BIOLOGY Volume 1 - A Guide to General Biology - 2004

7. AUTOTROPHIC NUTRITION

7.4. Leaf Structure

In flowering plants, the leaf serves as the primary photosynthetic organ. As in all living Organs, Structure and function in the leaf are closely interrelated. From the equation for Photosynthesis

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it can be deduced that, firstly, the leaf requires sources of carbon dioxide and Water; secondly, it must contain chlorophyll and be capable of absorbing sunlight; thirdly, oxygen is released as a byproduct; and finally, CARBOHYDRATES, as useful products, must be transported to other PARTS OF THE plant or stored. The structure of the leaf is exquisitely adapted to meet these requirements. Fig. 7.2 illustrates the External structure of a leaf. Fig. 7.3 provides An Overview of the microscopic structure of a dicotyledonous leaf. Fig. 7.4 shows the Fine Structure of a palisade mesophyll Cell. Fig. 7.5 is a schematic diagram of a transverse section of a dicotyledonous leaf. (Guidelines for drawing objects under a Light Microscope are given in Section 5.13.) The STRUCTURE OF THE epidermis in various leaf types is shown in Fig. 6.3, and the detailed structure and function of Stomata are discussed in Chapter 13.

The STRUCTURE AND Functions of the various Tissues of a dicotyledonous leaf are summarized in Table 7.1.

Fig. 7.2. External structure of a dicotyledonous leaf.

Fig. 7.3. Transverse section of a privet (Ligustrum) leaf through the midrib. Privet is a typical dicotyledonous plant.

Fig. 7.4. Transmission electron micrograph of a palisade mesophyll cell (×3000).

Fig. 7.5. Diagrammatic transverse section of a typical dicotyledonous leaf.

Table 7.1. Structure and functions of a dicotyledonous leaf

7.2. List the Structural Features of a leaf that ensure its successful functioning.

Finally, mention should be made of leaf arrangement (phyllotaxy), which ensures minimal overlap. Such mosaic-like arrangements are particularly noticeable in certain plants, such as ivy.

7.4.1. Chloroplasts

In eukaryotes, photosynthesis takes place within Organelles known as chloroplasts. Their number can range from a single chloroplast (as in the unicellular alga Chlorella) to around a hundred per cell (as in palisade mesophyll cells). Chloroplasts typically measure 3–10 µm in diameter (averaging about 5 µm), making them clearly visible under a light microscope (Figs. 5.2 and 7.3). They are bounded by a double membrane system termed the chloroplast envelope. Chloroplasts invariably contain chlorophyll and other Photosynthetic Pigments embedded within a membrane system. These membranes are immersed in a ground substance known as the stroma. The fine Structure of Chloroplasts can be revealed using an Electron microscope. Low-resolution electron micrographs (Figs. 5.11, 5.13, and 7.4) show the typical appearance of chloroplasts within a mesophyll cell. Figures 7.6 and 7.8 show detailed Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF chloroplasts, while Fig. 7.7 illustrates the diagrammatic structure of a chloroplast and its membrane systems. The light-dependent reactions of photosynthesis occur on these membranes (Section 7.6.2). Here, chlorophyll and other pigments, Enzymes, and electron carriers are located. The system consists of numerous fluid-filled flattened sacs called thylakoids, which are stacked to form grana (singular: granum), interconnected by lamellae (intergranal thylakoids). Each granum resembles a stack of coins, and each lamella acts as a connecting membrane (Fig. 7.8). Under a light microscope, grana are barely discernible as tiny granules.

Fig. 7.6. Transmission electron micrograph of a chloroplast (×15,800).

Fig. 7.7. Structure of a chloroplast. For simplicity, the membrane system has been somewhat reduced in scale (* indicates a prokaryotic-like protein-synthesizing apparatus).

Fig. 7.8. Scanning electron micrograph of a 'naked' chloroplast (with its outer membrane removed), viewed from above. The lamellae and grana are shown in a three-dimensional view. Note that the lamellae are flat, sheet-like structures connecting the grana. The preparation was coated with a metal replica.

The stroma is the site of the Light-Independent Reactions of photosynthesis (section 7.6.3). It has a gel-like consistency and contains soluble enzymes, notably those of The Calvin Cycle, as well as sugars and organic acids. Excess carbohydrates produced during photosynthesis are stored as starch grains. Spherical lipid droplets are frequently associated with the membranes. These droplets increase in size as the membranes break down during senescence, suggesting they accumulate Membrane Lipids.

Protein-synthesizing machinery and endosymbiotic theory

An interesting feature of chloroplasts, apart from photosynthesis, is their protein-synthesizing machinery. During the 1960s, it was demonstrated that both chloroplasts and Mitochondria contain DNA and Ribosomes. This led to the suggestion that chloroplasts and mitochondria might be prokaryotic organisms that invaded Eukaryotic cells early in the evolutionary history of life. Thus, According to the endosymbiotic theory, these organelles represent an extreme form of Symbiosis. Some evidence supporting this theory is summarized in Table 7.2.

Table 7.2. Comparison of prokaryotes, chloroplasts, and mitochondria with eukaryotes

Photosynthetic Bacteria (prokaryotes) do not contain chloroplasts. Their photosynthetic pigments are located on membranes scattered throughout the cytoplasm, effectively making the whole cell function like a single chloroplast of similar dimensions. It is now widely accepted that chloroplasts are descendants of photosynthetic bacteria (section 2.6.1).

Chloroplasts and mitochondria have been shown to synthesize some of their own Proteins independently. However, for this to occur, some of their genes must have migrated to the Cell Nucleus, where they interact with nuclear DNA. This explains why neither chloroplasts nor mitochondria can maintain an independent existence any longer.