1. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Chapter 5  Cell Membrane Structure and Function

2. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e head (hydrophilic) tails (hydrophobic) How Is the Structure of a Membrane Related to Its Function?  The phospholipid bilayer is the fluid portion of the membrane –A polar, hydrophilic head –Two nonpolar, hydrophobic tails

3. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e  Plasma membranes face both exterior and interior watery environments –Hydrophobic and hydrophilic interactions drive phospholipids into bilayers phospholipid hydrophilic heads hydrophobic tails hydrophilic heads extracellular fluid (watery environment) cytoplasm (watery environment) bilayer The Phospholipid Bilayer of the Cell Membrane

4. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e  The phospholipid bilayer’s flexible, fluid membrane allows for cellular shape changes –Individual phospholipid molecules are not bonded to one another more fluidless fluid Kinks Increase Fluidity unsaturated saturated

5. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e How Is the Structure of a Membrane Related to Its Function?  Maintaining fluidity –Membranes become more fluid at high temperatures (more movement) and less fluid at low temperatures (less movement) –Cell membranes of organisms living in low- temperatures tend to be more unsaturated (more kinks help them maintain fluidity)

6. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e glycoprotein pore extracellular fluid (outside) cytoplasm (inside) cholesterol phospholipid bilayer transport protein receptor protein phospholipid recognition protein extra- cellular matrix enzyme cytoskeleton carbohydrate binding site protein connection protein The Plasma Membrane Fig. 5-1

7. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e How Is the Structure of a Membrane Related to Its Function?  A variety of proteins form a mosaic within the membrane –Categories of membrane proteins –Enzymatic proteins - proteins that promote chemical reactions –Receptor proteins – hormones trigger cellular responses –Recognition proteins - glycoproteins that serve as identification tags on the surface of a cell –Connection proteins - anchor the cell membrane –Transport proteins – channel or carrier proteins

8. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Receptor Protein Activation Fig. 5-5 receptor hormone (cytoplasm) (extracellular fluid) A hormone binds to the receptor Hormone binding activates the receptor, changing its shape The activated receptor stimulates a response in the cell 1 2 3

9. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e How Is the Structure of a Membrane Related to Its Function?  There are two types of transport proteins –These proteins regulate the movement of hydrophilic molecules through the plasma membrane –Channel proteins form channels whose central pores allow specific ions or water molecules to pass through the membrane –Carrier proteins have binding sites that can temporarily attach to specific molecules on one side of the membrane and then move them through the membrane to the other side

10. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Review Questions 1.Membranes consist of a bilayer of _______________ 2.Explain why these molecules arrange themselves this way. 3.What is the other principal molecule in the plasma membrane? 4.What functions do these two molecules provide?

11. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e How Do Substances Move Across Membranes?  Molecules in fluids move in response to gradients –Definitions relevant to substance movement are: –A fluid is a substance whose molecules can flow past one another and, therefore, have no defined shape –A solute is a substance that can be dissolved (dispersed as atoms, ions, or molecules) in a solvent –A solvent is a fluid capable of dissolving a solute –The concentration of a substance defines the amount of solute in a given amount of solvent –A gradient is a physical difference (in temperature, pressure, charge, or concentration of a particular solute in a fluid) between two adjoining regions of space

12. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e How Do Substances Move Across Membranes?  Molecules in fluids move in response to gradients –Gradients in concentration or pressure cause molecules to move from one place to another –to equalize the difference –Why gradients cause molecules to move from one place to another: –Molecules and ions in solution are in constant random motion –An increase in temperature increases the rate of this random motion

13. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e How Do Substances Move Across Membranes?  Movement through membranes occurs by passive transport and energy-driven transport –Passive transport is the diffusion of substances across cell membranes down concentration gradients –includes simple diffusion, facilitated diffusion, and osmosis –Energy-requiring transport is transport that requires the use of cellular energy (ATP) –includes active transport, endocytosis, and exocytosis

14. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Simple Diffusion of a Dye in Water A drop of dye is placed in water Dye molecules diffuse into the water; water molecules diffuse into the dye Both dye molecules and water molecules are evenly dispersed drop of dye water molecule 2 1 3 Passive transport Small molecules move across membranes by simple diffusion (water, oxygen, carbon dioxide, and lipid-soluble molecules like alcohol and vitamins A, D, and E) Fig. 5-6

15. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e water glucose carrier protein aquaporin channel protein Cl – channel proteinsaquaporinscarrier proteins Facilitated Diffusion  Water soluble molecules (ions, amino acids, and sugars) require the aid of channel and carrier transport proteins –Down gradient –Many cells have specialized water channel proteins called aquaporins –small size and positive charges that attract the negative pole of water molecules

16. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Osmosis  The diffusion of water across selectively permeable membranes –Down gradient –Dissolved substances displace water molecules, lowering water concentration

17. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e The Effect of Solute Concentration on Osmosis No net flow of water Water flows out; the balloon shrinks Water flows in; the balloon expands (a) A balloon in an isotonic solution (b) A balloon in a hypertonic solution (c) A balloon in a hypotonic solution –Isotonic solutions have equal concentrations of water and solutes –A hypertonic solution is one with a greater solute concentration –A hypotonic solution has a lower solute concentration Fig. 5-7

18. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e How Do Substances Move Across Membranes?  Osmosis explains why fresh water protists have contractile vacuoles –Water leaks in continuously because the cytosol is hypertonic to fresh water –Salts are pumped into the vacuoles, making them hypertonic to the cytosol –Water follows by osmosis and is then expelled by contraction

19. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e The Effects of Osmosis on Red Blood Cells Fig. 5-9

20. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Active Transport  Membrane proteins use cellular energy to move molecules or ions across plasma membranes –against their concentration gradients –They often have a molecule binding site and an ATP binding site –Active transport proteins are often referred to as pumps The transport protein binds both ATP and Ca 2  Energy from ATP changes the shape of the transport protein and moves the ion across the membrane The protein releases the ion and the remnants of ATP (ADP and P) and closes ATP binding site recognition site ATP P ADP Ca 2  (extracellular fluid) (cytoplasm) ATP 1 2 3

21. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Endocytosis  Cells engulf particles or fluids –The engulfed particles are transported within the cell inside vesicles –Requires energy  Three types: –Pinocytosis (“cell drinking”) moves liquids into the cell –Receptor-mediated endocytosis moves specific molecules into the cell –Phagocytosis (“cell eating”) moves large particles into the cell

22. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Phagocytosis Fig. 5-14 food particle pseudopods food vacuole (a) Phagocytosis (b) An Amoeba engulfs a Paramecium (c) A white blood cell ingests bacteria (extracellular fluid) (cytoplasm) The plasma membrane extends pseudopods toward an extracellular particle (for example, food). The ends of the pseudopods fuse, encircling the particle. A vesicle called a food vacuole is formed containing the engulfed particle. 1 2 3 1 2 3 1 2 3

23. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Exocytosis  Moves material out of the cell –Requires energy –Disposes of undigested particles of waste or to secrete substances into the extracellular fluid

24. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e  Exchange of material across membranes influences cell size and shape –Most cells range in size from about 1 to 100µm (micrometers) in diameter –Cells need to exchange nutrients and wastes with the environment –As a spherical cell enlarges, its innermost parts get farther away from the plasma membrane –No part of the cell can be too far away from the external environment –Nerve and muscle cells and microvilli overcome size restraints by elongating, thus keeping the ratio of surface area to volume relatively high Why are most cells so small?

25. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e distance to center (r) surface area (4  r 2 ) Volume (4/3  r 3 ) area/volume 1.0 2.0 4.0 12.650.3 4.233.5 3.0 1.5 201.1 268.1 0.75 r r r Surface Area and Volume Relationships Fig. 5-16

26. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Review Questions 1.A membrane that is permeable to some substances but not to others is described as being ______________________ 2.The movement of a substance through a membrane down its concentration gradient is called ________________ 3.When applied to water, this process is called ____________ 4.The general process by which fluids or particles are transported into cells is called ___________________ 5.Does this process require energy? 6.The specific term for engulfing particles is ______________

27. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e  Desmosomes attach cells tightly together –under the stresses of movement in animal cells –Examples include the skin, intestine, and heart desmosome microvilli small intestineplasma membranes (edge view) In desmosomes, protein filaments hold cells together cells lining the small intestine (a) Desmosomes intermediate filaments in the cytoplasm Cell Attachment Structures

28. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e  Tight junctions make cell attachments leakproof –found in animal cells –where tubes and sacs must hold contents without leaking –Examples include the skin and the urinary bladder Cell Attachment Structures urinary bladder plasma membranes (edge view) cells lining the bladder (b) Tight junctions In tight junctions, proteins seal cells together

29. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e In gap junctions, channel proteins connect the insides of adjacent cells (a) Gap junctions plasma membranes liver cells  Gap junctions allow direct communication between cells –Cell-to-cell protein channels allowing for passage of hormones, nutrients, and ions in animal cells –Examples include heart and smooth muscle Cell Communication Structures

30. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e (b) Plasmodesmata In plasmodesmata, membrane-lined channels connect the insides of adjacent cells plasma membranes root cells cell walls root  Plasmodesmata allow direct communication between cells –Plant cells have holes in the walls of adjacent cells forming cytoplasmic connections –Similar to gap junctions in function Cell Communication Structures

31. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Fig. 4-1 frog embryo most eukaryotic cells mitochondrion most prokaryotic cells virus proteins diameter of DNA double helix chicken egg adult human tallest trees atoms Units of measurement: 1 meter (m) = 39.37 inches 1 centimeter (cm) = 1/100 m 1 millimeter (mm) = 1/1,000 m 1 micrometer (  m) = 1/1,000,000 m 1 nanometer (nm) = 1/1,000,000,000 m Size 100 m 10 m 1 m 10 cm 1 cm visible with unaided human eye visible with light microscope 1 mm 100  m 10  m 1  m 100 nm 10 nm 1 nm 0.1 nm visible with conventional electron microscope visible with special electron microscope Relative Sizes

32. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Pinocytosis Fig. 5-12 (extracellular fluid) (cytoplasm) vesicle containing extracellular fluid (a) Pinocytosis (b) TEM of pinocytosis A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle. extracellular fluid cytoplasm 1 2 3 1 2 3 1 2 3

33. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e protein coating coated pit coated vesicle extracellular particles bound to receptors plasma membrane (extracellular fluid) (cytoplasm) 0.1 micrometer (cytoplasm) nutrient molecule (extracellular fluid) receptor coated vesicle coated pit (a) Receptor-mediated endocytosis Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. A vesicle ("coated vesicle") containing the bound molecules is released into the cytoplasm. The coated pit region of the membrane encloses the receptor-bound molecules. The receptors bind the molecules and the membrane dimples inward. (b) TEM of receptor-mediated endocytosis 1 2 3 4 1 2 3 4 1 2 3 4 Receptor-Mediated Endocytosis Fig. 5-13