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#1 2021-12-13 20:03:30

ganesh
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Registered: 2005-06-28
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Plant Cell

Plant Cell

Definition

Plant cells are the basic unit of life in organisms of the kingdom Plantae. They are eukaryotic cells, which have a true nucleus along with specialized structures called organelles that carry out different functions. Plant cells have special organelles called chloroplasts, which create sugars via photosynthesis. They also have a cell wall that provides structural support.

Overview of Plant Cells

Animals, fungi, and protists are made of at least one eukaryotic cell. In contrast, bacteria and archaea are made up of a single prokaryotic cell. Plant cells are differentiated from the cells of other organisms by their cell walls, chloroplasts, and central vacuole.

Chloroplasts are organelles that are crucial for plant cell function. These are the structures that carry out photosynthesis, using the energy from the sun to produce glucose. In doing so, the cells use carbon dioxide, and they release oxygen.

Other organisms, such as animals, rely on this oxygen and glucose to survive. Plants are considered autotrophic because they produce their own food and do not have to consume any other organisms. Specifically, plant cells are photoautotrophic because they use light energy from the sun to produce glucose. Organisms that eat plants and other animals are considered heterotrophic.

The other components of a plant cell, the cell wall, and central vacuole, work together to give the cell rigidity. The plant cell will store water in the central vacuole, which expands the vacuole into the sides of the cell. The cell wall then pushes against the walls of other cells, creating a force known as turgor pressure. While animals rely on a skeleton for structure, turgor pressure in plant cells allows plants to grow tall and reach more sunlight.

Plant Cells vs. Animal Cells

Plant and animal cells are both eukaryotic cells, meaning they possess a defined nucleus and membrane-bound organelles. They share many common features, such as a cell membrane, nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, ribosomes, and more.

However, they have some apparent differences. Firstly, plant cells have a cell wall that surrounds the cell membrane, whereas animal cells do not. Plant cells also possess two organelles that animal cells lack: chloroplasts and a large central vacuole.

These additional organelles allow plants to form an upright structure without the need for a skeleton (cell wall and central vacuole), and also allow them to produce their own food through photosynthesis (chloroplasts).

Parts of a Plant Cell

The plant cell has many different features that allow it to carry out its functions. Each of these structures, called organelles, carry out a specialized role.

Animal and plant cells share many common organelles. However, there are some specialized structures in plant cells, including chloroplasts, a large vacuole, and the cell wall.

Chloroplasts

Chloroplasts are specialized organelles found only in plants and some types of algae. These organelles carry out the process of photosynthesis, which turns water, carbon dioxide, and light energy into nutrients from which the plant can obtain energy. There can be over one hundred chloroplasts in certain plant cells.

Chloroplasts are disk-shaped organelles that are surrounded by a double membrane. The outer membrane forms the external surface of the chloroplast and is relatively permeable to small molecules, allowing substances entry into the organelle. The inner membrane lies just beneath the outer membrane and is less permeable to external substances.

Between the outer and inner membrane is a thin intermembrane space that is about 10-20 nanometers wide. The center of the chloroplast that is enclosed by the double membrane is a fluid matrix called the stroma (you can think of this like the cytoplasm of the chloroplast).

Within the stroma, there are many structures called thylakoids, which look like flattened disks. Thylakoids are stacked on top of one another in vascular plants in stacks called grand. Thylakoids have a high concentration of chlorophyll and carotenoids, which are pigments that capture light energy from the sun. The molecule chlorophyll is also what gives plants their green color.

Vacuoles

Plant cells are unique in that they have a large central vacuole. A vacuole is a small sphere of plasma membrane within the cell that can contain fluid, ions, and other molecules. Vacuoles are essentially just large vesicles. They can be found in the cells of many different organisms. However, plant cells characteristically have a large vacuole that can take up anywhere from 30% to as much as 90% of the total cell volume.

