Cell biology

• Cells are the microscopic fundamental units of all living things
• Every living thing has cells: bacteria, protozoans, fungi, plants, and animals
• Some organisms are made up of just one cell (e.g. bacteria and protozoans), but animals, including human beings, are multi-cellular
• An adult human body is composed of about 100 trillion cells
• Cell theory timeline:

  • 1665, Robert Hooke makes drawings of cork and sees tiny structures that he calls 'cells'
  • 1674, Anton van Leeuwenhoek sees living, moving unicellular organisms in a drop of water
  • 1824, Rene Dutrochet concludes that all organisms are composed of cells
  • 1839, Matthias Schleiden and Theodor Schwann put forward the first clearly stated cell theory
  • 1858, Rudolf Virchow declares that a cell can only arise from another cell like it
• Cell theory states:

  • The cell is the unit of structure, physiology, and organization in living things
  • All organisms are made up of one or more cells
  • All cells come from pre-existing cells
  • Cells contain hereditary information passed from cell to cell during division
  • All cells have the same chemical composition
  • All energy flow occurs within cells
• Cell shapes and sizes:

  • Most human cells range from 10 to 15 micrometers in diameter
  • The human egg cell is 100 μm in diameter
  • Nerve cells can be over a meter long and muscle cells up to 30 cm long
• Units to measure cells:

  • Millimeters (mm) - 1/1000 of a meter
  • Micrometers (μm) - 1/1000 of a millimeter, and 1/1,000,000 of a meter
  • Nanometers (nm) - 1/1000 of a micrometer, 1/1,000,000 of a millimeter, and 1/1,000,000,000 of a meter
• As cells increase in size, the surface-area-to-volume ratio decreases, affecting their ability to obtain resources for metabolism
• Prokaryotic cells:

  • First type of cells formed when life first evolved
  • Much smaller and simpler than eukaryotic cells
• Eukaryotic cells:

  • More complex than prokaryotic cells
  • Have many organelles and membranes
  • Division of labor within the cell
  • Originated from the endosymbiont theory proposed by Lynn Margulis
• Differences between prokaryotic and eukaryotic cells:

  • Prokaryotic cells are simpler and smaller
  • Eukaryotic cells are more complex with many organelles and membranes
• Cell membrane:

  • Controls what enters and leaves the cell
  • Cell signaling allows recognition by hormones and immune system
  • Consists of fluid phospholipid bilayer with a mosaic pattern of associated proteins
• Membrane proteins:

  • Integral proteins extend across the lipid bilayer
  • Peripheral proteins span only one layer of the membrane
• Functions of cell membrane:

  • Shape and protection
  • Communication
  • Electro-chemical gradient
  • Selective permeability
• Substances cross the plasma membrane through:

  • Passive transport (along concentration gradient, no energy required)
  • Active transport (against concentration gradient, requires extra energy)
• Passive processes:

  • Simple diffusion driven by concentration gradient
  • Facilitated diffusion aided by carrier or channel proteins
• Facilitated diffusion:

  • Rate affected by carrier proteins (or channel proteins) present
  • Rate affected by the concentration gradient
  • Rate affected by the thickness of the membrane
• Osmosis:

  • Process by which water moves across a partially permeable membrane
  • Movement of water from a system with a high (less negative) water potential to one with a lower (more negative) water potential, across a partially permeable membrane
  • Water potential is measured in units of pressure – Pa, kPa, or mPa
  • Pure, liquid water has a higher water potential than any other system
  • Water potential values for cells, solutions, and suspensions are negative
• Types of cells in different solutions:

  • Animal cell in hypertonic solution: Loss of water, shrinks, called hemolysis
  • Animal cell in hypotonic solution: Gain water, may burst
  • Animal cell in isotonic solution: Remain unchanged
  • Plant cell in hypertonic solution: Loss of water, shrinks, and said to be flaccid or plasmolysed
  • Plant cell in hypotonic solution: Gains water, turgid
  • Plant cell in isotonic solution: Remain unchanged
• Active processes:

  • Active transport moves substances against a concentration gradient, requiring metabolic energy
  • Endocytosis involves engulfing large particles by a cell through phagocytosis, pinocytosis, or receptor-mediated endocytosis
  • Exocytosis moves substances from the inside to the outside of the cell
• Summary of transport processes:

  • Simple diffusion: No ATP requirement, lipid-soluble particles, small, non-polar
  • Facilitated diffusion: No ATP requirement, ions and medium-sized particles, requires transport proteins
  • Osmosis: No ATP requirement, water
  • Active transport: Requires ATP, ions, lipid-soluble particles
  • Endocytosis: Requires ATP, very large particles
  • Exocytosis: Requires ATP, very large particles
• Cytosol:

  • Cellular content inside the cell’s plasma membrane and external to the nucleus
  • Jelly-like substance, semi-fluid portion of cytoplasm
  • Function: Place for chemical reactions, receives raw materials, site for new substance synthesis
• Organelles:

  • Play specific roles in growth, maintenance, repair, and control
  • Some surrounded by one or two layers of unit membrane (membranous organelles)
  • Others not surrounded by membranes
• Ribosomes:

  • Small granules of protein and RNA
  • Sites of protein synthesis
• Endoplasmic Reticulum (ER):

  • Rough ER: Network composed of parallel, flattened sacs covered with ribosomes, temporary storage area for molecules
  • Smooth ER: Lacks ribosomes, site of fatty acid, phospholipid, and steroid synthesis
• Golgi Complex:

  • Located near the nucleus, consists of flattened sacs called cisterns stacked upon each other
  • Function: Receives newly synthesized proteins and lipids, processes, sorts, packages, and delivers them
• Lysosomes:

  • Membrane-enclosed vesicles that form in the Golgi complex
  • Contain powerful digestive enzymes
  • Function: Digest bacterial and other substances, recycle the cell’s own structure
• Peroxisomes:

  • Resemble lysosomes but contain different enzymes
  • Function: Neutralize free radicals, detoxify alcohol and other drugs, decompose fatty acids
• Mitochondria:

  • Organelles specialized for synthesizing ATP
  • Surrounded by a double unit membrane
  • Function: Extract energy from organic compounds and transfer it to ATP
• Centrioles:

  • Short cylindrical assembly of microtubules
  • Play a role in cell division
• Cytoskeleton:

  • Collection of protein filaments and cylinders
  • Determines cell shape, lends structural support, organizes cell contents, moves substances through the cell
• Vacuoles:

  • Membranous sacs that function as storage chambers
  • May contain liquid or solid materials
• Chloroplast:

  • Surrounded by two membranes
  • Contains grana for light-dependent reactions and stroma for light-independent reactions
• Nucleus:

  • Largest organelle, usually the only one visible with a light microscope
  • Surrounded by a double membrane called the nuclear envelope
  • Contains nucleoli that produce ribosomes
• Cell fractionation:

  • Technique used to study organelles
  • Cells are homogenized, filtered, and spun in an ultracentrifuge to separate organelles based on size and mass
• The cell membrane is the outermost layer that surrounds all cells.
• Cell membranes are made up of phospholipids, cholesterol, proteins, and carbohydrates.
• Phospholipids have hydrophilic heads (water-loving) and hydrophobic tails (water-hating).
• Science is sometimes called experimental science, heavily relying on experimentation to obtain information