Respiration in Plants — Revision Notes

⚡ 30-Second Revision

Respiration: — Catabolic process, releases energy (ATP).
Aerobic Respiration: — Requires $O_2$, high ATP yield (30-32 ATP/glucose).
Anaerobic Respiration: — No $O_2$, low ATP yield (2 ATP/glucose), fermentation.
Glycolysis: — Cytoplasm, Glucose $\rightarrow$ 2 Pyruvate, Net 2 ATP, 2 NADH.
Link Reaction: — Mitochondrial matrix, Pyruvate $\rightarrow$ Acetyl-CoA, 2 $CO_2$, 2 NADH (per glucose).
Krebs Cycle: — Mitochondrial matrix, Acetyl-CoA oxidized, 4 $CO_2$, 6 NADH, 2 $FADH_2$, 2 ATP (per glucose).
ETS: — Inner mitochondrial membrane, $e^-$ flow, $H^+$ pumping, $O_2$ final acceptor.
Oxidative Phosphorylation: — ATP synthesis via chemiosmosis, driven by proton gradient.
ATP Yield: — 1 NADH $\approx$ 2.5 ATP, 1 $FADH_2 \approx$ 1.5 ATP.
RQ: — $V_{CO_2}/V_{O_2}$. Carbohydrates = 1, Fats < 1, Proteins < 1, Organic Acids > 1, Anaerobic = $\infty$.

2-Minute Revision

Respiration in plants is the process of breaking down organic food, primarily glucose, to release energy in the form of ATP. It's a continuous, catabolic process occurring in all living cells. Aerobic respiration, requiring oxygen, is highly efficient.

It starts with glycolysis in the cytoplasm, yielding 2 net ATP and 2 NADH. Pyruvate then enters the mitochondrial matrix for the link reaction, forming Acetyl-CoA, 2 $CO_2$, and 2 NADH. Acetyl-CoA then enters the Krebs cycle, producing 4 $CO_2$, 6 NADH, 2 $FADH_2$, and 2 ATP.

Finally, the Electron Transport System (ETS) on the inner mitochondrial membrane uses electrons from NADH and $FADH_2$ to pump protons, creating a gradient. This proton motive force drives ATP synthase to produce a large amount of ATP (oxidative phosphorylation), with oxygen as the final electron acceptor.

Total aerobic ATP yield is 30-32 ATP per glucose. Anaerobic respiration, in the absence of oxygen, only performs glycolysis, followed by fermentation (e.g., alcoholic fermentation in plants), yielding only 2 ATP and regenerating $NAD^+$.

The Respiratory Quotient (RQ) is a key indicator ($V_{CO_2}/V_{O_2}$), with values like 1 for carbohydrates, <1 for fats/proteins, and >1 for organic acids.

5-Minute Revision

Respiration is the cellular process where plants break down complex organic molecules, mainly glucose, to release chemical energy stored as ATP. This energy fuels all cellular activities. It's a continuous process, distinct from photosynthesis, which builds food. The two main types are aerobic (with oxygen) and anaerobic (without oxygen).

Aerobic Respiration: This is the most efficient pathway, occurring in four main stages:

1
Glycolysis (Cytoplasm): — Glucose (6C) is broken into two pyruvate (3C) molecules. Net yield: 2 ATP (via substrate-level phosphorylation) and 2 NADH. This stage is oxygen-independent.
2
Link Reaction (Mitochondrial Matrix): — Each pyruvate is decarboxylated and oxidized to Acetyl-CoA (2C), releasing 1 $CO_2$ and 1 NADH per pyruvate. So, per glucose, 2 Acetyl-CoA, 2 $CO_2$, and 2 NADH are formed.
3
Krebs Cycle (Mitochondrial Matrix): — Acetyl-CoA combines with oxaloacetate to form citrate. Through a cyclic series of reactions, it's completely oxidized, releasing 2 $CO_2$, 3 NADH, 1 $FADH_2$, and 1 ATP (or GTP) per Acetyl-CoA. Thus, per glucose (two turns): 4 $CO_2$, 6 NADH, 2 $FADH_2$, and 2 ATP.
4
Electron Transport System (ETS) & Oxidative Phosphorylation (Inner Mitochondrial Membrane): — NADH and $FADH_2$ donate electrons to protein complexes. Electron flow pumps protons ($H^+$) into the intermembrane space, creating a proton gradient. Oxygen is the final electron acceptor, forming water. Protons flow back through ATP synthase, driving ATP synthesis (chemiosmosis). Each NADH yields $\approx 2.5$ ATP, and each $FADH_2$ yields $\approx 1.5$ ATP. The total theoretical yield is 30-32 ATP per glucose.

