CELLS and TRANSPORT Student Packet SUMMARY CELL MEMBRANES ARE SELECTIVELY PERMEABLE DUE TO THEIR STRUCTURE Hydrophilic head Hydrophobic tail Hydrophobic regions of protein Hydrophilic regions of protein Cell membranes separate the internal environment of the cell from the external environment. Cell membranes consist of a structural framework of phospholipid molecules, embedded proteins, cholesterol, glycoproteins and glycolipids. Surface area-to-volume ratios affect a biological system s ability to obtain necessary resources or eliminate wastes; the surface area of the plasma membrane must be large to adequately exchange materials. GROWTH AND HOMEOSTASIS ARE MAINTAINED BY THE MOVEMENT OF MOLECULES ACROSS MEMBRANES Molecules and atoms from the environment cross cell membranes and build new molecules. Carbon is used in storage molecules and cell formation in all organisms. Small nonpolar and uncharged molecules pass freely across the membrane. Hydrophilic substances such as large polar molecules and ions move across the membrane through embedded channel and carrier proteins. Water moves across membranes mostly through channel proteins called aquaporins. Diffusion of water is called osmosis. The diagrams below compare osmosis in animal and plant cells. Passive transport (a.k.a. diffusion) does not require the input of energy and the net movement of molecules is from high to low concentration. Active transport is a process where free energy (often provided by ATP) is used by proteins embedded in the membrane to transport molecules and/or ions across the membrane and to establish and maintain concentration gradients. 1
Endocytosis and exocytosis move large molecules from the external environment to the internal environment and vice versa. EUKARYOTIC CELLS MAINTAIN INTERNAL MEMBRANES THAT PARTITION THE CELL INTO SPECIALIZED REGIONS Prokaryotic cells of domains Archaea and Bacteria lack nuclei and other membrane-bound organelles while eukaryotic cells have internal membranes that compartmentalize cellular functions and specific enzymatic reactions. Examples of membranes include the endoplasmic reticulum, mitochondria, chloroplasts, Golgi, lysosomes and nuclear envelope. THE STRUCTURE AND FUNCTION OF SUBCELLULAR COMPONENTS PROVIDE ESSENTIAL CELLULAR PROCESSES Ribosomes are small structures found in all organisms and consist of ribosomal RNA and proteins. They are the sites of protein synthesis. Rough ER provides site-specific protein synthesis with bound ribosomes and plays a role in intracellular transport. Smooth ER mostly synthesizes lipids. The Golgi complex consists of a series of flattened membrane sacs. It packages and sorts proteins for transport in vesicles and it also produces lysosomes. Lysosomes contain hydrolytic enzymes which are important in intracellular digestion, recycling of cell s organic materials and programmed cell death (apoptosis). A vacuole functions in intracellular digestion, releases cellular wastes and maintains water balance. In plants, a large vacuole stores pigments or poisonous substances and plays a role in cell growth. Mitochondria specialize in energy capture and formation of ATP. Chloroplasts are found in algae and higher plants and capture energy through photosynthesis. Cell walls of plants, fungi, and prokaryotes provide structural boundary, as well as a permeability barrier. Eukaryotic Cell Prokaryotic Cell 2
MULTIPLE CHOICE QUESTIONS 1. In order to harvest energy through cellular respiration, mammals require uptake of oxygen and glucose into their red blood cells. Which of the following statements best describes the transport of these molecules? a. Cell membranes are not permeable to both of these molecules and therefore both oxygen and glucose are transported by proteins embedded in membranes. b. Both oxygen and glucose are transported down concentration gradients, but glucose requires a protein transporter while oxygen passes through the phospholipid bilayer. c. Glucose and oxygen move by simple diffusion down their concentration gradients. d. Oxygen moves by facilitated diffusion and glucose moves by simple diffusion. Questions 2-3 2. Scientists wanted to trace the path of a protein from its synthesis to its final destination in the cell. They radioactively labeled amino acids required for the synthesis of this protein and analyzed cell contents every ten minutes to determine the cellular location of the protein. The table below summarizes the results. Time (min) Rough Endoplasmic Reticulum Percentage of Radioactive Protein Golgi Plasma Ribosomes Apparatus Membrane Secretory Vesicles 10 10 0 0 90 0 20 75 25 0 0 0 30 0 50 10 0 40 40 0 0 85 0 15 50 0 0 0 0 0 Which of the following depicts the pathway of this protein? a. Ribosomes secretory vesicles rough ER Golgi apparatus cell membrane b. Ribosomes Golgi apparatus rough ER secretory vesicles cell membrane c. Ribosomes rough ER Golgi apparatus secretory vesicles cell membrane d. Ribosomes rough ER secretory vesicles Golgi apparatus cell membrane 3. Which of the following is the most likely function of the protein described in question 2? a. transport of hydrophilic substances in/out of the cell b. electron carrier of an electron