ENERGY AND THE CELL. 5.10 Cells transform energy as they perform work Cells are small units, a...
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Transcript of ENERGY AND THE CELL. 5.10 Cells transform energy as they perform work Cells are small units, a...
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ENERGY AND THE CELL
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5.10 Cells transform energy as they perform work
Cells are small units, a chemical factory, housing thousands of chemical reactions
– Some chemical reactions “build” molecules: anabolic reactions
– Some chemical reactions “break down” molecules: catabolic reactions
– The entire sum of all chemical reactions: cell metabolism
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5.10 Cells transform energy as they perform work
Energy is the capacity to do work and rearrange matter (i.e. chemical bonds)
– There are 2 kinds of energy
– Kinetic energy is the energy of motion
– Potential energy is energy that an object possesses as a result of its location
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5.10 Cells transform energy as they perform work
Kinetic energy performs work by transferring motion to other matter
– Heat, or thermal energy, is kinetic energy associated with the random movement of atoms
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5.10 Cells transform energy as they perform work
Potential energy is stored energy that an object possesses because of its location or structure
– Chemical energy is potential energy because of its energy available for release in a chemical reaction. It is stored in covalent bonds.
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5.10 Cells transform energy as they perform work
When chemical bonds are broken, potential energy can be converted to kinetic energy that is available for work.
– We will revisit this idea when we get to ATP (adenosine triphosphate) as an energy molecule.
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5.11 Two laws govern energy transformations
Thermodynamics: The study of energy transformations (changes)
– System and Surroundings
– Living organisms exchange both energy and matter with its surroundings. Living organisms are “open systems.”
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5.11 Two laws govern energy transformations
It is important to understand two laws that govern energy transformations in organisms
– First law of thermodynamics: Energy in the universe is constant, neither created nor destroyed.
– Second law of thermodynamics: Energy conversions increase entropy.
– Entropy is the measure of disorder, or randomness
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Entropy
Decrease in Entropy
Monosaccharides bonded together make a polysaccharide.
Amino acids joined by peptide bonds to make a protein.
Increase in Entropy
Triglycerides hydrolyzed into glycerols and fatty acids
DNA broken down into nucleotides.
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Fuel
Gasoline
Waste productsEnergy conversion
Combustion
Energy conversion in a car
Oxygen Water
Carbon dioxide
Kinetic energyof movement
Heatenergy
75%
25%
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Energy conversion in a cell
Energy for cellular work
Cellular respiration
Heat
Glucose
Oxygen Water
Carbon dioxide
Fuel Energy conversion Waste products
40%
60%
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5.12 Chemical reactions either release or store energy
An exergonic reaction is a chemical reaction that releases energy (has a negative G)
G
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Exergonic Reactions
Activation Energy (Ea)
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5.12 Chemical reactions either release or store energy
An endergonic reaction requires an input of energy and yields products rich in potential energy
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Endergonic Reactions
Photosynthesis
6 CO2 + 6 H2O C6H12O6 + 6 O2
Activation Energy (Ea)
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5.12 Chemical reactions either release or store energy
A living organism produces thousands of endergonic and exergonic chemical reactions
A cell does 3 main types of cellular work
– Chemical work—driving endergonic reactions
– Transport work—pumping substances across membranes
– Mechanical work—beating of cilia, muscle contraction
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5.12 Chemical reactions either release or store energy
To accomplish work and get things done, a cell must use “energy coupling.”
Energy coupling is using an exergonic (energy releasing) reaction to drive an endergonic (energy requiring) reaction.
ATP molecules are the key to energy coupling.
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ATP, adenosine triphosphate, is the energy currency of cells.
– ATP is the immediate source of energy that powers most forms of cellular work.
– It is composed of adenine (a nitrogenous base), ribose (a five-carbon sugar), and 3 phosphate groups.
