The term respiration has two
relatively distinct meanings in biology. First, respiration is the process by
which an organism takes oxygen into its body and then releases carbon dioxide
from its body. In this respect, respiration can be regarded as roughly equivalent
to "breathing." In some cases, this meaning of the term is extended
to mean the transfer of the oxygen from the lungs to the bloodstream and,
eventually, into cells. On the other hand, it may refer to the release of
carbon dioxide from cells into the bloodstream and, thence, to the lungs.
Aerobic respiration: Respiration that requires the presence of oxygen.
Anaerobic respiration: Respiration that does not require the presence of oxygen.
ATP (adenosine triphosphate): High-energy molecule that cells use to drive
energy-requiring processes such as biosynthesis (the production of chemical
compounds), growth, and movement.
Capillaries: Very thin blood vessels that join veins to arteries.
Diffusion: Random movement of molecules that leads to a net movement of
molecules from a region of high concentration to a region of low concentration.
Fermentation: A chemical reaction by which carbohydrates, such as sugar,
are converted into ethyl alcohol.
Gill: An organ used by some animals for breathing consisting of
many specialized tissues with infoldings. It allows the animal to absorb oxygen
dissolved in water and expel carbon dioxide to the water.
Glucose: also known as blood sugar, a simple sugar broken down in
cells to produce energy.
Glycolysis: A series of chemical reactions that takes place in cells by
which glucose is converted into pyruvate.
Hemoglobin: Blood protein that can bind with oxygen.
Lactic acid: Similar to lactate, a chemical compound formed in cells from
pyruvate in the absence of oxygen.
Pyruvate: The simpler compound glucose is broken down into during the
process of glycolysis.
Trachea: A tube used for breathing.
Second, respiration also refers to
the chemical reactions that take place within cells by which food is
"burned" and converted into carbon dioxide and water. In this
respect, respiration is the reverse of photosynthesis, the chemical change that
takes place in plants by which carbon dioxide and water are converted into
complex organic compounds. To distinguish from the first meaning of
respiration, this "burning" of foods is also referred to as aerobic
respiration.
All animals have some mechanism for
removing oxygen from the air and transmitting it into their bloodstreams. The
same mechanism is used to expel carbon dioxide from the bloodstream into the
surrounding environment. In many cases, a special organ is used, such as lungs,
trachea, or gills. In the simplest of animals, oxygen and carbon dioxide are
exchanged directly between the organism's bloodstream and the surrounding
environment. Following are some of the mechanisms that animals have evolved to
solve this problem.
Direct diffusion. In direct diffusion, oxygen passes from the environment
through cells on the animal's surface and then into individual cells inside.
Sponges, jellyfish, and terrestrial flatworms use this primitive method of
respiration. These animals do not have special respiratory organs. Microbes,
fungi, and plants all obtain the oxygen they use for cellular respiration by direct
diffusion through their surfaces.
Diffusion into blood. In diffusion into the blood, oxygen passes through a moist
layer of cells on the body surface. From there, it passes through capillary
walls and into the blood stream. Once oxygen is in the blood, it moves
throughout the body to different tissues and cells. This method also does not
rely upon special respiratory organs and is thus quite primitive. However, it
is somewhat more advanced than direct diffusion. Annelids (segmented worms) and
amphibians use this method of respiration.
Tracheae. In tracheal respiration, air moves through openings in the
body surface called spiracles. It then passes into special breathing tubes
called tracheae (singular, trachea) that extend into the body. The tracheae divide
into many small branches that are in contact with muscles and organs. In small
insects, air moves into the tracheae simply by molecular motion. In large
insects, body movements assist tracheal air movement. Insects and terrestrial
arthropods (organisms with external skeletons) use this method of respiration.
Gills. Fish and other aquatic animals use gills for respiration.
Gills are specialized tissues with many infoldings. Each gill is covered by a
thin layer of cells and filled with blood capillaries. These capillaries take
up oxygen dissolved in water and expel carbon dioxide dissolved in blood.
Lungs. Lungs are special organs in the body cavity composed of many
small chambers filled with blood capillaries. After air enters the lungs,
oxygen diffuses into the blood stream through the walls of these capillaries.
It then moves from the lung capillaries to the different muscles and organs of
the body. Humans and other mammals have lungs in which air moves in and out
through the same pathway. In contrast, birds have more specialized lungs that
use a mechanism called crosscurrent exchange. Crosscurrent exchange allows air
to flow in one direction only, making for more efficient oxygen exchange.
Movement
of gases through the body
In direct diffusion and tracheal
systems, oxygen and carbon dioxide move back and forth directly between cells
and the surrounding environment. In other systems, some mechanism is needed to
carry these gases between cells and the outside environment. In animals with
lungs or gills, oxygen is absorbed by the bloodstream, converted into an
unstable (easily broken down) chemical compound, and then carried to cells.
