Energy Transfer Review
AP Biology
Overview: Ecosystems, Energy, and Matter
An ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they
interact
Ecologists view ecosystems as transformers of energy and processors of matter
Energy flows through ecosystems while matter cycles within them
Trophic Relationships
Energy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) and then to
secondary consumers (carnivores)
Energy flows through an ecosystem, entering as light and exiting as heat
Nutrients cycle within an ecosystem
Atoms from the environment are needed to build new molecules
Photosynthetic organisms capture free energy from sunlight
Chemosynthetic organisms capture free energy from small inorganic molecules and can occur without oxygen
Heterotrophic organisms metabolize carbon compounds produced by other organisms through hydrolysis
reactions as a source of energy
Decomposition
Decomposition connects all trophic levels
Detritivores, mainly bacteria and fungi, recycle essential chemical elements by decomposing organic material and
returning elements to inorganic reservoirs
The student is able to use visual representations to analyze situations or solve problems qualitatively to illustrate
how interactions among living systems and with their environment result in the movement of matter and energy.
In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction
Overall, the coupled reactions are exergonic
Learning Objective 2.1
The student is able to explain how biological systems use free energy based on empirical data that all organisms
require constant energy input to maintain organization, to grow and to reproduce.
Learning Objective 2.2
The student is able to justify a scientific claim that free energy is required for living systems to maintain
organization, to grow or to reproduce, but that multiple strategies exist in different living systems.
Ecosystem Energy Budgets
The extent of photosynthetic production sets the spending limit for an ecosystem’s energy budget
A change in the size of the producer trophic level can affect the number and size of higher trophic levels
Food webs and food chains are dependent on primary productivity
Learning objective 2.3
The student is able to predict how changes in free energy availability affect organisms, populations and
ecosystems.
Learning objective 2.8
The student is able to justify the selection of data regarding the types of molecules that an animal, plant or
bacterium will take up as necessary building blocks and excrete as waste products.
Learning objective 2.9
The student is able to represent graphically or model quantitatively the exchange of molecules between an
organism and its environment, and the subsequent use of these molecules to build new molecules that facilitate
dynamic homeostasis, growth and reproduction.
Overview: The Energy of Life
Metabolism is the totality of an organism’s chemical reactions
Metabolism is an emergent property of life that arises from interactions between molecules within the cell
The cell extracts energy and applies energy to perform work
The First Law of Thermodynamics
According to the first law of thermodynamics, the energy of the universe is constant
Energy can be transferred and transformed
Energy cannot be created or destroyed
The first law is also called the principle of conservation of energy
The Second Law of Thermodynamics
During every energy transfer or transformation, some energy is unusable, often lost as heat
According to the second law of thermodynamics, every energy transfer or transformation increases the entropy
(disorder) of the universe
Living cells unavoidably convert organized forms of energy to heat
For a process to occur without energy input, it must increase the entropy of the universe
Energy input must exceed free energy lost to maintain order and power cellular processes
Biological Order and Disorder
Cells create ordered structures from less ordered materials using a constant supply of free energy into the system
Loss of order or free energy results in death
The evolution of more complex organisms does not violate the second law of thermodynamics
Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases
Increases in entropy are offset by biological processes that increase order
Catabolic pathways release energy by breaking down complex molecules into simpler compounds
An exergonic reaction proceeds with a net release of free energy and is spontaneous
Anabolic pathways consume energy to build complex molecules from simpler ones
An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous
ATP powers cellular work by coupling exergonic reactions to endergonic reactions
A cell does three main kinds of work:
Mechanical
Transport
Chemical
To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an
endergonic one
The Structure and Hydrolysis of ATP
ATP provides energy for cellular functions
The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis
Energy is released from ATP when the terminal phosphate bond is broken
In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction
Overall, the coupled reactions are exergonic
How ATP Performs Work
ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such
as a reactant
The recipient molecule is now phosphorylated
All types of cell work are powered by the hydrolysis of ATP
The Regeneration of ATP
ATP is a renewable resource that is regenerated by addition of a phosphate group to ADP
The energy to phosphorylate ADP comes from catabolic reactions in the cell
The chemical potential energy temporarily stored in ATP drives most cellular work
Energy flows into an ecosystem as sunlight and leaves as heat
Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration
Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work
Chloroplast structure
Double outer membrane
Thylakoid membranes inside containing pigment molecules
Thylakoids are arranged into stacks called grana, where production of ATP and NADPH occurs
Fluid around the membranes is called stroma, where carbon dioxide is converted into carbs
Structure of the chloroplast allows cells to capture the energy of sunlight and convert it to chemical bond energy
Chlorophylls and other pigments embedded in the thylakoid membranes are the key light trapping molecules
Chlorophyll a is the predominant pigment
Tracking Atoms Through Photosynthesis:
Photosynthesis can be summarized as the following equation:
Two sets of reactions
Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
The light reactions (in the thylakoids) split water, release O 2, produce ATP, and form NADPH
