Cellular Respiration and Photosynthesis: An Overview

Posted on Aug 23, 2024 in Biology

Universal Energy Currency

ATP acts as a ‘Uniercersal Energy Currency’ as it transfers energy to biochemical reactions in all living organisms.

Hydrolysis of ATP

Energy release = hydrolysis of ATP > ADP + inorganic phosphate (Pi)

Exergonic reaction catalysed by ATPase = removal of terminal phosphate

Always coupled with an endergonic reaction where energy is transferred

Soluble molecule which can be transported within a cell but can’t leave it

Transfers energy > processes requiring energy: nervous impulse/muscle contraction/Active Transport

Releases small amounts of energy (30.6kjmol) closely matched to that required in the coupled reaction

Energy transferred quickly as only 1 enzyme needed for hydrolysis

Formation of ATP

ADP + Pi > ATP via condensation = endergonic using energy from cellular respiration/ transduction of light energy from photosynthesis using ATPsynthetase

Cellular Respiration

Glycolysis (Cytoplasm)

Phosphorylation of Glucose [6c]>Hexose Bisphosphate [6c]

2ATP needed > activates the glucose = more reactive

Hexose Bisphosphate splits into 2TP [3c]

2TP is oxidised via dehydrogenation > 2Pyruvate [3c]

2NAD > NADH2 (reduced NAD) = Exergonic

This energy is used to synthesise 4 ATP via substrate-level phosphorylation

(using energy from breakdown of a high energy substrate molecule)

Net gain of 2ATP

Link Reaction (Mitochondrial Matrix)

Pyruvate > Acetate [2c] via oxidative decarboxylation

1. NAD > NADH2 + loss of CO2

Acetate[2c] + CoenzymeA > AcetylCoA [2c] (Occurs twice per glucose molecule)

Krebs Cycle (Mitochondrial Matrix)

AcetylCoA releases Acetate [2c] which combines with a [4c] Acid>A [6c] Acid

[6c] Acid > [5c] compound via decarboxylation (loses CO2) and dehydrogenation (1NAD > 1NADH2)

[5c] compound > back to [4c] Acid by decarboxylation (loses CO2)

2NAD > 2NADH2 + 1FAD > 1FADH2

1ADP+Pi > 1ATP via substrate-level phosphorylation

Electron Transport Chain (ETC) (Inner Mitochondrial Membrane)

Cristae provide large SA = more e- carriers = increased ATP synthesis

Reduced coenzymes NADH2 & FADH2 act as a source of e- & H+

Each NADH2 molecule produces 3ATP while FADH2 produces 2ATP (only 2H+ pumps involved)

As e- move along the chain of e- carriers, energy they release is used to synthesise ATP via oxidative phosphorylation (using energy released from redox reactions)

Energy from the e- is used to pump H+ from matrix via the inner-mitochondrial membrane > intermembrane space

A proton gradient is produced across the inner mitochondrial membrane which is impermeable to H+

H+ diffuse down this gradient into the matrix via the chemiosmotic protein channel attached to ATPsynthetase

This flow of H+ provides the energy to synthesise ADP+Pi > ATP

Oxygen is the final e- acceptor (1/2O2)+(2H+)+(2e-)>H2O this allows FAD & NAD to be regenerated

Net ATP: (Total = 38)

Anaerobic Respiration (Cytoplasm)

ETC can’t occur as NADH2/FADH2 aren’t made as they aren’t oxidised by O2

Thus dehydrogenation can’t occur on Link reaction or Krebs Cycle so only glycolysis takes place

Lactate Fermentation (Animals)

2pyruvate enters a different pathway & is reduced by NADH2 >2Lactate

2NAD is then recycled to oxidise 2TP>2Pyruvate where 4ADP+Pi (2ATP per TP)

Alcohol Fermentation (Plants/Fungi)

2Pyruvate is decarboxylated (loses 2co2) >Ethanal which is reduced by 2NADH2 >2Ethanol