transport chain c. hydrolysis of cells organic materials during apoptosis d. regulation of blood glucose level 4. Which of the following best describes an advantage that eukaryotic cells have over prokaryotic cells? a. Eukaryotic cells have internal membranes that facilitate cellular processes by minimizing competing interactions and by increasing surface area where reactions occur. b. Eukaryotic cells are able to regulate what substances enter and leave cells through the cell membrane while prokaryotic cells cannot. c. Eukaryotic cells reproduce faster than prokaryotic cells due to the presence of cytoskeleton. d. Eukaryotic cells contain lysosomes and vacuoles which allow them to break down and release poisonous substances. Prokaryotic cells are inefficient in releasing wastes. 3
Questions 5-6 Use the information in the table below to answer questions 5 and 6. Cell Side length of a cube-shaped cell Surface area (µm 2 ) Volume (µm 3 ) Surface areato-volume ratio A 1 6 1 6:1 B 2 24 8 3:1 C 3 54 27 2:1 D 4 96 64 1.5:1 5. Which cube-shaped cell would be the least efficient in obtaining oxygen? a. Cell A b. Cell B c. Cell C d. Cell D 6. Which of the following statements is incorrect about how surface area-to-volume ratios affect biological systems? a. The surface area of the plasma membrane must be large enough to adequately exchange materials. b. As cells increase in volume, the relative surface area decreases. c. Larger cells have a more favorable area-to-volume ratio for exchange of materials with the environment. d. As cells increase in volume, demand for material resources increases. Questions 7-8 The diagram below shows a mechanism plants cells use to absorb nitrogen from the soil. 7. Which of the following best explains how nitrate is transported in plants? a. The membrane potential and concentration gradient of H + are used to move nitrate down its concentration gradient. b. Nitrate transport couples passive transport of H + with facilitated diffusion of nitrate. c. Nitrate transport is a type of simple diffusion. d. Nitrate transport uses the membrane potential and the ph gradient to move nitrate against its concentration gradient. 4
8. Which of the following is the most likely reason plant roots need to absorb nitrate? a. Nitrate is required in building of proteins and nucleic acids. b. Nitrate is necessary to build carbohydrates, proteins, lipids and nucleic acids. c. Nitrate is necessary for synthesis of nucleic acids and certain lipids. d. Nitrate is absorbed through roots in order to build nucleic acids and ATP. 9. All animals balance water uptake and water loss by osmoregulation. Which of the following describes a mechanism used by freshwater fish to maintain osmotic balance? a. Cells of freshwater fish contain contractile vacuoles that remove extra water. b. Freshwater fish drink almost no water and excrete large amounts of dilute urine. c. Freshwater fish excrete large amounts of calcium, magnesium and sulfate ions in their urine. d. Freshwater fish lose salt ions by gills. MATH GRID IN 1. Cells vary in sizes. For example, a human egg has a diameter of 130 µm, a typical skin cell has a diameter of 30 µm and a lymphocyte has a diameter of 14 µm. How many times is the surface area of a human lymphocyte smaller than the surface area of a human egg, assuming that these cells are spherical? Give the answer to the nearest whole number. 2. Water potential is a measure of the potential energy of water. Calculating water potential is helpful in order to understand the tendency of water to move from one area to another. Calculate the water potential of water in an estuary with NaCl concentration of 0.3 M at 12 C. Give your answer to the nearest tenth. 5
SHORT FREE RESPONSE QUESTIONS 1. The relationship of structure to function is one of the major themes in biology. Describe the structure of membranes and then explain how the function of active transport or facilitated diffusion is related to the structure. 2. Prokaryotic cells lack membrane-bound organelles found in eukaryotes. However, prokaryotes must perform many of the same functions as eukaryotes. From the list below, choose TWO subcellular structures found in eukaryotic cells and describe their functions. Then explain how prokaryotic cells carry out the associated functions. Mitochondria Chloroplasts Golgi complex 6
LONG FREE RESPONSE QUESTION Water potential in potato cells was determined in the following manner. The initial masses of six groups of potato cores were measured. The potato cores were placed in sucrose solutions of various molarities. The masses of the cores were measured again after 24 hours. Percent changes in mass were calculated. The results are shown below. Molarity of sucrose in beaker Percent change in mass 0.0 M 18.0 0.2 5.0 0.4-8.0 0.6-16.0 0.8-23.5 1.0-24.0 a. Graph these data on the axes provided. From your graph, find the apparent molar concentration (osmolarity) of the potato core cells. b. Name the components of water potential and explain why water potential is important for the movement of water in plants. c. Predict what would happen to typical animal cell placed in 0.0 M and 1.0 M sucrose solutions and explain your predictions. 7
8