Cell Structure and Organelles

Cells are the basic structural and functional units of all known organisms. They are broadly categorised into two main types: prokaryotic and eukaryotic cells. Prokaryotic cells are simpler and smaller, lacking a true nucleus and other membrane-bound organelles. Their genetic material (DNA) is located in a region called the nucleoid, and they possess ribosomes for protein synthesis. Examples include bacteria and archaea. In contrast, eukaryotic cells are larger and more complex, characterised by the presence of a membrane-bound nucleus that encloses their genetic material, as well as numerous other membrane-bound organelles. These organelles compartmentalise various cellular processes, allowing for greater efficiency and specialisation. All multicellular organisms, including plants, animals, fungi, and protists, are composed of eukaryotic cells. Understanding this fundamental distinction is crucial for comprehending cellular organisation and function.

The nucleus is the largest organelle in animal cells and is often considered the control centre of the eukaryotic cell. It is enclosed by a double membrane called the nuclear envelope, which contains nuclear pores regulating the passage of molecules in and out. Inside, the nucleus houses the cell's genetic material, DNA, organised into chromosomes. The nucleolus, a dense region within the nucleus, is responsible for synthesising ribosomal RNA (rRNA) and assembling ribosomes. Ribosomes are tiny, non-membrane-bound organelles responsible for protein synthesis (translation). They can be found free in the cytoplasm, where they synthesise proteins for use within the cell, or attached to the endoplasmic reticulum, where they synthesise proteins destined for secretion or insertion into membranes. Their fundamental role in translating genetic information into functional proteins makes them essential for all cellular activities.

The endomembrane system is a network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles. The endoplasmic reticulum (ER) is an extensive network of interconnected membranes. The Rough Endoplasmic Reticulum (RER) is studded with ribosomes and is involved in the synthesis, folding, modification, and transport of proteins destined for secretion or insertion into membranes. The Smooth Endoplasmic Reticulum (SER) lacks ribosomes and is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions. The Golgi apparatus (or Golgi complex) receives proteins and lipids from the ER, further modifies them, sorts them, and packages them into vesicles for transport to various destinations. Lysosomes are membrane-bound organelles containing hydrolytic enzymes that break down waste materials and cellular debris, as well as pathogens.