Unit 1 – Cell Compounds & Homeostasis Comprehensive Notes

Homeostasis and Feedback Mechanisms

• Core idea: Living systems expend energy to keep their internal environment within narrow limits (homeostasis) despite external change.
• Single-celled vs. Multicellular
  • Unicellular life (e.g., euglena) survives as long as the environment remains within tolerable limits and nutrients are available.
  • Humans are composed of \text{~10^{14}} cells bathed in extracellular fluid (ECF).
  • Cells co-operate to keep ECF variables (temperature, pH, osmotic pressure, nutrient & gas levels) nearly constant.
• 4 universal components of any feedback loop
  1. Stimulus / Change – any deviation in a monitored variable (temperature, pressure, solute concentration…).
  2. Receptor / Sensor – detects the stimulus and relays input to the control centre.
  3. Control Centre (Integrator) – possesses a set point (e.g., $37^{\circ}C$ for core body temperature); compares actual value with set point; sends commands.
  4. Effector – muscle, gland or organ that carries out corrective action and restores the variable toward set point.

Negative feedback

• Definition: Response counteracts the original change → system stability.
• Blood-pressure example
  • Baroreceptors in carotid/aortic walls detect P.
  • Signals travel via glossopharyngeal nerve to cardiovascular centre in medulla.
  • If $P↓$ → medulla ↑ heart rate & contractility ⇒ ↑ cardiac output ⇒ $P↑$ toward set point; stimulus vanishes.
  • If $P↑$ → opposite adjustments.
• Other instances: thermoregulation, blood glucose control, osmoregulation, blood $p\text{O}<em>2$ / $p\text{CO}</em>2$, calcium balance.

Positive feedback

• Definition: Response amplifies the deviation; drives the system out of homeostatic range until an external stop signal removes stimulus.
• Only course-required example: childbirth.
  • Fetal head stretches cervix → stretch receptors signal hypothalamus → posterior pituitary releases oxytocin → stronger uterine contractions → more stretch… loop continues until delivery, after which oxytocin falls and stability returns.

Applied activities

• Patient-X glucose graph – data remain 70-110 mg·dL⁻¹; demonstrates intact negative feedback for glycemia.
• Hyperthermia case (Joy & Mary)
  • Homeostasis explanation must reference: heat receptors in skin & core; hypothalamic set point; effectors (sweat glands, vasodilation).
  • Elderly w/ poor circulation have compromised vasodilation ⇒ less heat loss.
  • Evaporative cooling: water absorbs heat (endothermic) during phase change; fan ↑ convection + evaporation.
  • Avoid shivering/vasoconstriction because these raise thermogenesis and reduce peripheral heat loss.

Water: Molecular Structure and Properties

• Bonding inside one H₂O: polar covalent bonds between O and each H (e⁻ shared unequally).
• Intermolecular bonding: hydrogen bonds – attraction between $\delta^{-}$ O of one molecule and $\delta^{+}$ H of a neighbour; each H₂O can form up to 4 H-bonds.

Emergent properties (relevant to biology)

1. Excellent solvent
   • “Universal solvent” for ionic & polar solutes; forms hydration shells.
   • Blood plasma, cytosol ≈ 90 % water – allows transport & biochemical reactions.
   • Solution definitions: solute = lesser quantity; solvent = greater. Examples given (baking soda in water, acetone removing nail polish, antifreeze mix, metal in acid).
2. High specific heat / heat of vaporisation
   • c_{water} = 1\;\text{cal·g}^{-1}\,^{\circ}C^{-1}; buffers body & climate.
   • Evaporative cooling: high-energy molecules leave → average kinetic energy of remaining water ↓.
3. Density anomaly
   • Ice < liquid water density ⇒ floats, forming insulating layer; aquatic life survives winter.
4. Transparency – lets light reach photosynthetic organisms in water.
5. Cohesion & Adhesion
   • Cohesion ⇒ surface tension (water strider, alveolar lining).
   • Adhesion ⇒ capillary rise; alveolar surfactant reduces excessive tension.
   • Everyday examples: water beading on waxed car (cohesion); meniscus in glass cylinder (adhesion).

Functions of water in humans

• Medium for metabolic reactions.
• Transport & lubricant (blood, lymph, synovial fluid, mucus, CSF).
• Temperature regulation via perspiration & blood flow.
• Reactant/product in hydrolysis & condensation reactions.

pH, Acids, Bases, Buffers

• pH definition: $\text{pH} = -\log[H^{+}]$ ; scale 0-14 (logarithmic).
  • [H^{+}] > [OH^{-}] → acidic (pH < 7).
  • $[H^{+}] = [OH^{-}]$ → neutral (pH = 7).
  • $[H^{+}] < [OH^{-}]$ → basic (pH > 7).
• Acid: proton donor (e.g., $HCl \rightarrow H^{+}+Cl^{-}$).
• Base: proton acceptor / OH⁻ donor (e.g., $NaOH \rightarrow Na^{+}+OH^{-}$).
• Every pH unit = 10-fold [H⁺] change; pH 4 is $10^{2}$ × more acidic than pH 6.

