2. Lipid

Lipids

Lipids are a broad group of organic compounds that are not solubilized in water. In other words, any hydrophobic organic macromolecule is classified as a lipid. Due to this, many types of molecules are classified into this group, though they have different structures and functions. 

Here, I will classify them into two types: hydrolyzable and non-hydrolyzable lipids. Hydrolysis is a chemical reaction in which a molecule is divided into smaller pieces by adding a water molecule between them. Hydrolyzable lipids are lipids that can be broken down into smaller molecules through hydrolysis. On the other hand, hydrolysis cannot divide non-hydrolyzable lipids.

I will go over four of the many varieties of lipids that can and cannot be hydrolyzed in this article.

Neutral fats

Neutral fats (or triacylglycerols, or triglycerides) are a type of hydrolyzable lipid. It’s the main constituent of animal and plant fats.

A molecule of glycerol joined to three fatty acids by ester linkages makes up neutral fats. (Esterified fatty acids are called acyl groups, so there are three (tri) acyl groups with glycerol.) (That's the reason for the name “triacylglycerol”.)

The primary function of triacylglycerols is energy storage. When the body needs energy, enzymes break down the easter bonds in neutral fats, releasing fatty acids that can be used for cellular respiration (which we will see in a later topic). 

They are one of the most efficient ways to store energy because they are nonpolar and pack tightly together. From the same amount of carbohydrates, lipids, proteins, and nucleic acids, lipids can generate the largest amount of energy. 

The fatty acids are long-chain carboxylic acids (R-COOH). The chain of carbons can be saturated or unsaturated. Saturation of carbon atoms means the carbon atoms are fully connected with a possibly maximal amount of hydrogen atoms. To achieve this condition, all carbon atoms should be connected only via single covalent bonds. If some of them are connected via double bonds, then those fatty acids are called unsaturated fatty acids. 

Saturated and unsaturated fatty acids have different physical properties. The carbon chains of saturated fatty acids have a relatively straight structure, so they fill relatively smaller spaces. Therefore, they can be packed densely, and the distance between each neutral fat should become narrower. Since the distance between molecules determines the strength of the intermolecular forces, neutral fats with saturated fatty acids have strong intermolecular forces. If the intermolecular forces are strong, then the shape of the material will become stable, so neutral fatty acids tend to be solid at room temperature. As you can see in supermarkets, the fats of animal meats, like beef and pork, are solid. The reason behind it is that animal-origin fats are mostly composed of neutral fats with saturated fatty acids.

The carbon chains of unsaturated fatty acids are bent at the sites of double bonds, so they occupy more spaces in comparison with those with saturated fatty acids. In the same logic, but in the opposite way, neutral fats with unsaturated fatty acids have weak intermolecular forces and tend to be liquid at room temperature. Imagine the lipids in the form of liquid. You might come up with cooking oils, such as sunflower oil and sesame oil. As you guess, the plant-origin lipids are mostly composed of neutral fats with unsaturated fatty acids.

Phospholipids

Phospholipids are also a type of hydrolyzable lipid whose structure is similar to that of neutral fats. They also contain glycerol and fatty acids, with a phosphate group in place of one of the three fatty acids. Phosphate groups are hydrophilic functional groups, in other words, they are water-loving. This indicates that a single molecule contains both hydrophilic and hydrophobic components at the same time. Such a characteristic is known as "amphipathic." Phospholipids are therefore amphipathic molecules. 

Phoshpolipid is the primary component of the biological membrane that covers or divides cells, as we shall see later.

Steroids

Steroids are non-hydrolyzable lipids, or simply saying, they do not contain ester bonds which can be broken by hydrolysis. The term “steroid” does not refer to a specific molecule, but rather it refers to a group of molecules. Each type have different molecular structure, but they share a core structure consisting of four fused rings of carbon atoms, called a gonane. Specific functional groups attache to this core give rise to different steroid types.

The funciton of steroids vary depending on their type. Here, I will give you some important examples of functions of steroids, which is essential for out body.

• Hormone function: Hormones are used as communication tools by the cells. A cell produces a hormone which flow inside the blood, then the hormone attach to another cell. This cell receives information and reacts in a corresponding way to manage the complex biological system. Communication between cells like this is called cellular signaling. For example, sex hormones regulate reproduction of the organism.
• Membrane structure: Cholesterol, a type of steroid, is a component of cell membranes. It is also an amphipathic molecule similar to phospholipids, but it takes much more space than a phospholipid. Due to this, cholesterols function as spacers for the biological membrane, meaning they make distances between molecules in the biological membrane. As we discussed in the topic of neutral fats, the molecular distances determine the fluidity of that substance. We can say cholesterol gives fluidity for the biological membrane to maintain normal function.
• Other examples: bile acid, vitamin D, etc. (We will see later.)

Carotenoids

Carotenoids are a group of yellow, orange, and red pigments found naturally in plants, algae, and some bacteria and fungi. Most carotenoids are composed of isoprene units, five-carbon building blocks linked together to form a long chain. The specific arrangement and presence of oxygen atoms within this chain determine the type of carotenoid and its color.

In plants and algae, carotenoids play several roles, including:

• Light absorption: They play a crucial role in photosynthesis by absorbing light energy.
• Photoprotection: They protect chlorophyll, the main pigment involved in photosynthesis, from damage by excessive light.
• Attractants: Some carotenoids contribute to the bright colors of fruits and flowers, attracting pollinators.

The body can convert some carotenoids, like beta-carotene, into vitamin A, which is necessary for cell growth, immunity, and vision. On top of that, many carotenoids act as antioxidants, helping to protect cells from damage caused by free radicals (a toxic material for organisms).