3.1 Cell structure
The cell is the fundamental functional unit of the human body, with an estimated 75 to 100 trillion cells comprising various forms and functions. Cells are incredibly small, usually only about 0.01 mm across, making even our largest cells no bigger than the width of a human hair. Yet, despite their size, cells exhibit immense versatility in their functions and structures.
Some cells form sheets, like those found in our skin or lining our mouths. Others, such as striated muscle cells, can be several inches long and possess the unique ability to shorten in length, leading to muscle contractions. Fat cells are small and round and store fatty acids for energy needs during lean times.
A remarkable characteristic of cells is their ability to reproduce themselves, as they can only arise from pre-existing cells. The complex human body originates from the union of two existing cells, the female egg and the male sperm. These sex cells merge to form a larger cell called the zygote, which is the starting point for developing a multi-trillion-celled human body (Roberts, 2016).
The zygote divides and forms two cells, which then divide to form four cells, and so on. This process continues throughout our lives as cells constantly die and are replaced by new ones. Even when the total number of cells reaches a relatively fixed amount, cell division persists to ensure the replacement of old or dead cells, maintaining the intricate organization and functionality of the human body.
3.1 Cellular components
Each cell type possesses its unique anatomy and physiology, which is achieved through subcellular structures known as organelles. Despite the incredible diversity among cells, there are certain features that all cells share, including an outer membrane, a control center called a nucleus, and tiny powerhouses called mitochondria. These common organelles contribute to the overall functionality of each cell within the human body.
3.1.1 Plasma membrane
The plasma membrane, also called the cell membrane, is the membrane found in all cells that separate the interior of the cell from the outside environment. It is a complex structure comprising mostly proteins and a phospholipid bilayer. The phospholipid bilayer (which is made up of glycerol, two fatty acids, and a phosphate group) forms a double-walled balloon-like structure with proteins embedded in these bilayer sheets. The nutritional significance of this structure is that the cell membrane is made up of fatty acids, which are part of the phospholipid bilayer. For this reason, fats are an important part of the diet.
The plasma membrane, or the cell membrane, provides protection for a cell. It also provides some structural support for a cell as it forms a fixed environment inside the cell.
The membrane has several different functions. One is to transport function: The plasma membrane regulates the transport of materials entering and exiting the cell. It can selectively allow the transport of molecules through it and also actively transport certain compounds across it via special mechanisms. It is therefore referred to as a semi-permeable plasma membrane. This gives the cell control over what substance and how much of a substance it allows inside.
Additionally, the cell can rid itself of undesirable compounds while retaining desirable ones. Insulin is an important hormone responsible for stimulating glucose and amino acid uptake across the plasma membrane. Insulin levels increase in the body after a meal to ensure that these vital nutrients get into the cell. There are ways to maximize the function of insulin through supplementation and timing of meals around training, which will be discussed later.
3.1.2. Nucleus
A nucleus is a membrane-enclosed organelle within a cell that contains chromosomes. Simplified, the nucleus is the center of the cell. Usually, it is situated approximately in the center of each cell and is slightly darker than the surrounding cytoplasm. Essentially, a nucleus is a cell within a cell, which has a membrane of its own and contains all of the cell’s chromosomes, which encode the genetic material
Strands of DNA (deoxyribonucleic acid) form chromosomes. The human cell contains two sets of 23 chromosomes, making a matching set of 23 pairs and a total of 46 chromosomes. Each parent contributes one set of chromosomes from sex cells (the sperm and egg). Chromosomes contain the genetic information responsible for the way we look. They are suspended in a liquid called the nucleoplasm. The liquid between the plasma membrane and the nuclear membrane is called cytoplasm or cytosol.
The X and Y chromosomes, which determine the biological sex of an individual, were only discovered in the early 20th century
The nucleus typically functions to initiate the production of substances needed by the cell. The process is initiated by an intercellular signal which causes specific genes or certain chromosomes to produce exact copies of the gene sequence being activated. These pieces of material carrying genetic information are called messenger RNA (Ribonucleic Acid). The information contained on the messenger RNA strands may be the sequence of amino acids needed for a protein molecule such as insulin. The messenger RNA is then transported from the nucleus through pores in the nuclear membrane and onto the cytoplasm. Once in the cell’s cytoplasm, the messenger RNA strand is used as a template to make molecules in the cytoplasm. For this to occur, ribosomes must be connected to the messenger RNA strand.
