Cellular respiration is the powerhouse of plant metabolism, breaking down organic molecules to release energy as ATP. This process fuels essential functions like growth, development, and survival, occurring in both aerobic and anaerobic conditions.

Glycolysis, the Krebs cycle, and the electron transport chain form the core of cellular respiration. These stages work together to extract energy from glucose, producing ATP, NADH, and FADH2 while releasing carbon dioxide as a byproduct.

Cellular respiration overview

• Cellular respiration is a metabolic process that breaks down organic molecules to release energy in the form of ATP
• Plays a crucial role in providing energy for various cellular processes essential for plant growth, development, and survival

Aerobic vs anaerobic respiration

• Aerobic respiration requires oxygen and yields more ATP (38 ATP per glucose molecule)
• Anaerobic respiration occurs in the absence of oxygen and produces less ATP (2 ATP per glucose molecule)
• Aerobic respiration is more efficient but anaerobic respiration allows survival in oxygen-deprived conditions (waterlogged soils)

Importance of cellular respiration

• Generates ATP to power energy-demanding processes (cell division, growth, and transport)
• Provides carbon skeletons for biosynthesis of various compounds (amino acids, nucleotides)
• Helps maintain cellular homeostasis by removing excess reducing equivalents (NADH)

Glycolysis

• Glycolysis is the first stage of cellular respiration and occurs in the cytosol
• Involves a series of enzymatic reactions that break down glucose into two pyruvate molecules

Glycolysis steps

• Glucose is phosphorylated by ATP to form glucose-6-phosphate
• Fructose-6-phosphate is phosphorylated by ATP to form fructose-1,6-bisphosphate
• Fructose-1,6-bisphosphate is split into two three-carbon molecules (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate)
• Glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase
• 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase, producing ATP
• 3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase
• 2-phosphoglycerate is converted to phosphoenolpyruvate by enolase
• Phosphoenolpyruvate is converted to pyruvate by pyruvate kinase, producing ATP

Glycolysis products

• Two pyruvate molecules
• Two ATP molecules (net gain)
• Two NADH molecules

Glycolysis location

• Glycolysis takes place in the cytosol of plant cells
• Enzymes involved in glycolysis are soluble and not associated with any organelles

Glycolysis regulation

• Glycolysis is regulated by the activity of key enzymes (hexokinase, phosphofructokinase, pyruvate kinase)
• Allosteric regulation by ATP, ADP, AMP, and citrate modulates enzyme activity
• Phosphofructokinase is the main regulatory enzyme and is inhibited by high ATP levels

Krebs cycle

• The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is the second stage of cellular respiration
• Occurs in the matrix of mitochondria and involves a series of enzymatic reactions that oxidize acetyl-CoA derived from pyruvate

Krebs cycle steps

• Acetyl-CoA combines with oxaloacetate to form citrate, catalyzed by citrate synthase
• Citrate is converted to isocitrate by aconitase
• Isocitrate is oxidized to α-ketoglutarate by isocitrate dehydrogenase, producing NADH and CO2
• α-ketoglutarate is oxidized to succinyl-CoA by α-ketoglutarate dehydrogenase complex, producing NADH and CO2
• Succinyl-CoA is converted to succinate by succinyl-CoA synthetase, producing ATP or GTP
• Succinate is oxidized to fumarate by succinate dehydrogenase, reducing FAD to FADH2
• Fumarate is hydrated to malate by fumarase
• Malate is oxidized to oxaloacetate by malate dehydrogenase, producing NADH

Krebs cycle products

• Three NADH molecules
• One FADH2 molecule
• One ATP or GTP molecule
• Two CO2 molecules

Krebs cycle location

• The Krebs cycle takes place in the matrix of mitochondria in plant cells
• Enzymes involved in the Krebs cycle are associated with the inner mitochondrial membrane or soluble in the matrix

Krebs cycle regulation

• The Krebs cycle is regulated by the availability of substrates (acetyl-CoA, NAD+, FAD) and the activity of key enzymes (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase)
• Allosteric regulation by ATP, NADH, and calcium ions modulates enzyme activity
• Citrate synthase is inhibited by high levels of ATP and NADH, while isocitrate dehydrogenase is stimulated by ADP and inhibited by NADH and ATP

Electron transport chain

• The electron transport chain (ETC) is the final stage of cellular respiration and is responsible for the majority of ATP production
• Consists of a series of protein complexes embedded in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen

Electron transport chain components

• Complex I (NADH dehydrogenase) oxidizes NADH and transfers electrons to ubiquinone
• Complex II (succinate dehydrogenase) oxidizes FADH2 and transfers electrons to ubiquinone
• Complex III (cytochrome bc1 complex) transfers electrons from ubiquinone to cytochrome c
• Complex IV (cytochrome c oxidase) transfers electrons from cytochrome c to oxygen, forming water
• ATP synthase (Complex V) generates ATP through chemiosmosis

Chemiosmosis in electron transport chain

• As electrons are transferred through the ETC, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space
• This creates a proton gradient across the inner mitochondrial membrane, with a higher concentration of protons in the intermembrane space
• The proton gradient drives the synthesis of ATP by ATP synthase

ATP synthase role

• ATP synthase is a protein complex that uses the proton gradient generated by the ETC to synthesize ATP
• Protons flow down their concentration gradient through ATP synthase, causing it to rotate
• The rotational energy is used to catalyze the formation of ATP from ADP and inorganic phosphate (Pi)

Oxidative phosphorylation process

• Oxidative phosphorylation refers to the coupling of electron transport and ATP synthesis in the mitochondria
• NADH and FADH2 generated from glycolysis and the Krebs cycle are oxidized by the ETC
• The energy released from electron transport is used to pump protons across the inner mitochondrial membrane
• The resulting proton gradient drives ATP synthesis by ATP synthase

