Digestive Absorption & Cell Division – Key Points

Digestive System & Absorption

• Main function: break down food, absorb nutrients, eliminate undigested waste.

• Key absorption site: small intestine (duodenum, jejunum, ileum).

• Structural adaptations for maximum absorption

  • Circular folds (plicae circulares)

  • Villi: finger-like projections (increase area $\sim 3\times$)

  • Microvilli on epithelial cells (“brush border” adds another $\sim 20\times$)

  • Net result: >200\,\text{m}^2 total surface area.

• Process

  • Digested nutrients diffuse/actively transport through epithelial cells.

  • Enter blood capillaries (monosaccharides, amino acids, water-soluble vitamins) → portal vein → liver.

  • Enter lacteals (fatty acids, glycerol, fat-soluble vitamins) → lymph → bloodstream.

Interaction with Other Body Systems

• Circulatory: transports absorbed nutrients & $O<em>2$ to cells; carries $CO</em>2$ and wastes away.

• Respiratory: supplies $O<em>2$ needed for cellular respiration; removes $CO</em>2$ produced from nutrient oxidation.

• Excretory: kidneys filter metabolic wastes from blood formed after nutrient use.

Surface Area Analogy Activity (Paper Towel / Sponge)

• Flat sheet < crumpled sheet < cotton ball in water absorption.

• More folds/projections ⇒ greater surface area ⇒ more water/nutrient uptake.

• Smooth small intestine would drastically reduce nutrient absorption efficiency.

Cell Division Cycle: Mitosis vs Meiosis

• Purpose

  • Mitosis: growth, repair, asexual reproduction.

  • Meiosis: gamete formation, genetic diversity.

• Chromosome number

  • Mitosis: $2n \rightarrow 2n$ (diploid conserved).

  • Meiosis: $2n \rightarrow n$ (haploid gametes).

• Divisions

  • Mitosis: $1$ division.

  • Meiosis: $2$ sequential divisions (I & II).

• Daughter cells

  • Mitosis: $2$ genetically identical cells.

  • Meiosis: $4$ genetically unique cells.

• Crossing-over & independent assortment occur only in meiosis I.

Quick Recall

• Small-intestine adaptations raise surface area $\Rightarrow$ efficient nutrient uptake.

• Absorbed nutrients enter blood/lymph, then circulate to all cells.

• Healthy interaction of digestive, circulatory, respiratory, and excretory systems sustains energy production.

• Remember: Mitosis = maintenance ($2 \rightarrow 2$); Meiosis = gametes ($2 \rightarrow n$, diversity).