Do Now 1.What is the cell membrane composed of? 2.Which type of macromolecule is this an example of? 3.Pick another example of the same type of macromolecule and write down what you know about that molecule. Tuesday, October 5, 2021 Obj: Review Macromolecules CW: Biological Molecules Packet HW: Finish packet Do Now Tuesday, October 5, 2021 Obj: Introduction to Cellular Transport CW: Notes on Cellular Exchange HW: None 1.Draw and label a phospholipid bi layer. 2.Which part is hydrophobic? Which part is hydrophilic? 3.Why is the cell membrane known as being semipermeable? Unit 4: Cellular Exchange with the Environment Cell Membrane • Phospholipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings. • Regulates what enters and leaves the cell and protects and supports the cell. • Selectively permeable – some substances can pass across and others cannot. • Large molecules, polar molecules and ions generally cannot pass through. Composition of Cell Membrane • Mostly composed of phospholipids. • Amphipathic – contains hydrophilic and hydrophobic regions. • Also includes proteins, cholesterols, glycoproteins and glycolipids. • Proteins aid in transport, receptors, cell-cell adhesion • The cell membrane is known as the fluid-mosaic model. • “Fluid” = parts of membrane are motile (move) • “Mosaic” = many different types of molecules floating in the membrane. Cell Growth and Division • Why don’t cells grow large enough to see? • 2 reasons why cells have to divide: • DNA overload – as a cell grows larger, the demand on DNA to provide information for protein synthesis becomes to great. • Material exchange across membrane becomes too much in large cells • Bigger cells need more food and oxygen, etc. • The ability to transport oxygen, food, and waste across cell membranes depends on surface area. • As a cell grows larger, the volume of the cell increases at a faster rate than the surface area. • Surface area to volume ratio = surface area/volume 1 mm cube 5 mm cube 125 1 mm cubes Surface Area 6 mm2 150 mm2 750 mm2 Volume 1 mm3 125 mm3 125 mm3 Surface Area to Volume Ratio 6:1 = 6 1.2:1 = 1.2 6:1 = 6 Solute, Solvent, Solution • A solution is a mixture consisting of a solute dissolved into a solvent. • The solute is the substance that is being dissolved, while the solvent is the dissolving medium. • Water is an excellent solvent as it is able to dissolve many materials in solution. Passive Transport • The cell membrane must keep the cell’s internal conditions relatively constant by regulating the movement of molecules from one side of the membrane to the other. • Passive transport is the movement of materials across the cell membrane without using cellular energy (ATP). • Materials move with the concentration gradient. • Examples of passive transport: diffusion, facilitated diffusion, osmosis Diffusion • Diffusion is the process by which particles move from an area of high concentration to an area of low concentration. • Small and uncharged molecules pass through the membrane most easily • Oxygen and carbon dioxide Facilitated Diffusion • Facilitated diffusion is the process in which molecules that cannot directly diffuse across the membrane pass through special protein channels • Ions like Cl- and large molecules like glucose Osmosis • Because the inside of the lipid bilayer is hydrophobic, water has a hard time passing through the cell membrane. • Special water channel proteins, aquaporins, allow water to pass through them. • The movement of water through the cell membrane is an example of facilitated diffusion. Osmosis • Osmosis is the diffusion of water through a selectively permeable membrane. • Just like in diffusion, there is movement from high concentration to low concentration. • However, in osmosis, water molecules move, not solute molecules. How Osmosis Works • Water will move across the membrane until equilibrium is reached. • Equilibrium = the concentrations of water and solute will be the same on both sides. • An isotonic solution occurs when the concentration of the solute inside of the cell and outside of the cell are equal. = solvent = solute How Osmosis Works • Water will move across the membrane until equilibrium is reached. • Equilibrium = the concentrations of water and solute will be the same on both sides. • Hypertonic solutions occur when the concentration of the solute is higher outside of the cell than inside. = solvent = solute How Osmosis Works • Water will move across the membrane until equilibrium is reached. • Equilibrium = the concentrations of water and solute will be the same on both sides. • Hypotonic solutions occur when the concentration of the solute is lower outside of the cell than inside. = solvent = solute What happens to cells when placed in different solutions? • Isotonic solution – Amount of water entering the cell is equal to the amount leaving the cell • cell remains the same size What happens to cells when placed in different solutions? • Hypertonic solution – more water exits than enters the cell • Animal cells shrink • Cell membrane pulls away from plant cell wall as central vacuole loses water What happens to cells when placed in different solutions? • Hypotonic solution – more water enters the cell than leaves the cell • Animal cell swells and may burst • Plant cell wall withstands osmotic pressure Osmotic Pressure • Osmotic pressure is pressure that must be applied to prevent osmotic movement (water) across a selectively permeable membrane. Osmotic Pressure • Cells contain dissolved materials and are almost always hypertonic to fresh water. • As a result, water will move into a cell, causing it to swell. • In plant cells, osmotic pressure causes the central vacuole to shrink or swell, cell wall maintains the shape of the cell. Active Transport • A cell must sometimes move materials against the concentration gradient. • Moving more sugar into the mitochondrion for cellular respiration • Active transport is the movement of materials against a concentration difference and requires energy (ATP). Active Transport • Can move molecules faster than diffusion • Movement of Na+ and K+ ions generate electrochemical impulses along nerve cells Types of Active Transport • Protein pumps: Sodium-Potassium, Protons • Endocytosis • Exocytosis Molecular Transport • Small molecules and ions are carried across membranes by protein pumps. • Calcium, potassium, sodium ions • Changes to the shape of the protein pump play an important role in the pumping process. • Considerable portion of the energy used by cells is spent on keeping these pumps going. • Enables cells to concentrate substances in a particular location. Proton Pumps • Move protons (H+) across membranes • Essential to cellular respiration and photosynthesis Bulk Transport • Movements of the cell membrane allow for large molecules and clumps of solid material to be transported into and out of the cell. Endocytosis • Endocytosis is the process of taking materials into the cell by means of in-folding of the cell membrane • Vesicles pinch off of the membrane and transport the molecules into the cell. Exocytosis • To release large amounts of materials, cells perform exocytosis. • The membrane of the vesicle fuses with the cell membrane, forcing the contents out of the cell. • Golgi body releasing materials out of the cell • Contractile vacuole removing excess water • Secretory vesicle releasing insulin (protein hormone) from a pancreas cell
