An Overview of Cytoplasm and Cell Organelles – Types, Structure and their Functions

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The Role of Cytoplasm in a Cell

Nature and Occurrence: The part of the cell which occurs between the plasma membrane and nuclear envelope is called the cytoplasm. The inner granular mass of the cytoplasm is often called endoplasm, while the outer, clearer (glassy) layer is called the cell cortex or ectoplasm.

Cytoplasm consists of an aqueous ground substance, the cytosol, containing a variety of cell organelles and other inclusions such as insoluble waste and storage products (starch, glycogen, lipid, etc.).

Cytosol – Definition and Examples

It is the soluble part of the cytoplasm. It forms the ground substance or “background material” of the cytoplasm and is located between the cell organelles. Cytosol contains a system of protein fibers called cytoskeleton but otherwise appears transparent and structureless in the electron microscope.

Cytosol is about 90 percent, water and forms a solution that contains all biochemicals of life. Some of these are ions and small molecules forming true solutions such as salts, sugars, amino acids, nucleotides, vitamins, and dissolved gases. Others are large molecules such as proteins which form colloidal solutions. A colloidal solution may be a sol (non-viscous) or a gel (viscous); often ectoplasm is more gel-like.

Structure and Functions of Cytoskeleton
Recently complex networks of fibrous protein structures have been shown to exist in the cytosol of eukaryotic cells. These networks collectively form the cytoskeleton which contains three types of protein fibres:

• Microtubules (of tubulin protein),
• Microfilaments (of actin protein),
• Intermediate filaments (of keratin and other types of proteins).

These fibrous proteins help in cellular movement i.e., amoeboid movement and cyclosis). They also help the cells to maintain their shapes.

Chemical Constituents of the Cytoplasm or Cytosol

Chemical	Percentage
1. Oxygen	64
2. Carbon	18
3. Hydrogen	10
4. Nitrogen	0.3
5. Trace Elements (Ca, P, Cl, S, K, Na, Mg, I, Fe)	0.5

Functions of Cytosol

• Cytosol (cytoplasm) acts as a store of vital chemicals such as amino acids, glucose, vitamins, ions, etc.
• It is the site of certain metabolic pathways, such as glycolysis. Synthesis of fatty acids, nucleotides, and some amino acids also takes place in the cytosol.
• Living cytoplasm is always in a state of movement.

Cell Organelles – Types, Structure, and Their Functions

A cell has to perform different functions with the help of its various membrane-bound organelles:

• It has to synthesize substances, e.g., protein synthesis by ribosomes, lipid synthesis on the surface of smooth endoplasmic reticulum (SER), and photosynthesis of food (e.g., glucose, starch) by chloroplasts.
• It has to secrete cell products, e.g., enzymes, hormones, mucus, etc.
• It has to digest those substances which are taken up by the cell during endocytosis. Such intracellular digestion is done by enzymes of lysosomes.
• It has to generate energy, e.g., the synthesis of energy-rich ATP (adenosine triphosphate) by mitochondria.

The membrane is a remarkable cellular structure. Every cell is bounded by a membrane and thus, keeps its own contents separate from the external environment. Larger or more evolved cells, or cells from multicellular organisms, have a great deal of metabolic activities to support their complicated structure or function. To keep metabolic activities of different types separate from each other, cells have membrane-bound organelles within themselves.

Cell organelles are “small organs” of the cell and are found embedded in the cytosol. They form a living part of the cell and each of them has a definite shape, structure, and function. Examples of such organelles are the nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, etc. We have already discussed the nucleus in a previous section. In this section, we will discuss the cellular organelles one by one.

Endoplasmic Reticulum – Structure and its Functions

Nature and Occurrence: Inside the cell, there exists a membranous network enclosing a fluid-filled lumen that almost fills up the intracellular cavity. It is called the endoplasmic reticulum (ER). At one end ER is connected to the outer membrane of the nucleus and at the other end to the plasma membrane. ER occurs in three forms: cisternae (i.e., closed, fluid-filled sacs), vesicles, and tubules. It is of two types:

• Rough endoplasmic reticulum (RER) with ribosomes attached on its surface for synthesizing proteins.
• Smooth endoplasmic reticulum (SER) which is without ribosomes and is meant for secreting lipids.