2NAD is recycled to oxidise 2TP >Pyruvate (4ADP+Pi >4ATP)

Other Respiratory Substrates

-Other respiratory substrates e.g. fats, carbs, proteins as reserves

Glycerol> TP for glycolysis

Long-chain fatty acids split into [2c] fragments & enter pathways as AcetylCoA

During starvation, tissue protein are hydrolysed >amino acids which are deaminated (loses nh2) into an organic acid which is then used in the Krebs Cycle

Photosynthesis

Light Energy > Chemical Energy

Photosynthetic pigments e.g. chlorophyll/carotenoids (cerotene/xanthophyll) arranged in clusters in the thylakoid membrane called photosystem

Chlorophyll a absorbs at blue-violet & red region of spectrum

Carotenoids only in blue-violet region

Chlorophyll a (main) found in reaction centre, chlorophyll b & carotenoids in antenna complex & channel light to reaction centre

PSI max absorption>700nm / PSII max> 680nm

Absorption Spectra – Show % of light absorbed by a particular pigment at different light wavelengths

Action Spectra – Rate of photosynthesis at different light wavelengths

Light-Dependent Stage

Photosynthetic pigments in PSII absorb light energy & pass it to Chlorophyll a in reaction centre

where pair of e- are displaced to a higher energy level & are received by an e- acceptor

e- are passed along the ETC via e- carriers (by redox) to a lower energy level

energy lost by the e- is used to convert ADP+Pi>ATP

Light energy is absorbed by PSI & a pair of e- from chlorophyll a are displaced to an even higher energy level and are received by another e- acceptor

These e- and H+ (from the photolysis of h20) are combined to reduce NADP>NADPH2

e- lost from PSII are also replaced by the photolysis of the h2o molecule

Non-Cyclic Photophosphorylation

As e- move down the ETC energy they lose is used to pump H+ via proton pumps from the stroma > thylakoid space through the thylakoid membrane where a high concentration of H+ creates an electrochemical gradient

*maintained by photolysis of water within thylakoid spaces creating more H+ & reduction of NADP in stroma (decreasing H+ concentration)

H+ move down the gradient back to stroma via the chemiosmotic protein channel containing ATPSynethetase creating energy to synthesise ATP from ADP+Pi

e- lost from PSI and H+ (from h2o) reduce NADP>NADPH2

These e- are replaced indirectly by e- from the photolysis of h2o

Cyclic-PhotoPhosphorylation

Some e- at the highest energy state can return to chlorophyll a in PSI via the ETC & produce small amounts of ATP

Light-Independent Stage (Calvin Cycle) (Stroma)

Atmospheric co2 enters stroma via stomata

Co2 combines with RuBP [5c] > 2GP[3c] via carboxylation catalysed by Rubisco

(also called carbon fixation)

2GP>2TP[3c] as 1NADPH2 reduces 2GP using energy from 1ATP molecule

5/6 TP is converted back to RuBP using energy from ATP

1/6 TP is converted to glucose, carbs, amino acids, lipids or nucleic acids

6co2+6h2o > c6h12o6+6co2

For every co2>2TP are made so 6co2>12TP[3c] of which 2 TP molecules leave the cycle & combine to form 1 Glucose molecule

Calvin’s Lollipop Experiment

Unicellular algae Chlorella is placed in a glass ‘lollipop’ vessel & supplied with radioactive co2

The algae is allowed to photosynthesise & ‘fix’ the radioactive co2 & incorporate it into organic molecules which become radioactive also

At specific time intervals, samples of the algae are put into boiling alcohol to denature their enzymes killing them & stopping the light-independent stage reactions occurring

Compounds that the radioactive co2 had reached at a particular moment were determined by chromatography/autoradiography

The order of which each compound is produced is found by identifying molecules & analysing results

Importance of Minerals

Mg is needed to synthesise chlorophyll, Mg deficiency leads to chlorosis & death

Nitrogen is also needed to synthesise amino acids from TP obtained from either no3- or nh4+ from soil