ATP - Energy Currency of the Cell, Structure, Functions, FAQ

What is ATP - Energy Currency of Cell

Adenosine triphosphate or ATP is the energy currency of the cell. It is a nucleotide made up of an adenine base, a ribose sugar and three phosphate groups. This is the reason why it is known as the energy currency of the cell, as it plays a central role in energy fulfilment.

ATP was first discovered in 1929 by Karl Lohmann. Later, Fritz Albert Lipmann explained its role in metabolism in the 1940s. His work earned the Nobel Prize in 1953.

ATP serves as an energy shuttle that imports and exports energy within the cells. In cellular respiration, energy from the nutrients is harnessed and used to produce ATP from ADP (adenosine diphosphate). This high-energy phosphate bond in ATP can then be hydrolysed to ADP and phosphate, and energy is released for cellular activities such as muscle contraction, transport of molecules across cell membranes, and synthesis of proteins and lipids, among others.

Structure of ATP Molecule

ATP as energy currency of cell has a unique structure that makes it the perfect energy carrier. It is built from adenine, ribose sugar, and three phosphate groups. The high‑energy phosphate bonds in ATP release instant energy for cellular functions.

The ATP structure is discussed below:

Molecular Structure

ATP as energy currency of cell is made of -

• Adenine base - the nitrogenous organic bases

• Ribose sugar - ribose is a five-carbon sugar or pentose

• Three phosphate radicals - bound to the backbone of the ATP molecule

High Energy Phosphate Bonds

The phosphate bonds in ATP are called high‑energy bonds. The breaking of the terminal phosphate bond releases a large amount of energy. This happens because:

• The phosphate groups carry negative charges and repel each other.

• Resonance in the molecule adds instability.

ATP Hydrolysis and Energy Release

ATP hydrolysis is the conversion of ATP to ADP (adenosine diphosphate and Pi – Inorganic phosphate).

This process liberates energy, which the cells need to fuel several biochemical reactions and cellular activities.

ATP is hydrolysed by enzymes called ATPases that help in the transfer of the phosphate group to other molecules, usually a substrate in metabolic pathways.

ATP Synthesis in Mitochondria

ATP synthesis is a process that operates in contrast to ATP hydrolysis, adenosine diphosphate (ADP) and inorganic phosphate (Pi) to make ATP.

This synthesis is mainly done in cellular respiration in the mitochondria from nutrients used in the synthesis of the phosphate group back to ADP.

The enzyme that catalyses this reaction is ATP synthase, which proceeds through the use of the proton motive force across the inner mitochondrial membrane to synthesise ATP.

ATP Production in Respiration and Photosynthesis

ATP as energy currency of cell is produced in two major ways - cellular respiration and photosynthesis. Both processes generate ATP. They are different organelles with different inputs.

Glycolysis

In the cytoplasm, glycolysis works and splits the glucose into pyruvate. It produces 2 ATP and 2 NADH.

Krebs cycle

Krebs cycle, which occurs in the mitochondrial matrix, oxidises 2 acetyl-CoA (derived from pyruvate) to produce 2 ATP, 6 NADH, and 2 FADH₂ per cycle, along with the release of two molecules of CO₂.

Electron Transport Chain

Located in the inner mitochondrial membrane, the ETC takes electrons from NADH and FADH₂, and transfers them to oxygen to produce a huge amount of ATP by oxidative phosphorylation.

Photosynthesis

Photosynthesis provides chemical energy in the form of glucose. It is the process through which plants, algae and certain bacteria convert light energy into chemical energy in the form of glucose through the production of ATP.

Light-dependent Reactions: Light-dependent reactions occur in the thylakoid membranes of chloroplasts. Photophosphorylation processes involve light energy to bring about the cleavage of two water molecules to give 3 ATP and 2 NADPH.

Calvin Cycle: Occurs in the stroma of chloroplasts, the Calvin cycle uses ATP and NADPH products of the light-dependent processes, to fix carbon dioxide into glucose through several enzyme-catalysed steps. To fix 6 CO₂ molecules, it uses 18 ATP and 12 NADPH

Comparison of ATP yield from different processes

Functions of ATP in Cells

ATP as energy currency of cell is vital because it provides energy for almost every cellular activity. The functions of ATP are discussed below:

1. Muscle Contraction

ATP is significant for muscle contraction because it is required to produce tension between actin and myosin filaments of the muscle fibre to enable contraction and bring movement.