Three-Dimensional Endoplasmic Reticulum

The ER is absent in the red blood cells of mammals.

Functions of Endoplasmic Reticulum:

• It forms the supporting skeletal framework of the cell.
• ER provides a pathway for the distribution of nuclear material from one cell to the other.
• Certain enzymes present in smooth ER synthesize fats (lipids), steroids, and cholesterol.
• Rough ER is concerned with the transport of proteins that are synthesized by ribosomes on their surface.

The endoplasmic reticulum performs the following important functions:
1. Smooth ER of the liver of vertebrates helps in detoxification. It metabolizes various toxic or poisonous substances such as drugs, aspirin, insecticides (DDT), petroleum products, and pollutants. These toxic substances make their entry into the animal’s body through food, air, or water.

2. Smooth ER plays an important role in the biosynthesis of glycolipids, phospholipids, and cholesterol. These lipids are used in the formation of plasma or cell membranes and various steroid hormones.

3. Hormones are either steroids or proteins. Smooth ER synthesizes steroid hormones such as estrogen, testosterone, and cortisol.

4. Enzymes are proteins. Digestive (hydrolytic) enzymes of lysosomes are produced by rough ER. Any enzyme which is meant for the lysosomes is synthesized on the ribosomes attached to the surface of rough ER. It then enters the lumen of the rough ER. From the rough ER, this enzymatic protein is transported to the Golgi apparatus where it is marked to be included in the lysosome.

5. Plasma membranes and other cellular membranes are also formed by the endoplasmic reticulum. The lipid molecules for the cell membrane are formed and inserted into the smooth ER membrane by the smooth ER itself. The protein molecules of the cell membrane are mostly synthesized and inserted into the membrane at the level of rough ER. In the process of glycosylation, short chains of sugars, called oligosaccharides, are added to molecules of proteins and lipids at the level of the Golgi apparatus.

For example, the formation of the plasma membrane, called membrane biogenesis, involves the following organelles, all forming the so-called endomembrane system:
Rough ER → Smooth ER → Golgi apparatus → Secretory vesicle → Plasma membrane.

6. Proteins that are synthesized by the cell and then released into the outer medium of the cell, are called secretory proteins. Examples of secretory proteins include mucus, digestive enzymes, and hormones (ie.g., insulin). These proteins are synthesized by rough ER.

Differences in Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).

Rough Endoplasmic Reticulum	Smooth Endoplasmic Reticulum
1. It contains flattened sacs called cisternae.	1. It is mainly formed of vesicles and tubules.
2. Ribosomes are attached to the outer surface of their membrane.	2. It does not contain ribosomes.
3. It is specialized to synthesize proteins.	3. It is specialized to synthesize lipids and steroids.
4. It is abundant in exocrine pancreatic cells and antibodies secreting plasma cells.	4. It is abundant in the liver and the testicular cells (e.g., Leydig cells) synthesizing steroid hormones.

Ribosome – Definition and Examples

Nature and Occurrence: Ribosomes are dense, spherical, and granular particles that occur freely in the matrix (cytosol) or remain attached to the endoplasmic reticulum (RER). Chemically, the major constituents of ribosomes are ribonucleic acid (RNA) and proteins. Lipids are not present in ribosomes. Ribosomes are not bounded by a membrane. They are present both in prokaryotic and eukaryotic cells (except mammalian RBC).

Function: Ribosomes play an important part in the synthesis of proteins.

Golgi Apparatus and its Functions in Animal Cells (Golgi Body or Golgi Complex)

Nature and Occurrence: Golgi apparatus consists of a set of membrane-bounded, fluid-filled vesicles, vacuoles, and flattened cisternae (closed sacs). Cisternae are usually stacked together (placed one above the other) in parallel rows. The Golgi apparatus exists as an extensive network near the nucleus in animal cells. However, the plant cells contain many freely distributed subunits of the Golgi apparatus, called dictyosomes. Cisternae are formed at one end of the stack, called the cis face of Golgi. They are budded off as vesicles at the other face of the Golgi apparatus, called the trans face of the Golgi.

