Chloroplasts: Ultrastructure and Functions | Cytoplasm

In this article we will discuss about the ultrastructure and functions of chloroplasts in the cytoplasm.

Ultrastructure of Chloroplast:

Chloroplasts of higher plants have disc-shaped or oval structure, 10 µm in length and 2-4 µm in diameter. Algal species contain larger chloroplasts of different shapes, e.g., spiral, cup-shaped, circular bands. The number of chloroplasts per cell is one to two in algae, while in higher plants, several chloroplasts occur in each cell.

Chloroplasts are enclosed by a double membrane envelope (Fig. 2.16). The space between the outer and inner membranes is called inter-membrane space. Light microscope shows the chloroplast to be composed of two parts: grana (small granules) and amorphous stroma in which grana is suspended. A single chloroplast may contain 50 or more grana, each granum measuring 0.5-2 µm diameter.

Grana:

Electron microscopy has revealed that in grana, flattened membranous sacs called thylakoids (thylakos = pouch) are closely stacked together (Fig. 2.16) Within each thylakoid, there is space known as thylakoid space. Membranes called stromalemellae (un-stacked thylakoids) join the thylakoids of grana.

Spherical particles (8-11 nm diameter) emerge from the surface of thylakoid membranes into stroma space (Fig. 2.16, 2.17); these particles are similar to the spherical particles found on the mitochondrial cristae. These particles contain CF₁ (a protein complex) which plays an important role in ATP formation.

CF₁ is attached to the thylakoid membrane by a complex of hydrophobic proteins called CF₀. The complex CF₀ functions as a trans-membrane channel through which protons (H⁺) pass to CF₁ ATP synthesis takes place.

Each un-stacked thylakoid is a large flattened sheet which connects many or all of the individual thylakoids of the granum. The thylakoid spaces are interconnected. Stacked thylakoids are found in all higher plants and green algae. But lower algae, e.g., red algae, possess parallel thylakoids which are completely separated from each other.

In red and blue-green algae, pigment containing granules called phycobilisomes are attached to the thylakoid membranes. Different types of chlorophyll pigments are found in the thylakoids. A distribution of these pigments in different prokaryotes and eukaryotes is given in Table 2.5. Freeze-fracturing electron microscopy shows the presence of 10-18 nm particles in the stacked membranes of grana (Fig. 2.17).

The larger particles correspond to the photosystem II (PS II), while the smaller ones correspond to different protein complexes, e.g., photosystem I (PS I), cytochrome b-f complex and CF₀. The light reaction of the photosynthetic process occurs in grana. PS I and ‘CF₁-CF₀‘are more frequent in un-stacked (stroma) thylakoids.

Stroma:

Under the electron microscope, the stroma appears to be composed of fine granules. Starch grains and lipid deposits called plastoglobuli are present in the stroma; they serve as lipid reservoirs for the formation of thylakoids membranes. In higher plants, large granules called stroma-centres are found in the stroma; they are composed of fibrils of 8-9 nm diameter which are tightly packed.

The stroma-centres may contain “ribulose-bi-phosphate carboxylase” (RuBP carboxylase) enzyme, which is involved in the carboxylation of RuBP (addition of CO₂ to RuBP) during the dark reaction. All the enzymes required for dark reaction are present in the stroma. Besides the above substances, DNA, RNA, ribosomes and the factors involved in nucleic acid and protein syntheses are also present in the stroma.

Chloroplasts develop from “pro-plastids” which are small double membrane vesicles of about 1 pm in diameter without any internal organization. New plastids are produced by fusion of pre-existing pro-plastids. Fusion of pre-existing chloroplasts also produces new chloroplasts.

Pro-plastids give rise through differentiation, to different types of plastids, e.g., leucoplast, chromoplast, chloroplast etc. Light is necessary for the development of chloroplast from pro-plastids (Fig. 2.18).

