CELLULAR ORGANELLES IN ANIMAL AND PLANT CELLS: CHARACTERISTICS, FUNCTIONS - BIOLOGY - 2024

The cell organelles are the structures that make up the cells - as a "small bodies" - they perform, metabolic, synthetic, structural functions of production and energy consumption.

These structures are contained in the cell cytoplasm and in general, all eukaryotic cells are composed of a basic set of intracellular organelles. These can be differentiated between membranous (they have a plasma membrane) and nonmembranous (they lack a plasma membrane).

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Each organelle has a unique set of proteins that are generally found on the membrane or inside the organelle.

There are organelles responsible for the distribution and transport of proteins (lysosomes), others carry out metabolic and bioenergetic functions (chloroplasts, mitochondria and peroxisomes), of cell structure and movement (filaments and microtubules), and there are those that are part of the surface cell (plasma membrane and cell wall).

Prokaryotic cells lack membranous organelles, while in eukaryotic cells we can find both types of organelles. These structures can also be classified according to their function in the cell.

Organelles: membranous and non-membranous

Membranous organelles

These organelles have a plasma membrane that allows the internal environment to be separated from the cell cytoplasm. The membrane has vesicular and tubular shapes and can be pleated as in the smooth endoplasmic reticulum or folded into the organelle as in the mitochondria.

This organization of the plasma membrane in the organelles makes it possible to increase their surface area and also form intracellular sub-compartments where various substances such as proteins are stored or secreted.

Among the membrane organelles we find the following:

-Cell membrane, which delimits the cell and other cellular organelles.

-Rough endoplasmic reticulum (RER), place where protein synthesis and modification of newly synthesized proteins takes place.

-Smooth endoplasmic reticulum (REL), where lipids and steroids are synthesized.

-Golgi apparatus, modifies and packages proteins and lipids for transport.

-Endosomes, participate in endocytosis and also classify and redirect proteins to their final destinations.

-Lysosomes, contain digestive enzymes and participate in phagocytosis.

-Transport vesicles, translate material and participate in endocytosis and exocytosis.

-Mitochondria and chloroplasts, produce ATP providing the cell with energy.

-Peroxisomes, involved in the production and degradation of H ₂ O ₂ and fatty acids.

Nonmembranous organelles

These organelles do not have a plasma membrane that defines them, and in them the exclusive proteins generally self-assemble into the polymers that are part of the structural elements of the cytoskeleton.

Among the nonmembranous cytoplasmic organelles we find:

-Microtubules, which constitute the cytoskeleton together with actin microfilaments and intermediate filaments.

-Filaments, are part of the cytoskeleton and are classified into microfilaments and intermediate filaments.

-Centrioli, cylindrical structures from which the basal bodies of the cilia derive.

-Ribosomes, are involved in protein synthesis and are composed of ribosomal RNA (rRNA).

Organelles in animal cells

Animal cell (Source: Animal_cell_structure_en.svg: LadyofHats (Mariana Ruiz) derivative work: Mel 23 talk via Wikimedia Commons)

Animals carry out daily activities of protection, feeding, digestion, movement, reproduction and even death. Many of these activities are also carried out within the cells that make up these organisms, and are carried out by the cellular organelles that make up the cell.

In general, all cells in an organism have the same organization and use similar mechanisms to carry out all their activities. However, some cells can specialize so much in one or more functions that they differ from the others by having a greater number or size of certain cellular structures or regions.

Two main regions or compartments can be differentiated within cells: the nucleus, which is the most prominent organelle of eukaryotic cells, and the cytoplasm that contains the other organelles and some inclusions in the cytoplasmic matrix (such as solutes and organic molecules).

Core

The nucleus is the largest organelle in the cell and represents the most outstanding characteristic of eukaryotic cells, being what differentiates them from prokaryotic cells. It is well delimited by two nuclear membranes or envelopes that have pores. Within the nucleus is the DNA in the form of chromatin (condensed and lax) and the nucleolus.

Nuclear membranes allow the interior of the nucleus of the cell cytoplasm to be isolated, in addition to serving as the structure and support of said organelle. This envelope is made up of an outer and an inner membrane. The function of the nuclear envelope is to prevent the passage of molecules between the nuclear interior and the cytoplasm.

The pore complexes in the nuclear membranes allow the selective passage of proteins and RNAs, keeping the internal composition of the nucleus stable and also fulfilling key roles in the regulation of gene expression.

