3 Types of Cell Surface Projection | Cell Biology

The following points highlight the three types of cell surface projection. The types are: 1. Cilia and Flagella 2. Pseudopodia 3. Microvilli.

Type # 1. Cilia and Flagella:

Cilia and flagella are similar whip-like mobile structures that extend from the plasma mem­branes of unicellular organisms (Paramoecium, Chlamydomonas) and many animal cells (tra­cheal cell, gut of worm, spermatozoon etc.) Cilia and flagella are essentially identical in ultrastructure and molecular organisation.

The only distinctions that can be made be­tween cilia and flagella:

1. Cilia axe shorter than flagella.

2. The number of flagella is very few whereas the number of cilia on cell surface is nu­merous.

3. Cilia beat backward and forward, flagella typically rotate in a screw-like manner.

Both cilia and flagella are made of bunch of microtubules that run its entire length of the central core. Microtubules are collectively called axoneme and they are surrounded by plasma membrane. The cross-section of these structures seen under electron microscope re­veals that the fundamental structure in 9 + 2, i.e., 9 pairs of peripheral microtubules and 2 single and central micro-tubes.

The beating motion of cilia and flagella are responsible for cell movement, hence they are the locomotors organelles on the cell surface. Besides locomotory function they also help in capturing food.

Most animals possess ciliated cells in the interior of their body system; for example cells of sense organ, epithelial cells lin­ing the alimentary canal, reproductive organs. In all such locomotion’s these organelles help in moving fluid, water and other solid particles over the cell surfaces.

Type # 2. Pseudopodia:

The cell surface of many protozoans, vertebrate macro-phase cells and some plant cells develops blunt finger-like protrusions called pseudopodia. These are temporary extension formed at any part of the cell surface. Pseudopodia are also locomotory organelles of plasma membrane.

The movement that takes place by pseudopodia is commonly known as amoeboid movement. The pseudopodia continuously extend and re­tract on the substratum and, as a result, the cell moves. The elongation-retraction process involves transition of “regions of the cytoplasm from a fluid-like state called a sol to a semisolid state called a gel.

Pseudopods develop as a re­sult of endoplasm being forced into low pressure region to create finger. The ectoplasm forms a hyaline cap at the advancing end.

It is generally agreed that the basis of movement resides in the activity of the contractile protein namely actin and myosin. Pseudopodia are responsible for cell movement and help in capturing food. In feeding, pseudopodia flow around the particle and engulf it, forming a food vacuole.

Different types of blood cells, macrophages are capable of engulfing foreign particles, such as harmful bacteria, viruses and other microbes through a process called phagocytosis.

Type # 3. Microvilli:

In many animal cells specialised for the ab­sorption of nutrients, the plasma membrane is folded into large number of finger-like or hair-­like projections called microvilli which greatly increase the surface area and, therefore, the absorption rate of the cell.

The surface of the epithelial cells lining the lumen of the small intestine, which is studded with microvilli, are collectively called the brush border. Microvilli are also associated with the oviduct, mammary gland, lining of kidney tubules etc.

Unlike cilia, microvilli form a rather polymorphic class of surface protuberances that are regularly packed in some tissues and loosely positioned in oth­ers. Generally they are shorter and smaller in diameter than cilia. They are commonly about 0.1 µm diameter and range in length from a fraction of micrometre to about 2 µm. The core of the microvillus contains a bundle of 20 to 30 actin filaments.

At the cytoplasmic end of the bundle, the filaments are called the terminal web. This region contains myosin filaments as well as those intermediate filaments called tonofilaments. Microvilli perform several types of func­tions.

The plasma membrane of the microvilli contains many enzymes that assist in the degra­dation of sucrose to glucose and fructose and of protein into amino acids. This membrane also contains specific permeases that allow the epithelial cells to absorb these nutrients from intestine.

A second type of proposed function is to store membrane and microfilament mate­rials. Motility is another function of microvilli. The microvilli on the cell surface may sweep unwanted materials toward a resorptive area of the cell.