Chloroplasts are usually large organelles (1 to 10 µm) found in plant cells. A cell from a leaf may contain from 20 to 100 chloroplasts (Figure 1). The shape of chloroplasts is variable, from round to ellipsoid, or much more complex. Chloroplasts are members of the plastids family and contain DNA with around 250 genes derived from the prokaryotic bacterial ancestor. These genes code for ribosomal RNA, transference RNA and messenger RNA. Messenger RNAs are translated in the chloroplast and give proteins needed for organelle growth and division, and for photosynthesis. Chloroplasts produce chlorophyll for catching the light energy.
1. Morphology
Chloroplasts are made up of several compartments (Figure 2). In the periphery, there are an outer membrane, an inner membrane and an intermembrane space between them. Unlike mitochondria, the inner membrane has no folding. Inside chloroplasts there are membrane sacs known as thylakoids, which are usually arranged in stacks referred as granum. Thylakoids of different grana (plural of granum) may have continuous membranes. Proteins in charge of photosynthesis are located in the thylakoid membranes. The space between thylakoids and inner membrane is known as stroma, where DNA, enzymes, ribosomes, and other types of molecules are found.
2. Division and movement
Chloroplasts need to divide in proliferating cells for getting a right number of chloroplasts and perform a proper photosynthetic activity. Chloroplast division may be synchronized with cell division. This happens in some alga species with cells having just one chloroplast. Chloroplast division usually happens during S phase of the cell cycle, where replication of the DNA takes place. The process is less known in plant species with cells containing many chloroplasts. In some cells, the number of chloroplasts is not related to cell division. For example, in parenchyma cells of leaves, chloroplasts divide to increase their number although the cell is not going to divide anymore. Curiously, the number of chloroplasts is related to the surface of the leaf. In leaf parenchyma cells, it is though that the number of chloroplasts is regulated by the size of the cell. In proliferating cells, the chloroplast population of the mother cell should be split in two more or less equal populations for the two daughter cells. Chloroplasts distribution during mitosis is mediated by actin filaments. The division of chloroplast to increase their number in a cell depends on some proteins synthesized by the nucleus and other synthesized by the chloroplast itself (all proteins involved in mitochondria division are coming from nuclear genes). The division starts with two protein rings, one inner ring composed of chloroplast proteins and an outer ring containing dynamin related proteins coded by nuclear genes. Both rings are connected through transmembrane proteins. These two rings drive the chloroplast division.
Changing the position of chloroplasts in the cytoplasm is a cell strategy to get adapted to variable light conditions (Figure 3). The movement of chloroplasts is slow, around 1 µm/min. A high intensity of light may be harmful for chloroplasts and a faint light decreases the photosynthesis activity. Cells of the parenchyma of leaves can move the chloroplasts from the periclinal walls (parallel to the leave surface) to the anticlinal walls (perpendicular to the leave surface). Light intensity is detected by receptors located in the outer chloroplast membrane, but there are plasma membrane photoreceptors as well. There are two types of movements. Under low light intensity, chloroplasts are moved to the periclinal walls, and under high light intensity they are moved to the anticlinal walls (Figure 3). Photoreceptors in plasma membrane triggers the first movement, and those located in the chloroplast are responsible for the second movement.
The motor drive that move chloroplasts is provided by actin filaments and myosin motor proteins, which are found around the chloroplast in a more or less complex organization. Actin filaments may also be involved in anchoring chloroplasts to the plasma membrane.
3. Functions
Photosynthesis
Photosynthesis is the main role of chloroplast. It transforms the electromagnetic energy of light into chemical bonds thanks to chlorophyll, ATP synthase (Figure 4), and ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO). Photosynthesis is divided in two stages: a light-dependent part, where the energy of light is transformed in a gradient of protons used for producing ATP and NADPH, and a light-independent part (it does not need light but the products generated during the light-dependent stage) where carbon from CO2 is fixed on phosphorylated carbohydrates by RUBISCO. The first stage of the photosynthesis takes place in the thylakoidal membranes and the second stage in the stroma.
Photosynthesis consists of several steps. a) Photosystem II splits two molecules of water that yield one O2 molecule and four protons. This reaction releases four electrons that are moved to the chlorophyll molecules of the photosystem II. There, the energy of light raises the energy of these electrons and so they are released from the photosystem II. b) The electrons are caught by a plastoquinone and quickly donated to cytochrome b6/f complex that uses the energy of electrons to enter four protons into the thylakoid. c) Cytochrome b6/f complex gives the electrons to plastocyanin that transfers them to the photosystem I. Here, again, thanks to the energy of light, chlorophyll raises the energy of electrons. Ferredoxin-NADP reductase is associated with the photosystem I and transforms molecules of NADP+ into NADPH, which remains in the stroma. The protons which are removed from the stroma and those that are produced or entered into the thylakoid create altogether a proton gradient across the thylakoidal membrane. This gradient is used by the ATP synthase to produce ATP. Since the catalytic center of the ATP synthase is facing the stroma, the new ATP molecules are synthesized into the stroma. Both NADPH and ATP are eventually used in the Calvin cycle, which is a metabolic chain of reactions where CO2, ribulose-1,5-bisphosphate, and RUBISCO make possible the fixation of carbon into phosphoglycerate.
Another functions
Besides photosynthesis, chloroplasts carry out many other salient functions, such as the synthesis of amino acids, nucleotides and fatty acids, the production of hormones, vitamins and other secondary metabolites. They are also involved in the metabolism of nitrogen and sulfur. Nitrate is the main source of nitrogen for plants. The last step to get assimilated nitrogen from nitrate is transforming nitrite into ammonium, which happens in chloroplasts by the nitrite reductase. Nitrite is formed from nitrate. Nitrite and ammonium may be toxic for the cell above a concentration, but not nitrate, which can be transiently stored in vacuoles. Some metabolites produced by chloroplasts are involved in protecting against pathogens and in the adaptation of plants to stress, excess of water, and extreme heat. By releasing hormones, chloroplasts may also influence cell found far away in the body plant.
Chloroplasts are in permanent communication with other compartments of the cell, either by sending molecular signals or through direct contact between their membranes. It is relatively frequent to observe physical contacts between the chloroplast membrane and the membranes of the endoplasmic reticulum and mitochondria. However, the most intense communication is with the nucleus, because it contains many genes for proteins that must work inside the chloroplast, some of them needed for photosynthesis. In this regard, nucleus and chloroplast must be well coordinated because they have to work together.
Bibliography
Wada M, Kong S-G. 2018. Actin-mediated movement of chloroplasts. Journal of cell science. 131. doi: 10.1242/jcs.210310.
Plastids