Q.What is Cell? Explain in brief the structure of Eukaryotic cell with suitable diagram. OR What are Proteins? Explain in brief the structure of proteins.

The cell is the smallest living unit; a eukaryotic cell is distinguished from a prokaryotic cell by having a true nucleus enclosed in a nuclear envelope and a range of membrane-bound organelles that divide the cell into specialised compartments.

What is a Cell?

The cell is the basic structural and functional unit of all living organisms (the Cell Theory: all organisms are composed of cells, and cells arise only from pre-existing cells). Organisms may be unicellular (a single cell performs all functions, e.g. Amoeba, Paramecium) or multicellular (division of labour among many cells, e.g. humans).

Structure of a Eukaryotic Cell

A eukaryotic cell is much larger and more complex than a prokaryotic cell and shows three broad regions:

1. Plasma membrane — the outer, selectively permeable lipid-bilayer boundary that regulates the movement of substances into and out of the cell.
2. Cytoplasm — the semi-fluid matrix between the plasma membrane and the nucleus, in which all the organelles are suspended. Key organelles include:
   • Nucleus — enclosed by a double-membraned nuclear envelope with nuclear pores; contains the nucleolus and chromatin (DNA + histone proteins), the control centre of the cell.
   • Endoplasmic reticulum (ER) — a network of membranous tubules; Rough ER (studded with ribosomes) makes proteins, Smooth ER makes lipids.
   • Golgi apparatus — stacks of flattened sacs (cisternae) that package and modify proteins/lipids received from the ER for secretion or use elsewhere.
   • Mitochondria — double-membraned, the "powerhouse of the cell," site of aerobic respiration and ATP production; the inner membrane is folded into cristae.
   • Ribosomes — non-membranous granules (free or ER-bound) that are the site of protein synthesis.
   • Lysosomes — membrane-bound vesicles containing hydrolytic enzymes for intracellular digestion.
   • Plastids and a large central vacuole are additionally present in plant cells (not in animal cells). …