The central vacuole of a plant cell helps maintain its turgor pressure, which is the pressure of the contents of the cell pushing against the cell wall. A plant thrives best when its cells have high turgidity, and this occurs when the central vacuole is full of water. If turgor pressure in the plants decreases, the plants begin to wilt. Plant cells fare best in hypotonic solutions, where there is more water in the environment than in the cell. Under these conditions, water rushes into the cell by osmosis, and turgidity is high.

Animal cells, in comparison, can lyse if too much water rushes in; they fare better in isotonic solutions, where the concentration of solutes in the cell and in the environment is equal, and the net movement of water in and out of the cell is the same.

Many animal cells also have vacuoles, but these are much smaller and tend to play a less crucial function.

Cell Wall

The cell wall is a tough layer found on the outside of the plant cell that gives it strength and also maintains high turgidity. In plants, the cell wall contains mainly cellulose, along with other molecules like hemicellulose, pectin, and lignins. The composition of the plant cell wall differentiates it from the cell walls of other organisms.

For example, fungi cell walls contain chitin, and bacterial cell walls contain peptidoglycan. These substances are not found in plants. Importantly, the main difference between plant and animal cells is that plant cells have a cell wall, while animal cells do not.

Plant cells have a primary cell wall, which is a flexible layer formed on the outside of a growing plant cell. Plants can also have a secondary cell wall, a tough, thick layer formed inside the primary plant cell wall when the cell is mature.

Other Organelles

Plant cells have many other organelles that are essentially the same as organelles in other types of eukaryotic cells, such as animal cells.

* The nucleus contains deoxyribonucleic acid (DNA), the cell’s genetic material. DNA contains instructions for making proteins, which controls all of the body’s activities. The nucleus also regulates the growth and division of the cell.

* Proteins are synthesized in ribosomes, modified in the endoplasmic reticulum, and folded, sorted, and packaged into vesicles in the Golgi apparatus.

* Mitochondria are also found in plant cells. They produce ATP through cellular respiration. Photosynthesis in the chloroplasts provides the nutrients that mitochondria break down for use in cellular respiration. Interestingly, both chloroplasts and mitochondria are thought to have formed from bacteria being engulfed by other cells in an endosymbiotic (mutually beneficial) relationship, and they did so independently of each other.

* The liquid within the cells is the cytosol. It is mostly made of water, and also contains ions, proteins, and small molecules. Cytosol and all the organelles within it, except for the nucleus, are called the cytoplasm.

* The cytoskeleton is a network of filaments and tubules found throughout the cytoplasm of the cell. It has many functions; it gives the cell shape, provides strength, stabilizes tissues, anchors organelles within the cell, and has a role in cell signaling. The cell membrane, a double phospholipid layer, surrounds the entire cell.

Plant Cell Types

There are five types of tissue formed by plant cells, each with different functions. Parenchyma, collenchyma, and sclerenchyma are all simple plant tissues, meaning they contain a single cell type. In contrast, xylem and phloem contain a mixture of cell types and are referred to as complex tissues.

Plant tissue types

* Parenchyma tissue represents the majority of cells in a plant. They are found in leaves and carry out photosynthesis and cellular respiration, along with other metabolic processes. They also store substances like starches and proteins and have a role in plant wound repair.
* Collenchyma tissue provides support to growing parts of a plant. They are elongated, have thick cell walls, and can grow and change shape as a plant grows.
* Sclerenchyma tissue contains hard cells that are the main supporting cells in the areas of a plant that have ceased growing. Sclerenchyma cells are dead and have very thick cell walls.
* Xylem cells transport mostly water and a few nutrients throughout a plant, from the roots to the stem and leaves.
* Phloem cells transport nutrients made during photosynthesis to all parts of a plant. They transport sap, which is a watery solution high in sugars.

Plant cells are eukaryotic cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capability to perform photosynthesis and store starch, a large vacuole that regulates turgor pressure, the absence of flagella or centrioles, except in the gametes, and a unique method of cell division involving the formation of a cell plate or phragmoplast that separates the new daughter cells.

