Biomolecules and Metabolism

 

All living organisms need energy to sustain life. Living cells and organisms can extract energy from their environment and use it to power various biological activities like growth, movement, development and reproduction. This unique ability to harness chemical energy from the environment and channel it to perform biological work defines life. Life can perform a remarkable variety of energy conversions. Chemical energy present in food is converted to different forms of energy for various purposes through metabolic reactions. 

Metabolism refers to a set of chemical reactions occurring in a living cell for sustaining life. Metabolic reactions generate energy from food, synthesise organic compounds that serve as building blocks of biomolecules and excrete toxic wastes. Metabolic reactions can broadly be classified as catabolic or anabolic. Catabolic reactions break down larger molecules into smaller ones and release energy in the process. Anabolic reactions synthesise larger biomolecules from smaller ones by consuming energy. Metabolism can be considered as the sum of anabolic and catabolic reactions. Catabolic reactions produce the energy required for all cellular activity, while anabolic reactions help in formation and growth of cells and tissues.

Metabolism is a complex process requiring interplay between several different biomolecules. Each metabolic reaction is catalysed by a specific enzyme. The interplay between different biomolecules often regulates metabolism and balances energy generation and energy consumption. Let us now take a brief look at the important biomolecules of life. 

The different biomolecules

ATP: A molecule known as ATP (Adenosine Triphosphate) is the biological unit of energy. ATP is the universal energy currency of all living cells. This molecule is very important as it can store energy. Just like money can be deposited in a bank as savings, ATP can be stored inside a living cell as energy currency. Whenever the cell needs energy, ATP can be utilised to generate it. The breakdown of ATP leads to release of energy.

Carbohydrates: Carbohydrates are organic compounds containing carbon, hydrogen and oxygen. Most carbohydrates are naturally occurring sugars. Carbohydrates may be simple or complex. Simple sugars like glucose, fructose (present in honey) and lactose (present in milk) are made up of one or two types of sugars. Simple sugars are also called monosaccharides (mono: one; saccharide: sugar). Complex sugars are made up of many individual sugar units linked together, forming a polymer. Complex sugars are also called as polysaccharides (poly: many; saccharide: sugar). Starch, glycogen and cellulose are examples of polysaccharides.  The main function of sugars is to provide energy. When carbohydrates are digested, they are broken down into simpler molecules by stripping them of electrons (a process called oxidation). This process releases heat and generates ATP molecules that can be used for fulfilling energy requirements. Carbohydrates like cellulose and chitin can also provide structural support in plants and insects.

Proteins: Like many carbohydrates, proteins are also polymers. The individual units (monomers) constituting proteins are known as amino acids. There are 20 amino acids. The protein structure depends on the amino acid sequence and the overall 3-D shape of the polymer. There are many remarkable structural configurations a protein can acquire. Proteins play several crucial roles. Some of the important roles of proteins are as follows:

a. Structural: These proteins provide support. Keratin (hair, nails), collagen (tendons and cartilage), elastin/fibroelastin (ligaments) and fibroin (silk and webs) are some important structural proteins;

b. Enzymes: Nearly all enzymes are proteins. Enzymes are essential for catalysing all metabolic reactions;

c. Defensive: Many antibodies that provide immunity against diseases are proteins;

d. Proteins like actin and myosin help in muscle contraction and movement;

e. Proteins can also carry nerve impulses, and control cell growth and differentiation

Lipids: Lipids are a class of organic compounds containing carbon, hydrogen and oxygen (similar to carbohydrates), but with lesser water content. The defining feature of lipids is that they are insoluble in water. Lipids are soluble in non-polar organic solvents. Lipids may be saturated or unsaturated based on the chemical bonding. Fats and oils are common examples of lipids. Lipids too play an important role in living cells. All living cells, whether plant, bacterial, fungal, animal or human, contain a lipid membrane that acts as the cellular boundary. Lipids are high energy compounds, and their breakdown leads to a greater energy release than carbohydrates. Certain lipids act as chemical messengers, and relay important signals between cells to regulate their functioning. Many hormones are also composed of lipids.

Nucleic acids: Nucleic acids carry genetic information. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are nucleic acids. The DNA double helix allows genetic information to be read and expressed. Genetic information present in DNA is passed on to the next generation. I have covered DNA structure and function here. 

Catabolism and Anabolism

Figure 1 depicts a simplified schematic diagram of metabolism. How do living organisms derive energy from food? When food is consumed, it is digested via catabolic reactions. During this process, the larger carbohydrates, proteins and lipids present in food are oxidised into smaller units. Thus, complex carbohydrates like starch and glycogen are oxidised to glucose, lipids are oxidised to smaller fatty acids and glycerol, while proteins are oxidised to amino acids. Relatively smaller amount of energy is released during these reactions as heat. However, heat cannot be used to do work by the cell. In the next stage of digestion, glucose, fatty acids and amino acids are further oxidised completely via different pathways. In each step of oxidation, electrons are removed from the intermediate compounds that are formed. In the final stage, the electrons are transferred to oxygen (which we breathe) while ATP gets synthesised. The cell can use ATP to do biological work. Unused ATP molecules are stored safely for future use. It is worth noting that glucose is the first choice of energy generation. In the absence of carbohydrates, the cells use lipids as the energy source and oxidise them. That is why, exercise performed in the morning before breakfast is considered best for burning fats. Since proteins have other crucial roles, they are not used for energy generation. Only under conditions of extreme starvation does the body resort to using proteins for generating energy.

Anabolic pathways use certain intermediate compounds to build large biomolecules like proteins, lipids and nucleic acids. These reactions consume ATP and hence energy is required for these reactions. 

                                     Figure 1: A schematic overview of metabolism

Weight gain/loss is regulated by metabolism

Metabolism plays an important role in how our bodies gain or lose weight. The food that we consume has a certain calorific value. A calorie is a unit that estimates how much energy a particular foodstuff can provide. If we exercise, we burn calories. Regular exercise conditions our metabolic rate and makes us energetic. If we consume more calories than what is required, we gain weight and experience lethargy. It is also important to note that the type of food we eat has a large bearing on metabolic activity. “Junk” food has a lot of calories, but they aren’t particularly useful. For example, excess consumption of chocolate, ice cream or pizza only leads to accumulation of fat as the excess carbohydrates in them are converted to fats. Likewise, deep fried foods too contain lots of useless calories that are simply stored as fat. It is essential to consume a healthy diet not only for improving metabolism, but also for maintaining good health and energy.

Finally, for some fun, which biomolecule are you?