Chemistry of Life

All life on Earth is composed of chemicals arranged in specific structures. While the periodic table includes many elements, only a particular subset—like carbon, hydrogen, oxygen, and nitrogen—plays a central role in the chemistry of life, forming stable and complex molecules that are essential for biological processes.

This tile will track through the chemical compositions that make up living organisms, starting with perhaps the most important of all: carbon.

The image above is a graphical representation of a kind of structure known as graphene, an extraordinary material made of a single layer of carbon atoms. The carbon atoms are arranged in a hexagonal lattice, resembling a honeycomb.

Despite being only one atom thick, graphene is incredibly strong, lightweight, and highly conductive, and these days it is being used in cutting-edge technologies, such as nanoscale machines, ultra-sensitive sensors, and advanced electronic devices.

But long before these technologies, nature used the unique properties of carbon to build life.

Carbon is a fundamental element in the chemistry of life, acting as the backbone for a wide variety of biological molecules.

This is because carbon has a unique ability to form stable bonds with many other elements, including with itself. This characteristic allows for the creation of complex molecules essential for life, such as proteins, nucleic acids, carbohydrates, and lipids.

Specifically, carbon atoms can form four covalent bonds with other atoms. This means that a single carbon atom can connect with up to four other atoms, creating a three-dimensional structure.

For example, in a methane molecule (CH4), carbon forms four bonds with hydrogen atoms, which gives the molecule a stable structure.

This ability to form multiple bonds makes carbon incredibly versatile, allowing it to be the foundation for large and complex molecules, known as macromolecules.

Carbon's ability to form double and triple bonds with other carbon atoms adds another layer of diversity to molecules. These types of bonds change the shape and properties of the molecules. For example, double bonds make a molecule more rigid and less flexible, affecting how it behaves and reacts with other substances.

In addition to bonding with other carbon atoms, carbon can also form stable bonds with elements like oxygen, nitrogen, sulfur, and phosphorus.

When carbon bonds with these elements, it creates functional groups, which are specific clusters of atoms that give a molecule certain properties and make it behave in a predictable way.

Functional groups are important because they determine how a molecule interacts in chemical reactions. For example:

• The hydroxyl group (–OH) makes a molecule more likely to dissolve in water.
• The carboxyl group (–COOH) makes a molecule acidic, like in fatty acids.

These functional groups are essential for processes in living organisms, such as metabolism, gene expression, and cell communication.

Carbohydrates are a prime example of the complexity and diversity that carbon can create in biological systems.

Carbohydrates are composed of carbon, hydrogen, and oxygen, and they play crucial roles in energy storage, structural integrity, and cellular communication in living organisms.

Simple carbohydrates, like glucose, serve as a primary energy source for cells.

When multiple glucose molecules bond together, they can form complex carbohydrates, such as starch and cellulose, which are essential for energy storage in plants and structural support in plant cell walls, respectively.

Humans and other animals consume starchy plants, like potatoes, to access this stored energy, breaking down the starch into glucose for use in metabolic processes.

Proteins are essential molecules that play a crucial role in the characteristics of life. They are made up of smaller units called amino acids, which are linked together in long chains.

The sequence of these amino acids determines the protein's shape and function.

Proteins are involved in almost every process within living organisms, making them vital for life.

One of the primary roles of proteins is to act as enzymes. Enzymes are proteins that speed up chemical reactions in the body, which is essential for metabolism. Without enzymes, reactions would occur too slowly to sustain life.

For example, digestive enzymes help break down food into nutrients that the body can use for energy, growth, and repair.

Proteins also provide structural support to cells and tissues. For instance, collagen is a protein that gives strength and structure to skin, bones, and connective tissues.

Keratin, another protein, is found in hair, nails, and the outer layer of skin, providing protection and durability.

These structural proteins help maintain the shape and integrity of cells and tissues, which is crucial for the proper functioning of an organism.

Transport proteins are another important type of protein. They help move substances across cell membranes and throughout the body.

Hemoglobin, a transport protein in red blood cells, carries oxygen from the lungs to the rest of the body and brings carbon dioxide back to the lungs for exhalation. This transport of gases is vital for respiration and energy production in cells.

Proteins also play a key role in cell signaling and communication.

Receptor proteins on the surface of cells bind to signaling molecules, such as hormones, and transmit messages into the cell.

This process helps cells respond to changes in their environment and coordinate activities within the organism.

For example, insulin is a regulatory protein or hormone, that controls blood sugar levels by signaling cells to take up glucose.

In addition to these roles, proteins are involved in the immune response.

Antibodies are proteins that recognize and neutralize foreign invaders, such as bacteria and viruses, protecting the body from infections. This defense mechanism is essential for maintaining health and preventing disease.

