Respiration
Cellular Respiration (細胞呼吸作用)
The term cellular respiration refers to the biochemical pathway by which cells release
energy from the chemical bonds of food molecules and provide that energy for the
essential processes of life. All living cells must carry out cellular respiration. It can be
aerobic respiration in the presence of oxygen or anaerobic respiration. Prokaryotic cells
carry out cellular respiration within the cytoplasm or on the inner surfaces of the cells.
More emphasis here will be placed on eukaryotic cells where the mitochondria are the site
of most of the reactions. The energy currency of these cells is ATP, and one way to view
the outcome of cellular respiration is as a production process for ATP. (細胞呼吸作用是
將食物分子化學鍵結所含的能量釋放出來提供給生命運作過程之所需，分為有氧
呼吸和無氧呼吸，真核生物的呼吸作用在粒腺體發生，主要用於產生 ATP，也就
是腺核苷三磷酸)
The graphic below can serve as a reminder of some of the processes involved in cellular
respiration.
1
These
are active graphics: click anywhere.
Cellular respiration produces CO2 as a metabolic waste. This CO2 binds with water to form
carbonic acid, helping to maintain the blood's pH. Since too much CO2 would lower the
blood's pH too much, the removal of the excess CO2 must be accomplished on an ongoing
basis.(二氧化碳可以協助調整血液酸鹼度，身體內過多二氧化碳會降低血液酸鹼
度)
Aerobic Respiration
Aerobic respiration, or cell respiration in the presence of oxygen, uses the end product of
glycolysis (pyruvate) in the TCA cycle to produce much more energy currency in the form
of ATP than can be obtained from any anaerobic pathway. Aerobic respiration is
characteristic of eukaryotic cells when they have sufficient oxygen and most of it takes
place in the mitochondria. (有氧呼吸透過檸檬酸循環將丙酮酸能量轉至 ATP，有氧
呼吸是真核生物的特點之一)
Anaerobic Respiration
The first step in cellular respiration in all living cells is glycolysis, which can take place
without the presence of molecular oxygen. If oxygen is present in the cell, then the cell can
subsequently take advantage of aerobic respiration via the TCA cycle to produce much
more usable energy in the form of ATP than any anaerobic pathway. Nevertheless, the
anaerobic pathways are important and are the sole source of ATP for many anaerobic
2
bacteria. Eukaryotic cells also resort to anaerobic pathways if their oxygen supply is low.
For example, when muscle cells are working very hard and exhaust their oxygen supply,
they utilize the anaerobic pathway to lactic acid to continue to provide ATP for cell
function. (無氧呼吸是許多細菌的能量唯一來源，但真核生物也可能有無氧呼吸，
肌肉細胞是常見的例子)
Glycolysis itself yields two ATP molecules, so it is the first step of anaerobic respiration.
Pyruvate, the product of glycolysis, can be used in fermentation to produce ethanol and
NAD+ or for the production of lactate and NAD+. The production of NAD+ is crucial
because glycolysis requires it and would cease when its supply was exhausted, resulting in
cell death. A general sketch of the anaerobic steps is shown below. It follows Karp's
organization. (呼吸的第一步驟是醣解，其產物是丙酮酸，而 NAD 是關鍵酵素)
3
Anaerobic respiration (both glycolysis and fermentation) takes place in the fluid portion of
the cytoplasm whereas the bulk of the energy yield of aerobic respiration takes place in the
mitochondria. Anaerobic respiration leaves a lot of energy in the ethanol (乙醇) or lactate
(乳酸) molecules that the cell cannot use and must excrete. (無氧呼吸的常見產物是乙
醇和乳酸，仍保有細胞無法利用的能量，必須排出體外)
Glycolysis
Glycolysis, part of cellular respiration, is a series of reactions that constitute the first phase
of most carbohydrate catabolism, catabolism meaning the breaking down of larger
molecules into smaller ones. The word glycolysis is derived from two Greek words and
4
means the breakdown of something sweet. Glycolysis breaks down glucose and forms
pyruvate with the production of two molecules of ATP. The pyruvate end product of
glycolysis can be used in either anaerobic respiration if no oxygen is available or in aerobic
respiration via the TCA cycle which yields much more usable energy for the cell. (醣解將
葡萄糖分解產生丙酮酸和兩個 ATP)
The following general outline of glycolysis follows the organization of Audesirk &
Audesirk.
A glucose molecule
is energized by the
addition of a
high-energy
phosphate from
ATP, forming
glucose-6-phosphate
.
A rearrangement of
the molecule forms
fructose-6-phosphate
.
Using the available
energy of a second
ATP molecule, a
second phosphate is
added to the
fructose.
5
The
fructose-1,6-biphospha
te is split into two
three-carbon
molecules, each having
one phosphate group
attached. The
dihydrooxacetone
(DHAP) quickly
rearranges to form
another G3P molecule,
so the net result is two
G3P molecules.
In
near-simultaneous
reactions, each G3P
molecule gains an
inorganic
phosphorous while
contributing two
electrons and a
hydrogen ion to
NAD+ to form the
energized carrier
molecules NADH.
The resulting
molecules have two
high-energy
phosphates.
6
Two molecules of
low energy ADP are
elevated to ATP
molecules by
phosphates from the
biphosphoglycerates
. This recovers the
energy invested in
the first step of the
glycolysis. The
remaining
phosphorous is
relocated to the
center position.
The final phosphate
is transferred to
ADP to form ATP,
and this step
represents the net
yield of 2 ATP for
the glycolysis
process as a whole.
The TCA Cycle
The tricarboxylic acid cycle (TCA cycle) is a series of enzyme-catalyzed chemical
reactions that form a key part of aerobic respiration in cells. This cycle is also called the
Krebs cycle and the citric acid cycle. The greatly simplified cycle below starts with
pyruvate, which is the end product of gylcolysis, the first step of all types of cell
respiration. (克氏循環；檸檬酸循環；三羧酸循環)
7
Since it is well known that ATP is the energy currency that is used for nearly every task in
the human body and other organisms, it may seem surprising that so little ATP is produced
directly by the TCA cycle. But the TCA cycle as shown here represents the process taking
place in the matrix of the mitochondria using the reactions with NAD+ and FAD which
carry high energy electrons to the electron transport systems in the inner mitochondrial
membrane. The reactions there are responsible for the production of most of the ATP yield
by chemiosmosis.
8
This shows the TCA cycle in the context of what is happening in the inner mitochondrial
membrane. It's role can be seen as giving energy to the reduced coenzyme NADH which
then powers the electron transport process in the membrane. The energy is used by the
protein complexes to produce a proton gradient which in turn powers ATP synthase in its
role of producing the needed ATP. (粒腺體內膜的電子傳遞系統)
9