Characteristics of plant cells

* Plant cells have cell walls, constructed outside the cell membrane and composed of cellulose, hemicelluloses, and pectin. Their composition contrasts with the cell walls of fungi, which are made of chitin, of bacteria, which are made of peptidoglycan and of archaea, which are made of pseudopeptidoglycan. In many cases lignin or suberin are secreted by the protoplast as secondary wall layers inside the primary cell wall. Cutin is secreted outside the primary cell wall and into the outer layers of the secondary cell wall of the epidermal cells of leaves, stems and other above-ground organs to form the plant cuticle. Cell walls perform many essential functions. They provide shape to form the tissue and organs of the plant, and play an important role in intercellular communication and plant-microbe interactions.

* Many types of plant cells contain a large central vacuole, a water-filled volume enclosed by a membrane known as the tonoplast that maintains the cell's turgor, controls movement of molecules between the cytosol and sap, stores useful material such as phosphorus and nitrogen and digests waste proteins and organelles.

* Specialized cell-to-cell communication pathways known as plasmodesmata, occur in the form of pores in the primary cell wall through which the plasmalemma and endoplasmic reticulum[5] of adjacent cells are continuous.

* Plant cells contain plastids, the most notable being chloroplasts, which contain the green-colored pigment chlorophyll that converts the energy of sunlight into chemical energy that the plant uses to make its own food from water and carbon dioxide in the process known as photosynthesis. Other types of plastids are the amyloplasts, specialized for starch storage, elaioplasts specialized for fat storage, and chromoplasts specialized for synthesis and storage of pigments. As in mitochondria, which have a genome encoding 37 genes, plastids have their own genomes of about 100-120 unique genes and are interpreted as having arisen as prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.

* Many cellular structures are membranous and their composition includes lipids.

* Cell division in land plants and a few groups of algae, notably the Charophytes and the Chlorophyte Order Trentepohliales, takes place by construction of a phragmoplast as a template for building a cell plate late in cytokinesis.

* The motile, free-swimming sperm of bryophytes and pteridophytes, cycads and Ginkgo are the only cells of land plants to have flagella similar to those in animal cells, but the conifers and flowering plants do not have motile sperm and lack both flagella and centrioles.

Types of plant cells and tissues

Plant cells differentiate from undifferentiated meristematic cells (analogous to the stem cells of animals) to form the major classes of cells and tissues of roots, stems, leaves, flowers, and reproductive structures, each of which may be composed of several cell types.

Parenchyma

Parenchyma cells are living cells that have functions ranging from storage and support to photosynthesis (mesophyll cells) and phloem loading (transfer cells). Apart from the xylem and phloem in their vascular bundles, leaves are composed mainly of parenchyma cells. Some parenchyma cells, as in the epidermis, are specialized for light penetration and focusing or regulation of gas exchange, but others are among the least specialized cells in plant tissue, and may remain totipotent, capable of dividing to produce new populations of undifferentiated cells, throughout their lives. Parenchyma cells have thin, permeable primary walls enabling the transport of small molecules between them, and their cytoplasm is responsible for a wide range of biochemical functions such as nectar secretion, or the manufacture of secondary products that discourage herbivory. Parenchyma cells that contain many chloroplasts and are concerned primarily with photosynthesis are called chlorenchyma cells. Others, such as the majority of the parenchyma cells in potato tubers and the seed cotyledons of legumes, have a storage function.

Collenchyma

Collenchyma cells are alive at maturity and have thickened cellulose cell walls. These cells mature from meristem derivatives that initially resemble parenchyma, but differences quickly become apparent. Plastids do not develop, and the secretory apparatus (ER and Golgi) proliferates to secrete additional primary wall. The wall is most commonly thickest at the corners, where three or more cells come in contact, and thinnest where only two cells come in contact, though other arrangements of the wall thickening are possible. Pectin and hemicellulose are the dominant constituents of collenchyma cell walls of dicotyledon angiosperms, which may contain as little as 20% of cellulose in Petasites. Collenchyma cells are typically quite elongated, and may divide transversely to give a septate appearance. The role of this cell type is to support the plant in axes still growing in length, and to confer flexibility and tensile strength on tissues. The primary wall lacks lignin that would make it tough and rigid, so this cell type provides what could be called plastic support – support that can hold a young stem or petiole into the air, but in cells that can be stretched as the cells around them elongate. Stretchable support (without elastic snap-back) is a good way to describe what collenchyma does. Parts of the strings in celery are collenchyma.

