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Stained yeast cells

Oral groove

Yeast Cell Membrane

A food vacuole forms around yeast cells.

The change in color shows that the vacuole has become acidic, like your stomach; acid helps digest the yeast cells.

As products of digestion move into the cytosol, the pH increases in the vacuole. The dye becomes red again.

A food vacuole forms around yeast cells.

The change in color shows that the vacuole has become acidic, like your stomach; acid helps digest the yeast cells.

As products of digestion move into the cytosol, the pH increases in the vacuole. The dye becomes red again.

Conclusion: Digestion, assisted by low pH, takes place in the food vacuole.

28.6 Food Vacuoles Handle Digestion and Excretion An experiment with Paramecium demonstrates the function of food vacuoles. Paramecium ingests food by way of the oral groove at the left.The dye Congo red turns green at acidic pH and red at neutral or basic pH.

Contractile vacuole

Contractile vacuole

28.5 Contractile Vacuoles Bail Out Excess Water

Water constantly enters freshwater protists by osmosis. A pore in the cell surface allows the contractile vacuole to expel the water it accumulates.

Plasma membrane

Inside of cell

Plasma Membrane

which periodically fuses with the plasma membrane, . . .

Inside of cell

28.5 Contractile Vacuoles Bail Out Excess Water

Water constantly enters freshwater protists by osmosis. A pore in the cell surface allows the contractile vacuole to expel the water it accumulates.

Plasma membrane ij Water passes from the cytoplasm to radiating canals and to the central vesicle . . .

which periodically fuses with the plasma membrane, . . .

digested food pinch away from the food vesicle and enter the cytoplasm. These tiny vesicles provide a large surface area across which the products of digestion may be absorbed by the rest of the cell.

The cell surfaces of protists are diverse

A few protists, such as some amoebas, are surrounded by only a plasma membrane, but most have stiffer surfaces that maintain the structural integrity of the cell. Many protists have cell walls, which are often complex in structure. Other protists that lack cell walls have a variety of ways of strengthening their surfaces. Some have internal "shells," which the organism either produces itself, as foraminiferans do, or makes from bits of sand and thickenings immediately beneath the plasma membrane, as some amoebas do (Figure 28.7).

(c) Paramecium aurelia

(c) Paramecium aurelia

Protist Cell Surfaces

28.7 Diversity among Protist Cell Surfaces (a) Foraminiferan shells are made of protein hardened with calcium carbonate. Several species are shown in this photograph. (b) This genus of shelled amoeba is commonly found in freshwater ponds and puddles. (c) The proteins in this Paramecium's surface—known as its pellicle—make it flexible but resilient.

28.7 Diversity among Protist Cell Surfaces (a) Foraminiferan shells are made of protein hardened with calcium carbonate. Several species are shown in this photograph. (b) This genus of shelled amoeba is commonly found in freshwater ponds and puddles. (c) The proteins in this Paramecium's surface—known as its pellicle—make it flexible but resilient.

Many protists contain endosymbionts

Endosymbiosis is very common among the protists, and in some instances both the host and the endosymbiont are pro-tists. Many radiolarians, for example, harbor photosynthetic protists (Figure 28.8). As a result, these radiolarians appear greenish or golden, depending on the type of endosymbiont they contain. This arrangement is beneficial to the radiolar-ian, for it can make use of the organic nutrients produced by its photosynthetic guest. The guest, in turn, may make use of metabolites made by the host, or it may simply receive physical protection. In other cases, the guest may be a prisoner, exploited for its photosynthetic products while receiving no benefit itself. We will take a more detailed look at the history of endosymbiosis among photosynthetic protists later in this chapter.

Both asexual and sexual reproduction occur among the protists

Although most protists practice both asexual and sexual reproduction, some groups lack sexual reproduction. As we will see, some asexually reproducing protists also engage in genetic recombination that does not directly result in reproduction.

Asexual reproductive processes in the protists include binary fission (splitting of the cell, with mitosis followed by cytokinesis), multiple fission (splitting into more than two

20 |m cells), budding (the outgrowth of a new cell from the surface of an old one), and the formation of spores (cells that are capable of developing into new organisms). Sexual reproduction also takes various forms. In some protists, as in animals, the gametes are the only haploid cells. In some other protists, by contrast, both diploid and haploid cells undergo mitosis, giving rise to alternation of generations, which will be described later in this chapter.

Astrolithium sp.

28.8 Protists within Protists Photosynthetic dinoflagellates (see Figure 28.1a) are living as endosymbionts within this radiolarian, providing organic nutrients for the radiolarian and imparting the golden-brown pigmentation seen at the center of its glassy skeleton. Both the dinoflagellates and the radiolarian are protists.

28.1 Major Protist Clades

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Responses

  • Weronika
    When organic materis is digested in a food vacuole the ph in the vacuole increase?
    8 years ago

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