Plant Cell : Defination, Structure and Functions, Types

Each living Organism is made up of Cell. Cell is called a structural and functional
unit of life of all living organisms capable of independent existence and can perform all
functions of life like reproduction, excretion etc. Living organisms are grouped into two main categories the Prokaryotes and Eukaryotes.

Cells in which the nucleus has a definite
nuclear membrane are known as Eukaryotic cells. These cells exhibit presence of membrane bound cell organelles. e.g. Cells of Protists, Plants, Animals and Fungi. The eukaryotic cells have different shape, size and physiology but all the cells are typically composed of plasma membrane, cytoplasm and its organelles viz. Mitochondria, Endoplasmic Reticulum, Ribosomes, Golgi complex, etc. and a true nucleus.

Defination of plant cell 
Answer : Plant cells are the basic unit of life of the kingdom Plantae enclosed by a cell wall, containing a membrane-bound nucleus and other cell organelles and perform photosynthesis.

What is plant Cell ?
Answer : Plant cells are eukaryotic cells or cells with a membrane-bound nucleus. Unlike prokaryotic cells, the DNA in a plant cell is housed within a nucleus that is enveloped by a membrane.

In addition to having a nucleus, plant cells also contain other membrane-bound organelles (tiny cellular structures) that carry out specific functions necessary for normal cellular operation. Organelles have a wide range of responsibilities that include everything from producing hormones and enzymes to providing energy for a plant cell.

Structure of Plant cell :
1. Cell wall:
  1. It is rigid, supportive and protective outer covering of plasma membrane of plant cells, fungi and some protists. Algae show presence of cellulose, galactans, mannans and minerals like calcium carbonate in cell wall. 
  2. In other plants, it is made up of hemicelluloses, pectin, lipids and protein. 
  3. Microfibrils of plant cell wall show presence of cellulose which is responsible for rigidity. Some of the depositions of cell wall are silica (grass stem), cutin (epidermal walls of land plants), suberin (endodermal cells of root), wax, lignin. 
  4. It gives shape to the cell and protects from  mechanical injury and infections. In plants, cell wall shows middle lamella, primary wall and secondary wall.

2. Cell membrane / Plasma membrane/ 
 biomembrane :
  1. It is thin, quasifluid structure present both extracellularly and intracellularly. Extracellularly, it is present around protoplast and intracellularly, it is present around most of the cell organelles in eukaryotic cell. It separates cell organelles from cytosol. 
  2. Thickness of biomembrane is about 75A° . Under electron microscope, cell membrane appears trilaminar (made up of three layers). 
  3. It shows presence of lipids (mostly phospholipids) arranged in bilayer. Lipids posses one hydrophilic polar head and two hydrophobic non-polar tails. So, phospholipids are amphipathic. 
  4. Lipid molecules are arranged in two layers (bilayer) in such a way that their tails are sandwitched in between heads. 
  5. Due to this, tails never come in direct contact with aqueous surrounding.
  6. Cell membrane also shows presence of proteins and carbohydrates. Ratio of proteins and lipids varies in different cells. 
  7. For example-in human beings, RBCs show approximately 52% protein and 40% lipids. 
  8. Fluid mosaic model : It is most accepted model of cell membrane. It was proposed by Singer and Nicholson in 1972. 
  9. According to this model, it is made up of phospholipid bilayer and proteins. Proteins are like icebergs in the sea of lipids. Proteins can change their position. Some proteins are intrinsic i.e. occur at different depths of bilayer.

3. Cytoplasm :
  1. The cell contains ground substance called cytoplasmic matrix or cytosol. This colloidal jelly like material is not static. 
  2. It shows streaming movements called cyclosis. 
  3. The cytoplasm contains water as major component along with organic and inorganic molecules like sugars, amino acids, vitamins, enzymes, nucleotides, minerals and waste products. 
  4. It also contains various cell organelles like endoplasmic reticulum, Golgi complex, mitochondria, plastids, nucleus, microbodies and cytoskeletal elements like microtubules. 
  5. Cytoplasm acts as a source of raw materials as well as seat for various metabolic activities taking place in the cell.

