Cell structure

• Cell structure

  Each cell is a highly organized unit
• Organelles


  Specialized structures within cells that perform specific functions
• Nucleus


  • Contains the cell's genetic material
• Cytoplasm


  • Living material surrounding the nucleus; contains many types of organelles
• Cell membrane / plasma membrane


  • Encloses the cytoplasm
• Four important functions performed by body cells

  • Cell metabolism and energy use
  • Synthesis of molecules
  • Communication
  • Reproduction and inheritance
• Cell membrane


  • Also known as the plasma membrane, is the outermost component of a cell
  • Encloses the cytoplasm and forms the boundary between material inside and outside of the cell
  • Supports the cell contents, acts as a selective barrier that determines what moves into and out of the cell
  • Plays a role in communication between cells
• Extracellular


  Substance outside the cell
• Intracellular


  Substance inside the cell
• Molecules that make up the cell membrane

  • Phospholipids
  • Proteins
  • Cholesterol
  • Carbohydrates
• Fluid-mosaic model


  • Phospholipids form a double layer
  • Polar, phosphate-containing ends are hydrophilic (water-loving)
  • Nonpolar, fatty acid ends are hydrophobic (water-fearing)
  • Double layer has a fluid quality
  • Cholesterol gives added strength and stability
  • Protein molecules "float" among the phospholipid molecules
• Membrane proteins


  Function as membrane channels, carrier molecules, receptor molecules, enzymes or structural support
• Membrane channels & carrier molecules

  Involved with the movement of substances through the cell membrane
• Receptor molecules


  Part of an intercellular communication system that enables cell recognition and coordination of activities
• Cell membranes


  • Are selectively permeable
  • Allow some substances to pass into or out of the cells
• Enzymes, glycogen, potassium ion (K+) are found at higher concentrations intracellularly
• Na+, Ca2+, and Cl- are found in greater concentration extracellularly
• Nutrients enter cells continually, waste products exit
• Movement through the cell membrane

  • Passive
  • Active
• Passive membrane transport


  Does not require the cell to expend energy
• Passive membrane transport processes

  • Diffusion
  • Osmosis
  • Facilitated diffusion
• Active membrane transport


  Does require the cell to expend energy, usually in the form of ATP
• Active membrane transport processes

  • Active transport
  • Secondary active transport
  • Endocytosis
  • Exocytosis
• Diffusion


  The movement of solutes from an area of higher concentration to an area of lower concentration
• Concentration gradient


  The difference in the concentration of a solute in a solvent between two points divided by the distance between the two points
• Certain small, water-soluble substances can diffuse between the phospholipid molecules of cell membranes
• Other water-soluble substances, such as ions, can diffuse across the cell membrane only by passing through cell membrane channels
• Molecules that are lipid-soluble, such as O2, CO2, and steroids, pass easily through the phospholipid bilayer
• Cell membrane channels


  • Leak channels
  • Gated channels
• Osmosis


  The diffusion of water (a solvent) across a selectively permeable membrane from a region of higher water concentration to one of lower water concentration
• Osmotic pressure


  The force required to prevent the movement of water across a selectively permeable membrane
• Carrier-mediated transport mechanisms


  Move large, water-soluble molecules or electrically charged ions across the cell membrane
• Carrier molecules


  Proteins within the cell membrane that transport substances
• Facilitated diffusion


  A carrier-mediated transport process that moves substances across the cell membrane from an area of higher concentration to an area of lower concentration
• Active transport


  A carrier-mediated process that moves substances across the cell membrane from a region of lower concentration to one of higher concentration, against a concentration gradient
• Cystic Fibrosis is a genetic disorder that affects the active transport of Cl- into cells
• Secondary active transport


  Involves the active transport of one substance establishing a concentration gradient, which then provides the energy to transport a second substance
• Secondary active transport processes

  • Cotransport
  • Countertransport
• Endocytosis


  The uptake of material through the cell membrane by the formation of a vesicle
• Receptor-mediated endocytosis


  When a specific substance binds to the receptor molecule, endocytosis is triggered, and the substance is transported into the cell