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 Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings Figure 5.11

 In water, phospholipids form a stable bilayer Figure 5.11B Hydrophilic heads Hydrophobic tails Water –The heads face outward and the tails face inward 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, CO 2, and O 2 dissolve in and cross membrane  Very small polar molecules, including H 2 O 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 (CO 2, and O 2 ) 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

 What kind of solution is inside the cell?  Hypertonic  Outside the cell?  Hypotonic  Which direction will water move?  Into the cell

 What kind of solution is inside the cell?  Hypotonic  Outside the cell?  Hypertonic  Which direction will water move?  Out of the cell

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

 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 Passive transport—no energy used Uses transport proteins embedded in the plasma membrane (ion channels) 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) ct/images/sppump.gif

 Active transport in two solutes across a membrane Figure 5.18 Transport protein 1 FLUID OUTSIDE CELL First solute First solute, inside cell, binds to protein Phosphorylated transport protein 2 ATP transfers phosphate to protein 3 Protein releases solute outside cell 4 Second solute binds to protein Second solute 5 Phosphate detaches from protein 6 Protein releases second solute into cell Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

1. 3 Na+ ions inside the cell bind to the pump. ATP donates a phosphate to the pump 2. The pump changes shape, transporting 3 Na+ across the membrane, and are released outside of the cell 3. 2 K+ ions outside the cell bind to the pump 4. The phosphate group is released and 5. 2 K+ ions are transported across the membrane 6. 2 K+ are released inside the cell

 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

Vesicle forms from a localized region of cell membrane that sinks inward; pinches off into cytoplasm Vesicle usually budded from the ER or Golgi and migrates to cell membrane Used by cells to incorporate extracellular substances Used by secretory cells to export products (insulin in pancreas; neurotransmitter from neuron) Process of importing macromolecules into a cell by forming vesicles derived from the cell membrane Process of exporting macromolecules from a cell by fusion of vesicles with the cell membrane EndocytosisExocytosis Figure 5.19A Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Figure 5.19B

 Phagocytosis— endocytosis of solid particles  Forms food vacuoles that fuse with lysosome to be digested  Pinocytosis— endocytosis of fluid droplets  Takes in solutes dissolved in the droplet ls/eustruct/images/phagocyt.gif 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 Figure 5.19C Pseudopod of amoeba Food being ingested Plasma membrane Material bound to receptor proteins PIT Cytoplasm

 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, 2 nd 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.