PLANT CELL: CHARACTERISTICS, PARTS (ORGANELLES) AND FUNCTIONS - BIOLOGY - 2024

Plant cells are the fundamental units that make up the organisms belonging to the kingdom of plants (kingdom Plantae).

Like all living things, plants are also made up of cells and these are known as plant cells. For any living organism considered, a cell represents the most basic unit, that is, the smallest part of an individual that preserves the characteristics of everything living.

In its interior, as well as in the interior of animal cells, since it is a type of eukaryotic cell, there is a type of "liquid" (the cytosol), in which a series of compartments delimited by membranes are submerged, which we know as organelles or organelles.

The organelles of any cell can be considered analogous to the body organs of an animal (heart, liver, kidneys, lungs, stomach, etc.) but on a significantly smaller scale, that is, smaller (plant cells can measure up to 100 microns).

Onion plant cells with their nuclei. Source: Laurararas / CC BY (https://creativecommons.org/licenses/by/4.0)

Thus, each cell can be viewed as a community of subcellular components, each with its own functions, which make life possible, but unable to survive on its own outside the cell.

Some organelles of plant cells are not present in animal cells, hence a special distinction is always made between the two types. Among these organelles only present in plant cells, the cell wall, the vacuole and the chloroplasts stand out, the latter responsible for the incredible process of photosynthesis.

Features

Plants, conceived, like all multicellular organisms, as a large cell community, have cells of different types that perform different functions.

There are cells specialized in:

- the protection, - the mechanical support, - the synthesis of food reserves, - transport, absorption and secretion, - meristematic activity and reproduction and

- the connection between specialized tissues

General characteristics

Plant cells share many characteristics with each other, but in turn they share some characteristics with animal cells, characteristics that are inherent in all eukaryotic cells.

Photograph of the microscope view of the tissue of an aquatic grass (Image by Andrea Vierschilling www.pixabay.com)

Next, we will present a list of some of the shared characteristics and characteristics of plant cells:

- They are eukaryotic cells: they have their genetic material enclosed within a membranous nucleus and have other compartments surrounded by double or single membranes.

- They all have a cell wall: the plasma membrane (the one that encloses the cytosol with its organelles) is surrounded and protected by a rigid wall, made up of complex networks of polysaccharides such as cellulose (a polymer of glucose molecules).

- They have plastids: among the special organelles that only plant cells have are the plastids specialized in different functions. The chloroplasts (where chlorophyll is a photosynthetic pigment) are the most important, since they are the principal site occurs photosynthesis, the process by which plants take advantage of sunlight, water, and carbon dioxide to synthesize organic matter and produce oxygen.

- They are autotrophic cells: the presence of chloroplasts inside them provides plant cells with the ability to "synthesize their own food", so they are a little more autonomous than animal cells for obtaining energy and carbon.

- They have a vacuole: in the cytosol of plant cells there is a special organelle, the vacuole, where water, sugars and even some enzymes are stored.

- They are totipotent: in certain circumstances, many differentiated plant cells have the ability to produce a new individual asexually.

Parts (organelles) of the plant cell and their functions

Plant cell organelles

Cytosol and plasma membrane

The cytosol is everything that is around the nucleus. It is a kind of fluid that includes membranous compartments and other structures. Occasionally the term "cytoplasm" is used to refer to this fluid and the plasma membrane at the same time.

Cellular membrane. Source: Jpablo cad / CC BY (https://creativecommons.org/licenses/by/3.0)

Such a "liquid" is surrounded and contained by a membrane, the plasma membrane, which is nothing more than a lipid bilayer with hundreds of associated proteins, integral or peripheral, that mediate the exchange of substances between the cell and the environment that surrounds it.

As plant cells are surrounded by a cell wall, many authors have coined the term protoplast to refer to everything that is inside this wall, that is to say, the plant cell: the plasma membrane and the cytosol with its organelles.

Cytoskeleton

Cytoskeleton, a network of filamentous proteins in the cell cytoplasm. Source: Alice Avelino / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Plant cells, like animal cells, have a cytoskeleton. The cytoskeleton consists of a series of molecular "scaffolds" that traverse the cell and that organize all the internal components of the cytosol.

They work in the movement of vesicles, in the transport of substances and molecules through the cell and, in addition, in the structuring and support of the cell.

This cytoskeleton is made up of filaments of a protein called F-actin and microtubules, which are polymers of another protein known as tubulin.

Chromatin nucleus and nuclear envelope

Eukaryotic cell nucleus. Source: Mariana Ruiz Villarreal (LadyofHats), translation by Kelvinsong. / CC0

The nucleus is the organelle that contains the genetic material, DNA (deoxyribonucleic acid), which is packaged in the form of chromatin (what chromosomes are made of). It is an organelle covered by a membranous system known as the nuclear envelope.

Nucleolus

Inside it there is also a region known as the nucleolus, in which some proteins and genes that code for ribosomal RNA (ribonucleic acid) are found.