Proteins are incredibly versatile because they can fold into a wide variety of shapes, allowing them to interact specifically with other molecules.

This specificity is crucial for the regulation of cellular processes.

For example, enzymes have active sites that bind to specific substrates, ensuring that biochemical reactions occur with precision and efficiency.

Moreover, proteins can change their shape in response to environmental conditions or interactions with other molecules.

These changes can activate or deactivate the protein's function, providing a mechanism for regulating cellular activities.

For instance, the binding of a hormone to its receptor can trigger a series of events inside the cell, leading to a specific response.

We already touched upon carbohydrates in the orb focused on carbon. This section is going to go into a little more depth.

Carbohydrates are essential molecules that play a crucial role in the characteristics of life, particularly in energy processing, growth and development, and response to stimuli.

They are composed of carbon, hydrogen, and oxygen, and are found in foods like fruits, vegetables, grains, and dairy products.

One of the primary roles of carbohydrates is to provide energy.

Simple carbohydrates, such as glucose, are quickly absorbed by the body and used as a direct source of energy. Glucose is especially important because it is the main fuel for our cells.

During cellular respiration, glucose is broken down to produce adenosine triphosphate (ATP), which cells use to power various activities, such as muscle contraction and nerve function.

This immediate energy supply is vital for everyday activities and overall bodily functions.

Carbohydrates also come in more complex forms, such as starches and glycogen, which serve as energy storage molecules.

Plants store energy in the form of starch, which is found in foods like potatoes, rice, and corn.

When we eat these foods, our bodies break down the starch into glucose, which can then be used for energy.

Animals, including humans, store energy in the form of glycogen, primarily in the liver and muscles.

When the body needs energy, glycogen is broken down into glucose to maintain blood sugar levels and provide fuel for cells.

This stored energy is crucial for sustaining bodily functions during periods when food intake is low.

In addition to energy storage, carbohydrates provide structural support, which is closely related to the growth and development of living organisms.

For example, as we saw in the previous orb on carbon, cellulose is a carbohydrate that makes up the cell walls of plants. It gives plants their rigidity and strength, allowing them to stand upright and grow.

Although humans cannot digest cellulose, it is an important part of our diet as dietary fiber, which helps maintain a healthy digestive system by promoting regular bowel movements and preventing constipation.

Carbohydrates also play a role in cell recognition and communication.

On the surface of cells, carbohydrates are often attached to proteins and lipids, forming structures known as glycoproteins and glycolipids.

These structures help cells recognize and interact with each other, which is important for the immune system and other cellular processes.

For instance, they help the immune system distinguish between the body's own cells and foreign invaders, such as bacteria and viruses.

Nucleic acids are essential molecules in all living things because they store and pass on genetic information, guiding how cells function and reproduce.

The two main types of nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

The "acid" in nucleic acids comes from the phosphate group, which can lose a hydrogen ion, leaving it with a negative charge.

This negative charge gives the molecule its acidic properties and allows it to interact with water and other charged molecules in the cell.

This interaction is important for the nucleic acid's ability to dissolve in water and function properly in the cell.

Nucleic acids are made up of smaller units called nucleotides. Each nucleotide has three parts: a sugar, a phosphate group, and a nitrogenous base.

DNA is structured like a twisted ladder, known as a double helix. The sugar and phosphate groups form the sides of the ladder, creating a long chain, while the nitrogenous bases form the rungs.

The term "nitrogenous" refers to the fact that these bases (the rungs on the ladder) contain nitrogen atoms.

The nitrogenous bases are the key to the genetic code because they pair up in specific ways to store information. In DNA, the bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

In RNA, thymine is replaced by uracil (U).

In the ladder structure of DNA, adenine pairs with thymine, and cytosine pairs with guanine.

This stable structure allows DNA to securely store genetic information. When needed, the two strands of the DNA can separate to allow the information to be copied or used to make proteins.

RNA, on the other hand, is usually single-stranded and more flexible. Because of this flexibility, RNA can fold into various shapes, enabling it to perform different functions within the cell.

There are three main types of RNA, each with a specific role:

Messenger RNA (mRNA): mRNA carries genetic instructions from DNA to the ribosomes, where proteins are synthesized. It acts as a template for assembling amino acids in the correct order to produce a specific protein.

Ribosomal RNA (rRNA): rRNA is a key component of ribosomes, the cellular structures where proteins are made. rRNA helps to align the mRNA and the ribosomes and catalyzes the formation of the bonds between amino acids.

Transfer RNA (tRNA): tRNA transports amino acids to the ribosome during protein synthesis. Each tRNA molecule recognizes specific sequences of mRNA and ensures that the correct amino acid is added to the growing protein chain.