4. Endoplasmic Reticulum (ER):
  1. This little network within the cytosol is present in all eukaryotic cells except ova and mature red blood corpuscles.
  2. Membrane of Endoplasmic reticulum is continuous with nuclear envelope at one end and extends till cell membrane. 
  3. It thus acts as intracellular supporting framework and helps in maintaining position of various cell organelles in the cytoplasm. 
  4. The outer surface of endoplasmic reticular membrane may or may not be studded with ribosomes. Accordingly, it is called rough or smooth ER. 
  5. Smooth and rough ER differ in their functions. 
  6. Smooth ER is involved in various processes in different cells. Depending on cell type, it helps in synthesis of lipids (ex. steroid secreting cells of cortical region of adrenal gland, testes and ovaries), detoxification of drugs and poisons (liver cells) and storage of calcium ions (muscle cells). 
  7. Rough ER is primarily involved in protein synthesis. (e.g. pancreatic cells that secrete insulin). These proteins are secreted by ribosomes attached to rough ER and are called secretory proteins. These proteins get wrapped in membrane that buds off from transitional region of ER. Such membrane bound proteins depart from ER as transport vesicles.

5. Golgi complex :
  1. Golgi complex or Golgi apparatus or Golgi body; various terms are used to denote this assembly, manufacturing cum packaging and transport unit of cell.
  2. Golgi complex essentially consists of stacks of membranous sacs called cisternae. 
  3. Diameter of cisternae varies from 0.5 to 1 µm. A cell may have few to several cisternae depending on its function. 
  4. The Golgi sacs show specific orientation in the cell. Each cisterna has a forming or ‘cis’ face (cis: on the same side) and maturing or ‘ trans’ face (trans: the opposite side).
  5. Transport vesicles that pinch off from transitional ER merge with cis face of Golgi cisterna and add its contents into the lumen. 
  6. Golgi body carries out two types of functions, modification of secretions of ER and production of its own secretions.
  7. Cisternae contain specific enzymes for specific functions. Refining of product takes place in an orderly manner. 
  8. Golgi bodies also manufacture their own products. Golgi bodies in many plant cells produce non-cellulose polysaccharides like pectin. 

6. Lysosomes :
  1. Lysosomes can be considered as dismantling and restructuring units of a cell. 
  2. These are membrane bound vesicles containing hydrolytic enzymes. The enzymes in lysosomes are used by most eukaryotic cells to digest (hydrolyse) macromolecules. The lysosomal enzymes show optimal activity in acidic pH.
  3. Lysosomes are polymorphic in nature. 
  4. We can classify lysosomes as, Primary lysosomes; which are nothing but membrane bound vesicles in which enzymes are in inactive state.
  5. Secondary lysosomes or hybrid lysosomes, which are formed by fusion of lysosome with endocytic vesicle containing materials to be digested, represented as heterophagic vesicle.
  6. Lysosomes which bring about digestion of cells own organic material like a damaged cell organelle, are called autophagic vesicles (or suicide bags).

7. Vacuoles :
  1. Vacuoles are membrane bound sacs prominently found in plant cells. In animal cells, whenever present they are few in number and smaller in size. Generally, there are two or three permanent vacuoles in a plant cell.
  2. vacuole can occupy as much as 90% of the total volume of the cell.
  3. The vacuoles are bound by semipermeable membrane, called tonoplast membrane. 
  4. This membrane helps in maintaining the composition of vacuolar fluid; the cell sap, different from that of the cytosol.
  5. Intake of food or foreign particle by phagocytosis involves formation of food vacuole. In fresh water unicellular forms like Paramoecium, excretion and osmoregulation takes place by contractile vacuoles. 
  6. Vacuoles maintain turgidity of the cell.

9. Mitochondria (Singular : 
 Mitochondrion) :

  1. These are important cell organelles involved in aerobic respiration. 
  2. Mitochondria are absent in prokaryotic cells and red blood corpuscles (RBCs). Their shape may be oval or spherical or spiral strip like. 
  3. It is a double membrane bound organelle. Outer membrane is permeable to various metabolites due to presence of a protein -Porin or Parson’s particles. 
  4. Inner membrane is selectively permeable to few substances only. 
  5. Both membranes are separated by a space-outer chamber. 
  6. Inner membrane shows several finger like or plate like folds- cristae. 
  7. Inner membrane bears numerous particles- oxysomes and cytochromes / electron carriers. 
  8. Inner membrane encloses a cavity- inner chamber, containing a fluid- matrix. Matrix contains few coils of circular DNA, RNA, 70S types of ribosomes, lipids and various enzymes of Krebs cycle and other pathways.
Read MoreMitochondria Structure and Functions

