生物 Energy and respiration

Alevel 第12章 Energy and respiration 生物知识导图，主要包含：ATP的结构及功能，线粒体的结构，有氧呼吸及无氧呼吸，糖酵解，Kerbs循环及电子传递链等重要知识点。

Energy and respiration

• ATP
  • ATP as energy‘currency’
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      • the energy does not come simply from breaking those bonds, but rather from changes in chemical potential energy of all parts of the system.
    • These reactions are all reversible
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        • the rate of interconversion is enormous（a resting human uses about 40kg of ATP in 24 hours）
    • ATP is the universal intermediary molecule between energy-yielding and energy-requiring reactions used in a cell
      • Not only is it readily hydrolysed to release energy, it is also small and water-soluble.
    • Energy transfers are inefficient
      • Any excess energy is converted to thermal energy;any extra energy will be released as thermal energy
  • Synthesis of ATP
    • 1.In respiration, energy released by reorganising chemical bonds during glycolysis and the Krebs cycle is used to make some ATP
    • 2.most ATP in cells is generated using electrical potential energy
      • This energy is from the transfer of electrons by electron carriers in mitochondria and chloroplasts.
        • Protons are then allowed to flow down their concentration gradient (by facilitated diffusion) through a protein that spans the phospholipid bilayer(Part of this protein acts as an enzyme that synthesises ATP and is called ATP synthase）
        • ATP synthase-- This produces structural changes in the binding sites and allows them to pass sequentially through three phases: 1.binding ADP and Pi ;2. forming tightly bound ATP ;3.releasing ATP
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  • The role of ATP in active transport
    • The sodium–potassium pump is a protein that spans the cell surface membrane; It has binding sites for sodium ions (Na+) and for ATP on the inner side, and for potassium ions (K+) on the outer side.
      • The protein acts as an ATPase and catalyses the hydrolysis of ATP to ADP and inorganic phosphate, releasing energy to drive the pump
        • For each ATP used, two potassium ions move into the cell and three sodium ions move out of the cell.This potential difference is most clearly seen as the resting potential of a nerve cell
          • About 50% of the ATP used by a resting mammal is devoted to maintaining the ionic content of cells.
• Respiration
  • Respiration is a process in which organic molecules act as a fuel--The organic molecules are broken down to release chemical potential energy, which is used to synthesise ATP.
    • substrate--carbohydrate( glucose),fatty acids, glycerol and amino acids
    • Glucose breakdown can be divided into four stages:1.glycolysis;2.the link reaction;3.the Krebs cycle;4.oxidative phosphorylation
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    • Oxidative phosphorylation and the electron transport chain
  • The glycolytic pathway
    • It is a multi-step process in which a glucose molecule with six carbon atoms is eventually split into two molecules of pyruvate, each with three carbon atoms.
      • the first steps-ATP is needed;later steps-- energy is released
      • Glycolysis takes place in the cytoplasm of a cell
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      • Th e end-product of glycolysis, pyruvate, still contains a great deal of chemical potential energy.
  • The link reaction
    • Pyruvate passes by active transport from the cytoplasm, through the outer and inner membranes of a mitochondrion and into the mitochondrial matrix.
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    • Here it is decarboxylated,dehydrogenated and combined with coenzyme A (CoA) to give acetyl coenzyme A.
      • Coenzyme A is a complex molecule composed of a nucleoside (adenine plus ribose) with a vitamin (pantothenic acid), and acts as a carrier of acetyl groups to the Krebs cycle.
  • The Krebs cycle
    • a closed pathway of enzymecontrolled reactions-- was discovered in 1937 by Hans Krebs
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        • Acetyl coenzyme A combines with a four-carbon compound (oxaloacetate) to form a six-carbon compound (citrate).
        • The citrate is decarboxylated and dehydrogenated in a series of steps, to yield carbon dioxide, which is given off as a waste gas, and hydrogens which are accepted by the carriers NAD and FAD
        • Oxaloacetate is regenerated to combine with another acetyl coenzyme A.
      • For each turn of the cycle, two carbon dioxide molecules are produced, one FAD and three NAD molecules are reduced, and one ATP molecule is generated via an intermediate compound.
        • For each turn of the cycle, two carbon dioxide molecules are produced, one FAD and three NAD molecules are reduced, and one ATP molecule is generated via an intermediate compound.
