaphase II: Sister chromatids separate
4. Telophase II: Four haploid cells form

Sources of genetic variation:

• Crossing over: Exchange of genetic material between homologous chromosomes during Prophase I
• Independent assortment: Random orientation of homologous pairs during Metaphase I
• Random fertilisation: Any sperm can fertilise any egg

Worked Example: Comparing mitosis and meiosis.

A cell in the G2 phase of the cell cycle has 46 chromosomes (diploid, human cell).

After mitosis: each daughter cell has 46 chromosomes (same as parent). The two daughter cells are Genetically identical.

After meiosis: each of the four daughter cells has 23 chromosomes (haploid). The four daughter cells Are genetically different (due to crossing over and independent assortment).

---

Microscopy

Light Microscopy

• Maximum resolution: approximately 0.2 \mu\mathrm{m (200 nm)
• Maximum magnification: approximately $\times 1500$
• Uses visible light and glass lenses
• Staining required to increase contrast (e.g., iodine, methylene blue)

Electron Microscopy

• Transmission EM (TEM): Electrons pass through a thin specimen; shows internal structure. Resolution: approximately 0.5 \mathrm{ nm.
• Scanning EM (SEM): Electrons bounce off the surface; shows 3D surface structure. Resolution: approximately 5 \mathrm{ nm.

Preparation for EM:

• Specimens must be fixed (preserved), dehydrated, and embedded in resin
• Thin sections needed for TEM
• Metal coating needed for SEM
• Specimens are viewed in a vacuum (dead specimens only)

Cell fractionation: The process of breaking cells open and separating organelles by differential Centrifugation:

1. Homogenise cells in isotonic, buffered solution
2. Filter to remove debris
3. Centrifuge at low speed: nuclei pellet
4. Centrifuge at medium speed: mitochondria pellet
5. Centrifuge at high speed: ribosomes pellet

Worked Example: Calculating magnification.

A cell appears 5 mm wide in a micrograph. The actual cell width is 10 $\mu$M.

Magnification = image size / actual size = 5 \mathrm{ mm / 10 \mathrm{ \mu m.

Convert to the same units: 5 \mathrm{ mm = 5000 \mathrm{ \mu m.

Magnification = $5000 / 10 = \times 500$.

---

Common Pitfalls

1. Confusing mitosis and meiosis: Mitosis produces 2 identical diploid cells; meiosis produces 4 non-identical haploid cells.

2. Crossing over timing: Crossing over occurs in Prophase I of meiosis, not during mitosis.

3. Osmosis direction: Water moves from high water potential to low water potential (not from high to low solute concentration directly).

4. 70S vs 80S ribosomes: Prokaryotic ribosomes are 70S; eukaryotic cytoplasmic ribosomes are 80S. Mitochondrial ribosomes are 70S (supporting endosymbiotic theory).

5. Resolution vs. Magnification: Resolution is the ability to distinguish two close objects; magnification is how much larger the image appears. Resolution is more important.

6. Forgetting that prophase I is the longest stage of meiosis. Homologous chromosomes must pair up and crossing over must occur, which takes longer than any other stage.

7. Confusing plasmolysis and crenation. Plasmolysis occurs in plant cells (cell membrane pulls away from the cell wall); crenation occurs in animal cells (cell shrinks and becomes wrinkled).

8. Thinking that osmosis only involves water. While water is the most common solvent, osmosis technically refers to the movement of any solvent across a selectively permeable membrane.

9. Forgetting the role of the spindle fibres. Spindle fibres attach to centromeres and pull chromatids apart during anaphase. Without spindle fibres, chromosomes cannot be correctly distributed to daughter cells.

10. Confusing interphase with mitosis. Interphase (G1, S, G2) is the longest part of the cell cycle and includes DNA replication. Mitosis is only the division phase.

