Cellular Organelles

The Fluid Mosaic Model was proposed by Seymour Jonathan Singer and Garth L. Nicolson (1972).

Ribosomes are mainly composed of rRNA and proteins.

The tonoplast is the membrane that covers the vacuole.

Transport across the plasma membrane occurs by passive transport, facilitated movement, and active transport.

- Passive transport: Molecules move along the concentration gradient from higher to lower concentration without energy expenditure. It occurs by diffusion and osmosis.
- Facilitated movement: Molecules such as glucose, ions and amino acids cannot cross by simple diffusion. They move with the help of carrier proteins and channel proteins.
- Carrier proteins help transport substances like glucose.
- Channel proteins form channels for movement of substances, such as ion channels and aquaporins for water.
- Facilitated diffusion is specific and uses membrane proteins, but no ATP.
- Active transport: Molecules move against the concentration gradient from lower to higher concentration and require ATP energy. Example: **Na$^+$-K$^+$ pump.
- Some active transports are ATP-independent and use the energy of an existing ion gradient; this is coupled transport.
- If two molecules move in the same direction, it is symport.
- If two molecules move in opposite directions, it is antiport.
- Movement of a single molecule by facilitated diffusion is called uniport.

A student should draw labelled diagrams for:
1. Facilitated transport of glucose through a carrier protein,
2. Transport through ion-gated channel, and
3. Na$^+$-K$^+$ pump** showing active transport.

The correct matching is:

- (a) Nucleolus → (v) rRNA synthesis
- (b) Mesosome → (vi) Membranous extensions of plasma membrane
- (c) Vacuoles → (vii) Storage and structural support
- (d) Cristae → (ii) Infoldings of inner mitochondrial membrane
- (e) Ribosomes → (iii) Protein synthesis
- (f) Thylakoid → (viii) Membranous sacs in chloroplast
- (g) Peroxisomes → (i) Alcohol detoxification
- (h) Cisternae → (iv) Disc shaped sacs in Golgi

The ratio of protein and lipids in membranes is important because it determines the structure and functioning of the membrane. Membranes are mainly made of phospholipids and proteins, and their proportions vary in different cells.

If the lipid concentration changes, the fluidity and function of the membrane change:
- A membrane with more lipids tends to be more fluid.
- Too much or too little lipid affects the membrane’s selective permeability, transport, and other functions.
- The membrane must remain in a proper quasifluid state so that lateral movement of lipids and proteins and normal cellular activities can occur.

The cell wall in prokaryotic cells is important because it:
- gives the cell rigidity and shape,
- provides protection from mechanical damage,
- prevents the cell from bursting due to osmotic pressure,
- helps in cell–cell interaction,
- provides mechanical strength and protection from infection.

In bacteria, it is a sturdy covalent shell of peptidoglycan cross-linked by peptides, and many antibiotics act by inhibiting this cross-linking, thereby stopping bacterial growth.

Eukaryotic organelles can be classified as follows:

### Single-membrane bound organelles
- Endoplasmic reticulum
- Golgi apparatus
- Lysosomes
- Vacuoles
- Peroxisomes / microbodies

### Double-membrane bound organelles
- Nucleus
- Mitochondria
- Plastids such as chloroplasts, chromoplasts, and leucoplasts

### Non-membrane bound organelles
- Ribosomes
- Centrioles / centrosome
- Cytoskeleton components such as microtubules, actin filaments, and intermediate filaments

The different types of vacuoles mentioned are lytic vacuoles, protein storage vacuoles (PSV), and other storage vacuoles.