Energy and Metabolism

Chapter 4Part 1

4.1 Impacts/IssuesA Toast to Alcohol Dehydrogenase

Metabolic processes break down organic molecules such as ethanol and other toxins – binge drinking is currently the most serious drug problem on college campuses

Video: Alcohol, enzymes, and your liver

Alcohol dehydrogenase (ADH) converts ethanol to toxic acetaldehyde, which is then converted to acetate by ALDH

4.2 Life Runs on Energy Laws of Thermodynamics

Energy• The capacity to do work

Law #1 Energy can be converted from one form to another, but cannot be created or destroyed –

Law #2 Energy disperses spontaneously

Material Recycle

Energy inputs drive a cycling of materials among producers and consumers

Producers and then consumers use energy to assemble, rearrange, and break down organic molecules that cycle among organisms throughout ecosystems

Fig. 4-2, p. 63

Light energy radiating from the sun reaches Earth. Producers capture some of it by converting it to chemical energy. They and all other organisms use chemical energy to drive cellular work.

ENERGY IN

PRODUCERSplants and other self-

feeding organisms

nutrient cycling

CONSUMERSanimals, most fungi,

many protists, bacteria

ENERGY OUT

With each conversion, there is a one- way flow of a bit of energy back to the environment, mainly in the form of heat.

One way Flow of energy

Matter recycling and energy flow

4.3 Energy in the Molecules of Life

Cells store and retrieve energy by making and breaking chemical bonds in metabolic reactions

Some reactions require a net input of energy – others end with a net release of energy

Chemical Reactions

Reaction• Process of chemical change

Reactant• Molecule that enters a reaction

Product• A molecule remaining at the end of a reaction

A Chemical Reaction

Energy Inputs and Outputsin Chemical Reactions

Chemical bonds hold energy – the amount depends on which elements take part in the bond

Cells store energy in chemical bonds by running energy-requiring reactions, and access energy by running energy-releasing reactions

Energy Inputs and Outputsin Chemical Reactions

Why the World Doesn’t Go Up in Flames

Molecules of life release energy when combined with oxygen – but not spontaneously – energy is required to start even energy-releasing reactions

Activation energy• Minimum amount of energy required to start a

reaction

Fig. 4-4, p. 65

Reactants: 2 H2 + O2

Activation energy

Ener

gy

Difference in energy between

reactants and products

Products: 2 H2O

TimeStepped Art

Activation Energy

Animation: Chemical equilibrium

ATP – The Cell’s Energy Currency

Energy carriers accept energy from energy-releasing reactions and deliver energy to energy-requiring reactions

ATP (Adenosine triphosphate)• Main energy carrier between reaction sites in

cells

Phosphorylation

Phosphate-group transfers (phosphorylation) to and from ATP couple energy-releasing reactions with energy-requiring ones

ATP: The Energy Currency of Cells

4.4 How Enzymes Work

Enzymes make chemical reactions proceed much faster than they would on their own

Enzyme• Protein or RNA that speeds a reaction without

being changed by it

Substrates

An enzyme’s particular substrates bind at its active site

Substrate• A reactant molecule that is specifically acted

upon by an enzyme

Active Sites

Active site• Pocket in an enzyme where substrates bind and

a reaction occurs

Factors That Influence Enzyme Activity

Each enzyme works best within a characteristic range of temperature, pH, and salt concentration

When conditions break hydrogen bonds, an enzyme changes its characteristic shape (denatures), and stops working

Enzymes, Temperature, and pH

Fig. 4-6a, p. 66

Organized, Enzyme-Mediated Reactions

Cells concentrate, convert, and dispose of most substances in enzyme-mediated reaction sequences

Metabolic pathway• Series of enzyme-mediated reactions by which

cells build, remodel, or break down an organic molecule

Linear and Cyclic Metabolic Pathways

Control of Metabolic Pathways

Various controls over enzymes allow cells to conserve energy and resources by producing only what they require • Concentrations of reactants and products• Feedback inhibition

Control of Metabolic Pathways

Feedback inhibition• Mechanism by which a change that results from

some activity decreases or stops the activity

Feedback Inhibition

Electron Transfers

Electron transfer chains allow cells to harvest energy in manageable increments

Electron transfer chain• An array of membrane-bound enzymes and other

molecules that accept and give up electrons in sequence

Fig. 4-9, p. 68

glucose++

oxygen water

spark

A Glucose and oxygen react (burn) when exposed to a spark. Energy is released all at once as light and heat when CO2 and water form.

carbon dioxide

carbon dioxide

glucose e–

oxygen H+

e–

water

B The same overall reaction occurs in small steps with an electron transfer chain. Energy is released in amounts that cells can harness for cellular work.

1 Energy input splits glucose into carbon dioxide, electrons, and hydrogen ions (H+).2 Electrons lose energy as they move through an electron transfer chain.3 Energy released by electrons is harnessed for cellular work.

4 Electrons, hydrogen ions, and oxygen combine to form water.

Stepped Art

Uncontrolled and Controlled Energy Release