How Cells Obtain Energy From Food Flashcards
(31 cards)
Briefly define cellular respiration.
= process by which sugar molecules get broken down into energy (stored in its chemical bonds) and oxidized into CO2 and H2O
- energy released during this reaction get captured in “high-energy” covalent bonds in activated carriers e.g. ATP, NADH
What is the diference between breaking down sugars with fire as opposed to how the cell does it?
- Fire is able to break the bonds with just one step -> releases significantly more energy than any activated carrier could ever hold (but also would be impossible to do under normal living conditions/temperature)
- Cell does so in a series of organized steps which stores the energy in activated carriers bit by bit
What are the two ways by which cells can produce ATP (just briefly)?
- The unfavourable reaction of ADP+Pi => ATP gets coupled to a favourable reaction of breakdown/oxidation of food = Direct
- Oxidative phosphorylation = process that also need an intermediate where the reaction gets driven by other activated carriers
- note: takes place in the inner mitochondrial membrane
What is catabolism? What is the first stage of catabolitic break down of food?
= process in which complex organic molecules get degraded by enzymes into simpler molecules
- Stage = Digestion
= the breakdown of polymeric molecules (e.g. proteins) into monomeric (e.g. amino acids)- takes place either outside the cells (i.e. intestines) or in specialized organelles (i.e. lysosomes)
=> enters the cell’s cytosol
- takes place either outside the cells (i.e. intestines) or in specialized organelles (i.e. lysosomes)
What is the second stage of catabolitic break down of food?
- Stage
- Glycolysis = series of steps splitting glucose into smaller molecules = pyruvates
- we can use also other sugars if they get converted into some substep within this chain
- produces also some ATP and NADH
-> Pyruvates get transported from the cytosol inside the internal segment of mitochondria (matrix)
-> enzyme complex converts pyruvate into acetyl (CoA) ad CO2 - some acetyl gets also produced by breakdown of fatty acids
- Glycolysis = series of steps splitting glucose into smaller molecules = pyruvates
What is the third stage of catabolitic break down of food?
- Stage
-acetyl group of acetyl is transferred to an oxaloacetate molecule fo form citrate -> enters the citric acid cycle
-> the acetyl group gets oxidized to CO2 + production of large amounts of NADH -> its electrons are passed along an enzymatic chain of mitochonria = elecron-transport chain
-> energy from that is used for oxidative phosphorylation -> produces ATP while consuming O2
NOTE: about half of the energy from glucose is used to fuel ADP+Pi reaction - remaining energy is released as heat
Here is a summary of the whole action:
Where does the word “glycolysis” come from? What happens during it? What are our gains?
- Greek for “glykys” = sweet, “lysis” = splitting
- Takes place in cytosol, breaks 6 carbon glucose into 2 3-carbon pyruvates
- releases energy into carriers (without O2)
- in order to start the splitting - consumption of 2 molecules of ATP
-> at the end of the process we get 4 ATP (gain of 2) and 2 NADH
Here are all the steps of glycolysis:
And enzymes that help out:
NOTE:
- Form of ATP creation with a direct transfer of phosphate group from a substrate (without O2) = substrate-level phosphorylation
- Although O2 is absent oxidation takes place - i.e. there is a transfer of electrons from H2O to NAD+ to form NADH
- when electron is transfered from high to lowe energy they release energy utilized for other reactions
- NADH looses its electrons during the electron-transport chain -> NAD+ returns to cytosol to be used in glycolysis (or elsewhere) again
What if an organism lives in an environment depleted of O2?
= Fermentation
- we call them anaerobic organisms, plus some animal cells can also function without O2 e.g. skeletal muscle cells
- pyruvate and NADH remain in the cytosol
-> pyruvate is converted into a different compound and release from the cell e.g. lactate from mucle cells
-> NADH gives up electrons and becomes NAD+ again needed in the glycolysis
NOTE: there is also anerobic respiration - unlike fermetation it does use electron-transport chain, but instead of O2 it just takes a different molecule
What happens in the 6 and 7 step of glycolysis?
= conversion of 3-carbon sugar intermediate glyceraldehyde 3-phosphate (an aldehyde) into 3-phosphoglycerate (a carboxilic acid)
- oxidation of aldehyde to carboxylic acid
- releases enough energy to transfer 2 electrons from aldehyde to NAD+ (to make NADH) and transfer phosphate group to ADP (example of substrate-level phosphorylation)
- also releases enough heat to make the overall reaction energetically favourable
NOTE: molecules that contain phosphate bonds and have more energy than ATP (are “high energy”) will readily transfer the group (are energetically favourable)
Learn the following steps of glycolysis:
What happens right after glycolysis (and before citric acid cycle)?
