Bio 107: Enzymes and Membrane Transport

• Bio 107 covers enzymes, the structure and function of the cell membrane, and membrane transport.

Membrane Transport Learning Objectives

• Predict how molecules are transported through cellular membranes based on their hydrophobic or hydrophilic properties.
• Based on a molecule’s size, permeability, and concentration gradients, predict its ability to cross a biological membrane and its direction of movement.
• Differentiate between channels and pumps based on energy requirements and movement relative to the concentration gradient.
• Predict how cells use Na+/K+ ATPase pump-established concentration gradients to transport substances using a specific transporter.
• Explain how substances enter cells when there is no specific channel, transporter, or pump available.

Enzymes

• Enzymes are organic molecules that catalyze chemical reactions without being consumed in the process.
• Enzymes generally have names ending in "-ase."
• Most enzymes are proteins, but some are composed of RNA.
• Enzymes help sustain life by facilitating chemical reactions quickly.
• DNA synthase is an enzymes used for copying DNA.
• Ligase is an enzymes used for building molecules.
• Amylase is an enzymes used for digesting food.
• Oxidoreductase is an enzymes used as a catalyst for redox reactions.
• Enzymes work by bringing reactants together.
• They have high specificity because substrates bind to a particular active site on the enzyme.
• Enzymes aren't altered during a reaction.
• Once the enzyme releases the products of a reaction, it's free to undergo another reaction.
• Enzymes accelerate chemical reactions by reducing the activation energy required to start the reaction.
• Negative feedback loops that can inhibit enzyme activity are used to control the rates of chemical reactions.

Metabolic Pathway Regulation

• Competitive inhibition occurs when product molecules bind directly to the active site of an enzyme, preventing the substrate from binding.
• Noncompetitive inhibition happens when product molecules bind elsewhere on the enzyme, changing the shape of the active site.
• Enzymes are very sensitive to the conditions in the cell.
• If pH, temperature, or salt concentration is too high or too low, an enzyme can become denatured and stop working.
• Denaturation involves a change in the shape of the protein, which stops it from working properly.

Cell Membranes

• Cell membranes separate cells from their surroundings, allow cells to communicate, and regulate the movement of substances in and out of the cell.
• Phospholipids are the main components of cell membranes.
• Water intoxication can lead to death.

Membrane Structure

• Cell membranes separate cells from their surroundings and regulate substance movement.
• Phospholipids are the main components of cell membranes; they have a hydrophilic head and a hydrophobic tail.
• The fluid mosaic model describes the structure of the cell membrane
• Cell membranes are made of fluid mosaics of a phospholipid bilayer and proteins.
• Phospholipids are amphipathic molecules with hydrophobic and hydrophilic regions.
• Cell membranes create separate inside and outside environments.
• Sometimes cells need to transport things into or out of the cell.
• The side chain of the amino acid phenylalanine has nonpolar bonds (C-C and C-H) within a membrane.
• Phenylalanine would be attracted to the similarly nonpolar phospholipid tails.
• Unsaturated hydrocarbon tails can help prevent packing, creating a more fluid membrane.
• Cholesterol reduces membrane fluidity at moderate temperatures, but hinders solidification at low temperatures.
• Cell membranes are selectively permeable.

Plasma Membrane

• The plasma membrane is the boundary between the cell's internal and external environments.
• It regulates the transport of substances into and out of the cell.
• The plasma membrane receives external stimuli and initiates responses.

Membrane Transport

• A cell's interior is chemically different from its exterior.
• Solute movement across membranes relies on concentration gradients and the chemical properties.
• Membranes regulate traffic of substances.
• A concentration gradient is the difference in concentration of a substance.
• Molecules get across plasma membranes through gaps in the phospholipid bilayer, transmembrane proteins, and vesicles.

Passive & Active Transport

• Passive transport includes simple diffusion, facilitated diffusion, and osmosis and does not require energy.
• Active transport requires energy.
• Some substances pass through a lipid bilayer on their own by simple diffusion.
• Some substances require a protein to help it through the membrane by facilitated passive transport.
• Some substances need energy to get a protein to transport in the right direction via active transport.
• Some substances are too large for a single protein to transport and require bulk transport.

Passive Transport

• Passive transport is the diffusion of a substance across a membrane with no energy investment.
• Diffusion is the movement of particles of any substance so that they spread out evenly into the available space.
• At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other.
• Substances diffuse down their concentration gradient.
• Simple diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration.
• This results from dissipation of a chemical gradient and the kinetic energy of molecules
• The goal of passive transport is when equilibrium has been reached.
• The concentration gradient represents potential energy that drives diffusion.
• The rate of diffusion also depends on membrane permeability to the specific substance.
• The molecules can simply diffuse across a phospholipid bilayer if it consist of O2, CO2, water or the molecule being small, uncharged, steroids or fatty acids.
• Gas diffusion in lungs is an example of simple diffusion.
• O2 is inhaled while CO2 is exhaled.
• Hydrophobic things go through membranes a lot more easily than hydrophilic things.
• Charged molecules are the worst at easily simply diffusing across membranes.
• Facilitated passive transport has hydrophilic substances cross membranes more quickly by passing through transport proteins
• The concentration of electrolytes influences osmosis.
• Aquaporins in cell membranes allow water to pass in this type of passive transport.
• Channel proteins create a tunnel through the membrane for water to pass
• Carrier protein or transporters bind to substances and change shape to shuttle them across the membrane.

