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Plant Cell: Its Parts and Structure With Functions

What is a Plant Cell

A cell is the basic unit of all living organisms that are capable of independent existence. Plant cells are thus defined as the basic unit of life in organisms of the kingdom Plantae. They are eukaryotic cells that have their genetic material enclosed within a membrane.

Structure of Plant Cell

A labeled plant cell diagram shows it as a rectangular shape that is comparably larger than animal cells and ranging in size from 10 to 100 µm. Even though plant cells share many characteristics of an animal cell, they are unique in some ways.

Under the microscope, a typical model of a plant cell shows many different parts that perform distinct functions and together keep the cell functional; they are known as organelles. A plant cell contains some unique organelles that are absent in all animal cells.

Different parts of a plant cell

Among all other organelles, the three distinct parts or features found only in a plant cell are Cell wall, Chloroplast, and Central Vacuole. Apart from all the intracellular organelles found within the cell, plasmodesmata are the only intercellular organelle that is found outside the cell forming connections between two adjacent cells.

Plant Cell Diagram

1) Cell wall

It is a tough layer, composed of non-living cells found outside the plant cell that has a thickness of 20-80 nm. Cell walls are made up of mainly cellulose, the main structural component, along with other carbohydrates like hemicellulose and pectin, but their actual composition varies depending on the growth conditions. Apart from carbohydrates, the cell wall also contains lignin, the complex organic polymer that is particularly important in the formation of the cell wall.

A plant cell wall contains three layers, an outside primary cell wall, which is thin and flexible, an inner secondary cell wall, which is tough and thick. The secondary cell wall develops from the primary cell wall when the cell becomes mature. The third layer that is found between the primary and secondary layers is called the middle lamella.

Functions

  • Providing mechanical strength, support, and rigidity to the cell
  • Providing shape to the plant
  • Allowing selective entry of small molecules inside the cell by preventing large molecules
  • Protecting the delicate inner organelles from outer shock
  • Providing turgidity to plants that helps the movement of water and minerals absorbed by the roots to be distributed throughout the plant body
  • Helping to retain water and thus preventing wilting of plants

2) Plastids

They are membrane-bound organelles that have their genetic material.

Plastids are mainly of three types:

Chloroplasts

They are the oval-shaped structure that contains two distinct membranes. The outer membrane forms the external surface of the chloroplast, and the inner membrane faces the inner side that lies beneath the outer membrane. Between the two membranes, there is a thin intermembrane space. A third membrane present inside of the chloroplast is highly folded to form closed disk-shaped structures known as thylakoid. The space between the inner membrane of the chloroplast and the thylakoid membrane is called stroma, which contains dissolved enzymes, starch, granules, and its genetic material, DNA along with ribosomes, the organ necessary for its protein synthesis.

The presence of green pigment, the chlorophyll in the chloroplast, makes cells green, which is the site photosynthesis in plants.

Leucoplasts

They are found in the non-photosynthetic tissues of plants. Leucoplasts are much smaller than chloroplast, having a variable morphology often called as amoeboid.

Chromoplasts

They are heterogeneous colored plastid found in photosynthetic eukaryotes. Chromoplasts normally have red, orange, and yellow-colored pigments, thus providing color to fruits and flowers.

Functions

  • Absorbing light energy (chloroplast)
  • Helping in the conversion of light energy into chemical energy that produces food in the form of sugar for plants by a process called photosynthesis (chloroplast)
  • Providing defense against infections (chloroplast)
  • Helping in the generation of energy in the form of high energy molecules (chloroplast)
  • Storing protein, lipid, and starch (leucoplast)
  • Synthesizing and storing of pigments (chromoplast)

3) Central Vacuole

They are large vesicles derived from the plasma membrane that contains fluid, ions, and other molecules. A plant cell has a characteristic large central vacuole that takes up almost 30 to 80% of the total plant cell volume. The central vacuole is often the largest organelle in the cell. Apart from plants, they are also present in algae.

Functions

  • Helping to maintain turgidity of the cell, this is the pressure of the contents of the cell pushing against the cell wall, that prevents plants from wilting
  • Storing of reserve food, water and waste materials for the cell
  • Breaking of macromolecules into simpler ones using specific enzymes present inside the vacuole

4) Cell Membrane or Plasma Membrane

It is a thin layer of membrane that is present within the cell wall in plantshaving a thickness of 7.5-10 nm. A cell membrane is mainly composed of proteins and lipid molecules, besides some carbohydrates arranged in a double membrane. Phospholipids are the main component of the cell membrane

Functions

  • Protecting the cell from the outside environmental shock and thus providing additional structural support
  • Maintaining the shape of the cell
  • Regulating the entry and exit of nutrients, essential minerals, and toxic waste products within the cell
  • Helping the cell to attach to the extracellular matrix and thus in forming tissues
  • Helping in the cell to cell communication
  • Helping in regulating cell growth

5) Endoplasmic Reticulum

It is an extensive network of membranes having flattened sac-like structures that are distributed throughout the cell, known as cisternae. The membranes of endoplasmic reticulum consist of a phospholipid bilayer that encloses a space, the lumen from the cytosol, which forms a continuous channel from the cell membrane through the cytoplasm to the outer membrane of the nucleus.