This envelope actually consists of a series of specialized cisterns that surround the nucleus and control the exchange of materials between the nucleus and the cytosol, which occurs through the complexes of the nuclear pore.

It is formed by two membranes that delimit the lumen or nucleoplasm, one internal and one external, the latter continuing with the membranes of the rough endoplasmic reticulum (the one with embedded ribosomes).

The inner membrane is associated with some internal components of the nucleus and probably organizes them spatially. Some authors point out the existence of a nucleus-skeleton, whose protein filaments (as well as those of the cytoskeleton in the cytosol) allow the organization of chromatin.

Endoplasmic reticulum

1-Nuclear membrane. 2-Nuclear pore. 3-Rough endoplasmic reticulum (RER). 4-Smooth endoplasmic reticulum (SER). 5-Ribosome attached to the rough endoplasmic reticulum. 6-Macromolecules. 7-Transport vesicles. 8-Golgi apparatus. 9-Cis face of the Golgi apparatus. 10-Trans face of the Golgi apparatus. 11-Cisternae of the Golgi apparatus. Source: Nucleus ER golgi.jpg: Magnus ManskeDerivative work: Pbroks13 / CC BY (https://creativecommons.org/licenses/by/3.0)

It is a very dynamic membrane system, whose abundance is variable, as well as its structure, its organization and its arrangement in the cytosol.

It is usually divided into a "smooth" portion and another "rough" portion, continuing with the outer nuclear envelope already in which multiple ribosomes are embedded, which are part of the molecular machinery in charge of protein synthesis.

Cellular proteins are processed and distributed in the endoplasmic reticulum, especially those destined for the lipid membranes (secretory pathway). If it occurs, it is one of the sites where some post-translational modifications of proteins occur, such as glycosylation.

In many of the cells that form glands, this organelle is very abundant and works in the secretion of fats, oils and fragrant oils.

It is also abundant in epidermal cells that make lipids that are deposited as waxes on the surface of leaves and other plant organs.

Golgi apparatus

This organelle, also membranous, consists of a series of flattened circular cisterns, delimited by a single membrane. The content of these tanks, their chemical composition and their functions change from one "face" to the other.

In some "lower" plants, an "outer" cistern is associated with the endoplasmic reticulum and is known as the cis compartment or "face" of the Golgi complex, while the more "remote" cisterns form part of the trans face..

In the middle between the cis and trans cisterns are "middle" cisterns and secretory vesicles are formed on the trans side.

The Golgi complex is responsible for the processing and packaging of different macromolecules, as well as for their transport (export) to the cell surface or into vacuoles. Such macromolecules include lipids and proteins.

Unlike animal cells, the Golgi of plant cells has important synthesis activities, since they participate in the de novo synthesis of glycoproteins, pectins, hemicelluloses and some secretory products and components of cell walls.

Ribosomes

Scheme of a ribosome

Ribosomes are very small organelles, with a spherical shape. They are usually on the rough endoplasmic reticulum, but some are free in the cytoplasm. They are made up of RNA and proteins.

These are involved in the synthesis of macromolecules, mainly proteins.

Vacuole and Tonoplast

The vacuole is a multifunctional organelle that participates in the storage, digestion, osmoregulation, and maintenance of the shape and size of plant cells.

Many substances can be stored inside these organelles: colored pigments such as anthocyanins that color leaves and petals, some organic acids that function to regulate pH, some “defense” chemicals against herbivores and secondary metabolites.

Under the microscope they can be seen as "empty sites" in the cytosol, with a spherical appearance and sometimes very large, since they can occupy up to 90% of the cell volume.

As it is an organelle, we must assume that it is surrounded by a membrane, the tonoplast. This membrane is responsible for regulating the passage of substances between the vacuolar lumen and the cytosol, for which it has some specialized proteins.

Vacuoles also function as "digestive organelles" of cells, so they often fulfill functions that are analogous to those of lysosomes in animal cells.

Mitochondria

As in the rest of eukaryotic cells, plant cells have mitochondria, which are organelles surrounded by two membranes, one internal and the other external, which enclose a matrix, they are specialized in the synthesis of energy in the form of ATP and respiration cellular.

They are cylindrical or elliptical organelles, a bit elongated and, in some cases, branched. They have their own genome, so they are capable of encoding and synthesizing many of their proteins, although not all, since the nuclear DNA of the cell codes for others.

Plastids

Plastids are a group of different cellular components, which arise from precursors known as proplastidia. They are normally larger than mitochondria, with a double membrane and a dense matrix called the stroma. They also have their own genome.

Chloroplasts, ethioplasts, amyloplasts, and chromoplasts belong to this family of organelles. Thus, these are the main organelles that distinguish plant cells from animals.

- Chloroplasts are the plastids responsible for photosynthesis and are the ones that house chlorophyll, the photosynthetic pigment par excellence.

Scheme of a chloroplast. Source: Kelvinsong / CC0, wikimedia commons

- Amyloplasts are plastids that function in the storage of starch in different tissues.

