the cell membrane & passive transport. the cell membrane is the boundary that separates the...
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The Cell Membrane & Passive Transport
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The cell membrane is the boundary that separates the living cell from its surroundings
Life has an inside and an outside
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The Phospholipid BilayerPhospholipids are
the most abundant molecule in the cell membrane
Hydrophobic tails & a hydrophilic head
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The Fluid Mosaic Model
• The membrane is a fluid structure with a mosaic of proteins, glycolipids and sterols embedded in it
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The Fluid Mosaic Model
• Phospholipids in the plasma membrane can move within the bilayer
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Selective Permeability
• A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
• Plasma membranes are selectively permeable, they only allow some substances to cross the membrane.
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Hydrophobic (nonpolar) molecules, can dissolve in the lipid bilayer and pass through the membrane rapidly
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Polar molecules, such as sugars, do not cross the membrane easily
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Passive Transport
Passive transport is diffusion of a substance across a membrane with no energy investment
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Concentration Gradient
• A difference in the number per unit volume of ions or molecules of a substance between adjoining regions
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DiffusionThe net movement of like molecules or ions
down a concentration gradient
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(b) Diffusion of two solutes
Fig. 7-11b
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
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Diffusion Rate
• Determined by steepness of gradient, molecular size, temperature & electric or pressure gradients
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Osmosis
• Osmosis is the diffusion of water across a selectively permeable membrane
• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration
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Lowerconcentrationof solute (sugar)
Fig. 7-12
H2O
Higher concentrationof sugar
Selectivelypermeablemembrane
Same concentrationof sugar
Osmosis
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Tonicity
• Tonicity is the ability of a solution to cause a cell to gain or lose water
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Hypertonic Solution
Solute concentration is greater than that inside the cell; cell loses water
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Hypotonic Solution
Solute concentration is less than that inside the cell; cell gains water
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Isotonic Solution
Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
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• Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments
• The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
Video: Chlamydomonas Video: Paramecium Vacuole
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Fig. 7-14
Filling vacuole 50 µm
(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling fluid from the cell.
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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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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