---

Practice Questions

1. Compare and contrast prokaryotic and eukaryotic cell structure in a table.

2. Describe the role of the Golgi apparatus in protein processing and secretion.

3. Explain how the fluid mosaic model accounts for the properties of the cell membrane.

4. Calculate the magnification of a cell that appears 5 \mathrm{ mm wide in a micrograph when the actual cell is 10 \mu\mathrm{m wide.

5. Describe what happens to an animal cell and a plant cell when placed in (a) a hypertonic solution and (b) a hypotonic solution.

6. Explain two ways in which meiosis generates genetic variation.

7. Describe the process of cell fractionation and explain why the homogenisation medium must be ice-cold and isotonic.

8. Explain the evidence for the endosymbiotic theory of mitochondrial origin.

9. A student observes that red blood cells placed in a 0.3% NaCl solution swell and burst. Explain why this happens.

10. Describe the role of the sodium-potassium pump in maintaining the resting potential of a neurone.

11. Explain why the cell fractionation medium must be buffered (kept at a constant pH).

12. Compare light microscopy and electron microscopy in terms of resolution, magnification, specimen preparation, and the type of image produced.

13. Describe the process of plasmolysis in a plant cell and explain what happens when the cell is returned to a hypotonic solution.

14. Explain why chromosomes must condense before cell division and describe the role of histone proteins in this process.

15. A cell in G2 has 20 chromosomes. After meiosis, how many chromosomes will each daughter cell have? How many chromatids?

16. Explain the significance of mitosis in the context of cancer, describing what goes wrong when cell division is not properly controlled.

17. Describe the structure and function of smooth ER in (a) liver cells and (b) muscle cells.

18. Explain why the nuclear pore is a selectively permeable structure and describe what types of molecules can pass through it.

19. Calculate the actual size of a mitochondrion that appears 8 mm long in an electron micrograph at a magnification of $\times 10,000$.

20. Explain how cholesterol affects the fluidity of the cell membrane at both high and low temperatures.

21. A plant cell has a solute potential of $-800$ kPa and a pressure potential of $400$ kPa. It is placed in a solution with a water potential of $-300$ kPa. Calculate the cell water potential and predict the direction of water movement.

22. Explain the role of spindle fibres during mitosis, including how they attach to chromosomes and what happens when spindle fibres are disrupted (e.g., by colchicine).

23. Describe three differences between cell division in plant cells and animal cells.

24. Explain why TEM requires very thin sections of the specimen while SEM does not.

25. A student observes 200 cells and counts the following: interphase 150, prophase 25, metaphase 12, anaphase 8, telophase 5. If the total cell cycle time is 20 hours, calculate the duration of each stage.

Review: Cell Fractionation and Ultracentrifugation

Cell fractionation is a technique used to separate the organelles of a cell so that their individual Functions can be studied.

Procedure:

1. Cells are homogenised (broken open) in a cold, isotonic, buffered solution.

• Cold: to reduce the activity of digestive enzymes that could damage organelles.
• Isotonic: to prevent organelles from bursting or shrinking due to osmotic effects.
• Buffered: to maintain a constant pH, as enzyme activity is pH-dependent.

2. The homogenate is filtered to remove unbroken cells and debris.
3. The filtrate is subjected to differential centrifugation:

• Low speed: heavy organelles (nuclei) pellet first.
• Higher speed: mitochondria, chloroplasts, lysosomes pellet next.
• Very high speed: ribosomes, membrane fragments pellet last.
• The supernatant (liquid above the pellet) is poured off and centrifuged at a higher speed at each stage.

Worked Example: Designing a cell fractionation experiment.

A student wants to isolate mitochondria from liver cells. They homogenise liver tissue in cold, Isotonic, buffered sucrose solution. They centrifuge at 600 g for 10 minutes and collect the Supernatant. They then centrifuge the supernatant at 10,000 g for 15 minutes. The pellet at this Stage contains the mitochondria. The student resuspends the mitochondrial pellet and can now study Mitochondrial function (e.g., measuring the rate of aerobic respiration).

