Introduction to Aerobic Respiration and its link to Photosynthesis
We know by now that life requires a concstan input of energy. In almost all ecosystems, that energy
comes from the sun. The figure to the right illustrates how the combination of photosynthesis and
cellular respiration provide energy for living organisms.
In photosynthesis, which takes place in a plant
cell’s chloroplasts, the energy from sunlight is
used to rearrange the atoms of carbon dioxide
(CO2) and water (H2O) to produce sugars and
oxygen.
In cellular respiration, O2 is consumed as sugar
is broken down to CO2 and H2O; the cell
captures the energy released from sugar in ATP
molecules. When O2 is present, pyruvate
continues to be broken down in in the
mitochondria of almost all eukaryotic cells
(plants, animals, fungi, protists).
This figure also shows that as energy is
converted, some energy is lost as heat. For the
most part, life on earth is solar powered, and
energy makes a ONE-WAY trip through the
ecosystem.
The atoms (matter), however, are recycled. The CO2 and H2O released by cellular respiration are
converted through photosynthesis to sugar and O2, which are then used in respiration.
Breathing (Respiratory System) and Circulation Supply O2 for Cellular Respiration and Remove CO2
Respiration is often used as a synonym for
“breathing”. It is not incorrect as “spirare”is
Latin for “to breathe”. When used as a synonym,
it basically refers to an exchange of gases. An
organism obtains O2 from its environment and
released CO2 as a waste product.
Biologists also define respiration as the aerobic
(oxygen-requiring) harvesting of energy cells
from food molecules by cells. . This process is
called cellular respiration (and we will call it
aerobic cellular respiration) to distinguish it from
breathing.
Breathing and cellular respiration are closely
related. As a runner breathes in air, lungs take up O2 and pass it to the bloodstream. The bloodstream
carries the O2 to the muscle cells (and really to all cells). Mitochondria in the cells use the O2 in cellular
1
respiration to harvest energy from glucose and other organic molecules and generate ATP. Muscle cells
use ATP to fuel contractions, nerve cells use ATP for active transport, pancrease cells use ATP when
producing insulin. The blood stream and lungs also perform the vital function of disposing of the CO2
waste which is produced in cellular respiration. So the “standard equation” that we see, refers to
cellular respiration in the presence of O2.
The Process of Cellular Respiration Banks Energy in ATP molecule
So, you breathe air and eat food to supply your cells with the reactants for cellular respiration (when we
use this term, unless anaerobic is used with it, the assumption is that oxygen is present).
So, our equation is Glucose + 6O2  6CO2 + 6H2O
ATP +Heat
The simple sugar glucose is the fuel the cells use most often, although other organic molecules can also
be “burned” or broken down. The equation tells us that when glucose is utilized for energy (remember,
it can be stored or used to build other things) all of the atoms from the glucose are converted to waste
products (CO2 and H2O and ONLY the energy from the bonds is used to form ATP from ADP + Pi. This
means that the ADP and Pi molecules are present in the cell. The multiple arrows in the equation
indicate there are many steps in cellular respiration. Molecules for each glucose molecule, which means
about 34% of the energy originally stored in glucose is captured. The rest (66%) is released as heat.
While it may seem “inefficient”, it compares well with the energy conversion of most systems (like cars
and motors). Heat is not only given off as sweat, it also helps you to maintain your body temperature.
The Human Body Uses Energy from ATP for all its activities
Your body requires a continuous supply of
energy just to stay alive – to keep your heart
pumping and to keep you breathing.
Your brain especially requires a HUGE amount
of energy; its cells burn about 120 g (about ¼
lb of glucose) each day, and accounts for about
15% of total oxygen consumption.
Maintaining brain cells and other lifesustaining activities uses as much as 75% of
the energy that a person takes in as food
during a typical day.
Above and beyond the energy that you need
for body maintenance, cellular respiration
provides energy for voluntary activities (see the chart above). The energy units state calories, but as we
discussed during our nutrition section, these calories are actually capital “C” Calories or kcals. The
average adult needs to take in food that provides about 2200 calories per day (this does go down as you
get older).
2
So…lets look at how your cells release the energy stored in food molecules (such as glucose) to produce
the ATP required to power the work of your cells (and therefore the activities of your body).
Cells Capture Energy Due to the Movement of Electrons and Hydrogen Ions!
Before discussing the steps in aerobic respiration, it is important to understand how the energy is
extracted from glucose. To understand this we need to spend a few paragraphs discussing redox
reactions, electron carriers, and the electron transport chains.
REDOX REACTIONS
During cellular respiration, electrons are transferred from glucose to oxygen, releasing energy. Oxygen
attracts electrons very strongly, and an electron loses energy as it moves through many reactions and is
ultimately transferred to oxygen.
By using many steps to transfer energy, energy can be released in small amounts, which is why ATP is
created in various steps throughout aerobic respiration.
The movement of electrons from one molecule to the next is an oxidation- reduction or REDOX
reaction. In a redox reaction, the loss of electrons from one substance is called oxidation and the
addition of electrons to another substance is called reduction. A molecue is said to become oxidized
when it loses one or more electrons and reduced when it gains one or more electrons. Because an
electron transfer requires both a donor AND an acceptor, oxidation and reduction always occur
together.
When we look at the cellular respiration equation, you can’t really see the electron transfers. What you
do see are the changes in the location of the hydrogen atoms. The movement of the hydrogen ions
represent electron transfers because each hydrogen atom consists of an electron (e-) and a proton (H+).
