Chapter 4

• OUTLINE


  • INTRODUCTION TO PLASMA MEMBRANE
  • PASSIVE TRANSPORT
  • ACTIVE TRANSPORT
  • PHAGOCYTOSIS & PINOCYTOSIS

  
• Why is movement across cell membrane important?

  • To obtain nutrients
  • Excretion of waste products
  • Secretion of useful products
  • Generate ionic gradient for nerve & muscle action
  • Maintain suitable pH and ionic concentration in cell for enzyme action

  
• Passive transport


  • This type of transport requires NO energy because particles travel from where they are highly concentrated to a low concentrated area
  • They travel DOWN the concentration gradient
  • Substances pass by diffusion and osmosis

  
• Simple diffusion


  • Requires NO energy
  • Molecules move from area of HIGH to LOW concentration, until equilibrium is reached
  • Diffusion is a PASSIVE process which means no energy is used to make the molecules move, they have a natural KINETIC ENERGY
  • Movement of molecules is random and spontaneous

  
• Small, Non-polar Molecules and Lipid-Soluble Substances crossing the plasma membrane


  • Small, Non-polar Molecules: These molecules, such as oxygen, carbon dioxide, and some lipids, can freely diffuse through the lipid bilayer because the hydrophobic interior of the membrane is more favorable to their passage
  • Lipid-Soluble Substances: Certain substances that are lipid-soluble, such as steroid hormones, also diffuse through the plasma membrane. They dissolve in the lipid bilayer and pass through it easily

  
• Facilitated diffusion


  • Another form of passive transport
  • Used for molecules that are too large to cross the membrane by diffusion (i.e. glucose)
  • With the help of transport protein: Carrier proteins bind to larger molecules and change their shape so molecules can diffuse through, Channel proteins provide water filled pores for charged ions to pass through

  
• Channel protein


  • Definition: Channel proteins are integral membrane proteins that form aqueous pores or channels across the lipid bilayer of the cell membrane
  • Example: One common example of a channel protein is the aquaporin. Aquaporins facilitate the passage of water molecules across the cell membrane, allowing water to move freely in and out of cells based on osmotic gradients
  • Energy Requirement: These channels facilitate the passive movement of ions or small molecules along their concentration gradient, without requiring energy input. The process is driven solely by the concentration gradient of the molecules being transported

  
• Carrier protein


  • Definition: Carrier proteins are also integral membrane proteins involved in the transport of molecules across the cell membrane. Unlike channel proteins, carrier proteins bind to specific molecules on one side of the membrane, undergo a conformational change, and release the molecules on the other side
  • Example: An example of a carrier protein is the glucose transporter (GLUT). GLUT proteins facilitate the transport of glucose molecules across the cell membrane, ensuring the uptake of glucose into cells for energy production
  • Energy Requirement: Carrier proteins can either require or not require energy, depending on the type of transport they facilitate
• Osmosis


  • Osmosis is the passive movement of water, across a selectively permeable membrane from a region of higher water potential to a region of lower water potential, driven by the concentration gradient of solute molecules
  • Higher Water Potential: This refers to a solution with fewer solute particles and more free water molecules, resulting in a higher tendency for water to move into the solution
  • Lower Water Potential: This refers to a solution with more solute particles and fewer free water molecules, resulting in a lower tendency for water to move out of the solution

  
• Aquaporins


  • Membrane protein or water channels that enhances osmosis
  • Present in plasma membrane of both plant and animal cells
  • Speed up osmosis without changing the direction of water movement

  
• Tonicity & Water Potential

  • Tonicity is the ability of cells to gain or lose water
  • Water potential, Ψwater is the tendency of water molecules to move from one place to another
  • Pure water has the highest water potential
  • Water potential of pure water is zero at atmospheric pressure
  • Hypotonic: more water than in the cell
  • Hypertonic: less water than in the cell
  • Isotonic: same amount of water
• Solute Potential


