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3.5: True Respiration - Biology

3.5: True Respiration - Biology


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The common misconception about plants is that their only energy-related metabolic process is photosynthesis:

CO(_2) + H(_2)O + energy (longrightarrow) carbohydrates + O(_2)

However, as most eukaryotes, plants have mitochondria in cells and use aerobic (oxygen-related) respiration to obtain energy:

carbohydrates + O(_mathbf{2}) (longrightarrow) CO(_mathbf{2}) + H(_mathbf{2})O + energy

Typically, plants spend much less oxygen in respiration than they make in photosynthesis. However, at nights plants do exactly the same as animals, and make only carbon dioxide!


AQA A Level Respiration (3.5.2) - Full Lesson/ Revision PowerPoint

pptx, 63.65 MB

AQA A Level Respiration (3.5.2) - Full Lesson/ Revision PowerPoint. The PowerPoint is designed for teaching the NEW 2015 AQA A Level Biology Specification.

The PowerPoint incudes:
-An introduction to Respiration
-An overview of Glycolysis, the Link Reaction and the Krebs Cycle
-An overview of the electron transport chain
-The effect of mitochondria poisons on respiration
-Alternative substrates used for respiration
-Past Paper Questions taken from 2018-2010

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How Is the Rate of Cellular Respiration Measured?

Cellular respiration is measured primarily through the use of two different methods: one that involves measuring changes in temperature over time and another that utilizes the exchanges and consumption of different gases through the use of a respirometer. Heat can be used to measure cellular respiration because it is an exergonic process. Because respiration is so closely tied to different gases, respirometers and measuring gases can also be used effectively.

Cellular respiration is a process consisting of many different metabolic reactions that take place inside of the cells of organisms. These processes are able to convert different types of nutrients into energy and produce different waste products.

Measuring the rate at which a cell goes through respiration can be done in multiple ways. One simple way is to track the heat of the cell. This is possible because the process of converting nutrients into energy creates heat. The fluctuations in a cell's heat patterns can be analyzed to give scientists an idea of the rate at which it is respiring. Additionally, the measurement of certain gases, particularly oxygen and carbon dioxide, can be used. This is because oxygen is consumed during this process, and carbon dioxide is produced as a waste product. Different levels of oxygen and carbon dioxide can indicate whether or not a cell has recently gone through respiration.


Cellular Respiration

The oxygen obtained from internal respiration is used by cells in cellular respiration. In order to access the energy stored in the foods we eat, biological molecules composing foods (carbohydrates, proteins, etc,) must be broken down into forms that the body can utilize. This is accomplished through the digestive process where food is broken down and nutrients are absorbed into the blood. As blood is circulated throughout the body, nutrients are transported to body cells. In cellular respiration, glucose obtained from digestion is split into its constituent parts for the production of energy. Through a series of steps, glucose and oxygen are converted to carbon dioxide (CO2), water (H2O), and the high energy molecule adenosine triphosphate (ATP). Carbon dioxide and water formed in the process diffuse into the interstitial fluid surrounding cells. From there, CO2 diffuses into blood plasma and red blood cells. ATP generated in the process provides the energy needed to perform normal cellular functions, such as macromolecule synthesis, muscle contraction, cilia and flagella movement, and cell division.


Gas Exchange across the Alveoli

Differences in partial pressures of O2 create a gradient that causes oxygen to move from the alveoli to the capillaries and into tissues.

Learning Objectives

Explain the process of gas exchange across the alveoli

Key Takeaways

Key Points

  • The change in partial pressure from the alveoli (high concentration) to the capillaries (low concentration) drives the oxygen into the tissue and the carbon dioxide into the blood (high concentration) from the tissues (low concentration), which is then returned to the lungs and exhaled.
  • Once in the blood of the capillaries, the O2 binds to the hemoglobin in red blood cells which carry it to the tissues where it dissociates to enter the cells of the tissues.
  • The lungs never fully deflate, so air that is inhaled mixes with the residual air left from the previous respiration, resulting in a lower partial pressure of oxygen within the alveoli.