Fermentation

• Fermentation is an anaerobic process that allows cells to generate ATP in the absence of oxygen
• Occurs in the cytosol and involves the reduction of pyruvate to regenerate NAD+ for glycolysis

Lactic acid fermentation

• In lactic acid fermentation, pyruvate is reduced to lactate by lactate dehydrogenase
• This process regenerates NAD+ for glycolysis to continue
• Occurs in plant tissues under anaerobic conditions (waterlogged soils, post-harvest storage)

Alcoholic fermentation

• In alcoholic fermentation, pyruvate is decarboxylated to acetaldehyde by pyruvate decarboxylase
• Acetaldehyde is then reduced to ethanol by alcohol dehydrogenase, regenerating NAD+
• Occurs in yeast and some plant tissues (fruits, seeds) under anaerobic conditions

Fermentation in plant cells

• Fermentation allows plant cells to generate ATP in the absence of oxygen, although it is less efficient than aerobic respiration
• Lactic acid fermentation is common in plant tissues exposed to anaerobic conditions (roots in waterlogged soils)
• Alcoholic fermentation occurs in some fruits and seeds, contributing to their flavor and aroma

Respiration in plants

• Plant respiration involves the uptake of oxygen and the release of carbon dioxide
• Occurs in all living plant tissues, including leaves, roots, and seeds

Respiration in leaves

• Leaves are the primary site of photosynthesis but also undergo respiration
• During the day, respiration is masked by photosynthesis, as the CO2 released is used for carbon fixation
• At night, when photosynthesis stops, respiration becomes evident, and leaves release CO2

Respiration in roots

• Roots depend on respiration for energy to support growth, nutrient uptake, and transport
• Oxygen is required for aerobic respiration in roots, and its availability can be limited in waterlogged or compacted soils
• Under anaerobic conditions, roots may switch to fermentation to generate ATP

Respiration in seeds

• Seeds are metabolically active during development and germination, requiring energy from respiration
• Respiration rates in seeds are influenced by factors such as temperature, moisture, and oxygen availability
• During seed storage, respiration rates are kept low to maintain viability and prevent deterioration

Factors affecting plant respiration

• Temperature: Respiration rates increase with rising temperatures, roughly doubling for every 10°C increase (Q10 effect)
• Oxygen availability: Adequate oxygen is necessary for aerobic respiration; low oxygen levels can lead to fermentation
• Water status: Water stress can reduce respiration rates by limiting metabolic activity and gas exchange
• Developmental stage: Respiration rates vary depending on the growth stage and tissue type (meristems, fruits, senescing leaves)

Respiration and photosynthesis

• Respiration and photosynthesis are complementary processes in plants
• Photosynthesis is an anabolic process that uses light energy to synthesize organic compounds, while respiration is a catabolic process that breaks down organic compounds to release energy

Respiration vs photosynthesis

• Photosynthesis occurs in chloroplasts and requires light, while respiration occurs in mitochondria and does not require light
• Photosynthesis consumes CO2 and releases O2, while respiration consumes O2 and releases CO2
• Photosynthesis stores energy in the form of organic compounds, while respiration releases energy from organic compounds

Interplay of respiration and photosynthesis

• Respiration and photosynthesis are interconnected processes in plants
• Organic compounds produced by photosynthesis are used as substrates for respiration
• Oxygen released by photosynthesis is used for aerobic respiration
• Carbon dioxide released by respiration can be used for photosynthesis during the day

Respiration measurement techniques

• Various methods are used to measure respiration rates in plants
• These techniques help researchers understand the factors affecting respiration and its role in plant metabolism

Oxygen consumption measurement

• Oxygen electrodes or Clark-type electrodes can be used to measure oxygen uptake by plant tissues
• Tissues are placed in a closed chamber, and the decrease in oxygen concentration over time is recorded
• Oxygen consumption rates are calculated based on the volume of the chamber and the fresh weight or dry weight of the tissue

Carbon dioxide production measurement

• Infrared gas analyzers (IRGA) can be used to measure CO2 release from plant tissues
• Tissues are placed in a closed chamber, and the increase in CO2 concentration over time is recorded
• CO2 production rates are calculated based on the volume of the chamber and the fresh weight or dry weight of the tissue

Calorimetry in respiration studies

• Calorimetry measures the heat released during respiration
• Plant tissues are placed in a calorimeter, and the heat produced by respiration is measured over time
• The heat released is proportional to the amount of oxygen consumed or CO2 produced during respiration

Respiration and plant metabolism

• Respiration plays a crucial role in plant metabolism, providing energy and carbon skeletons for various processes
• Understanding the relationship between respiration and plant metabolism is essential for optimizing plant growth and productivity

Respiration and plant growth

• Respiration provides the energy needed for cell division, elongation, and differentiation
• Higher respiration rates are often associated with rapidly growing tissues (meristems, expanding leaves)
• Efficient respiration can lead to faster growth rates and increased biomass accumulation

Respiration and stress responses

• Respiration is involved in plant responses to various biotic and abiotic stresses
• Stress conditions (drought, salinity, extreme temperatures) can alter respiration rates and energy metabolism
• Enhanced respiration can provide energy for stress-related processes (osmotic adjustment, antioxidant synthesis)

Respiration and crop yield

• Respiration is a key determinant of crop yield, as it influences the balance between carbon gain and loss
• Efficient respiration can lead to higher net carbon assimilation and increased yield
• Breeding or engineering crops with optimized respiration rates could potentially improve yield and resource use efficiency