Glucose loses hydrogen atoms (electrons) as it becomes oxidized to CO2. Simultaneously, O2 gains
hydrogen atoms (electrons) as it becomes reduced to H2O. As they pass from glucose to oxygen, the
electrons lose some of its energy, with each transfer.
3
WHAT ARE ELECTRON CARRIERS?
An important player in the process of oxidizing glucose is an electron acceptor (which is really a
coenzyme) called NAD+, which accepts electrons and becomes reduced to NADH. NAD (nicotinamide
adenine dinucleotide) is an organic
molecule that cells make from the vitamin
niacin. NAD is used to shuttle (transport)
electrons in redox reactions.
STEP 1. Glycolysis
NADH is produced in glycolysis (2 NADH)
when glucose is broken down two
pyruvates Two ATP (net) are also formed.
STEP 2a. Prep Step (Grooming step)
When oxygen is present, the pyruvates are
oxidized (CO2 is formed), hydrogens and
electrons are transferred to NAD (forming NADH) and Acetyl CoA is formed. For each pyruvate, one
NADH and 1 CO2 is produced.
STEP 2b.The Citric Acid Cycle (Krebs Cycle) completes the oxidation of organic molecules
Acetyl CoA enters the Krebs cycle and is further oxidized. In
the process, 3 more NADH, and 1 FADH2 are formed per
pyruvate, as well as 1 ATP.
We can see that the Krebs cycle makes a small amount of ATP,
but for both glycolysis and the Krebs cycle, the amount of ATP
that will be produced from the electron carriers will be much
greater than the ATP produced in each of these steps. ATP
produced during glycolysis and the Krebs cycle are produced
through substrate phosphorylation. An enzyme transfers a
phosphate group directly from a substrate molecule to ADP in
order to form ATP.
Compared with glycolysis, the Krebs cycle pays big energy
dividends to the cell. Adding all the energy molecules from each step we arrive at the following:
So lets take a moment to sum up how many energy molecules we have.
In Glycolysis, put in 2 ATP
In Glycolysis
Prep Step (2 Pyruvate 2 Acetyl CoA +2 CO2)
Krebs Cycle ( 2 Acetyl CoA  4 CO2)
TOTAL
-2 ATP
+4 ATP
+2 ATP
4 ATP
+2 NADH
+ 2 NADH
+ 6 NADH
10 NADH
+ 2 FADH
2 FADH
SO… how do we extract energy from NADH and FADH2 and put it into about 32 ATP molecules?
4
STEP 3. Oxidative Phosphorylation
Oxidative phosphorylation involves electron transport and a
process known as chemiosmosis. NADH and a related
electron carrier, FADH2 (Flavin adenine dinucleotide) shuttle
electrons to an electron transport chain (ETC) embedded in
the inner mitochondrial membrane. On this membrane,
energy is released by the downhill fall of electrons (which are
donated by NADH and FADH2).
This energy pumps hydrogen ions across the inner membrane
of the mitochondria into the inter-membrane space. This
concentrates H+ on one side of the membrane. Hydrogens
that are not pumped into the intemembrane space combine
with “dead electrons (at the end of the ETC) to form water
(H2O). As H+ accumulate in the inter-membrane space a
gradient is set up.
This concentration gradient across the membrane stores potential energy, much the same way a dam
stores energy, by holding back the elevated water behind the dam. The energy stored by a dam can be
harnessed to do work (such as generating electricity). There is one type of protein on the membrane
(ATP synthase) which permits the hydrogen ions to pass back into the matrix (inner space of the
mitochondria) via facilitated diffusion. The ATP synthase acts like a miniature turbine, and as the H+
ions flow back into the matrix, the ATP synthase spins, which in turn activates the catalytic sites (“active
sites) that allow the phosphorylation of ADP to ATP.
So the 10 NADH and
2 FADH that are
produced from one
glucose molecule
during glycolysis and
the Krebs cycle end
up producing 32-34
ATP in step 3
(Oxidative
phosphorylation).
The amount of ATP
produced in this step
depends on the
organism.
If we add this amount of ATP to the net amount produced during glycolysis (2 ATP) and the 2 ATP
produced in the Krebs cycle, our total net number of ATP produced are 36-38 ATP
5
QUESTIONS:
1. What is misleading about the following statement:
Plant cells perform photosynthesis and animal cells perform cellular respiration
2. How is your breathing related to your cellular respiration?
3. Why are sweating and other body-cooling mechanisms necessary during vigorous exercise?
4. SHOW WORK FOR THE FOLLOWING CALCULATIONS: Consider your gender. If you eat a slice of
pizza (475 Calories) ,
a. how long would you need to “walk briskly” to “burn off” the calories you have just
eaten? ?
b. How many kilometers would you have walked?
c.
If you ran instead, how many kilometers would you need to run? How much time?
6
5. Of the three main stages of cellular respiration, which is the only one that uses oxygen?
6. For each glucose molecule processes, what are the net molecular products of glycolysis?
7. The step that converts pyruvate to acetyl CoA is often included with the Krebs cycle. If
we do this, what are the net molecular products of the Krebs cycle (be sure to include
ALL molecules that are produced. Place these in the correct location on the diagram
below.
Which carry energy?
Which are electron carriers?
Which are considered waste products
8. Use the diagram of the mitochondria below to label the three major steps of cellular respiration
AND THEIR LOCATIONS.
9. Look at the diagram on page 4 ( the one beside the electron carriers paragraph). Which
molecule is being reduced? How do you know?
10. What effect would the absence of O2 have on oxidative phosphorylation? (bottom of page 5).
7