Chapter 5 The Working Cell

Cool “Fires” Attract Mates and Meals Fireflies use light to send signals to potential mates Instead of using chemical signals like most other insects

The light comes from a set of chemical reactions That occur in light-producing organs at the rear of the insect Many of the enzymes that control a firefly's ability to produce light energy from chemical energy are located in membranes.

Females of some species Produce a light pattern that attracts males of other species, which are then eaten by the female

5.1 Energy is the capacity to perform work ENERGY AND THE CELL 5.1 Energy is the capacity to perform work Crash Course-Energy All organisms require energy Energy is defined as the capacity to do work Energy is the capacity to rearrange matter. Energy can only be described and measured by how it affects matter. We can’t see energy.

Kinetic energy is the energy of motion Heat- (thermal energy)-the movement of molecules or atoms in a body of matter.

And can be converted to kinetic energy (the energy of a moving object) Potential energy is stored energy that an object possesses as a result of its location or structure. And can be converted to kinetic energy (the energy of a moving object) Figure 5.1A–C

Chemical Energy-the potential energy of molecules Life depends on the fact that energy can be converted from one form to another. Ex. Glucose molecules provide energy to power the swimming motion of sperm. In this example, the sperm are changing chemical energy into kinetic energy.

5.2 Two laws govern energy transformations Thermodynamics Is the study of energy transformations that occur in a collection of matter. System-the collection of matter under study in thermodynamics Ex. Single cell or Earth Surroundings-everything outside the system A living thing is an open system-it exchanges both energy and matter with its surroundings.

The First Law of Thermodynamics According to the first law of thermodynamics Energy can be changed from one form to another Energy cannot be created or destroyed Figure 5.2A

The Second Law of Thermodynamics States that energy transformations increase disorder or entropy, and some energy is lost as heat Entropy-the amount of disorder in a system. Ex. A steer must eat at least 100 pounds of grain to gain less than 10 pounds of muscle tissue. Crash Course-Entropy Figure 5.2B

Living systems decrease their entropy while increasing the entropy of the universe. Ex. Protein Synthesis The more heat released by a reaction, the greater the increase in entropy.

Energy transfers possible in living systems: Light energy to chemical energy (photosynthesis) Chemical energy to kinetic energy (cellular respiration) Potential energy to kinetic energy Light energy to potential energy

5.3 Chemical reactions either store or release energy Endergonic reactions-absorb energy and yield products rich in potential energy Ex. The synthesis of glucose from carbon dioxide and water Products Amount of energy required Energy required Potential energy of molecules Reactants Figure 5.3A

Exergonic reactions Release energy and yield products that contain less potential energy than their reactants Reactants Amount of energy released Energy released Potential energy of molecules Products Figure 5.3B

Cells carry out thousands of chemical reactions The sum of which constitutes cellular metabolism Energy coupling Uses exergonic reactions to fuel endergonic reactions When a cell uses chemical energy to perform work, it couples an exergonic reaction with an endergonic reaction.

5.4 ATP shuttles chemical energy and drives cellular work ATP contains a nitrogenous base called adenine & 3 phosphate groups. ATP powers nearly all forms of cellular work Anything that prevents ATP formation will most likely result in cell death. ATP can be used as the cell's energy currency because endergonic reactions can be fueled by coupling them with the hydrolysis of high-energy phosphate bonds in ATP.

The energy in an ATP molecule Lies in the bonds between its phosphate groups Adenosine Triphosphate Adenosine diphosphate Phosphate groups H2O P P P P P + P + Energy Hydrolysis Adenine Ribose ATP ADP Figure 5.4A

ATP drives endergonic reactions by phosphorylation Transferring a phosphate group to make molecules or compounds ATP Chemical work Mechanical work Transport work Membrane protein Solute P + Motor protein P Reactants P P P Product P Molecule formed Protein moved Solute transported ADP + P Figure 5.4B

3 Main Types of Cellular Work: (ATP drives all three) Chemical Ex. Protein Synthesis Mechanical Ex. Muscle contraction Transport Ex. Movement of molecules across membranes

Cellular work can be sustained Because ATP is a renewable resource that cells regenerate ATP Dehydration Synthesis Hydrolysis Energy from exergonic reactions Energy for endergonic reactions ADP + P Figure 5.4C

HOW ENZYMES FUNCTION 5.5 Enzymes speed up the cell’s chemical reactions by lowering energy barriers Energy of Activation-the amount of energy that reactants must overcome to start a chemical reaction. An energy barrier prevents the spontaneous decomposition of ATP in the cell.

