Membranes and Cell Transport Lab Exercise – Week # 8
Student Readings • Lecture textbook – Campbell and Reece, Seventh Edition Pages 124 – 138
Objectives For the student to: • Define the term selective permeability and describe the role of the plasma membrane in regulating the movement of materials into and out of the cell 2. Contrast and compare passive and active transport. Identify each of the following as either active or passive processes: diffusion; osmosis; phagocytosis; filtration; and exocytosis.
Objectives: 3) Define the terms diffusion and osmosis and understand their significance in living systems. 4) Define the terms isotonic, hypertonic and hypotonic and describe the effect of tonicity on cell volume.
Objectives 5) Explain the effect of temperature, medium (liquid versus semi-solid) and molecular weight on the rates of diffusion and osmosis. 6) Define the term phagocytosis and describe several factors that may effect the ability of a single celled organism (tetrahymena) in ingest material in this way.
Homeostasis Selective permeability Passive transport Active transport Hydrophobic Hydrophilic Diffusion Osmosis Phagocytosis Filtration Exocytosis Solute pump Isotonic Hypertonic Hypotonic Hemolysis Crenation Plasmolysis Microtubule Microfilament Vacuole Elodea Tetrahymena VocabularyWrite out definitions for the following terms.
Introduction • To perform their functions, cells must maintain a steady state in the midst of an ever-changing environment. • Steady state is maintained by regulating the movement of materials into and out of the cell.
Introduction • Regulating the movement of materials into and out of the cell is a major role of the cell membrane. • The cell membrane is selectively permeable – allowing some substances to cross more easily than others.
Introduction • The molecular structure of the cell membrane is a phospholipid bilayer (hydrophilic exterior and interior and a hydrophobic core). • Because of its structure, factors such as the size of the molecule, whether it is hydrophobic or hydrophilic, and the tonicity of the solution are all potential factors affecting transport across it.
Introduction • This laboratory experiment explores the mechanism that move molecules across cell membranes. • The means by which molecules move across membranes are either active (energy expended) or passive (no energy expended) transport.
Passive Transport Definition:The movement of a substance across the membrane with no energy investment. • Includes: Diffusion Osmosis Facilitated Diffusion
Active Transport Definition Process uses energy to move solutes across the membrane (against their gradients) Includes:Solute pumps Exocytosis Endocytosis – Phagocytosis and Pinocytosis
Diffusion • Definition Spontaneous process by which molecules move from a region where they are highly concentrated to a region in which their concentration is Iower. • Results from the kinetic activity of molecules (constant motion)
Diffusion Passive process that moves molecules from high concentration to low (across cell membranes when permeable). Click here to view Diffusion Video Clip
Osmosis • Definition Diffusion of water across a selectively permeable membrane. Water moves from its highest concentration or potential to its lowest concentration or potential. Pure water has the highest potential. The more solute dissolved in water the lower the potential. .
Osmosis • Maintaining the water content of the cell is CRITICAL to the functioning of all plant and animal cells. Click here to view Osmosis Video Clip
Tonicity • Definition:The ability of a solution to cause a cell within it to gain or lose water • Tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane.
Isotonic • "ISO" means the same • If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won't have any result on the overall amount of water on either side.
Hypotonic • The word "HYPO" means less, in this case there are less solute (salt) molecules outside the cell, (and more water molecules). Therefore, water moves from its highest concentration outside the cell into the cell. • The cell will gain water and grow larger.
Hypotonic • When placed in a hypotonic solution:Animal cells, lacking a rigid cell wall, are in danger of bursting, called hemolysis. In some cases organelles called contractile vacuoles will pump water out of the cell to prevent this. (Tetrahymena –protist with this cabability will be discussed later). • In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid (called turgor), the cell wall keeps the plant cell from bursting
Hypertonic • The word "HYPER" means more, in this case there are more solute (salt) molecules outside the cell, which causes the water to leave the cell (again moving from high water concentration inside the cell to a lower concentration outside the cell).
When Placed in Hypertonic Solution • Plant cells, the central vacuole loses water and the cells shrink, causing wilting. • Animal cells, the cells also shrink, called crenation. • In both cases, the cell may die.
Tonicity Click the icon to begin the movie. Clicking on the screen at any point will pause the movie.
