Powering the cell Flashcards
Prokaryotic and eukaryotic fuelled
Cells need fuel to sustain their activities – growth, production of macromolecules, other cellular activities
To fuel cell needs 2 things – carbon source (organic or inorganic) and a means of capturing energy from chemical reactions or light to produce usable energy
Heterotrophs – carbon source from organic sources
Autotrophs – use co2 as a carbon source
Chemoheterotrophs – energy source and carbon source from organic sources. Consume organic building blocks that they are unable to make themselves. Most of their energy is from organic molecules such as sugars. Very common among eukaryotes, including humans
Photoheterotrophs – capture light energy to convert to chemical energy in the cells, get carbon from organic sources (other organisms) – e.g purple non-sulphur bacteria, heliobacteria
Chemoautotrophs – break down inorganic molecules to supply energy for the cell, use co2 as a carbon source. Prokaryotes break down hydrogen sulphide (H2S) and ammonia (NH4). Many chemoautotrophs also live in extreme environments such as deep sea vents (extremophiles)
Photoautotrophs – capture light energy, use co2 as carbon source – e.g cyanobavteria. Use similar compounds to those of plants to trap light energy
How ATP stores energy
Capacity to harvest, store and use energy is a universal feature of all cells (pro and euk) inherited from LUCA
Energy is conserved intracellularly in the energy-rich phosphate bond of ATP
ATP can transfer its high energy phosphate bonds to GTP, CTP and UTP
Processes to generate ATP
- Aerobic oxidation – oxidation of glucose, oxidation of fatty acids (beta oxidation)
- Anaerobic metabolism of glucose (fermentation)
- Photosynthesis
- Electron transport + proton motive force (oxidative phosphorylation) – anaerobes, chemoautotrophs
ATP is generated by the adding of a phosphate to ADP (phosphorylation)
When electrons are lost from a donor (ox) in a reaction involving chemical substrates or light the energy released is harvested to phosphorylate ADP and generate ATP. This is because electrons exist at different energy levels and movement from one energy level to a lower energy level releases energy
Oxidative phosphorylation = electron moves from a high energy level in a chemical molecule (electron donor) to a lower energy level in another molecule (electron acceptor), when one substrate gains an electron, another loses one
Photophosphorylation = light is used to excite photosynthetic pigments to move the electron to higher energy state. This excited state is unstable and the electron is transferred a lower energy level in another molecule (electron acceptor)
Reduction – substrate gains electrons
Oxidation – substrate loses electrons
Reducing power
Potential of a substance to reduce another substance. Can be either by loss or gain of electrons by addition or removal of hydrogen
Transfer of hydrogen
- Electrons in an organic redox reaction often are transferred in the form of a hydride ion – a proton and two electrons
- Because they occur in conjunction with the transfer of a proton, these are commonly referred to as hydrogenation and dehydrogenation reactions
- Dehydrogenation – a substrate loses electrons and protons (H+) simultaneously and they are transferred to an H-acceptance molecule – e.g NAD+ or NAD(P)+
How reducing power is used
- NADH+ can be oxidised to NAD+. By extracting the electrons (ox) of NADH+ and transporting these electrons sequentially through several membrane proteins (etc) a proton gradient is generated across the cell membrane, which is then used to generate energy
Energy calculations
Amount of usable energy released or consumed in a reaction can be calculated as the change in Gibbs free energy
J = Joules, measure of mechanical work that can be performed with energy
G pos = reaction consumed energy, endergonic reaction, formation of molecules is endergonic
G neg = reaction released energy, exergonic reaction, breakdown of molecules is exergonic
How much energy is needed to make reducing power
- NAD+ + 2H+ + 2e- = NADH+ + H+ (G = 219.25 kJ/mol)
