Module 2 – Organisation of Living Things Comprehensive Notes

Cellular Arrangement of Organisms

• Unicellular organisms

  • One cell only – oldest living forms.
  • Cell performs all life functions: nutrient uptake, gas exchange, waste removal, reproduction.
  • May live in groups but survive alone.
  • Mostly prokaryotic; some eukaryotic.
  • Microscopic; size limited by surface-area-to-volume ratio (SA:V).
  • Short lifespan because a single cell bears entire energetic load.
  • Mostly asexual (clonal) reproduction; whole cell = whole organism.

• Colonial organisms

  • Many eukaryotic cells; intermediate level of organisation.
  • Form colonies that confer survival advantages.
  • Facultative colonies – independent individuals aggregate for social benefit (e.g. honey-bees).
  • Obligate colonies – zooids are morphologically/physiologically specialised and inter-dependent (e.g. Portuguese man-o’-war).
  • Functions divided among zooids; macroscopic size; longer lifespan as workload shared.
  • Predominantly asexual clonal reproduction; some sexual stages carried out by special zooids.

• Multicellular organisms

  • All eukaryotic; enormous diversity.
  • Many specialised cells with identical genomic DNA (except gametes) but differential gene expression.
  • Functions executed at cellular, tissue, organ and system levels.
  • Macroscopic; increased cell number allows larger body size.
  • Workload divided → efficient metabolism, long lifespan.
  • Mostly sexual reproduction; only specialised gamete-producing cells divide for reproduction.

Advantages and Disadvantages of Multicellularity

• Advantages

  • Energy–efficient specialisation – cells avoid wasting energy performing all tasks.
  • Longer lifespan & resilience; cell death ≠ organism death.
  • Sexual reproduction & recombination ↑ genetic diversity → adaptability.
  • Larger size ⇒ mobility, predator avoidance, complex behaviours.
  • Multiple systems buffer short-term environmental change.
  • Capacity for diverse functions.

• Disadvantages

  • Larger energy demand for maintenance & reproduction.
  • Cellular inter-dependence – individual cells lose autonomy.
  • Mate-finding / gamete production costly.
  • Longer generation times slow evolutionary rate.

Levels of Organisation in Multicellular Organisms

• Hierarchy: Organelles → Specialised Cells → Tissues → Organs → Systems → Organism.
• Simple multicellular organisms
  • Organised only at cellular level (no true tissues); thin body permits diffusion; high regenerative capacity (e.g. sponges).
• Complex plants
  • Organs: roots, stems, leaves, flowers, fruits.
  • Two main systems
  • Root system (support, water & mineral absorption/storage).
  • Shoot system – vegetative (leaves, stems) & reproductive (flowers, fruits).
• Complex animals
  • Specialised cells: neurons, muscle fibres, RBCs, WBCs, epithelial, sperm etc.
  • Four primary tissues: epithelial, connective (bone, blood, loose), muscle (skeletal, cardiac, smooth), nervous.
  • Organs (eye, skin, heart …) integrate multiple tissues.
  • 11 major organ systems: respiratory, circulatory, digestive, excretory, immune, nervous, endocrine, reproductive, muscular, skeletal, integumentary.

Cell Differentiation & Specialisation

• All somatic cells carry same genome; stem cells differentiate via selective gene expression.
• Animals
  • Fertilisation → zygote → cleavage stages → blastocyst.
  • Embryonic stem cells of blastocyst differentiate into three germ layers:
  • Endoderm (internal linings, liver, pancreas …).
  • Mesoderm (muscle, bone, blood …).
  • Ectoderm (skin, nervous system …).
• Plants
  • Meristematic tissue (apical meristems in shoots/roots) contains unspecialised cells that differentiate into all plant tissues.
• Specialisation increases efficiency of function (division of labour).

Autotroph & Heterotroph Requirements

• Autotrophs – perform carbon fixation, converting inorganic $CO<em>2$ and $H</em>2O$ into organic molecules.

  • Photoautotrophs – use sunlight (majority; plants, algae, cyanobacteria).
  • Chemoautotrophs – use energy from inorganic chemical reactions (e.g. $NH<em>4^+ \rightarrow NO</em>2^-$ oxidation); all are prokaryotic extremophiles (e.g. methanogens $CO<em>2 + 4H</em>2 \rightarrow CH<em>4 + 2H</em>2O$; poisoned by $O_2$).

• Heterotrophs – cannot fix carbon; depend on organic compounds from other organisms.

  • Photoheterotrophs – capture light for ATP but need organic carbon.
  • Chemoheterotrophs – obtain both energy and carbon from organic molecules via cellular respiration.
  • Herbivores, carnivores, omnivores, saprotrophs, parasites.

