Chapter 5

1. Chapter 5 • Cell Membrane Structure and Function

2. How Is the Structure of a Membrane Related to Its Function? • The phospholipidbilayer is the fluid portion of the membrane • A polar, hydrophilic head • Two nonpolar, hydrophobic tails tails (hydrophobic) head (hydrophilic)

3. The Phospholipid Bilayer of the Cell Membrane extracellular fluid (watery environment) phospholipid • Plasma membranes face both exterior and interior watery environments • Hydrophobic and hydrophilic interactions drive phospholipids into bilayers hydrophilic heads hydrophobic tails bilayer hydrophilic heads cytoplasm (watery environment)

4. Kinks Increase Fluidity • The phospholipid bilayer’s flexible, fluid membrane allows for cellular shape changes • Individual phospholipid molecules are not bonded to one another saturated unsaturated more fluid less fluid

5. 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. The Plasma Membrane carbohydrate extracellular fluid (outside) protein glycoprotein extra- cellular matrix binding site receptor protein phospholipid bilayer transport protein connection protein phospholipid pore cholesterol recognition protein enzyme cytoskeleton Fig. 5-1 cytoplasm (inside)

7. 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. Receptor Protein Activation (extracellular fluid) 1 A hormone binds to the receptor hormone receptor 2 Hormone binding activates the receptor, changing its shape 3 The activated receptor stimulates a response in the cell Fig. 5-5 (cytoplasm)

9. 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. Review Questions Membranes consist of a bilayer of _______________ Explain why these molecules arrange themselves this way. What is the other principal molecule in the plasma membrane? What functions do these two molecules provide?

11. 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. 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. 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. Simple Diffusion of a Dye in Water 2 Dye molecules diffuse into the water; water molecules diffuse into the dye 3 Both dye molecules and water molecules are evenly dispersed 1 A drop of dye is placed in water drop of dye Fig. 5-6 • 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) water molecule

15. 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 water Cl– glucose aquaporin channel protein carrier protein channel proteins aquaporins carrier proteins

16. Osmosis • The diffusion of water across selectively permeable membranes • Down gradient • Dissolved substances displace water molecules, lowering water concentration

17. The Effect of Solute Concentration on Osmosis • 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 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 Fig. 5-7

18. 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. The Effects of Osmosis on Red Blood Cells Fig. 5-9

20. Active Transport • Membrane proteins use cellular energy to move molecules or ions across plasma membranes • againsttheir concentration gradients • They often have a molecule binding site and an ATP binding site • Active transport proteins are often referred to as pumps (extracellular fluid) Energy from ATP changes the shape of the transport protein and moves the ion across the membrane The transport protein binds both ATP and Ca2 The protein releases the ion and the remnants of ATP (ADP and P) and closes 2 3 1 ADP ATP binding site recognition site ATP P ATP Ca2 (cytoplasm)

21. 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. Phagocytosis (extracellular fluid) food particle pseudopods 1 2 food vacuole (cytoplasm) 3 1 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 (b) An Amoeba engulfs a Paramecium (c) A white blood cell ingests bacteria 2 2 3 3 (a) Phagocytosis Fig. 5-14

23. Exocytosis • Moves material out of the cell • Requires energy • Disposes of undigested particles of waste or to secrete substances into the extracellular fluid

24. Why are most cells so small? • 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

25. Surface Area and Volume Relationships r r r distance to center (r) 1.0 4.0 2.0 surface area (4r2) 12.6 50.3 201.1 Volume (4/3r3) 4.2 33.5 268.1 Fig. 5-16 3.0 0.75 area/volume 1.5

26. Review Questions A membrane that is permeable to some substances but not to others is described as being ______________________ The movement of a substance through a membrane down its concentration gradient is called ________________ When applied to water, this process is called ____________ The general process by which fluids or particles are transported into cells is called ___________________ Does this process require energy? The specific term for engulfing particles is ______________

27. Cell Attachment Structures small intestine plasma membranes (edge view) intermediate filaments in the cytoplasm • Desmosomes attach cells tightly together • under the stresses of movement in animal cells • Examples include the skin, intestine, and heart desmosome microvilli cells lining the small intestine In desmosomes, protein filaments hold cells together (a) Desmosomes

28. Cell Attachment Structures • 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 plasma membranes (edge view) urinary bladder cells lining the bladder In tight junctions, proteins seal cells together (b) Tight junctions

29. Cell Communication Structures plasma membranes liver • Gap junctionsallow 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 liver cells In gap junctions, channel proteins connect the insides of adjacent cells (a) Gap junctions

30. Cell Communication Structures • 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 plasma membranes cell walls In plasmodesmata, membrane-lined channels connect the insides of adjacent cells root root cells (b) Plasmodesmata