The so called cell hypothesis is a biological science foundational theory. Cells are the primary units of structure and reproduction for all creatures, according to this hypothesis.

In terms of human biology, this means that understanding cells is critical to comprehending both the structure and function of the human body. There are approximately 100 trillion cells in the human body. Their architecture, functions, and life spans are all very different.


Despite their vast differences in form and function, all human cells have certain characteristics. To begin, keep in mind that human cells are eukaryotic, meaning they have a nucleus and other structures that are separated by a membrane. The nuclear and cytoplasmic compartments of the cell are separated by a nuclear envelope. The substance that lies between the nuclear and plasma membranes is known as cytoplasm. It is made up of cytosol, a gelatinous fluid, and organelles, which are diverse structures. Water, electrolytes, proteins, lipids, and carbohydrates make up the cytosol.



The plasma membrane is the cell’s outermost layer. It’s a thin (7.5–10 nm) structure made up of 55 percent proteins, 25% phospholipids, 13 percent cholesterol, 4% other lipids, and 3% carbohydrates. Figure 2.3 depicts the arrangement of various chemical components. The fluid mosaic model is a means of representing the plasma membrane. According to this paradigm, a fluid membrane made up of lipids contains a mosaic of different types of proteins (i.e., a mosaic).

Structure (a) and composition (b) of the plasma membrane


The lipid component of the cell membrane has a viscosity similar to olive oil or another sort of cooking oil. Lipids are a diverse group of compounds that can be dissolved in nonpolar liquids. The biological significance of this feature is that they are hydrophobic (water-hating) molecules since they are not soluble in water. Proteins and carbohydrates, on the other hand, are soluble in polar liquids, including water, and belong to the hydrophilic (water-loving) chemical family. The major lipids that make up the membrane are phospholipids.

They combine to produce a two-layered film known as a lipid bilayer. Understanding the structure and function of the plasma membrane requires an understanding of phospholipids’ chemical structure. A phospholipid molecule is made up of a polar head with a phosphate group, an organic component like choline, and two nonpolar tails with fatty acid chains.

Chemical structure of a phospholipid. R is a simple organic molecule such as

Cholesterol is a steroid type of lipid that is also found in the plasma membrane. These lipid molecules are distributed throughout the membrane’s nonpolar area. The plasma membrane has an essential functional consequence in that it establishes a lipid barrier between two aqueous environments, preventing water and water-soluble solutes (such as glucose, ions, and urea) from moving into and out of the cell. This is not to argue that water and its solutes do not flow between the intracellular and extracellular compartments; rather, particular transport proteins are required to convey these molecules across the cell membrane. The lipid bilayer does not obstruct the passage of lipid-soluble substances through the membrane (for example, oxygen and carbon dioxide).


Membrane proteins are globular masses that are connected to the lipid bilayer. Peripheral proteins are connected to the membrane’s inner or outer surfaces and do not pass through the lipid part. Integral proteins penetrate the membrane’s nonpolar core and extend from each of its surfaces. Some peripheral proteins are lightly coupled to the polar heads of phospholipids or the polar surfaces of integral proteins, whereas others are more securely bound to the nonpolar tails.

Major types of proteins are found in the plasma membrane.

Integral proteins are divided into four functional groups channels; 2) carriers; 3) enzymes; and 4) receptors are the four types of receptors. Through the cell membrane, channels form holes. They let water and other water-soluble compounds readily circulate between intracellular and extracellular fluids. A molecule’s ability to travel through a channel is determined by its size; molecules larger than the channel’s diameter cannot enter or leave the cell this way. Carrier proteins provide a second method for transporting water-soluble compounds across the plasma membrane. These proteins change the form of molecules to move them across the membrane.


Sugars make up the majority of membrane-associated carbohydrates. Cn (H2 O)n is the general chemical formula for these compounds. Sugars are any monosaccharide or disaccharide that is used to store energy in biology. Glucose and fructose are the most common monosaccharides. Dextrose, which is made up of two glucose molecules, and sucrose, which is made up of one glucose molecule and one fructose molecule, are two common disaccharides. These small, water-soluble molecules are usually found linked to proteins and lipids on the plasma membrane’s external surface. They produce the glycocalyx, which aids in membrane structural stabilization, cell identification, and immunology.


The cytoplasm consists of an intracellular fluid known as cytosol, membrane-bound compartments known as organelles, and non-membranous objects known as inclusions.


The cytosol is a transparent, semi-gelatinous fluid that contains dissolved nutrients, proteins, ions, and waste products (e.g., glucose and amino acids). Because the plasma membrane controls the passage of chemicals into and out of the cell, its chemical composition differs from that of the extracellular fluid.


The cytoplasm of cells contains a variety of tiny entities or structures. Some of these have their own border membranes (such as organelles), while others don’t (i.e., inclusions). Membrane-bound structures seen in cytoplasmic organelles include cytoplasmic vesicles; mitochondria; endoplasmic reticulum; Golgi complex; mitochondria; and endoplasmic reticulum. These structures’ membranes are comparable to the plasma membrane in that they are all made up of a phospholipid bilayer.

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Mitochondria are found throughout the cytoplasm and vary in number depending on the cell type. Their principal function is energy metabolism, which entails performing biochemical operations that take energy from metabolic fuels like glucose and store it in the phosphate bonds of adenosine triphosphate as chemical energy (ATP). Mitochondria differ in size and shape depending on the cell type, but they all have two membranes. Many folds (crests, or cristae) emerge inward from the inner membrane. These folds are home to a variety of oxidative enzymes.

Other types of enzymes involved in the extraction of energy from metabolic fuels are found in the mitochondrion’s inner cavity (matrix). The oxidative enzymes make ATP in collaboration with matrix enzymes, which is then distributed throughout the cell to power cellular functions. Mitochondria are self-replicating organelles that can grow in number in response to cellular ATP demands.

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