Bioenergetics made easy!

Bioenergetics studies how a living organism obtains, transforms, and utilizes energy to grow and reproduce. Technically, we have various sources of energy (primary)- plants (autotrophs) have sun, and humans and animals (heterotrophs) get energy from the food we eat.
But, can these sources be directly used as an energy source?
No.. our body requires energy in the form of chemical compounds like ATP and NADH. In fact, ATP can be seen as a currency unit of energy in our body.
Bioenergetics not just focusses on how to use the ATP efficiently (catabolism), but also how to build ATP (anabolism). The ratio of ATP: ADP is called energy charge. if there is too much ATP, the cell needs to use ATP to do the work. If ADP is high, your body needs to synthesise ATP through oxidative phosphorylation.

Free energy (G)

Every organism should perform work in order to stay alive, grow, and reproduce.
The amount of utilizable energy available to perform the work is called free energy.
Change in free energy (ΔG) predicts the feasibility of a chemical reaction.
A low ΔG, indicates the reaction can occur spontaneously. Meanwhile,

A high ΔG indicates that the reaction is non spontaneous.
Cells require a source of free energy. That is given by the primary sources of energy- sun for plants and food for us.

Our cells being an isothermal system needs to convert this free energy to ATP or any other energy rich compounds that provide energy to do the work.

Principles of bioenergetics

Bioenergetics follows laws of thermodynamics

Law 1- Law of conservation of energy- where energy is neither created nor destroyed

Law 2- Universe tends towards disorder / entropy- entropy keeps increasing unless the energy required for the process counteracts it.

Let’s understand some important terminologies before diving deeper

1. Gibbs free energy (G)- As mentioned previously, the total amount of energy capable of doing work at constant temperature (T) and pressure (P).

• 2. Enthalpy (H)- it refers to the heat content of the system. it is the amount of heat transferred from or to the system. under constant pressure.

• 3. Entropy- It refers to the degree of randomness or disorder. it measures the amount of heat dispersed or transferred during a chemical reaction. If ΔS is positive, entropy is high, if ΔS is negative, entropy is low.

The above three terms are related through an equation.

Examples of Bioenergetics

1. Glycolysis where glucose is converted to 2 molecules of pyruvate, 2 ATPs, and 2 NADH. pyruvate enters TCA cycle or enters gluconeogenesis. NADH provides electrons to the electron transport chain (ETC).
   
2. Proteins are broken down to amino acids which are required for glucose production if ATP is less (during starvation).
   
3. Citric acid cycle- where acetyl CoA combines with oxaloacetate undergoes 8 reactions to finally give citrate. Each intermediate from the 8 steps is oxidized to energy precursors like FADH2 and NADH
   
4. Ketosis- where ketone bodies are used by your body as an energy source in case there is no glucose available.
   
5. Photosynthesis- where plants convert energy from the sun to glucose from CO2 and H2O. Plants cells create ATP through phosphorylation.
   

Coupled processes in Bioenergetics

It refers to the sequence if reactions where energy from an exergonic reaction drives an endergonic process. This way, a non spontaneous reaction (like muscle contraction, glycolysis, or photosynthesis) can be easily performed by a cell. Most of the energy is obtained using ATP from spontaneous processes like cellular respiration. This coupling is very important becuase-

1. Energy conservation
2. Metabolic efficiency
3. Regulation
   

Integration of central metabolism

When it comes to bioenergetics, three main pathways are very crucial-

1. Glycolysis
2. Citric Acid Cycle
3. Electron Transport Chain
   

These three pathways break down complex organic compounds to ATP. They are the central metabolism. They are called so because almost all bioenergetic pathways (be it catabolic or anabolic) intersect here.

The three pathways help maintain a steady supply of energy and precursor molecules for the cell.

Integration of the central metabolism

As bioenergetics involves several pathways all connecting to a central metabolic network, our cells require a biochemical logic to integrate the three pathways. A common hub for that is acetyl CoA. Alongside acetyl CoA, feedback loops, enzyme regulators and hormone signals control the flow of molecules within and between cells. because the three pathways are interconnected, one type of fuel can be easily converted to another.

Breakdown of carbohydrates, fats, and proteins results in their respective products which are accepted by acetyl CoA. the Acetyl CoA then enters the TCA cycle to produce NADH and FADH2, prime sources of energy. Meanwhile, intermediates from TCA cycle exit the cycle at different stages to be used for synthesis of fatty acids and amino acids. Our main source of energy ATP is produced by oxidative phosphorylation using NADH and FADH2. In addition, the Hexose Mono Phosphate Shunt provides the cell with NADPH which is used in biosynthesis.

Entry points

Carbohydrates- glucose via glycolysis. Reaction takes place in cytoplasm and glucose is broken down to pyruvates

Lipids- fatty acids are broken down to acetyl CoA through beta oxidation in mitochondria. Acetyl CoA enters TCA cycle. Glycerol enters through glycolysis.

Proteins- amino acids enter at various points in the pathway either by converting to pyruvate or directly entering TCA cycle as acetyl CoA

Exit points

Main exit points- production of ATP and NADH- these are used in oxidative phosphorylation.

Intermediates of TCA cycle can exit at various points to help in amino acid and fatty acid synthesis

Regulation and control

1. So many different pathways and products—- without regulation will be extremely chaotic. So, our cells have key enzymes that regulate direction and rate of reactions.
2. Compartmentalization- each specialized cell or organelle is designed according to their metabolic function. For instance, mitochondria or liver cells.
3. Hormones like glucagon and insulin help in the big picture. they coordinate metabolic responses between different tissues based on the body’s energy status.