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Ch. 5 Cytoplasm Figure 5.10 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Membranes are selectively permeable – They control the flow of substances into and out of a cell Membranes can hold teams of enzymes that function in metabolism Phospholipid Bilayer 2 layers of phospholipids Proteins Transport Receptors Enzymes Cholesterol Maintains fluidity Figure 5.11 Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings In water, phospholipids form a stable bilayer – The heads face outward and the tails face inward Water Hydrophilic heads Hydrophobic tails Water Figure 5.11B Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Membrane composed of different molecules Phospholipids Cholesterol Proteins Phospholipid molecules form a flexible bilayer Cholesterol and protein molecules are embedded in it Carbohydrates act as cell identification tags Molecules move laterally Because of polar and nonpolar regions of the phospholipid bilayer, the membrane allows certain materials in and certain materials out of the cell Permits exchange of nutrients, waste products, oxygen, and inorganic ions. Allows some substances to cross more easily than others: Hydrophobic molecules—hydrocarbons, CO2, and O2 dissolve in and cross membrane Very small polar molecules, including H2O can cross easily Diffusion caused by the random movement of particles across a membrane Movement due to concentration gradient Moving from a higher concentration to a lower concentration No energy used Movement continues until equilibrium reached Concentration is the same on both sides of the membrane A substance will diffuse from where it is more concentrated to where it is less concentrated Only small, uncharged particles diffuse without assistance (CO2, and O2) http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundari es%20ONLINE.ppt#276,25,Section G Diffusion of water across a selectively permeable membrane Water diffuses down its own concentration gradient (from hypotonic solution to hypertonic solution) Does not use energy Hypertonic solution Solution contains more solute than the solution it is compared with Hypotonic solution Solution contains less solute than the solution it is compared with Isotonic solution Solution contains same amount of solute as the solution it is compared with What kind of solution is inside the cell? Isotonic Outside the cell? Isotonic Which direction will water move? In and out at the same rate 10 % NaCl 10 % NaCl What kind of solution is inside the cell? Hypertonic Outside the cell? Hypotonic Which direction will water move? Into the cell 10 % NaCl 20 % NaCl What kind of solution is inside the cell? Hypotonic Outside the cell? Hypertonic Which direction will water move? Out of the cell 30 % NaCl 10 % NaCl Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings Water balance between cells and their surroundings is crucial to organisms The control of water balance is osmoregulation Passive transport—no energy used Uses transport proteins embedded in the plasma membrane (ion channels) Aides transport of many polar molecules and ions that are inhibited by phospholipid bilayer (sugar, some hormones, ions such as K+ and Na+) Pores—always open Gated channels— open/close in response to stimuli Solute molecule Transport protein http://sussexhigh.nbed.nb.ca/swift/biology11/Cell%20Boundari es%20ONLINE.ppt#276,25,Section G Transport proteins share similar properties with enzymes: They are specific for the solutes they transport They can be saturated with solute— maximum rate occurs when all binding sites are occupied They can be inhibited by molecules that resemble the solute (similar to competitive inhibition) Substances are transported across a membrane, against the concentration gradient Use carrier proteins embedded in the membrane Use energy (ATP) http://student.ccbcmd.edu/~gkaiser/biotutorials/eustru ct/images/sppump.gif Active transport in two solutes across a membrane FLUID OUTSIDE CELL Phosphorylated transport protein Transport protein First solute 1 First solute, inside cell, binds to protein 2 ATP transfers phosphate to protein 3 Protein releases solute outside cell 5 Phosphate detaches from protein 6 Protein releases second solute into cell Second solute Figure 5.18 4 Second solute binds to protein Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings 1. 2. 3. 4. 5. 6. 3 Na+ ions inside the cell bind to the pump. ATP donates a phosphate to the pump The pump changes shape, transporting 3 Na+ across the membrane, and are released outside of the cell 2 K+ ions outside the cell bind to the pump The phosphate group is released and 2 K+ ions are transported across the membrane 2 K+ are released inside the cell http://gotoknow.org/file/somluckv/Cell_membrane06.jpg Substances that are too large for carrier proteins (proteins and polysaccharides) Endocytosis—movement of substances into the cell (a.k.a. phagocytosis) Exocytosis—movement of substance out of the cell http://www.stanford.edu/group/Urchin/GIFS/exocyt.gif Exocytosis Process of exporting macromolecules from a cell by fusion of vesicles with the cell membrane Endocytosis Process of importing macromolecules into a cell by forming vesicles derived from the cell membrane Vesicle usually budded from the ER or Golgi and migrates to cell membrane Vesicle forms from a localized region of cell membrane that sinks inward; pinches off into cytoplasm Used by secretory cells to export Used by cells to incorporate products (insulin in pancreas; extracellular substances neurotransmitter from neuron) Figure 5.19A Figure 5.19B Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Phagocytosis— endocytosis of solid particles Forms food vacuoles that fuse with lysosome to be digested http://student.ccbcmd.edu/~gkaiser/biotutoria ls/eustruct/images/phagocyt.gif Pinocytosis— endocytosis of fluid droplets Takes in solutes dissolved in the droplet http://student.ccbcmd.edu/~gkaiser/biotutoria ls/eustruct/images/pinocyt.gif Imports specific macromolecules into the cell by inward budding of vesicles formed from coated pits Occurs in response to binding specific ligands to receptors on cell’s surface Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors Figure 5.19 Three kinds of endocytosis Pseudopod of amoeba Food being ingested Plasma membrane Material bound to receptor proteins PIT Cytoplasm Figure 5.19C Protein that binds a specific signal molecule, allowing the cell to respond to the signal molecule Some receptor proteins are attached to ion channels Changes permeability to a specific ion Some may cause the formation of a second messenger, which acts as a signal molecule in the cytoplasm Figure 5.20 Essential Biology with Physiology, 2nd ed., by Campbell, Reece, and Simon, ©2007. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher. BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher. BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.