Carbohydrates and Lipids in Cells

Section3:Carbohydrates and Lipids in Cells

Just as any machine requires external energy to operate, the life activities of cells also require energy to sustain. Many organic substances can provide energy for cellular life, with carbohydrates being the main energy source.

Carbohydrates in Cells

When it comes to sugars, we are familiar with names like granulated sugar, cane sugar, rock candy, and glucose, among others. Besides these well-known sugars, substances like starch and cellulose also belong to the carbohydrate group. What are the similarities and differences in the molecules of these carbohydrates? Starch and cellulose are not sweet, so why are they also classified as carbohydrates?

Carbohydrate molecules are typically composed of the elements C, H, and O. Most carbohydrate molecules have a hydrogen to oxygen ratio of 2:1, similar to water molecules, hence carbohydrates are also known as "hydrates of carbon," abbreviated as (CH2O).

Carbohydrates can generally be divided into monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

During acute enteritis, patients often receive intravenous fluids containing glucose (C6H12O6). Glucose is the main energy substance needed for cellular life activities and is often described as the "fuel of life." Glucose cannot be hydrolyzed and can be directly absorbed by cells. Such sugars that cannot be hydrolyzed are known as monosaccharides. Common monosaccharides include fructose, galactose, ribose, and deoxyribose.

Disaccharides

Disaccharides (C12H22O11) are formed by the dehydration synthesis of two monosaccharide molecules and generally need to be hydrolyzed into monosaccharides before they can be absorbed by cells. The most common disaccharide in daily life is sucrose, found in sugars like brown sugar, white sugar, and rock candy, all of which are types of sucrose. Sugarcane and sugar beets contain abundant sucrose, and it is also found in most fruits and vegetables. Other common disaccharides include maltose found in germinating grains like wheat, and lactose found abundantly in the milk of humans and animals.

Polysaccharides

The majority of carbohydrates in organisms exist in the form of polysaccharides [(C6H10O5)n]. Starch is the most common polysaccharide (Figure 2-2). Green plants produce starch through photosynthesis, serving as stored energy in plant cells. Seeds of cereal crops such as corn, wheat, and rice are rich in starch, which is also abundant in tubers like potatoes, yams, and sweet potatoes, as well as in the fruits of certain plants. Starch ingested by humans must be digested and broken down into glucose before it can be absorbed and utilized by cells.

Food starch, when hydrolyzed, converts into glucose, which serves as the raw material for animals and humans to synthesize animal polysaccharides—glycogen (Figure 2-3). Glycogen is primarily stored in the liver and muscles of animals and humans, serving as their energy storage material. When cellular activities deplete energy, and the glucose content in the blood falls below normal levels, glycogen in the liver breaks down to promptly supplement glucose.

Have you ever noticed cotton, palm, and hemp plants? They all have long fibrous filaments, as well as fibers distributed in the stems, branches, and leaves of other plants, and the cell walls of all plant cells, mainly composed of cellulose. Cellulose is also a polysaccharide, insoluble in water, and difficult to digest in the bodies of humans and animals. Even herbivores, despite having developed digestive organs, require the aid of certain microorganisms to break down such polysaccharides. Like starch and glycogen, cellulose is composed of many glucose molecules connected together. As shown in Figure 2-3, their basic units are glucose molecules.

Chitin is also a polysaccharide, also known as shell polysaccharide, widely present in the exoskeletons of crustaceans and insects (Figure 2-4). Chitin and its derivatives have wide applications in medicine, chemical engineering, and more. For example, chitin can effectively bind with heavy metal ions in solution and is thus used in wastewater treatment; it can be used to make packaging paper and food additives; it can be used to create artificial skin; and so on.

Lipids in Cells

Have you ever noticed the fat in meat products? The main component of fat is lipids (Figure 2-5); vegetable oils are extracted from oil crops, and their main component is also fat. Fat is a type of lipid (lipid) found in all cells and is an important organic compound in cell and organism composition. Similar to carbohydrates, lipids consist primarily of the chemical elements C, H, and O, with some lipids also containing P and N. Common lipids include fats, phospholipids, and steroids, which differ greatly in molecular structure, usually insoluble in water but soluble in lipophilic organic solvents such as acetone, chloroform, and ether.

