Cell The Unit of Life - Notes | Class 11 | Part 3: Eukaryotic Cell

Cell: The Unit of Life - Eukaryotic Cells

Eukaryotic Cells have a well-organized, membrane-bound nucleus and organelles.
Presence of membranes gives clear compartmentalization of cytoplasm.
Their genetic material is organized into chromosomes.
They have complex locomotory and cytoskeletal structures.

Cell Organelles in Eukaryotic Cells

Chemical studies on human RBCs show that the cell membrane is composed of a lipid bilayer, protein, and carbohydrate.
Lipids (mainly phosphoglycerides) have an outer polar head and inner hydrophobic tails, protecting the non-polar tail of saturated hydrocarbons from the aqueous environment.
The ratio of protein and lipid varies in different cells. E.g., in human RBCs, the membrane has 52% protein and 40% lipids.
Based on ease of extraction, membrane proteins are two types:
- Integral proteins: Partially or totally buried in the membrane.
- Peripheral proteins: Lie on the surface of the membrane.
Fluid mosaic model of cell membrane: Proposed by Singer & Nicolson (1972). The quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.

Functions:

Transport of molecules. The membrane is selectively permeable to some molecules present on either side of it.
Due to its fluid nature, the plasma membrane can help in cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division.

Types of Transport:

Passive transport: Movement of molecules across the membrane along the concentration gradient (from higher to lower concentration) without energy expenditure. It is two types:
1. Simple diffusion: Movement of neutral solutes across the membrane.
2. Osmosis: Movement of water by diffusion across the membrane.
Polar molecules cannot pass through the non-polar lipid bilayer, so they require membrane carrier proteins for transport.
Active transport: Movement of molecules against the concentration gradient (from lower to higher concentration) with energy expenditure (ATP is utilized). E.g., Na+/K+ pump.

A non-living rigid structure found outer to the plasma membrane of fungi and plants.
Cell wall of algae is made of cellulose, galactans, mannans, and minerals like CaCO3. In other plants, it consists of cellulose, hemicellulose, pectins, and proteins.
The cell wall of a young plant cell (primary wall) is capable of growth. It gradually diminishes as the cell matures, and the secondary wall forms on the inner side (towards the membrane).
The middle lamella is a layer containing calcium pectate, which glues neighboring cells together. Cell wall and middle lamellae may be traversed by plasmodesmata, connecting the cytoplasm of neighboring cells.

Functions:

A group of membranous organelles with coordinated functions.
Includes endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles.

A network of tiny tubular structures scattered in the cytoplasm.
Divides the intracellular space into two compartments: luminal (inside ER) and extra-luminal (cytoplasm).
Endoplasmic reticulum is two types:
1. Rough endoplasmic reticulum (RER): Bears ribosomes on its surface. Frequently observed in cells actively involved in protein synthesis and secretion. Extends to the outer membrane of the nucleus.
2. Smooth endoplasmic reticulum (SER): Lacks ribosomes. Major site for lipid synthesis. In animal cells, steroidal hormones are synthesized in SER.

Densely stained reticular structures near the nucleus.
First observed by Camillo Golgi (1898).
Consists of flat, disc-shaped sacs (cisternae) of 0.5–1.0 µm diameter, stacked parallelly.
Cisternae are concentrically arranged with a convex cis (forming) face and a concave trans (maturing) face. Cis and trans faces are totally different but interconnected.

Functions:

Secretes materials to intra-cellular targets or outside the cell.
Materials to be packaged as vesicles from the ER fuse with the cis face and move towards the trans face, explaining the close association with the endoplasmic reticulum.
Proteins synthesized by ribosomes on the ER are modified in the cisternae before release from the trans face.
Formation of glycoproteins and glycolipids.

Membrane-bound vesicular structures formed by packaging in the Golgi apparatus.
Contain almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases), active at acidic pH, which digest carbohydrates, proteins, lipids, and nucleic acids.

Membrane-bound spaces in the cytoplasm containing water, sap, excretory products, and other materials not useful for the cell.
Bound by a single membrane called tonoplast.
In plant cells, vacuoles can occupy up to 90% of the cell volume.
In plants, the tonoplast facilitates transport of ions and other materials against concentration gradients into the vacuole, resulting in higher concentrations in the vacuole than in the cytoplasm.
In Amoeba, the contractile vacuole aids in excretion.
In many cells (e.g., protists), food vacuoles form by engulfing food particles.