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Cell Membranes



Every cell in every living thing is bound by a cell membrane. This membrane functions as a gatekeeper, regulating what enters or exits the cell. Despite this selectivity in what is allowed to cross the cell membrane, there are many processes a cell can use to transport molecules in and out the cell. Sometimes these processes disrupt the cell's shape when imbalances are created. Other times these processes play a role in cell-to-cell communication, helping a cell respond to its immediate environment.

At A Glance

  • Cells have a unique cell membrane that serves many functions, including controlling the passage of substances into and out of the cell and cell communication.
  • The cell membrane structure is a lipid bilayer with proteins, carbohydrates, and other lipids embedded within.
  • Proteins associated with cell membranes are either associated with one side of the membrane or are embedded in it and will have several roles in membrane activity.
  • The fluid mosaic model describes the nature of the cell membrane in that the membrane is not rigid and it contains many components.
  • As a selectively permeable surface, the cell membrane controls the movement of material into and out of the cell.
  • Bulk transport is the movement of many molecules into (endocytosis) or out of (exocytosis) the cell in membrane-bound vesicles.
  • Passive transport is the movement of material across the cell membrane from an area of high concentration to an area of low concentration, through diffusion or facilitated diffusion, without the expenditure of cellular energy.
  • Several factors affect the rate of diffusion: the nature of the diffusing substance, the physical environment in which diffusion is occurring, the cellular conditions in which diffusion is occurring, and the concentration gradient.
  • The passage of water across the cell membrane, through osmosis, is based on relative concentrations of solutes on either side of the membrane. Cellular integrity can be compromised if too much water enters or leaves the cell.
  • The cell must expend its own energy to drive active transport mechanisms, such as ion pumps, that move molecules against the concentration gradient.
  • Cells receive information from direct interaction with neighboring cells and from chemicals in the environment.
  • Cell communication involves a signal cascade that includes reception of the stimulus signal, transduction of the signal to cellular components, and response of the cell to the stimulus.
  • Changes in the chemical or electrical concentrations of cell membranes drive the formation of an electrochemical gradient on each side of the membrane.
  • The sodium-potassium pump maintains the ideal ion concentrations within living cells, so that cells can control cell volume, pH, and nutrient balance.