  • Oxidative phosphorylation and the electron transport chain
    • the energy for the phosphorylation of ADP to ATP comes from the activity of the electron transport chain; takes place in the inner mitochondrial membrane
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        • the hydrogens are removed from the two hydrogen carriers(Reduced NAD and reduced FAD) and each is split into its constituent proton (H+) and electron (e−).
        • Most of the carriers are associated with membrane proteins, of which there are four types--a respiratory complex（consists of one of each of these proteins) electrons can be passed from one to another down an energy gradient.
        • As an electron moves from one carrier at a higher energy level to another one at a lower level, energy is released
        • protons pass back into the mitochondrial matrix through protein channels in the inner membrane.their electrical potential energy is used to synthesise ATP in the process called chemiosmosis
        • oxygen has a role to play as the final electron acceptor reducing it to water
    • Theoretically,3 ATP can be produced from 1 reduced NAD;2 ATP from 1 reduced FAD
      • About 25% of the total energy yield of electron transfer is used to transport ADP into the mitochondrion and ATP into the cytoplasm
        • The number of ATP molecules actually produced varies in different tissues and different circumstances, largely dependent on how much energy is used to move substances into and out of the mitochondria
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  • Hydrogen carrier molecules
    • NAD is made of two linked nucleotides(One nucleotide contains the nitrogenous base adenine;The other has a nicotinamide ring, which can accept a hydrogen ion and two electrons)
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    • FAD is made of one nucleotide containing ribose and adenine and one with an unusual structure involving a linear molecule, ribitol, instead of ribose.
• Adaptations of rice for wet environments
  • Most plants cannot grow in deep water because their roots do not get enough oxygen
    • gases diffuse much more slowly in water than they do in air; the concentrations of dissolved oxygen and dissolved carbon dioxide in water are much less than they are in air.
  • Some varieties of rice respond to flooding by growing taller
    • This aows oxygen and carbon dioxide to be exchanged through the stomata on the leaves
  • The stems of the rice plants contain loosely packed cells forming a tissue known as aerenchyma
    • Gases are able to diffuse through the aerenchyma to other parts of the plant, including those under the water.
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  • the cells in the submerged roots do still have to use alcoholic fermentation at least some of the time
    • 1.the cells in rice roots can tolerate much higher levels than most plants;They also produce more alcohol dehydrogenase
• Respiratory substrates
  • Energy values of respiratory substrates
    • the greater the number of hydrogens in the structure of the substrate molecule, the greater the energy value.
    • energy density -energy value per unit mass
      • The energy value of a substrate is determined by burning a known mass of the substance in oxygen in a calorimeter
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    • The energy liberated by oxidising the substrate can be determined from the rise in temperature of a known mass of water in the calorimeter
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  • glucose, lipids and amino acids
    • amino acids--The carbon– hydrogen skeletons of amino acids are converted into pyruvate or into acetyl coenzyme A.
  • Respiratory quotient (RQ)
    • the ratio of oxygen taken in and carbon dioxide released is 1 : 1.
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    • respiratory quotient (RQ), shows what substrate is being used in respiration. It can also show whether or not anaerobic respiration is occurring.
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    • when respiration is not aerobic
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        • no RQ can be calculated for muscle cells using the lactate pathway, as no carbon dioxide is produced
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• Respiration without oxygen
  • When free oxygen is not present, hydrogen cannot be disposed of by combination with oxygen
    • There are two different anaerobic pathways that solve the problem of ‘dumping’ this hydrogen-- Both pathways take place in the cytoplasm of the cell.
  • alcoholic fermentation
    • In various microorganisms such as yeast, and in some plant tissues, the hydrogen from reduced NAD is passed to ethanal (CH3CHO)
      • 1.pyruvate is decarboxylated to ethanal;2. the ethanal is reduced to ethanol (C2H5OH) by the enzyme alcohol dehydrogenase
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  • lactic fermentation
    • In other microorganisms, and in mammalian muscles when deprived of oxygen, pyruvate acts as the hydrogen acceptor and is converted to lactate by the enzyme lactate dehydrogenase
      • the NAD is released and allows glycolysis to continue in anaerobic conditions
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  • the products of anaerobic reaction, ethanol or lactate, are toxic, the reactions cannot continue indefinitely
    • The ethanol pathway cannot be reversed and the remaining chemical potential energy of ethanol is wasted
    • The lactate pathway can be reversed in mammals;is carried by the blood plasma to the liver and converted back to pyruvate(20%--is oxidated by the liver to carbon dioxide and water via aerobic respiration when oxygen is available;other-- is converted by the liver to glycogen)
  • the oxygen debt
    • This post-exercise uptake of extra oxygen(When exercise stops, the person continues to breathe deeply and absorb oxygen at a higher rate than when at rest)
      • The oxygen is needed for:1.conversion of lactate to glycogen in the liver.2.reoxygenation of haemoglobin in the blood.3.a high metabolic rate, as many organs are operating at above resting levels.