Why sucrose solution is used instead of water: A sucrose solution is used because it provides an Isotonic medium. If water were used, the organelles would absorb water by osmosis and burst, Destroying their structure and function. The sucrose solution has the same water potential as the Cytoplasm, preventing osmotic damage.

Review: Prokaryotic Cell Division — Binary Fission

Prokaryotes reproduce by binary fission, a form of asexual reproduction:

1. The circular DNA is replicated, producing two identical copies.
2. The cell grows, separating the two DNA molecules.
3. A new cell wall and plasma membrane form across the middle of the cell (septum formation).
4. The cell splits into two daughter cells, each with a complete copy of the DNA.

Key differences from eukaryotic cell division:

• No mitosis (no spindle apparatus, no chromosomes to separate).
• DNA replication and cell division are continuous processes (no distinct cell cycle phases).
• Reproduction can be very rapid under optimal conditions (E. Coli can divide every 20 minutes).

Review: The Importance of Membrane Fluidity

The fluidity of the cell membrane is essential for its function:

• Membrane proteins must be able to move laterally to perform their functions (e.g., receptors clustering together during cell signalling).
• Endocytosis and exocytosis require the membrane to be flexible enough to form vesicles.
• Cell division requires the membrane to be remodelled.

Factors affecting fluidity:

• Temperature: Higher temperature increases kinetic energy of phospholipids, increasing fluidity. Lower temperature decreases fluidity.
• Fatty acid composition: Shorter fatty acid tails increase fluidity (fewer interactions between tails). Unsaturated (kinked) fatty acids increase fluidity by preventing tight packing.
• Cholesterol: Acts as a fluidity buffer. At high temperatures, it restrains phospholipid movement (reducing fluidity). At low temperatures, it prevents tight packing (maintaining fluidity).

Review: The Cytoskeleton in Detail

The cytoskeleton is a network of protein fibres that provides structural support, facilitates Intracellular transport, and enables cell movement and division.

Microtubules (25 nm): Hollow tubes made of tubulin. They radiate from the centrosome and form The mitotic spindle during cell division. Motor proteins (kinesin and dynein) move along Microtubules to transport vesicles and organelles. Kinesin moves towards the plus end (away from the Centrosome); dynein moves towards the minus end (towards the centrosome).

Microfilaments (7 nm): Solid rods made of actin. They form a meshwork beneath the cell membrane (cortical actin) that provides mechanical support. In muscle cells, actin filaments interact with Myosin filaments to generate contraction. During cytokinesis, a contractile ring of actin and myosin Pinches the cell in two.

Intermediate filaments (10 nm): Rope-like fibres made of various proteins (e.g., keratin, Vimentin, lamins). They provide mechanical strength and resist shear stress. Nuclear lamins (a type Of intermediate filament) line the inside of the nuclear envelope, providing structural support.

Worked Example: Motor proteins and vesicle transport.

A vesicle containing a secretory protein must travel from the Golgi apparatus to the plasma Membrane. The vesicle is attached to a motor protein (kinesin) that walks along a microtubule Towards the plus end (the cell periphery). Each step of kinesin requires the hydrolysis of one ATP Molecule. The motor protein moves in a hand-over-hand fashion, taking steps of approximately 8 nm Per ATP hydrolysed. This process allows directed, ATP-dependent transport of materials within the Cell.

Review: Summary Comparison of Transport Mechanisms

Mechanism	Energy Required	Direction	Mediator	Example
Simple diffusion	No	High to low conc.	None	\mathrm{O_2, \mathrm{CO_2
Facilitated diffusion	No	High to low conc.	Channel/carrier	Glucose, \mathrm{Na^+
Osmosis	No	High to low $\psi$	Aquaporins	Water
Active transport	Yes (ATP)	Low to high conc.	Carrier protein	\mathrm{Na^+/\mathrm{K^+ pump
Cotransport	Yes (gradient)	Low to high conc.	Carrier protein	\mathrm{Na^+-glucose symporter
Exocytosis	Yes (ATP)	Out of cell	Vesicles	Hormone secretion
Endocytosis	Yes (ATP)	Into cell	Vesicles	Phagocytosis

Review: Plant Cell Adaptations

Plant cells have several adaptations not found in animal cells:

Cell wall: Provides structural support and protection. Made of cellulose microfibrils in a Matrix of hemicellulose and pectin. Fully permeable to water and dissolved substances. The cell wall Prevents plant cells from bursting in hypotonic solutions.