There are two types of ER: i) rough endoplasmic reticulum (RER) and ii) smooth endoplasmic reticulum (SER). The surface of RER is studded with ribosomes, which gives a rough appearance to it, while in SER, the tubules are devoid of ribosomes, thus providing a smooth shape.

Functions

  • Helping in the formation of nuclear membrane during cell division
  • Producing trans-membrane proteins, lipids, glycogen and other steroids like cholesterol for its membrane and other parts of a cell
  • Packaging and transporting of proteins and carbohydrates to other organelles
  • Providing an increased surface area for the cell to cell interactions
  • Forming the skeletal framework of the cell

6) Golgi Apparatus

They are comprised of a series of five to eight cup-shaped membrane-covered sacs called cisternae. Cisternae are flattened membranes consisting of disk-shaped pouches that make the Golgi apparatus, also known as Golgi body, Golgi complex, and dictyosome. This collection of cisternae is broken down into cis, medial, and trans compartments, making up two main networks: the cis-Golgi network (CGN) and the trans-Golgi network (TGN). The Golgi apparatus packages proteins into membrane-bound vesicles called Golgi vesicles inside the cell before being transported to their destination. The number of Golgi apparatus varies in cells according to their functions.

Functions

  • Synthesizing complex polysaccharides of the cell wall
  • Processing, packaging and transporting or secretion of the proteins generated in the endoplasmic reticulum thus acts as a post office inside the cell
  • Performing modifications such as phosphorylation and glycosylation of proteins
  • Performing breakdown of proteins and thus forming small active fragments

7) Microfilaments or Actin filament

They are a network of long and thin protein fibers present in the cytoplasm of the cell having a diameter of 3-6 nm. They are primarily composed of a bundle of monomeric proteins called actin but are modified by and interact with other proteins to form a network. Microfilaments are usually 7nm in diameter but are extremely strong and flexible.

Functions

  • Helping in the contraction of muscles
  • Helping in cell movement
  • Aiding in cell division
  • Maintaining cell shape
  • Helping in the flow of cytoplasmic contents including nutrients throughout the cell (cytoplasmic streaming)

8) Microtubules

They are fibrous hollow rods composed of a network of a long chain of monomeric proteins called tubulin. Microtubules have an outer diameter of 23 to 27 nm and an inner diameter of about 11 to 15 nm. They can grow to as long as 50 µm and thus are highly dynamic.

Functions

  • Maintaining the structure of the cell
  • Helping in cell movement
  • Aiding in cell division
  • Helping in the movement of cytoplasmic organelles except for nucleus throughout the cell
  • Helping in communication and intracellular transport within the cell

9) Intermediate Filaments

They complete the cytoskeletal structural components of the cell along with thin microfilaments and the thick microtubules. Intermediate filaments are stable and durable composed of a pair of two intertwined proteins called a coiled-coil structure. They range in diameter from 8-10 nm that are intermediate compared to microfilaments and microtubules. They are prominent in cells that withstand mechanical stress and are the most insoluble part of the cell. Microtubules, together with microfilaments and intermediate filaments, form the cytoskeleton of the cell.

Functions

  • Maintaining structural integrity of the cell
  • Maintaining cell shape
  • Helping in organelle motility

10) Mitochondria

It is an oval-shaped double membrane-bound organelle having an area of 0.75 and 3 μm² that is found in most plant cells. They are often called the powerhouse or energy factories of the cell. The outer membrane of mitochondria surrounds the entire organelle, and the inner one has many inward protrusions called cristae that enclose the inward compartment known as the mitochondrial matrix, the two membranes were separated by a narrow region known as intermembrane space.

The number of mitochondria in a cell varies widely depending upon the organism, cell type, and tissue. Although most of the cell’s genome is contained within the nucleus, mitochondria have their DNA, along with the ribosomes, the organ necessary for its protein synthesis. The DNA of mitochondria shows substantial similarity to the chloroplast and the bacterial genomes.

Functions

  • Producing the energy currency of the cell, the high energy molecule, adenosine triphosphate (ATP)
  • Helping in cellular respiration, which involves breaking down of nutrients to generate energy

11) Nucleus

It is a spherical shaped double membrane-bound organelle found mostly at the center of the eukaryotic cells that accounts for about 10 percent of the total cell volume. Generally, eukaryotes, including plant cells, have a single nucleus, also called uninucleated that acts as the storehouse of the genetic material, the deoxyribonucleic acid, or DNA of the cell. The division of the nucleus of a parental cell resulting in two daughter cells is called karyokinesis.

