The Mitochondrion - Molecular Biology of the Cell - NCBI Bookshelf
Examines the role of mitochondria in cellular respiration. compartments: the intermembrane space (between outer and inner membranes). I. Cellular Respiration: breaking down sugar in the presence of oxygen The mitochondria is another organelle in eukaryotic cells. like the chloroplast, . V. A comparison between Photosynthesis and Respiration: What's the connection?. Mitochondria; The Citric Acid Cycle; The Electron Transport Chain; Chemiosmosis Cellular respiration is the process of oxidizing food molecules, like glucose, an inner membrane that encloses a fluid-filled matrix; between the two is the . an endosymbiotic relationship with the ancestors of eukaryotic cells early in the.
Most of the proteins involved are grouped into three large respiratory enzyme complexes, each containing transmembrane proteins that hold the complex firmly in the inner mitochondrial membrane.
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- Mitochondria - Turning on the Powerhouse
- Mitochondria and chloroplasts
Each complex in the chain has a greater affinity for electrons than its predecessor, and electrons pass sequentially from one complex to another until they are finally transferred to oxygen, which has the greatest affinity of all for electrons. The proteins guide the electrons along the respiratory chain so that the electrons move sequentially from one enzyme complex to another—with no short circuits.
It generates a pH gradient across the inner mitochondrial membranewith the pH higher in the matrix than in the cytosolwhere the pH is generally close to 7. Since small molecules equilibrate freely across the outer membrane of the mitochondrion, the pH in the intermembrane space is the same as in the cytosol.
It generates a voltage gradient membrane potential across the inner mitochondrial membrane, with the inside negative and the outside positive as a result of the net outflow of positive ions. Figure The two components of the electrochemical proton gradient. The electrochemical proton gradient exerts a proton-motive forcewhich can be measured in units of millivolts mV.
In a typical cell, the proton-motive force across the inner membrane of a respiring mitochondrion is about mV and is made up of a membrane potential of about mV and a pH gradient of about -1 pH unit. How the Proton Gradient Drives ATP Synthesis The electrochemical proton gradient across the inner mitochondrial membrane is used to drive ATP synthesis in the critical process of oxidative phosphorylation Figure This is made possible by the membrane-bound enzyme ATP synthasementioned previously.
This enzyme creates a hydrophilic pathway across the inner mitochondrial membrane that allows protons to flow down their electrochemical gradient. The ATP synthase is of ancient origin; the same enzyme occurs in the mitochondria of animal cells, the chloroplasts of plants and algae, and in the plasma membrane of bacteria and archea.
Figure The general mechanism of oxidative phosphorylation. The structure of ATP synthase is shown in Figure A large enzymatic portion, shaped like a lollipop head and composed of a ring of 6 subunits, projects on the matrix side of the inner mitochondrial membrane. As protons pass through a narrow channel formed at the stator-rotor contact, their movement causes the rotor ring to spin. This spinning also turns a stalk attached to the rotor blue in Figure Bwhich is thereby made to turn rapidly inside the lollipop head.
As a result, the energy of proton flow down a gradient has been converted into the mechanical energy of two sets of proteins rubbing against each other: Figure ATP synthase. Both F1 and F0 are formed from multiple subunits, as indicated. A rotating stalk turns with a rotor formed by a ring of 10 to more Three of the six subunits in the head contain binding sites for ADP and inorganic phosphate. These are driven to form ATP as mechanical energy is converted into chemical bond energy through the repeated changes in protein conformation that the rotating stalk creates.
Three or four protons need to pass through this marvelous device to make each molecule of ATP.Fermentation
In mitochondria, many charged small molecules, such as pyruvate, ADP, and Pi, are pumped into the matrix from the cytosolwhile others, such as ATP, must be moved in the opposite direction. Since an ATP molecule has one more negative charge than ADP, each nucleotide exchange results in a total of one negative charge being moved out of the mitochondrion.
Figure Some of the active transport processes driven by the electrochemical proton gradient across the inner mitochondrial membrane. The charge on each of the transported more In eucaryotic cells, the proton gradient is thus used to drive both the formation of ATP and the transport of certain metabolites across the inner mitochondrial membrane.
In bacteria, the proton gradient across the bacterial plasma membrane is harnessed for both types of functions. And in the plasma membrane of motile bacteria, the gradient also drives the rapid rotation of the bacterial flagellum, which propels the bacterium along Figure The flagellum is attached to a series of protein rings orangewhich are embedded in the outer and inner membranes and rotate with the flagellum.
The rotation is driven by a flow of protons more Proton Gradients Produce Most of the Cell's ATP As stated previously, glycolysis alone produces a net yield of 2 molecules of ATP for every molecule of glucose that is metabolized, and this is the total energy yield for the fermentation processes that occur in the absence of O2 discussed in Chapter 2.
During oxidative phosphorylationeach pair of electrons donated by the NADH produced in mitochondria is thought to provide energy for the formation of about 2. Oxidative phosphorylation also produces 1. From the product yields of glycolysis and the citric acid cycle summarized in Table Aone can calculate that the complete oxidation of one molecule of glucose—starting with glycolysis and ending with oxidative phosphorylation—gives a net yield of about 30 ATPs.