Plasmodesmata: Narrow channels through the cell wall that connect the cytoplasm of adjacent Plant cells. They allow the transport of small molecules and signalling molecules, facilitating Communication and coordination between cells.

Central vacuole: A large, fluid-filled organelle that can occupy up to 90% of the cell volume. It maintains turgor pressure, stores water and dissolved substances, stores pigments, and stores Defensive compounds (toxins that deter herbivores). The vacuole is surrounded by a membrane called The tonoplast.

Chloroplasts: The site of photosynthesis. They have a double membrane, internal thylakoid Membranes (stacked into grana), and stroma. Chloroplasts have their own DNA and ribosomes (70S), Supporting the endosymbiotic theory.

Review: Endosymbiotic Theory

The endosymbiotic theory proposes that certain organelles, particularly mitochondria and Chloroplasts, originated as free-living prokaryotes that were engulfed by ancestral eukaryotic Cells.

Evidence supporting endosymbiotic theory:

1. Double membrane: Both mitochondria and chloroplasts have a double membrane. The outer membrane is thought to be derived from the host cell’s phagocytic vesicle, and the inner membrane is thought to be the original prokaryotic plasma membrane.
2. Own DNA: Both organelles contain circular DNA, similar to prokaryotic DNA.
3. Own ribosomes: Both have 70S ribosomes (prokaryotic size), not 80S (eukaryotic size).
4. Binary fission: Both replicate by binary fission, independently of the host cell division.
5. Transcription and translation: Both can synthesise some of their own proteins using their own machinery, similar to prokaryotes.
6. Antibiotic sensitivity: Protein synthesis in mitochondria and chloroplasts is inhibited by antibiotics that target prokaryotic ribosomes (e.g., chloramphenicol, tetracycline) but not by antibiotics that target eukaryotic ribosomes (e.g., cycloheximide).

Sequence of events (proposed):

An ancestral eukaryotic cell (which already had a nucleus and endomembrane system) engulfed an Aerobic prokaryote. Instead of digesting it, the host cell formed a symbiotic relationship: the Prokaryote provided ATP through aerobic respiration, and the host provided protection and nutrients. Over millions of years, the prokaryote lost many of its genes (some were transferred to the host Nucleus) and became the mitochondrion. A similar event occurred later when a photosynthetic Prokaryote (cyanobacterium) was engulfed, giving rise to chloroplasts in the lineage that led to Plants and algae.

Review: Cell Surface Receptors and Cell Signalling

Cells communicate with each other through signalling molecules that bind to specific receptors on The cell surface or inside the cell.

Cell surface receptors: Transmembrane proteins that span the cell membrane. When a signalling Molecule (ligand) binds to the extracellular domain of the receptor, it triggers a conformational Change that activates an intracellular signalling pathway.

Types of cell surface receptors:

• G-protein coupled receptors (GPCRs): The largest family of cell surface receptors. When a ligand binds, the receptor activates a G-protein, which then activates an enzyme (e.g., adenylate cyclase, which produces cAMP as a second messenger). Example: adrenaline receptors.
• Receptor tyrosine kinases (RTKs): When a ligand binds, the receptor dimerises and autophosphorylates, triggering a signalling cascade (e.g., the MAP kinase pathway). Example: insulin receptor.
• Ion channel receptors: Ligand-gated ion channels that open when a neurotransmitter binds, allowing ions to flow through the membrane. Example: nicotinic acetylcholine receptor at the neuromuscular junction.

