5.2.2 Respiration

ATP

ATP

Adenosine triphosphate

ADP

adenosine diphosphate

why do living organisms need to respire?

it releases the energy stored in organic molecules such as glucose

this energy is immediately used to synthesis molecules of ATP from ADP and inorganic phosphate

the ATP can then be hydrolysed to release energy needed to drive biological processes

all living organisms respire to obtain energy

so why do living organisms need energy 1

energy is the capacity to do work

energy is sorted in complex organic molecules is potential energy

it is also chemical energy converted from light by photosynthesis

when this energy is released it can be used to drive biological processes such as;

active transport

endocytosis/exocytosis

synthesis of large molecules such as collagen

DNA replication

cell division

movement

activation of chemicals- glucose is phosphorylated at the beginning of respiration so that it become more reactive and able to be broken down to release more energy

so why do living organisms need energy 2

metabolic reactions are the collective name for all the chemical reactions inside living cells

there are two types; anabolic and catabolic

when larger molecules are made from smaller it is anabolic

when smaller molecules are made by the hydrolysis of larger ones it is catabolic

the role of ATP

ATP is the intermediary between energy-releasing and energy consuming metabolic reactions

ATP is relatively stable in solutions in cells, however, it can be readily hydrolysed by enzyme catalysis

the energy releasing hydrolysis of ATP is coupled with an energy consuming metabolic reaction

this makes ATP an immediate energy source

when ATP is hydrolysed to ADP and Pi, a small quantity of energy is released

this means the cell obtains energy in small manageable amounts

ATP is often referred to as the universal energy currency as all living things use it

some energy is released by hydrolysis of ATP as heat

whilst this may appear wasteful it helps keep living organisms warm and helps with enzyme reactions

respiration formula

C₆H₁₂0₆+60₂ → 6CO₂+6H₂O + 32 ATP

respiration definition

respiration is a series of reactions in which energy is transferred from organic compounds, such as carbohydrates, to the temporary energy store, ATP

glycolysis

the first stage of respiration

a biochemical pathway that occurs in the cytoplasm of all living organism that respire, including prokaryotes

it is anaerobic as it doesn’t require oxygen

oxidative phosphorylation

the formation of ATP using energy released in the electron transport chain and in the presence of oxygen. it is the last stage in aerobic respiration

4 stages of respiration

glycolysis (in both aerobic and anaerobic respiration), link reaction, Krebs cycle, electron transport chain (only in aerobic respiration)

mitochondria

found in all cell types

found in higher numbers in cells that have higher energy demands#

1um diameter, 10um long

what happens where?

glycolysis = cytoplasm of the cell

link reaction = matrix of the mitochondria

Krebs cycle = also in the matrix

electron transfer chain = utilises proteins found in the membrane of the crista

glycolysis

this is the first stage

it occurs in the cytoplasm

first stage of aerobic respiration and anaerobic respiration

doesn’t need O₂ so this stage is anaerobic

occurs in the cytoplasm of all living cells

there are 2 stages of glycolysis

1. phosphorylation

2. oxidation

it is a pathway involving a sequence of 10 reactions, each catalysed by a different enzyme, some with the help of the coenzyme, NAD

the main points;

1. phosphorylation of glucose to hexose bisphosphate

2. splitting each hexose bisphosphate molecule into two triose phosphate molecules

3. oxidation of triose phosphate to pyruvate

cofactor

enzymes involved in catalysing oxidation and reduction reactions need the help of coenzymes (a type of cofactor) to accept the hydrogen atoms removed during oxidation

NAD is a non-protein molecule that helps dehydrogenase enzymes to carry out oxidation reactions

NAD oxidises substrate molecules during glycolysis, the link reaction and the Krebs cycle

NAD in the living cells synthesised from nicotinamide (vitamin B₃), the five carbon sugar ribose, the nucleotide base adenine and two phosphoryl groups

stage 1- phosphorylation

glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP

glucose is split using water (hydrolysis)

