Structure of the Plant Cell

The cell is a structural and functional unit of an organism. A plant cell is composed of a spherical or oval body called nucleus in a tiny mass of protoplasm, surrounded by a cell wall. The protoplast of the cell comprises living parts such as protoplasm, nucleus, and other living bodies. Hence, plant cells consist of a living protoplast and a non-living framework called the cell wall.

The Protoplast

It consists of protoplasmic bodies like 1) Cytoplasm, 2) Nucleus, 3) plastids.

1. Cytoplasm: the mass of the cell except nucleus and plastid is called cytoplasm or protoplasm of the cell. Endoplasm comprises of the inner granular mass of cytoplasm and a fluidic portion of the cytoplasm is called hyaloplasm. Vacuoles, cavities filled with fluid are also present in the cytoplasm. Vacuoles are the storage house of soluble food, anthocyanins, various salts, and water.
2. Nucleus: Nucleus is a denser, oval or spherical body in protoplasm is called a nucleus. The nucleus is enclosed by a thin transparent membrane called nuclear membrane. The nucleus is composed of nucleoplasm a dense mass of protoplasm. Function: Chromosomes or thin chromatin threads containing DNA are suspended in the nucleoplasm. DNA replication occurs during cell division and reproduction.
3. Plastids: plastids are protoplasmic bodies besides nucleus and cytoplasm. Plastid consist of the ground matrix known as stroma and stroma is filled with large numbers of granules call grana. Each grana or granum consists disc in varying numbers.

There are three types of plastids namely

• Leucoplast (colorless): They are found in roots and underground stems. Function: Storage of starch
• Chloroplasts (green color): The green color of chloroplasts is due to a pigment called chlorophyll. It is generally present in leaves, stems, or green parts of plants. Function: Chlorophyll absorbs carbon dioxide from the air, and the water from the soil and manufacture sugar and starch and liberates oxygen in the air.
• Chromoplasts (colored plastids): They are of various colors like orange, yellow (xanthophyll), and red or orange-red (carotene) generally present in fruits and flowers. Function: Colored flowers attract insects for cross-pollination.   

4. Mitochondria: It comprises of the outer membrane, inner membrane, makes folds called cristae forming the cavities filled in a granular matrix. Mitochondria are composed of proteins and phospholipids. Function: The main function of mitochondria is to make ATP, to provide energy. ATP is formed by a respiratory cycle called the Krebs cycle.  Hence mitochondria are called a powerhouse of the cell.

5. Golgi bodies: they are elongated and flattened sacs in the cytoplasm of some types of cells. Golgi complex comprises of, Golgi bodies consist of a stack of flat sacs, with two membranes, and vacuoles are embedded between two membranes. Tiny vesicles form a cluster and are resent at the end of the sacs. Functions: To store proteins and secret enzymes and hormones.
6. Endoplasmic reticulum (ER): They are tube-like structures present throughout the cytoplasm. Some of these tubules touch the nucleus and extend through the cytoplasm and open at the cell membrane. Functions: protein synthesis, the formation of enzymes, and for storage and transport of metabolic products.

7. Ribosomes: They are minute particles attached to a membrane of the ER. ER with ribosomes is called rough ER, whereas ER without ribosomes is called a smooth ER. ER consist of nucleoprotein i.e. RNA and protein. Function: Ribosomes act as a seat for protein synthesis.  

8. Lysosomes: They are tiny, spherical particles with outer membrane present in the cytoplasm. They are mainly found in animal cells and few meristematic cells of the plant. Lysozymes are the enzymes present in lysosomes (e.g. hydrolases). Function: Their main function is intracellular digestion.

9. Cell wall: The outer rigid and protective covering outside the plasma membrane is called a cell wall. The plant cell wall is made of cellulose, hemicellulose, lignin, cutin, pectin, suberin, and hemicellulose.