Key Terms

  • hemoglobin: iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body it consists of a protein (globulin) and heme (a porphyrin ring with iron at its center)
  • mole: in the International System of Units, the base unit of amount of substance

Gas Exchange across the Alveoli

In the human body, oxygen is used by cells of the body’s tissues to produce ATP, while carbon dioxide is produced as a waste product. The ratio of carbon dioxide production to oxygen consumption is referred to as the respiratory quotient (RQ), which typically varies between 0.7 and 1.0. If glucose alone were used to fuel the body, the RQ would equal one, as one mole of carbon dioxide would be produced for every mole of oxygen consumed. Glucose, however, is not the only fuel for the body both proteins and fats are used as well. Since glucose, proteins, and fats are used as fuel sources, less carbon dioxide is produced than oxygen is consumed the RQ is, on average, about 0.7 for fat and about 0.8 for protein.

The RQ is a key factor because it is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung: the alveolar PO2 (PALVO2). The lungs never fully deflate with an exhalation therefore, the inspired air mixes with this residual air, lowering the partial pressure of oxygen within the alveoli. This results in a lower concentration of oxygen in the lungs than is found in the air outside the body. When the RQ is known, the partial pressure of oxygen in the alveoli can be calculated: alveolar PO2 = inspired PO2−((alveolar PO2)/RQ)

In the lungs, oxygen diffuses out of the alveoli and into the capillaries surrounding the alveoli. Oxygen (about 98 percent) binds reversibly to the respiratory pigment hemoglobin found in red blood cells. These red blood cells carry oxygen to the tissues where oxygen dissociates from the hemoglobin, diffusing into the cells of the tissues. More specifically, alveolar PO2 is higher in the alveoli (PALVO2=100mmHg) than blood PO2 in the capillaries (40mmHg). Since this pressure gradient exists, oxygen can diffuse down its pressure gradient, moving out of the alveoli and entering the blood of the capillaries where O2 binds to hemoglobin. At the same time, alveolar PCO2 is lower (PALV CO2=40mmHg) than blood PCO2 (45mmHg). Due to this gradient, CO2 diffuses down its pressure gradient, moving out of the capillaries and entering the alveoli.

Oxygen and carbon dioxide move independently of each other they diffuse down their own pressure gradients. As blood leaves the lungs through the pulmonary veins, the venous PO2=100mmHg, whereas the venous PCO2=40mmHg. As blood enters the systemic capillaries, the blood will lose oxygen and gain carbon dioxide because of the pressure difference between the tissues and blood. In systemic capillaries, PO2=100mmHg, but in the tissue cells, PO2=40mmHg. This pressure gradient drives the diffusion of oxygen out of the capillaries and into the tissue cells. At the same time, blood PCO2=40mmHg and systemic tissue PCO2=45mmHg. The pressure gradient drives CO2 out of tissue cells and into the capillaries. The blood returning to the lungs through the pulmonary arteries has a venous PO2=40mmHg and a PCO2=45mmHg. The blood enters the lung capillaries where the process of exchanging gases between the capillaries and alveoli begins again.

Partial pressures: The partial pressures of oxygen and carbon dioxide change as blood moves through the body.

In short, the change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs and oxygen into the blood.


What is an Internal Respiration?

Internal respiration occurs within cells of the body and involves all body cells, not just cells of the lungs. It uses oxygen to break down molecules in order to release energy in the form of adenosine triphosphate (ATP). Internal respiration is often also called cellular respiration since it occurs within the cell.

Internal cellular respiration can occur in two forms:

  • Aerobic respiration which requires oxygen
  • Anaerobic respiration (also known as fermentation) which does not require oxygen

The cells of most living organisms cannot survive long periods of anaerobic respiration, and thus oxygen is needed. Aerobic respiration generates large amounts of energy as ATP while anaerobic respiration cannot produce very much energy (ATP).

Aerobic respiration involves three stages:

  1. Glycolysis (splitting of sugar) which occurs in the cytoplasm
  1. Kreb’s cycle which occurs in the matrix of the mitochondrion
  1. Oxidative phosphorylation which occurs across the membrane of the mitochondrion.

The oxygen is the final electron acceptor of what is known as the electron transport chain found in the last stage, oxidative phosphorylation, of aerobic cellular respiration. Oxygen provides a force to drive the transport of electrons down the chain. As electrons move across the membrane, ATP is formed from ADP.

Water and carbon dioxide are produced as waste products of internal cellular respiration. Water is formed when protons combine with oxygen at the end of the electron transport chain.


Words to Know

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 ȫurned" 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 ȫurning" of foods is also referred to as aerobic respiration.


Cellular Respiration in Plants

Cellular respiration in plants involves three major pathways to oxidize glucose into energy (ATP). In plants, ATP formed during cellular respiration serves as the “Energy currency” that helps in the forming and functioning of different cells.

