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What is Cytoplasm?

Cytoplasm is the gel-like material that fills a cell and surrounds the organelles. It provides the supporting framework and optimum internal environment for organelles.

In biology, the cytoplasm is a rich, semi-solid material present in the cells of organisms.

The cytoplasm is enclosed by the plasma membrane. It contains various components, such as the cytosol and different organelles. Some scientists consider the nucleus a part of the cytoplasm.

Cytoplasm Diagram

Analogy for Cytoplasm

Here are a few analogies to understand cytoplasm in better way:

1. Analogy of cytoplasm in a school

The cytoplasm is like the hallways of a school. It is a jelly-like substance that contains all of the organelles, which are like the different classrooms and offices in the school. The cytoplasm allows the organelles to move around and interact with each other, just like the hallways allow students and teachers to move around and interact with each other.

2. Analogy of cytoplasm in a house

The cytoplasm is like the air in a house. It is everywhere and it surrounds all of the furniture and appliances, which are like the organelles in the cell. The cytoplasm allows the organelles to communicate with each other and to transport materials, just like the air allows people to communicate with each other and to move around the house.

3.  Analogy of cytoplasm in comparison to a person

The cytoplasm is like the blood and lymph in a person’s body. It is a fluid that transports nutrients and oxygen to all of the cells in the body, and it removes waste products from the cells. The cytoplasm also helps to regulate the body’s temperature and to protect the cells from infection.

4.      Analogy of cytoplasm in a city  

The cytoplasm is like the streets and sidewalks in a city. It allows people and goods to move around from one place to another. The cytoplasm also contains all of the infrastructure, such as the power grid and the water system that supports the city.

Structure of Cytoplasm

The cytoplasm is composed of a gel-like cytosol containing dissolved molecules and suspended organelles that carry out specialized functions within the cell.

Here are the main components of cytoplasm:

1.    Cytosol

Cytosol is the liquid part of cytoplasm. Components of cytosol are water, ions, carbohydrates, lipids, proteins, and nucleotides.

Water makes up majority of cytosol and creates aqueous environment for biochemical reactions.

Ions like sodium, potassium, calcium, magnesium, chloride, phosphate, and sulfate are also present in cytosol.

Structural, enzymatic, signaling, transport proteins are dissolved in cytosol.

Carbohydrates, and lipids like Phospholipids, and triglycerides are present in small amounts.

2.    Organelles

The major organelles suspended in the cytosol are:

1. Nucleus – Has nuclear envelope and contains chromatin with DNA and the nucleolus.
2. Endoplasmic Reticulum – Network of membranous tubules and sacs involved in lipid synthesis and protein modification.
3. Golgi apparatus – Flattened stacked sacs that sort, package, and modify proteins.
4. Mitochondria– Double-membraned powerhouses that produce ATP via cellular respiration. Have inner folded cristae.
5. Lysosomes – Spherical vesicles containing digestive enzymes to break down waste.
6. Peroxisomes – Small vesicles housing enzymes for lipid metabolism.
7. Vacuoles – Fluid-filled sacs that store metabolites, waste, water, or food.
8. Cytoskeleton – Protein filaments including microfilaments, microtubules, and intermediate filaments. Provide cell shape and structure.

3. Inclusions

The cytoplasm also contains inclusions like:

• Glycogen granules – Storage form of glucose
• Lipid droplets – Storage form of lipids

Do all cells have cytoplasm?

All cells (e.g animal, plant, algae, fungi) have cytoplasm. However, the size of the cytoplasm varies from cell to cell. For example, sperm cells in humans lose much of their organelles to achieve a complex nucleus for a more streamlined structure for its motility at maturity.

Thus it possesses small-sized cytoplasm.  While the ovum is the largest cell of the human body as it possesses a larger-sized cytoplasm.

Physical Nature of Cytoplasm

The cytoplasm serves both the purpose of mobility of the cell and structure maintenance for the cell. It allows organelles to move throughout the cell while ensuring that the cell itself has a solid structure.