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      • lipids -- carbon atoms are removed in pairs, as acetyl coenzyme A, from the fatty acid chains and fed into the Krebs cycle
• Mitochondrial structure and function
  • the site of the Krebs cycle and the electron transport chain
    • rod-shaped or filamentous organelles; 0.5–1.0 μm in diameter; not rigid,can change their shape
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    • 子主题
  • structure
    • an envelope of two phospholipid membranes(outer-smooth;inner-much folded; form cristae--a large total surface area)
      • in general, mitochondria from active cells have longer, more densely packed cristae
  • Function
    • The two membranes have different compositions and properties
      • The outer membrane
        • relatively permeable to small molecules
      • The inner membrane
        • studded with the enzyme ATP synthase, about 9 nm in diameter;the site of the electron transport chain and contains the proteins necessary for this.
      • The space
        • a lower pH than the matrix of the mitochondrion as a result of the protons that are released into the intermembrane space by the activity of the electron transport chain
    • The matrix of the mitochondrion
      • the site of the link reaction and the Krebs cycle; contains small (70 S) ribosomes and several identical copies of looped mitochondrial DNA
  • ATP is formed in the matrix by the activity of ATP synthase on the cristae
    • The energy comes from the proton gradient between the intermembrane space and the matrix. The ATP can be used for both inside and outside the mitochondrion
• Work
  • Work in a living organism includes
    • 1.the synthesis of complex substances from simpler ones (anabolic reactions)
    • 2.the active transport of substances against a diffusion gradient
    • 3.mechanical work such as muscle contraction and other cellular movements
    • 4.in a few organisms, bioluminescence and electrical discharge
  • thermal energy
    • ectotherms --The thermal energy that warms them comes from outside their bodies.
    • endotherms -- releasing enough thermal energy within their bodies; maintain a constant body temperature through negative feedback loops
  • In the complete oxidation of glucose (C6H12O6) in aerobic conditions, a large quantity of energy is made available:
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      • Multi-step reactions allow precise control via feedback mechanisms
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    • the reaction does not happen easily
      • the activation energy that has to be added before any reaction takes place
        • the activation energy is overcome by lowering it using enzymes , and also by raising the energy level of the glucose by phosphorylation
    • a much more flexible system actually occurs in which energy-yielding reactions in all organisms are used to make an intermediary molecule, ATP.
• The need for energy in living organisms
  • The process of photosynthesis transfers light energy to chemical potential energy, and so almost all life on Earth depends on photosynthesis
    • Photosynthesis supplies living organisms with two essential requirements: an energy supply and usable carbon compounds.
  • All living organisms therefore need a source of carbon
    • autotrophs --Organisms that can use an inorganic carbon source in the form of carbon dioxide
    • heterotrophs --Those needing a ready-made organic supply of carbon
      • depend on autotrophs for both materials and energy
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  • Organic molecules can be used by living organisms in two ways
    • as ‘building bricks’ for making other organic molecules
    • represent chemical potential energy that can be released by breaking down the molecules in respiration
• 1.outline the need for energy in living organisms 2.describe the role of ATP as the universal energy currency 3.explain how energy from complex organic molecules is transferred to ATP in the process of respiration 4.compare the different energy values of respiratory substrates 5.describe how to use simple respirometers to calculate respiratory quotients (RQs) or to measure the effect of temperature on the rate of respiration 6.use a redox dye to investigate respiration in yeast 7.explain how the structure of a mitochondrion fits it for its functions 8.compare how respiration takes place in aerobic and anaerobic conditions 9.describe how rice plants are adapted for growing with their roots submerged in water