Fiche de révision : Fundamentals of Cell Structure and Division

📋 Course Outline

1. Cell discovery and microscope
2. Cell membrane and osmosis
3. Nucleus, DNA and chromosomes
4. Ribosomes, ER and Golgi apparatus
5. Lysosomes, mitochondria and plastids
6. Cell division: mitosis and meiosis

📖 1. Cell discovery and microscope

🔑 Key Concepts & Definitions

• Cell : A cell is the basic structural and functional unit of life where all living activities begin.
• Limit of resolution of human eye : The limit of resolution is the smallest separation a human eye can distinguish as two distinct points.
• Light microscope : A light microscope uses an objective lens and eyepiece to magnify objects viewed under visible light.
• Electron microscope : An electron microscope forms highly magnified images using a beam of electrons to reveal nanometre-scale details.

📝 Essential Points

• The human eye near point is about 25 cm, and points separated by about 0.1 mm can be seen as distinct with resolution 0.1 mm.
• Robert Hooke in 1665 observed cork under a self-designed microscope (~200–300×) and named the box-like compartments ‘cells’.
• A light microscope’s total magnification equals eyepiece magnification multiplied by objective magnification (e.g., 10X × 10X = 100X).
• Improvements in microscopes focus on resolution (clarity), contrast (brightness difference), and magnification (size increase).
• Electron microscopes use electrons instead of light, enabling cell structures to be seen at the nanometre scale.
• Unit conversion for size estimation is 1 millimetre (mm) = 1000 micrometre (μm).

💡 Memory Hook

RCM for microscope improvements: Resolution = clarity, Contrast = brightness difference, Magnification = size boost.

📖 2. Cell membrane and osmosis

🔑 Key Concepts & Definitions

• Plasma membrane : The plasma membrane is a thin boundary around the cell that protects its contents and defines the cell’s individuality.
• Selective permeability : Selective permeability is the membrane property that lets some substances pass through while blocking others.
• Osmosis : Osmosis is the diffusion of water across a selectively permeable membrane from higher water concentration to lower water concentration until concentrations balance.
• Solution tonicity : Solution tonicity describes how the extracellular solute concentration compares with the intracellular solute concentration using isotonic, hypotonic, or hypertonic conditions.

📝 Essential Points

• Osmosis moves water from a dilute solution to a concentrated solution until the solute concentrations on both sides become equal.
• In the potato experiment, water-treated cells swell and salt/sugar-treated cells shrink after about an hour due to osmosis.
• Diffusion is the net movement of particles from higher to lower concentration, while osmosis is specifically diffusion of water across a selectively permeable membrane.
• In the fluid-mosaic model, the membrane has a lipid bilayer with water-attracting heads outside and water-repelling tails inside, with proteins embedded and able to move sideways, flip, and rotate.
• Proteins embedded in the membrane act as gatekeepers that help specific substances cross the membrane.
• Cell membrane thickness is about 7 to 10 nm, and it is made of lipids and proteins.

💡 Memory Hook

Osmosis goes from “more water” to “more solute”: water moves dilute → concentrate across a selectively permeable membrane.

📖 3. Nucleus, DNA and chromosomes

🔑 Key Concepts & Definitions

• Nuclear membrane : A double-layered nucleus covering with pores that lets materials move between the nucleus and the cytoplasm.
• Nucleolus : A dense round body inside the nucleus where ribosomal subunits are synthesized.
• Chromatin : A non-dividing cell form of DNA-protein material that appears as an entangled mass of thread-like structures.
• Chromosome : A rod-shaped DNA-protein structure that becomes visible when a cell is about to divide and carries inheritance information.

📝 Essential Points

• Water passes into cells by osmosis, moving from dilute to concentrated regions across a selectively permeable membrane until concentrations equalize.
• In eukaryotic cells, DNA is packaged as chromatin in non-dividing cells and becomes organized into chromosomes before cell division.
• Chromosomes contain DNA and specific proteins, and genes are the functional segments of DNA.
• The nucleolus produces ribosomal subunits that leave the nucleus and assemble with other subunits in the cytoplasm to form ribosomes.
• Mature human red blood cells are enucleate, so they lack a nucleus and cannot repair or divide, surviving for about 120 days.

