Adenosine 5′-triphosphate, or ATP, is the principal molecule for storing and transferring energy in cells. It is often referred to as the energy currency of the cell and can be compared to storing money in a bank. ATP can be used to store energy for future reactions or be withdrawn to pay for reactions when energy is required by the cell. Animals store the energy obtained from the breakdown of food as ATP. Likewise, plants capture and store the energy they derive from light during photosynthesis in ATP molecules. ATP is a nucleotide consisting of an adenine base attached to a ribose sugar, which is attached to three phosphate groups. These three phosphate groups are linked to one another by two high-energy bonds called phosphoanhydride bonds. When one phosphate group is removed by breaking a phosphoanhydride bond in a process called hydrolysis, energy is released, and ATP is converted to adenosine diphosphate (ADP). Likewise, energy is also released when phosphate is removed from ADP to form adenosine monophosphate (AMP). This free energy can be transferred to other molecules to make unfavourable reactions in a cell favourable. AMP can then be recycled into ADP or ATP by forming new phosphoanhydride bonds to store energy once again. In the cell, AMP, ADP, and ATP are constantly interconverted as they participate in biological reactions.

The First Law of Thermodynamics

The first law of thermodynamics deals with the total amount of energy in the universe. It states that this total amount of energy is constant. In other words, there has always been, and always will be, exactly the same amount of energy in the universe. Energy exists in many different forms. According to the first law of thermodynamics, energy may be transferred from place to place or transformed into different forms, but it cannot be created or destroyed. The transfers and transformations of energy take place around us all the time. Light bulbs transform electrical energy into light energy. Gas stoves transform chemical energy from natural gas into heat energy. Plants perform one of the most biologically useful energy transformations on earth: that of converting the energy of sunlight into the chemical energy stored within organic molecules. 

The challenge for all living organisms is to obtain energy from their surroundings in forms that they can transfer or transform into usable energy to do work. Living cells have evolved to meet this challenge very well. Chemical energy stored within organic molecules such as sugars and fats is transformed through a series of cellular chemical reactions into energy within molecules of ATP. Energy in ATP molecules is easily accessible to do work. Examples of the types of work that cells need to do include building complex molecules, transporting materials, powering the beating motion of cilia or flagella, contracting muscle fibers to create movement, and reproduction.

The Second Law of Thermodynamics   

A living cell’s primary tasks of obtaining, transforming, and using energy to do work may seem simple. However, the second law of thermodynamics explains why these tasks are harder than they appear. None of the energy transfers we’ve discussed, along with all energy transfers and transformations in the universe, is completely efficient. In every energy transfer, some amount of energy is lost in a form that is unusable. In most cases, this form is heat energy. Thermodynamically, heat energy is defined as the energy transferred from one system to another that is not doing work. For example, when an airplane flies through the air, some of the energy of the flying plane is lost as heat energy due to friction with the surrounding air. This friction actually heats the air by temporarily increasing the speed of air molecules. Likewise, some energy is lost as heat energy during cellular metabolic reactions. This is good for warm-blooded creatures like us, because heat energy helps to maintain our body temperature. Strictly speaking, no energy transfer is completely efficient, because some energy is lost in an unusable form.

An important concept in physical systems is that of order and disorder (also known as randomness). The more energy that is lost by a system to its surroundings, the less ordered and more random the system is. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder and low energy (Figure 2). To better understand entropy, think of a student’s bedroom. If no energy or work were put into it, the room would quickly become messy. It would exist in a very disordered state, one of high entropy. Energy must be put into the system, in the form of the student doing work and putting everything away, in order to bring the room back to a state of cleanliness and order. This state is one of low entropy. Similarly, a car or house must be constantly maintained with work in order to keep it in an ordered state. Left alone, the entropy of the house or car gradually increases through rust and degradation. Molecules and chemical reactions have varying amounts of entropy as well. For example, as chemical reactions reach a state of equilibrium, entropy increases, and as molecules at a high concentration in one place diffuse and spread out, entropy also increases.