For a chemical reaction to begin Reactants must absorb some energy, called the energy of activation EA barrier Enzyme Reactants Products 1 2 Figure 5.5A

Enzymes: Are usually proteins Can decrease the energy of activation needed to begin a reaction Does not add energy to a cellular reaction; it speeds up a reaction by lowering the activation energy barrier. Catalyze reactions & lower the activation energy of the reaction Without enzymes, most metabolic reactions would occur too slowly to sustain life.

5.6 A specific enzyme catalyzes each cellular reaction Enzymes have unique three-dimensional shapes due to their protein nature That shape determines which chemical reactions occur in a cell. That shape does not change after the reaction.

Substrate-a specific reactant that an enzyme acts on. Fits into an active site-the region of an enzyme that attaches to a substrate. When a substrate binds to an enzyme, the active site changes shape slightly so that it embraces the substrate more snugly. We call this induced fit.

The catalytic cycle of an enzyme 1 Enzyme available with empty active site Active site Substrate (sucrose) 2 Substrate binds to enzyme with induced fit Enzyme (sucrase) Glucose Fructose H2O 4 Products are released 3 Substrate is converted to products Figure 5.6

5.7 The cellular environment affects enzyme activity Some enzymes require: Nonprotein, inorganic cofactors, such as metal ions organic molecules called coenzymes, which are usually vitamins.

The following can affect the rate of an enzyme-catalyzed reaction: Heating, inactivates enzymes by changing the enzyme's three-dimensional shape. The following can affect the rate of an enzyme-catalyzed reaction:  temperature   pH   competitive inhibitors   noncompetitive inhibitors

5.8 Enzyme inhibitors block enzyme action Inhibitors interfere with an enzyme’s activity Competitive inhibitors bind to the active site of the enzyme; noncompetitive inhibitors bind to a different site Inhibition of an enzyme is irreversible when bonds form between inhibitor and enzyme.

Normal binding of substrate A competitive inhibitor Takes the place of a substrate in the active site A noncompetitive inhibitor Alters an enzyme’s function by changing its shape Substrate Active site Enzyme Normal binding of substrate Competitive inhibitor Noncompetitive inhibitor Enzyme inhibition Figure 5.8

5.9 Many poisons, pesticides, and drugs are enzyme inhibitors CONNECTION 5.9 Many poisons, pesticides, and drugs are enzyme inhibitors

MEMBRANE STRUCTURE AND FUNCTION 5.10 Membranes organize the chemical activities of cells Membranes Provide structural order for metabolism Cell Membrane-Hippocamus

The plasma membrane of the cell is selectively permeable Controlling the flow of substances into or out of the cell Outside of cell TEM 200,000  Cytoplasm Figure 5.10

Functions of the Plasma Membrane: Forms a selective barrier around the cell. Plays a role in signal transduction. Has receptors for chemical messages. Is involved in self-recognition.

5.11 Membrane phospholipids form a bilayer Have a hydrophilic head and two hydrophobic tails Are the main structural components of membranes Sometimes have a kink due to a double bond with carbon Hydrophilic head + CH3 CH2 N CH3 CH2 CH3 Phosphate group O O P O– O CH2 CH CH2 O O C O C O CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 Symbol CH2 CH CH2 CH CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH3 Hydrophobic tails Figure 5.11A

Phospholipids form a two-layer sheet Called a phospholipid bilayer, with the heads facing outward and the tails facing inward Water Hydrophilic heads Hydrophobic tails Water Figure 5.11B

5.12 The membrane is a fluid mosaic of phospholipids and proteins A membrane is a fluid mosaic which describes the plasma membrane as consisting of individual proteins and phospholipids that can drift in a phospholipid bilayer. The cholesterol associated with cell membranes helps to stabilize the cell membrane at body temperature. http://www.youtube.com/watch?v=Qqsf_UJcfBc Figure 5.12

Unsaturated lipids have kinks, which make the membrane more fluid by keeping phospholipids from packing tightly together. Glycoprotein-a protein with attached sugars Glycolipid-lipid with attached sugars A major function of glycoproteins and glycolipids in the cell membrane is to allow the cells of an embryo to sort themselves into tissues and organs.