Signal transduction pathways: The process by which an extracellular signal is converted into an Intracellular response. Key features include amplification (one activated enzyme can activate many Downstream molecules) and regulation (the signal can be modulated at multiple steps).

Worked Example: The adrenaline signalling pathway.

When adrenaline binds to a GPCR on a liver cell:

1. The receptor changes shape and activates a G-protein.
2. The G-protein activates adenylate cyclase.
3. Adenylate cyclase converts ATP to cyclic AMP (cAMP), a second messenger.
4. CAMP activates protein kinase A (PKA).
5. PKA phosphorylates target enzymes, activating glycogen phosphorylase and inhibiting glycogen synthase.
6. The net effect: glycogen is broken down to glucose (glycogenolysis), increasing blood glucose levels.

This pathway demonstrates signal amplification: one molecule of adrenaline can lead to the Activation of thousands of glycogen phosphorylase molecules through the cascade.

Review: The Endomembrane System

The endomembrane system is a network of membranes and organelles that work together to modify, Package, and transport lipids and proteins. It includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles, lysosomes, and the plasma membrane.

Protein trafficking through the endomembrane system:

1. Proteins destined for secretion or for the plasma membrane are synthesised by ribosomes on the rough ER.
2. The proteins enter the ER lumen, where they are folded and may be modified (e.g., glycosylation — addition of carbohydrate chains).
3. Transport vesicles bud from the ER and carry proteins to the cis face of the Golgi apparatus.
4. In the Golgi, proteins are further modified (e.g., additional glycosylation, sorting), packaged into vesicles, and sent to their final destination.
5. Secretory vesicles fuse with the plasma membrane (exocytosis), releasing the proteins outside the cell.
6. Lysosomal enzymes are tagged with mannose-6-phosphate in the Golgi, which directs them to lysosomes.

Worked Example: What happens when protein trafficking goes wrong.

In I-cell disease (mucolipidosis II), the enzyme that adds the mannose-6-phosphate tag to lysosomal Enzymes in the Golgi is deficient. Without this tag, the lysosomal enzymes are secreted outside the Cell instead of being directed to lysosomes. The lysosomes therefore lack their digestive enzymes And cannot break down waste materials. Substances accumulate inside the cell, causing severe Developmental problems and early death.

Review: Cell Adhesion and the Extracellular Matrix

Cell junctions:

• Desmosomes: Button-like junctions that bind adjacent cells together, providing mechanical strength. They are common in tissues subject to mechanical stress (e.g., skin, heart muscle). Intermediate filaments (keratin) anchor into desmosomal plaques inside the cell.
• Gap junctions: Channels that connect the cytoplasm of adjacent cells, allowing ions and small molecules to pass directly between cells. They enable rapid cell-to-cell communication (e.g., between cardiac muscle cells for coordinated contraction).
• Tight junctions: Seal the space between adjacent cells, preventing leakage. They are important in epithelial tissues (e.g., the gut lining) where they maintain the barrier between the external environment and the internal tissue.

Extracellular matrix (ECM): A network of proteins and polysaccharides secreted by cells that Provides structural support, regulates cell behaviour, and facilitates cell adhesion.

Components of the ECM:

• Collagen: The most abundant protein in the animal body. Provides tensile strength.
• Fibronectin: Connects cells to the ECM by binding to integrin receptors on the cell surface.
• Proteoglycans: Proteins with attached glycosaminoglycan (GAG) chains that attract water, providing hydration and resistance to compression.
• Laminin: A component of the basal lamina (a specialised ECM layer beneath epithelial cells).

Integrins: Transmembrane receptors that connect the ECM to the cytoskeleton inside the cell. They transmit mechanical signals and also participate in cell signalling, affecting cell growth, Migration, and differentiation.

Worked Example: The role of the ECM in wound healing.