It involves glycolysis, Krebs cycle and electron transport system for the complete oxidation of a glucose molecule into 38 ATP molecules. The whole process of cellular respiration is depicted in a diagram:

Process

In cellular respiration, glucose first oxidizes into pyruvate by a series of enzymes. Then, pyruvate undergoes oxidation and converts into acetyl coenzyme-A by an enzyme pyruvate dehydrogenase.

The acetyl coenzyme-A enters the Kreb’s cycle and oxidizes to carbon dioxide, protons and electrons through the combination of different enzymes. To know more about glycolysis and the Krebs cycle, one can look at the article difference between glycolysis and Krebs cycle.

Then, the protons and electrons released during the Krebs cycle participate in the electron transport system. The oxygen accepts an electron and combines with a proton to release a water molecule. 38 ATP molecules are produced per glucose molecule after the completion of cellular respiration.

Types of Respiration in Plants

Based on the oxygen requirement, respiration in plants is categorized into two types:

Aerobic Respiration

It occurs in the presence of atmospheric oxygen. A plant uses oxygen for oxidizing high energy organic compound (glucose) into low energy molecules (like water and carbon dioxide). Aerobic respiration in plants releases a high amount of energy, which the plants utilise to drive ATP synthesis.

The ATP further breaks down into ADP and inorganic phosphate and releases energy. Plants utilize the energy to perform cellular functions. Aerobic respiration occurs in the mitochondria with a net production of 38 ATP molecules by the complete oxidation of one glucose molecule.

Anaerobic Respiration

It is intramolecular respiration that occurs in succulent plants like cacti, meristematic tissue, and germinating seeds. It occurs in the absence of oxygen. Anaerobic respiration results in incomplete oxidation of the respiratory substrates into carbon dioxide and ethyl alcohol by releasing little energy.

Thus, anaerobic respiration in plants is related to alcoholic fermentation. The energy released during anaerobic respiration is harnessed to maintain the protoplasmic activity. Anaerobic respiration occurs in the cytoplasm with a net production of 2 ATP molecules. It involves incomplete oxidation of a glucose molecule.

Mechanism of Respiration

Like other living organisms, plants also need oxygen to respire and produce energy. Then, a plant supplies energy to the different parts. In plants, respiration occurs in the roots, stems and leaves. Root hairs, stomata, lenticels are the respiratory parts through which a plant facilitates gaseous exchange.

Respiration in Roots

Root respires by getting the oxygenated air from the soil particles via root hairs. Root hairs are tubular structures and in direct contact with the soil particles. Diffusion of oxygen occurs from the soil particles to the root hairs and finally to the other parts.

During respiration, roots consume oxygen and release carbon dioxide into the atmosphere. A plant again uses the released carbon dioxide to prepare food and release oxygen.

The soil must not be soggy, or soil with excessive water can block free oxygen availability in the soil. There is no oxygen requirement during seed germination in the initial stage, as the testa or seed covering does not allow oxygen to enter.

Respiration is Stems

Stems of herbaceous plants respire through a stomatal pore found in the epidermis of the stem. Stems of woody plants respire via lenticels found in the periderm of the stem, which usually exists as loosely packed dead cells.

Thus, both stomatal pore and lenticels allow the entry of oxygen inside the plant’s intercellular spaces and release carbon dioxide into the atmosphere.

Respiration in Leaves

Small pores present in the lower epidermis layer of the leaf called stomata allow gaseous exchange between the leaves and the environment. Guard cells control the stomatal activity and control the stomatal opening and closing for the gaseous exchange.


Open Research

Soil respiration data from the SAF-01, SAF-02, SAF-03, SAF-04, SAF-05, DAN-04, DAN-05, MLA-01 and MLA-02 plots in will be openly available in Zenodo, at https://doi.org/10.5281/zenodo.3266770. Soil respiration data from the LAM-06 and LAM-07, and the auxiliary data from all the plots will be available from the corresponding author upon reasonable request. Plot-level estimates of the components of the below-ground carbon cycle presented in Figure 4 are available for all plots in Zenodo, at https://doi.org/10.5281/zenodo.3266770.

Filename Description
gcb15522-sup-0001-FigS1.pdfPDF document, 157.1 KB Fig S1
gcb15522-sup-0002-FigS2.pdfPDF document, 90.1 KB Fig S2
gcb15522-sup-0003-FigS3.pdfPDF document, 670.9 KB Fig S3
gcb15522-sup-0004-FigS4.pdfPDF document, 237.1 KB Fig S4
gcb15522-sup-0005-FigS5.pdfPDF document, 628.8 KB Fig S5

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.