A nucleus has four main parts:

Nuclear membrane or Nuclear envelopethe double-layered membrane that separates the content of the nucleus from the cell cytoplasm. Within the membrane are numerous microscopic pores called nuclear pores that regulate the passage of molecules between the nucleus and the cytoplasm

Chromatin threads or Chromosomesthe genetic material of the cell that takes part in cell division or mitosis

Nuclear sap or Nucleoplasmclear, homogeneous, and transparent liquid found within the nucleus that contains the genetic material of the cell, the nucleic acid, along with proteins, sugars, and enzymes.

Nucleolusa membrane-less organelle within the nucleus that manufactures the cell’s protein-producing organelle, the ribosomes

Some plant cells being exception have no nucleus and are referred to as enucleated cells, whereas with more than one nucleus are called multinucleated.

Functions

  • Storing of cell’s hereditary material
  • Controlling cell growth and reproduction
  • Acting as the home of some of the vital cellular processes and coordinating them
  • Creating two identical copies of the host or duplicating one’s genetic material or DNA (replication)
  • Creating other forms of genetic material from DNA that acts as instructions to the rest of the cell (transcription)

12) Peroxisomes

They are single membrane-bound vesicular organelles having a diameter of 0.1 to 1 mm that are found in all eukaryotic cells. They vary widely in shape, size, and number depending upon the energy requirement of the cell. Peroxisomes contain several digestive and oxidative enzymes required for cellular metabolism of the plant cell.

Functions

  • Helping in photosynthesis
  • Helping in seed germination
  • Preventing loss of energy during photosynthesis
  • Performing oxidation of specific biomolecules
  • Synthesizing specific lipids, bile acids, and cholesterol

13) Ribosomes

They are complex molecular machines that are found in all living cells. A ribosome is a complex of the hereditary material, the ribonucleic acid (RNA), and protein that binds to a messenger ribonucleic acid (mRNA) and decodes the information present in the mRNA with the help of the transfer RNAs (tRNAs). Since a ribosome is a complex of RNA and protein, it is also known as a ribonucleoprotein. Ribosomes are composed of two subunits; the smaller subunit binds to mRNA and decodes its information, whereas the larger subunit adds up amino acids.

Functions

  • Producing mRNA from the genetic code, the DNA (transcription)
  • Acting as catalysts in the process of synthesizing proteins from amino acids (translation)

14) Cytoplasm

They are jelly-like fluid that fills up the entire space of the cell and is mainly composed of water, salts, and proteins. The portion of the cytoplasm that is not contained in the organelles is called the cytosol. Although cytoplasm may appear to have no form or structure, it is highly organized. The cytoplasm contains cytoskeletal elements, ions, proteins, and macromolecular structures and also other organelles such as mitochondria, ribosomes, vacuoles, and endoplasmic reticulum, that are suspended in the cytosol except for the nucleus. The division of the cytoplasm of a parental cell into two daughter cells is called cytokinesis.

Functions

  • Maintaining turgidity thus keeping cell shape
  • Assisting several metabolic activities
  • Keeping the cell organelles in their positions
  • Providing the raw materials necessary for the chemical reactions within the cell

The whole cellular content of a living cell is called protoplasm. The cytoplasm, nucleus, and all other living components of the cell together make up the protoplasm of a cell.

15) Plasmodesmata

They are microscopic channels that traverse the cell wall of plant cells. Plasmodesmata are approximately 50–60 nm in diameter that form intercellular cytoplasmic bridges between plant cells. They are specialized channels formed from the close association between the plasma membrane and the endoplasmic reticulum of the cell. There are two types of plasmodesmata: primary plasmodesmata, which are made during cell division, and secondary plasmodesmata, which develop between mature cells.

Functions

  • Allowing intercellular movement of water, nutrients and small signaling molecules between the cells
  • Transporting viral genomes between cells
  • Helping in the cell to cell communication

Different types of plant cell

There are five different types of plant cells, each performing different functions:

Parenchyma cells

They are the primary living cells of plants that are mainly found in leaves. Parenchyma cells carry out photosynthesis and cellular respiration along with other cellular processes. They also store substances like starches and proteins and have a role in plant cell repair.

Collenchyma cells

They are elongated cells with thick cellulosic cell walls that can grow and change shape as the plant grows. Collenchyma cells provide support to growing parts of a plant.

Sclerenchyma cells

They are dead cells that have very thick cell walls. Sclerenchyma cells are hard cells that are providing support to the plant in the areas that have ceased growing.

Xylem cells

They are hard, water-conducting cells that help to transport water and few nutrients throughout the plant. Xylem is a complex tissue composed of water-conducting vessel elements, together with fibers and parenchyma cells.

Phloem cells

They are specialized, complex tissues that transport food and sap, mainly in the form of sucrose to all parts of a plant. Phloem cells consist of two main cell types, the sieve tubes, and the intimately associated companion cells, together with parenchyma cells and fibers.

Article was last reviewed on Tuesday, January 21, 2020

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