Human body cell structure

2021-06-11 02:12 PM

Most of the cell organelles are covered by membranes consisting of lipids and proteins. These membranes include the cell membrane, nuclear membrane, endoplasmic reticulum, mitochondrial membrane, lysosomes, and Golgi apparatus.

Cells contain many structures, called organelles. The nature of the organelles is just as important as the chemical structures of the cell for cell function. For example, without one of the organelles, such as mitochondria, more than 95% of the energy a cell releases from nutrients disappear immediately. The most important organelles and other structures are shown in Fig.

Figure. Structure of a typical cell

The membrane structure of the cell

Most of the cell organelles are covered by membranes consisting of lipids and proteins. These membranes include the cell membrane, nuclear membrane, endoplasmic reticulum, mitochondrial membrane, lysosomes, and Golgi apparatus.

The lipid composition of the membrane forms a barrier that impedes the movement of water and water-soluble substances from one compartment of the cell to another because water is not lipid-soluble. However, the protein molecules on the membrane cross the cell membrane, creating a pathway for substances to pass through. In addition, many membrane proteins are enzymes that catalyse various chemical reactions.


The cell membrane (also called the plasma membrane) covers the cell and is a thin, flexible, flexible membrane, only 7.5-10 nm thick. They are composed almost entirely of lipids and proteins. About 55% protein, 25% phospholipid, 13% cholesterol, 4% other lipids, 3% carbohydrates.

The membrane lipid barrier prevents the penetration of water-soluble substances:

The figure shows the structure of the cell membrane. Its basic structure is the lipid bilayer, which is a thin membrane, consisting of two lipid layers, each only one molecule thick, on the surface of the entire cell. Scattered on the lipid layer are large protein molecules.

The lipid bilayer is made up of three main lipids: phospholipids, sphingolipids, and cholesterol. Phospholipids are the most abundant components. One end of the phospholipid molecule is water-soluble, the hydrophilic end. The other end is fat soluble, the hydrophobic end. The phosphate head is the hydrophilic end and the fatty acid end is the hydrophobic end.

Since the hydrophobic ends of the phospholipid molecules are filled with water but attract each other, they have a natural tendency to stick together in the middle of the membrane, as shown. The hydrophilic phosphate head, therefore, forms the two sides of the complete cell membrane, with intracellular fluid on the membrane and extracellular fluid on the outer surface.

The lipid layer in the middle of the membrane is impermeable to water-soluble substances such as ions, sugars, and urea. In contrast, fat-soluble substances such as oxygen, CO2, alcohol can penetrate this part of the membrane easily.

Sphingolipids, derived from sphingosine, also have hydrophilic and hydrophobic groups, occupying a small number of cell membranes, especially neurons. Complex membrane sphingolipid molecules are thought to have several functions, including protection from harmful environmental factors, signal transduction, and as attachment site for extracellular proteins.

The membrane cholesterol molecule is also lipid because its steroid nucleus is also fat-soluble.

These molecules, as if soluble in the bilayer of the membrane. They mainly help determine the permeability of membranes to the water-soluble components of body fluids. Cholesterol also controls the fluidity of membranes.

Transmembrane and peripheral proteins:

The figure also shows raised spherical masses on the surface of the lipid membrane. These membrane proteins are mainly glycoproteins. There are two types of membrane proteins: transmembrane proteins that penetrate the entire membrane and peripheral proteins that attach to only one side of the membrane and do not penetrate the membrane.

Figure. The structure of the cell membrane shows that it is composed mainly of a lipid bilayer of phospholipid molecules, but many protein molecules protrude through the layer. In addition, carbohydrate radicals are attached to protein molecules on the outside of the membrane and additional protein molecules on the inside.

Many transmembrane proteins form transmembrane channel structures where water molecules and water-soluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. Channel proteins are also selective, thus favouring the diffusion of some substances over others.

Other transmembrane proteins act as carrier proteins to transport substances that cannot diffuse across the lipid bilayer. Sometimes these carrier proteins can transport substances against a concentration gradient, called active transport. Others act as enzymes.