Buffers

• Mixtures that minimise pH change by reversible binding of H⁺/OH⁻.
• Composed of weak acid/conjugate base pairs (e.g., $H<em>2CO</em>3/HCO_3^{-}$ in blood).
• Critical because enzymes/proteins denature outside narrow pH ranges (blood ≈ 7.35–7.45).

Biological Molecules Overview

• Carbon versatility: 4 valence electrons ⇒ can form up to 4 covalent bonds, chains & rings.
• Terminology
  • Monomer: single subunit.
  • Polymer: large molecule of repeating monomers.
  • Dehydration synthesis (condensation): links monomers; removes $H^{+}+OH^{-}$ → $H_2O$ (energy required).
  • Hydrolysis: breaks polymers by adding water (energy released; catabolic).
• 4 classes studied: Carbohydrates, Lipids, Proteins, Nucleic Acids.

Carbohydrates

• Elements & general formula: $C, H, O$ with H:O = 2:1; empirical $CH_2O$.

Monosaccharides (C₅ or C₆)

• Hexoses (C₆H₁₂O₆):
  • Glucose (blood sugar).
  • Fructose (fruit sugar; sweetest).
  • Galactose (part of lactose).
• Pentoses: ribose (RNA) & deoxyribose (DNA; 1 O less).
• Isomers share formula but differ in arrangement.

Disaccharides (C₁₂H₂₂O₁₁) – formed by glycosidic linkage

1. Maltose = glucose + glucose.
2. Sucrose = glucose + fructose.
3. Lactose = glucose + galactose.

Polysaccharides ((C6H{10}O5)n)

• Starch (plant glucose storage; moderately branched).
• Glycogen (animal storage; highly branched; liver & muscle).
• Cellulose (plant cell walls; β-1,4 linkages; indigestible fibre “roughage”).

Functions

• Rapid energy source via respiration ($C<em>6H</em>{12}O<em>6 + 6O</em>2 \rightarrow 6CO<em>2 + 6H</em>2O + ATP$).
• Structural (cell walls).
• Energy storage (starch, glycogen).

Lipids

• Include: neutral fats (triglycerides), phospholipids, steroids (plus oils, waxes, soaps).
• Elements $C, H, O$ (H:O > 2:1) ⇒ non-polar, hydrophobic.

Fatty acids

• Long hydrocarbon chain + terminal carboxyl (\ce{-COOH}).
• Saturated: only C–C single bonds; straight → pack tightly (solid fat).
• Unsaturated: ≥1 C=C double bond; kinked → liquid oils.

Neutral fats (Triglycerides)

• 1 glycerol + 3 fatty acids (mono- & di-glycerides have 1 or 2).
• Major energy reserve (9 kcal·g⁻¹); insulation & organ protection.

Phospholipids

• Glycerol + 2 fatty acids + phosphate-N head.
• Amphipathic: hydrophilic head / hydrophobic tails.
• Form phospholipid bilayer of cell membranes; heads face ECF & cytosol, tails interior.

Steroids (sterols)

• 4 fused carbon rings (e.g., cholesterol, testosterone, estrogen).
• Cholesterol stabilises plasma membranes & is precursor for steroid hormones; excess → atherosclerosis ((\uparrow) BP).

Proteins

• Elements $C, H, O, N$ (+ S, P, Fe occasionally).
• Monomer: amino acid (central C bonded to H, amino \ce{-NH_2}, carboxyl \ce{-COOH}, variable R-group).
• Peptide bond: \ce{-COOH + NH2 \rightarrow -CONH- + H2O} (dehydration synthesis).

Structural hierarchy

1. Primary – linear a.a. sequence (covalent peptide bonds).
2. Secondary – α-helix or β-sheet via H-bonds between backbone groups.
3. Tertiary – 3-D folding; interactions among R-groups: H-bonds, ionic, hydrophobic interactions, disulfide bridges.
4. Quaternary – assembly of >1 polypeptide (e.g., hemoglobin = 4 subunits + heme).

Denaturation

• Loss of native shape (and active site) due to heat, pH change, heavy metals, alcohol, radiation… → loss of function; sometimes irreversible.

Functional categories

• Enzymes (biological catalysts) – lower activation energy; e.g.,
  • Maltase: maltose → 2 glucose.
  • Carbonic anhydrase: $CO<em>2 + H</em>2O \leftrightarrow H<em>2CO</em>3$ (blood gas transport).
• Transport proteins – hemoglobin, membrane channels.
• Immune proteins – antibodies.
• Structural proteins – keratin (hair, nails), collagen (connective tissue), actin & myosin (muscle contraction).

Nucleic Acids and ATP

• Monomer: nucleotide = phosphate + pentose sugar + nitrogenous base.
• Types: DNA, RNA, ATP.

DNA/RNA (genetic information)

• DNA stores hereditary info; RNA involved in protein synthesis.

ATP – Adenosine Triphosphate

• Adenosine (adenine + ribose) + 3 phosphates.
• High-energy phosphoanhydride bonds ((~) symbol).
• Energy currency:
  \text{ATP} + H2O \xrightarrow{ATPase} \text{ADP} + Pi + \text{Energy} \; (\approx 7 \text{ kcal·mol}^{-1})
• ADP + $P_i$ can be re-phosphorylated (e.g., by cellular respiration).
• Analogy: fats = savings bonds; glycogen = bank account; glucose = piggy bank; ATP = pocket cash.