Function of Chloroplast:

Chloroplasts produce carbohydrates through photosynthesis, the overall reaction being as:

The process of photosynthesis occurs in two main steps called (i) light reaction and (ii) dark reaction (Fig. 2.19).

(i) Light Reaction:

Hundreds of chlorophyll molecules are organized to make a photosynthetic unit; these molecules are called antennae. Light absorbed by a chlorophyll molecule causes excitation. The excitation is passed by resonance transfer from chlorophyll-to-chlorophyll and reaches the reaction centre which is a “chlorophyll a-protein-complex”.

The reaction centres are of two types:

(1) Photosystem I (PS I):

This system is more efficient at the longer wavelengths (700 nm) of light and is called P₇₀₀

(2) Photosystem II (PS II):

This system is more active at relatively shorter wavelengths (680 nm) of light and is called P₆₈₀.

The antenna pigments of the photosystem II contain higher concentration of chlorophyll b as compared to that of photosystem I. The excitation energy reaches the PSII and as a result, an electron is ejected from it (P₆₈₀). The excited electron from P₆₈₀ passes to “O” which is a protein- bound plastoquinone molecule (Fig. 2.19).

Due to this loss of electron, PSII becomes positively charged; as a result, it attracts electrons from water; a protein containing manganese (Mn) acts as the carrier of electron between H₂O and PSII. The water molecule splits into oxygen and free protons (H⁺).

From “Q”, the electron is transferred to “B” (complex of freely mobile plastoquinone molecule). Then the electron passes through a series of carriers (cytochrome b-f –> plastocyanin or PC) and reaches the PSI. The PSI absorbs light energy (through the passage of resonance transfer between adjacent molecules), and the excitation causes the ejection of an electron from P₇₀₀; the excited electron is transferred to iron-sulphur proteins from where it is transferred to ferredoxin. The P700 regains electrons coming from plaslocyanin or PC (i.e. from PSII). Finally, it is passed on to NADP ⁺ which is reduced to NADPH, the reaction being catalysed by NADP- reductase.

Some of the energy released during the electron flow is utilized for the synthesis of ATP; this process is called photophosphorylation.

Photophosphorylation is of two types:

(1) Non- cyclic and

(2) Cyclic.

In non-cyclic photophosphorylation, O₂ is produced and NADP⁺ is reduced to NADPH, the flow of electrons being in one direction.

In the cyclic photophosphorylation, however, the electrons are activated by light in the PSI but they do not pass to NADP⁺; they return to PSI. As the electrons return to a lower energy level, ATP is formed. Thus is cyclic photophosphorylation, neither O₂ is produced nor NADP⁺ is reduced. ATP formation also occurs in the spherical bodies (CF₁-CF₀ complex) when protons pass through CF₀.

(ii) Dark Reaction:

Dark reaction occurs within the stroma where energy stored in forms of NADPH and ATP (generated through light reaction) is utilized in fixation and reduction of CO₂; this process is called the Calvin cycle (Fig. 2.19B). The CO₂ accepter in Calvin cycle is a 5-carbon compound known as ribulose-bi-phosphate (RuBP). After joining with CO₂, RuBP is converted into two molecules of 3-phosphoglycerate (a 3-carbon compound).

Thus the initial product of CO₂ fixation is 3-phosphoglycerate which is then converted into 3-phospho-glyceraldehyde. Glucose is ultimately produced from 3-phosphoglyceraldehyde through a series of reactions. This pathway of CO₂ fixation is called the C3 Pathway, since the first product is a 3-carbon compound.

In contrast, some plants contain a C4 pathway in which CO₂ is fixed in the mesophyll cells and then transferred to the bundle sheath where it enters the Calvin cycle.

In the C4 system, the CO₂ is accepted by a 3-carbon compound, phosphoenolpyruvic acid (PEP), to yield a 4-carbon compound, “oxaloacetate” which is then converted into malate. Malate is broken-down to release CO₂ and pyruvate; this CO₂ is utilized in the Calvin cycle. (Fig. 2.20).