10. Plastids :

  1. Like mitochondria, plastids too are double walled organelles containing DNA, RNA and 70S ribosomes. But they are larger in size and can be observed under light microscope. 
  2. Plastids are classified according to the pigments present in it as leucoplasts, chromoplasts and chloroplasts. 
  3. Leucoplasts do not contain any pigments, they are of various shapes and sizes. These are meant for storage of nutrients. e.g. Amyloplasts that store starch, Elaioplasts that store oils and Aleuroplasts that store proteins. 
  4. Chromoplasts contain pigments like carotene and xanthophyll, etc. They impart red, yellow or orange colour to flowers and fruits.
  5. Plant cells, cells of algae and few protists like Euglena contain chloroplasts. Inner membrane of double walled chlorophyll is comparatively less permeable. 
  6. Inside the cavity of inner membrane, there is another set of membranous sacs called thylakoids. 
  7. Thylakoids are arranged in the form of stacks called grana (singular: granum). 
  8. The grana are connected to each other by means of membranous tubules called stroma lamellae. Space outside thylakoids is is filled with stroma. 
  9. The stroma, and the space inside thylakoids contain various enzymes essential for photosynthesis. Like other plastids, stroma of chloroplast also contains DNA and ribosomes.

11. Ribosomes :
  1. You are aware that ribosomes are protein factories of the cell. They use the genetic information to synthesise proteins.
  2. Ribosomes were first observed as dense particles in electron micrograph of a cell by scientist Pallade in 1953. 
  3. Ribosomes are made up of Ribosomal RNA and proteins. They do not have any membranous covering around them.

12. Nucleus :

  1.  Structure of nucleus of a plant cell becomes distinct in a non-dividing cell or during interphase. Such an interphase nucleus is made up of nuclear envelope, nucleoplasm, nucleolus and chromatin network.
  2. Nuclear envelope is a double walled delimiting membrane of nucleus. 
  3. Two membranes are separated from each other by perinuclear space (10 to 50nm). Outer membrane is connected with endoplasmic reticulum at places. 
  4. The nucleoplasm or karyolymph contains various substances like nucleic  acids, protein molecules, minerals and salts. 
  5. It contains chromatin network and nucleolus. 
  6. Nucleolus is another component which is not bound by cell membrane.
  7. Nucleolus is made up of rRNA and ribosomal proteins and it is best known as the site of ribosome biogenesis. 
  8. They appear as dense spherical bodies present near chromatin network. They produce rRNA and ribosomal proteins which are then transported to cytoplasm and are assembled together to form ribosomes.

Chromatin material :
  1. Nucleus contains genetic information in the form of chromosomes which are nothing but DNA molecules associated with proteins.
  2. The chromatin material contains DNA, histone and non-histone proteins and RNA. 
  3. In some regions of chromatin, DNA is more and is genetically active called euchromatin. Some regions that contain more of proteins and less DNA and are genetically inert, are called heterochromatin
The nucleus contains entire genetic
information, hence play important role in
heredity and variation. It is the site for synthesis of DNA, RNA and ribosomes. It plays important role in protein synthesis. Chromosome number being constant for a species, it is important in phylogenetic studies. Nucleus thus is the master
cell organelle.

Functions of plant cell 

  1. As the basic living part of a plant, the plant cell is involved in many physiological and structural activities. 
  2. They range from synthesis of proteins and energy to maintaining structural integrity and tautness.
  3. Various organelles (sub-cell parts) within the plant cell are dedicated to specific tasks. 
  4. cell wall provides support for the internal parts and protects them from external forces. 
  5. The chloroplast is involved in photosynthesis. 
  6. The mitochondria specialize in energy synthesis within the cell. 
  7. The endoplasmic reticulum is used in the synthesis of ions and proteins, packaging them and storing them. 
  8. The cell nucleus is the control center from which all the various cellular activities are coordinated.

Frequently Asked Questions

Who discoverd the plant cell ?
Answer : Robert Hooke, The cell was first discovered and named by Robert Hooke in 1665. He remarked that it looked strangely similar to cellula or small rooms which monks inhabited, thus deriving the name. However what Hooke actually saw was the dead cell walls of plant cells (cork) as it appeared under the microscope.