Anaerobic Respiration (Fermentation): Occurs when oxygen is limited. Only glycolysis takes place, producing 2 net ATP. Pyruvate is then converted to other products (e.g., ethanol and $CO_2$ in alcoholic fermentation, or lactic acid) to regenerate $NAD^+$ for glycolysis to continue. It's far less efficient.

Respiratory Quotient (RQ): $RQ = \frac{Volume,of,CO_2,evolved}{Volume,of,O_2,consumed}$. It indicates the substrate: Carbohydrates (RQ=1), Fats (RQ<1, e.g., 0.7), Proteins (RQ<1, e.g., 0.8-0.9), Organic Acids (RQ>1, e.g., 1.33 for malic acid), Anaerobic (RQ=$\infty$).

Prelims Revision Notes

1
Definition: — Respiration is the catabolic breakdown of organic food to release energy (ATP) for cellular activities. Occurs continuously in all living cells.
2
Types:

* Aerobic Respiration: Requires $O_2$. High ATP yield (30-32 ATP/glucose). Occurs in cytoplasm and mitochondria. * Anaerobic Respiration (Fermentation): No $O_2$. Low ATP yield (2 ATP/glucose). Occurs only in cytoplasm.

1
Stages of Aerobic Respiration:

* Glycolysis (EMP Pathway): * Location: Cytoplasm. * Input: 1 Glucose. * Output: 2 Pyruvate, Net 2 ATP (substrate-level), 2 NADH. * Oxygen: Independent. * Oxidative Decarboxylation (Link Reaction): * Location: Mitochondrial matrix.

* Input: 2 Pyruvate (from 1 glucose). * Output: 2 Acetyl-CoA, 2 $CO_2$, 2 NADH. * Krebs Cycle (TCA Cycle/Citric Acid Cycle): * Location: Mitochondrial matrix. * Input: 2 Acetyl-CoA (from 1 glucose).

* Output: 4 $CO_2$, 6 NADH, 2 $FADH_2$, 2 ATP (or GTP, via substrate-level). * Electron Transport System (ETS) & Oxidative Phosphorylation: * Location: Inner mitochondrial membrane. * Process: Electrons from NADH and $FADH_2$ pass through complexes (I, II, III, IV).

Energy released pumps $H^+$ into intermembrane space, creating proton gradient. $O_2$ is final electron acceptor, forming $H_2O$. * **ATP Synthase ($F_0F_1$):** $H^+$ flow back through it drives ATP synthesis (oxidative phosphorylation).

* ATP Yield: 1 NADH $\rightarrow 2.5$ ATP; 1 $FADH_2 \rightarrow 1.5$ ATP. * Total ATP (Aerobic): 2 (Glycolysis) + 5 (NADH from Glycolysis) + 5 (NADH from Link) + 2 (Krebs) + 15 (NADH from Krebs) + 3 ($FADH_2$ from Krebs) = 32 ATP (modern value).

1
Anaerobic Respiration (Fermentation):

* Purpose: Regenerate $NAD^+$ for glycolysis. * Types: Alcoholic fermentation (Pyruvate $\rightarrow$ Acetaldehyde $\rightarrow$ Ethanol + $CO_2$); Lactic acid fermentation (Pyruvate $\rightarrow$ Lactic acid). * ATP Yield: Only 2 ATP (from glycolysis).

1
Respiratory Quotient (RQ):

* Formula: $RQ = \frac{Volume,of,CO_2,evolved}{Volume,of,O_2,consumed}$. * Values: Carbohydrates (e.g., Glucose): RQ = 1. Fats (e.g., Tripalmitin): RQ < 1 (approx. 0.7). Proteins: RQ < 1 (approx. 0.8-0.9). Organic Acids (e.g., Malic acid): RQ > 1 (e.g., 1.33). * Anaerobic Respiration: RQ = $\infty$ (no $O_2$ consumed).

1
Factors Affecting Respiration: — Temperature, $O_2$ concentration, water, $CO_2$ concentration, respiratory substrates.

Vyyuha Quick Recall

To remember the sequence of major events in aerobic respiration: Good Little Kids Eat Oranges.

Glycolysis
Link Reaction
Krebs Cycle
Electron Transport System
Oxidative Phosphorylation