2 molecules of triose phosphate are created and 2 molecules of ATP are used up

as glucose is very stable it needs to be activated before it can be split into two three carbon compounds

one molecule of ATP is hydrolysed and the released phosphoryl group is added to glucose to make hexose monophosphate

another molecule of ATP is hydrolysed and the phosphoryl group is added to the hexose phosphate to form a molecule of hexose bisphosphate. it now had phosphates on carbon 1 and 6

the energy from the hydrolysed ATP activates the sugar

the hexose bisphosphate is then split into two three carbon molecules, triose phosphate. each has a phosphate group attached

stage 2- oxidation

4 ATP are produced, but 2 were used at the start, so there’s a net gain of 2ATP

coenzyme NAD collects the hydrogen ions forming 2 Reduced NAD

the triose phosphate is oxidised (loses hydrogen), forming 2 molecules of pyruvate

the sums

the ned products of glycolysis are pyruvate and reduced NAD

energy is released during this reaction

4 molecules of ADP + Pi converted to 4 molecules of ATP

so net gain of 2 molecules of ATP

2 pairs of hydrogen atoms produced

2 molecules of pyruvate

the fate of pyruvate

this depends on the availability of oxygen

the stages of respiration

the link reaction, the Kerbs cycle and oxidative phosphorylation only happen in aerobic conditions

under aerobic conditions, the pyruvate molecules from glycolysis are actively transported into the mitochondria for the link reaction

in the absence of oxygen, pyruvate is converted in the cytoplasm to lactate or ethanol

cristae

inner highly-folded mitochondrial membrane

mitochondrial matrix

fluid filled inner part of mitochondria

how the structure enables function

the matrix is where the link reactions and the Kerbs cycle takes place

the matrix contains:

-enzymes that catalyse the stages of these reactions

-molecule of the coenzymes NAD and FAD (flavine adenine dinucleotide)

-oxaloacetate- the four-carbon compound that accepts the acetyl group from the link reaction

-mitochondrial DNA- some of which codes for enzymes and other proteins

-mitochondrial ribosomes- structurally similar to prokaryotic ribosomes

the outer membrane

the phospholipid composition of the outer membrane is similar to that of the membranes around other organelles

proteins from channels to allow passage of molecules such as pyruvate into the mitochondrion

the inner membrane

the lipid composition of the inner membrane differs from the outer

this bilayer is less permeable to small ions such as hydrogen

the folds give a large surface area for the electron carries and ATP synthase enzyme embedded in them

the electron carriers are protein complexes arranged in electron transport chains. electron transport chains are involved in the final stage of aerobic respiration

the intermembrane space

this space is also involved in oxidative phosphorylation

the inner membrane is in close contact with the mitochondrial matrix, so the molecules of reduced NAD and FAD can easily deliver hydrogen to the electron transport chain

electron transport chain

each electron carrier protein contains a haem group

the iron ion can accept and donate electrons as it can alternate between Fe₂+ (reducing- gaining an electron) and Fe₃+ (oxidised giving an electron to the next carrier)

these carrier proteins are oxido-reductase enzymes

the electron carriers also have a coenzyme that pumps protons from the matrix to the intermembrane space using energy released from the electrons

this leads to a build up of protons in the intermembrane space and a proton gradient forms

this leads to a flow of protons through the channels in the ATP synthase enzymes to make ATP

decarboxylation

removal of a carboxyl group from substrate molecule

dehydrogenation

removal of hydrogen atoms from a substrate molecule

substrate-level phosphorylation

production of ATP from ADP and Pi during glycolysis and the Krebs cycle

symport

a transport protein that transports two ions or molecules in the same direction

aerobic respiration

if 0₂ is present ₃C pyruvate passes into mitochondria

here it is completely oxidised forming CO₂ and the H₂0

the second stage of aerobic respiration is the link reaction

link reaction- pyruvate

this is transported via a specific pyruvate-H+ symport

pyruvate is converted to a two-carbon acetyl group during the link reaction

the acetyl group is oxidised during the Krebs cycle

the link reaction

links glycolysis to the Krebs cycle

the end product of the link reaction can enter the Krebs Cycle

-one carbon atom is removed from pyruvate in the form of CO₂

-the remaining 2-carbon molecule combines with coenzyme A to produce Acetyl Coenzyme (acetyl CoA)

-another oxidation reaction occurs when NAD+ collects more hydrogen ions. this forms reduced NAD

-no ATP is produced in this reaction

the link reaction occurs twice for every glucose molecule

for each glucose molecule used in glycolysis, two pyruvate molecules are made

but the link reaction uses only one pyruvate molecules, so the link reaction and the Krebs cycle happen twice for every glucose molecule which goes through glycolysis

overall equation for one link reaction

pyruvate + NAD + CoA → acetyl CoA + reduced NAD + CO₂

the products of the link reaction go to the Krebs Cycle and the ETC

so for each glucose molecules:

-2 acetyl coenzyme A (go into the Krebs cycle)

-2 carbon dioxide (released as waste products)

-2 reduced NAD (go to the electron transport chain)