Question

The law of the conservation of energy states that energy can neither be created or destroyed within a closed system, but we also know that any transfer of energy is not 100% efficient and some energy “escapes”. Write briefly about the transfer of energy within our cells …. how do we obtain, store, and use energy? Be sure to give specific examples of the way we convert chemical energy to mechanical and transport/potential energy within the cell. Also address how the “loss” of energy is observable within the cell.

Answer 1

Transfer of energy within cell :

Organisms ingest large molecules, like carbohydrates, proteins, and fats, and convert them into smaller molecules like carbon dioxide and water. This process is called cellular respiration, a form of catabolism, and makes energy available for the cell to use. The energy released by cellular respiration is temporarily captured by the formation of adenosine triphosphate (ATP) within the cell. ATP is the principle form of stored energy used for cellular functions and is frequently referred to as the energy currency of the cell.

The nutrients broken down through cellular respiration lose electrons throughout the process and are said to be oxidized. When oxygen is used to help drive the oxidation of nutrients the process is called aerobic respiration. Aerobic respiration is common among the eukaryotes, including humans, and takes place mostly within the mitochondria. Respiration occurs within the cytoplasm of prokaryotes. Several prokaryotes and a few eukaryotes use an inorganic molecule other than oxygen to drive the oxidation of their nutrients in a process called anaerobic respiration. Electron acceptors for anaerobic respiration include nitrate, sulfate, carbon dioxide, and several metal ions.

The energy released during cellular respiration is then used in other biological processes. These processes build larger molecules that are essential to an organism’s survival, such as amino acids, DNA, and proteins. Because they synthesize new molecules, these processes are examples of anabolism.

In living systems, a small class of molecules functions as electron shuttles: they bind and carry high-energy electrons between compounds in cellular pathways. The principal electron carriers we will consider are derived from the vitamin B group, which are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) is derived from vitamin B3, niacin. NAD+ is the oxidized form of niacin; NADH is the reduced form after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron). It is noteworthy that NAD+must accept two electrons at once; it cannot serve as a one-electron carrier.

NAD+ can accept electrons from an organic molecule according to the general equation:

RH (Reducing agent) + NAD+ (Oxidizing agent) → NADH (Reduced) + R (Oxidized)

When electrons are added to a compound, the compound is reduced. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent and NAD+ is reduced to NADH. When electrons are removed from a compound, the compound is oxidized. In the above equation, NAD+ is an oxidizing agent and RH is oxidized to R. The molecule NADH is critical for cellular respiration and other metabolic pathways.

Similarly, flavin adenine dinucleotide (FAD+) is derived from vitamin B2, also called riboflavin. Its reduced form is FADH2. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD+ and FAD+ are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis.

A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would lead to excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the “energy currency” of the cell and can be used to fill any energy need of the cell.

Conversion of chemical energy to other forms :

Chemical energy stored within organic molecules such as sugars and fats is transferred and transformed through a series of  reactions into energy within molecules of ATP (adenosine triphosphate). Energy in ATP molecules is easily accessible to do work.

Loss of energy :

From the living cells ,some energy is lost as heat energy during cellular metabolic reactions.