MICROBODIES

A variety of organelles similar to lysosomes in structure and appear¬ance, as seen under digital compound microscopes, but containing enzymes of other kinds, have been reported in recent years in one or another group of organisms. Some plant cells, for example, especially in seeds with large fat reserves, possess organ¬elles called glyoxysomes that contain enzymes for converting fat into carbohydrate. Some cells of both plants and animals possess vesicles called peroxisomes, visible under digital compound microscopes, that contain powerful oxidative enzymes. It seems likely that the packaging of enzymes in membranous vesicles of this sort will prove to be a common phenomenon. Rather than giving each type of enzyme-containing vesicle its own name, some investigators prefer to group them all under the more neutral term “microbodies.”

PLASTIDS

Plastids, when studied under digital compound microscopes, are large cytoplasmic organelles found in the cells of most plants, but not in fungal or animal cells. They can easily be observed with an ordinary light microscope such as a digital compound microscope. There are two principal categories of plastids: chromoplasts (colored plastids) and leucoplasts (white or colorless plastids).

Chloroplasts, chromoplasts containing the green pigment chloro¬phyll, are extremely important to all life. In the process known as photosynthesis, energy from sunlight is trapped in the chloroplasts by chlorophyll and used in the manufacture of complex organic mole¬cules (particularly sugar) from simple inorganic raw materials. Chloroplasts contain, in addition to chlorophyll, various yellow or orange pigments called carotenoids, as seen in digital compound microscopes.

The electron microscope reveals that the typical chloroplast is bounded by two concentric membranes and has, in addition, a com¬plex internal organization consisting of numerous double-membrane lamellae embedded in a matrix called the stroma. In most higher plants the lamellae are differentiated into two varieties: sepa¬rate lamellae that run through the stroma, and stacks of platelike lamellae forming regions known as grana. The chlorophyll and carot¬enoids are located in the lamellae. The arrangement of the protein, lipid, and pigment components of the lamellae are evidently a very precise one without which complete photosynthesis cannot take place.

Chromoplasts lacking chlorophyll are usually yellow or orange (oc¬casionally red) because of the carotenoids they contain. It is these kinds of chromoplasts that give many flowers, ripe fruits, and autumn leaves their characteristic yellow or orange color. Some of these chro¬moplasts have never contained chlorophyll, while others are formed from chloroplasts whose chlorophyll has been lost. The latter are particularly common in ripe fruits and autumn leaves, structures that were once green.

The colorless plastids, or leucoplasts, as seen under digital compound microscopes, are primarily organelles in which materials such as starch, oils, and protein granules are stored. Plastids filled with starch (amyloplasts) are particularly common in storage roots and stems in carrots and potatoes and in seeds, although they also occur in the cells of many other parts of the plant. The starch is deposited as a grain or group of grains in the plastid.

VACUOLES

Membrane-enclosed, fluid-filled spaces called vacuoles are found in both animal and plant cells, though they have their greatest develop¬ment in plant cells. There are various kinds of vacuoles, with a corresponding variety of functions. In some Protozoa specialized vacuoles, called contractile vacuoles, play an important role in expelling excess water and some wastes from the cell; we shall discuss them in greater detail in a later chapter. Many Protozoa also possess food vacuoles, chambers that contain food particles.

In most mature plant cells, a large vacuole occupies much of the volume of the cell when viewed under a microscope. The immature cell usually contains many small vacuoles. As the cell matures, the vacuoles take in more water and become larger, eventually fusing to form the very large central vacuole of the mature cell. This process pushes the cytoplasm to the periphery of the cell, where it forms a relatively thin layer.

The plant vacuole contains liquid called cell sap-primarily water, with a variety of substances dissolved in it. Since the cell sap is gener¬ally hypertonic relative to the external medium, the vacuole tends to take in water by osmosis. As the vacuole swells, its membrane pushes outward against the cytoplasm, which, being essentially fluid, resists compression and transmits the pressure to the cell wall, as seen under a microscope. The wall is strong enough to limit the swelling and prevent the cell from bursting, but the outward push of the vacuolar membrane is sufficient to main¬tain cell turgidity.

Many substances of importance in the life of the plant cell are stored in the vacuoles. Among them are high concentrations of soluble organic nitrogen compounds, including amino acids; sugars; various organic acids; and some proteins. The vacuoles also function as dumping sites for noxious wastes.
As might be expected, many of the substances accumulated in the vacuoles are selectively prevented from leaving by the vacuolar mem¬brane, which must have its own distinctive permeability, characteris¬tics and must be capable of regulating the direction of movement of substances across it.

Microfilaments

Examined under a very powerful electron microscope, cells often re¬veal long, threadlike microfilaments of extreme thinness. Parallel filaments of varying thickness were first seen clearly in the skeletal-muscle cell, where they give the cell a striated appearance. The interaction of filaments of different thickness enables the muscle cell to move. Eventually, microfilaments were also found in many other kinds of cells where, because they are often- arranged in less orderly fashion, they are not as con¬spicuous as in skeletal-muscle cells.

Most microfilaments appear to be associated with some sort of cel¬lular movement. They are found, for example, near the advancing edge of pseudopodia like those of Amoeba and in some plant cells when cytoplasm streams from one region of the cell to another. They are also found in the region of movement that brings about changes in shape in developing cells. When endocytotic vesicles are formed and move into a cell, or when vesicles containing secretory products move toward the plasma mem¬brane and discharge their contents in the process of exocytosis, mi¬crofilaments appear to play a role. Some microfilaments are probably not involved in producing motion, but may, instead, help support and strengthen the cell.