-Only those exergonic reactions that release at least 219 kJ/mol can be used to generate reducing power (NADH+)
What reducing power is used for
Cell can use NADH+ to ultimately phosphorylate ADP and generate ATP
ADP to ATP requires 31 kJ/mol – about 7 ATPs can be generated with one NADH+
Accessing power in lab - bomb calorimeter
Accessing power in lab – bomb calorimeter
Lab instrument used to measure the amount of a samples combustion heat or heat power when excess o2 combustion occurs
Used to work out the amount of energy stored in food by heating the food until it burns
Excess heat released by the reaction is directly proportional to the amount of energy contained in the food
Oxidation of glucose
Glucose metabolised to CO2 and H2O and ATP is generated
1. Glycolysis = metabolic pathway that converts glucose into pyruvic acid
- During glycolysis, one molecule of glucose is converted into 2 molecules of pyruvate
- Early in the glycolytic pathway, 2 ATP molecules are consumed
- In the payoff phase energy is produced (4 ATP, 2 NADH, H+)
- 10 water soluble cytosolic enzymes catalyse the reactions of the glycolytic pathway
2. Pyruvate oxidation and citric acid cycle
- In eukaryotes = pyruvate is transported from the cytosol across the mitochondrial membranes to the matrix, pyruvate reacts with CoA forming co2 and acetyl CoA, acetyl group of acetyl CoA is oxidised to co2 via a set of 9 reactions in citric acid/krebs cycle, for each acetyl group entering the cycle as acetyl CoA, two molecules of co2 produced
3. Electron transport and proton motive force
- Eukaryotes = localised in the internal membrane of the mitochondria and the chloroplasts
- Prokaryotes = electron transport chains associated with cell
- Transporters in the inner mitochondrial membrane allow the uptake of electrons from cytosolic NADH
- Most of the free energy released during the oxidation of glucose to co2 is retained in the reduced coenzymes NADH and FADH2 generated during glycolysis and the citric acid cycle
- ETC = cytosolic NADH+ and FADH2 generated during glycolysis and the citric cycle is oxidsed to NAD+ and FAD+, electrons transferred from NADH+ and FADH2 through a series of electron carriers (multiprotein complexes) in the inner membrane of the mitochondria (eukaryotes) or the plasma membrane (prokaryotes), step-by-step transfer of electrons via the etc allows the free energy in NADH+ and FADH2 to be released in small increments and stored as the proton-motive force
- Proton motive force occurs when the cell membrane becomes energised due to electron transport reactions by the electron carriers embedded in it, protons (10 total) are pumped at several sites during electron transport from NADH to o2 (or other electron acceptor), protons from the mitochondrial matrix are pumped across the inner mitochondrial membrane or the plasma membrane creating a proton concentration gradient across the membrane
- ATP synthase = an enzyme complex that uses proton motive force (proton gradient) to generate ATP from ADP – oxidative phosphorylation (production of ATP via electron transport and pmf), prokaryotes = in plasma membrane, mitochondrion = in inner membrane, chloroplast = in thylakoid membrane, membrane protein formed of 2 subunits – F0 and F1, catalyses ATP synthesis on the cytosolic face of the membrane – ATP always formed on cytosol
- F0 is hydrophobic and is embedded int the membrane
- F1 is exposed to the cytosol
- F0 has a cavity that takes in the protons – lower portion rotates in response to ions moving down the concentration gradient across the membrane [H+] exoplasmic greater than [H+] cytosolic
- Rotation of the lower component of F0 activates F1 that binds and phosphorylates ADP into ATP
Oxidation of fatty acids (beta-oxidation)
- TAGs hydrolysed
- Fatty acids stored as triacylglycerols (TAG) primarily as lipid droplets in eukaryotes or as free TAG in the cytoplasm of prokaryotic cells
- Triacylglycerols are hydrolysed in the cytosol to free fatty acids and glycerol - Formation of fatty acyl-coA
- Transportation of fatty acyl-coAs (carnitine transporter)
- Fatty acyl-coAs transported into mitochondria and/or peroxisomes in eukaryotes, stay in cytosol in prokaryotes - Oxidation of fatty acyl-coAs