The cell wall is made up of three parts

• Middle lamella: It is a first thin layer composed of calcium, magnesium pectates, and some proteins between two adjacent cells.
• Primary layer: It is a thin and elastic layer inner to the middle lamella layer. It consists of cellulose micro-fibrils, embedded in a matrix of hemicellulose, water, pectin, and glycoprotein.
• Secondary layer: It is a layer inner to the primary layer. It is composed of microfibrils embedded in the matrix of hemicellulose, suberin, pectin, minerals, lignin, waxes, tannins, calcium carbonate, and resins.
• Plasmodesmata: It is a fine strand, carried by narrow pores of cytoplasm, by which the cell contents are interlinked.
• Functions: To protect the protoplasm, gives rigidity, the shape of the cell, for transport (xylem), and prevent cell burst from endosmosis

10. Plasma membrane: It is a thin elastic membrane covering the cell. The outer and inner layer is made up of proteins, whereas the middle layer phospholipid bilayer. According to the fluid mosaic model of the cell membrane, the lipid bilayer is made up of a double layer of phospholipid molecules. It is amphipathic in nature, with polar heads (hydrophilic) and non-polar tails (lipophilic). This arrangement only allows lipid-soluble substances to cross the membrane. Proteins attached on the surface of the membrane are called extrinsic proteins, whereas the proteins penetrated in the lipid bilayer are called intrinsic proteins. These intrinsic proteins are tightly held by strong bonds.  The plasma membrane also contains glycoproteins, proteins, steroids, cholesterol, and glycolipids.

• Functions: Transport metabolic wastes out of the cell and nutrients into the cell.  It also acts as a barrier for unwanted material to enter the cell. Substances are transported across the membrane by Active (carrier-mediated) or Passive (diffusion or osmosis) transport mechanism.

Cell inclusions

There are several products of metabolism and by-products appear in plant cells, called cell inclusions.

Carbohydrates: sugars (glucose, fructose, maltose), starch, dextrin, inulin, and glycogen.

Nitrogenous materials: Proteins and amino compounds

Fats and oils: fixed oils and aromatic oils e.g. coconut oil

Other cell products: tannins, essential oils, Resins, Gums, Mineral crystals ( silica, calcium carbonate, calcium oxalate), latex, Organic acids, Alkaloids

Multiple-choice questions

1. Protoplasm of the cell comprises of _________

a) Cell wall

b) Nucleus

c) Cell membrane

d) Plasmodesmata

2. _________is called powerhouse of the cell

a) Mitochondria

b) Endoplasmic reticulum

c) Nucleus

d) Golgi complex

3. _______ acts as a seat for protein synthesis

a) Nucleus

b) DNA

c) Ribosomes

d) Rough Endoplasmic reticulum

4. ATP is synthesized in mitochondria by _______

a) Glycolysis

b) Gluconeogenesis

c) Krebs cycle

d) Glycogenolysis

5. Chloroplasts containing chlorophyll absorbs carbon dioxide from air and release ________

a) Oxygen

b) Nitrogen

c) Hydrogen

d) Neon

6. __________ gives colour to flower and fruits

a) Endoplasmic reticulum

b) Plastids

c) Golgi bodies

d) Cell wall

7. The fine strands carried by narrow pores of cytoplasm are called_________

a) Plasmodeta

b) Plasmoedeta

c) Plasmodesmata

d) Plasmodemata

8. Plasma membrane is responsible for __________type of transport mechanism

a) Carrier mediated

b) Diffusion

c) Osmosis

d) All

 9. ___________is responsible for Lipophilic molecules to cross across plasma membrane

a) Extrinsic proteins

b) Intrinsic proteins

c) Polar heads

d) Non-polar tails

 10. RNA is present in _________

a) Smooth endoplasmic reticulum

b) Rough endoplasmic reticulum

c) Ribosomes

d) lysosomes

Answer key.

1. B
2. A
3. C
4. C
5. A
6. B
7. C
8. D
9. D
10. C

Reference:

A. C. Dutta. Botany for degree students, 33rd e.d. New Delhi: Oxford University Press, 2014, Page: 117-143.

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