Page 76 - Demo 1

Figure 36. In a hypotonic soluon, cells gain water. Red blood cells swell to bursng, and
plant cells become turgid. In an isotonic soluon, cells neither gain nor lose
water. In a hypertonic soluon, cells lose water. Red blood cells shrivel, and
plant cell cytoplasm shrinks away from the cell wall.
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ACTIVE TRANSPORT

In acve transport, molecules or ions move through the cell membrane,
accumulang on one side of the cell. For example, iodine collects in the cells of
the thyroid gland; glucose is completely absorbed from the digesve tract by
the epithelial lining; and sodium can be almost completely drawn out from
urine by cells lining the kidney tubules. In these situaons, substances move
from low to high concentraon.
Acve transport moves substances against a concentraon gradient and
requires both a transport protein and energy. Energy is needed because the
substance is moving from an area of lower concentraon to an area of higher
concentraon. The energy for acve transport comes from the energy-carrying
molecule called ATP. Like passive transport, acve transport may also involve
transport proteins.

An example of acve transport is the sodium-potassium pump. When
this pump is in operaon, sodium ions are pumped out of the cell, and potassium
ions are pumped into the cell. Both ions move from areas of lower to higher
concentraon, so ATP is needed to provide energy for this “uphill” process. The
sodium-potassium pump is vital in nerve conducon.

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