Active Transport • Energy is expended by the cell to move a molecule across its membrane against its concentration gradient. (moving it from low concentration to high). • Important in maintaining ion concentrations and water balance in many cells.
Active Transport • Organisms living in fresh water are immersed in a hypotonic solution. • Water moves into their cells. Some cells handle this by actively removing the water. • ATP is expended to move water out of the cell against its concentration gradient (low concentration of water inside the cell, high concentration of water outside the cell).
Active Transport • Phagocytosis – “Cell Eating” • Active transport process by which particles are brought into the cell and packaged within a membrane enclosed sac called a vacuole. • Action of immune cells that ingest and destroy foreign particles, and a way of attaining nutrients for some cells.
Tetrahymena • Tetrahymena - a teardrop-shaped, unicellular, ciliated freshwater protozoan about 50 µm long. • Tetrahymena -species are very common in aquatic habitats and are non-pathogenic Scanning electron micrograph of single tetrahymena cell. http://www.lifesci.ucsb.edu/~genome/Tetrahymena/genetics.htm
Tetrahymena Actively pumps water out of the cell Therefore, can be used to demonstrate what happens to water transport out of the cell when the cells are placed in solutions of differing tonicity. This organism also phagocytizes materials, and will be used in lab experiments to investigate multiple factors affecting phagocytosis (active transport)
TetrahymenaPhagocytosis – shown after ingesting ink Image: Drs. Barnett, Strange, Summers and Hopper
Elodea • Aquatic plant; commonly used in freshwater aquariums • Important part of lake ecosystems • http://www.ecy.wa.gov/programs/wq/plants/native/elodea.html
Elodea • Normal conditions: It is not possible to see the cell membrane because it is pushed up against the cell wall. Also, the central vacuole cannot be clearly distinguished from the cell cytoplasm. In a living elodea cell you can “roughly” tell where the vacuole is by where the moving chloroplasts are not able to go. • viewed using light microscope • http://science.exeter.edu • /jekstrom/WEB/CELLS/Elodea/
Elodea • Placed in hypertonic solution. • Water inside the plant moves from where there is more water (less salt) through the cell wall and membrane to the outside where there is less water (more salt). This process of water movement is called osmosis, or In plants commonly referred to as plasmolysis. (note choloroplasts clumped in the middle as cell membrane collapses toward the center, while the cell wall remains rigid) • http://science.exeter.edu • /jekstrom/WEB/CELLS/Elodea/
Lab Procedures • Writing hypothesis statements • Applying the Scientific Method • Use of metric system • Use of pipettes • Wet Mount preparation • Use of light Microscope • Basic Statistical Analysis
Writing Hypotheses • Remember the “If” - “Then” rule For example: If a red blood cell is placed in a hypertonic solution, then the red blood cells internal volume will decrease.
Scientific Method In lab, be prepared to follow the scientific method: • Observation of Phenomena(already decided by your instructor and introduced in this presentation) • Stating your hypothesis • Data Collection • Analysis of Data • Writing your results and conclusions
Metrics For this weeks lab - please review basic metric units for Volume One Liter (L) = 1.05 Quarts Deciliter (dl) = 0.1 or 1/10 liter Centiliter (cl) = 0.01 or 1/100 liter Milliliter (ml) = 0.001 or 1/1,000 liter Microliter (ul) = 0.000001 or 1/1,000,000 liter Students will be measuring solutions in the microliter (ul) range For more on metrics visit: http://www.visionlearning.com/library/module_viewer.php?mid=47
Statistical Analysis Students should review procedures for determining: • Mean • Standard deviation • Standard error of the Mean (S.E.M.) See Appendix 6 in the Lab Manual These calculations will be completed in class to statistically analyze data. For more information visit http://davidmlane.com/hyperstat/A16252.html Click here to download a problem set for practice!
Review – Key Concepts • Regulating the movement of materials into and out of the cell is a major role of the cell membrane. • Membrane structure results in selective permeability. • Passive transport is diffusion of a substance across a membrane with no energy investment • Active transport uses energy to move solutes against their gradients. • Maintaining the water content of the cell is CRITICAL to the functioning of all plant and animal cells.
Don’t Forget Don’t forget to complete the pre-lab quiz titled Membrane and Cell Transport PRIOR to attending lab this week. Have a great week!