Metabolism

Cell Energetics

Metabolism = total of all the chemical reactions taking place in

an organism

Metabolism

• Anabolism = ‘build up’ processes; ‘consume’ (store) energy by assembling macromolecules (photosynthesis)

• Catabolism = ‘break down’ reactions; release energy by breaking down (lyse) molecules (digestion)

Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics

Metabolism

• Energy - the ability to do work

• Closed Systems - system without energy input

• Open systems – system with energy input

Types

• Potential = capacity to do work

– Caused by POSITION

– Stored chemical energy; (glucose, glycogen)

• Kinetic – energy of motion

• Equilibrium = energy runs outEquilibrium = energy runs out

Thermodynamics

• Thermodynamics - the study of energy transformations

• 1st Law of Thermodynamics - energy cannot be created nor destroyed transformed

• 2nd Law of Thermodynamics - in a closed system, when energy is transformed, some is ‘lost’ as heat

• Entropy – decreasing available energy of the universe is increasing (disorder)

• Energy systems become more disordered/random;

• Total entropy increases; ‘stuff’ runs down

Free Energy

• The energy in a system available for work

• A spontaneous change can cause free energy to ‘flow’– System becomes more stable– Less work capacity– Free energy decreases (entropy)

Biological Order and Disorder

• Organisms live at the expense of free energy

• Organisms are open systems with low entropy – Use free energy to maintain order and

organization – Convert complex molecules into simpler ones;

digestion – Trade organization for heat (increases

randomness and entropy)

Types of Reactions

• Exergonic reaction - net release of free energy

– Less stable, more work– Fire, respiration

• Endergonic reaction - energy-requiring reaction; energy is absorbed/stored

– Photosynthesis

Concept 8.3: ATP Powers Cellular Work by Coupling Exergonic

Reactions to Endergonic Reactions

• Cells do three main kinds of work

– Mechanical - movement

– Transport – of stuff

– Chemical – polymerization, bioluminescence

Coupled Reactions• Coupled Reactions - endergonic reactions

are coupled with exergonic reactions

• Energy from an exergonic reaction (respiration) is stored in phosphate bonds

• Phosphate group is added to a molecule– Phosphorylation

– Molecule ‘works’

ATP

• Adenine + ribose + phosphate group

• Phosphate bond is easily broken/formed

• Controlled by enzymes

Uses of Energy

• Mechanical - beating of cilia/flagella, muscle, cytoplasmic flow, movement of chromosomes (mitosis)

• Transport - H+ ‘pump’, receptors

• Chemical – polymerization, bioluminescence

Figure 8.1

Concept 8.4: Enzymes speed up metabolic reactions by lowering

energy barriers

• Enzymes - biological catalysts– Accelerate reactions without being changed – Proteins (700)

• Catabolic or anabolic• All chemical reactions require activation

energy– Activation energy, EA - the initial amount of

energy needed to start a chemical reaction

• Often supplied as heat from the environment (spontaneous)

EnzymesEnzymes

Enzymes • Cellular T needs to remain

low, but metabolism is too slow at low T

• Enzymes reduce activation energy

• Transition state - reactants have absorbed energy

Enzymes • Substrate = substance

enzyme acts upon• Active site = area on the

enzyme which the substrate binds to (attaches) – Verryyy specific– Groove, pocket = 3d shape

• 2 mechanisms describe how enzymes function:– ‘Lock and Key’– Induced Fit - enzyme

may change shape to allow better reaction on substrate

Enzymes

Active site

Induced fit

Chemical bonds broken

• Enzymatic action is reversible – E + S P + E – Enzyme unaffected by reaction– Dependent upon concentration of reactants vs

products

• Reaction rate of 1000’s per second

• Speeding up enzyme reactions: – Add more substrate; until saturated with substrate…– Add more enzyme

• DNA controls cell’s activities by storing the code for protein synthesis (enzymes)

Enzymes

Factors That Affect Enzyme Activity • Temperature and pH

• Inorganic salts – disrupt H, ionic bonds, hydrophobic interactions

• Cofactors

• Inhibitors

Cofactors• Cofactors - nonprotein enzyme helpers

– Metals – Fe, Zn, Cu

• Coenzymes - organic – Vitamins

• Inhibitors = substances that inhibit the actions of enzymes (2 kinds:)

– Competitive inhibitors

– Noncompetitive inhibitors

Competitive inhibitors - resemble substrate, block active site Neurotoxin, Disulfiram

• Noncompetitive inhibitors - causes enzyme to change shape– Destroys conformation

(active site)– DDT, nerve gas (DSF)– May be allosteric

regulation

Enzymes

Concept 8.5: Regulation of enzyme activity helps control

metabolism

• A cell’s metabolic pathways must be tightly regulated

Allosteric Regulation of Enzymes

• Allosteric regulation - a protein’s function at one site is affected by binding of a regulatory molecule at another site

• Receptor site located away from the active site (quaternary structure)

• Allosteric site has to be activated, (may be inhibited)

Allosteric activator

Allosteric inhibitor

• Cooperativity - one substrate molecule can activate all other subunits of an enzyme

• Only requires a small concentration of substrate to activate enzyme– Hemoglobin

Enzymes

Feedback Inhibition• Metabolic pathways –

series of enzymes creates small steps to a final product

• Controlling the enzymes (activity or production) controls the pathway and product(s)

• Feedback Inhibition - end product of the pathway inhibits the pathway

Isoleucine – allosteric inhibitor

• Feedback inhibition prevents cells from wasting resources– “don’t need gas if you don’t have a car.”

Enzymes

Structure and Metabolism

• Cells are organized • Enzymes are grouped into

complexes or incorporated into membranes

• Multi-enzyme complex =

enzymes are assembled in correct physical position for a sequence of events to happen– Mitochondria, chloroplasts