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ENERGY
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All living things require energy to survive. (remember, work requires energy)
Whether they are unicellular or multicellular, all organisms need energy to power the many
endothermic reactions that are essential to cell function.
Organisms that can make their own food are classified as AUTOTROPHS
Organisms that must consume other organisms to obtain their energy are classified as
HETEROTROPHS.
Heterotrophs can further be classified as
HERBIVORES - eat only plants
CARNIVORES - eat only animals
OMNIVORES - eat both plants and animals
SCAVENGERS - eat dead animals (a type of carnivore)
DECOMPOSERS - break down dead plants and animals
The major way that autotrophs produce chemical energy is known as PHOTOSYNTHESIS.
It is carried out in pants and many microorganisms.
Glucose
Radiant
Energy
Sun
PHOTOSYNTHESIS
(stored
chemical
energy)
CELLULAR
RESPIRATION
ATP
(usable
chemical
energy)
Cell
activities
Photosynthesis
6 CO2 + 6 H2O + light energy (sun)  C6H12O6 + 6 O2
The process by which glucose (and other food) molecules are broken down and their stored energy
released (i.e. breaking down food to yield energy) is known as CELLULAR RESPIRATION.
-
you (humans) get the energy out of food by using the oxygen you breathe to break down food
molecules you eat
in this way your body “liberates” (frees) the energy stored in the food
it is essentially the reverse of photosynthesis and it occurs within the cells of the body
all living things use cellular respiration to get energy from food molecules (even
organisms that make their own food through photosynthesis use cellular respiration to obtain
energy from the food)
Cellular Respiration
C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy (ATP)
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The ultimate source of energy (for almost all living things) is SUNLIGHT.
The Carbon Cycle
ADENOSINE TRIPHOSPHATE
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Both photosynthesis and respiration are processes that involve a series of biochemical reactions.
Such a series of reactions is called a biochemical pathway.
Usable energy produced by one reaction may be stored and used in a later reaction. In most
cases this usable energy is stored in a molecule called adenosine triphosphate, or ATP.
The ATP molecule has 3 parts:
1. ADENINE = a nitrogen-containing molecule
2. RIBOSE = a five-carbon sugar
3. 3 PHOSPHATE GROUPS
- the adenine bonds to ribose, forming the compound adenosine
- the phosphate groups attach in sequence to the adenosine
- Adenosine monophosphate, or AMP, has one attached phosphate group
- Adenosine diphosphate, or ADP, has two attached phosphate groups
- Adenosine triphosphate, or ATP, has three attached phosphate groups
ATP–ADP Cycle
A
P
P
Energy
P
Energy
+
(from catabolism)
+
P
P
A
P
P
Photosynthesis, respiration, the formation of ATP, and the breakdown of ATP form the
fundamental biological cycle.
- Photosynthesis stores energy in glucose molecules
- Respiration releases that energy
- The energy freed forms ATP as another storage molecule
- Energy from the breakdown of ATP fuels cell activity
PHOTOSYNTHESIS
6CO2 + 6H2O + light energy  C6H12O6 + 6O2
LIGHT-DEPENDENT REACTIONS = light energy is converted into chemical energy
- The energy in sunlight is trapped, O2 is released, and both ATP and the hydrogencarrier molecule, which is written as NADPH + H+, are formed.
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The end products of the light reactions are NADPH + H+, O2, and ATP
O2 is released into the atmosphere
NADPH + H+ and ATP are released into the stroma of chloroplasts and are used to drive
the dark reactions of photosynthesis
LIGHT-INDEPENDENT REACTIONS (formerly known as the Dark Reactions)
= molecules of ATP, produced in the light reactions, are used to produce glucose
- The ATP, NADPH + H+ act with CO2 from the atmosphere and form GLUCOSE.
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Also known as the CALVIN CYCLE
The overall process results in the transformation of light energy from the sun into energy
stored in the bonds of the glucose molecule.
Both of these photosynthetic reactions take place in the chloroplasts.
CHLOROPLASTS = membrane-bound organelles that contain both the pigment chlorophyll
and the enzymes necessary for photosynthesis
 They have a double membrane (an outer and inner membrane)
 The interior membrane is organized into flattened sacs of photosynthetic membranes
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called THYLAKOIDS
The thylakoid membrane has an internal reservoir called a lumen
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Stacks of thylakoids are called GRANA (sing. granum; pl. grana)
Each granum may contain up to several dozen thylakoids
The thylakoids are embedded in a protein-rich solution that is called the STROMA
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The light-dependent reactions take place in the thylakoid membranes
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The light-independent reactions take place in the stroma
CHLOROPHYLL = Light absorbing pigment found in the thylakoid membranes
of the chloroplasts that “trap” the energy in sunlight.