Cytoplasmic Streaming

Cytoplasmic streaming is also called protoplasmic streaming and cyclosis. It is the circular motion of cytosol and organelles around the interior of a cell. Cytoskeleton, including actin filaments and microtubules, and motor proteins support the movement of cytosol.

This streaming helps to distribute nutrients, proteins, and organelles throughout the cell. There are different types of cytoplasmic streaming, such as rotational streaming and shuttle streaming.

Streaming allows efficient transport and mixing within large stationary plants and algal cells. Disruption of cytoplasmic streaming can impair circulation and lead to defects.

Role of Cytoplasm in Cytokinesis

When cell division occurs during the cell cycle, the cytoplasm divides as well to become part of the new cells. This division of the cytoplasm is called cytokinesis. Cytokinesis occurs during meiosis or mitosis of the cell.

Functions of Cytoplasm

The cytoplasm is the site of cellular metabolism, molecular transport, structural support, storage, waste disposal, cell signaling, cell growth, and division.

Here are the main functions of cytoplasm in animal and plant cell:

1.    Cellular metabolism

Many metabolic pathways, including glycolysis, protein synthesis, and lipid metabolism, occur in the cytoplasm. Enzymes located in the cytoplasm catalyze these reactions.

2.    Molecular transport

The cytoplasm allows the movement of molecules such as proteins and RNAs between different compartments. For example, proteins synthesized by ribosomes in the cytoplasm need to be transported to various organelles.

3.    Structural support

The gel-like cytosol provides shape and support to the cell, while the cytoskeleton maintains the cell’s integrity.

4.    Storage

Metabolites such as glycogen and lipids can be stored in the cytoplasm for later use as an energy source.

5.    Waste disposal

Cellular waste products are broken down in the cytoplasm by lysosomes before being removed from the cell.

6.    Cell signaling

Receptor proteins on the cell surface relay signals to effector proteins inside the cytoplasm, enabling cell response to external stimuli.

7.    Cell growth and division

The cytoplasm increases in volume as the cell grows before dividing. The cytoplasm partitions between daughter cells during division.

8.    Site of translation

Messenger RNAs are translated into proteins by ribosomes present in the cytoplasm.

Why is cytoplasm important?

The cytoplasm is crucial for cell survival and function. This gel-like material that fills the cell provides the space, molecules, and proteins needed for essential metabolic reactions, transport, signaling, waste removal, and growth.

Enzymes suspended in the aqueous cytosol catalyze reactions like protein synthesis and cell respiration that the cell depends on.

Cytoskeletal elements give structural support and enable transport of substances from one area to another. The cytoplasm harbors the organelles and inclusions necessary for cell function while also storing metabolites for energy.

Communication between the exterior and interior of the cell relies on receptor proteins embedded in the cytoplasmic membrane that activate intracellular signaling cascades.

Without the cytoplasm and its components, the organelles would not have an optimal internal environment to carry out their jobs, and the cell could not conduct basic functions needed for maintaining life.

Frequently Asked Question-FAQs

The endoplasmic reticulum (ER) is an extensive network of membranous tubules and flattened sacs called cisternae that extend throughout the cytoplasm of eukaryotic cells. They are also in contact with the nuclear membrane. This entire system of channels is the endoplasmic reticulum.

The word reticulum means “network. The Endoplasmic Reticulum (ER) is located within the cytoplasm of eukaryotic cells.

History of Endoplasmic Reticulum

The ER was observed for the first time in the late 19th century when studies of stained cells indicated the presence of an extensive cytoplasmic structure.  Here is timeline of History of Endoplasmic Reticulum:

1. 1897: Edouard G. Gardiner observed a network of membranes within cells using light microscopy and coined the term “ergastoplasm,” laying the groundwork for future discoveries.
2. 1902: Emilio Veratti described a similar network in muscle fibers, terming it the “sarcoplasmic reticulum.” While significant, it wasn’t yet recognized as a universal cellular structure.
3. 1945: A breakthrough arrived with the invention of the electron microscope. Keith R. Porter, Albert Claude, and Ernest F. Fullam used this powerful tool to finally visualize the ER’s intricate network of membranes in detail across various cell types.
4. 1953: Keith Porter built upon the 1945 discovery by coining the now-familiar term “endoplasmic reticulum.” “Endoplasmic” refers to its location within the cell (endoplasm), and “reticulum” describes its net-like structure.

