Photosynthesis

PHOTOSYNTHESIS

Chloroplasts are the sites of photosynthesis in plants

All green parts of a plant, including green stems and unripened fruit, have chloroplasts, but the leaves are the major sites of photosynthesis in most plants (FIGURE 10.2).. There are about half a million chloroplasts per square millimeter of leaf surface. The color of the leaf is from chlorophyll, the green pigment located within the chloroplasts. It is the light energy absorbed by chlorophyll that drives the synthesis of food molecules in the chloroplast. Chloroplasts are found mainly in the cells of the meso- function phyll, the tissue in the interior of the leaf Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic as pores called stomata (singular, stoma; Gr. "mouth"). Water absorbed by the roots is delivered to the leaves in veins. Leaves also use veins to export sugar to roots and other nonphotosynthetic parts of the plant. A typical mesophyll cell has about 30 to 40 chloroplasts, each a lens-shaped organelle measuring a 2-4 ~tm by 4-7 gin. An envelope of two membranes closes the stroma, the dense fluid within the chlorop. An elaborate system of interconnected thylakoid branes segregates the stroma from another compment, the thylakoid space. In some places, thylakoid are layered in dense stacks called grana. Chlorop resides in the thylakoid membranes. (Photosynth prokaryotes lack chloroplasts, but, as you will see Chapter 25, they do have membranes that manner similar to the thylakoid membranes of chloroplasts.) Now that we have identified chloroplasts sites of photosynthesis in plants, we are ready to see these organelles convert the light energy absorbed chlorophyll to chemical energy.

Unit Two

The Cell