Why Idli Batter Puffs Up Overnight in a Warm Kitchen

Every living cell needs a steady supply of energy, and it gets it by breaking down food molecules such as glucose. Plant cells do this day and night, even though plants have no lungs.

This part covers gas exchange in plants, respiration versus burning and the role of ATP, the steps of glycolysis, and fermentation.

Plants need no respiratory organs because each part handles its own gas exchange, their demand for gases is low, gases travel only short distances, and most living cells lie close to air; gases diffuse through stomata in leaves and lenticels in stems.

The reasons:

- Each part looks after itself — roots, stems and leaves exchange gases separately, with little transport between them
- Low demand — plants respire far more slowly than animals; large volumes of gas are exchanged mainly in photosynthesis, when leaves make their own oxygen
- Short distances — in stems, living cells form thin layers under the bark; in leaves, most cells touch air spaces
- Loose packing — parenchyma cells leave connected air spaces

Openings:

- Stomata — pores in leaves, controlled by guard cells
- Lenticels — pores in the bark of woody stems

An everyday example. Small raised dots on the bark of a young neem or mango branch are lenticels, letting the living tissue inside exchange gases.

The substance. Plants respire all the time, not only at night — in daylight, photosynthesis simply hides the carbon dioxide they release.

Combustion releases all the energy of glucose at once as heat and light, while cellular respiration breaks glucose down in many enzyme-controlled steps and captures part of the energy in ATP, which cells spend wherever work must be done.

Combustion versus respiration:

- Speed — a sudden burst versus a stepwise release
- Control — no enzymes versus enzymes at every step
- Energy — lost as heat and light versus partly trapped in ATP
- Conditions — high temperature versus body temperature, in water

The overall equation is the same:

$${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2\to 6{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{energy}$$

ATP, the energy currency. Energy from respiration joins ADP and phosphate into ATP. When a cell needs energy — for active transport, muscle contraction or building proteins — ATP breaks back to ADP and releases it.

Respiratory substrates. Carbohydrates, usually glucose, are the common fuel; fats and proteins can also be used.

An everyday example. A salary paid into a bank account and then spent in many small purchases mirrors how ATP stores energy and releases it in usable amounts.

The substance. If glucose burned in one step inside a cell, the heat would damage it — stepwise release is what makes respiration safe.

Glycolysis splits one glucose into two pyruvic acid molecules in the cytoplasm, first spending 2 ATP to activate the sugar and then making 4 ATP and 2 NADH, for a net gain of 2 ATP and 2 NADH.

Features. Glycolysis, the EMP pathway, occurs in the cytoplasm of all living organisms, needs no oxygen, and is the only respiratory process in anaerobic organisms.

Investment steps:

- Glucose $\to$ glucose-6-phosphate, using 1 ATP (hexokinase)
- Glucose-6-phosphate $\to$ fructose-6-phosphate
- Fructose-6-phosphate $\to$ fructose-1,6-bisphosphate, using 1 ATP
- Fructose-1,6-bisphosphate splits into two 3-carbon triose phosphates, DHAP and PGAL

Payoff steps, for each 3-carbon molecule:

- PGAL $\to$ 1,3-bisphosphoglycerate, forming **1 NADH + H${}^{+}$**
- 1,3-bisphosphoglycerate $\to$ 3-phosphoglycerate, making 1 ATP
- 3-phosphoglycerate $\to$ 2-phosphoglycerate $\to$ PEP
- PEP $\to$ pyruvic acid, making 1 ATP

Worked example — the balance sheet.

$$\text{ATP made}=2\times 2=4\text{net ATP}=4-2=2\text{NADH}=2\times 1=2$$

An everyday example. Opening a small shop needs money invested before any profit comes in — glycolysis spends 2 ATP before earning 4.

The substance. Glycolysis releases only a small part of glucose's energy; most stays locked in pyruvic acid for later stages.

In alcoholic fermentation, yeast turns pyruvic acid into ethanol and carbon dioxide; in lactic acid fermentation, some bacteria and oxygen-starved muscle turn it into lactic acid; both only recycle NADH so glycolysis can continue, so they release less than seven per cent of the energy in glucose.

Alcoholic fermentation:

- Organism — yeast
- Steps — pyruvic acid $\to$ acetaldehyde + CO${}_2$ (pyruvic acid decarboxylase); acetaldehyde $\to$ ethanol (alcohol dehydrogenase), using NADH

Lactic acid fermentation:

- Organisms — some bacteria; animal muscle when oxygen is inadequate during strenuous exercise
- Step — pyruvic acid $\to$ lactic acid (lactate dehydrogenase), using NADH; **no CO${}_2$

Why so little energy:

- The only ATP gained is the net 2 ATP of glycolysis
- Most energy stays locked in ethanol or lactic acid**
- NADH is spent regenerating NAD${}^{+}$, not making ATP
- The products are harmful — yeast dies once alcohol reaches about 13 per cent

Worked example — count the gain. One glucose fermented by yeast gives **2 ATP, 2 ethanol and 2 CO${}_2$.

An everyday example. Idli and dosa batter left in a warm kitchen overnight puffs up and turns sour as microbes ferment it and release carbon dioxide.

The substance. Fermentation's real job is recycling NAD${}^{+}$** — without it, glycolysis would stall.