CELLULAR RESPIRATION = The process by which mitochondria break down food molecules to
molecules to produce ATP.
 There are two main types of respiration that exist in living organisms:
1. FERMENTATION (or anaerobic respiration) = The breakdown of pyruvic acid
without the aid of oxygen
2. AEROBIC RESPIRATION = The metabolism of pyruvic acid with the aid of oxygen
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Both types of respiration begin with glycolysis
GLYCOLYSIS = A process by which one glucose molecule (a 6-carbon compound) is broken down
into two pyruvic acid molecules (a 3-carbon compound)
 It occurs in the cytoplasm of a cell
 It does not require oxygen
 2 molecules of ATP are used to start glycolysis and only 4 molecules of ATP are produced
(therefore, there is a net gain of 2 ATP in the process)
 also makes 4 NADH molecules
 After glycolysis, there are 2 pathways for producing ATP; either fermentation, which also occurs in
the cytoplasm of the cell, or aerobic respiration, which occurs in mitochondria.
 The metabolism of pyruvic acid during fermentation does not actually produce ATP. The actual
function of fermentation is to break down pyruvic acid and regenerate NAD+ for reuse in glycolysis,
where ATP is formed.
ANAEROBIC RESPIRATION (AKA fermentation)
 There are two forms of fermentation:
1. LACTIC ACID Fermentation = The process by which pyruvic acid is converted into
lactic acid
Pyruvic acid + NADH + H+ —→ lactic acid + NAD+
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Occurs in animal cells and in some unicellular organisms when O2 is in short supply
Lactic acid makes muscles feel tired and sore
2. ALCOHOL Fermentation = The process by which pyruvic acid is converted to ethyl alcohol
Pyruvic acid + NADH + H+ —→ ethyl alcohol + CO2 + NAD+
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Occurs in some plant cells and some unicellular organisms, such as yeasts.
Just as lactic acid accumulates during lactic acid fermentation, ethyl alcohol
accumulates during alcoholic fermentation.
The alcohol in wine and beer is produced by fermentation carried out by some
microorganisms.
 Overall net yield of ATP from anaerobic respiration = 2 ATP
AEROBIC RESPIRATION
 Most organisms carry on aerobic respiration, which releases a great deal more energy from a glucose
molecule than anaerobic respiration does.
 Aerobic respiration occurs in mitochondrion.
Steps of aerobic respiration:
 In the 1st step, pyruvic acid is converted to a molecule called acetyl-CoA.
 Acetyl-CoA then enters a biochemical pathway called the Krebs cycle, which is also known as the
citric acid cycle.
 For each molecule of glucose, two molecules of acetyl-CoA enter the Krebs cycle
THE KREBS CYCLE = The central biochemical pathway of aerobic respiration.
(Because citric acid is formed in the process, it is also known as the
citric acid cycle.)
 For each molecule of acetyl-CoA the cycle produces 3 molecules of NADH + H+, one
molecule of FADH2, and 1 molecule of pyruvic acid, each of which in turn becomes a
molecule of acetyl-CoA.
 The breakdown of 1 molecule of glucose via the Krebs cycle produces
6 NADH , 2 FADH2 , and 2 ATP molecules.
 The above products (other than ATP) then go through an electron transport chain to yield ATP.
 Each molecule of NADH yields 3 molecules of ATP, from the electron transport chain.
 Each molecule of FADH2 yields 2 molecules of ATP, from the electron transport chain.
Total Energy Yield (from aerobic respiration)
Glycolysis = 2 ATP
4 NADH
2 PYRUVIC ACIDS
Krebs Cycle = 2 ATP
6 NADH
2 FADH2
ETC =
34 ATP
(4 NADH from glycolysis = 12 ATP)
(6 NADH from Krebs cycle = 18 ATP)
(2 FADH2 from Krebs cycle = 4 ATP)
Total =
2 ATP from glycolysis
2 ATP from Krebs
34 ATP from E.T.C.
Thus, aerobic respiration produces a maximum of
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38 ATP
molecules
Aerobic respiration is generally 19 times more efficient than anaerobic respiration.
The ATP produced represents only about ½ the total energy stored in a molecule of glucose.
(The remaining energy is unavailable for use by organisms.)