Based on composition, there are two types of proteins as follows:

1. Simple Proteins: These proteins are made up of only amino acids e.g.,  albumins, globulins, glutelin.
2.  Conjugated proteins: These are formed by the binding of a simple protein with a non-protein called the prosthetic group. Depending upon the type of prosthetic group, conjugated proteins are of several types few of which are mentioned below:

Based on shape, proteins are classified into two types:

1.Fibrous proteins: They are thread-like structural proteins that may occur singly or in groups.  Fibrous proteins generally possess secondary structures and are insoluble in water. The common example of fibrous proteins is collagen of connective tissue, actin and myosin of muscles, keratin of scales,  feathers,  hair,  claws,  nails, horns and hoofs, the silk of spider web. Fibrinogen is also a fibrous protein but soluble in blood plasma. It forms insoluble fibrin during the clotting of blood.

2.Globular proteins: They are spherical in shape and are involved in metabolic activities.  They have a tertiary or quaternary structure.  Smaller globular proteins are usually soluble in water and are not coagulated by heat, e.g., histones.  Egg albumin, serum globulins and glutelin (wheat,  rice)  are examples of large globular proteins, which get coagulated by heat.

Role/ Functions of proteins

Proteins perform several functions in organisms. The main functions of proteins are as follows:

1.Structural Proteins: Many proteins serve as a building material of cells and tissues. They take part in the formation of a colloidal complex of protoplast, cell membranes, organelles, extracellular matrices and fibres. Some proteins form supporting structures, e.g., elastin of ligaments, collagen of tendons, cartilages, bone and connective tissue. Keratin is the major constituent of the external protective structure of animals like hair, feathers, horny layer of skin, nails, claws, hoofs etc.

2.Enzymes: Many proteins function as enzymes to catalyse biochemical reactions that occur in the living world. Enzymes play a key role in metabolism. Every enzyme is specific in action.  E.g.,  Amylase,  protease,  RUBISCO  etc.  RUBISCO  (Ribulose Bisphosphate Carboxylase Oxygenase) is the most abundant enzyme found in plants. It the principal enzyme involved in photosynthesis.

3.Carrier  Proteins  (Transport proteins): Some proteins act as carriers that bind and transport specific molecules across a membrane or in a body fluid.  Hemoglobin of RBCs transports oxygen in the body.  RBCs contain haemoglobin  (Hb)  which is made up of 4 polypeptide chains and contains an iron ion. Iron has a strong affinity for oxygen. In the lungs, oxygen combines with haemoglobin to produce oxyhemoglobins. In body cells, oxygen is released and can diffuse into a body cell. Other examples include proteins such as the myoglobin of muscles that store oxygen. α-globulin of blood carries thyroxine and bilirubin,  β-globulin transport vitamins  A,  D  and  K, cholesterol and ions in the blood.

4.Receptor  Proteins: Several proteins present on the external surface of the cell membrane act as receptor molecules. A receptor protein molecule receives chemical signals from outside a cell. When such chemical signals bind to a  receptor, they cause some form of cellular/tissue response. For example, when insulin is released in the blood, the receptor proteins present on the cell membrane bind with this insulin. This binding changes the permeability of the cell membrane and glucose can enter the cell through glucose channels.

5.Hormones: Some hormones are proteinaceous, e.g., insulin(sugar metabolism regulating hormone} parathyroid (calcium and phosphate transport regulating hormones). Hormones play a key role in the regulation of metabolism.

6.Contractile Proteins: Myosin and actin make the muscle fibres contractile to bring about movements and locomotion. The contractile system is made up of protein actin but association with myosin is essential for contraction.

7.Defensive proteins: Some proteins act as antibodies (immunoglobulins) that participate in the defence mechanism of the body.

8.Storage Proteins: These occur in milk, eggs and seeds to nourish the young ones. They include casein of milk, albumin of egg white and glutelin in cereals.