- Chromoplasts are plastids that have yellowish or orange coloration or pigmentation, as they can contain different pigments inside.

- Ethioplasts, on the other hand, are found in “etiolated” tissues and are actually chloroplasts that have lost chlorophyll. In undifferentiated tissues they may be called leukoplasts.

Peroxisomes or Microbodies

Basic structure of a peroxisome

Peroxisomes or microbodies are organelles surrounded by a simple membrane, which are distinguished from vesicles by their size and content. They are usually known as peroxisomes, since a toxic chemical called hydrogen peroxide (H ₂ O ₂) is produced inside them, which is harmful to cells.

They are organelles with a large amount of oxidative enzymes inside and are responsible for the synthesis of some molecules, although their main function is the oxidation and decomposition of certain types of lipids, amino acids, nitrogenous bases, etc.

They are especially important in the cells of a seed, since they work in the conversion of fats and lipids stored in these into carbohydrates, which are the main source of energy for embryonic cells.

Some modified peroxisomes are known as glyoxysomes, since the glyoxylate cycle occurs within them, by which carbon atoms derived from photosynthetic processes are recycled.

Cellular wall

Plant cell wall. Source: Scuellar / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

This is another of the characteristic organelles of plant cells (fungi also have wall cells, but their composition is different).

The cell wall consists of an intricate network of a polymer called cellulose, which is made up of repeating units of a sugar called glucose. This structure has many functions, but the most important is to maintain the structure of plant cells and tissues and protect them from the outside.

Although viewed under the microscope it appears to be a relatively thin structure, it gives plant cells some mechanical rigidity and resistance to deformation, especially in different climates.

Plasmodesmata

In plant tissue, narrow cytoplasmic channels can be observed, surrounded by the plasma membrane and connecting neighboring cells through their protoplasts (everything that is inside the cell wall).

Plant cell types

Plant organisms have many different types of cells, which are the product of cell differentiation processes, which are controlled both genetically and environmentally.

Many scientists recognize a collection of plant cells, and here are some of them:

- Initial or meristematic cells: they are found in the meristems, which are the main centers of growth and division of all plants, as they are in constant mitotic division. From these the other cells of a plant's body are differentiated.

- Differentiated cells: all plants have three main types of differentiated cells that are derived from meristematic cells, parenchymal cells, collenchymal cells, and sclerenchyma cells.

Parenchymal or parenchymal cells

These are the most common cells. Some authors describe them as the "beasts of burden" of a plant, since they are the most abundant, but they are the least specialized, that is, the least differentiated.

They have a thin primary cell wall and do not develop a secondary wall. They are responsible for "filling" the available space in plant tissues and provide structure, so they can have different shapes and sizes.

Those parenchymal cells that specialize in photosynthesis are also known as chlorenchyma cells. These cells also participate in the storage of water in the roots, the stem, the leaves, the fruits and the seeds.

Cholenchymal or collenchymal cells

They are cells that provide "flexible support" to plant tissues. They are elongated and have various shapes, which can change during the growth of the plant. They have a primary wall that can be thickened by the deposition of additional cellulose.

They are "glue" cells, since they are the ones that provide greater support than parenchymal cells, while maintaining flexibility. They are always swollen, as their vacuoles are full of water.

Sclerenchyma cells

These cells, unlike the previous two, do have a secondary cell wall, which is fortified with lignin, a polymer composed of different acids and quite heterogeneous phenolic molecules. The term derives from the Greek "skleros" which means "hard".

They are less common cells than parenchymal and colenchymal cells and die when they reach maturity. They provide structural strength to tissues that stop growing in length.

Two types of sclerenchyma cells are known: fibers and sclereids. The former are long, with thick, lignified cell walls, making them strong and flexible.

Sclereids, on the other hand, are more varied, morphologically speaking, but are generally cubic or spherical. These cells are what make up the peels and pits of many fruits. They are not flexible, but rather hard.

Cells in vascular tissues

The vascular tissues of plants are made up of cells. These are those that are responsible for the conduction of water and nutrients and minerals through the body of vegetables.

The xylem tissue (xylem) is what transports water and mineral nutrients from the root to the rest of the plant. The phloem tissue (the phloem), on the other hand, conducts sugars and organic nutrients from the leaves to the rest of the plant. The sum of both fluids is known as sap.

The xylem is composed of tracheids, which are long cells, narrow at their ends. They are considered as a type of sclerenchyma cell. These cells die when they reach maturity so what is "left" is the "shell" formed by the thickened cell wall.

Other cells called vessel elements are also in this tissue, which transport water and minerals faster than tracheids. They also die at maturity, making them hollow "tubes", shorter and narrower than tracheids.

The phloem is composed of a cell type known as elements of the sieve tubes. These are living, metabolically active cells. They join at their ends to form the sieve tube, which is through which photosynthetic products are transported from the leaves to the rest of the body.

References

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