The cell genome is contained in these organelles, which is why it serves as a storehouse for the cell's genetic information. RNA transcription and processing and DNA replication occur within the nucleus, and only translation occurs outside of this organelle.

Plasma membrane

Plastic membrane

The plasma or cell membrane is a structure composed of two layers of amphipathic lipids, with a hydrophobic and a hydrophilic part (lipid bilayer) and some proteins (integral membrane and peripheral). This structure is dynamic and participates in various physiological and biochemical processes in cells.

The plasma membrane is responsible for keeping the cell interior isolated from the surrounding environment. It controls the passage of all substances and molecules that enter and leave the cell through various mechanisms such as simple diffusion (in favor of a concentration gradient), and active transport, where transport proteins are required.

Rough endoplasmic reticulum

The endoplasmic reticulum is made up of a network of tubules and sacs (cisterns) that are surrounded by a membrane that extends from the nucleus (outer nuclear membrane). It is also one of the largest organelles in cells.

The rough endoplasmic reticulum (RER) has a large number of ribosomes on its outer surface and also contains vesicles that extend to the Golgi apparatus. It is part of the protein synthesis system of the cell. The synthesized proteins pass into the RER tanks where they are transformed, accumulated and transported.

Secretory cells and cells with a large amount of plasma membrane, such as neurons, have well-developed rough endoplasmic reticulum. The ribosomes that make up the RER are responsible for the synthesis of secretory proteins and proteins that make up other cellular structures such as lysosomes, Golgi apparatus, and membranes.

Smooth endoplasmic reticulum

The smooth endoplasmic reticulum (REL) is involved in lipid synthesis and lacks membrane-associated ribosomes. It is made up of short tubules tending to have a tubular structure. It can be separate from the RER or be an extension of it.

Cells associated with lipid synthesis and steroid secretion have highly developed RELs. This organelle is also involved in the processes of detoxification and conjugation of harmful substances, being highly developed in liver cells.

They have enzymes that modify hydrophobic compounds such as pesticides and carcinogens, turning them into water-soluble products that are easily degraded.

Golgi apparatus

In the Golgi apparatus, the proteins synthesized and modified in the endoplasmic reticulum are received. In this organelle, these proteins can undergo other modifications to finally be transported to lysosomes, plasma membranes or destined for secretion. Glycoproteins and sphingomyelin are synthesized in the Golgi apparatus.

This organelle is made up of some species of bags surrounded by a membrane known as cisterns, and they present associated vesicles. Cells that secrete proteins by exocytosis and those that synthesize membrane and membrane-associated proteins have highly active Golgi apparatus.

The structure and function of the Golgi apparatus presents polarity. The portion closest to the RER is called the cis-Golgi network (CGN) and has a convex shape. Proteins from the endoplasmic reticulum enter this region to be transported within the organelle.

The Golgi stack constitutes the middle region of the organelle and is where the metabolic activities of that structure take place. The maturation region of the Golgi complex is known as the trans-Golgi network (TGN), it has a concave shape and is the point of organization and distribution of proteins towards their final destinations.

Lysosomes

Part of a cell, including the lysosome

Lysosomes are organelles that contain enzymes capable of degrading proteins, nucleic acids, carbohydrates, and lipids. They are basically the digestive system of cells, degrading biological polymers captured from the outside of the cell and the cells' own products (autophagy).

Although they can come in different shapes and sizes, depending on the product captured for digestion, these organelles are generally dense spherical vacuoles.

Particles captured by endocytosis are transported to endosomes, which later mature into lysosomes by the aggregation of acid hydrolases from the Golgi apparatus. These hydrolases are responsible for degrading proteins, nucleic acids, polysaccharides and lipids.

Peroxisomes

Graphic representation of a peroxisome.

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Peroxisomes are small organelles (microbodies) with a simple plasma membrane, which contain oxidative enzymes (peroxidases). The oxidation reaction carried out by these enzymes produces hydrogen peroxide (H ₂ O ₂).

In these organelles, catalase is responsible for regulating and digesting H ₂ O _{2 by} controlling its cellular concentration. Liver and kidney cells have significant amounts of peroxisomes, these being the main detoxification centers in the body.

The number of peroxisomes contained in a cell is regulated in response to diet, consumption of certain drugs, and in response to various hormonal stimuli.

Mitochondria

Mitochondria. Taken and edited from: LadyofHats.

