L4 GRQs Membrane Structure and Function Reading Objectives Describe cell membrane structure Differentiate molecules that can freely cross cell membranes vs those that can t Compare and contrast the different ways in which various substances cross membranes Make predictions about water movement during osmosis Apply the two mechanisms of chemical signaling to insulin signaling and sex hormone Module 5 1 5 2 Membranes are fluid mosaics of lipids and proteins with many functions The spontaneous formation of membrane was a critical step in the origin of life 1 What is meant by the terms fluid mosaic and selective permeability when describing a The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components including phospholipids cholesterol proteins and carbohydrates that give the membrane a fluid character b Selective permeability means that the membrane regulates what substances cross signaling membranes it evolution 2 Why was the spontaneous formation of membranes such an important step in cell a Membranes formed spontaneously can enclose a solution with a different composition than its surroundings Self replicating molecules like RNA and a useful variety of other molecules might be present in that fluid and transferred to daughter cells when new membrane enclosed vesicles budded off One essential component of life is a membrane that controls chemical interactions with the environment in addition to enclosing a successful molecular assembly Module 5 3 Passive transport is diffusion across a membrane with no energy investment 1 Use the words diffusion and concentration gradient in a sentence a Diffusion is the movement of particles along a concentration gradient moving from the side with more particles to the side with less particles 2 How does the structure of the membrane relate to its selective permeability Think about the chemical properties of the inner core of the bi layer a The membrane is a bilayer of phospholipids with hydrophilic heads and a hydrophobic inner core allowing substances with similar characteristics to pass through while differentiating polarity substances cannot 3 Can O2 and CO2 diffuse easily across a membrane Why or why not Review and define the words polar and nonpolar if you need to a Non polar a molecule in which all atoms have the same electronegativity and the electron distribution is equal i Molecules can cross freely through a membrane because the inner core tails of a membrane are also nonpolar b Polar Occur when there is an electronegativity difference between the bonded atoms negative charge on one side positive charge on the other unequal sharing of electrons i ii Molecules CANNOT cross freely through a membrane because inner core is nonpolar and like does not dissolve unlike c O2 and CO2 pass through the plasma membrane because they are small non polar molecules that can diffuse easily through the lipid bilayer They move down their concentration gradients O2 generally moves into cells while CO2 moves out 4 What do ions and polar molecules need to diffuse across the hydrophobic interior of a membrane a They need transport proteins Module 5 4 Osmosis is the diffusion of water across a membrane 1 When water moves through a selectively permeable membrane down its concentration gradient this is termed Osmosis free water less concentrated concentration 2 If I give you a 5 sucrose solution and a 10 sucrose solution in which solution is the a Water is less concentrated in the 10 solution because there is a higher solute 3 If the 5 and 10 solution were separated by a semi permeable membrane that sucrose could not move between the water would move into the 10 solution Using Figure 5 4 to check your answer Module 5 5 Water balance between cells and their surroundings is crucial to organisms 1 Tonicity refers to the ability of a surrounding solution to to cause a cell to gain or lose 2 Tonicity of a solution is dependent on its concentration of solutes relative to the water concentration of solutes inside the cell 3 Define a Isotonic Isotonic is referring to a solution that when surrounding a cell causes no net movement of water into or out of the cell b Hypertonic Referring to a solution that when surrounding a cell will cause the c Hypotonic Hypotonic is referring to a solution that when surrounding a cell will cell to lose water cause the cell to take up water 4 Practice If a cell is put into a solution that is hypertonic to its cytosol what would happen shrink or swell Why Where is the water more concentrated a The cell will shrink because water will have left the cell due to osmosis The hypertonic solution has a higher solute concentration meaning the water concentration is higher inside the cell Modules 5 6 Transport proteins can facilitate diffusion across membranes 1 How is facilitated diffusion similar and different from diffusion a Facilitated diffusion is the diffusion of polar molecules across the membrane with the help of transport proteins and it does not require any energy b It is similar because both do not require energy Both have the driving force of a concentration gradient 2 In our discussion of how cells use glucose as an energy resource we will see that cells can bring glucose in from the blood where the concentration is higher What kind of transport does glucose use a It uses facilitated diffusion with the use of transport proteins 3 Water is a polar molecule but it is very small so it can diffuse slowly into cells What type of membrane protein would you expect to find in a cell to make it highly permeable to water Aquaporin a Specialized proteins that form channels in the cell membrane allowing water molecules to pass through while blocking other substances i These proteins are crucial for regulating water flow in and out of cells Module 5 8 Cells expend energy in the active transport of solute 1 How do calcium ions move against their concentration gradient a Through the use of active transport transporting polar molecules in and out of the cell through the use of energy transporting the molecules against their concentration gradient 2 How is this similar or different from facilitated diffusion a It is similar to diffusion because molecule are moved across the membrane it is different because it uses energy and molecules can be transported against the gradient Module 5 9 Exocytosis and endocytosis transport large molecules across membranes 1 Compare and contrast exocytosis and endocytosis a
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