When the compound reaches a cell, it breaks down and releases the oxygen. The
oxygen then passes into the cell.
In the reverse process, carbon
dioxide is released from a cell into the bloodstream. There the carbon dioxide
is used to form another unstable chemical compound, which is carried by the
bloodstream back to the gills or lungs. At the end of this journey, the
compound breaks down and releases the carbon dioxide to the surrounding
environment.
Various animals use different
substances to form these unstable compounds. In humans, for example, the
substance is a compound known as hemoglobin. In the lungs, hemoglobin reacts
with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the
bloodstream to cells, where it breaks down to form hemoglobin and oxygen. The
oxygen then passes into cells.
On the return trip, hemoglobin
combines with carbon dioxide to form carbaminohemoglobin. In this (and other)
forms, carbon dioxide is returned to the surrounding environment.
Animals other than humans use
compounds other than hemoglobin for the transport of oxygen and carbon dioxide.
Certain kinds of annelids (earthworms, various marine worms, and leeches), for
example, contain a green blood protein called chlorocruorin that functions in
the same way that hemoglobin does in humans.
Whatever substance is used, the
compound it forms with oxygen and carbon dioxide must be unstable, it must
break down easily. This property is essential if the oxygen and carbon dioxide
are to be released easily at the end of their journeys into and out of cells,
lungs, and gills.
Cellular respiration. Cellular respiration is a process by which the simple sugar
glucose is oxidized (combined with oxygen) to form the energy-rich compound
adenosine triphosphate (ATP). Glucose is produced in cells by the breakdown of
more complex carbohydrates, including starch, cellulose, and complex sugars
such as sucrose (cane or beet sugar) and fructose (fruit sugar). ATP is the
compound used by cells to carry out most of their ordinary functions, such as
production of new cell parts and chemicals, movement of compounds through cells
and the body as a whole, and growth.
The overall chemical change that
occurs in cellular respiration can be represented by a fairly simple chemical
equation:
6C 6 H 12 O 6 +
6 O 2 → 6 CO 2 + 6 H 2 O
+ 36 ATP
That equation says that six
molecules of glucose (C 6 H 12 O 6 )
react with six molecules of oxygen (O 2 ) to form six
molecules of carbon dioxide (CO 2 ), six molecules of
water (H 2 O) and 36 molecules of ATP.
Cellular respiration is, however, a
great deal more complicated that this equation would suggest. In fact, nearly
two dozen separate chemical reactions are involved in the overall conversion of
glucose to carbon dioxide, water, and ATP. Those two dozens reactions can be
grouped together into three major cycles: glycolysis, the citric acid (or
Krebs) cycle, and the electron transport chain.
In glycolysis, glucose is broken
down into a simpler compound known as pyruvate. Pyruvate, in turn, is converted
in the citric acid cycle to a variety of energy-rich compounds, such as ATP and
NADH (nicotinamide adenine dinucleotide). Finally, all of these energy-rich
compounds are converted in the electron transport chain to ATP.
Anaerobic respiration. As the equation above indicates, cellular respiration
usually requires the presence of oxygen and is, therefore, often known as
aerobic (or "using oxygen") respiration. Another form of respiration
is possible, one that does not make use of oxygen. That form of respiration is
known as anaerobic (or "without oxygen") respiration.
Anaerobic respiration begins, as
does aerobic respiration, with glycolysis. In the next step, however, pyruvate
is not passed onto the citric acid cycle. Instead, it undergoes one of two
other chemical reactions. In the first of these reactions, the pyruvate is
converted to ethyl alcohol in a process known as fermentation. Fermentation is
a well-known chemical reaction by which grapes, barley, rice, and other grains
are used to make wine, beer, and other alcoholic beverages.
The second anaerobic reaction occurs
when cells are unable to obtain oxygen by methods they normally use. For
example, a person who exercises vigorously may not be able to inhale oxygen
fast enough to meet the needs of his or her cells. (Glucose is used up faster
than oxygen is supplied to the cells.) In that case, cells switch over to
anaerobic respiration. They convert glucose to pyruvate and then to another
chemical known as lactate or lactic acid (two forms of the same compound). As
lactic acid begins to build up in cells, it causes an irritation similar to
placing vinegar (acetic acid) in an open wound.
Most cells are able to switch from
aerobic to anaerobic respiration when necessary. But they are generally not
able to continue producing energy by this process for very long.
Scientists believe that the first
organisms to appear on Earth's surface were anaerobic organisms. Those
organisms arose when Earth's atmosphere contained very little oxygen. They had
to produce the energy they needed, therefore, by mechanisms that did not
require oxygen. As the composition of Earth's atmosphere changed to include
more oxygen, organisms evolved to adapt to that condition.