We’re going to briefly move away from the organic molecules crucial to life to examine a fundamental molecule that will enhance your understanding of later sections, such as lipids. This molecule is water.

Water is essential for life due to its unique properties that directly support living organisms.

One of the key characteristics of water is its polarity. A water molecule is made up of one oxygen atom and two hydrogen atoms. These atoms share electrons, but not equally, which gives the oxygen side of the molecule a slight negative charge and the hydrogen side a slight positive charge.

This makes water a polar molecule, meaning it has a positive and a negative side, similar to a magnet.

Because of its polarity, water can form hydrogen bonds with other water molecules and with other polar substances. These bonds are weak attractions that allow water to dissolve many substances, making it an excellent solvent.

For example, when salt is added to water, the water molecules surround and separate the salt's ions, causing the salt to dissolve.

This ability to dissolve substances is crucial for transporting nutrients and removing waste in living organisms, ensuring that cells get the materials they need to function and stay healthy.

Water also has a high specific heat capacity, meaning it can absorb a lot of heat before its temperature increases. This property helps stabilize temperatures in organisms and their environments.

For instance, the large amounts of water in the human body help maintain a stable internal temperature, which is vital for enzymes and metabolic processes to function properly.

Another important feature of water is its cohesion and adhesion.

Cohesion is the ability of water molecules to stick to other water molecules, which creates surface tension.

This property is illustrated by the way small insects, like water striders, can walk on water without sinking. In living organisms, this surface tension is important for processes like the movement of water in plants.

Adhesion, on the other hand, is water's ability to stick to different substances, which, along with cohesion, enables capillary action. In plants, capillary action helps water travel from the roots to the leaves, which is essential for transporting nutrients and maintaining plant health.

Finally, water helps maintain pH balance in living organisms. pH measures how acidic or basic a solution is, and maintaining a stable pH is crucial for life.

Water acts as a buffer, absorbing excess hydrogen or hydroxide ions by surrounding them with water molecules, which helps to keep pH levels stable.

This is important for maintaining the proper conditions for enzymes and other proteins to function, such as in human blood, where a stable pH of around 7.4 is necessary for the body to function effectively.

Lipids are a group of molecules that don't mix well with water, making them essential for many biological functions.

These molecules are mainly made up of hydrogen and carbon atoms.

Because of this composition, lipids are nonpolar, meaning they don't have a positive or negative charge that would allow them to interact with water molecules.

Water is polar, which means it has regions of slight positive and negative charge, allowing water molecules to stick together.

Since lipids are nonpolar, they don't interact well with water and, therefore, do not dissolve in it. Instead, they separate from water, similar to how oil separates from water in a mixture.

This property is what makes lipids useful for forming barriers, such as cell membranes, and storing energy in the body.

Besides storing energy and forming membranes, lipids also insulate and protect. For example, a layer of fat beneath the skin helps animals maintain their body temperature by reducing heat loss.

Additionally, lipids cushion organs, providing a protective layer that absorbs shock.

The main types of lipids include: fats, oils, waxes, phospholipids, and steroids, each with its own important role in living organisms.

One key function of lipids is storing energy.

Fats, also called triglycerides, are the primary way animals store energy.

A fat molecule consists of a glycerol backbone connected to three fatty acid chains. These fatty acids can be either saturated or unsaturated.

Saturated fatty acids have no double bonds between the carbon atoms, so they are fully saturated with hydrogen atoms. This makes them straight, allowing them to pack tightly together, which is why saturated fats are usually solid at room temperature, like butter.

Unsaturated fatty acids, on the other hand, have one or more double bonds, creating bends in the chain that prevent tight packing, making them liquid at room temperature, like olive oil.

Phospholipids are another important type of lipid, especially for forming cell membranes.

A phospholipid molecule has a glycerol backbone, two fatty acid tails, and a phosphate group.

The phosphate group is hydrophilic, meaning it attracts water, while the fatty acid tails are hydrophobic, meaning they repel water.

This unique structure allows phospholipids to form double layers in water, with the fatty acid tails facing inward and the heads facing outward.

This arrangement creates the cell membrane, a barrier that controls what enters and leaves the cell.

Steroids are a different kind of lipid with a structure made up of four connected carbon rings.

Cholesterol is a well-known steroid that is crucial for cell membrane stability and fluidity.

Cholesterol also serves as a building block for steroid hormones like testosterone and estrogen, which regulate many body functions.

Additionally, cholesterol is necessary for producing bile salts that help digest and absorb fats from food.

Lipids such as steroids also play a role in communication between cells. Some lipids act as signaling molecules, influencing how cells behave.

For example, steroid hormones derived from cholesterol can enter cells and bind to receptors, causing changes in how genes are expressed and how the cell functions.

This ability to signal is vital for maintaining balance in the body and coordinating complex biological responses.