Plant Respiration and Energy

• All living organisms, including plants and microbes, breathe to live

• Living organisms need energy for activities like absorption, transport, movement, and reproduction

• Energy for life processes comes from the oxidation of macromolecules, called food

• Green plants and cyanobacteria make their own food through photosynthesis, converting light energy into chemical energy stored in carbohydrates like glucose, sucrose, and starch

• Not all cells in green plants photosynthesize, only those with chloroplasts, mostly in superficial layers

• Non-green plant parts need food for oxidation, so it is translocated

• Animals are heterotrophic and obtain food from plants

• All food respired for life processes originates from photosynthesis

• Cellular respiration breaks down food materials to release energy, trapping it for ATP synthesis

• Photosynthesis occurs in chloroplasts while the breakdown of complex molecules happens in the cytoplasm and mitochondria

• Respiration breaks C-C bonds in cells through oxidation, releasing energy

• Respiratory substrates are compounds oxidized, usually carbohydrates, but also proteins, fats, and organic acids

• Energy from oxidation is released in enzyme-controlled steps and trapped as ATP

• ATP acts as the energy currency of the cell, broken down when energy is needed

• Carbon skeletons from respiration are used to synthesize other molecules in the cell

• The chapter will discuss cellular respiration

Do Plants Breathe?

• Plants need O₂ for respiration and release CO₂

• Plants have systems to ensure O₂ availability, like stomata and lenticels, but lack specialized respiratory organs

• Plant parts handle their own gas exchange needs, with little gas transport between parts

• Plants have lower gas exchange demands than animals

• Large gas volumes are exchanged during photosynthesis, and each leaf manages its needs

• Photosynthesizing cells release O₂ internally, while non-photosynthesizing cells obtain O₂ from the air

• Each plant cell is close to the surface for gas diffusion

• Thick, woody stems have living cells in thin layers with lenticels, and inner cells are dead

• Loose packing of parenchyma cells in leaves, stems, and roots creates interconnected air spaces

• Complete combustion of glucose yields CO₂, H₂O, and heat C6H12O6 + 6O₂ → 6CO₂ + 6H₂O + Energy

• Plant cells catabolize glucose to synthesize other molecules, preventing all energy from being released as heat

• Glucose is oxidized in small steps, coupling energy release to ATP synthesis

• During respiration, O₂ is utilized, and CO₂, water, and energy are released

• Combustion requires O₂, but some cells live where O₂ may not be available

• The first cells likely lived in an oxygen-free atmosphere, and some organisms are adapted to anaerobic conditions, such as facultative or obligate anaerobes

• All living organisms retain enzymatic machinery to partially oxidize glucose without oxygen

• This breakdown of glucose to pyruvic acid is called glycolysis

Glycolysis

• Glycolysis comes from the Greek words for sugar and splitting
• The glycolysis scheme, given by Gustav Embden, Otto Meyerhof, and J. Parnas, is called the EMP pathway
• In anaerobic organisms, it is the only process in respiration
• Glycolysis occurs in the cytoplasm of the cell, and is present in all living organisms
• Glucose undergoes partial oxidation to form two molecules of pyruvic acid
• In plants, glucose is derived from sucrose, which is the end-product of photosynthesis or storage carbohydrates
• Sucrose is converted into glucose and fructose by invertase
• Glucose and fructose are phosphorylated to glucose-6-phosphate by hexokinase
• This phosphorylated glucose then isomerizes to produce fructose-6-phosphate
• Subsequent metabolism steps of glucose and fructose are the same
• Glycolysis involves ten reactions, controlled by different enzymes, to produce pyruvate from glucose
• Be aware of the locations where ATP utilization or synthesis, or NADH + H+, occurs
• ATP is utilized twice: in the conversion of glucose into glucose 6-phosphate and fructose 6-phosphate to fructose 1, 6-bisphosphate
• Fructose 1, 6-bisphosphate is split into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL)
• NADH + H+ is formed when 3-phosphoglyceraldehyde (PGAL) is converted to 1, 3-bisphosphoglycerate (BPGA)
• Two redox-equivalents are removed from PGAL and transferred to NAD+
• PGAL is oxidized and turns into BPGA with inorganic phosphate
• The conversion of BPGA to 3-phosphoglyceric acid (PGA), is also an energy yielding process
• Another ATP is synthesized during the conversion of PEP to pyruvic acid
• Pyruvic acid is the key product of glycolysis

Fermentation

• There are three major ways cells handle pyruvic acid produced by glycolysis: lactic acid fermentation, alcoholic fermentation, and aerobic respiration
• Fermentation occurs under anaerobic conditions in many prokaryotes and unicellular eukaryotes
• Aerobic respiration (Krebs' cycle) is used for complete glucose oxidation to CO₂ and H₂O, requiring O₂
• In fermentation (by yeast), incomplete glucose oxidation happens under anaerobic conditions where pyruvic acid is converted to CO₂ and ethanol through reactions catalyzed by pyruvic acid decarboxylase and alcohol dehydrogenase
• Some bacteria produce lactic acid from pyruvic acid
• Muscles during exercise convert pyruvic acid to lactic acid due to inadequate oxygen for cellular respiration
• Lactate dehydrogenase reduces pyruvic acid to lactic acid
• The reducing agent is NADH+H+ which is reoxidized to NAD+ in both processes
• In both lactic acid and alcohol fermentation, little energy is released (less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP)
• The processes are hazardous because they produce either acid or alcohol
• Yeasts poison themselves to death when the alcohol concentration reaches about 13 per cent

