CELLS AND TISSUE

• The cell is the basic and smallest functional unit of all living things
• Cellular organelles


  Specialized structures within the cytoplasm
• Human cells


  • Vary considerably in size
  • All are microscopic
  • Differ notably in shape
• Cytoplasm


  A substance found only in cells
• Plasma membrane


  Forms outer boundary of cell<|>Composed of thin, two-layered membrane of phospholipids and embedded with proteins<|>Is selectively permeable
• Cytoskeleton


  Internal framework of cell<|>Made up of microfilaments and microtubules<|>Provides support and movement
• Ribosomes


  Made of two tiny subunits of mostly ribosomal RNA (rRNA)<|>Manufacture enzymes and other proteins; often called protein factories
• Endoplasmic reticulum (ER)


  • Network of connecting sacs and canals
  • Carries substances through cytoplasm
  • Rough ER collects, folds, and transports proteins
  • Smooth ER synthesizes chemicals; makes new membrane
• Golgi apparatus


  Group of flattened sacs near nucleus<|>Collects chemicals into vesicles that move from the smooth ER outward to the plasma membrane<|>Called the chemical processing and packaging center
• Mitochondria


  Involved with energy-releasing chemical reactions (cellular respiration)<|>Often called power plants of the cell<|>Each mitochondrion contains one DNA molecule
• Lysosomes


  Membrane-enclosed packets containing digestive enzymes<|>Have protective function (eat microbes)<|>Formerly thought to be responsible for apoptosis (programmed cell death)
• Centrosome


  Microtubule-organizing region of the cytoskeleton near the nucleus<|>Centrioles function in moving chromosomes during cell reproduction
• Cell extensions


  • Microvilli increase surface area and enhance absorption
  • Cilia serve sensory functions and can propel mucus
  • Flagella act as "tails" of sperm cells
• Nucleus


  Controls cell because it contains most of the genetic code (genome)<|>Contains nuclear envelope, nucleoplasm, nucleolus, and chromatin granules<|>DNA molecules become tightly coiled chromosomes during cell division
• 46 nuclear chromosomes contain DNA, which contains genetic code
• Every human cell has a designated function

  Some help maintain the cell, others regulate life processes
• Cell junctions


  Plasma membranes of adjacent cells are usually separated by extracellular fluids<|>In certain tissues, membranes may join and form a junction
• Kinds of cell junctions

  • Desmosomes
  • Tight junctions
  • Gap junctions
• Desmosomes


  Protein attachments between adjacent cells<|>Act like spot welds to hold together tissues that undergo considerable stress
• Tight junctions


  Tightly stitched seams between cells<|>Prevent the movement of material between the cell
• Gap junctions


  Narrow tunnels between cells consisting of proteins called connexons<|>Allow communication between cells through the exchange of materials or transmission of electrical impulses