💡 Memory Hook

Nucleolus makes ribosome pieces; chromatin is “threads,” and chromosomes are “rods” right before division.

📖 4. Ribosomes, ER and Golgi apparatus

🔑 Key Concepts & Definitions

• Ribosomes : Ribosomes are tiny cell structures where proteins are made, either free in cytoplasm or attached to the endoplasmic reticulum.
• Endoplasmic reticulum : The endoplasmic reticulum is a network-like organelle in the cytoplasm that is continuous with the outer nuclear membrane and helps make and move biomolecules.
• Rough endoplasmic reticulum : Rough endoplasmic reticulum is an ER type that has ribosomes attached and mainly carries out protein synthesis and protein secretion.
• Smooth endoplasmic reticulum : Smooth endoplasmic reticulum is an ER type without ribosomes on its surface that mainly synthesises and stores fats and some hormones.
• Golgi apparatus : The Golgi apparatus is a set of stacked flattened sacs that modifies, sorts, and packages proteins or lipids into vesicles for transport and secretion.

📝 Essential Points

• Ribosomes serve as the sites of protein synthesis in the cell.
• The endoplasmic reticulum is continuous with the outer membrane of the nuclear envelope.
• Rough ER appears rough under an electron microscope because ribosomes are attached to its surface and it is mainly for protein synthesis and secretion.
• Smooth ER appears smooth because it lacks surface ribosomes and it synthesises and stores fats and hormones.
• The Golgi apparatus modifies, sorts, and packages proteins and/or lipids into vesicles for transport, secretion, or lysosome formation.

💡 Memory Hook

Rough ER = Ribosomes on the surface = Proteins; Smooth ER = Smooth surface = Fats & Hormones.

📖 5. Lysosomes, mitochondria and plastids

🔑 Key Concepts & Definitions

• Mitochondria : Mitochondria are double-membrane organelles that supply energy for most cellular activities by releasing energy from glucose during cellular respiration.
• Cristae : Cristae are folded inner-membrane projections in mitochondria that increase surface area and support energy production reactions.
• Chloroplast : Chloroplasts are double-membrane plastids where chlorophyll absorbs light and photosynthesis forms sugars stored in the stroma.
• Chromoplasts : Chromoplasts are pigment-containing plastids that produce yellow, orange, or red colours in flowers and fruits.
• Leucoplasts : Leucoplasts are colourless plastids that store food such as starch, oils, or proteins and are grouped by the stored material.

📝 Essential Points

• Each mitochondrion has an outer smooth porous membrane and an inner folded membrane called cristae.
• Energy from glucose breakdown in mitochondria during cellular respiration is stored in ATP, which powers most cellular activities.
• Chloroplasts contain a semi-fluid stroma and disc-shaped membrane structures where chlorophyll captures light for photosynthesis.
• Plastids have DNA and ribosomes, suggesting an evolutionary relationship with bacteria.
• A mature plant cell usually has one large central vacuole with cell sap that stores water, minerals, sugars, and waste to maintain cell firmness.

💡 Memory Hook

Cristae = “crater ridges” that add surface area for making ATP (energy).

📖 6. Cell division: mitosis and meiosis

🔑 Key Concepts & Definitions

• Cell division : Cell division is the process where new cells are formed from pre-existing cells to support growth, repair and reproduction.
• Mitosis : Mitosis is a common cell division that produces two genetically identical daughter cells from one parent cell for growth and repair.
• Meiosis : Meiosis is a two-step cell division in reproductive organs that produces gametes and generates genetic variation.