Aerobic Respiration

• Eukaryotes need to synthesize a larger number of ATP molecules for cellular metabolism in the mitochondria, requiring O₂
• Aerobic respiration completely oxidizes organic substances with oxygen, releasing CO₂, water, and a large amount of energy
• For aerobic respiration, pyruvate is transported from the cytoplasm into the mitochondria

Key events:

• Complete oxidation of pyruvate through stepwise removal of all hydrogen atoms, producing three molecules of CO₂

• Passing removed electrons to molecular O₂ with simultaneous ATP synthesis

• The first process occurs in the matrix and the second on the inner membrane of the mitochondria

• Pyruvate, from glycolysis, undergoes oxidative decarboxylation in the mitochondrial matrix, catalyzed by pyruvic dehydrogenase

• The reactions require coenzymes, including NAD+ and Coenzyme A

• Two molecules of NADH are produced from the metabolism of two pyruvic acid molecules

• The acetyl CoA enters the tricarboxylic acid cycle (Krebs’ cycle), after Hans Krebs

• The TCA cycle starts with acetyl group condensation with oxaloacetic acid (OAA) and water, yielding citric acid due to citrate synthase

• CoA is released

• Citrate is then isomerized to isocitrate, followed by two decarboxylation steps, to produce a-ketoglutaric acid and then succinyl-CoA

• In the remaining steps of the citric acid cycle, succinyl-CoA is oxidized to OAA, allowing the cycle to continue

• During the conversion of succinyl-CoA to succinic acid, a molecule of GTP is synthesized (substrate level phosphorylation)

• In a coupled reaction, GTP converts to GDP with the synthesis of ATP from ADP

• There are three points in the cycle where NAD+ is reduced to NADH + H+ and one point where FAD+ is reduced to FADH₂

• Continued acetyl CoA oxidation requires oxaloacetic acid replenishment and regeneration of NAD+ and FAD+ from NADH and FADH₂

• We have now seen that glucose has been broken down to release CO₂ and eight molecules of NADH + H+; two of FADH₂ have been synthesised besides just two molecules of ATP in TCA cycle

Electron Transport System (ETS) and Oxidative Phosphorylation

• The succeeding steps in the respiratory process are to release and utilise the energy stored in NADH+H+ and FADH₂

• This happens when they are oxidised through the electron transport system and the electrons are passed on to O₂ resulting in the formation of H₂O

• The metabolic pathway through which the electron passes from one carrier to another, is termed the electron transport system (ETS)

• ETS is present in the inner mitochondrial membrane

• NADH produced in the mitochondrial matrix is oxidized by NADH dehydrogenase (complex I), transferring electrons to ubiquinone

• Ubiquinone also receives reducing equivalents via FADH₂ (complex II)

• Reduced ubiquinone (ubiquinol) is oxidized, transferring electrons to cytochrome c via cytochrome bc₁ complex (complex III)

• Cytochrome c is attached to the outer surface of the inner membrane and is a mobile carrier

• Complex IV (cytochrome c oxidase complex) contains cytochromes a and a₃ and two copper centers

• Electrons pass from one carrier to another via complex I to IV in the electron transport chain, coupled to ATP synthase (complex V) for ATP production from ADP and inorganic phosphate

• The number of ATP molecules synthesized relies on the nature of the electron donor

• Oxidation of NADH yields 3 ATP while FADH₂ yields 2 ATP

• Aerobic respiration has a limited role of oxygen in the terminal stage

• Oxygen drives the whole process by removing hydrogen from the system

• Oxygen acts as the final hydrogen acceptor

• The proton gradient required for phosphorylation needs energy of oxidation-reduction, thus it is known as oxidative phosphorylation

• Energy released during electron transport is utilized to produce ATP

• System of electron carriers is called electron transport system (ETS) located on the inner membrane of the mitochondria

• As the electrons move through the system, enough energy that are trapped is used to synthesise ATP, termed oxidative phosphorylation

• In this process, O₂ is the ultimate acceptor of electrons and is it reduced to water

The Respiratory Balance Sheet

• It is possible to make calculations of the net gain of ATP for every glucose molecule oxidised; but in reality this can remain only a theoretical exercise
• These calculations can be done only on these assumptions:
• There should be a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS pathway following one after another
• The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation
• None of the intermediates in the pathway are utilised to synthesise any other compound
• Only glucose is being respired – no other alternative substrates are entering in the pathway at any of the intermediary stages Yet, it is useful to do this exercise to appreciate the beauty and efficiency of the living system in extraction and storing energy.
• Hence, there can be a net gain of 38 ATP molecules during aerobic respiration of one molecule of glucose

Amphibolic Pathway

• Glucose is favored for respiration and is usually converted from carbohydrates
• Other substrates can be respired, entering the respiratory pathway at other steps
• Fats break down into glycerol and fatty acids
• Fatty acids degrade into acetyl CoA, and glycerol converts to PGAL
• Proteases degrade proteins into amino acids, entering the Krebs cycle as pyruvate or acetyl CoA
• Respiration is usually considered catabolic, breaking down substrates
• Respiratory pathway can have different substrates along with differing points of entry to be respired and gain energy
• Respiratory pathway is also used for the synthesis of substrates
• Acetyl CoA is withdrawn from the respiratory pathway when fatty acids are being synthesized
• Respiratory intermediates are used for both breakdown and synthesis
• Since the respiratory pathway involves anabolism and catabolism, it is an amphibolic pathway

Respiratory Quotient

• During aerobic respiration, O₂ is consumed and CO₂ is released
• The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed RQ = Volume of CO₂ evolved / Volume of O₂ consumed
• When carbohydrates are utilized completely; the RQ is 1 because equal amounts of CO₂ and O₂ are evolved and consumed
• When fats are used in respiration, the RQ is less than 1
• When proteins are respiratory substrates the ratio would be about 0.9