⚑ Panic box

Cell membrane — selectively permeable; water crosses by osmosis

Mitosis — 2 genetically identical cells; chromosome number unchanged

Meiosis — 4 cells after 2 divisions; each has half the chromosomes

Prokaryote vs eukaryote — no membrane-bound nucleus vs a true nucleus

Cell theory — cells come from pre-existing cells

1. Seeing and measuring cells

• Human-eye resolution limit — about $0.1\ \text{mm}$; closer points may appear as one
• Unit conversion — $1\ \text{mm} = 1000\ \mu\text{m}$
• Estimated cell size — $\dfrac{\text{field-of-view diameter}}{\text{cells across its diameter}}$
• Total microscope magnification — eyepiece magnification $\times$ objective magnification
• Microscope image quality — resolution = clarity · contrast = brightness difference · magnification = apparent enlargement
• Robert Hooke (1665) — observed cork compartments with a microscope; named them “cells”
• Electron microscope — electron beam; reveals finer cell details at nanometre scale

2. Cell boundaries and water movement

Fluid-mosaic membrane — lipid bilayer · water-attracting heads face out · water-repelling tails face in · proteins embedded · molecules move within the membrane

• Diffusion — net particle movement from higher to lower concentration; no membrane required
• Osmosis — water diffusion through a selectively permeable membrane, from dilute (more water) to concentrated (less water) solution

• Plant cell in concentrated solution → inner contents shrink away from wall; wall retains the cell’s outer shape
• Animal cell in concentrated solution → shrinks; no rigid wall
• Root cells take up soil water by osmosis · low vacuole water → plant cells lose firmness → wilting

3. Cell types and organelles

• Shared basics — cell membrane · cytoplasm · genetic material
• Cytoplasm — jelly-like interior; organelles and many cell activities
• Cytoskeleton — eukaryotic cell fibre network; support, shape, movement and internal transport
• Cell inclusions — stored materials, e.g. starch or crystals in some plant cells
• Nucleoid — prokaryotic region containing DNA, not enclosed by a membrane

Plastids in plants

• Chloroplast — chlorophyll absorbs light for photosynthesis; sugars stored in stroma
• Chromoplast — coloured pigments, often yellow / orange / red; colour attracts pollinators or fruit-eating animals
• Leucoplast — colourless food storage; may store starch, oils or proteins
• Mitochondria and plastids — own DNA and ribosomes; can make some of their proteins
• Plant cell wall — cellulose · animal cells — no cell wall

Protein / lipid route — ER synthesis → Golgi modification and packaging → vesicles → cell membrane or secretion

4. Cell division and cell culture

• Cell division — new cells form from pre-existing cells; supports growth, tissue repair and reproduction

• Fertilisation — two gametes combine → chromosome number is restored
• Cell cycle — controlled, orderly process of eukaryotic cell division
• Cell culture — cells grow outside the body in nutrient medium; maintain suitable temperature, pH and moisture under sterile conditions
• Cell culture uses — research and production of biochemicals, food, medicines and vaccines
• Division errors — faulty mitosis → uncontrolled division / tumours · faulty meiosis → genetic disorders, pregnancy loss or reduced fertility

5. Cell theory and growth control

• Classical cell theory — all organisms consist of one or more cells · cell is the basic structural and functional unit · cells arise from pre-existing cells
• Contact inhibition — many animal cells stop dividing when they contact neighbouring cells
• Cancer cells — may lose this control → uncontrolled division → tumours; malignant tumours can invade nearby tissues and spread
• Programmed cell death — genetically controlled cell removal; supports normal development and cell quality control
• Cell lifespan — cells perform specialised work, then die and are replaced; mature human red blood cells lack a nucleus and live about $120$ days
• Early-life clue — hot-spring thermophiles are unicellular heat-loving bacteria; mineral deposits may have protected early organic molecules and helped form a first membrane (proposed explanation)

6. Traps

• Osmosis is movement of any particle → water crosses a selectively permeable membrane
• Water moves from concentrated to dilute solution → net water movement is from more water / dilute to less water / concentrated
• Cell wall is selectively permeable → the cell membrane is selectively permeable; wall is permeable
• All cells have a nucleus → prokaryotes lack a membrane-bound nucleus; mature human red blood cells lack one
• RER and SER both have ribosomes → RER has ribosomes; SER does not
• Mitosis makes gametes / halves chromosomes → meiosis makes gametes and halves chromosome number
• Meiosis is one division → parent cell divides twice, producing four cells
• All plastids are green → chloroplasts are green; chromoplasts are coloured; leucoplasts lack pigment
• Mitochondria make food → mitochondria release energy from food; chloroplasts capture light for food synthesis
• Plant cells show contact inhibition like many animal cells → plant growth follows a different pattern because of rigid cell walls