The Golgi apparatus is absent in bacteria, blue-green algae, mature sperms, and red blood cells of mammals and other animals. The Golgi apparatus arises from the membrane of the smooth endoplasmic reticulum, which in turn originates from the rough endoplasmic reticulum. The proximal Golgi saccules (cisternae at cis face) are formed by the fusion of ER-derived vesicles, while distal saccules (cisternae at trans face) “give their all” to vesicle formation and disappear. Thus, Golgi saccules are constantly and rapidly renewed.

Functions of Golgi Apparatus:

• The main function of the Golgi apparatus is secretory. Golgi apparatus acts as a way-station or assembly area for the storage, processing, and packaging of various cellular secretions.
• It packages materials synthesized in the cell and dispatches them either to intracellular targets such as plasma membrane and lysosomes or extracellular targets (e.g., zymogens).
• It produces vacuoles or secretory vesicles which contain cellular secretions, e.g., enzymes, proteins, cellulose, melanin pigment, lactoprotein of milk, etc.
• Golgi apparatus is also involved in the synthesis of cell walls, plasma membranes, and lysosomes.

Lysosomes – Definition, Significance, Functions

Nature and Occurrence: Lysosomes are simple tiny spherical sac-like structures evenly distributed in the cytoplasm. Each lysosome is a small vesicle surrounded by a single membrane and contains powerful enzymes. These enzymes are capable of digesting or breaking down all organic materials. Lysosomal enzymes are made by RER.

Functions of Lysosomes:

• Lysosomes serve as intracellular digestive systems, hence, called digestive bags. They destroy any foreign material which enters the cell such as bacteria and viruses. In this way, they protect the cells from bacterial infection.
• Lysosomes also remove the worn-out and poorly working cellular organelles by digesting them to make way for their new replacements. In this way, they remove the cell debris and are also known as demolition squads, scavengers, and cellular housekeepers. Thus, lysosomes form a kind of garbage disposal system of the cell.
• During the breakdown of cell structure, when the cell gets damaged, lysosomes may burst and the enzymes eat up their own cells. Therefore, lysosomes are also known as the suicide bags of a cell.

Significance of Lysosomes:

• In WBC or leucocytes: Cells of leucocytes digest foreign proteins, bacteria, and viruses.
• In autophagy: During starvation, the lysosomes digest stored food contents such as proteins, fats, and glycogen of the cytoplasm and supply the necessary amount of energy to the cell.
• In metamorphosis (Frog): During the transformation of a tadpole into a frog, the embryonic tissues such as gills and tail are digested by the lysosomes and utilized by other body cells.
• In fertilization: The lysosomal enzymes present in the acrosome of sperm cells digest the limiting membrane of the ovum (egg). Thus, the sperm is able to enter the ovum and start the fertilization.

Mitochondria – Definition, Structure, Functions

Nature and Occurrence: The mitochondria (singular: mitochondrion) are tiny bodies of varying shapes (cylindrical, rod-shaped, spherical) and sizes (0.2 mm to 2 mm), distributed in the cytoplasm. Each mitochondrion is bounded by a double membrane envelope. The outer membrane is porous.

The inner membrane is thrown into folds and, therefore, has an area several times the surface of an area of the outer membrane. These folds are called cristae and are studded (dotted) with small rounded bodies known as Fa particles or oxysomes. The interior cavity of the mitochondria is filled with a proteinaceous (gel-like) matrix which contains a few small-sized ribosomes, a circular DNA molecule, and phosphate granules. Mitochondria are absent in bacteria and the red blood cells of mammals.