  • Also known as osmotic potential
  • Solutes make water potential negative (less than zero)
  • The amount of solute lowering the water potential of a solution is called solute potential
  • How to increase water potential? Apply physical pressure
  • Ψwater = Ψsolute + Ψpressure
• Active transport


  • Active transport is a biological process that moves molecules or ions across a cell membrane from an area of lower concentration to an area of higher concentration (against their concentration gradient) using the input of energy, usually in the form of adenosine triphosphate (ATP)
  • Divided into two main types: Primary Active Transport- Direct use of ATP, Secondary Active Transport- Indirect involvement of ATP

  
• Primary active transport


  • Energy is derived directly from ATP hydrolysis
  • Ex.: Sodium-potassium pump
  • Maintains the concentration of sodium ions (Na+) and potassium ions (K+) inside cell
  • Cells must maintain a lower concentration of Na+ and higher concentration of K+ inside the cells
  • To maintain this, active transport is needed to move the substances against the concentration gradient
• Primary active transport


  1. Binding of Intracellular Sodium (Na⁺)

  2. ATP Hydrolysis

  3. Phosphorylation of pump and Conformational Change

  4. Extracellular Release of Sodium (Na⁺)

  5. Binding of Extracellular Potassium (K⁺)

  6. Dephosphorylation and Conformational Change

  7. Intracellular Release of Potassium (K⁺)

  8. Resetting the Pump
• Secondary active transport


  Involves 2 processes: Symport & Antiport
• Cotransport/Symport


  • A pump that transports 2 substances in the same direction across a membrane
  • Example: Transport of glucose in intestinal cells, Transport of iodine in thyroid gland using Iodine pump
  • Glucose is transported from the gut into the intestinal epithelial cells
  • To ensure that glucose always flows into intestinal cells and get transported into the blood stream
• Cotransport/Symport (Mechanism)


  1. Two molecules, like glucose and sodium ions, bind to a couple protein in the cell membrane simultaneously

  2. The energy released from one molecule moving down its concentration gradient (like sodium ions) powers the transport of the other molecule (like glucose) against its concentration gradient

  3. Both molecules are carried across the membrane in the same direction

  4. Inside the cell, the molecules are released, and the protein resets to transport more molecules

  5. This process allows the cell to efficiently take up essential substances, like glucose, even when they're at lower concentrations outside the cell
• Cotransport/Symport (Important Aspect)


  • Glucose will hitch a ride together with Na+ to be transported into the intestinal cell
  • Na+ is transported down the concentration gradient
  • Glucose is transported against the concentration gradient
  • Energy does not come directly from ATP to transport glucose against its concentration gradient
  • Energy comes from the Na+ concentration gradient created by Na+/ K+ pump present in the plasma membrane of the intestinal cells
• Countertransport (Antiport)


  • A pump that transports 2 substances simultaneously in opposite directions across a membrane
  • Ex: sodium-calcium exchanger in cardiac cells
  • Na+ is transported into the cell down the concentration gradient
  • Ca2+ are transported out of the cell against the concentration gradient
  • Energy does not come directly from ATP to transport Ca2+ against the concentration gradient
  • Energy comes from the Na+ concentration gradient created by the Na+/K+ pump present in the plasma membrane of the cardiac cells
• Membrane trafficking


  • Flow of membrane materials between organelles and the plasma membrane
  • Essential for transport of protein and other macromolecules to various places inside and outside of cells
  • Ex: flowing of secretory products from the Golgi apparatus into the secretory vesicles to be passed on to the plasma membrane for release
• Membrane trafficking


  • Endocytosis
  • Exocytosis
• Endocytosis


  • A process where materials (fluids, large particles and even other cells) are transported into cells by formation of vesicles
  • The vesicle then pinches away from the plasma membrane
  • 3 types: Phagocytosis (cell eating), Pinocytosis (cell drinking), Receptor- mediated endocytosis

