Membrane Structure and Function

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fluid from the cell.

Slide 44

Water Balance of Cells with Walls

Cell walls help maintain water balance

A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

Slide 45

Video: Plasmolysis

Video: Turgid Elodea

Animation: Osmosis

In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Slide 46

Facilitated Diffusion: Passive Transport Aided by Proteins

In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane

Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane

Channel proteins include

Aquaporins, for facilitated diffusion of water

Ion channels that open or close in response to a stimulus (gated channels)

Slide 47

Fig. 7-15

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute

CYTOPLASM

Solute

Carrier protein

(b) A carrier protein

Slide 48

Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

Slide 49

Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria

Slide 50

Concept 7.4: Active transport uses energy to move solutes against their gradients

Facilitated diffusion is still passive because the solute moves down its concentration gradient

Some transport proteins, however, can move solutes against their concentration gradients

Slide 51

The Need for Energy in Active Transport

Active transport moves substances against their concentration gradient

Active transport requires energy, usually in the form of ATP

Active transport is performed by specific proteins embedded in the membranes

Animation: Active Transport

Slide 52

Active transport allows cells to maintain concentration gradients that differ from their surroundings