Maintaining Homeostasis A regular arrangement of microtubules called
Maintaining Homeostasis: • A regular arrangement of microtubules called pellicle, underlies the plasma membrane of many protozoa • Pellicle is rigid enough to maintain the shape of the protozoan, but it is also flexible • The cytoplasm of the protozoan is differentiated into two regions • The portion of cytoplasm just beneath the pellicle is called ectoplasm • It is relatively clear and firm • The inner cytoplasm is called endoplasm • It is usually granular and more fluid • Most marine protozoa have solute concentrations similar to that of their environment • Freshwater protozoa however must regulate the water and solute concentrations of their cytoplasm
• Water enters freshwater protozoa by osmosis because of higher solute concentration in protozoa than in environment • Contractile vacuoles remove the excess water • In some protozoa, contractile vacuole formed by coalescence of smaller vacuoles • In others, the vacuoles are permanent organelles that collecting tubules radiating into the cytoplasm fill • Contracting microfilaments have been implicated in the emptying of contractile vacuoles • Most protozoa absorb dissolved nutrients by active transport or ingest whole or particulate food through endocytosis • Some protozoa ingest food in a specialized region analogous to mouth, called the cytopharynx
• Digestion and transport of food occurs in food vacuoles that form during endocytosis • Enzymes and acidity changes mediate digestion • Food vacuoles fuse with enzyme-containing lysosomes and circulate through the cytoplasm, distributing the products of digestion • After digestion is complete, the vacuoles are called egestion vacuoles • They release their contents by exocytosis, sometimes at a specialized region of the plasma membrane or pellicle called, the cytopyge • Because protozoa are small, they have large surface area in proportion to their volume • This high surface-area-volume ratio facilitates two other maintenance functions gas exchange and excretion
• Gas exchange involves acquiring oxygen for cellular respiration and eliminating the carbon dioxide produced as b by-product • Excretion is the elimination of the nitrogenous by-products of protein metabolism, which in protozoa is primarily ammonia • Both gas exchange and excretion occur by diffusion across the plasma membrane
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