  
• Phagocytosis


  • Folding of plasma membrane around the engulfed material/ molecule
  • Followed by formation of intracellular vesicles (phagosomes)
  • Phagosome will fuse with lysosome
  • The material/ molecule will then be digested by the hydrolytic enzymes released by the lysosome

  
• Pinocytosis


  • Intake of fluid material into cell by formation of pinocytic vesicles (pinosome)
  • Cells form an invagination
  • Dissolved materials (organic molecules & nutrients) will be brought into cells in pinosomes
• Receptor-mediated endocytosis


  • Receptor-mediated endocytosis is a highly specific process by which cells internalize specific molecules from the extracellular environment
  • It involves the uptake of ligands (molecules) that bind to specific receptors on the cell surface
  • This mechanism allows cells to regulate the intake of substances like hormones, growth factors, enzymes, and lipoproteins
• Exocytosis


  • Exocytosis, the process by which secretory vesicles release their contents outside the cell
  • Wastes, proteins, hormones and secretory products are released from the cell
  • Large molecules that are manufactured in the cell are released through the plasma membrane through exocytosis
  • Divided into two types: Regulated exocytosis, Unregulated exocytosis

  
• Biocatalyst


  Enzymes, Enzyme Kinetics, Enzyme Inhibition, Proenzymes and Isoenzymes
• Catalyst


  A substance that speeds up a chemical reaction by providing an alternative pathway with a lower activation energy, without being consumed or permanently altered in the process
• Enzyme


  Specialized proteins that act as biological catalysts, accelerating chemical reactions in living organisms without being consumed in the process
• Enzymes


  • Primarily composed of protein molecules, though some RNA molecules called ribozymes also exhibit catalytic activity
  • Each enzyme typically catalyzes a specific reaction or a group of similar reactions, owing to its unique three-dimensional structure and active site
  • Have a region called the active site where the substrate binds, undergoes a chemical reaction, and then releases the product
  • Lower the activation energy required for a chemical reaction to occur, thereby increasing the rate of the reaction
  • Are not consumed in the reactions they catalyze; they remain unchanged after the reaction and can be reused multiple times
  • Enzyme activity can be influenced by factors such as temperature, pH, substrate concentration, and the presence of inhibitors or activators
  • Enzyme activity is tightly regulated within cells to maintain metabolic pathways and cellular homeostasis
  • Are characterized by their remarkable efficiency and specificity in catalyzing chemical reactions, often increasing reaction rates by factors of millions to billions compared to uncatalyzed reactions
• How enzymes work


  1. Lowering activation energy (free energy)

  2. Role of active site of enzyme
• Enzyme specificity


  Enzymes catalyse only one type of reaction and act on only one or one related group of substrates
• Lock and key model

  The enzyme is like a lock, and its active site is the keyhole. The substrate molecule is compared to a key that fits into the lock (the enzyme's active site).
• Induced fit model


  Initially, the enzyme and substrate exist in different shapes. When the substrate binds to the enzyme's active site, the enzyme undergoes a conformational change to better accommodate the substrate.
• Enzyme classification


  • Oxidoreductase
  • Transferase
  • Hydrolase
  • Lyase
  • Isomerase
  • Ligase
• Enzyme kinetics


  The study of the rate at which an enzyme works
• Maximal velocity (Vmax)


  The maximum velocity of the reaction, when 100% of active sites of enzymes are saturated with substrate
• Effect of substrate concentration on enzyme activity

  Initially the velocity of reaction is proportional to the substrate concentration, then the velocity increases, and finally the velocity becomes independent of substrate concentration because all enzyme molecules are saturated with substrate molecules
• Michaelis-Menten constant (Km)


  The substrate concentration at which half the enzyme's active sites are occupied by substrate, or 50% of active sites of enzymes are saturated with substrates. Inversely proportional to affinity of enzyme for substrates.