Life Processes in Animals and Plants

Modes of nutrition

Punjab Textbook Chapter 11 organises nutrition into autotrophic (photosynthetic plants and chemosynthetic bacteria) and heterotrophic (holozoic animals, saprophytic fungi, parasitic, symbiotic). Holozoic nutrition has five steps — ingestion, digestion, absorption, assimilation, egestion — that the MDCAT loves to test as ordering MCQs. Saprophytes secrete enzymes externally and absorb the products (extracellular digestion); parasites such as Taenia live inside hosts and absorb pre-digested nutrients across their tegument.

The human digestive system

Salivary amylase (pH 6.8) starts starch digestion in the mouth, hydrolysing α-1,4 bonds to maltose. The stomach's pepsin (pH 2, activated from pepsinogen by HCl) cleaves proteins into peptides. The small intestine receives bile (which emulsifies fats but does not digest them) and pancreatic juice carrying trypsin, chymotrypsin, pancreatic amylase, and lipase, all working at pH 8. Final brush-border enzymes (maltase, sucrase, lactase, peptidases) finish hydrolysis at the villi, which absorb monosaccharides and amino acids into capillaries and fatty acids/glycerol into lacteals. The large intestine reabsorbs water and houses gut flora that synthesise vitamin K and some B-vitamins.

Photosynthesis and plant nutrition

Plants are autotrophs that convert light into chemical energy via 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, captured by chlorophylls a (blue ~ 430 nm and red ~ 660 nm absorption peaks) and accessory pigments. The light-dependent reactions on the thylakoid generate ATP and NADPH and split water (the source of O₂); the Calvin cycle in the stroma fixes CO₂ via Rubisco. Macronutrients required are N, P, K, Ca, Mg, S; micronutrients include Fe, Mn, Zn, Cu, B, Mo, Cl. Nitrogen deficiency yellows older leaves first (chlorosis); magnesium deficiency disrupts chlorophyll directly because Mg²⁺ sits at the centre of the porphyrin ring.

Gas exchange surfaces

Efficient gas exchange surfaces share four properties — large surface area, thin walls, moist, well ventilated and well perfused — first articulated by Krogh. In humans the alveolar epithelium (~ 70 m² total area, < 1 μm wall thickness) sits next to capillaries; O₂ diffuses down a partial-pressure gradient (alveolar pO₂ ~ 100 mm Hg, venous blood ~ 40 mm Hg). Insects use a tracheal system delivering air directly to tissues; fish use counter-current gill exchange that extracts ~ 80% of dissolved O₂; amphibians supplement lungs with cutaneous respiration. Plants use stomata (guard cells responding to K⁺ flux and turgor changes) for daytime gas exchange and lenticels in woody stems for night-time exchange.

Recurring MCQ traps

Bile contains no enzymes — it only emulsifies fats, lowering surface tension so lipase can act. Pepsin works at pH 2, while trypsin works at pH 8 — never confuse the two. The villi absorb most nutrients; the large intestine mainly reabsorbs water. Photosynthesis releases O₂ from H₂O (van Niel's isotope experiment), not from CO₂. Stomata open in the light when guard cells take up K⁺ and water and become turgid. The bow-shaped (kidney-bean) guard cell is a classic diagram MCQ. Always cross-reference Campbell Chapter 41 (animal nutrition) and Chapter 36 (plant resource acquisition) with the Punjab Textbook tables.

Disorders of nutrition and gas exchange

Deficiency diseases the MDCAT keeps testing: kwashiorkor (severe protein deficiency in children, oedematous belly, low albumin), marasmus (calorie + protein deficiency, wasting), rickets (vitamin D deficiency in children, bowed legs from impaired Ca²⁺ absorption), osteomalacia (the adult equivalent), beri-beri (thiamine B1, peripheral neuropathy), pellagra (niacin B3, the "3D" dermatitis-diarrhoea-dementia), pernicious anaemia (B12 absorption failure due to lack of intrinsic factor), and goitre (iodine deficiency, thyroid enlargement). Disorders of gas exchange include emphysema (alveolar wall destruction, mostly from cigarette smoke; reduced surface area), asthma (bronchoconstriction reversible by β2 agonists), and pulmonary fibrosis (thickened alveolar walls). Each of these directly degrades one of Krogh's four respiratory-surface properties (area, thickness, moisture, ventilation), which is exactly the framing past papers exploit.