How etioplasts transform into chloroplasts
You can observe that the newly germinated seeds are yellow because plastids develop into pale yellow etioplasts in the dark. Etioplasts enclosed by a double membrane are usually ellipsoidal. They are smaller than chloroplasts and about 3 micrometers long. There is no flattened thylakoid in the stroma. Their endomembrane system folded inward to form crystal-like tubular structures called prolamellar body.
Although lacking in chlorophyll, they are rich in chlorophyll precursors, carotenoids and POR (an enzyme). When etioplasts are exposed to light, the prolamellar body begins to disintegrate. Chlorophyll precursors are made into chlorophyll via POR. Vesicles increase in number and merge into flattened thylakoids. This is a very rapid process. Photosystem I is assembled within 15 minutes. Photosystem II starts to work in about 2 hours. Proteins such as electron transport chain and ATP synthase are also assembled within 2 to 3 hours. Most of these proteins are encoded by cell nucleus and enter the stroma via transmembrane proteins in chloroplast. If the plant is placed in a dark environment, chloroplast will degrade to etioplasts.
Functions and morphology of chloroplasts
Etioplasts are considered intermediate in the dim condition. If they are exposed to sunlight, chloroplasts are derived from proplastids directly. This process is similar to the differentiation of etioplasts and has already been discussed in the previous paragraph. Chloroplasts make glucose from light energy, carbon dioxide and water, and release oxygen. Glucose is made into sucrose and transported to other tissues, or it is synthesized into starch that is stored in chloroplasts temporarily. Starch is broken down at night to provide energy and materials for life activities. In addition to carbohydrates, amino acids, and fatty acids are also produced here.
Some algae have one or several large chloroplasts whose shapes are like cups, bands, spirals, etc. The mature chloroplasts in higher plants are small ellipsoids. They are pushed against the cell wall by turgid central vacuole, and covers the cell as a single layer. Their size is about 5-10 micrometers. Each mesophyll cell generally contains 20 to 200 chloroplasts. It is estimated that there are astonishingly 500,000 ones per square millimeter in a leaf. This greatly increases the area for photosynthesis. Chloroplasts change their distribution according to light intensity. Weak light prompts them to distribute at the top of mesophyll cells to receive more sunlight. When the light is too strong, they are located in the side to avoid excessive energy input.
Even in the same plant, chloroplasts differ in different tissues
Although both stems and leaves contain chloroplasts, their size, abundance, and thylakoid vary greatly. Each chloroplast can independently complete photosynthesis in C3 plant mesophyll cells. Palisade tissue in leaf upper surface is the main site of photosynthesis. In each palisade mesophyll cell, chloroplasts have hundreds or thousands of thylakoids that are often stacked into dozens of grana for larger area. The spongy tissue located on leaf lower surface is mainly responsible for gas exchange and transpiration. They can only utilize the residual sunlight filtered by the upper cells, so there are fewer chloroplasts and thylakoids. Although the mesophyll cells in here can also complete photosynthesis independently, their efficiency is lower than palisade tissue. Chloroplasts in other cells are even fewer and their development is inhibited. They can’t complete light reaction, but they are still the center of organic synthesis.
C4 plant cells can only perform certain steps of photosynthesis. Mesophyll cells have highly developed thylakoids for light-dependent reaction, but its stroma is mainly involved in fixing carbon dioxide into malic acid. The degenerated thylakoids in bundle sheath cells are not adept at absorbing light energy, so the main biochemical reaction is the release of carbon dioxide from malic acid and the Calvin cycle (C3 cycle). Division of labor and cooperation is an evolutionary strategy of C4 plants for arid and high-temperature environments. The C4 pathway utilizes water and carbon dioxide more efficiently, and the loss of energy and carbon is decreased in photorespiration, but the cost is that extra energy is required to transport and convert carbon molecules.