Osmosis

• Osmosis and facilitated diffusion are passive transport processes.
• Substance diffuses through protein channels in the cell membrane during facilitated diffusion.
• Intestinal cells transport glucose, so the glucose enters the cell.

Osmosis & Water Permeability

• Cells need to regulate their permeability to water.
• Cells needing to be less permeable to water does so with fewer aquaporins and more cholesterol.
• During osmosis water diffuses from an area of low solute concentration to an area of high solute concentration.
• In a hypotonic solution animal cells may be lysed. In plant cell it may be turgid.
• In a isotonic solution animal cells may be normal. In plant cell it may be flaccid.
• In a hypertonic solution animal cells may be shriveled. In plant cell it is is plasmolyzed.
• Salt water will cause water in elodea to move out of the cell.

Osmosis Problems

• If a freshwater fish is placed into a saltwater tank, it dies because water leaves its cells, causing them to shrivel up.
• Less solute concentration in solution relative to the inside of the cell represents a hypotonic solution.
• Net diffusion of water is from a surrounding solution to the inside of the cell because of a hypotonic solution.
• Cells without cell walls will gain water, swell, and lyse, or burst in a hypotonic solution.
• Equal concentration represents an isotonic solution.
• There is net diffusion of water to the surrounding solution.
• Water diffuses across the membrane at the same rate in both directions when inside an isotonic solutions.
• Greater solute concentration relative to inside of the cell is an hypertonic solution.
• Cells without cell walls will lose water, shrivel, and likely die in hypertonic conditions.
• An animal cell put in a solution with a higher than average concentration of solutes will shrivel.
• If Cynthia Lucero drank so much Gatorade that it diluted her blood, water will move into the cell from the blood causing water intoxication.
• This type of water intoxication can be caused by hyponatremia, a deficiency of sodium in the blood
• Water intoxication can lead to cerebral edema, or swelling of the brain.
• Most cells have aquaporin channels which allows water to move freely in and out
• Osmosis is the diffusion of water.

Diffusion & Transport Proteins

• If only diffusion and osmosis occurred, the concentration of things inside and outside the cell would be nearly equal. That's not the case, with most cells only letting salts move sometimes.
• Gatorade is less salty than blood and other internal body fluids
• Before the marathon, cells were about equal level of saltiness inside and out; but due to dilution of fluid and low salt content in Gatorade water will move into cells
• Not all substances can pass through a lipid bilayer on their own in simple diffusion.
• Some substances cannot pass on their own, but if they are helped through the membrane using a protein, they will go through in the direction the cell wants through facilitated passive transport.

Active Transport

• If a molecule is moving down its concentration gradient, it does not need need energy.
• If a molecule is moving against its concentration gradient, it does need energy.
• Some small molecules use active transport, which requires energy and a transport protein.
• Pump proteins use energy from ATP hydrolysis to bind to substances and change shape to shuttle them across the membrane.

Sodium-Potassium Pump

• Active transport enables cells to maintain solute concentrations that differ from the environment.
• The sodium-potassium pump is a specific case of active transport.
• Inside an animal cell, there are a low concentration of sodium ions (Na+) and high concentration of potassium ions (K+).
• The sodium-potassium pump uses ATP to energize transport of 2 K+ into the cell and 3 Na+ out of the cell.
• It creates concentration gradient by pumping 3 Na+ out of the cell and 2 K+ into the cell.
• If a potassium channel is open, potassium will tend to exit the cell because the sodium-potassium pump causes there to be a higher concentration of K+ inside the cell.
• Cotransport occurs when active transport of a solute indirectly drives transport of other substances.
• If intestinal cells have a transporter that transports Na+ down its gradient which will allow glucose to be transported from the outside to the inside of the cell.
• The Na/K pump creates a higher concentration of sodium outside the cell.

Bulk Transport

• Some things are too big for a single protein to transport, this requires bulk transport.
• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis.
• Large molecules such as polysaccharides and proteins cross the membrane in bulk inside vesicles.
• Exocytosis is the secretion of large molecules when a vesicle fuses with the plasma membrane.
• Endocytosis involves molecules taken in when the plasma membrane pinches inward, forming a vesicle.
• Transport vesicles migrate to the membrane and fuse with it during exocytosis, then releasing their contents outside the cell.
• Many secretory cells use exocytosis to export their products.
• Cells in the pancreas secrete insulin by exocytosis.
• In an endocytosis, a pocket of membrane folds in to engulf and take in large substances.
• The immune cells engulf and kill bacteria in a type of endocytosis called phagocytosis.
• Cells take in molecules by forming transport vesicles during endocytosis such as phagocytosis, pinocytosis and receptor-mediated endocytosis.

Membrane Transport Summary

• Passive transport does not require energy and substances go with their gradient from high to low concentration.
  • In simple diffusion, substances diffuse through a membrane on their own.
  • In facilitated diffusion, substances move through the membrane using a channel or transporter.
  • Osmosis is diffusion of water.
• Active transport needs ATP energy and substances go against their concentration gradient, from low to high concentration.
  • Pumps that are used for small things.
  • Endocytosis and exocytosis for the transport of big molecules.