Sclerenchyma

Sclerenchyma is a tissue composed of two types of cells, sclereids and fibres that have thickened, lignified secondary walls  laid down inside of the primary cell wall. The secondary walls harden the cells and make them impermeable to water. Consequently, sclereids and fibres are typically dead at functional maturity, and the cytoplasm is missing, leaving an empty central cavity. Sclereids or stone cells, (from the Greek skleros, hard) are hard, tough cells that give leaves or fruits a gritty texture. They may discourage herbivory by damaging digestive passages in small insect larval stages. Sclereids form the hard pit wall of peaches and many other fruits, providing physical protection to the developing kernel. Fibres are elongated cells with lignified secondary walls that provide load-bearing support and tensile strength to the leaves and stems of herbaceous plants. Sclerenchyma fibres are not involved in conduction, either of water and nutrients (as in the xylem) or of carbon compounds (as in the phloem), but it is likely that they evolved as modifications of xylem and phloem initials in early land plants.

Xylem

Xylem is a complex vascular tissue composed of water-conducting tracheids or vessel elements, together with fibres and parenchyma cells. Tracheids are elongated cells with lignified secondary thickening of the cell walls, specialised for conduction of water, and first appeared in plants during their transition to land in the Silurian period more than 425 million years ago. The possession of xylem tracheids defines the vascular plants or Tracheophytes. Tracheids are pointed, elongated xylem cells, the simplest of which have continuous primary cell walls and lignified secondary wall thickenings in the form of rings, hoops, or reticulate networks. More complex tracheids with valve-like perforations called bordered pits characterise the gymnosperms. The ferns and other pteridophytes and the gymnosperms have only xylem tracheids, while the flowering plants also have xylem vessels. Vessel elements are hollow xylem cells without end walls that are aligned end-to-end so as to form long continuous tubes. The bryophytes lack true xylem tissue, but their sporophytes have a water-conducting tissue known as the hydrome that is composed of elongated cells of simpler construction.

Phloem

Phloem is a specialised tissue for food transport in higher plants, mainly transporting sucrose along pressure gradients generated by osmosis, a process called translocation. Phloem is a complex tissue, consisting of two main cell types, the sieve tubes and the intimately associated companion cells, together with parenchyma cells, phloem fibres and sclereids.  Sieve tubes are joined end-to-end with perforated end-plates between known as sieve plates, which allow transport of photosynthate between the sieve elements. The sieve tube elements lack nuclei and ribosomes, and their metabolism and functions are regulated by the adjacent nucleate companion cells. The companion cells, connected to the sieve tubes via plasmodesmata, are responsible for loading the phloem with sugars. The bryophytes lack phloem, but moss sporophytes have a simpler tissue with analogous function known as the leptome.

Epidermis

The plant epidermis is specialised tissue, composed of parenchyma cells, that covers the external surfaces of leaves, stems and roots. Several cell types may be present in the epidermis. Notable among these are the stomatal guard cells that control the rate of gas exchange between the plant and the atmosphere, glandular and clothing hairs or trichomes, and the root hairs of primary roots. In the shoot epidermis of most plants, only the guard cells have chloroplasts. Chloroplasts contain the green pigment chlorophyll which is needed for photosynthesis. The epidermal cells of aerial organs arise from the superficial layer of cells known as the tunica (L1 and L2 layers) that covers the plant shoot apex, whereas the cortex and vascular tissues arise from innermost layer of the shoot apex known as the corpus (L3 layer). The epidermis of roots originates from the layer of cells immediately beneath the root cap. The epidermis of all aerial organs, but not roots, is covered with a cuticle made of polyester cutin or polymer cutan (or both), with a superficial layer of epicuticular waxes. The epidermal cells of the primary shoot are thought to be the only plant cells with the biochemical capacity to synthesize cutin.

Plant-Cell-diagram-1.jpg


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