Chapter 5 The Working Cell

Fireflies use light to send signals to potential mates Intro: 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.

Energy is the capacity to perform work ENERGY AND THE CELL Energy is the capacity to perform work All organisms require energy! (“ability to obtain and use energy” was a defining characteristic of living things!) What is Energy?

Kinetic energy is the energy of motion Potential energy is stored energy, which can be converted to kinetic energy. Rollercoaster animation Figure 5.1A–C

5.2 Two laws govern energy transformations Thermodynamics- the study of energy transformations The First Law of Thermodynamics Energy can be changed from one form to another, but energy cannot be created or destroyed. Figure 5.2A

Energy for cellular work The Second Law of Thermodynamics energy transformations increase disorder or entropy, and some energy is lost as heat. Heat Chemical reactions Carbon dioxide Glucose + + ATP ATP water Oxygen Figure 5.2B Energy for cellular work

Chemical reactions either store or release energy

Endergonic reactions - absorb energy and yield products rich in potential energy. Potential energy of molecules Reactants Energy required Products Amount of energy required Figure 5.3A

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

Energy coupling- Uses exergonic reactions to fuel endergonic reactions Energy coupling- Uses exergonic reactions to fuel endergonic reactions. This is how cells carry out thousands of chemical reactions! (cellular metabolism) Endergonic and exergonic

Currency of the cells Energy is like a bank account, with ATP being the “currency” for the cells. You either save $ or you withdraw $! You cells can either store (save $) energy by forming bonds OR release (withdraw $) energy by breaking bonds! Can only do one or the other at any given time!

ATP (adenosine triphosphate) powers nearly all forms of cellular work The energy in an ATP molecule lies in the bonds between its phosphate groups. Phosphate groups ATP Energy P Hydrolysis Adenine Ribose H2O Adenosine diphosphate Adenosine Triphosphate + ADP Figure 5.4A

Take off a phosphate; breaks the bond = release of energy. Phosphorylation = adding a phosphate group to make molecules more reactive by storing energy. ATP Chemical work Mechanical work Transport work P Molecule formed Protein moved Solute transported ADP + Product Reactants Motor protein Membrane protein Solute Take off a phosphate; breaks the bond = release of energy. Add a phosphate; builds a bond = stores/absorbs energy. Animation Figure 5.4B

Cellular work can be sustained because ATP is a renewable resource that cells constantly regenerate! ADP + P Energy for endergonic reactions Energy from exergonic reactions Phosphorylation Hydrolysis Figure 5.4C

HOW ENZYMES FUNCTION For a chemical reaction to begin reactants must absorb some energy, called the energy of activation (energy needed to get the reaction going) Enzymes speed up the cell’s chemical reactions by lowering energy barriers. EA barrier Reactants Products 1 2 Enzyme Figure 5.5A

Progress of the reaction A catalyst speeds up a reaction, by decreasing the energy of activation needed to begin a reaction. Reactants EA without enzyme EA with enzyme Net change in energy Products Energy Progress of the reaction Figure 5.5B

THEREFORE, ALL ENZYMES are catalysts!

An enzymes specific shape determines which chemical reactions it will catalyze. 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 Animation 1 Figure 5.6

What affects enzyme activity?? Temperature, salt concentration, and pH influence enzyme activity and can affect its function. How?? Some enzymes require nonprotein cofactors such as metal ions or organic molecules called coenzymes. Ex. Coenzyme Q 10

Normal binding of substrate Enzyme inhibitors can block enzyme action Competitive inhibitors- take the place of a substrate in the active site Noncompetitive inhibitors alters an enzyme’s function by changing its shape Substrate Enzyme Active site Normal binding of substrate Enzyme inhibition Noncompetitive inhibitor Competitive inhibitor Many poisons, pesticides, and drugs are enzyme inhibitors. Figure 5.8

Phospholipids have a hydrophilic head and 2 hydrophobic tails. The plasma membrane of the cell is selectively permeable controlling the flow of substances into or out of the cell Phospholipids have a hydrophilic head and 2 hydrophobic tails. Phospholipid bilayer, with the heads facing outward and the tails facing inward CH2 CH3 CH N + O O– P C Phosphate group Symbol Hydrophilic head Hydrophobic tails Water Hydrophilic heads Hydrophobic tails Figure 5.11A

A membrane is a fluid mosaic with proteins, lipids and carbohydrate embedded in a phospholipid bilayer or along each side. Fibers of the extracellular matrix Carbohydrate (of glycoprotein) Glycoprotein Microfilaments Phospholipid Cholesterol Proteins Plasma membrane Glycolipid Cytoplasm Figure 5.12

Membrane proteins can function as: Enzymes - they help speed up chemical reactions for the cell. Receptors – bind and transmit chemical messages from other cells Transport- moving substances across the membrane ATP Messenger molecule Receptor Activated molecule Receptor site animation Figure 5.13A

TYPES OF TRANSPORT ACROSS A MEMBRANE

NO ENERGY REQUIRED! 1) Passive transport- substances diffuse through membranes without work by the cell; spread from areas of high concentration to areas of low concentration Small nonpolar molecules such as O2 and CO2 Diffuse easily across the phospholipid bilayer of a membrane (Diffusion animation) Equilibrium Membrane Molecules of dye Figure 5.14A Figure 5.14B

Many kinds of molecules DO NOT diffuse freely across membranes NO ENERGY REQUIRED! Many kinds of molecules DO NOT diffuse freely across membranes For these molecules, transport proteins MUST provide passage across membranes through a process called facilitated diffusion. Solute molecule Transport protein Facilitated diffusion animation Figure 5.15

NO ENERGY REQUIRED! 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 (higher water conc. to lower water conc.) Lower concentration of solute Higher concentration of solute Equal concentration of solute H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water Osmosis animation Figure 5.16

Shrink in hypertonic solutions Swell in hypotonic solutions The control of water balance is called Osmoregulation. Shrink in hypertonic solutions Swell in hypotonic solutions Equal movement of particles in isotonic solutions (Animal cells are normal, but plant cells are limp) Plant cell H2O Plasma membrane (1) Normal (2) Lysed (3) Shriveled (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed) Isotonic solution Hypotonic solution Hypertonic solution Animal cell Figure 5.17

Active transport- ENERGY REQUIRED! Transport proteins can move solutes against a concentration gradient through active transport, which requires ATP (One example of active transport is the sodium-potassium pump used in nerve impulses) P Protein changes shape Phosphate detaches ATP ADP Solute Transport protein Solute binding 1 Phosphorylation 2 Transport 3 Protein reversion 4 Figure 5.18

ENERGY REQUIRED! To move large molecules or particles through a membrane, a vesicle may fuse with the membrane and expel its contents (exocytosis) Membranes may fold inward enclosing material from the outside (endocytosis) Fluid outside cell Cytoplasm Protein Vesicle Vesicle forming Figure 5.19A

Endocytosis can occur in 3 ways: Phagocytosis – cellular eating (animation) Pinocytosis – cellular drinking Receptor-mediated endocytosis Pseudopodium of amoeba Food being ingested Phagocytosis Pinocytosis Receptor-mediated endocytosis Material bound to receptor proteins PIT Cytoplasm Plasma membrane TEM 54,000 TEM 96,500  LM 230 Figure 5.19C

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. Overview animation (We’ll go. into more detail on each process in the upcoming chapters!)