Why are the structures that make up the endomembrane system grouped together?

Cell is the fundamental and structural unit of all living entity. It is the  biological, structural, and functional unit of all plants and animals. cells are the ‘Building Blocks of Life’ or the ‘Basic units of Life’. Organisms made up of a single cell are ‘unicellular’ whereas organisms made up of many cells are ‘multicellular’. Cells perform many different functions within a living organism such as digestion, respiration, reproduction, so on. cells give rise to a tissue → multiple tissues make up an organ → many organs create an organ system → several organs systems functioning together make up the human body.

Cell Theory

1838, a German botanist, Matthias Schleiden was quick to express that cells are the structure blocks, everything being equal. In the next year, another German botanist, Theodor Schwann expressed that cells are the crucial units of animals as well. These assertions finished the idea that plants and creatures have fundamental differences of structure.

Their discoveries led the detailing of the ‘Cell Theory’. Cell Theory expresses that cells are the essential units of every single living organic entity. Yet, the cell hypothesis neglected to make sense of how new cells emerge. In 1855, Rudolf Virchow, a German physiologist expressed in German ‘Omnis cellula e cellula’ which implies that new cells come from previously existing cells.

Types of cell

Cell are two types, prokaryotic cell, eukaryotic cell

Prokaryotic cell:Prokaryotic cells need both, a distinct nucleus and membrane bound cell organelles. Instances of prokaryotes are blue green growth, microbes and mycoplasma.

Among prokaryotes, microbes are the most well-known and increase extremely quick. They are single-celled and range in size from 0.2 to 10 microns (multiple times less than most plant and creature cells). Microscopic organisms are found all over the place – in rocks, soil, sea water

Eukaryotic cell:Eukaryotic cells are characterized as cells containing organized nucleus and organelles which are enveloped by layer bound organelles. Instances of eukaryotic cells are plants, animals, protists, parasites. Their hereditary material is coordinated in chromosomes. Golgi contraption, Mitochondria, Ribosomes, Nucleus are portions of Eukaryotic Cells.

Functions of cell

They provide structure for the body, use in nutrients from food to convert the nutrients into energy, and carry out specialized functions. Cells also consist the body’s hereditary material(genetic material)and can make copies of themselves.

Endomembrane System

While each of the membranous organelles is distinct in terms of its structure and function. A large number of these are considered about together as an endomembrane system since their functions are coordinated.The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria chloroplast and peroxisomes are not compose with the above parts, these are not considered as a part of the endomembrane system.

Endoplasmic Reticulum

Endoplasmic reticulum is a membranous structure which forms significant part  of the endomembrane arrangement of the Eukaryotic cell that separate the intracellular space into luminal  and additional luminal (cytoplasm) compartments. 

Structure of Endoplasmic Reticulum

The structure of the endoplasmic reticulum is shaped like a  ER is of two types, smooth endoplasmic reticulum, rough endoplasmic reticulum. 

Rough endoplasmic reticulum structure

• The rough endoplasmic reticulum is named so on because of its appearance.
• It is a  associated flattened sacs having a few ribosomes on its external surface, subsequently the name.
• It synthesis and secretes proteins in the liver, chemicals, and different substances in the glands.
• Rough ER is conspicuous in cells where protein synthesis occurs. 

Function

• Most of the elements of rough ER is related with protein synthesis.
• The rough endoplasmic reticulum likewise assumes a crucial part in protein folding.
• Additionally makes quality control. 
• The second most significant capability after protein synthesis and protein folding is protein arranging.

Smooth Endoplasmic Reticulum structure

• The smooth endoplasmic reticulum, then again, doesn’t have ribosomes.
• The smooth endoplasmic reticulum has a tubular structure.
• It participates in the development of phospholipids, the main lipids in cell membranes and are essential during the  digestion.
• Smooth ER transport the results of the rough ER to other cell organelles, particularly the Golgi device

Function

• Smooth ER is essential in  the synthesis of essential lipids such as phospholipids and cholesterol.
• Smooth ER is also responsible for the  secretion of steroid hormones.
• It is also responsible for the metabolism of carbohydrates.
• The smooth ER store and releases calcium ions. These are  important for the nervous system and muscular systems.

Golgi complex

It is a single membrane-bound organelle that forms a part of the endomembrane system. Golgi complex is mainly found in the cytosol of the eukaryotic cells  membrane sacs known as cisternae. A Golgi stack  contains 4-8 cisternae. Each Golgi stack has two faces- the cis face and the trans face. Both faces are also called the entry face and exit face, respectively. The main functions of the Golgi apparatus include protein packaging and secretion. 

Lysosomes

It is a single membrane enclosed  organelle that contains hydrolytic catalysts that are responsible for the breakdown of different biomolecules. These hydrolytic proteins incorporate nucleases, proteases, lipases, glycosidases, phosphatase, phospholipases, and sulphatases. For ideal action, the chemical requires an acidic climate inside the lysosomes with a pH of around 5.0. There stays present a proton siphon inside the lysosomal layer. This proton siphon ships the proton from inside the film involving ATP as a wellspring of energy. Lysosomes are liable for the absorption of both intracellular as well as extracellular materials as they can separate infection particles or microbes in the phagocytosis of macrophages.