Structure of Endoplasmic Reticulum

Endoplasmic Reticulum has a unique structure that allows it to perform various functions in the cell. The structure of the endoplasmic reticulum can be described as following parts:

1. Membrane system – The endoplasmic reticulum is a membrane-bound organelle, consisting of a continuous network of flattened sacs (cisternae) and tubules. The membrane of the ER is a phospholipid bilayer, similar to the structure of other cellular membranes.
2. Ribosomes – The rough ER (RER) is characterized by the presence of ribosomes attached to its cytoplasmic surface. These ribosomes are responsible for the synthesis of proteins that are either destined for secretion or incorporation into the membrane of the ER itself or other organelles.
3. Lumen – The interior space of the ER is called the lumen or cisternal space. The lumen of the ER is continuous with the lumen of the nuclear envelope, as the ER is an extension of the nuclear envelope.
4. Membrane proteins – The ER membrane contains various proteins that are involved in different functions, such as protein folding, protein transport, and enzymatic activities.
5. Transitional ER – This region of the ER is involved in the transport of newly synthesized proteins from the rough ER to the Golgi apparatus. It is characterized by the presence of small vesicles that bud off from the ER and move toward the Golgi complex.
6. Tubular network – The ER forms a continuous, interconnected network of tubules that extends throughout the cytoplasm. These spherical or tubular membranes are called Cisternae. This network allows for efficient transport of materials and communication between different regions of the cell.

Types of Endoplasmic Reticulum

There are two forms of the endoplasmic reticulum. These are:

1. Smooth Endoplasmic Reticulum

The cytoplasmic surface of the SER lacks ribosomes. So they are known as SER. SER of various cell types functions in diverse metabolic processes like the synthesis of lipids, carbohydrate metabolism, the detoxification of drugs, and other poisons.

Functions of SER

Here are some main functions of the Smooth Endoplasmic Reticulum:

1. Lipid Synthesis

The SER plays a vital role in the synthesis of lipids, including phospholipids and cholesterol. These lipids are essential components of cell membranes and serve various other functions within the cell.

2. Detoxification

The SER is involved in the detoxification of harmful substances. It contains enzymes that catalyze reactions to break down toxic compounds, making them less harmful or easier to excrete.

3. Calcium Storage and Regulation

The SER serves as a storage site for calcium ions (Ca2+). It can release or sequester calcium ions, which are important for various cellular processes, such as muscle contraction and signal transduction.

4. Steroid Synthesis

In specialized cells, such as those in the gonads and adrenal glands, the SER is responsible for the synthesis of steroid hormones, including estrogen, testosterone, and cortisol.

5. Metabolism of Carbohydrates

The SER plays a role in the metabolism of carbohydrates, particularly in liver cells. It is involved in the process of gluconeogenesis, which is the synthesis of glucose from non-carbohydrate precursors.

6. Transmission of Nerve impulses

SER transmit nerve impulse in some cells like muscles and nerve cells.

 7. Transport of material

The SER plays an important role in the transport of material within the cell.

 8. Mechanical Support

The endoplasmic reticulum provides mechanical support to the cell. So the shape of the cell is maintained.

2. Rough Endoplasmic Reticulum

 It is named for its rough appearance because of the attached Ribosome. Ribosomes are attached to the cytoplasmic surface of the membrane of RER.

So RER appears rough through an electron microscope. Rough ER lies adjacent to the nucleus of the cell. Its membrane is continuous with the outer membrane of the nuclear envelope.

Functions of RER

RER performs the following functions in the cell:

1. Protein Synthesis

Specialized cells secrete proteins produced by the rough ER. For example, white blood cells secrete antibodies. Proteins are synthesized by ribosomes attached to the rough ER.

The polypeptide chain grows and enters through the ER membrane into the cisternal space. The protein enters the cisternal space and it is folded. Secretary proteins are glycoproteins. Proteins are covalently bonded to carbohydrates in SER.

