Topic 4: Basics of Cell Energy

How do organisms transform energy?

Absorb energy (light or chemical) → Releases energy (thermal energy + metabolic wastes)

How are organisms governed by the law of thermodynamics?

▪ Energy is neither created nor destroyed, only transferred and

transformed

▪ Every energy transfer or transformation increases the entropy of

the surroundings

What happens when some energy in any transformation from aa cell becomes heat

→  Heat = most disorganized form of energy, therefore release of heat = increase in entropy

→ Not available to do work, and is lost from system

What happens if an energy transformation results in an increase in entropy

Chemical reactions will proceed spontaneously (without adding energy) 

Can a non-spontaneous process happen in the cell?

 Yes! But… energy has to be added

 Creates localized area of lowered entropy 

Exergonic Reaction

Releases free energy (ΔG less than o)

Occurs spontaneously

Ex: Breaking polymers into monomers

Endergonic Reaction

Absorbs free energy from the surroundings to power the reaction (ΔG greater than 0)

Not spontaneous

Ex: Making a polymer from monomers

Metabolism

All of an organism’s chemical reactions

Organized into pathways

Metabolic Pathway

The chemical reactions from a starting material to an ending material

Each step catalyzed by a different enzyme

Catabolic Pathway

Series of chemical reactions that break down complex molecules into simpler molecules

→Releases energy = exergonic

→Requires enzymes

What are two examples of a catabolic pathway?

Breakdown of starch into glucose and Cellular respiration

Anabolic Pathways

Definition: Series of chemical reactions that

build complex molecules from simpler molecules

▪ Requires energy = endergonic

▪ Requires enzymes

What’s are two examples of anabolic pathways?

Synthesis of polypeptide from amino acid monomers

-Dehydration reaction

And photosynthesis

Energy coupling

Energy released from one reaction is used to power another reaction that requires energy

Energy released from carbolic (exergonic) reactions used to power anabolic (endergonic) reactions

Why is ATP important?

Powers cellular work by coupling exergonic/catabolic processes to endergonic/anabolic processes

What are the 3 cellular work that relies on ATP?

1. Movement:

→Movement of transport vesicles around cell

→Flagella and cilia

2. Transport:

→Moves substances across membrane against concentration gradient

3. Chemical:

→Driving endergonic chemical reactions

What is the structure of ATP?

Three phosphate groups

Ribose sugar

Adenine nitrogenous base (purine, a monomer of a long polymer [nucleic acid])

Nucleotide

Energy is released when the terminal (end) phosphate bond is removed

Why is ATP hydrolysis exergonic?

Repulsive forces between negatively charged phosphate groups make ATP less stable than ADP

How does ATP perform cellular work?

ΔG = -7.3 kcal/mol → energy is available to drive endergonic processes

Energy is harnessed by phosphorylation

▪ Phosphate from ATP hydrolysis is added to another molecule

What does phosphorylation make molecules?

Phosphorylation makes molecule more reactive causing endergonic chemical reactions to occur

Phosphorylation can cause a protein to change shape

o Example: Transport proteins

Binding and hydrolyzing ATP can cause a protein to change shape

o Example: Motor proteins

What does the storing of covalent bonds depend on?

Electrons in covalent bonds store different amounts of energy depending on the electronegativity of of atoms involved

How does energy re

Electrons release energy going from a less electronegative atom to a more electronegative atom

Redox reaction 

Chemical reaction where there is a transfer of electrons from one reactant to another

Oxidation 

Loss of electrons (LEO)

Reduction

Gain of electrons (GER)

Cell stores in electron shuttle molecules

Electron shuttle molecules

Temporary storage site for high energy electrons

Ex: NAD⁺

Enzyme that removes 2 hydrogen atoms from high energy substrate and transfers them to shuttle molecules

A series of electron-carrying membrane proteins that transfer electrons and release energy 

-Receive high energy electrons (from electron shuttles)

Energy is released

What is an enzyme?

Catalytic proton that speed up a reaction w/o being consumed itself

→ Increases reaction rate

→ Does not change the reaction

→ Does not provide energy to the reaction

→ Used in both exergonic and endergonic reactions

→ Activity can be regulated

How do enzymes impact a reaction without changing the overall delta G?

It lowers E_a 

Reaction occurs at safe temp. for cells

Reaction occurs rapidly 

How does an enzyme work?

• Substrate: Reactant that an enzyme acts upon

• Product: Molecules produced at the end of the reaction

What is the active site in an enzyme?

→ Pocket on the surface of the enzyme where the substrate binds and where the reaction happens

→ Enzyme specific active site to its substrate due to of active site

▪ Shape due to the amino acid sequence of the protein

→ Amino acids in the active site specifically interact with the substrate

What is the 1st mechanism that is part of how an enzyme works?

Substrate enters active site

→ Enzyme changes shape to interact with substrate

→ Called induced fit

What is the 2nd mechanism that is part of how an enzyme works?

Substrate is held in place in the active site of the enzyme

→ H-bonds, hydrophobic interactions, charge interactions form between active site amino acids and substrate

What is the 3rd mechanism that is part of how an enzyme works?

Activation energy is lowered by the active site (3 ways this can be done)

a) Enzyme lines up the substrates correctly for new bonds to form

b) Bonds in the substrate are stressed, making them easier to break

c) Creates a charge environment that favours the reaction

What is the 4th mechanism that is part of how an enzyme works?

Substrate is converted into products that are released from active site

→ Products have a different shape than substrates and don’t fit in active site

Cycle repeats

What determines enzyme activity?

Concentration of substrate

→ More substrate = faster reaction rate

→ Rate increases until a active sites in all enzymes are full = maximum rate

What does more enzyme in concentration mean for the reaction?

→ More enzyme = faster reaction rate

As long as there is enough substrate for all enzymes 

What happens with temperature when an enzyme is used in a reaction?

↑ temperature = ↑ enzyme/substrate interactions = ↑ reaction rate

→ Too much heat = enzymes denature = lower reaction rate

→ Optimal temperature produces

fastest rate (refer to graph)

What happens with pH when an enzyme is used in a reaction?

Affects charges on amino acid side chains

→ Changes in pH = changes protein shape and ability of substrate to bind in active site

= decreased reaction rate

→ Optimal pH produces fastest rate

Cofactors and Coenzymes

Cofactors = metal ions or inorganic minerals

Coenzymes = organic molecules (vitamins)

Non-protein helpers that are required for catalytic activity of enzymes 

Enzyme Inhibitors 

Chemicals that selectively decrease activity of an enzyme

May be:

chemical, drugs, poisons

normal molecules in the cell

Two types of inhibitors?

1)  Irreversibile Inhibitor

-Bind to enzyme using covalent bond

-Permanent inactivation

2) Reversible Inhibitor  

-Binds to enzymes using hydrogen bonds

-Temporary inactivation (enzyme will return to normal when inhibitor is removed)

-Two types

A) Competitive Inhibitor 

Competes w/ substrate for binding to active site

Inhibitor block the active site & prevents formation of enzyme-substrate complex = decreased reaction rate

Can be overcome by increasing substrate concentration

B) Non-Competitive Inhibitor

Bind to region of enzyme other than active site

Changes the shape of the enzyme to make it less able to bind substrate

Does NOT compete with substrate for active site

CANNOT be overcome by adding more substrate