Functions of Mitochondria:
Mitochondria are sites of cellular respiration. They use molecular oxygen from the air to oxidize the carbohydrates and fats (lipids) present in the cell to carbon dioxide and water vapor. Oxidation releases energy, a portion of which is used to form ATP (adenosine triphosphate). Since the mitochondria synthesize, energy-rich compounds (ATP), they are known as the ‘power house’ of the cell. The energy stored in ATP is used by the cell.

ATP stands for the organic compound adenosine triphosphate. ATP is generally known as the energy carrier or energy currency of the cell. It is a common cellular fuel, i.e., it is used to drive numerous energy-requiring processes of the cell. The body of an organism uses the energy stored in AJP for 1. synthesis of chemical compounds (e.g., DNA replication, transcription of RNAs, and synthesis of proteins, carbohydrates, and lipids) and 2. mechanical work, such as contraction of muscles (for movement, locomotion, peristalsis), movement of cilia and flagella, conduction of nerve impulse and production of heat, electricity (e.g., electric eel), and light (e.g., fireflies). Mitochondria are able to make some of their own proteins; so, they are regarded as semiautonomous organelles.

Different Types of Plastids and Their Functions in Plants Cell

Nature and Occurrence: Plastids occur in most plant cells and are absent in animal cells. Like the mitochondria, the plastids also have their own genome (i.e., DNA) and ribosomes. They are self-replicating organelles like the mitochondria, i.e., they have the power to divide. Plastids are of the following three types :

• Chromoplasts: Coloured plastids (except green colour).
• Chloroplasts: Green-coloured plastids.
• Leucoplasts: The colorless plastids.

Differences between Leucoplasts and Chromoplasts (nongreen plastids)

Leucoplasts	Chromoplasts
1. They are colourless.	1. They range from brownish to reddish in colour.
2. They are cylindrical or rounded in shape.	2. They are irregular in shape.
3. They are found in unexposed cells.	3. They are found in both exposed and unexposed cells.
4. They can change to other types of plastids.	4. They do not change into other types of plastids.
5. They take part in the storage of food, e.g., amyloplasts (carbohydrates), elaioplasts (lipids), and aleuroplasts (proteins).	5. They provide colour to organs to attract pollinators and disseminators.

Differences between Chloroplasts and Chromoplasts

Chloroplasts	Chromoplasts
1. They are green plastids.	1. They are non-green coloured plastids.
2. They contain chlorophylls and carotenoids.	2. Chlorophylls are absent. Only carotenoids are present.
3. Lamellae are present.	3. Lamellae are absent.
4. Chloroplasts are sites of photosynthesis.	4. They add colour to the organs (e.g., flowers, fruits) for attracting animals to perform pollination and fruit dispersal.

Structure, and Functions of Chloroplast

Nature and occurrence. Chloroplasts are present in green algae and higher plants. They have a green pigment called chlorophyll and they are involved in the photosynthesis of food. So chloroplasts are the “kitchens of the cells”. Each chloroplast is bounded by two unit membranes like the mitochondria. It shows two distinct regions:

• Grana are stacks of membrane-bounded, flattened discoid sacs (called thylakoids) containing the molecules of chlorophyll. They are the main functional units of chloroplasts.
• Stroma is the homogeneous matrix in which grana are embedded. Stroma contains a variety of photosynthetic enzymes, starch grains, DNA, and ribosomes.

Granum is the site of light reactions during photosynthesis, while stroma is the site of dark reactions during photosynthesis.

Functions of Chloroplasts:
Plastids perform the following functions:

• Chloroplasts trap solar energy and utilize it to manufacture food for the plant.
• Chromoplasts impart various colours to flowers to attract insects for pollination.
• Leucoplasts store food in the form of carbohydrates (starch), fats, and protein.

Differences between Mitochondria and Chloroplasts

Mitochondria	Chloroplasts
1. They occur in the cells of aerobic organisms (plants and animals) with the exception of mammalian RBCs.	1. They occur in the cells of green photosynthetic parts (e.g., leaves) of plants.
2. They are colourless.	2. They are green in colour.
3. The shape is rod-like or sausage-shaped.	3. They are generally disc-like in outline.
4. Inner membrane of each mitochondrion is thrown into folds called cristae.	4. Their inner membrane forms flattened sacs called thylakoids or lamellae.
5. They liberate energy.	5. They trap solar energy and convert it into chemical energy.
6. They perform oxidation of food.	6. They synthesize food by photosynthesis.
7. They consume O2 and liberate CO2.	7. They consume CO2 and liberate O2.

