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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 bywhich 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.

Respiration mechanisms

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.


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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 cellson 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.


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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 reactswith 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 waythat 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 orbeet 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:

6C6

H12

O6

+ 6 O26 CO

2

+ 6 H2

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


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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 respirationbegins, 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 aprocess 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 adaptto that condition.

[See also Bacteria ; Blood ; Diffusion ; Fermentation ; Metabolism ; Oxygen family ;

Respiratory system ;Yeast ]


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User Contributions:

keerthana Jun 9, 2010 @ 10:10 am

useful for the students for primary, middle,higher schools

belma l. valerio Aug 1, 2010 @ 3:03 am

During the process of respiration, the end products are carbondioxide and water.Water isused by the parts of the plant, transpiration, and photosynthesis where do excess water go?

John Aug 9, 2010 @ 8:08 am

"6C 6 H 12 O 6 + 6 O 26 CO 2 + 6 H 2 O + 36 ATP"I believe its just 1 glucose molecule not 6

April Aug 18, 2010 @ 4:04 am

...i like it... it can gain more knowledge... it can help students analyze and answer theirprojects and assignments properly...

L-r Mamalias Aug 19, 2010 @ 6:06 am

@it helps all the student's like me, who are studying the general biology..

roopa Apr 29, 2011 @ 9:09 am

i wanted more information about respiration in plants.

steph anie May 22, 2011 @ 2:02 am

its very good and useful to students and i need more information about respiration in plants

Jessica Delate Howell May 22, 2011 @ 12:12 pm

Why is it important for the blood to have a specialised type of cell for oxygen absorption?

sarah willy Jun 23, 2011 @ 2:02 am

may you show me the diagram represent the arrangement of a tube and structure through

which oxgen passes as it travel from the atmosphere into the blood of mammals

benny Nov 5, 2011 @ 1:01 am

this is good for those in higher,primary,college,may lead to a good interest for a project..


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Alexis Sep 15, 2013 @ 4:16 pm

This is very helpful in many ways. I needed some information, and here is where I got all of it!

Reptiles Respiratory System


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