Eukaryotic Cells (Part-II)

2. Chloroplast-Chloroplasts are found in plant, algae and other lower invertebrates such as euglena. Contrasting to mitochondria, chloroplast has outer membrane, an inner membrane and then light pigment containing inner most thylakoid membrane (Figure 23.1, A). Outer membrane is porous to the small molecules but protein or large molecule are transported by TOC (translocon on the outer chloroplast membrane) complex. Movement of material passed through outer membrane enters into the inner membrane through TIC (translocon on the inner chloroplast membrane) complex. In between outer and inner membrane is intermembrane space filled with aqueous liquid. 

Figure 23.1: (A) Structure of Chloroplast, (B) Arrangement of thylakoid membrane in chloroplast

The inner membrane of the chloroplast further folds into flattend membrane system known as thylakoids. The photosynthsis machinery such as light absorbing pigments, electron carriers and ATP synthesizing machinery is present on inner membrane as intergral protein complex. Thylakoid membranes are arranged like stack of coin to form granum (Figure 23.1, B). The granum through out the chloroplast are connected by tubule to share the material. Over-all structure of chloroplast is similar to mitochondria but it has few significant structural and biochemical differences. Thylakoid membrane contains photosynthetic green colored pigment chlorophyll.  

 

Photosynthesis is a assimilation reaction involving CO₂ and water to produce sugar in the presence of solar energy (photons) to catalyze fusion reaction as given Eq. 23.1 and Figure 23.2. The photo system present on thylakoid membrane consists of two photo system, photo system-I (PS-I) and photo system complex II (PS-II). PS-II absorbs the photon from solar energy to excite the electron to the higher energy state, and catalyze water break down into the proton and oxygen.