Fibers of the extracellular matrix Carbohydrate (of glycoprotein) Glycoprotein Glycolipid Plasma membrane Phospholipid Proteins Microfilaments of cytoskeleton Cholesterol Cytoplasm

5.13 Proteins make the membrane a mosaic of function Proteins perform most of the functions of a membrane. Many membrane proteins Function as enzymes Provide cellular identification tags Attach the membrane to the cytoskeleton. Figure 5.13A

Other membrane proteins Function as receptors for chemical messages from other cells Messenger molecule Receptor Activated molecule Figure 5.13B

Membrane proteins also function in transport Moving substances across the membrane ATP Figure 5.13C

5.14 Passive transport is diffusion across a membrane In passive transport, substances diffuse through membranes without work by the cell Spreading from areas of high concentration to areas of low concentration Molecules of dye Membrane Equilibrium Figure 5.14A Equilibrium Figure 5.14B

Small nonpolar molecules such as O2 and CO2 Diffusion: Is a result of the kinetic energy of atoms and molecules.   is driven by entropy.   requires no input of energy into the system.   proceeds until equilibrium is reached. Small nonpolar molecules such as O2 and CO2 Diffuse easily across the phospholipid bilayer of a membrane

5.15 Transport proteins may facilitate diffusion across membranes Many kinds of molecules Do not diffuse freely across membranes For these molecules, transport proteins Provide passage across membranes through a process called facilitated diffusion Transport Proteins Solute molecule Transport protein Figure 5.15

5.16 Osmosis is the diffusion of water across a membrane In osmosis Water travels from a solution of lower solute concentration to one of higher solute concentration Equal concentration of solute Lower concentration of solute Higher concentration of solute Solute molecule H2O Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Figure 5.16 Net flow of water

5.17 Water balance between cells and their surroundings is crucial to organisms Hypotonic- the concentration of solute outside the cell is lower than the concentration inside the cytosol. Hypertonic-the concentration of solute outside the cell is higher than the concentration in the cytosol. A cell that neither gains nor loses water when it is immersed in a solution is isotonic to its environment. Osmosis Figure 5.17

Osmosis causes cells to shrink in hypertonic solutions The control of water balance is called osmoregulation. Osmosis causes cells to shrink in hypertonic solutions And swell in hypotonic solutions In isotonic solutions Animal cells are normal, but plant cells are limp A plant cell in a hypotonic solution is turgid. http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Isotonic solution Hypotonic solution Hypertonic solution H2O H2O H2O H2O Animal cell (1) Normal (2) Lysed (3) Shriveled Plasma membrane H2O H2O H2O H2O Plant cell (6) Shriveled (plasmolyzed) (4) Flaccid (5) Turgid

5.18 Cells expend energy for active transport Transport proteins can move solutes against a concentration gradient Through active transport, which requires ATP Transport protein P P ATP P Protein changes shape Phosphate detaches Solute ADP 1 Solute binding 2 Phosphorylation 3 Transport 4 Protein reversion Figure 5.18

Active transport: Uses ATP as an energy source. Can move solutes up a concentration gradient. Requires the cell to expend energy. Is necessary to allow nerves to function properly. http://www.youtube.com/watch?v=STzOiRqzzL4&feature=PlayList&p=8447AE4E545686AA&index=14

5.19 Exocytosis and endocytosis transport large molecules To move large molecules or particles through a membrane A vesicle may fuse with the membrane and expel its contents (exocytosis) Fluid outside cell Vesicle Protein Cytoplasm Figure 5.19A

Membranes may fold inward Enclosing material from the outside (endocytosis) Vesicle forming Figure 5.19B

Endocytosis can occur in three ways Phagocytosis-“cellular eating” Pinocytosis-“cellular drinking” Receptor-mediated endocytosis Plasma membrane Food being ingested Pseudopodium of amoeba Material bound to receptor proteins PIT TEM 96,500  TEM 54,000 Cytoplasm LM 230 Phagocytosis Pinocytosis Receptor-mediated endocytosis Figure 5.19C

http://www. youtube. com/watch http://www.youtube.com/watch?v=4gLtk8Yc1Zc&feature=PlayList&p=8447AE4E545686AA&index=13&playnext=2&playnext_from=PL http://www.youtube.com/watch?v=W6rnhiMxtKU&feature=PlayList&p=8447AE4E545686AA&playnext=1&playnext_from=PL&index=12

5.20 Faulty membranes can overload the blood with cholesterol CONNECTION 5.20 Faulty membranes can overload the blood with cholesterol Harmful levels of cholesterol Can accumulate in the blood if membranes lack cholesterol receptors Cells acquire LDLs by receptor-mediated endocytosis. An inherited lack of functional LDL receptors causes hypercholesterolemia.   Figure 5.20

Phospholipid outer layer LDL particle Vesicle Cholesterol Protein Plasma membrane Receptor protein Cytoplasm

5.21 Chloroplasts and mitochondria make energy available for cellular work Enzymes 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