The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH
The light reactions convert solar energy to the chemical energy of ATP and NADPH
The thylakoids transform light into the chemical energy of ATP and NADPH which power the production of
carbohydrates
Pigments absorb free energy from light boosting electrons to a higher energy level in Photosystems II and I
These photosystems are embedded in the thylakoids and are connected by the transfer of excited electrons
through an ETC
Noncyclic Electron Flow
Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH
Electron acceptors are photosystem 2 and NADPH
When electrons are passed through the ETC an electrochemical gradient of hydrogen ions (protons) is created
across the thylakoid membrane
Chemiosmosis
The proton gradient allows hydrogen ions to equalize by flowing through ATP synthase
This powers the production of ATP from ADP and inorganic phosphate
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy
Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical
energy of ATP
The spatial organization of chemiosmosis differs in chloroplasts and mitochondria
The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase
ATP and NADPH are produced which powers the production of sugars in the Calvin cycle
The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)
For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO 2
The Calvin cycle has three phases:
Carbon fixation (catalyzed by rubisco)
Reduction
Regeneration of the CO2 acceptor (RuBP)
The Importance of Photosynthesis: A Review
The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules
of cells
In addition to food production, photosynthesis produces the oxygen in our atmosphere
Prokaryotes
Photosynthesis first evolved in prokaryotes and was responsible for an oxygenated atmosphere
Prokaryotic photosynthesis pathways were the foundation for eukaryotic photosynthesis
Chemiosmosis is accompanied by outward movement of proteins across the plasma membrane
Learning objective 2.4
The student is able to use representations to pose scientific questions about what mechanisms and structural
features allow organisms to capture , store or use free energy.
Learning objective 2.5
The student is able to construct explanations of the mechanisms and structural features of cells that allow
organisms to capture, store or use free energy.
Catabolic Pathways and Production of ATP
The breakdown of organic molecules is exergonic
Fermentation is a partial degradation of sugars that occurs without oxygen
Cellular respiration is a series of reactions that consumes oxygen and organic molecules and yields ATP
Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with
the sugar glucose:
C6H12O6 + 6O2  6CO2 + 6H2O + Energy (ATP + heat)
The Stages of Cellular Respiration
Cellular respiration has three stages:
Glycolysis (breaks down glucose into two molecules of pyruvate)
The citric acid cycle (completes the breakdown of glucose)
Oxidative phosphorylation (accounts for most of the ATP synthesis)
The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox
reactions
Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration
A small amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation
Glycolysis harvests energy by oxidizing glucose to pyruvate
Glycolysis (“splitting of sugar”) rearranges the bonds of glucose into two molecules of pyruvate, forming 2 ATP, 2
NADH and 2 pyruvate molecules
Glycolysis occurs in the cytoplasm and has two major phases:
Energy investment phase
Energy payoff phase
The citric acid cycle completes the energy-yielding oxidation of organic molecules
Before the citric acid cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis
1 CO2 is released
An acetyl group and 1 NADH are produced
The acetyl group is attached to coenzyme A for transport into the Krebs cycle
NADH goes to the electron transport chain
The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix
The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH 2 and 2 CO2 per
turn
ATP is produced by substrate-level phosphorylation
Electrons are stored in NADH and FADH2 which go to the ETC to powers ATP synthesis via oxidative
phosphorylation
Mitochondrion structure
Has a double membrane which allows compartmentalization within the mitochondria
Outer membrane is smooth but inner membrane is highly folded into cristae
Cristae increases the surface area for ATP production
Cristae contain the ATP synthase enzyme
The Pathway of Electron Transport
The electron transport chain is in the cristae of the mitochondrion
Most of the chain’s components are proteins, which exist in multi-protein complexes
The carriers alternate reduced and oxidized states as they accept and donate electrons
Electrons drop in free energy as they go down the chain and are finally passed to the electron acceptor, O2,
forming water
The electron transport chain generates no ATP
The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in
manageable amounts
Chemiosmosis: The Energy-Coupling Mechanism
Electron transfer in the electron transport chain causes proteins to pump H + from the mitochondrial matrix to the
intermembrane space
H+ then moves back across the membrane, passing through channels in ATP synthase
ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP
This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work
The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain
to ATP synthesis
The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work
An Accounting of ATP Production by Cellular Respiration
During cellular respiration, most energy flows in this sequence:
glucose NADH electron transport chain proton-motive force ATP
About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38
ATP
Fermentation enables some cells to produce ATP without the use of oxygen
Cellular respiration requires O2 to produce ATP
Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)
In the absence of O2, glycolysis couples with fermentation to produce ATP
Types of Fermentation
Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis
Two common types produce the organic molecules ethanol and lactic acid
Learning objective 2.4
The student is able to use representations to pose scientific questions about what mechanisms and structural
features allow organisms to capture, store or use free energy.
Learning objective 2.5
The student is able to construct explanations of the mechanisms and structural features of cells that allow
organisms to capture, store or use free energy.