Gas Exchange & Photosynthesis in Autotrophs

• Stomata (plural) – pores in lower epidermis surrounded by guard cells; regulate $CO<em>2$ uptake, $O</em>2$ release and transpiration.
  • Guard cells turgid (high $K^+$, water influx) → pore opens.
  • Guard cells flaccid (water loss) → pore closes.
• Chloroplast structure – outer & inner envelope, stroma (fluid), thylakoid membranes forming grana.
• Photosynthesis overall equation:
  $6CO<em>2 + 6H</em>2O \xrightarrow[chlorophyll]{light} C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Inputs & limiting factors
  • $CO_2$ – controlled by number/open state of stomata.
  • Water – plentiful but stomatal closure under drought lowers rate.
  • Light – rate rises with intensity to saturation plateau.
• Outputs
  • $O_2$ production used to gauge photosynthetic rate.
  • Glucose – converted to starch, cellulose; biomass/starch assays estimate rate.
• Leaf anatomy
  • Cuticle → upper epidermis (transparent, no chloroplasts) → palisade mesophyll (dense chloroplasts, photosynthesis) → spongy mesophyll (air spaces, gas diffusion) → lower epidermis with stomata → cuticle.
  • Veins: xylem (water) & phloem (sugars).
• Vascular tissues
  • Xylem: water/inorganic ions root→shoot.
  • Phloem: sucrose/amino acids source→sink.
• Root hairs increase SA for absorption; three root layers: epidermis, cortex (parenchyma), vascular cylinder.

Transpiration-Cohesion-Tension Mechanism (Xylem Flow)

• Water evaporates from mesophyll → tension pulls column upward.
• Cohesion (water-water H-bonds) maintains continuous column; adhesion (water-xylem) counters gravity.
• Water potential gradient: soil (≈$-0.2\,MPa$) → root → stem (≈$-0.6\,MPa$) → leaf tip (≈$-1.5\,MPa$) → atmosphere (≈$\le -100\,MPa$).
• Factors ↑ transpiration: daytime, high temperature, low humidity, wind.

Translocation of Sugars (Phloem)

• Source–sink model (pressure-flow hypothesis)
  1. Companion cells actively load sucrose into sieve-tube elements at source (requires ATP).
  2. Water enters by osmosis from adjacent xylem, raising hydrostatic pressure.
  3. Bulk flow toward sinks where sucrose unloaded (active/passive) → water potential rises.
  4. Water re-enters xylem; pressure gradient maintained.
  • Phloem sap ≈90 % sucrose.

Nutrient Acquisition & Digestive Systems in Heterotrophs

• Macromolecule needs

  • Carbohydrates – quick ATP (glycogen storage in animals).
  • Lipids – dense energy, membranes, hormones, vitamins.
  • Amino acids – protein synthesis; 9 essential AA listed (isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, histidine) with food sources.
  • Vitamins (13 needed) – organic cofactors; Minerals – inorganic ions (structural & enzymatic roles).

• Digestion definitions

  • Physical/mechanical: chewing, churning, peristalsis, bile emulsification.
  • Chemical: enzymatic hydrolysis – amylases, proteases, lipases from gut wall, salivary glands, pancreas.
  • Extracellular vs intracellular digestion (e.g. sea stars vs protozoans).

• Mammalian dietary categories

  • Carnivores: meat only; short, simple gut; small caecum.
  • Herbivores: plant matter; long gut, large caecum; cellulose digestion via cellulase-producing microbes; foregut or hindgut fermenters.
  • Omnivores: mixed diet; versatile system (humans, bears).

• Human digestive tract (mouth → anus)

  • Mouth (pH 6-8) – teeth, salivary amylase.
  • Epiglottis guards airway.
  • Oesophagus – peristalsis.
  • Stomach (pH 1-3) – proteases, acid, churning → chyme.
  • Liver – bile production, metabolism, detox, glycogen storage.
  • Gallbladder – bile storage.
  • Pancreas – enzymes, insulin/glucagon, $NaHCO_3$ neutralisation.
  • Small intestine – major absorption; villi & microvilli provide vast SA; lipid absorption via lacteals; 90-95 % water reabsorbed.
  • Large intestine – water/vitamin absorption; faeces formation.
  • Coeliac disease – autoimmune destruction of villi by gluten.

• Energy storage

  • Glycogen (~300 g in humans) + fats (primary reserve).
  • Fat advantages: $+25\%$ more ATP/C, $\approx 2\times$ energy density compared to carbs, no water binding, virtually unlimited capacity.

• Basal Metabolic Rate (BMR)

  • $50{-}70\%$ of daily energy expenditure; influenced by composition, sex, age, genetics.