- pyruvate from glycolysis is pumped into the matrix of the mitochondria -> rapidly decarboxylated by a complex of enzymes = pyruvate dehydrogenase complex
-> products = CO2 (waste), NADH, acetyl CoA
What else is used as energy source in the mitochondrial matrix?
- Fatty acids are also converted into acetyl CoA
- broken down by a cycle of reactions that trims two carbons at a time from their carboxyl end -> generating one molecule of CoA for each turn of the cycle
- produces two activated carriers i.e. NADH, FADH2 (another high-energy electron carrier)
NOTE: additionally, some amino acids can also be taken up into mitochondria and converted to either acetyl or an intermediate of the citric acid cycle
How else can we call citric acid cycle? Where do we find it? What are it’s products?
And does it require O2 or NOT?
- other name: Krebs cycle
- location = mitochondrial matrix
- products = CO2 (waste), NADH -> passes its electrons into the electron-trasport chain (which will eventually combine O2 to produce H2O)
- Requires to proceed O2 although NOT directly - it doesn’t use it within itself BUT O2 is the final acceptor of electrons in the electron-transport chain which allowd NADH to loose the electrons and return as NAH+ into the citric acid cycle
What exactly happens within the cycle?
Complete oxidation of carbon atoms of the acetyl groups in acetyl CoA -> which converts it into CO2
- acetyl -> large 4-carbon molecule = oxaloacetate -> 6-carbon tricarboxylic acid = cytric acid -> citrid acid molecule (=citrate) is progressively oxidized
-> energy is harnessed to produce activated carriers
- 8 reactions forming a cycle as oxaloacetate (start) is regenerated at the end
What activated carriers are produced in citric acid cycle?
- NADH
- FADH2 (reduced flavin adenine dinucleotide) - also transfers readily available high-energy electrons (during the electron-transport chain)
- GTP (guanosine triphosphate) from GDP
- transfer of its phosphate group can produce ATP
What is a common misconception about the citric acid cycle?
That the O2 required for the function of the cycle is directly used to produce the waste product CO2
- NOT the case, the O2 we breath is reduced into water during the electron-transfer chain
- while the O2 that forms CO2 actually comes from water (3 molecules of H2O are split each round of the cycle -> ultimetely CO2)
Look at the full scheme of the citric acid cycle:
Now look at the specific steps of the cycle:
What else does the glycolysis and citric acid cycle engage in?
Anabolic pathways = chemical processes in which series of enzymatic steps intermediates of these mechanisms are converted into amino acids, necleotides, lipids, and other small organic molecules
- E.g. oxaloacetate and alpha-ketoglurate from Krebs cycle are transfered into the cytosol wehere they function as precursors for aspartate and glutamate
Define again oxidative phosphorylation. How does it work?
= stage at which the chemical energy captured by the activated carriers (generated in the previous phases) is used to produce ATP
- NADH and FADH2 transfer their high-energy electrons to the electron-transport chain = series of electrons embedded in the inner mitochondrial membrane
-> by the series of acceptor and donor actions the electrones fall to lower-energy states
-> at specific sites the energy released is used to drive H+ across the inner membrane from mitochondrial matrix to the intermembrane space
-> generation of proton gradient across inner membrane
-> drives variaty of reactions there
-> mainly ATPase
-> at the end electrons are added to O2 that diffused into the mitochondrion to become H2O
NOTE: about 30 molecules of ATP are produced per glucose
What is a “problem” that need to be overcome for cells to survive in terms of anabolic and catabolic pathways?
- To maintain order within cells, the cell needs to keep on producing ATP -> however, animals don’t eat all the time, or plants cannot photosynthesize during the night
- they could for instance, synthesize food reserves when there is plenty to be prepared for scarce resources
-> at any point the cell needs to be able to decide whether to use molecules in anabolic or catabolic pathway
- e.g. pyruvate is used in many different pathways - so where should it go at any given point?
How does the cell choose between catabolic and anabolic reactions?
The cell utilizes an elaborate network of control mechanisms coordinating activity of enzymes that catalyze or synthetize
- e.g. done by covalent modification such as addition or removal of phosphate group
