All the organelles embedded in the cytoplasm of the cell are surrounded by the plasma membrane, which separates them from the extracellular environment, gives shape to the cell and also transports substances across the cell.
The plasma membrane is a bilipid layer, with the polar hydrophilic head of both the layers pointing outwards, towards the aqueous environment and the non-polar hydrophobic tail pointing inwards.
Apart from lipids, proteins and carbohydrates are also parts of the cell membrane, with the proteins arranged in two forms – peripheral and intrinsic. Singer and Nicolson put forth the Fluid Mosaic Model, which was widely accepted. This model stated that lipids are found in the form of a fluid bilayer. The fluid like nature of the membrane allows the cell to perform functions such as cell growth, formation of intercellular junctions, secretion and cell division. The cell membrane is selectively permeable, which selectively permits the entry or exit of ions and other polar molecules, either through passive or active transport.
In passive transport, substances move across the membrane either through diffusion (simple diffusion and facilitated diffusion) or osmosis. Osmosis refers to the movement of water molecules across the cell membrane through diffusion. In active transport, transmembrane proteins act as pumps and utilise the energy from ATP to move substances against their concentration gradient.
The cilia and flagella are hair-like structures that project from the cell surface and help it to propel or move the fluid surrounding it. These structures emerge from basal bodies. Their core, called the axoneme, is covered by the cell membrane and has numerous microtubules running parallel to its axis.
Two microtubules join to form a doublet. If we observe a cross-section of the axoneme, we will see nine pairs of radially arranged peripheral doublets and a pair of centrally located microtubules, which are interconnected by a bridge. The central microtubules are enclosed by a central sheath, which is connected to one of the microtubules of each peripheral doublet by a radial spoke. The peripheral doublets too are interconnected by linkers called an interdoublet bridge.