Cellular and molecular mechanisms of mitochondrial function
In conclusion, the vast majority of the ATP produced from the oxidation of glucose in an animal cell is produced by chemiosmotic mechanisms in the mitochondrial membrane. ADP Ratio in Cells Because of the carrier protein in the inner mitochondrial membrane that exchanges ATP for ADP, the ADP molecules produced by ATP hydrolysis in the cytosol rapidly enter mitochondria for recharging, while the ATP molecules formed in the mitochondrial matrix by oxidative phosphorylation are rapidly pumped into the cytosol, where they are needed.
A typical ATP molecule in the human body shuttles out of a mitochondrion and back into it as ADP for recharging more than once per minute, keeping the concentration of ATP in the cell about 10 times higher than that of ADP. As discussed in Chapter 2, biosynthetic enzymes often drive energetically unfavorable reactions by coupling them to the energetically favorable hydrolysis of ATP see Figure The ATP pool is therefore used to drive cellular processes in much the same way that a battery can be used to drive electric engines.
If the activity of the mitochondria is blocked, ATP levels fall and the cell's battery runs down; eventually, energetically unfavorable reactions are no longer driven, and the cell dies. The poison cyanide, which blocks electron transport in the inner mitochondrial membranecauses death in exactly this way. It might seem that cellular processes would stop only when the concentration of ATP reaches zero; but, in fact, life is more demanding: To explain why, we must consider some elementary principles of thermodynamics.
We showed on p. The rate constants in boxes 1 and 2 are determined from experiments in which product accumulation is measured as a function of time. The equilibrium constant more Without this large disequilibrium, ATP hydrolysis could not be used to direct the reactions of the cell; for example, many biosynthetic reactions would run backward rather than forward at low ATP concentrations.
It thus acts as a reversible coupling device, interconverting electrochemical proton gradient and chemical bond energies. Figure The ATP synthase is a reversible coupling device that can convert the energy of the electrochemical proton gradient into chemical-bond energy, or vice versa.
Although the exact number of protons needed to make each ATP molecule is not known with certainty, we shall assume that one molecule of ATP is made by the ATP synthase for every 3 protons driven through it. The following example will help explain how the balance between these two free-energy changes affects the ATP synthase. ADP ratio in the matrix to fall. In many bacteria, ATP synthase is routinely reversed in a transition between aerobic and anaerobic metabolismas we shall see later.
And how does energy end up stored in the broccoli to begin with, anyway? The answers to these questions have a lot to do with two important organelles: Chloroplasts are organelles found in the broccoli's cells, along with those of other plants and algae.
They capture light energy and store it as fuel molecules in the plant's tissues. Mitochondria are found inside of your cells, along with the cells of plants. They convert the energy stored in molecules from the broccoli or other fuel molecules into a form the cell can use. Let's take a closer look at these two very important organelles.
Chloroplasts Chloroplasts are found only in plants and photosynthetic algae. Humans and other animals do not have chloroplasts. The chloroplast's job is to carry out a process called photosynthesis.
In photosynthesis, light energy is collected and used to build sugars from carbon dioxide. The sugars produced in photosynthesis may be used by the plant cell, or may be consumed by animals that eat the plant, such as humans. The energy contained in these sugars is harvested through a process called cellular respiration, which happens in the mitochondria of both plant and animal cells.
Chloroplasts are disc-shaped organelles found in the cytosol of a cell. They have outer and inner membranes with an intermembrane space between them. Diagram of a chloroplast, showing the outer membrane, inner membrane, intermembrane space, stroma, and thylakoids arranged in stacks called grana. Thylakoid discs are hollow, and the space inside a disc is called the thylakoid space or lumen, while the fluid-filled space surrounding the thylakoids is called the stroma. You can learn more about chloroplasts, chlorophyll, and photosynthesis in the photosynthesis topic section.
Mitochondria Mitochondria singular, mitochondrion are often called the powerhouses or energy factories of the cell. The process of making ATP using chemical energy from fuels such as sugars is called cellular respirationand many of its steps happen inside the mitochondria. The mitochondria are suspended in the jelly-like cytosol of the cell. They are oval-shaped and have two membranes: Electron micrograph of a mitochondrion, showing matrix, cristae, outer membrane, and inner membrane. Modification of work by Matthew Britton; scale-bar data from Matt Russell.
The matrix contains mitochondrial DNA and ribosomes. We'll talk shortly about why mitochondria and chloroplasts have their own DNA and ribosomes. The multi-compartment structure of the mitochondrion may seem complicated to us. That's true, but it turns out to be very useful for cellular respirationallowing reactions to be kept separate and different concentrations of molecules to be maintained in different "rooms.
Electrons from fuel molecules, such as the sugar glucose, are stripped off in reactions that take place in the cytosol and in the mitochondrial matrix. These electrons are captured by special molecules called electron carriers and deposited into the electron transport chaina series of proteins embedded in the inner mitochondrial membrane. As protons flow back down their gradient and into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to generate ATP.
This process of generating ATP using the proton gradient generated by the electron transport chain is called oxidative phosphorylation. The compartmentalization of the mitochondrion into matrix and intermembrane space is essential for oxidative phosphorylation, as it allows a proton gradient to be established.