Transmembrane proteins can act as a receptor for water-soluble substances, such as peptide hormones, which cannot easily cross cell membranes. The interaction of the membrane receptor with the specific binding radicals causes changes in the receptor's conformation. This process activates enzymes on the intracellular part of the protein or causes an interaction between the receptor and the protein in the cytoplasm, which acts as a second signal, relaying information from the extracellular part of the receptor to the interior of the cell. cell. In this way, transmembrane proteins bridge the cell membrane providing a means of transporting environmental information into the cell.

Peripheral proteins are usually bound to transmembrane proteins. These proteins mostly function as enzymes or as controllers for the transport of substances across membrane channels.

Carbohydrate -Glycocalyx:

Membrane carbohydrates are found almost unchanged in combination with proteins or lipids as glycoproteins or glycolipids. The truth is that most transmembrane proteins are glycoproteins, and about one-tenth of membrane lipids are glycolipids. The "glycol" portion of these molecules protrudes almost unchanged on the outside of the cell, dangling outward from the cell surface. Many other carbohydrates called proteoglycans - mainly carbohydrates attached to a small protein nucleus - are loosely attached to the outside of the cell. Thus, the entire outer surface of the cell has a loose carbohydrate coat called the glycocalyx.

Carbohydrate moieties attached to the outside of cells have several functions:

Many of them are negatively charged, making most cells have their entire outer surface negatively charged thereby repelling other negatively charged objects.

The glycocalyx layer of some cells attaches to the glycocalyx layer of other cells thereby attaching one cell to another.

Many carbohydrate molecules act as receptors for attached hormones, such as insulin, which, upon binding, complexes that activate proteins bound in the membrane, which then activates a cascade of intracellular enzymes.

Cytoplasm and organelles

The cytoplasm is filled with large and small granules and organelles. The jelly-like portion of the cytoplasm in which the particles are scattered is called the cytoplasmic fluid and contains mainly proteins, electrolytes, and glucose.

Scattered throughout the cytoplasm are neutrophils, glycogen granules, ribosomes, secretory vesicles, and five particularly important organelles: the endoplasmic reticulum, the Golgi apparatus, mitochondria, lysosomes, and peroxisomes.

Endoplasmic reticulum

The figure shows the network of tubular and planar systems in the cytoplasm, that is, the endoplasmic reticulum. This organ helps to process molecules made by cells and transport them to specific locations inside or outside the cell. These bags and tubes are connected to each other. In addition, their walls are made up of a lipid bilayer containing a large amount of protein, like a cell membrane. The total area of ​​this structure in some cells—for example, liver cells—can be 30 to 40 times the area of ​​the cell membrane.

The detailed structure of the small parts of the endoplasmic reticulum is shown in Fig. The spaces within the tubules and vesicles are filled with the endoplasmic matrix, a dilute medium different from the cytoplasmic fluid outside the endoplasmic reticulum. Electron microscopy shows that the space within the endoplasmic reticulum connects to the space between the two membranes of the nuclear membrane.

Figure. Structure of the endoplasmic reticulum

Substances made in some parts of the cell enter the space in the endoplasmic reticulum and are then sent to other parts of the cell. Thus, the vast area of ​​this network and its many membrane-bound enzyme systems provide the main machinery for the cell's metabolic function.

Ribosomes and endoplasmic reticulum:

Attached to the outer surface of many endoplasmic reticulum are a large number of small particles called ribosomes. Where these particles are present, the endoplasmic reticulum is known as the granular endoplasmic reticulum. The ribosome is composed of a mixture of RNA and proteins, and its function is to synthesize new proteins in the cell.

Smooth endoplasmic reticulum:

Parts of the endoplasmic reticulum that do not have ribosomes attached. This part is called the grain less or smooth endoplasmic reticulum. The function of the smooth endoplasmic reticulum is to synthesize lipids and several other functions are promoted by enzymes.

Golgi apparatus

The Golgi apparatus, shown in the figure, is closely related to the endoplasmic reticulum. It has a membrane resembling that of a smooth endoplasmic reticulum. The Golgi apparatus normally consists of four or more thin, flat, vesicles adjacent to one side of the cell nucleus. This apparatus develops in secretory cells, which are located on the side of the cell from which secretions are expelled.