Cells that consume and generate significant amounts of energy (such as striated muscle cells) have abundant amounts of mitochondria. These organelles play a critical role in the production of metabolic energy in cells.

They are responsible for the production of energy in the form of ATP from the degradation of carbohydrates and fatty acids, through the oxidative phosphorylation process. They can also be described as mobile power generators capable of moving around the cell, providing the necessary energy.

Mitochondria are characterized by containing their own DNA and can encode tRNA, rRNA and some mitochondrial proteins. Most of the mitochondrial proteins are translated on the ribosomes and transported to the mitochondria by the action of specific signals.

The assembly of mitochondria involves proteins encoded by their own genome, other proteins encoded in the nuclear genome, and proteins imported from the cytosol. The number of these organelles increases by division during interphase, although these divisions are not synchronized with the cell cycle.

Ribosomes

Ribosomes are small organelles that participate in protein synthesis. These are made up of two subunits superimposed one on top of the other, containing proteins and RNA. They play an important role in the construction of polypeptide chains during translation.

Ribosomes can be found free in the cytoplasm or associated with the endoplasmic reticulum. By actively participating in protein synthesis, they are bound by mRNA in chains of up to five ribosomes called polyribosomes. Cells specialized in protein synthesis have large amounts of these organelles.

Organelles in plant cells

Morphoanatomy of a plant cell (Source: Ævar Arnfjörð Bjarmason / gallery via Wikimedia Commons)

Most of the organelles previously described (nucleus, endoplasmic reticulum, Golgi apparatus, ribosomes, plasma membrane, and peroxisomes) are found as part of plant cells, where they basically perform the same functions as in animal cells.

The main organelles in plant cells that differentiate them from other organisms are the plastids, vacuoles, and the cell wall. These organelles are surrounded by a cytoplasmic membrane.

Cellular wall

The cell wall is a glucoprotein network existing in practically all plant cells. It plays an important role in the cellular exchange of substances and molecules and in the circulation of water at different distances.

This structure is composed of cellulose, hemicelluloses, pectins, lignin, suberin, phenolic polymers, ions, water, and various structural and enzymatic proteins. This organelle originates in cytokinesis by the insertion of the cell plate, which is a partition formed by the fusion of Golgi vesicles in the center of the mitotic figure.

Complex cell wall polysaccharides are synthesized in the Golgi apparatus. The cell wall, also known as the extracellular matrix (ECM) not only provides toughness and defined shapes to the cell, but also participates in processes such as cell growth, differentiation and morphogenesis, and responses to environmental stimuli.

Vacuoles

Vacuoles are one of the largest organelles present in plant cells. They are surrounded by a simple membrane and are shaped like sacks, storing water and reserve substances such as starches and fats or waste substances and salts. They are composed of hydrolytic enzymes.

They intervene in the processes of exocytosis and endocytosis. Proteins transported from the Golgi apparatus enter vacuoles, which assume the function of lysosomes. They also participate in maintaining turgor pressure and osmotic balance.

Plastids

Plastids are organelles surrounded by a double membrane. They are classified into chloroplasts, amyloplasts, chromoplasts, oleinoplasts, proteinoplasts, proplasts, and etioplasts.

These organelles are semiautonomous, as they contain their own genome known as a nucleoid in the organelle matrix or stroma, as well as a replication, transcription and translation machinery.

Plastids fulfill various functions in plant cells, such as the synthesis of substances and the storage of nutrients and pigments.

Types of plastids

Chloroplasts are considered the most important plastids. They are among the largest organelles in cells and are found in various regions within it. They are present in green leaves and tissues, containing chlorophyll. They intervene in the capture of solar energy and the fixation of atmospheric carbon in the photosynthesis process.

-Amyloplasts are found in reserve tissues. They lack chlorophyll and are full of starch, serving as a store for these and also participate in the gravitropic perception in the root cap.

-The chromoplasts store pigments called carotenes, which are associated with the orange and yellow colorations of autumn leaves, flowers and fruits.

-Oleinoplasts store oils while proteinoplasts store proteins.

-Proplastidia are small plastids found in meristematic cells of roots and stems. Their function is not very clear, although they are believed to be precursors to the other plastids. The reformation of the proplastids is associated with the re-differentiation of some mature plastids.

-The etioplasts are found in cotyledons of plants grown in the dark. When exposed to light they quickly differentiate into chloroplasts.

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