Week 14

• Welcome to MCB 181R
• Today, we will talk about lipids and cell membranes
• Dr. Corin Gray, she/her/they/them
• Week 14 - Lipids & Cell Membranes
• Mid-semester check-in survey & Exam 3
• Exam 3 Scores Updated
• 181R Grade Reminder


  Quizzes and reflections are 17%, in-class participation is 15%, the final project is 8% and exams are 60%. With a 65% exam average, you can still get a B in this class if you receive full credit in these other categories
• Post the slides earlier please - I adapt my slides to the needs of the class and edit them after most students have completed the pre-class quizzes
• Students have mixed feelings about groupwork and a "flipped classroom". The goal of groupwork is to give you thought partners and people to work with in May for the final project. We only have 2 more group work activities minus the final project!
• More videos and tutorials please - Mason (TA) has been posting tutorials and I have posted some videos that should help for exam 4
• More SI and review sessions please - There are 9 SI sessions every week and several review sessions several days before the exam. For exam 4 (April 22nd), the LT will host an in-class review session on April 17th
• Why aren't all SI and review sessions recorded? We like to encourage questions from students and don't want people not to participate because of the recording. Also, sometimes, we don't have the capability to record
• Exam keys and office hours. We don't post exam keys because I don't want exam materials distributed. Most of the learning team is available to meet one-on-one if you reach out to schedule a meeting with them several days in advance
• More metaphors, analogies and different ways of explaining things. This is a work in progress! If you find one that is useful to you, please let us know so we can share it with the class
• Gradient


  A difference across a distance
• Concentration gradient


  High to low, "with" or "down" the gradient. Low to high, "against" or "up" the gradient

  
• If there is a path available, the net (total) movement of solute particles via diffusion will be with or down the solute's gradient

  
• Diffusion (Passive transport)


  The net (total) movement of solute particles will be from areas of high concentration to areas of low concentration. Diffusion is due to the random, undirected motion of the solutes and surrounding water. Particles DON'T "know" they are in an area of high concentration it takes a l-o-n-g time for particles to diffuse much

  
• Osmosis


  A special case of diffusion when there are solutes that cannot quickly pass across the cell membrane and there is a higher solute concentration on one side of the membrane than the other. The net (total) movement of water will be to the area of higher solute concentration i.e. lower water concentration. Water moves into and out of cells by passive diffusion. Water can also move into a cell more readily by facilitated diffusion using channels (aquaporins)

  
• Proteins are important components of biological membranes
• Peripheral membrane proteins


  Temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions
• Integral membrane proteins


  Permanently associated with cell membranes and can be transmembrane proteins spanning the entire lipid bilayer
• Passive transport


  Works to the cell's advantage only when the concentration gradient is in the right direction e.g. from higher concentration on the outside to lower concentration on the inside. Molecules that are small and nonpolar (e.g. O2, CO2) cross the plasma membrane by simple diffusion. Molecules that cannot move across the lipid bilayer directly can move passively toward a region of lower concentration by facilitated diffusion

  
• Simple Diffusion


  Moving "down" the concentration gradient
• Facilitated Diffusion


  Molecules that cannot move across the lipid bilayer directly can move passively toward a region of lower concentration. During facilitated diffusion, the molecule moves through a membrane protein channel or carrier. Carriers exist in two shapes: one that is open to one side of the cell and another that is open to the other side. Binding of the transported molecule induces a conformational change allowing the molecule to be transported across the lipid bilayer

  
• Ion channels


  Provide a hydrophilic pathway altering the flow of ions across the plasma membrane. Channels (ligand binding, voltage) open and close by "gating" mechanisms
• Active Transport


  The movement of substances against their concentration gradient (from areas of lower concentration to areas of higher concentration) requires energy and is known as active transport. The sodium/potassium pump is a membrane protein that uses the energy stored in ATP to move sodium and potassium ions against their concentration gradients

  
• Secondary Active Transport


  Active transporters can drive the transport of molecules through a different transporter through the creation of an electrochemical gradient. The active transporter actively pumps protons across the cell membrane using ATP which creates an electrochemical gradient because of the charge of the protons. The movement of protons down their concentration gradient drives the movement of other molecules against their gradients

  
• Nonpolar covalent bond


  Atoms have the same or similar electronegativity e.g. bonds between C or C and H
• Polar covalent bond


  Atoms have different electronegativity e.g. bonds between C and O or O and H
• Hydrophilic


  Water-loving, water-soluble, have polar covalent bonds
• Hydrophobic


  Water-fearing, not water-soluble, have primarily nonpolar covalent bonds
• Lipids


  Generally defined by having large nonpolar regions that are hydrophobic. Some lipids have polar (hydrophilic) and nonpolar (hydrophobic) regions, referred to as amphipathic
• Saturation and length of lipid tails


  Affect packing and association of lipids. More saturation generally means packed more tightly with a decrease in fluidity
• Selectively permeable membrane


  All cells are enclosed by a plasma membrane that serves as a boundary defining the space of the cell. The membrane separates the internal contents from the surrounding environment, keeping the internal environment within a persistent and narrow window of conditions. The maintenance of a constant environment is known as homeostasis. The cell membrane is selectively permeable, allowing some items to move in and out freely, and others to move in and out under certain conditions usually with the help of a protein

  
• Permeability


  How easily solutes pass across the phospholipid bilayer, is also directly related to fluidity

  
• To compensate and maintain consistent fluidity at all temperatures, the composition changes
• As temperature increases so does fluidity
• Fun Fact: The following represent some of the organisms that have been shown to have changes in lipid composition (bilayer) as a response to decreasing temperatures
• Cells need to maintain homeostasis and membrane fluidity, how might the microbial species respond to the stress of having to adapt to cooler temperatures? Ideally, membrane fluidity would match what was observed at room temperature