Figure. A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and nucleus.

The function of the Golgi apparatus is related to the endoplasmic reticulum. As shown, the small transport particles (also called endoplasmic reticulum vesicles) continue to separate from the endoplasmic reticulum and soon merge into the Golgi apparatus. In this way, substances in the endoplasmic reticulum are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and many other cytoplasmic components discussed in this chapter.


Lysosomes, shown in figure, are vesicular organelles, formed by the Golgi apparatus and dispersed throughout the cytoplasm. Lysosomes provide an intracellular digestive system that allows the cell to digest structures that are harmful to the cell, food that has been eaten by the cell, and unwanted substances such as bacteria. Lysosomes are different in different cell types, but they are usually 250-750 nm in diameter. They are surrounded by a characteristic lipid bilayer and are filled with many small particles with a diameter of 5-8 nm, which are aggregate proteins of 40 different digestive enzymes. Hydrolytic enzymes are capable of separating compounds into 2 or more parts by combining hydrogen from a water molecule with one part of the compound and attaching the hydroxyl part of the water molecule to the rest of the compound. For example, proteins are hydrolysed to amino acids,

Hydrolytic enzymes are concentrated in lysosomes. Normally, the lysosomal membrane prevents hydrolytic enzymes from combining with other substances in the cell and thus inhibits their digestive activity. However, some cellular conditions damage the membrane of the lysosome, allowing the release of digestive enzymes. These enzymes then separate the organic matter into smaller, highly diffusible substances such as amino acids and glucose.


Peroxisomes are like lysosomes, but they differ in two important points. First, they are thought to be formed by self-regeneration (or possibly budding from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second, it contains more oxidase enzymes than hydrolases. Some oxidase enzymes can combine oxygen with hydrogen obtained from various chemicals in the cell to form hydrogen peroxide (H2O2). Hydrogen peroxide is a strong oxidant and is used in combination with catalase, another oxidizing enzyme present in large quantities in peroxisomes, to oxidize substances that may be cytotoxic. For example, about half of the alcohol a person drinks is detoxified to acetaldehyde by the peroxisomes of liver cells in this way. The main function of peroxisomes is to metabolize long-chain fatty acids.

Savings bag

One of the important functions of cells is to secrete special chemicals. Nearly all secretions are formed by the endoplasmic reticulum and the Golgi apparatus and then released from the Golgi apparatus into the cytoplasm as vesicles called secretory vesicles or secretory granules. The figure shows typical secretory vesicles inside pancreatic cells, which contain proenzymes (inactivated enzymes). These proenzymes are then secreted extracellularly into the pancreatic duct and from there into the duodenum, where they are activated and perform their function of digesting food in the digestive system.

Figure. Secretory granules (secretory vesicles) in the acinar cells of the pancreas


Mitochondria, shown in the figure, are called the energy factories of the cell. Without them, the cell is incapable of obtaining energy from nutrients, and essentially all cellular function stops.

Figure. The structure of a mitochondria

Mitochondria are present everywhere in the cytoplasm, but the number of mitochondria in cells varies from less than 100 to several thousand, depending on the energy needs of the cell. Cardiac muscle cells, for example, use large amounts of energy, so have a much larger number of mitochondria than fat cells, which perform little activity and use less energy. Furthermore, mitochondria are concentrated in the part of the cell that is primarily responsible for energy metabolism. They also vary widely in size and shape. Some mitochondria are only a few nanometres in diameter and spherical, while some are elongated and 1micrometer in diameter, 7micrometers long, others are branched or thread-like.

The basic structure of mitochondria, as shown, consists of two lipid bilayers, an outer membrane, and an inner membrane. The inner membrane has many inverted sections that form compartments or small tubes called crests into which the oxidizing enzyme attaches. The mitochondrial crest provides a large area for chemical reactions to take place. In addition, the cavity in the mitochondria is filled with a matrix containing a large amount of the breakdown enzymes needed to derive energy from nutrients. These enzymes work in conjunction with the oxidizing enzyme on the mitochondrial crest to oxidize nutrients, thereby creating CO2 and water at the same time releasing energy. The released energy is used to synthesize an energy-rich substance called adenosine triphosphate (ATP). ATP is then transported out of the mitochondria and diffused throughout the cell to release its energy wherever it is needed for cellular function.

