Living organisms require a continuous input of energy to maintain cellular and organismal functions such as growth, repair, movement, defense, and reproduction. Cells can only use chemical energy to fuel their functions, therefore they need to harvest energy from chemical bonds of biomolecules, such as sugars and lipids. Autotrophic organisms, namely plants, algae, and photosynthetic and chemosynthetic bacteria, convert inorganic materials into such biomolecules by harnessing energy from the environment, such as from sunlight during photosynthesis.
Heterotrophic organisms are unable to synthesize high-energy biomolecules from inorganic materials, so they obtain energy by consuming carbon compounds produced by other organisms, primarily from autotrophs. When energy is needed, chemical bonds of carbon compounds are broken to harvest the energy stored in these bonds.
The processes to harvest energy from biomolecules are called cellular respiration. Cellular respiration occurs in both autotrophic and heterotrophic organisms, where energy becomes available to the organism most commonly through the conversion of adenosine diphosphate ADP to adenosine triphosphate ATP.
There are two main types of cellular respiration—aerobic respiration and anaerobic respiration. Aerobic respiration is a specific type of cellular respiration, in which oxygen O 2 is required to create ATP. In this case, glucose C 6 H 12 O 6 can be oxidized completely in a series of enzymatic reactions to produce carbon dioxide CO 2 and water H 2 O. Aerobic respiration occurs in three stages. A process called glycolysis splits glucose into two three-carbon molecules called pyruvate.
This process releases energy, some of which is transferred to ATP. Next, pyruvate molecules enter the mitochondria to take part in a series of reactions called the Krebs cycle, also known as the citric acid cycle. This completes the breakdown of glucose, harvesting some of the energy into ATP and transferring electrons onto carrier molecules. In the last stage, known as oxidative phosphorylation, electrons pass through an electron transport system in the mitochondrial inner membrane, which maintains a gradient of hydrogen ions.
Cells harness the energy of this proton gradient to generate the majority of the ATP during aerobic respiration. Aerobic respiration requires oxygen, however, there are many organisms that live in places where oxygen is not readily available or where other chemicals overwhelm the environment. Extremophiles are bacteria that can live in places such as deep ocean hydrothermal vents or underwater caves. Rather than using oxygen to undergo cellular respiration, these organisms use inorganic acceptors such as nitrate or sulfur, which are more easily obtainable in these harsh environments.
The Presence of Oxygen There are two types of cellular respiration see Cellular Respiration concept : aerobic and anaerobic. Summary Cellular respiration always begins with glycolysis, which can occur either in the absence or presence of oxygen.
Cellular respiration that proceeds in the absence of oxygen is anaerobic respiration. Cellular respiration that proceeds in the presence of oxygen is aerobic respiration. Anaerobic respiration evolved prior to aerobic respiration. Explore More Use this resource to answer the questions that follow. Aerobic vs. Respiration is only around 40 per cent efficient. As animals respire, heat is also released.
In birds and mammals, this heat is distributed around the body by the blood. It keeps these animals warm and helps to keep a constant internal temperature. ATP is also required:. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next pathway step, the Citric Acid Cycle.
Before you read about the last two stages of cellular respiration, you need to review the structure of the mitochondrion, where these two stages take place. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix.
The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane. Recall that glycolysis produces two molecules of pyruvate pyruvic acid.
Pyruvate, which has three carbon atoms, is split apart and combined with CoA, which stands for coenzyme A. The product of this reaction is acetyl-CoA. These molecules enter the matrix of a mitochondrion, where they start the Citric Acid Cycle.
The third carbon from pyruvate combines with oxygen to form carbon dioxide, which is released as a waste product. High-energy electrons are also released and captured in NADH.
This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. This energy is captured in molecules of ATP and electron carriers. Carbon dioxide is also released as a waste product of these reactions.
This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvate molecules when it splits glucose. After the second turn through the Citric Acid Cycle, the original glucose molecule has been broken down completely.
All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:. Oxidative phosphorylation is the final stage of aerobic cellular respiration. There are two substages of oxidative phosphorylation, Electron transport chain and Chemiosmosis. During this stage, high-energy electrons are released from NADH and FADH 2 , and they move along electron-transport chains found in the inner membrane of the mitochondrion.
An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by chemical reactions. This ion transfer creates an electrochemical gradient that drives the synthesis of ATP. The electrons from the final protein of the ETC are gained by the oxygen molecule, and it is reduced to water in the matrix of the mitochondrion. The pumping of hydrogen ions across the inner membrane creates a greater concentration of these ions in the intermembrane space than in the matrix — producing an electrochemical gradient.
This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. The ATP synthase acts as a channel protein, helping the hydrogen ions across the membrane. The flow of protons through ATP synthase is considered chemiosmosis.
0コメント