When the skin is wounded, the ECM plays several critical roles. Collagen fibres provide a scaffold For cell migration into the wound. Fibronectin guides the migration of fibroblasts, which synthesise New ECM components. Growth factors bound to the ECM are released and stimulate cell proliferation. As healing progresses, the ECM is remodelled: initially disorganised collagen is replaced by more Organised fibres, and excess ECM is broken down by matrix metalloproteinases (MMPs). If this Remodelling process is dysregulated, it can lead to excessive scarring (fibrosis) or delayed wound Healing.

Review: Stem Cells and Cell Specialisation

Stem cells are undifferentiated cells that have the capacity to divide and differentiate into Specialised cell types.

Types of stem cells:

Type	Source	Potential
Totipotent	Early embryo (up to 8-cell stage)	Can become any cell type, including placental
Pluripotent	Blastocyst (inner cell mass)	Can become any cell type (not placental)
Multipotent	Adult tissues (e.g., bone marrow)	Can become a limited range of cell types
Unipotent	Adult tissues	Can become only one cell type

Cell specialisation: All cells in an organism contain the same genetic information, but Different genes are expressed in different cell types. This differential gene expression is Controlled by transcription factors and epigenetic modifications (DNA methylation, histone Modification).

Worked Example: Specialisation of red blood cells.

Red blood cells are derived from stem cells in the bone marrow. During differentiation:

1. The cell produces large amounts of haemoglobin (the oxygen-carrying protein).
2. The nucleus is extruded from the cell, creating more space for haemoglobin.
3. The cell adopts a biconcave disc shape, increasing the surface area to volume ratio for efficient gas exchange.
4. The cell loses its mitochondria and other organelles, relying entirely on anaerobic respiration for energy.

This specialisation is irreversible — mature red blood cells cannot divide or synthesise new Proteins because they have no nucleus.

Review: Apoptosis — Programmed Cell Death

Apoptosis is a highly regulated process of programmed cell death that occurs as a normal part of Development and tissue homeostasis. Unlike necrosis (accidental cell death), apoptosis is an orderly Process that does not cause inflammation.

Process of apoptosis:

1. The cell receives a signal to undergo apoptosis (either from outside the cell via death receptors, or from inside the cell due to DNA damage or cellular stress).
2. Caspases (cysteine proteases) are activated. These are the “executioner” enzymes of apoptosis.
3. Caspases break down key cellular proteins, including nuclear lamins (causing the nucleus to condense) and cytoskeletal proteins (causing the cell to shrink and form blebs).
4. DNA is fragmented by endonucleases.
5. The cell breaks into membrane-bound apoptotic bodies, which are phagocytosed by macrophages.

Importance of apoptosis:

• Embryonic development: removes webbing between fingers, shapes the nervous system.
• Immune system: removes self-reactive lymphocytes (preventing autoimmune disease).
• Tissue homeostasis: balances cell division with cell death to maintain tissue size.
• DNA damage: removes cells with damaged DNA that could become cancerous.
• Viral infection: infected cells may undergo apoptosis to limit the spread of the virus.

Worked Example: Apoptosis in cancer.

Cancer cells often evade apoptosis, allowing them to divide uncontrollably. One mechanism is the Overexpression of Bcl-2, a protein that inhibits apoptosis by preventing the release of cytochrome c From mitochondria. Normally, cytochrome c release activates caspases and triggers apoptosis. By Blocking this pathway, cancer cells survive and proliferate despite having DNA damage. Some cancer Therapies aim to restore the apoptotic pathway by inhibiting Bcl-2 or activating pro-apoptotic Proteins.

Worked Examples

Example 1: Comparing mitosis and meiosis

Compare mitosis and meiosis in terms of the number of divisions, daughter cells produced, and genetic variation.

Solution:

Feature	Mitosis	Meiosis
Divisions	1	2
Daughter cells	2	4
Chromosome number	Same as parent	Half of parent
Genetic variation	No	Yes (crossing over, independent assortment)
Function	Growth, repair, asexual reproduction	Production of gametes