Mitochondria are self-replicating, which means that a mitochondrion can produce a second, a third, and more, wherever the cell requires large amounts of ATP. Indeed, mitochondria contain DNA like the DNA in the cell nucleus. We will see that DNA is the basic chemical component of the nucleus that controls cell replication. Mitochondrial DNA plays a similar role, controlling mitochondrial replication. Cells facing increased energy demands - when that happens, for example in skeletal muscle during repetitive exercise - can increase mitochondrial density to meet energy needs. .

Cytoskeleton - tubular and fibrous structures

The cytoskeleton is a network of fibrous proteins arranged in fibres or tubes. This structure originates from protein molecules synthesized by ribosomes in the cytoplasm. These molecules then polymerize to form fibres. For example, large amounts of actin filaments are frequently found in the outer region of the cytoplasm, called the extra cytoplasm, to give cell membrane flexibility. In addition, in muscle cells, actin and myosin filaments form the special contractile apparatus that is the basis for muscle contraction.

A special kind of stiff filament of polymerized tubular molecules is used in all cells to form strong tubular structures, called microtubules. The figure shows a typical microtubule in the sperm tail.

Figure. Microtubules

Another example of a microtubule is the bony tubular structure in the centre of the microvilli that radiates outward from the cytoplasm to the apex of the microvilli. This structure is discussed later in this chapter and illustrated in Fig. In addition, both the centrosomes and mitotic spindles of dividing cells also contain microtubules.

Thus, the basic function of microtubules is to act as a cytoskeleton, providing a strong structure for the cell. The cytoskeleton not only determines cell shape, but also participates in cell division, allows cell movement, and provides a rail-like system to control the movement of organelles. in the cell.

Cell nucleus

The nucleus, which is the control centre of the cell, sends signals to the cell to grow and mature, to replicate, or to die. The nucleus of a cell contains a large amount of DNA, including genes. Genes determine the specificity of cellular proteins, including structural proteins, as well as intracellular enzymes that control cytoplasmic and nuclear activity.

Genes also control and promote cell reproduction. The gene first copies to form two sets of genes, then the cell divides by a special process called meiosis to form two daughter cells, each of which receives one of the two sets of genes.

Unfortunately, the appearance of the cell nucleus under the microscope does not provide any clues as to the mechanism by which the cell nucleus performs its regulatory action. The figure shows the cell nucleus during interphase under the light microscope (intermittent interval), showing darkly stained chromatin throughout the nucleus. During meiosis, chromatin makes up a chromosomal structure, which can be easily seen under an optical microscope.

nuclear membrane

The facial membrane, also called the nuclear envelope, is two double membranes, one inside the other. The outer membrane is continuous with the endoplasmic reticulum of the cytoplasm, and the space between the two membranes also connects to the space in the endoplasmic reticulum, as shown in Fig.

Figure. The structure of the nucleus

The nuclear membrane is pierced by thousands of nuclear pores. The complex protein molecules attach to the edge of the hole so that the central region of the hole is only 9 nm in diameter. However, the hole is large enough for molecules with a weight of 44000 to pass through easily.

Nuclei and ribosome formation

The nucleus of most cells contains one or more pigmented structures called the nucleus. The nucleus, unlike most of the other organelles mentioned here, does not have a limiting membrane. Instead, it is simply a concentration of large amounts of RNA and proteins such as those found in ribosomes. The nucleus becomes larger when the cell actively synthesizes proteins.

The formation of the nucleus (and ribosomes in the extranuclear cytoplasm) begins in the nucleus. First, the separate DNA genes of the chromosomes synthesize RNA. Some of the synthesized RNA is stored in the nucleus, but most of it is transported out through the nuclear pores to the cytoplasm. There, they are combined with specific proteins to form mature ribosomes that play a major role in cytoplasmic protein synthesis.