Membrane Transport

1. Membrane Transport • Cells need to move substances both in and out (of the cell) • The transport of water and other types of molecules across membranes is the key to many processes in living organisms • Permeable membrane – allows substances through • Semi-permeable membrane (differentially permeable) – allow some substances through (smaller molecules such as H2O, CO2, O2), but not others (larger molecules) do not get through easily

2. Passive Transport • Passive transport – movement of substances that requires no energy (e.g. diffusion) • Diffusion – natural process where substances move from an area of high concentration to an area of low concentration (requires no energy) • Eventually diffusion results in an equilibrium (even distribution of molecules throughout) • Osmosis – diffusion of water across a semi-permeable membrane. The direction of water movement (in or out of cell) depends on concentration of solute. • A solute (e.g. NaCl) is dissolved in a solvent (water)

3. Isotonic condition – concentration of solute and water inside cell EQUALS concentration of solute and water outside • Hypertonic condition - concentration of solute outside cell GREATER than solute concentration inside cell • Hypotonic condition - concentration of solute outside cell LESS than solute concentration inside cell • Check it out: http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm • http://www.coolschool.ca/lor/BI12/unit4/U04L06/rbc.html • http://science.nhmccd.edu/biol/membrane/osfram1.html

4. Question 1-3, page 87 • Explain what a concentration gradient is and what it means for a molecule to diffuse down its concentration gradient. • a concentration gradient is the difference in concentration of a substance from one location to another. A molecule diffuses down its a concentration gradient by moving from a region of higher concentration to a region of lower concentration. • Explain why facilitated diffusion does not require energy from a cell. • No energy is needed because the molecules move down a concentration gradient. • Apply – A cell is bathed in fluid. However, you noticed that water is flowing out of the cell. In what kind of solution is the cell immersed? Isotonic, hypotonic or hypertonic? • Hypertonic

5. Questions 4 & 5, p 87 • Compare – How are receptors and transport proteins similar? • Both are proteins and may work with only specific molecules. In addition, both may require a change in shape to accomplish their function. • Health – When a person becomes dehydrated due to a loss of fluids and solutes, saline solutions (water and salts) is infused into the bloodstream by medical personnel. Why is saline solution used instead of pure water? • Pure water would be hypotonic relative to the contents of blood cells, so water would rush into the blood cells and could cause the cells to burst. The saline solution is isotonic relative to the cell contents.

6. Active Transport • Active transport – energy (from ATP) is needed to move substances from an area of low concentration to an area of high concentration (opposes diffusion) • Active transport is used to:  1. Generate charge gradients. For example in the mitochondrion, hydrogen ion pumps pump hydrogen ions into the intermembrane space of the organelle as part of making ATP.  2. Concentrate ions, minerals and nutrients inside the cell that are in low concentration outside.  3. Keep unwanted ions or other molecules out of the cell that are able to diffuse through the cell membrane. 

7. Movement of large molecules • 2 important processes (require energy from ATP): 1. Endocytosis – bringing in materials into cell a. pinocytosis – brings in liquid material b. phagocytosis - brings in solid material 2. Exocytosis – expel wastes and useful substances needed elsewhere, such as hormones or enzymes

8. Phagocytosis

9. Questions 1-3, page 91 • How do transport proteins that are pumps differ from those that are channels? • Pumps require energy, transport a molecule against its concentration gradient, and change shape upon binding. A protein channel does not change shape or require energy. It allows certain molecules to diffuse through it, down their concentration gradient. • How do endocytosis and exocytosis differ from diffusion? • They require energy input; diffusion does not. They also enable large particles to enter a cell, particles that are too large to diffuse across a cell membrane. • Apply – small lipid molecules are in high concentration outside a cell. They slowly cross the membrane into the cell. What term describes this action? Does it require energy? • diffusion, No

10. Questions 4 & 5, p 91 • Apply – ions are in low concentration outside a cell. They move rapidly into the cell via protein molecules. What term describes this action? Does it require energy? • active transport, Yes • Diffusion – Suppose molecules were unable to diffuse into and out of cells. How might life be different if cells had to use active transport to move every substance? Explain your reasoning. • Cells would require vast amounts of energy to perform even simple functions. Perhaps organisms would have to take in more food to provide more energy. If food were limited, perhaps only photosynthetic organisms would be able to survive. Perhaps organisms would move and respond more slowly and be more sedentary. Perhaps organisms would be simpler, and highly specialized organs would not be developed.