Gas Exchange in Heterotrophs (Animals)

• Basic principles

  • Cells need $O<em>2$ for aerobic respiration; produce $CO</em>2$ (acidic in solution) to be excreted.
  • Diffusion across membranes down concentration gradients.

• Human respiratory system

  • Nasal cavity → pharynx → trachea → bronchi → bronchioles → alveoli.
  • Trachea/bronchi lined with ciliated, mucus-secreting epithelium.
  • Alveoli: 30-70 $m^2$ total area; single-cell epithelium rich in capillaries → rapid gas diffusion.
  • Ventilation (negative-pressure pump): diaphragm contracts down & ribs lift (inhalation); recoil (exhalation). Forced exhalation active.
  • Lung volumes: tidal volume (rest ≈500 mL), vital capacity, residual volume.

• Diseases

  • Asthma – hypersensitive bronchioles swell & constrict with mucus.
  • Emphysema – destruction of alveolar walls (smoking/age) ↓ surface area.
  • Pneumonia – infection fills alveoli with fluid/WBCs, thickens diffusion path.

Transport Systems in Plants

• Xylem components
  • Vessels: dead, lignified, end-to-end tubes; large lumens.
  • Tracheids: dead, overlapping; water moves laterally via pits.
• Root uptake pathways
  • Extracellular (apoplastic) – along cell walls/intercellular spaces.
  • Cytoplasmic (symplastic) – through plasmodesmata; must cross membranes; blocked by Casparian strip forcing entry into symplast before xylem.
• Phloem components
  • Sieve-tube elements (living, no nucleus, no lignin), companion cells (metabolic support), parenchyma, sclerenchyma.

Transport Systems in Animals

• Mammalian cardiovascular system – closed; pulmonary + systemic circuits.
  • Heart: 4 chambers – right atrium/ventricle (deoxygenated), left atrium/ventricle (oxygenated).
  • Vessels: arteries (away, thick muscular walls), arterioles, capillaries (one-cell-thick), venules, veins (toward, thin walls, valves).
• Capillary exchange
  • Hydrostatic (blood) pressure vs osmotic pressure; net filtration of fluid (~15 %) collected by lymphatic system, remainder reabsorbed.
• Blood composition
  • Plasma (92 % water, ions, gases, proteins, hormones, nutrients, wastes).
  • RBCs/erythrocytes (40 %; biconcave, no nucleus, Hb, 120-day lifespan).
  • WBCs/leukocytes (immune; fewer, larger).
  • Platelets (cell fragments; clotting).

Open vs Closed Circulatory Systems

• Open systems (arthropods, insects)

  • Heart pumps haemolymph into body sinuses; no distinct vessels; direct cell contact; low pressure, slow O$_2$ delivery.
  • Insects breathe via tracheal system (spiracles → tracheae → tracheoles); circulation not used for gas exchange.

• Closed systems

  • Blood confined to vessels; higher pressure; efficient O$_2$ transport.
  • Single circuit (fish): heart → gills (O$_2$ uptake) → body → heart.
  • Double circuit (birds, mammals): pulmonary (heart–lungs–heart) + systemic (heart–body–heart); maintains high pressure in systemic circuit.

Gas Transport – Haemoglobin & Bohr Effect

• Hb + $O_2 \rightleftharpoons$ oxyhaemoglobin; reversible binding.
  • Each Hb carries 4 O$_2$; Fe²⁺ in haem group gives red colour.
• CO$_2$ transport: 7 % dissolved, 23 % carbamino-Hb, 70 % as bicarbonate in RBCs.
• Bohr effect – Hb affinity for O$2$ decreases with ↑ CO$2$ / ↓ pH; curve shifts right → O$2$ released at working tissues; high affinity (curve left) in lungs where CO$2$ low.

Disorders & Malfunctions

• Cardiovascular – Marfan syndrome (connective-tissue defect), arteriosclerosis/atherosclerosis (arterial hardening/plaque buildup).
• Lymphatic – deep-vein thrombosis (clots block venous return; lymphatics help fluid balance).
• Respiratory – asthma, emphysema, pneumonia (see above).

Study & Revision Tasks (From Transcript)

• Atomi videos & quizzes: cellular organisation, multicellularity, plant & animal structure, transport processes, digestive, respiratory, circulatory systems.
• Worksheets: 2.1–2.7, skills book pages.
• Textbook reviews: Ch 4 (p 193), 4.2 (p 207), 4.3 (p 213), 5.1 (p 227), 5.2 (p 238), 5.3 (p 252), 5.4 (p 259), 6.1 (p 275), 6.2 (p 295), Module 2 review (p 300-305).
• Draw/label diagrams: leaf cross-section with tissues, source-to-sink model, heart anatomy, single vs double circulation, plant vascular tissues.
• Create flash cards for terminology, equations, structures, functions.