Transcript Notes
Chapter 4 – Cell Transport • Plasma Membrane Structure and Function • Separates the internal environment of the cell from its surroundings. • Regulates what materials enter and leave a cell • Made of 2 layers of phospholipids (phospholipid bilayer) with proteins embedded throughout. • Fluid consistency (like a light oil) and a mosaic pattern of proteins. http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane fluidity.swf •Because of consistency and pattern of components, referred to as fluid-mosaic model of membrane structure http://home.earthlink.net/~shalpine/anim/Life/memb.htm • Cells live in fluid environments, with water inside and outside the cell. • Components of plasma membrane: –2 layers of phosphlipids • Polar head and nonpolar tail –Proteins –Cholesterol –Carbohydrates •Phospholipid bilayer: •Polar heads (hydrophilic) (waterloving) •Face outside and inside of cell. •Nonpolar tails (hydrophobic) (waterfearing) •Extend to the interior of the plasma membrane. • Proteins: • Some pass through entire membrane, others go part-way through • Help large molecules pass through cell membrane •Cholesterol: •Strengthens the plasma membrane. •Carbohydrates: •Help cells send messages and recognize one another The Permeability of the Plasma Membrane • Selectively permeable – meaning only certain materials can cross the membrane. • Two mechanisms of transport: –Active – requires energy –Passive – does not require energy •How do materials get across the plasma membrane? •Small, uncharged molecules pass through the membrane, following their concentration gradient •Gradual change in chemical concentration from one area to another •Molecules tend to move from area of high to low concentration •Larger macromolecules or tiny charged molecules rely on proteins to help them to get across. How molecules cross the plasma membrane Diffusion and Osmosis • Diffusion is the movement of molecules from a higher to a lower concentration until equilibrium is reached. – Equilibrium – • State in which all materials are evenly concentrated • Movement of molecules still occurs, but there is no NET movement of molecules • Gases move through plasma membrane by diffusion. Osmosis • Defined as the diffusion of water across a selectively permeable membrane due to concentration differences. • Molecules always move from higher to lower concentration. Osmosis in cells • A solution contains a solute (solid) and a solvent (liquid). • Cells are normally isotonic (iso = same) to their surroundings, and the solute concentration is the same inside and out of the cell. – Cell is in equilibrium – There is no net movement of water across the cell membrane. Fresh water = Isotonic solution X = Water Cells in isotonic solutions http://www.linkpublishing.com/V ideos/transport/blood_isotonic. wmv http://www.linkpublishing.com/ Videos/transport/elodea_norm al.wmv • Hypotonic (“hypo” means less than) – Lower concentration of solute, higher concentration of water outside than inside cell. – Net movement of water into the cell. – Animal cells - may burst – Plant cells – • Increased turgor pressure (makes plant cell rigid) • Plant cells do not burst because they have a cell wall. Distilled water = Hypotonic solution X = Water Cells in a hypotonic solution http://www.linkpublishing.com/Vide os/transport/blood_hypo.wmv http://www.linkpublishing.com/Videos/t ransport/elodea_hypo.wmv • Hypertonic (“hyper” means more than) – Higher concentration of solute, lower concentration of water outside than inside cell. – Net movement of water out of the cell. – Animal cells - shrivel – Plant cells - shrink • Turgor pressure is lost as plant cells shrink – plant will wilt or feel soft and bendable. Salt water = Hypertonic solution X = Water Cells in a hypertonic solution http://www.linkpublishing.com/Videos/t ransport/blood_hyper.wmv http://www.linkpublishing.com/Video s/transport/elodea_hyper.wmv Transport by Carrier Proteins • Some materials cannot enter or leave cell due to their size and/or nature. • Some of those molecules use the channel proteins and carrier proteins that span the membrane in order to enter or leave the cell. • Carrier proteins are specific and combine with only a certain type of molecule. • Facilitated transport and active transport both require carrier proteins. Facilitated transport • Substances pass through a carrier protein following their concentration gradients (high low concentration). • Does not require energy. • Brings in materials such as glucose & amino acids; proteins are specific to molecules taken in. • Some molecules can be taken in faster than others – differential permeability. http://www.stolaf.edu/people/giannini/flashanimat/transport/channel.swf Active transport • Ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration. • Energy in the form of ATP is required for the carrier protein to combine with the transported molecule. • Occurs in cells such as kidney cells (taking sodium from urine), thyroid gland cells (taking in iodine), and cells in digestive tract (absorbing nutrients). Active transport http://www.brookscole.com/chemistry_d/templates/student_resources/shared_re sources/animations/ion_pump/ionpump.html Exocytosis and Endocytosis • For molecules that are too large to be transported with carrier proteins – requires energy. • Use vesicles – small storage units • Exocytosis - vesicles fuse with the plasma membrane for secretion. • Causes cell membrane to enlarge – process occurs during growth. Exocytosis http://academic.brooklyn.cuny.edu/biology/bio4fv/page/exocy.htm • Endocytosis - cells take in substances when a portion of the plasma membrane folds in, and forms a vesicle around the substance. • When vesicle fuses with lysosome, digestion occurs. • Endocytosis occurs as: • Phagocytosis – large particles (food or other cells) • Pinocytosis – small particles or liquids • Receptor-mediated endocytosis – form of pinocytosis for specific particles such as vitamins, hormones, or lipoproteins http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/animations.html Phagocytosis •Seen in unicellular organisms like amoebas. •White blood cells can use this process to take in bacteria & worn out red blood cells Pinocytosis •Used by root cells of plants. •Seen in blood cells, cells that line kidneys and intestines. Receptor-mediated endocytosis •Seen during exchange of maternal & fetal blood in placenta; & intake of cholesterol in body cells (failure to do so results in high blood pressure, blocked arteries, and heart attacks).