Study Notes on Respiration in Plants

RESPIRATION IN PLANTS

All living organisms breathe to live; breathing is essential to life for energy creation.
Need for energy:
- Daily activities: Absorption, transport, movement, reproduction, and breathing.
- Source of energy: Comes from the oxidation of macromolecules called 'food'.

Photosynthesis: Green plants and cyanobacteria convert light energy into chemical energy.
- Produces carbohydrates such as glucose, sucrose, and starch.
- Not all plant cells can photosynthesize; only those containing chloroplasts can do so.
- Non-green parts of the plant rely on translocated food for oxidation.

Animals obtain food:
- Directly (herbivores) or indirectly (carnivores) from plants.
- Saprophytes like fungi depend on dead and decaying matter.
Ultimately, all food used for respiration stems from photosynthesis.

Definition: Mechanism of breaking down food materials within the cell to release energy; traps this energy to synthesize ATP.
Photosynthesis occurs in chloroplasts; breakdown occurs in cytoplasm and mitochondria (in eukaryotes).
Respiration involves the oxidation of compounds resulting in the release of energy.
Oxidised compounds during respiration are known as respiratory substrates; chiefly carbohydrates, but proteins, fats, and organic acids can also be used under certain conditions.
- Energy release occurs through a series of enzyme-controlled reactions, and not all energy is released at once.

Energy released during respiration is utilized to synthesize ATP, the energy currency of the cell.
ATP breaks down for energy in various life processes.
Carbon skeletons from respiration are used as precursors in biosynthesis of other cellular molecules.

Oxygen Requirement: Plants require O2 for respiration.
- CO2 Release: They also give out CO2 as a byproduct.
Gas Exchange Structures: Plants lack specialized organs for gas exchange but utilize stomata and lenticels.

Each plant part exchanges gases independently; low gas exchange rate relative to animals.
- Roots, stems, and leaves have minimal gas exchange requirements.
- Large volumes of gases are primarily exchanged during photosynthesis.
Cells in stems (including woody stems) and roots have direct contact with air due to their structure and arrangement.

Complete combustion of glucose reaction:
C_{6}H_{12}O_{6} + 6O_{2}
ightarrow 6CO_{2} + 6H_{2}O + ext{Energy}
Plants oxidize glucose in small steps to trap energy for ATP synthesis, preventing loss of energy solely as heat.

Some cells can survive in environments lacking oxygen and can undergo partial oxidation of glucose, resulting in processes like glycolysis.

Definition: Origin of the term: Greek words 'glycos' (sugar) and 'lysis' (splitting).
Metabolic pathway comprising ten enzymatic steps resulting in the conversion of glucose to pyruvate.
Occurs in the cytoplasm; present in all living organisms.
Enzymatic Steps:
- Glucose is converted via invertase into glucose and fructose; enters glycolysis as glucose-6-phosphate.
- Phosphorylation occurs producing fructose-6-phosphate, which further leads to fructose-1,6-bisphosphate.
- Fructose-1,6-bisphosphate splits into dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).
ATP Yield from Glycolysis:
- Two ATP used in glycolysis; several ATP molecules produced in subsequent steps.
Key products:
- Pyruvate as the main product of glycolysis, with potential metabolic fates dependent on oxygen availability.

Process Overview: Involves anaerobic conversion of pyruvate into CO2 and ethanol or lactic acid.
Enzymes catalyzing fermentation:
- Alcohol fermentation via yeast: Pyruvic acid decarboxylase, alcohol dehydrogenase.
- Lactic acid fermentation: Lactate dehydrogenase reduces pyruvate to lactic acid.
Energy Yield: Less than 7% of available energy captured as ATP; toxic byproducts may accumulate (e.g., alcohol).

Defined as complete oxidation of organic substances in oxygen presence, yielding CO2, water, and substantial energy.
Key Steps in Aerobic Respiration:
- Pyruvate transported to mitochondria for oxidation.
- Krebs’ Cycle: Acetyl CoA formed from pyruvate, yielding additional NADH and FADH2.

Begins with condensation of acetyl group with oxaloacetic acid.
Citrate formed and undergoes transformations generating CO2 and producing ATP, NADH, and FADH2.

Following TCA cycle, NADH and FADH2 oxidized through the ETS embedded in the mitochondrial membrane.
Process Overview:
- Electron transfer leads to proton gradient generation and ATP synthesis via ATP synthase.
- O2 serves as the terminal electron acceptor.

Each NADH oxidized yields approximately 3 ATP; FADH2 yields approximately 2 ATP.

The theoretical net gain of ATP under ideal conditions can be calculated but may not entirely represent real biological systems.
Net Gain: Generally estimated at 38 ATP per glucose molecule during aerobic conditions.
Assumptions include sequential metabolic pathways and no other substrates interfering.

Fermentation results in partial oxidation with limited ATP yield (2 ATP per glucose).
Aerobic respiration yields a significantly higher ATP output.

Definition: Respiratory processes involved in both catabolism (breaking down substrates) and anabolism (synthesizing substrates).
Various substrates (carbohydrates, fats, proteins) can enter at different points in the respiratory pathway for either energy release or synthesis.

Definition: Ratio of CO2 evolved to O2 consumed.
RQ values vary with the type of substrate:
- Generally, RQ = 1 for carbohydrates, less than 1 for fats, and approximately 0.9 for proteins.

Plants do not require specialized respiratory organs; gas exchange occurs via stomata and lenticels.
Cellular respiration begins with glycolysis, leading to aerobic respiration or fermentation depending on O2 availability.
The electron transport chain drives ATP synthesis in aerobic conditions, with O2 as the final electron acceptor, leading to efficient energy extraction.

Differentiate between respiration and combustion, glycolysis and Krebs’ cycle, aerobic respiration and fermentation, and more outlined topics for students to explore.