Ribosomes are also organelles that run along the messenger RNA strands while in the cytoplasm. As the ribosomes go along the messenger RNA strand, they function to connect each code point along the RNA to the corresponding transfer RNA which has an amino acid attached to it. In the same way, the ribosomes roll along the messenger RNA, amino acids are struck together to form proteins, enzymes, and other compounds. The protein chains cannot be completed if certain amino acids are missing. This is why adequate and effective protein intake is mandatory. Protein synthesis can be reduced or temporarily stopped if an essential amino acid is lacking. This concept of the limiting nutrient is important to consider. The diet can be abundant in calories, but if an essential nutrient is in short supply, it can limit certain reactions needed for the cell to thrive.
3.1.3. Ribosomes
Ribosomes are extremely small and spherical organelles made up of protein and RNA. They are the most numerous of cell organelles. They are found scattered throughout the cell’s cytoplasm and along the surface of another organelle, the endoplasmic reticulum. Ribosomes are located in the cytoplasm and are the cell’s protein synthesis site. The ribosome reads the messenger RNA (mRNA) sequence and translates that genetic code into a specified string of amino acids. That specified amino acid string grows into long chains that fold to form proteins. Ribosomes on the endoplasmic reticulum synthesize compounds for use outside the cell and can be channeled out of the cell for export, such as hormones and digestive enzymes.
3.1.4. Endoplasmic Reticulum (ER)
The endoplasmic reticulum is a network of membranes within the cytoplasm through which proteins and other molecules move. ER exists in two forms, rough ER and smooth ER. Rough ER has ribosomes attached. Here is where proteins and other biomolecules can be made and transported through the ER’s canal network to other parts of the cell and outside the cell. Smooth ER is without ribosomes, and it helps synthesize and concentrate various substances needed by the cell.
3.1.5. Golgi apparatus
It consists of stacks of tiny, oblong sacs embedded in the cell’s cytoplasm near the nucleus. It is a cell organelle that helps process and package proteins and lipid molecules, especially proteins destined to be exported from the cell.
Golgi sacs are responsible for the synthesis of carbohydrate biomolecules. These carbohydrates are then combined with the proteins made in the endoplasmic reticulum to form glycoproteins. Glycoproteins function as enzymes, hormones, antibodies, structural proteins, etc.
A Golgi body, also known as a Golgi apparatus, was named after its discoverer, Camillo Golgi.
3.1.6. Lysosome
Lysosomes are other sac-like structures whose size and shape change with the degree of their activity. They start out small, and as they become active, they increase in silver.
A lysosome is a membrane-bound cell organelle that contains digestive enzymes. Lysosomes are involved with various cell processes. They break down excess or worn-out cell parts like protein, fat, and nucleic acid. The broken-down products formed inside the lysosome can be used as raw material for synthesizing new biomolecules or as energy.
Lysosomes may be used to destroy invading viruses and bacteria. If the cell is damaged beyond repair, lysosomes can help it to self-destruct in a process called programmed cell death, or apoptosis.
3.1.7. Mitochondria
Mitochondria, often discussed in the context of athletic performance, is the second most well-known organelle after the nucleus due to its crucial role in energy generation. Commonly referred to as the cell’s powerhouse, mitochondria are small, complex organelles with a sausage-like shape. They feature a smooth outer membrane enclosing an inner membrane, creating a sac within a sac. The inner membrane is folded, resembling an accordion, and forms several inward extensions known as cristae.
Mitochondria house enzymes vital for producing one of the most important biomolecules, ATP (adenosine triphosphate). ATP stores energy within the mitochondria, which is then used to power various biological functions. More information about ATP will be provided in subsequent units. Catabolic enzymes within the inner mitochondrial membrane catalyze reactions that supply cells with life-sustaining energy.
Nutrients such as glucose and fatty acids consist of carbon atoms connected by chemical bonds. When these bonds are broken down, energy is released. This energy is captured and stored within ATP molecules inside the intricate environment of the mitochondria, allowing the body to utilize it. In essence, glucose’s energy is transferred to the ATP molecule, making it available for use by the body.
Mitochondria have their DNA separate from the nucleus because they were once free-living bacteria engulfed by ancestral cells in an endosymbiosis process, leading to their evolution into the mitochondria we know today.
These biological structures are among the primary components of cells. Other structures include glycogen granules, which are responsible for storing glycogen and housing enzymes for glycogen breakdown and synthesis. Although not a structure, the cytoplasm is noteworthy. This liquid portion of the cell serves as the site for numerous reactions, including gluconeogenesis (glucose and glycogen formation), fatty acid synthesis, amino acid activation, and glycolysis (the initial phase of breaking down glucose to produce ATP molecules for energy).