📝 Essential Points

• Mitosis produces two genetically identical daughter cells, with each daughter getting the same DNA and chromosome number as the parent cell.
• Meiosis occurs only in reproductive organs and parent cells divide twice to form four daughter cells.
• In meiosis, the first division reduces chromosomes to half in each daughter cell, and the second division makes four gametes with half the DNA.
• After fertilisation, combining gametes restores the original chromosome number compared with the parent cells.
• Errors in mitosis can cause uncontrolled cell division, leading to tumours and abnormal chromosome numbers in body cells.
• Errors in meiosis can cause genetic disorders and may be linked to early pregnancy loss or reduced fertility.

💡 Memory Hook

Mitosis = Mates alike (2 identical daughters); Meiosis = Makes variety (2 divisions → 4 gametes, half chromosomes).

📅 Key Dates

📊 Synthesis Tables

Mitosis vs meiosis

Prokaryotic vs eukaryotic cells

⚠️ Common Pitfalls & Confusions

1. Mixing up diffusion and osmosis: diffusion is net particle movement across a gradient, while osmosis is specifically water diffusion across a selectively permeable membrane.
2. Saying osmosis moves from concentrated to dilute solution: it moves water from dilute (more water) to concentrated (more solute) until concentrations balance.
3. Confusing isotonic/hypotonic/hypertonic: isotonic means extracellular and intracellular solute concentrations are equal; hypotonic is less external solute; hypertonic is more external solute.
4. Assuming plant cells swell/shrink in concentrated sugar like animal cells: the rigid, permeable cell wall keeps the cell from shrinking in size (though the membrane pulls away).
5. Thinking rough ER is rough because it stores fats: rough ER has ribosomes attached and is mainly for protein synthesis and secretion; smooth ER is ribosome-free for fats and some hormones.
6. Believing mitochondria and chloroplasts are identical: both have double membranes and their own DNA/ribosomes, but mitochondria do cellular respiration (ATP) while chloroplasts do photosynthesis.
7. Confusing meiosis outcomes with mitosis: meiosis produces gametes with half the chromosome number after two divisions, restoring the original number only after fertilisation.

✅ Exam Checklist

1. State the definition of limit of resolution of the human eye and use the 25 cm near-point and 0.1 mm separation idea to explain what can be distinguished.
2. Explain why Hooke could observe cells and what he named in 1665 when examining cork.
3. Calculate total magnification as eyepiece × objective magnification and interpret what a higher magnification/resolution means.
4. Convert sizes using 1 millimetre (mm) = 1000 micrometre (μm) and use the field-of-view method logic to estimate a cell’s real size.
5. Define plasma membrane (selectively permeable) and distinguish diffusion from osmosis using the potato experiment outcomes (swelling in plain water, shrinkage in salt/sugar).
6. Identify the fluid-mosaic model features: lipid bilayer head/tail orientation, embedded proteins, and protein gatekeepers; include the stated 7 to 10 nm thickness.
7. Explain cell wall necessity in plants and predict outcomes in concentrated sugar (plant rigidity vs animal shrinkage) using osmosis and cell-wall permeability.
8. List nucleus functions: nuclear membrane pores, nucleolus ribosomal subunits, chromosomes visible before division, and genes as functional DNA segments; include mature RBC lack of nucleus and ~120 days lifespan.
9. Match organelles to functions: ribosomes (protein synthesis), rough ER vs smooth ER, Golgi apparatus (modifies/sorts/packages), and lysosomes (enzymes break down waste/damaged parts).
10. Describe mitochondrion structure (outer smooth porous membrane; inner folded cristae) and how ATP is produced during cellular respiration.
11. Describe chloroplast structure and roles (stroma, chlorophyll absorbing light, photosynthesis and sugar storage) and classify plastids by pigments/stored material (chromoplasts, leucoplasts).
12. Contrast mitosis and meiosis: number of divisions, daughter cells, chromosome change, where they occur, and consequences of errors; then state the Cell Theory points (1838/1839/1855 work) and the statement that cancer is uncontrolled division.