Question

Pick one surface extension from the plasma membrane and discuss its structure and importance to cell function.

Answer 1

The plasma membrane have extensions such as whip-like flagella or brush-like cilia.

Cilia are slender protuberance which projects from much larger cell body.

Structure:-A cilium is made up of microtubules coated in plasma membrane and these microtubules are small hollow rods made of the protein tubulin, and each cilium contains nine pairs of microtubules forming the outside of a ring, and two central microtubules. This structure is known as an axoneme and this arrangement as 9+2, is an arrangement ubiquitous in motile cilia. The microtubules are held together by cross-linking proteins and between the nine outer pairs are motor proteins called dynein.

Usually cilia attach to the cell at a basal body which is made up of microtubules arranged as nine triplets, and these triplets are formed as the doublets from the cilia which are joined by an additional microtubule from the cell and the two central microtubules end before entering the basal body.

The dynein are large flexible molecules that allow the cilia to be motile and these proteins hydrolyze ATP for energy. As the proteins are activated, they undergo conformational changes which allow for complex movements and these dynein molecules essentially crawl along the microtubules, pulling the neighboring doublet up and reattaching further down. As the doublets are attached to each other through the cross-linking proteins they can only slide a short distance along each other and this movement causes bending in the cilium.The cilia act like oars by beating back and forth to create movement.

Functions:-Their main function is locomotion, either of the cell itself or of fluids on the cell surface and they are also involved in mechanoreception.

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