Ground Rules of Metabolism Chapter 5

5.1 Energy and the World of Life Energy: The capacity to do work Potential energy (Example: chemical bonds) Kinetic energy (Energy of motion)

Two Laws of Thermodynamics 1. Energy cannot be created or destroyed It can be converted from one form to another and transferred between objects or systems The energy transfer most pertinent to living systems in the conversion of light to chemical energy via photosynthesis 2. Entropy tends to increase Energy tends to disperse spontaneously Some energy disperses at each energy transfer, usually in the form of heat Therefore, living systems need a constant input of energy to maintain their organization

One-Way Energy Flow

Energy In, Energy Out Chemical reactions Reactants (molecules in) Products (molecules out) Endergonic reactions (energy-requiring) Exergonic reactions (energy-releasing) Chemical reactions in cells are nearly always coupled – an input of energy is coupled by an output of energy

Exergonic and Endergonic Reactions

5.2 ATP in Metabolism Adenosine triphosphate (ATP) Phosphorylation Main energy carrier between cell reaction sites ADP/ATP cycle Phosphorylation Phosphate-groups transfer to and from ATP When a phosphate is added to a molecule (endergonic reaction) it is coupled to the exergonic reaction of the lost of a phosphate from the molecule “giving up” the phosphate

ATP: Energy Currency of Life

5.3 Enzymes in Metabolism Activation energy Enzymes are catalysts Minimum energy needed to start a reaction Enzymes are catalysts Speed reaction rates by lowering activation energy Most are proteins (some RNAs) catalyze every synthesis (condensation reactions) and decomposition (hydrolysis) reaction in cells

Activation Energy

Enzyme Action Active site How enzymes lower activation energy Specific site of enzyme molecule where reactions occur Creates a microenvironment that is more favorable for the reaction How enzymes lower activation energy By concentrating substrate molecules By orienting substrates to favor reaction By inducing fit between substrate and active site Activation energy allows enzyme to bring substrate to transition state

Enzyme Action: Hexokinase

5.4 Controls Over Enzymes Enzymatic reactions are carefully controlled in cells via competitive inhibition = when a molecule similar to the substrate attaches to the active site and blocks its access to the actual substrate Allosteric sites = a physical site on the enzyme molecule distinct from the active site Molecular bonding to the allosteric site can enhance or inhibit enzyme function by changing the shape of the enzyme molecule

Allosteric Control

Feedback Inhibition – results from inhibition

Enzymes and the Environment Each enzyme functions best within a characteristic range of temperature, salt concentration, and pH

Enzymes and pH

5.5 Metabolism – Organized, Enzyme-Mediated Reactions Cells concentrate, convert, and dispose of most substances in orderly, enzyme-mediated reaction sequences called metabolic pathways Biosynthetic pathways Construct large molecules from smaller ones Require energy Photosynthesis Main biosynthetic pathway in the biosphere Autotrophs

Degradative Pathways Degradative pathways Aerobic respiration Break down molecules to smaller products Release usable energy Aerobic respiration Main degradative pathway in the biosphere Heterotrophs

Main Metabolic Pathways

REDOX Reactions Oxidation–reduction (redox) reactions Electron transfers used in metabolic pathways Electron transfer includes a transfer of energy Electron transfer chains Take part in organized sequences of reactions in photosynthesis and aerobic respiration Electrons are passed from one molecule to the next “moving’ energy along the way

Controlled Energy Release

Key Players in Metabolic Pathways

5.6 Diffusion, Membranes, and Metabolism Concentration of a substance Number of atoms or molecules in a given volume Concentration gradient of a substance A difference in concentration between two regions – a higher concentration versus a lower concentration Effects the movement of substances via diffusion

Diffusion Diffusion Diffusion rates are influenced by: Net movement of molecules to a region where they are less concentrated Diffusion rates are influenced by: Temperature Molecular size Gradients of pressure, charge, and concentration

Diffusion

Diffusion and Membrane Permeability Selective Permeability

How Substances Cross Membranes: Diffusion, Passive and Active Transport

How Substances Cross Membranes: Endocytosis and Exocytosis

Three Types of Endocytosis Receptor-mediated endocytosis Substance binds to surface receptors Pit forms endocytotic vesicle Phagocytosis (“cell eating”) Amoebas use pseudopods to engulf prey

Endocytosis and Exocytosis

Phagocytosis

5.7 Working With and Against Gradients Many solutes cross membranes through transport proteins (open or gated channels) Facilitated diffusion (passive transport) does not require energy input Solute diffuses down its concentration gradient through a transporter Example: Glucose transporters

Facilitated Diffusion

Active Transport Active transporters require ATP energy to move a solute against its concentration gradient Maintain gradients (concentration differences) across cell membranes Example: Calcium pumps Cotransporters move two substances at the same time Example: Sodium-potassium pump

Active Transport: Calcium Pump

5.8 Which Way Will Water Move? Osmosis The diffusion of water across a selectively permeable membrane Water molecules follow their concentration gradient, influenced by solute concentration – Water moves from an area of lower solute (higher water) concentration to an area of higher solute (lower water) concentration

Osmosis

Tonicity Relative concentrations of two solutes separated by a semipermeable membrane Hypertonic fluid (higher solute concentration) Hypotonic fluid (lower solute concentration) Isotonic solutions (two solutions with the same tonicity)

A Tonicity Experiment

Osmosis and Hydrostatic Pressure

5.10 Night Lights Bioluminescence Fluorescent light released by enzyme-mediated reactions in organisms