Cellular Organelles — NCERT Solutions

Correct Option: (d) Singer and Nicolson

The Fluid Mosaic Model of the plasma membrane was proposed by S.J. Singer and G.L. Nicolson in 1972. According to this model, the membrane is a fluid phospholipid bilayer in which proteins are embedded (like a mosaic), and both lipids and proteins can move laterally. Robert Brown discovered the nucleus; Schleiden and Schwann proposed the Cell Theory; Virchow proposed 'Omnis cellula e cellula'.

Correct Option: (b) rRNA and proteins

Ribosomes are ribonucleoprotein particles composed of ribosomal RNA (rRNA) and proteins. They do not contain DNA or lipids. Each ribosome consists of two subunits (large and small), both made of rRNA molecules associated with specific ribosomal proteins. They are the sites of protein synthesis in the cell.

Correct Option: (c) a membrane covering the vacuoles

The tonoplast is the single membrane that surrounds the central vacuole in plant cells. It regulates the movement of ions and molecules between the vacuole and the cytoplasm, thereby maintaining turgor pressure and playing a key role in osmoregulation.

Transport Across the Plasma Membrane

The plasma membrane is selectively permeable and regulates the movement of substances into and out of the cell. The major mechanisms are:

I. Passive Transport (No energy/ATP required; movement along concentration gradient)

(a) Simple Diffusion:

• Small, non-polar molecules (O₂, CO₂, ethanol) move directly through the lipid bilayer from a region of higher concentration to lower concentration.
• Formula for net flux: $J = -D \frac{d[C]}{dx}$ (Fick's Law)

(b) Facilitated Diffusion:

• Polar or charged molecules (glucose, amino acids, ions) cannot cross the lipid bilayer directly.
• They move through specific channel proteins (form pores) or carrier proteins (undergo conformational change).
• Movement is still along the concentration gradient; no ATP is needed.
• Example: Glucose transport into red blood cells via GLUT transporters.

(c) Osmosis:

• Movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).
• Water potential: $\Psi = \Psi_s + \Psi_p$

II. Active Transport (Requires energy/ATP; movement against concentration gradient)

• Substances move from lower to higher concentration using energy (ATP) and specific carrier proteins (pumps).
• Example: Sodium-Potassium pump (Na⁺/K⁺-ATPase) — pumps 3 Na⁺ out and 2 K⁺ into the cell per ATP molecule hydrolysed.

III. Bulk Transport (for large molecules/particles)

(a) Endocytosis: Cell engulfs external material by infolding of the plasma membrane.

• Phagocytosis ('cell eating'): Engulfment of solid particles (e.g., bacteria by macrophages).
• Pinocytosis ('cell drinking'): Engulfment of liquid droplets.
• Receptor-mediated endocytosis: Specific molecules bind to receptors on the membrane surface, triggering vesicle formation.

(b) Exocytosis: Vesicles fuse with the plasma membrane and release their contents outside the cell (e.g., secretion of hormones, neurotransmitters).

Summary Table:

(Note: Labelled diagrams should show the lipid bilayer with embedded proteins, arrows indicating direction of movement, and vesicle formation for bulk transport.)

Correct Matching:

Brief Justifications:

• Nucleolus: Site of rRNA transcription and ribosome assembly.
• Mesosome: Infoldings/extensions of the plasma membrane in prokaryotes; involved in cell wall formation and DNA segregation.
• Vacuoles: Store water, ions, nutrients; provide structural support (turgor).
• Cristae: Shelf-like infoldings of the inner mitochondrial membrane; increase surface area for oxidative phosphorylation.
• Ribosomes: Translate mRNA into proteins.
• Thylakoid: Flattened membranous sacs in chloroplasts; site of light reactions of photosynthesis.
• Peroxisomes: Contain oxidative enzymes; involved in detoxification of alcohol and other harmful substances.
• Cisternae: Flattened, disc-shaped membranous sacs stacked in the Golgi apparatus.

Significance of Protein : Lipid Ratio in Membranes:

The ratio of proteins to lipids in a membrane varies depending on the type of membrane and its function:

• Myelin sheath (electrically insulating): ~80% lipid, ~20% protein — high lipid content provides insulation.
• Plasma membrane: ~50% lipid, ~50% protein — balanced for selective permeability and signalling.
• Inner mitochondrial membrane: ~25% lipid, ~75% protein — high protein content due to numerous enzyme complexes (ETC, ATP synthase) needed for oxidative phosphorylation.

General Significance:

1. Proteins serve as channels, carriers, receptors, enzymes, and structural anchors. A higher protein content increases the membrane's functional capacity (transport, signalling, catalysis).
2. Lipids (mainly phospholipids and cholesterol) form the structural bilayer, determine fluidity, and act as a barrier to polar molecules.
3. The ratio thus determines the permeability, fluidity, and functional specialisation of the membrane.

Effect of Varying Lipid Concentration:

1. Increased lipid (especially cholesterol) concentration:

• Increases membrane rigidity and decreases fluidity at higher temperatures.
• Reduces permeability to small polar molecules.
• Cholesterol acts as a 'fluidity buffer' — prevents membranes from becoming too rigid at low temperatures or too fluid at high temperatures.

2. Decreased lipid concentration:

• Membrane becomes more fluid and permeable.
• Structural integrity is compromised.
• Transport of non-polar molecules may increase, but selective permeability is lost.
• The membrane may become leaky, disrupting ion gradients essential for cellular functions (e.g., nerve impulse transmission, ATP synthesis).

Conclusion: The protein-to-lipid ratio is a key determinant of membrane identity and function. Any alteration in lipid concentration directly impacts membrane fluidity, permeability, and the ability to carry out specialised functions.

Importance of Cell Wall in Prokaryotic Cells:

Prokaryotic cells (bacteria) possess a rigid cell wall, primarily composed of peptidoglycan (murein) — a polymer of sugars (NAG and NAM) cross-linked by short peptide chains.

Functions:

1. Mechanical Protection: The cell wall provides a rigid framework that protects the cell from physical damage and mechanical stress.

2. Osmotic Protection: It prevents the cell from bursting (lysis) due to osmotic pressure when the cell is in a hypotonic environment. The wall withstands the turgor pressure generated by water entering the cell.

3. Maintenance of Cell Shape: The cell wall gives the bacterium its characteristic shape (cocci, bacilli, spirilla), which is important for its identification and function.

4. Protection from Harmful Substances: It acts as a selective barrier, preventing entry of certain toxic molecules and large enzymes.

5. Role in Cell Division: The cell wall participates in septum formation during binary fission, ensuring proper division of daughter cells.

6. Pathogenicity and Virulence: The cell wall components (e.g., lipopolysaccharides in Gram-negative bacteria) can act as antigens and are involved in the pathogenic mechanisms of bacteria.

7. Target for Antibiotics: The unique composition of the prokaryotic cell wall (peptidoglycan absent in eukaryotes) makes it an ideal target for antibiotics like penicillin, which inhibit peptidoglycan synthesis without harming the host.

Note: Gram-positive bacteria have a thick peptidoglycan layer; Gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer lipopolysaccharide membrane.