1 - A leaf cell consists of several metabolic compartments

Abstract

In higher plants, photosynthesis occurs mainly in the mesophyll, the chloroplast-rich tissue of leaves. The cellular contents are surrounded by a plasma membrane called the plasmalemma and are enclosed by a cell wall. The cell contains organelles, each with its own characteristic shape, which divide the cell into various compartments. Each compartment has specialized metabolic functions, which will be discussed in detail in the following chapters. The largest organelle, the vacuole, usually fills about 80% of the total cell volume. Chloroplasts represent the next largest compartment, and the rest of the cell volume is filled with mitochondria, peroxisomes, the nucleus, the endoplasmic reticulum, the Golgi bodies, and, outside these organelles, the cell plasma, called cytosol. In addition, there are oil bodies derived from the endoplasmic reticulum. These oil bodies, which occur in seeds and some other tissues, are storage organelles for triglycerides. The nucleus is surrounded by the nuclear envelope, which consists of the two membranes of the endoplasmic reticulum. The space between the two membranes is known as the perinuclear space. Nuclear pores with a diameter of about 50 nm interrupt the nuclear envelope. The nucleus contains chromatin, consisting of DNA double strands that are stabilized by being bound to basic proteins (histones). The genes of the nucleus are collectively referred to as the nuclear genome. Within the nucleus, usually off-center, lies the nucleolus, where ribosomal subunits are formed. These ribosomal subunits and the messenger RNA formed by transcription of the DNA in the nucleus migrate through the nuclear pores to the ribosomes in the cytosol, the site of protein biosynthesis. The synthesized proteins are distributed among the different cell compartments according to their final destination.