Vacuole

A vacuole is a   membrane bound structure tracked down in the cytoplasmic membrane of a cell. The membrane encoded the vacuole is known as tonoplast. The components of the vacuole, known as the cell sap, differ from that of the surrounding cytoplasm. The layers are made out of phospholipids. The layers are implanted with proteins that help in transportation of molecule across the membrane. Various combination of these proteins assist the vacuoles with holding various materials. 

Frequently Asked Questions

Question 1: What is called the endomembrane system?

Answer:

The endomembrane system endo- = “within. a group of membranes and organelles in eukaryotic cells that works together to change, package, and transportation of lipids and proteins.

Question 2: Who discovered endomembrane system?

Answer:

The endomembrane system was first found in the late of the 1800s when researcher Camillo Golgi saw that a specific stain specifically marked just a few inside cell layers.

Question 3: Write the 3 parts of the endomembrane system?

Answer:

The 3 parts of endomembrane system is Golgi apparatus, vacuoles, and cytoplasmic vesicles. 

Question 4: What is the role of  endomembrane system?

Answer:

The endomembrane system permits different elements of the eukaryotic cell to be compartmentalized (e.g., protein corruption happens in the lysosome), permitting a more significant level of cell specialization. The system depends on unique cooperations between various compartments, worked with by vesicle dealing between them.

Question 5: Who discovered the cell?

Rober hook discovered the cell in the year 1665.

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Learning Outcomes

• Describe the structure and function of the nucleus and nuclear membrane
• Describe the structure, function, and components of the endomembrane system

The endomembrane system (endo = within) is a group of membranes and organelles (Figure 1) in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes (which only appear in animal cells), vesicles, the endoplasmic reticulum, and Golgi apparatus, which we will cover shortly.

The Nucleus

Typically, the nucleus is the most prominent organelle in a cell (Figure 1). The nucleus (plural = nuclei) houses the cell’s DNA in the form of chromatin and directs the synthesis of ribosomes and proteins. Let us look at it in more detail (Figure 1).

Figure 1. The outermost boundary of the nucleus is the nuclear envelope. Notice that the nuclear envelope consists of two phospholipid bilayers (membranes)—an outer membrane and an inner membrane—in contrast to the plasma membrane, which consists of only one phospholipid bilayer. (credit: modification of work by NIGMS, NIH)

The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 1). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm.

To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material, and proteins. This combination of DNA and proteins is called chromatin. In eukaryotes, chromosomes are linear structures. Every species has a specific number of chromosomes in the nucleus of its body cells. For example, in humans, the chromosome number is 46, whereas in fruit flies, the chromosome number is eight.

Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, the chromosomes resemble an unwound, jumbled bunch of threads, which is the chromatin.

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus, called the nucleolus (plural = nucleoli), aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported through the nuclear pores into the cytoplasm.

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) (Figure 1) is a series of interconnected membranous tubules that collectively modify proteins and synthesize lipids. However, these two functions are performed in separate areas of the endoplasmic reticulum: the rough endoplasmic reticulum and the smooth endoplasmic reticulum, respectively.

The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope.

The ribosomes synthesize proteins while attached to the ER, resulting in transfer of their newly synthesized proteins into the lumen of the RER where they undergo modifications such as folding or addition of sugars. The RER also makes phospholipids for cell membranes.

If the phospholipids or modified proteins are not destined to stay in the RER, they will be packaged within vesicles and transported from the RER by budding from the membrane (Figure 1). Since the RER is engaged in modifying proteins that will be secreted from the cell, it is abundant in cells that secrete proteins, such as the liver.

The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (see Figure 1). The SER’s functions include synthesis of carbohydrates, lipids (including phospholipids), and steroid hormones; detoxification of medications and poisons; alcohol metabolism; and storage of calcium ions.

The Golgi Apparatus

Figure 2. The Golgi apparatus in this transmission electron micrograph of a white blood cell is visible as a stack of semicircular flattened rings in the lower portion of this image. Several vesicles can be seen near the Golgi apparatus. (credit: modification of work by Louisa Howard; scale-bar data from Matt Russell)

We have already mentioned that vesicles can bud from the ER, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles need to be sorted, packaged, and tagged so that they wind up in the right place. The sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs (Figure 2).

The Golgi apparatus has a receiving (cis) face near the endoplasmic reticulum and a releasing (trans) face on the side away from the ER, toward the cell membrane. The transport vesicles that form from the ER travel to the receiving face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications. The most frequent modification is the addition of short chains of sugar molecules. The newly modified proteins and lipids are then tagged with small molecular groups so that they are routed to their proper destinations.

Finally, the modified and tagged proteins are packaged into vesicles that bud from the opposite face of the Golgi. While some of these vesicles, transport vesicles, deposit their contents into other parts of the cell where they will be used, others, secretory vesicles, fuse with the plasma membrane and release their contents outside the cell.

The amount of Golgi in different cell types again illustrates that form follows function within cells. Cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundant number of Golgi.

In plant cells, the Golgi has an additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell.

Figure 3. The endomembrane system works to modify, package, and transport lipids and proteins. (credit: modification of work by Magnus Manske)

Why does the cis face of the Golgi not face the plasma membrane?

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