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) • 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 • 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 • 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 • 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 • 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