2. Active Transport

ATP provides direct energy for transporting substances across the biological membranes by functioning as the direct energy source for membrane proteins such as pumps and carriers. These transport ions and molecules move from one area to another against a gradient, which could be concentration, electrical or both.

3. Biosynthesis

ATP provides the energy needed for the building of other large molecules like proteins, lipids and nucleic acids for the growth, repair and replication of cells.

4. Signal Transduction

ATP is directly involved in signal transduction by adding phosphate groups to proteins that alter enzyme activity and the genes involved in cell signalling and adaptation to environmental stimuli.

5. Thermogenesis

ATP hydrolysis both gains and loses energy. The energy is used to power cellular functioning and control heat and metabolic reactions.

Regulation of ATP Use

ATP as energy currency of cell is carefully regulated so that energy is produced and used only when needed. This control happens through enzymes, feedback mechanisms, and its role in metabolism. The regulation of ATP is discussed below:

Enzymes Involved

• ATP Synthesis: ATP synthase is an enzyme which facilitates the process of ATP synthesis through oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts.

• ATP Breakdown: ATPases reverse the process by breaking ATP into ADP and phosphate for another cellular process to demand energy.

Role of ATP

• Energy Currency: ATP is the energy-transporting process that fuels metabolic processes in a cell, such as glycolysis, the citric acid cycle, and oxidative phosphorylation.

• Activation Energy: ATP reduces the energy that is needed to start up metabolic reactions, hence making the metabolic process faster.

Feedback Control

• Negative Feedback: ATP reduces the pathways that produce ATP, such as glycolysis and the citric acid cycle, and thus conserves energy.

• Positive Feedback: ATP depletion activates feedback that can reenergize the cell through other energy-producing pathways.

ATP NEET MCQs (With Answers & Explanations)

Important topics for the NEET exam are:

• Structure of ATP

• ATP in Different Processes

Practice Questions for NEET

Q1. ATP has

1. Ribose sugar

2. Deoxyribose sugar

3. Both a and b

4. None of these

Correct answer: 1) Ribose sugar

Explanation:

ATP is a nucleotide molecule. Adenosine is made up of adenine and ribose. Adenosine triphosphate, or ATP, is referred to as the cell's energy currency. Phosphate groups, adenine, and sugar ribose make up this chemical molecule. These molecules give the body energy for several metabolic functions.

Hence, the correct answer is option 1) Ribose sugar.

Q2. The high energy of ATP is stored in

1. Ribose sugar

2. Adenine

3. Terminal phosphate bond

4. Axial phosphate bond

Correct answer: 3) Terminal phosphate bond

Explanation:

The high-energy molecule ATP (adenosine triphosphate) stores energy within its phosphoanhydride bonds, particularly the two between the second and third phosphates, known as beta and gamma phosphates. These bonds are high-energy due to their instability, allowing for straightforward hydrolysis which releases the energy stored. Upon hydrolysis, ATP typically converts to ADP (adenosine diphosphate) or AMP (adenosine monophosphate), thereby releasing energy utilized by the cell in critical functions such as muscle contraction, active transport, and biosynthesis.

Hence, the correct answer is option 3) Terminal phosphate bond.

Q3. Phosphorylation is the formation of ____ occurring during _____.

1. ADP; photosynthesis

2. ATP; respiration

3. ATP; digestion

4. ATP; circulation

Correct answer: 2) ATP; respiration

Explanation:

Phosphorylation is the formation of ATP occurring during respiration. It occurs in three main types: substrate-level phosphorylation, oxidative phosphorylation in mitochondria, and photophosphorylation in chloroplasts. Oxidative phosphorylation is driven by the electron transport chain and chemiosmosis, utilizing a proton gradient. Phosphorylation is essential for cellular energy metabolism, powering various biological processes like active transport and biosynthesis.

Hence, the correct answer is option 2) ATP; respiration

Also Read:

• MCQs on ATP

• Applications of Enzymes

• Metabolites

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