5.13 ATP shuttles chemical energy and drives cellular work
High-energy bonds
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5.13 ATP shuttles chemical energy and drives cellular work
Hydrolysis of ATP releases energy by transferring its 3rd phosphate from ATP to some other molecule
– The transfer is called phosphorylation
– In the process, ATP energizes molecules
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Ribose
Adenine
Triphosphate (ATP)Adenosine
Phosphategroup
Hydrolysis
Diphosphate (ADP)Adenosine
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Chemical work
Solute transportedMolecule formed
Product
Reactants
Motorprotein
Membraneprotein
Solute
Transport workMechanical work
Protein moved
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5.13 ATP shuttles chemical energy and drives cellular work
ATP is a renewable source of energy for the cell
– When energy is released in an exergonic reaction, such as breakdown of glucose, the energy is used in an endergonic reaction to generate ATP
– C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP)
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HOW ENZYMES FUNCTION
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5.14 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
Although there is a lot of potential energy in biological molecules, such as carbohydrates and others, it is not released spontaneously
– “Start up” energy must be available to break bonds and form new ones
– This energy is called energy of activation (EA)
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5.14 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
The cell uses catalysis to drive (speed up) biological reactions
– Catalysis is accomplished by enzymes, which are proteins that function as biological catalysts
– Enzymes speed up the rate of the reaction by lowering the EA , and they are not used up in the process
– Each enzyme has a particular target molecule called the substrate
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Reactionwithoutenzyme
EA with enzyme
En
erg
y Reactants
Reaction withenzyme
EA withoutenzyme
Netchangein energy(the same)
Products
Progress of the reaction
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Enzymes …
NEVER change the endergonic or exergonic nature of a reaction
NEVER change the G (free energy) of a reaction
ONLY change the activation energy (Ea)
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5.15 A specific enzyme catalyzes each cellular reaction
Enzymes have unique three-dimensional shapes
– The shape is critical to their role as biological catalysts
– As a result of its shape, the enzyme has an active site where the enzyme interacts with the enzyme’s substrate
– Consequently, the substrate’s chemistry is altered to form the product of the enzyme reaction
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Enzyme availablewith empty activesite
Active site
1
Enzyme(sucrase)
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Enzyme availablewith empty activesite
Active site
1
Enzyme(sucrase)
Substrate bindsto enzyme withinduced fit
2
Substrate(sucrose)
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Enzyme availablewith empty activesite
Active site
1
Enzyme(sucrase)
Substrate bindsto enzyme withinduced fit
2
Substrate(sucrose)
Substrate isconverted toproducts
3
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Enzyme availablewith empty activesite
Active site
1
Enzyme(sucrase)
Substrate bindsto enzyme withinduced fit
2
Substrate(sucrose)
Substrate isconverted toproducts
3Products arereleased
4
Fructose
Glucose
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5.15 A specific enzyme catalyzes each cellular reaction
For optimum activity, enzymes require certain environmental conditions
– Temperature is very important, and optimally, human enzymes function best at 37ºC, or body temperature
– High temperature will denature human enzymes
– Enzymes also require a pH around neutral for best results
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Rat
e o
f re
acti
on
pH109876543210
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5.16 Enzyme inhibitors block enzyme action and can regulate enzyme activity in a cell
Inhibitors are chemicals that inhibit an enzyme’s activity
– One group inhibits because they compete for the enzyme’s active site and thus block substrates from entering the active site
– These are called competitive inhibitors
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Substrate
Enzyme
Active site
Normal binding of substrate
Competitiveinhibitor
Enzyme inhibition
Noncompetitiveinhibitor
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5.16 Enzyme inhibitors block enzyme action and can regulate enzyme activity in a cell
Other inhibitors do not act directly with the active site
– These bind somewhere else and change the shape of the enzyme so that the substrate will no longer fit the active site
– These are called noncompetitive inhibitors
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5.16 Enzyme inhibitors block enzyme action and can regulate enzyme activity in a cell
Enzyme inhibitors are important in regulating cell metabolism
– Often the product of a metabolic pathway can serve as an inhibitor of one enzyme in the pathway, a mechanism called feedback inhibition
– The more product formed, the greater the inhibition, and in this way, regulation of the pathway is accomplished
– Biochemical Pathway
– Feedback Inhibition of Biochemical Pathways
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5.15 A specific enzyme catalyzes each cellular reaction
Some enzymes require nonprotein helpers
– Cofactors are inorganic, such as zinc, iron, or copper
– Coenzymes are organic molecules and are often vitamins (Ex. Vitamin B6, FAD, NADH, CoQ)