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Cell Membranes Problem Set

Problem 9: Water flow solution

Tutorial to help answer the question

The system used by protozoa to help control the flow of water into the cell is:

A. tugor pressure against the cell wall.
B. a pump in the cytoplasm that squeezes out water.
C. the Na+ K+ ATPase of the cell membrane.
D. the lysosome, used to secrete water in animal cells.


Mechanisms for dealing with water problem

There are three general mechanisms for dealing with the water problem. For most cells, the concentration of solutes is higher inside the cell than in the aqueous external environment.

Plant cells
Hypoertonic, Isotonic, Hypotonic plant cells Plants solve the water problem by having a rigid cell wall. The entering water is placed inside a large vesicle. The wall protects the cell against expansion, and the resulting pressure makes the plant rigid, a phenomena called tugor pressure.


Animal cells

Hypoertonic, Isotonic, Hypotonic Animal cells Animal cells lack a wall, and use active transport systems (especially the Na+K+ATPase that moves three Na+ out for each two K+ that move in) to move ions outside the cell, reducing the osmotic pressure.

Contractile mechanism

Most protozoa use a special contractile mechanism. Water collects in a vesicle, and microfilaments force a contraction that squeezes water back outside the cell. This pump mechanism protects the cell from osmotic pressure.

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