The secretory proteins are wrapped in the membranes of vesicles. They sprout from a particular region called the ER. These vesicles are named transport vesicles

2.  Membrane Production

Rough ER is a membrane factory. It grows through the addition of proteins and phospholipids. They are added to the ER membrane itself. The enzymes present in the RER synthesized their membranes from these phospholipids and proteins. The ER membrane can be transferred to the plasma membrane in the form of vesicles.

3. Protein Folding and Modification

The RER plays a crucial role in the folding and modification of proteins. Enzymes within the RER assist in the proper folding and addition of carbohydrate groups (glycosylation) to certain proteins.

4. Protein Sorting and Transport

The RER is responsible for sorting and transporting proteins to their correct destinations. Proteins destined for secretion or integration into cellular membranes are packaged into transport vesicles that bud off from the RER.

Difference Between Smooth Endoplasmic Reticulum and Rough Endoplasmic Reticulum (SER vs RER)

Frequently Asked Question-FAQs

Cytoplasm communicates with the outer environment by the means of the cell membrane. This communication or movement of molecules occurs through different mechanisms. Transport across plasma membrane occurs through diffusion, facilitated diffusion, osmosis, and many others.

Diffusion:

The movement of molecules in or outside the cell from higher concentration to lower concentration is called simple diffusion.

In diffusion, the molecules move randomly. They keep moving randomly until they are distributed in the cell evenly. They come in a state of dynamic equilibrium. Most of the short-distance transport takes place by simple diffusion.

Facilitated Diffusion:

 The movement of molecules through the assistance of protein molecules from higher concentration to lower concentration is called facilitated Diffusion.

Polar molecules are insoluble in lipids. So their diffusion takes place by the means of protein channel pores. The protein channels provide a pathway for the movement of specific molecules across the membrane. Thus these molecules do not come in contact with the hydrophobic layer.

Large molecules and lipid insoluble molecules need assistance for movement which is provided by protein carriers. It does not need energy. A carrier protein binds with a molecule in the plasma membrane. It is transported from an area of higher concentration to an area of lower concentration.

Osmosis:

The diffusion of water across a selectively permeable membrane from higher concentration to lower concentration is called osmosis.

Osmosis is a particular type of diffusion.

What is Tonicity?

The relative concentration of solutes in the water inside and outside the cell is called tonicity.

There are three different types of tonicity based on solvent and solute concentration.

(a) Isotonic solution:

The concentration of solute and solvent is the same inside and outside the cell.

(b) Hypertonic solution:

When the solute concentration is higher outside the cell than inside, this is called a hypertonic solution.

(c) Hypotonic solution:

In this case. The solute concentration is lower outside the cell than inside.

Filtration:

Filtration is a process that pushes small molecules across selectively permeable membranes by using hydrostatic pressure.

What is hydrostatic pressure?

The pressure generated due to water in the cell is called hydrostatic pressure.

For example, in the kidney blood pressure forces water and dissolved wastes out of the blood vessels and into the kidney tubules. This is the first step in urine formation.

Active transport:

The movement of molecules from lower concentration to higher concentration by the expenditure of energy is called active transport.

It takes place against the concentration gradient. That’s why movement against the concentration gradient requires ATP energy. The active transport process resembles facilitated diffusion. But this carrier protein in the plasma membrane uses ATP to move the molecules against a concentration gradient.

The sodium-potassium pump is an example of active transport.

Endocytosis:

The movement of bulky material into the cell by the formation of the vesicle is called endocytosis.

The movement of molecules does not take place individually. Different types of molecules like cholesterol are brought into cells in this manner.

Exocytosis:

The removal of cell secretions from the cells by out folding is called exocytosis. The Golgi apparatus box up cholesterol, proteins, and other molecules into vesicles for secretion. These secretory vesicles fuse with the cell membrane and release their constituents into the outer environment. This process adds new membrane material. This material replaces the plasma membrane lost during endocytosis.

Frequently Asked Questions-FAQs