2. Which is largest plant cell 
Answer : Xylem, the vascular tissue in plants which conducts water and nutrients, is the largest plant cell. Xylem elements are many millimetre(mm) long and they have a narrow lumen. Ovules of cycas is the largest plant cell in multicellular plants. A unicellular marine algae, acetabularia.

3. What is the function of cell wall in plant cell ?
Answer : The cell wall is a protective layer outside the cell membrane that also provides support for the cell's structure.
  1. The cell wall provides a definate shape and support to the cell.
  2. It acts a gaurdkeeper, which allows the entry and exit of only certain substances. The cell wall, is thus, selectively permeable.
  3. It protects the cell from over-expansion.
  4. The cell wall protects the plant from insects and pathogens.
  5. It prevents loss of water.

4. What are the orgnells present in plant cells and there functions  
Answer : Plant cells carry different organelles depending upon the type of cell considered. The most commonly presented organelles are as follows.

Cell membrane : barrier between cell protoplast and external environment.

Cell wall: A structure responsible plant strength and support.

Cytoplasm : The liquid medium inside cell.

Cytoskeleton : The set of microtubules microfilaments and intermediate filaments which give cell its shape.

Central vacuole : stores water for turgidity.

Nucleus : contains genetic code of cell and
responsibile for its functions.

Ribosome : Protein synthesis.

Golgi apparatus : packing and finishing of cell products. Basically addition of carbohydrates to proteins and lipids to form glycoprotien and glycolipid.

Endoplasmic reticulum : transport channels within cell for cell products.

Mitochondria : cell power house.

Peroxisomes : formation and decomposition of hydrogen peroxide.

Plasmodesmata : channels between cells for transport of molecules.

Chloroplasts : present in leaves for photosynthesis.

Chromoplasts : present in petals or fruit skin for giving colors to the parts.

Leucoplasts : found in underground parts of plant to store food.

Glyoxysomes : present in plant seedling for converting stored fatty acid to carbohydrates.

Is plant cells living or dead ? 
Answer : Plant have both living and dead cells. Simple tissue such as parenchyma and collenchyma are living but sclerenchyma is dead, whereas in complex tissues the xylem tracheids, vessels, and fibres are dead but xylem parenchyma is living while phloem fibres are dead but phloem parenchyma, companion cells, sieve tube elements of phloem are living.

What are the different types of plant cells ?
Answer : There are various types of plant cells which include: parenchyma cells, sclerenchyma cells, collenchyma cells, xylem cells, and phloem cells.

Why plant cell has large vacuoles ?
Answer : Vacuoles also play an important role in plant cell Structure. Plant cell contain large vacuole  To store food and water And To hold turgor pressure across the cell wall.

What is the power house of plant cell ?
Answer : Mitochondria are the powerhouse of the plant cell. These structures are nicknamed the powerhouse of the cell because they work to convert energy into forms that the cell is able to use.

What is the function of plasmodesmata in plant cell ?
Answer : Plasmodesmata transport water minerals and whatnot (which could be said to be large molecules)). The most important function of plasmodesmata is to connect cells together to facilitate water transport. Plasmodesmata are cytoplasmic bridges through the cell wall connecting the cytoplasm of adjacent plant cells. The cell membrane lines the pores and therefore does not separate the adjacent cells.

Which orgnells are only found in plant cell ?
Answer :
  1. All plants cells have cellulose cell wall
  2. All plants cells are separated by cell plate which later differentiates to form plasmodesmata
  3. All plants cells have fluid filled sap vacuole
  4. Many plants cells have plastids

Can Plant cell have Cell Membrane 
Answer : yes, Plants cells do have a cell membrane. At the time off cell division in plant cells, first appears a cell plate in the middle of the cell and then it grows towards outside; which makes the primary cell wall of the cell and then, after complete division of the cell, a thick layer of calcium pectate and some other compounds is deposited inside the cell to make the cell wall harder and then at last, the plasma membrane or cell membrane of the cell develops inside this wall.

What is the role of lysosome in plant Cell ?
Answer : Lysosomes are simple sac filled with fluid containing enzymes which enable the cell to process its nutrients and also are responsible for destroying the cell after it has died. they are the main sites of digestion, that is break down of structures.