CHAPTER 5 The Working Cell Modules 5.10 – 5.21

The Structure of the Plasma Membrane Glycoprotein Carbohydrate (of glycoprotein) Fibers of the extracellular matrix Glycolipid Phospholipid Cholesterol Microfilaments of the cytoskeleton Proteins CYTOPLASM Figure 5.12

Membrane phospholipids form a bilayer Phospholipids are the main structural components of membranes They each have a hydrophilic head and two hydrophobic tails Head Symbol Tails Figure 5.11A

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

The cell membrane is about 10 nm thick discernable only faintly with a transmission electron microscope. The membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for the cell.

The membrane is a fluid mosaic made of phospholipids and proteins Phospholipid molecules form a flexible bilayer Phospholipid molecules in the cell membrane are "fluid," in the sense that they are free to diffuse and exhibit rapid lateral diffusion. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane The plasma membrane lipids consist of 1/3 cholesterol and 2/3 phospholipids (65-80%) and sphingolipids (20-35%). The outer leaflet contains 5% glycolipids. Cholesterol is embedded in the plasma membrane Phospholipids flow at a rate of 2 micrometers/second

Membrane proteins can span the lipid bilayer while others are peripheral.

MEMBRANE FUNCTION Membranes organize the chemical reactions making up metabolism   Cytoplasm Figure 5.10

The membrane : Sorts what goes in and out of the cell. Membranes are selectively permeable. Anchors of the cytoskeleton to provide shape to the cell Attaches to the extracellular matrix to help group cells together in the formation of tissues Transports particles by way of ion pumps, ion channels, and carrier proteins Contains receptors that allow chemical messages to pass between cells and systems Participates in enzyme activity important in such things as metabolism and immunity

Membranes are selectively permeable They control the flow of substances into and out of a cell

Some membrane proteins form cell junctions The Membrane Proteins: allow chemical messages to pass between cells and systems Some membrane proteins form cell junctions Others transport substances across the membrane Figure 5.13 Transport

Many membrane proteins are enzymes Some proteins function as receptors for chemical messages from other cells The binding of a messenger to a receptor may trigger signal transduction Messenger molecule Receptor Activated molecule Figure 5.13 Enzyme activity Signal transduction

Passive and Active Transport Across the Membrane Passive Transport does not require energy 1. Simple diffusion 2. Osmosis 3. Diffusion by transport proteins (carrier proteins, channel proteins). B. Active Transport requires energy

Passive transport is diffusion across a membrane without expending energy In simple diffusion, substances diffuse through membranes without work by the cell They spread from areas of high concentration to areas of lower concentration Solutes move with the concentration gradient Solute specific Molecule of dye Membrane EQUILIBRIUM EQUILIBRIUM Figure 5.14A & B

Osmosis is the passive transport of water Hypotonic solution Hypertonic solution In osmosis, water travels from an area of lower solute concentration to an area of higher solute concentration Not solute specific Selectively permeable membrane Solute molecule HYPOTONIC SOLUTION HYPERTONIC SOLUTION Water molecule Selectively permeable membrane Solute molecule with cluster of water molecules NET FLOW OF WATER Figure 5.15

Water balance between cells and their surroundings is crucial to organisms Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution The control of water balance (osmoregulation) is essential for organisms ISOTONIC SOLUTION HYPOTONIC SOLUTION HYPERTONIC SOLUTION ANIMAL CELL (1) Normal (2) Lysing (3) Shriveled Plasma membrane PLANT CELL Figure 5.16 (4) Flaccid (5) Turgid (6) Shriveled

Transport proteins facilitate diffusion across membranes as passive transport. Small nonpolar molecules diffuse freely through the phospholipid bilayer but: Many other kinds of molecules pass through selective protein pores (carrier proteins) by facilitated diffusion Solute molecule Transport protein Figure 5.17

Facilitated Diffusion

Channel Proteins move ions passively through the membrane.

Cells expend energy for active transport Transport proteins can move solutes across a membrane against a concentration gradient This is called active transport Active transport requires ATP

Active transport in two solutes across a membrane FLUID OUTSIDE CELL Phosphorylated transport protein Active transport in two solutes across a membrane Transport protein First solute 1 First solute, inside cell, binds to protein 2 ATP transfers phosphate to protein 3 Protein releases solute outside cell Second solute 4 Second solute binds to protein 5 Phosphate detaches from protein 6 Protein releases second solute into cell Figure 5.18

Direction of transport Uniports Co-transports-symports and antiports

To move large molecules or particles through a membrane 5.19 Exocytosis and endocytosis transport large molecules---active transport To move large molecules or particles through a membrane a vesicle may fuse with the membrane and expel its contents (exocytosis) FLUID OUTSIDE CELL CYTOPLASM Figure 5.19A

or the membrane may fold inward, trapping material from the outside (endocytosis) Figure 5.19B

Material bound to receptor proteins Three kinds of endocytosis Pseudopod of amoeba Food being ingested Plasma membrane Material bound to receptor proteins PIT Cytoplasm Figure 5.19C

5.20 Connection: Faulty membranes can overload the blood with cholesterol Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors Phospholipid outer layer LDL PARTICLE Receptor protein Protein Cholesterol Plasma membrane Vesicle CYTOPLASM Figure 5.20

5.21 Chloroplasts and mitochondria make energy available for cellular work Enzymes and membranes are central to the processes that make energy available to the cell Chloroplasts carry out photosynthesis, using solar energy to produce glucose and oxygen from carbon dioxide and water Mitochondria consume oxygen in cellular respiration, using the energy stored in glucose to make ATP

Chemicals recycle among living organisms and their environment Sunlight energy Nearly all the chemical energy that organisms use comes ultimately from sunlight Chloroplasts, site of photosynthesis CO2 + H2O Glucose + O2 Mitochondria sites of cellular respiration Chemicals recycle among living organisms and their environment (for cellular work) Heat energy Figure 5.21