Welcome to topic 3.7, Cellular Respiration.
Cellular respiration is one of the most fundamental aspects of the metabolism of living things.
In essence, cellular respiration is how organisms obtain energy from glucose in order to perform
basic life functions, such as movement.
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From our study of eukaryotic cells (both plant and animal), you remember that they contained
an organelle responsible for the production of energy called the mitochondria. You also
remember that eukaryotic plant cells have chloroplasts, which are in turn responsible
for photosynthesis. The two processes are complementary: while photosynthesis uses energy
from the sun, water and carbon dioxide to produce glucose and oxygen, cellular respiration
does just the opposite, using oxygen and glucose to produce energy in the form of ATP, carbon
dioxide and water.
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Cellular respiration is the controlled release of energy from organic compounds in cells
to form ATP. Let’s take a look at some of the key terms in this definition.
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Cellular respiration is ‘controlled’ because it runs on enzymes that regulate whether it
takes place or not. The cell can regulate the production of enzymes through protein
synthesis, which we studied recently.
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The bonds in glucose and pyruvate, two molecules you’ll be hearing about in this lecture,
contain energy stored in their bonds. Once those bonds are broken, energy is released
and harvested by the cell by the formation of a molecule called ATP – adenosine triphosphate.
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Cellular respiration begins in the cytoplasm and then can move into the mitochondria if
oxygen is present. It cannot happen outside of cells.
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ATP is the universal energy currency and is common for all organisms. Unicellular and
multicellular organisms all use ATP to fuel different processes, such as muscular contraction
(studied in topic 11.3) and active transport in cell membranes (studied in topic 2.4).
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Glycolysis is always the first stage of cellular respiration. It takes place in the cytoplasm
of eukaryotic and prokaryotic cells. One glucose molecule can be broken down to produce two
pyruvate and two ATP molecules. This is a relative small amount of energy, considering
there’s still much energy left in the bonds of pyruvate. The advantage is that it does
not require oxygen, so it will take place regardless of oxygen availability (so long
as glucose is there!).
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If oxygen is not present, pyruvate will be further converted into ethanol and carbon
dioxide in yeast and some plants (an important process in the production of beer and wine).
In humans, pyruvate turns into lactate when no oxygen is present. A buildup of lactate
in the muscles is the cause of cramps.
Eukaryotes, which contain mitochondria, have a significant advantage when oxygen is present.
Oxygen will allow pyruvate to enter a process called the Krebs cycle, or the Citric Acid
Cycle, inside the mitochondria. Pyruvate takes part in a series of reactions, ultimately
leading to the production of large amounts of ATP (around 24 per turn of the cycle),
water and carbon dioxide (released when you exhale, a process studied in topic 6.4).
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And this concludes topic 3.7. If you’re revisiting this video prior to your exams,
consider how it relates to other areas of your study of Biology: the carbon cycle in
Ecology, ventilation, digestion, muscle movement and other mechanisms in Human Physiology,
membrane transport in Cells and probably some of your options as well. If you’re seeing
this for the first time, drawing concept maps showing the relationship between the processes
and the molecules they produce (such as on page 20 of Allott) is quite helpful. We’ll
reinforce these concepts in class through lab activities and as we see the other topics
that relate to cellular respiration.
