Chloroplast in Plant Cell

introduction image

Chloroplast in a plant cell is a membrane-bound cell organelle, contain chlorophyll pigments that are absent in the animal cell. The study of chloroplast was first carried by a scientist named Hugo Von Mohl in the year 1837. After his study, it was found that the chloroplasts were the type of plastids and hence it was named “Chloroplastids” by a scientist named Schimper in the year 1883. Then in the year 1884, a scientist named Eduard Strasburger termed the name “Chloroplast”.

After further study, it was found that the chloroplast has evolved from the cyanobacteria through a process of endosymbiosis. The morphology of chloroplast had shown considerable resemblance with the cyanobacteria and thus considered as the ancestors of cyanobacteria. This evolution of the chloroplast from the endosymbiosis of cyanobacteria was introduced in the year 1905, by a scientist named Konstantin Mereschkowski.

Content: Chloroplast in Plant Cell

    1. Definition
    2. Characteristics
    3. Structure
    4. Location
    5. Functions

Definition of Chloroplast in Plant Cell

Chloroplast in a plant cell can define as a type of plastid which exists as a double membrane-bound cell organelle that produces energy through photosynthesis. These shows “Dimorphism” as it can exist in two different forms, namely granal and agranal. Granal kind of chloroplast comprises of all the elements like grana, stroma and stroma lamellae in the internal membrane. Agranal kind of chloroplast lacks grana and only contains stroma and stroma lamellae in the inner layer.

Characteristics of Chloroplast

We can know the characteristic features of a chloroplast by knowing its size, shape and number.

Size of Chloroplast

Size of the chloroplast is variable, i.e. differs from one plant to other. The length of the chloroplast ranges 5-10 micrometres, and the diameter ranges between 2-4 micrometres.

Shape of Chloroplast

The shape of chloroplast also differs between the different photosynthetic organisms.

variable shapes of chloroplast

Cup-shaped chloroplast: Chlamydomonas comprises of a cup or horseshoe-shaped chloroplast.
Girdle shaped: Ulothrix contains a chloroplast in the middle of the vegetative cell.
Ribbon shaped: Spirogyra contains a filamentous, band-like chloroplast.
Discoidal shaped: Higher plants having oval or biconvex chloroplast, and this is the most common type.

Number of Chloroplasts

The number of chloroplasts can vary between different photosynthetic organisms.

Chlamydomonas and Ulothrix, there is only one chloroplast present per cell.
Spirogyra contains several chloroplast ranges 1-16 per cell.
The number of chloroplast increases in higher plants, containing 20-40 chloroplasts per cell.
Chara contains 500 chloroplasts per cell.

Structure of Chloroplast

A chloroplast is a kind of plastid which is double membrane-bound organelle and consists of the following structural elements:

labelled diagram of chloroplast

Outer membrane: The outer layer is smooth, contains less protein and more permeable to the passage of molecules or ions.

Inner membrane: It is also smooth but contains more protein and selectively permeable membrane, i.e. allows specific molecules to enter inside the cell.

Intermembrane space: It is the space in the middle of an outer and inner chloroplast membrane, having a thickness of 10-20nm.

Stroma: It is the colourless, alkaline, aqueous liquid that encloses the internal membrane structures like thylakoid and lamellae. The region of the stroma is rich in enzymes and contains coiled circular DNA and dispersed starch granules. Likewise mitochondrial cell, chloroplast also functions as “Semi-autonomous” by having its own circular DNA (cp-DNA) and ribosomes of 70-S type (Plastidozomes).

Specialized part

A chloroplast consists of a specialized part thylakoid, where the chlorophyll pigments are embedded.

Thylakoid: It is the specialized sac-like structures, which are enclosed by the aqueous stroma. When the thylakoids arrange one over the other, like a pile of coins, then it refers as “Grana”. The stack-like grana attach via stroma lamellae. Thylakoid is surrounded by the thylakoid membrane within which a unit of “Quantosome” is present.

All the 230 pigments like chlorophyll, carotenoids, xanthophyll and phycobilins in the thylakoid space collectively known as Quantosome. In the middle of the thylakoid membrane, a fluid is filled in a space refers to as “Thylakoid lumen or space”. The pigments in the thylakoid membrane absorb light of a particular wavelength from the sun and pass it to the light-harvesting complexes (PS-I and PS-II).

Thylakoid comprises of two parts, namely appressed and non-appressed part.

thylakoid

  • The region of thylakoid membrane which is in contact with the aqueous stroma refers as “Non-appressed part”. It contains a photosystem-I.
  • The region of thylakoid membrane which is in contact with the other thylakoid membrane refers as “Appressed part”. It contains a photosystem-II.

Stroma lamellae: It is the tubular structure that invaginates from the inner chloroplast membrane. Lamellae act as a skeleton of chloroplast by maintaining a proper distance between thylakoids or protects from overlapping.

Location of chloroplast in plant cell

Chloroplasts are the essential cell organelles in a plant cell that are located within the mesophyll cells of the leaf. Mesophyll cells consist of:

location of chloroplast

  • Palisade parenchyma
  • Spongy parenchyma

Palisade parenchyma is the upper layer that contains a high number of chloroplasts within the broad and elongated chlorenchymatous cells.

Spongy parenchyma is the lower layer that contains less number of chloroplasts within small, ovoid cells and also having prominent intercellular spaces.

Functions of Chloroplast in plant cell

A chloroplast performs three significant tasks like:

function of chloroplast in plant cell

Role in photosynthesis: Chloroplast acts as a location of photosynthetic activity, where the chlorophyll pigments inside it trap light energy and transform it into chemical energy. The light reaction of photosynthesis, i.e. conversion of H2O into O2, will occur in the thylakoids by the release of ATP and NADPH. Dark reaction or the fixation of CO2 into sugar will take place in the stroma by the release of ADP and NADP.

Energy metabolism:  The oxidation of water molecule releases ATP, and NADPH energy molecules produce that harnesses to convert the CO2 into three-carbon sugar. Glyceraldehyde 3-phosphate is a three-carbon sugar which helps to build a variety of carbohydrates and other organic molecules. By the reduction of CO2 into sugar, hydrolysis of ATP and NADPH molecule occurs that will produce ADP and NADP. Thus, chloroplast drives some of the energy for the breakdown of carbohydrates and other processes.

Food storage: In chloroplast, starch granules act as food storage molecules that disperse throughout the aqueous stroma.

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