Discovery and Functions of Vacuoles

Nature and Occurrence: Vacuoles are fluid-filled or solid-filled and membrane-bounded spaces. They are a kind of storage sacs. In animal cells, the vacuoles if present are small and temporary. They store water, glycogen, and proteins. The vacuolar membrane is typically a single-unit membrane and is often associated with the maintenance of water balance (e.g., they serve as osmoregulatory organelles in protozoans) or ingestion of nutrient material (food vacuole). Thus, the food vacuole of single-celled organisms such as Amoeba or Paramecium, contains the food item that the animal has consumed.

In plant cells, the vacuoles are large, distinct, and permanent. In mature plant cells, the vacuole occupies almost the entire (i.e., 90%) volume of the cell. Because of the central position of a vacuole, the nucleus and other cell organelles in plant cells are pushed near the boundary wall. The vacuole is bounded by a membrane, called tonoplast. The vacuole is filled with cell sap which is a watery solution rich in sugars, amino acids, proteins, minerals, and metabolic wastes (such as anthocyanins, and alkaloids).

Functions: Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation). They store toxic metabolic by-products or end products of plant cells. They provide turgidity and rigidity to the plant cells.

Structure and Functions of Peroxisome

Nature and Occurrence: Peroxisomes are small (0.3 to 1.5 mm in diameter) and spherical organelles containing powerful oxidative enzymes. They are bounded by a single membrane. Peroxisomes are mostly found in kidney and liver cells. The inner contents of peroxisomes are finely granular, but sometimes a crystalline core is visible by an electron microscope in the centre of peroxisomes. This crystalline core is a crystallized protein, called catalase enzyme.

Functions: Peroxisomes are specialized for carrying out some oxidative reactions, such as detoxification or removal of toxic substances from the cell.

Catalase enzyme of peroxisomes catalyzes the decomposition of hydrogen peroxide (H2O2) to water and oxygen (hence the name ‘peroxisome’). Hydrogen peroxide is a byproduct of certain cell oxidations and is also very toxic, so must be eliminated immediately.

Structure and Functions of Centrosome

Nature and Occurrence: Centrosome is found only in animal cells. It is not bounded by any membrane but consists of two granule-like centrioles. Centrioles are hollow and cylindrical structures that are made up of microtubules. In plant cells, the polar caps perform the function of centrioles.

Functions of Centrosome:

• Centrosome helps in cell division in animal cells. During cell division centrioles migrate to the poles of animal cells and are involved in the formation of the spindle.
• In plant cells, cell division involves polar caps for spindle formation.

Each cell, thus, acquires its distinct structure and function due to the organisation of its membrane and organelles in a specific way. As a result, each type of cell has a basic structural organisation. Such an organization helps different cells to perform some basic functions such as respiration, obtaining nutrition, clearing waste material, forming new proteins, etc. A cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life. This conclusion forms the main point of the cell theory.

Differences between Plant and Animal Cells

The cells of animals and plants have the following differences:

Animal Cell	Plant Cell
1. Animal cells are generally small in size.	1. Plant cells are larger than animal cells.
2. Cell wall is absent.	2. The plasma membrane of plant cells is surrounded by a rigid cell wall of cellulose.
3. Except the protozoan Euglena, no animal cell possesses plastids.	3. Plastids are present.
4. Vacuoles in animal cells are many, small and temporary.	4. Most mature plant cells have a permanent and large central sap vacuole.
5. Animal cells have a single highly complex and prominent Golgi apparatus.	5. Plant cells have many simpler units of Golgi apparatus, called dictyosomes.
6. Animal cells have centrosomes and centrioles.	6. Plant cells lack centrosomes and centrioles.