Chapter 5 - Cell Membrane Structure and Function 5.1 How is the structure of a membrane related to its function? 5.2 How do substances move across membranes? 5.3 How do specialized junctions allow cells to connect and communicate?

5.1 How Is the Structure of a Membrane Related to Its Function? 1.Selectively isolates cell’s contents from outside (phosph o lipids) 2.Regulates exchange of substances in and out (proteins) 3.Communicates with other cells (proteins) 4.Create attachments within & between cells (proteins) 5.Regulate many biochemical reactions (proteins)

Fluid Mosaic Model

head (hydrophilic) tails (hydrophobic) C CH CH 2 CHC O O NOPH3CH3C CH 3 O O OH2CH2C O HC CH 2 O CH 3 CH 2 CH 3 CH 2 CH 3 unsaturated

phospholipid hydrophilic heads hydrophobic tails hydrophilic heads extracellular fluid (watery environment) cytoplasm (watery environment) bilayer

(This picture is not in edition 7) Kinks increase membrane fluidity

Membrane Proteins Form a Mosaic Proteins are embedded in the phospholipid bilayer –Some proteins can float and drift –Other proteins are anchored by protein filaments in the cytoplasm –Many proteins have attached carbohydrates (glycoproteins)

5.1 How Is the Structure of a Membrane Related to Its Function? 5 major types of membrane proteins –Receptor Proteins –Recognition Proteins –Enzymatic Proteins –Attachment Proteins –Transport Proteins (This section has changed slightly from ed. 7)

Proteins in the Membrane Glycoprotein are a protein with a carbohydrate attached (diverse functions not mentioned in this book) Receptor Proteins: hormones or nutrients attach and trigger cellular responses

(Picture not in ed. 7) Receptor Proteins

Proteins in the Membrane Recognition Proteins –Many are glycoproteins –ID tags and surface attachment sites –Immune system Enzyme Proteins –Promote chemical reactions that synthesize or break apart biological molecules

Proteins in the Membrane Attachment Proteins –Anchor the cell membrane to inner cytoskeleton, to proteins outside the cell, and to other cells Transport Proteins –Channel Proteins create pores for water- soluble molecules to pass through –Carrier Proteins attach to molecules and help them across

5.2 How Do Substances Move Across Membranes? Know the following definitions: –Fluid: any substance whose molecules move freely past each other (both liquid and gas) –Solutes: a substance that can be dissolved –Solvent: Fluid capable of dissolving solute –Concentration: number of molecules of a substance in a given volume of fluid

More Definitions A gradient is a physical difference in properties such as temperature, pressure, electrical change and/or concentrations between 2 adjoining regions Diffusion: movement of particles from high to low concentration driven by a gradient

Principles of Diffusion Net movement of molecules down a gradient from high to low concentration The greater the concentration gradient, the faster the rate of diffusion The higher the temperature, the faster the rate If no other processes intervene, diffusion will continue until gradient is gone Diffusion cannot move molecules rapidly over long distances

5.2 How Do Substances Move Across Membranes? Movement Across Membranes Occurs by Both Passive and Active Transport Active transport requires energy to be added Passive transport doesn’t need energy

5.2 How Do Substances Move Across Membranes? Diffusion –Diffusion can happen in a fluid or across a membrane –Plasma membranes are selectively permeable –Some molecules can permeate (pass through) –To some molecules, the membrane is impermeable (cannot pass through)

Many ions are transported this way

Facilitated diffusion only works with (or down) the gradient

Principles of Osmosis Diffusion of water across selectively permeable membranes Water moves across a membrane down (or with) the concentration gradient From high to low water concentration Dissolved substances reduce the concentration of free water molecules in solution

“Having the same strength” Extracellular fluid of animals is usually isotonic to cytoplasmic fluid Gradients of dissolved particles equal (but not types)

“Having greater strength” Dissolved particles higher outside In this case, salt is higher on the outside

“Having lesser strength” Dissolved particles lower outside In this case, salt is lower or non-existent on the outside

Active Transport Active transport uses cellular energy to move molecules against their concentration gradients ATP (usually) donates energy to change the shape of the protein and move it across These are often called ATP pumps

Cells Engulf Particles or Fluids by Endocytosis Endocystosis is Greek for “into the cell” Three types: –Pinocytosis –Receptor-mediated endocytosis –Phagocytosis

A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle. Pinocytosis “cell drinking” 3 3 (extracellular fluid) (cytoplasm) vesicle containing extracellular fluid

Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. The receptors bind the molecules and the membrane dimples inward. The coated pit region of the membrane encloses the receptor-bound molecules. A vesicle ("coated vesicle") containing the bound molecules is released into the cytoplasm. (extracellular fluid) (cytoplasm) Receptor-mediated endocytosis nutrients receptors coated pit coated vesicle

protein coating coated pit extracellular particles bound to receptors plasma membrane (cyto- plasm) (extracellular fluid)

coated vesicle 0.1 micrometer

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. (extracellular fluid) (cytoplasm) pseudopods food vacuole food particle Phagocytosis “cell eating”

5.2 How Do Substances Move Across Membranes? Pinocytosis moves liquids into the cell Receptor-mediated endocytosis moves specific molecules into the cell Phagocytosis moves large particles into the cell Exocytosis moves material out of the cell (Greek for “out of the cell”)

plasma membrane (cytoplasm) vesicle secreted material (extracellular fluid)

plasma membrane vesicle 0.2 micrometer secreted material

4.3 How Are Cell Surfaces Specialized? Various Specialized Junctions Allow Cells to Connect and Communicate –Desmosomes (animals) attach cells together –Tight junctions (animals) make the cell leakproof –Gap junctions (animals) communication between cells –Plasmodesmata (plants) communication between cells

desmosome small intestine plasma membranes (edge view) cells lining small intestine protein filaments in cytoplasm Desmosome

urinary bladder plasma membranes (edge view) cells lining bladder T ight junction

liver liver cells plasma membrane Gap junctions Hormones, nutrients, ions and electrical signals

plasma membrane cell wall root cells root Plasmodesmata Water, nutrients and hormones