Excretion Systems

Contractile Vacuoles in Microorganisms

Contractile vacuoles absorb excess water and wastes from a microorganism's cell and excrete them into the environment by contracting.

Contractile Vacuoles in Microorganisms

A contractile vacuole (CV) is an organelle, or sub-cellular structure, that is involved in osmoregulation and waste removal. Previously, a CV was known as a pulsatile or pulsating vacuole. CVs should not be confused with vacuoles which store food or water. A CV is found predominantly in protists and in unicellular algae. In freshwater environments, the concentration of solutes inside the cell is higher than outside the cell. Under these conditions, water flows from the environment into the cell by osmosis. Thus, the CV acts as a protective mechanism against cellular expansion (and possibly explosion) from too much water; it expels excess water from the cell by contracting. However, not all species that possess a CV are freshwater organisms; some marine and soil microorganisms also have a CV. The CV is predominant in species that do not have a cell wall, but there are exceptions. Through the process of evolution, the CV was mostly eliminated in multicellular organisms; however it still exists in the unicellular stage of several multicellular fungi and in several types of cells in sponges, including amoebocytes, pinacocytes, and choanocytes.

The CV's phases of collecting water (expansion) and expelling water (contraction) are periodical. One cycle takes several seconds, depending on the species and the environment's osmolarity. The stage in which water flows into the CV is called diastole. The contraction of the CV and the expulsion of water from the cell is called systole. Water always flows from outside the cell into the cytoplasm; and only then from the cytoplasm into the CV. Species that possess a CV always use it, even in very hypertonic (high concentration of solutes) environments, since the cell tends to adjust its cytoplasm to become even more hyperosmotic (hypertonic) than the environment. The amount of water expelled from the cell and the rate of contraction are related to the osmolarity of the environment. In hyperosmotic environments, less water will be expelled and the contraction cycle will be longer.

The number of CVs per cell varies, depending on the species. Amoeba have one; Dictyostelium discoideum, Paramecium aurelia, and Chlamydomonas reinhardtii have two; and giant amoeba, such as Chaos carolinensis, have many. In some unicellular eukaryotic organisms (e.g., amoeba), cellular wastes, such as ammonia and excess water, are excreted by exocytosis as the contractile vacuoles merge with the cell membrane, expelling wastes into the environment. In Paramecium, which, presumably, has the most-complex and highly-evolved CV, the vacuole is surrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water, it is pumped into the vacuole. When the vacuole is full, it expels the water through a pore in the cytoplasm that can be opened and closed.

Flame Cells of Planaria and Nephridia of Worms

Flame cells and nephridia remove the waste from bodies through filtration in a manner similar to a kidney.

Flame Cells & Nephridia

As multi-cellular systems evolved to have organ systems that divided the metabolic needs of the body, individual organs evolved to perform the excretory function. Excretory cells known as flame cells developed in flatworms, while nephridia developed as excretory cells in annelids.

Flame Cells of Planaria

Planaria are flatworms that live in fresh water. Their excretory system consists of two tubules connected to a highly-branched duct system that leads to pores located all along the sides of the body. The filtrate is secreted through these pores. The cells in the tubules are called flame cells (or protonephridia) because they have a cluster of cilia that looks like a flickering flame when viewed under the microscope. Flame cells function like a kidney, removing waste materials through filtration. The cilia propel waste matter down the tubules and out of the body through excretory pores that open on the body surface; cilia also draw water from the interstitial fluid, allowing for filtration. After excretion, any useful metabolites are reabsorbed by the cell. Flame cells are found in freshwater invertebrates, such as flatworms, including parasitic tapeworms and free-living planaria.

Nephridia of Worms

Earthworms (annelids) and some other invertebrates, such as arthropods and mollusks, have slightly-more-evolved excretory structures called nephridia. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that they have tubules with cilia and function like a kidney to remove wastes, but they often open to the exterior of the organism. The ciliated tubules filter fluid from the body cavity and carry waste, including excess ions, through openings called nephrostomes. From the nephrostomes, excretion occurs through a pore called the nephridiopore. A nephridium is more evolved than a flame cell in that it has a system for reabsorption of some useful waste products, such as metabolites and ions, by a capillary network before excretion (unlike planaria that can only reabsorb useful metabolites after excretion).

Malpighian Tubules of Insects

Malpighian tubules remove wastes from insects by producing urine and solid nitrogenous waste, which are then excreted from the body.

Malpighian Tubules of Insects

Malpighian tubules line the gut of some species of arthropods, such as bees. They are usually found in pairs in the posterior regions of arthropod alimentary canals; the number of tubules varies with the species of insect. The system of malpighian tubules consists of branching tubules, which increase their surface area, near the hemolymph (a mixture of blood and interstitial fluid that is found in insects, other arthropods, and most mollusks) and fat tissues. They are lined with microvilli for reabsorption and maintenance of osmotic balance. They contain actin for support.

Malpighian tubules work cooperatively with specialized glands in the wall of the rectum. Body fluids are not filtered, as in the case of nephridia. Instead, urine is produced by tubular secretion mechanisms by the cells lining the malpighian tubules that are bathed in hemolymph. Metabolic wastes, such as urea and amino acids, freely diffuse into the tubules, while ions are transported through active pump mechanisms. There are exchange pumps lining the tubules which actively transport H⁺ ions into the cell and K⁺ or Na⁺ ions out; water passively follows to form urine. The secretion of ions alters the osmotic pressure, which draws water, electrolytes, and nitrogenous waste (uric acid) into the tubules. Water and electrolytes are reabsorbed when these organisms are faced with low-water environments and uric acid is precipitated and excreted as a thick paste or powder. By not dissolving wastes in water, these organisms are able to conserve water; this is especially important for life in dry environments.