4 Answers 4

Prof. Allen Gathman has a great 10-minutes video on Youtube, explaining the reaction of adding nucleotide in the 5' to 3' direction, and why it doesn't work the other way.

Briefly, the energy for the formation of the phosphodiester bond comes from the dNTP, which has to be added. dNTP is a nucleotide which has two additional phosphates attached to its 5' end. In order to join the 3'OH group with the phosphate of the next nucleotide, one oxygen has to be removed from this phosphate group. This oxygen is also attached to two extra phosphates, which are also attached to a Mg++. Mg++ pulls up the electrons of the oxygen, which weakens this bond and the so called nucleophilic attack of the oxygen from the 3'OH succeeds, thus forming the phospodiester bond.

If you try to join the dNTP's 3'OH group to the 5' phosphate of the next nucleotide, there won't be enough energy to weaken the bond between the oxygen connected to the 5' phosphorous (the other two phosphates of the dNTP are on the 5' end, not on the 3' end), which makes the nucleophilic attack harder.

Watch the video, it is better explained there.

DNA replications needs a source of energy to proceed, this energy is gained by cleaving the 5'-triphosphate of the nucleotide that is added to the existing DNA chain. Any alternative polymerase mechanism needs to account for the source of the energy required for adding a nucleotide.

The simplest way one can imagine to perform reverse 3'-5' polymerization would be to use nucleotide-3'-triphosphate instead of the nucleotide-5'-triphosphate every existing polymerase uses. This would allow for a practically identical mechanism as existing polymerases, just with different nucleotides as substrates. The problem with this model is that ribonucleotide-3'-triphosphates are less stable under acidic conditions due to the neighbouring 2'-OH (though this obviously only applies for RNA, not for DNA).

So any 3'-5' polymerase would likely need to use the same nucleotide-5'-triphosphates as the 5'-3' polymerase. This would mean that the triphosphate providing the energy for addition of a new nucleotide would be on the DNA strand that is extended, and not on the newly added nucleotide.

One disadvantage of this approach is that nucleotide triphosphates spontaneously hydrolyze under aqeuous conditions. This is no significant problem for the 5'-3' polymerase, as the triphosphate is on the new nucleotide and the polymerase just has to find a new nucleotide. For the 3'-5' polymerase spontaneous hydrolysis is a problem because the triphosphate is on the growing chain. If that one gets hydrolyzed, the whole polymerization needs to be either aborted or the triphosphate need to be readded by some mechanism.

You can take a look at the article "A Model for the Evolution of Nucleotide Polymerase Directionality" by Joshua Ballanco and, Marc L. Mansfield for more information about this. They created a model on early polymerase evolution, though they don't reach any final conclusion.

In my opinion, Prof. Allen Gathman's "great 10-minutes video on Youtube" is a pretty waste of time if you already know how hydrolysis happens. In fact, he has not considered the 3'->5' route in an unbiased manner he doesn't seem to look at the possibility of a triphosphate appearing at the growing 5' tip of the strand in the 3'->5' case.

Actually, the only difference between the two routes (5'->3' and 3'->5') is that the reacting triphosphate appears in different places. In the usual case, the triphosphate which is hydrolysed belongs to the added nucleotide, while in the latter case, the triphosphate which is hydrolysed belongs to the nucleotide on the growing strand. Both are feasible.

In fact, it is known that RNA polymerase has dual activity, but you see, RNA polymerase doesn't have proofreading activity!. Proofreading requires removal of the mismatched base, but in the 3'->5 direction the base's attachment had consumed the triphosphate at the 5' tip of the strand, so it is no longer available to add the replacement base. 3'->5' activity readily destroys proofreading capability of a polymerase So, basically, it is the need for proofreading that restricts the synthesis of DNA strands to 5'->3'. Why it is so, would need a lot more explanation (if in words) but I think a picture has far better explanatory power than a thousand words. I've added a picture from Essential Cell Biology that shows the answer to the 'WHY' question:

The other important consideration is repair. If one or more nucleotide is missing in one strand, repair of the missing nucleotide would be impossible for 3' to 5' synthesis, because no 5'-triphosphate is present. On the other hand, 5' to 3' synthesis does not require a 3'-triphosphate present at the repair site. This is important. That is 3' to 5' synthesis does not allow nucleotide repair.


Watch the video: Introduction to Respiration and Breathing. Dont Memorise (June 2022).


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