Chloroplasts are green plastids. These are the special protoplasmic organelles present only in the green cells of plants. Chloroplasts act as photosynthetic apparatus. The entire process of photosynthesis is completed in each chloroplast. Hence these are the site of photosynthetic reactions. Leaves are specialized photosynthetic organs and hence, they contain the maximum number of chloroplasts in their mesophyll cells. Moreover the morphology and the anatomy of leaves are most helpful during photosynthesis for :

(i) getting maximum absorption of sunlight,

(ii) providing steady exchange of water to green cells, and

The chloroplasts in higher plants are microscopic and mostly oval, spherical or discoid. Each chloroplast is bounded by two smooth and selectively permeable cytoplasmic membranes with an inter-membrane space. These are composed of
lipo-protein subunits.

The internal space of the chloroplast is filled with a colorless hydrophilic matrix called the stroma. A number of grana are suspended in the stroma. Each granum is a stack (compact bundle) of thylakoids. These are membrane-bound, flattened, disc-shaped vesicles. The thylakoid membranes are called grana lamellae. All grana are connected to one another by stroma lamellae (i.e. inter-grana lamellae or frets). The internal space of each thylakoid is called a fret channel.

The thylakoid lamellae are composed of alternating layers of lipids and aqueous proteins. There is a layer of chlorophyll and carotenoid molecules situated between the protein and lipid layers. The chlorophyll molecules are arranged in such a way that their hydrophilic heads extend into the aqueous protein layer while the lipophilic tails are embedded in the lipid layer. The pigments are organized into numerous photosynthetic units called quantasomes. Each quantasome contains about 230 to 300 chlorophyll molecules. Quantasomes are capable of trapping light energy and converting it into chemical energy (ATP) during the photochemical reactions (light reaction) of photosynthesis. The grana also contain various co-enzymes and electron acceptors necessary for the process. Hence, grana are the site of the light reactions (phase-I) in photosynthesis.

The stroma contains various enzymes required for the dark reaction( i.e., the biochemical reactions involving the reduction of carbon dioxide to form carbohydrates). Hence, stroma is the site of the dark reactions (phase-II) of photosynthesis. The "dark reactions" do not directly require light, but they do require energy produced from light during the light reactions.

The grana thylakoids and the stroma lamellae together form an intricate internal membrane system in the chloroplast. This system is derived from the inner limiting membrane during the development of chloroplast.

Pigments in photosynthesis : The most common photosynthetc pigments present in higher plants and green algae are:

(ii) Chlorophyll-b (yellow-green) = C₅₅H₇₀O₆N₄Mg

(iii) Carotenoids - Carotenes (orange-red) = C₄₀ H₅₆

(iv) Xanthophylls (yellow) = C₄₀H₅₆O₂

For photosynthesis, these pigments can absorb and use light belonging to the visible spectrum only.

Both chlorophyll-a and chlorophyll-b show maximum light absorption in the blue-violet and in the red regions of the visible range of wavelengths of light.

Carotenoids absorb light in the blue and blue-green regions. They also protect the chlorophyll from undergoing photo-oxidation when exposed to very high intensity light.

Chlorophyll-a is the essential pigment in photosynthesis, because only chlorophyll-a can utilize the absorbed light energy for the synthesis of chemical energy ATP. Other pigments act as accessory pigments. They collect the light energy and transfer it to chlorophyll-a for photosynthesis.

Thylakoids in prokaryotes : In prokaryotes like cyanobacteria, purple bacteria, etc., thylakoids are present but they lie naked in the cytoplasm. Chloroplasts are absent. In prokaryotes, pigments are distributed uniformly on or in the lamellae.

Chapter 5