Anatomy and Physiology

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Unit 1
Chapter 6 Physiology of Cells

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES:
PASSIVE TRANSPORT

  • Passive transport processes do not require any energy expenditure of the cell membrane (Table 4-1)
  • Diffusion: a passive process (Figure 4-1)
  • Molecules spread through the membranes.
  • Molecules move from an area of high concentration to an area of low concentration down a concentration gradient(Figure 4-2)

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT (cont.)

  • Simple diffusion (Figure 4-3)
  • Molecules cross the phospholipid bilayer.
  • Solutes permeate the membrane; therefore the membrane is called permeable.

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT (cont.)

  • Osmosis (Figure 4-4)
  • Diffusion of water through a selectively permeable membrane.

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT (cont.)

  • Osmosis (cont.)

  • Isotonic: when two fluids have the same potential osmotic pressure (Figure 4-5)
  • Hypertonic (higher pressure): cells placed in solutions that are hypertonic to intracellular fluid always shrivel as water flows out of them; if medical treatment causes the extracellular fluid to become hypertonic to the cells of the body, serious damage may occur.
  • Hypotonic (lower pressure): cells placed in a hypotonic solution may swell as water flows into them.

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT (cont.)

  • Facilitated diffusion (mediated passive transport)
  • A special kind of diffusion in which movement of molecules is made more efficient by the action of transporters embedded in a cell membrane.
  • Transports substances down a concentration gradient.

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT (cont.)

  • Channel-mediated passive transport (Figure 4-6)
  • Channels are specific; allow only one type of solute to pass through.
  • Gated channels may be open or closed (or inactive); may be triggered by any of a variety of stimuli.
  • Channels allow membranes to be selectively permeable.

  • Carrier-mediated passive transport (Figure 4-7)
  • Carriers attract and bind to the solute, change shape, and release the solute on the other side of the carrier.
  • Carriers are usually reversible depending on the direction of the concentration gradient.

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT (cont.)

  • Role of passive transport processes.
  • Move substances down a concentration gradient, thus maintaining equilibrium and homeostatic balance.
  • Types of passive transport: simple and facilitated diffusion (channels and carriers); osmosis is a special example of channel-mediated passive transport of water.

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: ACTIVE TRANSPORT

  • Active transport processes require the expenditure of metabolic energy by the cell (Table 4-2)
  • Transport by pumps
  • Pumps are membrane transporters that move a substance against their concentration gradient; opposite of diffusion.
  • Examples: calcium pumps (Figure 4-8) and sodium-potassium pumps (Figure 4-9)

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: ACTIVE TRANSPORT (cont.)

  • Transport by vesicles allows substances to enter or leave the interior of a cell without moving through its plasma membrane.
  • Endocytosis: the plasma membrane “traps” some extracellular material and brings it into the cell in a vesicle.
  • Two basic types of endocytosis (Figure 4-10)

Phagocytosis (“condition of cell eating”): large particles are engulfed by the plasma membrane and enter the cell in vesicles; the vesicles fuse with lysosomes, which digest the particles..

Pinocytosis (“condition of cell drinking”): fluid and the substances dissolved in it enter the cell.

  • Receptor-mediated endocytosis: membrane receptor molecules recognize substances to be brought into the cell (Figure 4-11)

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: ACTIVE TRANSPORT (cont.)

  • Exocytosis
  • Process by which large molecules, notably proteins, can leave the cell even though they are too large to move out through the plasma membrane.
  • Large molecules are enclosed in membranous vesicles and then pulled to the plasma membrane by the cytoskeleton, where the contents are released.

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: ACTIVE TRANSPORT (cont.)

  • Role of active transport processes
  • Active transport requires energy use by the membrane.
  • Pumps concentrated substances on one side of membrane, such as when storing an ion inside an organelle.
  • Vesicle-mediated (endocytosis, exocytosis): move large volumes of substances at once, such as in secretion of hormones and neurotransmitters.