Week 1: Cell Structure and Function

• Endoplasmic reticulum and Golgi apparatus

  Function together to synthesize proteins and lipids for transport to lysosomes or to the plasma membrane
• Lysosomes and peroxisomes


  Membrane-bound bags of digestive enzymes that degrade intracellular debris
• Mitochondria


  • Contain enzymes necessary for oxidative phosphorylation to produce ATP
  • Have their own small number of genes that code for some of the mitochondrial proteins
• Cellular respiration - Energy Production


  1. Digestion of food into small molecules

  2. Glycolysis - Glucose degraded into pyruvate

  3. Complete oxidation of glucose to CO2 and H2O - Citric Acid Cycle, Oxidative phosphorylation
• Mitochondria


  • Contain enzymes necessary for ATP production
  • Have outer and inner membrane
  • Inner space (matrix) is site of citric acid cycle (Kreb)
  • Electron transport chain
  • Contains mitochondrial DNA, inherited only from maternal line
  • Have a role in programmed cell death: apoptosis
• Cellular Respiration


  1. Glycolysis

  2. Citric Acid Cycle (Kreb cycle)

  3. Oxidative Phosphorylation (electron transport chain)
• Glycolysis - Glucose is broken down into 2 pyruvate molecules
• Glycolysis


  • Anaerobic process
  • Yields 2 ATP and 2 NADH
  • Pyruvate and fatty acid enter mitochondrial matrix and are converted to acetyl CoA
• Glycolysis provides ATP under anaerobic conditions
• Pyruvate can be converted to lactate under prolonged anaerobic conditions leading to lactic acidosis
• Metabolic acidosis can result from prolonged anaerobic conditions
• Kreb Cycle (Cytric Acid Cycle)


  • Yields ATP, NADH, FADH2
  • NADH and FADH2 are oxidized in electron transport chain to produce more ATP
  • Does not require molecular oxygen from respiration...However it will cease to function in the absence of oxygen
• A total of ~30 ATP molecules are formed from the complete oxidation of glucose into CO2 and H2O (mostly from oxidative phosphorylation)
• Electron Transport Chain


  In the inner membrane of mitochondria
• Glycolysis


  1. Anaerobic catabolism, to lactic acid and little ATP

  2. Aerobic catabolism, to carbon dioxide, water, and lots of ATP
• Cell Membrane


  • Are fluid - They contain gates, channels and pumps
  • Phospholipids - hydrophilic head, hydrophobic tail- charged particles cannot cross
  • Proteins - allow material into and out of a cell
• Plasma Membrane


  • All cells enclosed by a barrier composed primarily of protein and lipid- a fluid barrier where proteins move through the membrane
  • Three types of membrane lipids: Cholesterol, Phospholipids, Glycolipids
  • Lipid molecules arrange in bilayers due to hydrophilic and hydrophobic properties
• Membrane Proteins


  • Approximately 50% of typical cell membrane's mass is protein - carry out many functions
  • Receptors
  • Transport channels/carriers
  • Enzymes
  • Surface markers
  • Cell adhesion molecules (CAMs)
  • Catalysts - cell signaling and communication
• Transmembrane proteins


  • Extend across bilayer and have contact with internal and external environments
  • Functions: Transport charged and polar molecules in and out of cells, Transduction of extracellular signals into intracellular messages
• Plasma Membrane Functions


  • Transport nutrients and waste products
  • Generate membrane potentials
  • Cell recognition and communication
  • Growth regulation
  • Sensor of signals that enable cell to respond and adapt to changes in environment
• Lipid-soluble molecules


  Cross cell membrane due to simple diffusion because they can dissolve through membrane
• Water-soluble molecules


  • Cross through water-filled channels, or pores, or may be transported by carriers
  • Most transporter proteins are highly specific for a particular molecule
• Passive transport


  Facilitated diffusion through membrane proteins - ions or solutes move down electrochemical gradients
• Active transport


  Protein pumps move solutes across membrane against the electrochemical gradient; requires use of energy
• Water movement


  Across the membranes through channels called aquaporins due to osmotic pressure differences in a passive process
• Types of Transport Across the Membrane

  • Simple diffusion
  • Facilitated diffusion
  • Carrier-mediated
  • Active Transport
  • Secondary Active Transport
• Simple diffusion


  Occurs down an electrochemical gradient, does not require energy, thus it is passive
• Facilitated diffusion


  • Occurs down an electrochemical gradient, does not require energy, thus it is passive
  • Faster than simple diffusion
  • Is carrier-mediated
• Carrier-mediated transport


  Carrier binds molecules and moves them across the membrane by a conformational change
• Sodium-driven carriers


  • The Na+ gradient created by the Na+/K+ pump drives secondary carriers
  • Ex: Ca2+ transporters in heart rely on Na+ gradient
• Digitalis inhibits the Na+/K+ pump - causes Ca2+ removal from cell to be less efficient, and increases contraction
• Passive transport carriers


  • Move substances across the membrane passively
  • Glucose is passively transported into a cell when Glut-4 transporters move to the cell surface after insulin binds receptor on cell surface
• Insulin signaling for glucose transport


  Insulin binds to the receptor on cell membrane, which signals a secondary messenger to activate the Glut-4 transporters to move to the cell surface and allows glucose to enter the cell
• Active Transport


  • Occurs against an electrochemical gradient
  • Requires direct input of metabolic energy (ATP)
  • Is carrier-mediated
  • Examples: Na+/K+ pump, Ca2+ pump in the sarcoplasmic reticulum, Proton pump H+, K+ in gastric parietal cells
• Secondary Active Transport


  • Transport of 2 or more solutes is coupled
  • One solute (usually Na+) is transported down the electrochemical gradient and provides energy for the transport of a solute against the gradient
  • Solutes can move in the same direction or opposite directions
• Charged molecules are not lipid soluble, so it must be transported through a channel or by a carrier system such as active transport or facilitated diffusion
• Ion Channels


  • Integral proteins that span the membrane involved in transport of ions
  • Selective for particular ions
  • Opening and closing of channel controlled by gates
• Voltage-gated channels


  Changes in membrane potential
• Ligand-gated channels


  Binding of a ligand
• Sodium-driven carriers


  The Na+ gradient created by the Na+/K+ pump drives secondary carriers