Fat

Fat is the most common lipid. Please discuss the following issues based on your existing life experience.

Fat is formed by the reaction of three molecules of fatty acids with one molecule of glycerol, forming an ester known as triglyceride (also called glycerol triester, Figure 2-6). Glycerol molecules are relatively simple, while the types and lengths of fatty acids are not the same. Fatty acids can be saturated or unsaturated. Most plant fats contain unsaturated fatty acids and are liquid at room temperature, such as edible oils used for daily cooking (peanut oil, soybean oil, and rapeseed oil, etc.); most animal fats contain saturated fatty acids and are solid at room temperature.

The "backbone" of fatty acids is a long chain composed of carbon atoms. Carbon atoms bond with other atoms through covalent bonds. If each carbon atom on the long chain is singly bonded to adjacent carbon atoms, then the carbon atom can bond with two hydrogen atoms, and this carbon atom is saturated, forming a saturated fatty acid. Saturated fatty acids have high melting points and easily solidify. If there are double bonds in the long chain, then the number of hydrogen atoms bonded to the carbon atom cannot reach saturation, forming unsaturated fatty acids. Unsaturated fatty acids have lower melting points and do not easily solidify.

Phospholipids

The difference between phospholipids and fats is that one hydroxyl group (—OH) of glycerol does not form an ester with fatty acids, but rather combines with phosphoric acid and other derivatives. Therefore, in addition to containing C, H, and O, phospholipids also contain P and sometimes N.

Phospholipids are an important component of cell membranes and are also important components of various organelle membranes. In the seeds of humans and animals, phospholipids are abundant.

Steroids

Steroids include substances like cholesterol, which is an important component of animal cell membranes, involved in the transport of lipids in blood; sex hormones promote the development of reproductive organs in humans and animals, as well as the formation of reproductive cells; vitamin D effectively promotes the absorption of calcium and phosphorus in the intestines of humans and animals.

Carbohydrates and Lipids in Cells

Glucose not only supplies cells for utilization but also, when in surplus, can be converted into glycogen for storage; if glucose is still in excess, it can be converted into fat and certain amino acids. Providing livestock and poultry with feed rich in carbohydrates promotes their fattening because carbohydrates in their bodies convert into fat. After food fat is digested and absorbed, it can be stored in adipose tissue and other connective tissues as fat tissue. However, the degree of conversion between carbohydrates and fats differs significantly. For example, in situations where carbohydrates are abundant, they can be extensively converted into fat; whereas fats generally only break down to supply energy when there is a metabolic disorder affecting carbohydrate metabolism, and they cannot be extensively converted into carbohydrates. One gram of glycogen oxidizes to release approximately 17 kJ of energy, while one gram of fat can release approximately 39 kJ of energy. Fat is a good energy storage material in cells, and when life activities require it, it can be broken down and utilized.

Fat is not only an energy storage material but also an excellent insulator. Large marine mammals like whales and seals (Figure 2-7) have thick layers of fat under their skin, serving to maintain warmth. Penguins, living in the cold environment of Antarctica, have fat in their bodies up to 4 cm thick. Fat distributed around visceral organs also acts as a buffer and shock absorber, protecting internal organs.

Phospholipids differ from fats in that one hydroxyl group (—OH) of glycerol does not form an ester with fatty acids but combines with phosphoric acid and other derivatives. Therefore, in addition to containing C, H, and O, phospholipids also contain P and sometimes N.

Steroids include cholesterol, sex hormones, and vitamin D, among others. Cholesterol is an important component of animal cell membranes and is also involved in the transport of lipids in blood; sex hormones promote the development of human and animal reproductive organs and the formation of reproductive cells; vitamin D effectively promotes the absorption of calcium and phosphorus in the intestines of humans and animals.

Carbohydrates and lipids in cells can be converted into each other. Glucose in the blood not only supplies cells for utilization but also can be converted into glycogen for storage