How Cells Work

Chapter 5

Energy Laws

• Energy is the capacity to do work

• The total amount of energy in the universe is constant (1st law)

• Energy is flowing from high-energy forms to forms lower in energy. This is called ENTROPY

ENERGY• Energy is the capacity to do work

• Energy exists in multiple forms– Light– Heat– Electricity– Chemical bond energy– Etc.

• These various types of energy can be placed into two groups– Kinetic energy– Potential energy

KINETIC ENERGY• “Energy of motion”

• Anything that moves possesses kinetic energy– e.g., Heat, light, balls on a pool table,

flowing water, flowing electrons, etc.

POTENTIAL ENERGY• “Energy of location or structure”

• “Stored energy”

• Resting objects may still possess energy– e.g., A rock at the top of a hill, chemical

bond energy

Adenosine triphosphate - ATP

• Main energy carrier in cells

• Can give up phosphate group to another molecule

• Phosphorylation primes a molecule to react

• “Currency” our cells use

We take in large energy sources

glucose, starch, lipids, etc.

We break these down, take the energy that was stored in bonds, and store the energy as ATP

Adenosine triphosphate - ATP

Not the only waywe store energy

• ATP couples energy

inputs and outputs

• ATP/ADP cycle

regenerates ATP

ATP ATP ADP releases energy ADP releases energyADP ADP ATP requires energy ATP requires energy

The Cell’s Energy Currency• ATP couples energy inputs and outputs

• ATP/ADP cycle regenerates ATP

Regeneration of ATP happens quickly10 million/sec/cell in active muscle

ATP

The “high energy bond” is not so high in, energy, but very unstable.

Energy Changes

• Endergonic reactions require energy– Synthesis of glucose from carbon dioxide and

water during photosynthesis

• Exergonic reactions release energy– Breakdown of glucose to carbon dioxide and water

by aerobic respiration

Energy + 6H2O + 6CO2 C6H12O6+ 6O2

C6H12O6+ 6O2 Energy + 6H2O + 6CO2

Reactions

Electron Transfers

• Extracting energy in small pieces/less waste

• Oxidation: loss of an electron

• Reduction: gain of an electron

• Electron transfer chains are vital to the formation

of ATP during photosynthesis and aerobic

respiration

Vs.

If the reactionslowed, we canextract energyin more places

All energy lost at once

Participants in Metabolic Pathways

• Reactants – starting substances (also called

substrate)

• Intermediates – substances formed during the

reaction

• Products – what remains at the end of the

reaction

C6H12O6+ 6O2 Energy + 6H2O + 6CO2

Participants in Metabolic Pathways

• Energy carriers – provide energy to activate

enzymes

• Enzymes – speed reactions

• Cofactors – assist enzymes with reactions

• Transport proteins – help substances across cell

membranes

Metabolic Pathways• Biosynthetic (anabolic) pathways

– Require energy inputs– Assemble large molecules from

subunits– Photosynthesis

• Degradative (catabolic) pathways– Release energy– Breakdown large molecules to

subunits– Aerobic respiration

Enzymes

• Catalyze (speed up) reactions

• Recognize and bind specific substrates

• Act repeatedly – emerge unchanged

• Most are proteins

Activation Energy

• Minimum amount of energy required to get a reaction started

• For a reaction to occur, an energy barrier must be surmounted

• Enzymes make the energy barrier smaller

What are some reasons why it is a good thingthat enzymes lower the activation energy?

Why is it a good thing that enzymes usually onlybind to one type of substrate (reactant)?

How do enzymes lower activation energy?

Tough question, but they put the reactantsin an environment more favorable for a reaction.

increases concentration of substrate

reorients

excludes water

ENZYME EXAMPLE

Factors Influencing Enzyme Activity

Coenzymes and cofactors

Competitive and noncompetitive inhibitors

Allosteric regulators

Temperature

pH

Salt concentration

Coenzymes and cofactors

Cofactor – inorganic helpers that bind to theactive site or substrate that speed reactions

Coenzyme – organic helpers that bind to the active site or substrate that speed reactions

• Many enzymes require non-protein helpers for catalytic activity

• e.g., DNAse requires Mg2+ as a cofactor– Removal of Mg2+ inactivates the enzyme

Competitive and noncomp. Inhibitor

Binds to active siteand clogs

Binds somewhereelse and changesshape

Bio-warfare, toxins

Allosteric site

Allosteric or noncompetitive control

• Activator or inhibitor binds to an enzyme NOT in

the active site, like non-competitive inhib.

• Binding changes enzyme shape

• Change hides or exposes active site

• Your body does this on purpose

• Feedback inhibition– Product of pathway binds to and inhibits enzyme in the

pathway

Allosteric Control

inhibition

activation

Figure 4.8

Effect of Temperature

• Small increase in temperature increases molecular collisions, reaction rates

• High temperatures disrupt bonds and destroy the shape of active site

pH shifts and salts also denature proteins

Concentration Gradient

• Means the number of molecules or ions in one region is different than the number in another region

• In the absence of other forces, a substance moves from a region where it is more concentrated to one where it is less concentrated: “down” gradient

TRANSPORT

Diffusion• The net movement of like

molecules or ions down a concentration gradient

• Although molecules collide randomly, the net movement is away from the place with the most collisions (down gradient)

• e.g. perfume open in a room

Factors Affecting Diffusion Rate

• Steepness of concentration gradient– Steeper gradient, faster diffusion

• Molecular size– Smaller molecules, faster diffusion

• Temperature– Higher temperature, faster diffusion

• Electrical or pressure gradients

• Span the lipid bilayer

• Interior is able to open to both sides

• Change shape when they interact with solute, only let one type through

• Move water-soluble substances across a membrane

Transport Proteins

Passive and Active Transport

• Doesn’t require energy inputs

• Solutes diffuse through a channel inside the protein’s interior, or through cell membrane

• Net movement is down concentration gradient

Passive Transport Active Transport

• Requires ATP

• Protein is an ATPase pump

• Pumps solute against its concentration gradient

Active Transport of Na and K

Osmosis• Diffusion of water across a selectively permeable membrane

• Hypotonic – solution with a lower concentration of solute

• Hypertonic – solution with a higher concentration of solute

• Water always moves from a hypotonic solution to a

hypertonic solution

Osmosis

Hydrostatic Pressure

• Pressure that a fluid exerts against structure

enclosing it

• Increases with increased solute concentration

• Influences the osmotic movement of water

Think of a water balloon

Membrane Traffic

• Endocytosis– Membrane sinks inward around a

substance bringing it into the cell in a vesicle

• Exocytosis– Vesicle carrying substance fuses with

membrane releasing it into theintracellular fluid

Types of Endocytosis

• Pinocytosis – “drinking” fluids

• Phagocytosis – “eating” particles

• Receptor-mediated endocytosis - specific

Pinocytosis Phagocytosis

Receptor mediated