- Fatty acyl-coAs can be oxidsed by mitochondrial oxidation, peroxisomal oxidation
- Convert fatty acyl-coA molecule to acetyl coA and a shorter fatty acyl-coA (liberating a two-carbon unit) occurs in a sequence of four enzyme-catalysed reactions
- One FAD molecule is reduced to FADH2 and one NAD+ molecule is reduced to NADH
- Cycle is repeated on the shortened acyl-coA until this is completely converted to acetyl-coA
- In mitochondria acetyl-coA enters the citric acid cycle
- In peroxisomes oxidation of fatty acids yields no ATP - Electron transport and proton motive force
- Electrons from FADH2 and NADH+ are used in the etc generating the pmf and resulting in synthesis of additional ATP
Classification of organisms according to their mode of respiration
lassification of organisms according to their mode of respiration
1. Aerobe organisms (or strict/obligate aerobe)
- Use o2 for respiration to produce ATP (o2 last electron acceptor in etc and reduced to h2o)
- Die of no o2 available
2. Anaerobe organisms (or strict/obligate anaerobe)
- Use another molecule than o2 for respiration – no3^-, so4^2-
- Perform anaerobic respiration to produce ATP = a molecule, different from o2, is the last electron acceptor in etc
- Can ferment (anaerobic metabolism of glucose) to produce ATP
- Die in presence of o2
3. Facultative (optional) aerobe or facultative anaerobe
- Use o2 for respiration if available
- If o2 not available use fermentation or perform anaerobic respiration to obtain ATP
Microbes and oxygen
Oxygen highly reactive and some byproducts toxic. Ability of microbes to withstand o2:
- Depends on ability to breakdown the byproducts of oxygen metabolisms
- Some of these byproducts, ROS (reactive oxygen species), are h2o2 and o2^- (superoxide anion)
- Organisms that can live in the presence of oxygen have projective enzymes against ROS – e.g superoxide dismutases and catalases
If oxygen scarce
Most eukaryotes can generate some ATP by anaerobic metabolism
Few eukaryotes are facultative anaerobes – grow in either the presence or absence of o2
How cells generate ATP in absence of O2
- Ferment (prokaryotic and eukaryotic)
- In the absence of o2, facultative anaerobes convert glucose to one or more carbon compounds which are generally released into the surrounding medium
- It is not anaerobic respiration, does not use etc - Perform anaerobic respiration (mostly pro)
- Alternative terminal electron acceptor (organic or inorganic molecules)
- Transfer of electrons via the etc allows the free energy in NADH and FADH2 to generate a pmf
- Pmf fuels ATP synthase to phosphorylate ADP to ATP
- Sometimes pmf generated by o2 consuming H+ in cytoplasms
- Not all the alternative electron acceptors harness the same amount of energy
- High redox potential = more ATP
Fermentation different from anaerobic respiration
Fermentation = anaerobic metabolic process that consumes sugars in the absence of o2, products are organic acids, alcohol and gases
Fermentation generates less energy than anaerobic respiration
- ATP only produced in glycolysis , yielding 2
- Complete anaerobic breakdown of glucose to co2 and h2o gives 38 ATP (36 net ATP)
- Anaerobic respiration uses the etc but produces somewhat less ATP than aerobic respiration but more than fermentation
Fermentation uses intracellular electron carriers while anaerobic respiration uses membrane-bound electron carriers
Chemoautotrophs and photoautotrophs
Chemo – organisms that obtain energy by oxidising reduced inorganic compounds in the environment to obtain reducing power (needed for respiration and ATP production) and use co2 as a source of carbon
Photo – organisms that obtain energy from light (photophosphorylation) and use co2 as source of carbon
Photosynthesis
- Light energy converted to chemical energy that drives synthesis of organic molecules
Solar radiation drives reduction of co2 to produce carbohydrates, fats and proteins
- Thylakoid membranes site in plants and photosynthetic bacteria
- Thylakoids in bacteria = extensive invaginations of the plasma membrane form a set of internal membranes
