The cytoskeleton is a network of protein filaments and tubules that extends throughout the cytoplasm of all eukaryotic cells. It includes three main types of filaments: microfilaments (actin filaments), intermediate filaments, and microtubules. The cytoskeleton serves multiple essential functions: it provides mechanical support and maintains cell shape; it enables cell motility, including muscle contraction, cell division, and intracellular transport of organelles and vesicles; and it plays a role in cell signalling pathways. Microfilaments facilitate cell movement and are involved in muscle contraction and cell division. Intermediate filaments provide tensile strength for the cell, helping it withstand stress. Microtubules are responsible for separating chromosomes during cell division and are the tracks along which organelles move within the cell. The dynamic nature of the cytoskeleton is crucial for cellular responses to stimuli and for the adaptation to changing environments.

Peroxisomes are small, membrane-bound organelles present in virtually all eukaryotic cells. They contain enzymes that catalyse various biochemical reactions, most importantly the breakdown of very long chain fatty acids through β-oxidation. Peroxisomes also play a key role in the detoxification of harmful substances, including the breakdown of hydrogen peroxide – a byproduct of cellular metabolism that can be damaging to cells – into water and oxygen. In addition, peroxisomes are involved in the synthesis of certain important biomolecules, such as bile acids and plasmalogens (phospholipids essential for the normal function of the brain and lungs). The absence or malfunction of peroxisomes can lead to severe metabolic disorders, highlighting their importance in cell function and overall organism health.

The endoplasmic reticulum (ER) is a network of membranous tubules and sacs called cisternae, playing a central role in the synthesis, folding, modification, and transport of proteins and lipids. There are two types: rough ER, studded with ribosomes, and smooth ER, which lacks ribosomes. The rough ER is primarily involved in protein synthesis; newly synthesised proteins enter the ER lumen where they are folded and modified, for instance through glycosylation. These proteins are then transported to other parts of the cell, often in vesicles that bud from the ER. The smooth ER, on the other hand, is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions. The ER thus serves as a manufacturing and packaging system, playing a critical role in the maintenance of cellular function and health.

Lysosomes are small, spherical organelles found in animal cells that contain a variety of enzymes capable of breaking down all types of biological polymers – proteins, nucleic acids, carbohydrates, and lipids. These enzymes are active at a much lower pH than the cytoplasm, and thus lysosomes maintain an acidic environment for optimal enzyme activity. Lysosomes play several critical roles: they digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria; they break down complex molecules into simpler compounds, which are then either recycled within the cell or expelled as waste. This process is vital for cellular housekeeping and turnover. If cell debris accumulates, it can disrupt cellular functions and lead to diseases. Moreover, in multicellular organisms, lysosomes help in tissue remodelling during development and contribute to the immune response by destroying pathogens engulfed by white blood cells.

The Golgi apparatus, often referred to as the cell’s post office, is a series of flattened, stacked pouches called cisternae. It primarily functions in modifying, sorting, and packaging proteins and lipids for secretion or use within the cell. Proteins and lipids arrive from the endoplasmic reticulum in transport vesicles; once in the Golgi apparatus, they may undergo further modifications, such as glycosylation or the addition of other functional groups. The Golgi apparatus also produces lysosomes. The final products are then packaged into vesicles that bud from the edges of the Golgi cisternae. These vesicles may either fuse with the cell membrane to release their contents outside the cell or transport their contents to other locations within the cell. This organelle is crucial for correctly processing and targeting proteins and lipids, ensuring they reach their proper destinations, which is vital for the cell's functionality and overall health.