Cell Biology and Membrane Dynamics

Limit of resolution: Human eyes can distinguish two points as separate if they are at least $0.1\,mm$ apart when viewed from $25\,cm$ .
Robert Hooke: In $1665$ , he observed box-like compartments in cork using a self-designed microscope ( $200-300 \times$ magnification) and named them "cells."
Magnification: Convex lenses or combinations (objective and eyepiece) are used to make objects appear larger.

Thickness: Approximately $7$ to $10\,nm$ ( $1\,nm = 0.000001\,mm$ ).
Composition: Made of lipids (fats) and proteins.
Fluid-Mosaic Model: Molecules are arranged like tiles in a mosaic and can move sideways, flip, or rotate.
Lipid Bilayer: Consists of two layers with water-attracting heads facing outwards and water-repelling tails facing inwards.

Diffusion: Net movement of particles from higher to lower concentration due to a concentration gradient.
Osmosis: The diffusion of water across a selectively permeable membrane; plant roots use this to absorb water from soil.
Solution Types:
- Isotonic: Solute concentration of extracellular medium $=$ solute concentration of intracellular medium.
- Hypotonic: Solute concentration of extracellular medium $<$ solute concentration of intracellular medium (water enters cell).
- Hypertonic: Solute concentration of extracellular medium $>$ solute concentration of intracellular medium (water leaves cell).

Presence: Found in plants, fungi, and bacteria, but absent in animal cells (e.g., human cheek cells).
Function: Provides a rigid structure to withstand environmental stresses like wind and rain; helps plants stay upright.
Permeability: Unlike the selectively permeable membrane, the cell wall is permeable to water and dissolved minerals.
Observations: In a $20\%$ sugar solution, plant cells (like Rhoeo or onion peel) exhibit shrinking of inner content while the cell wall maintains the overall shape.

What do you think is the necessity of the cell wall in these cells? It provides rigidity and protection against environmental stress, maintaining the plant's shape even when cells lose water.
What do you observe when comparing human cheek cells and onion peel cells? Onion peel (or Rhoeo leaf) cells are box-shaped and regularly arranged, whereas human cheek cells are irregularly arranged.
What if mung bean seeds are kept in a concentrated solution after soaking in water for $12$ hours? What will happen to them? (This is presented as an inquiry into the effects of osmosis in hypertonic environments).