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Amyloplasts are organelles present in plant cells in which starch is synthesized and stored. In addition to being part of the energy storage system of plants, these organelles also fulfill essential functions for plant development and growth, by allowing the plant to be able to distinguish top from bottom and thus know where its roots should grow and which way to go. where their stems and leaves.
Amyloplasts are a particular type of leucoplast. These, in turn, are a class of plastid commonly found in tissues not exposed to sunlight, and which are characterized by not having any pigment. For this reason, they do not present any color when observed through the microscope.
Amyloplasts are very abundant in different types of plants and in different parts of plant tissue. For example, they are found in large quantities in potatoes and other tubers, and also in many fruits.
plastids
As mentioned a moment ago, amyloplasts are a type of plastid. Plastids are a group of organelles that are surrounded by a double membrane that separates their interior from the cell’s cytoplasm. There are several different types of plastids that have different functions, but they all share some basic characteristics:
- Plastids are organelles found in the cytoplasm of plant cells.
- All plastids come from a type of immature cell called a proplastid.
- All plastids have an outer membrane and one or more internal compartments, which are in turn surrounded by a second membrane. Both are phospholipid membranes similar to the cell membrane.
- Plastids have their own DNA and divide by binary fission independently of the cell of which they are a part.
types of plastids
Upon maturing, proplastids can become one of four different types of distinct plastids which are:
chloroplasts
They are green plastids in which glucose biosynthesis from carbon dioxide and water is carried out, through photosynthesis. These organelles are found primarily in plant leaves and contain the green pigment chlorophyll , which absorbs sunlight to provide the energy required for photosynthesis.
chromoplasts
They are called like this because they are organelles that have characteristic colors from the different pigments that they synthesize and store. They are responsible for the color of the flowers, fruits, roots and some types of leaves.
gerontoplasts
They correspond to the product of the degradation of other plastids, which occurs when the cell dies.
leukoplasts
As mentioned before, these are colorless plastids and their main function is to store nutrients for the cell. They can be found mainly in tissues not exposed to light (non-photosynthetic tissues) such as roots and seed germs.
There are four different types of leucoplasts depending on the type of nutrient they store. Some, called elaioplasts , synthesize and store fatty acids (lipids or plant oils). Others, termed etioplasts , synthesize and store chlorophyll precursors and can develop into chloroplasts on exposure to light. A third type of leucoplast is called a proteinoplast , and as the name suggests they store protein. Finally, amyloplasts synthesize and store starch.
Starch synthesis and storage in amyloplasts
Starch is synthesized in both chloroplasts and amyloplasts through the polymerization of glucose molecules. This storage compound is classified as a homopolysaccharide, since it is a polymer made up of only one type of sugar, in this case, glucose molecules.
Plants use starch as a way to store excess glucose produced during periods of intense light, in which photosynthesis produces more glucose than the plant needs. Depending on where it is stored, this starch is used by the plant as an alternative energy source when it is in the dark, or in situations where photosynthesis is not feasible.
The starch stored in the chloroplasts is transient and represents a quick source of glucose at times when the plant does not receive enough sunlight. Instead, the starch synthesized in the amyloplasts is stored for the long term. It is a reserve that is only used in certain situations, such as when a seed is about to germinate.
amylose and amylopectin
Starch can occur in one of two characteristic forms, amylose and amylopectin, both of which are synthesized and stored by amyloplasts.
Amylose consists of a linear (unbranched) chain of glucose molecules linked to one another by α1-4 glycosidic bonds (link carbon 1 of one glucose molecule to carbon 4 of the next).
Amylopectin, on the other hand, is a branched form of starch. In this case, the long chains formed by glucose molecules with α1-4 glycosidic bonds are linked to other chains through carbon 6, thus forming α1-6 glycosidic bonds.
Starch synthesis and storage in amyloplasts is particularly important for humans, since much of the carbohydrate we consume comes from this reserve polysaccharide. In fact, amylose is one of the first nutrients to begin to be metabolized when we eat, since saliva contains an enzyme called α-amylase whose function is to break down the α1-4 glycosidic bonds of amylose and amylopectin. α1-6 bonds are later broken down.
Storage in internal compartments of amyloplasts
Upon maturity, amyloplasts form internal compartments surrounded by membranes in which they store starch in the form of granules. The number and size of these granules depends both on the plant species and on the particular tissue involved. Some cells contain amyloplasts with several internal granules, while others contain a single large, spherical granule.
The granules are made up of a highly ordered combination of amylose and amylopectin, and the size of the granule is mainly determined by the amount of starch that the plant stores. In some cases, the granules can become very compact and dense, making the amyloplasts that contain them more dense than the cytosol in which they are suspended. This difference in density has important implications related to the direction of growth of stems and roots, as will be seen below.
Amyloplasts and gravitropism
As mentioned at the beginning, in addition to being involved in starch synthesis and storage, amyloplasts also play an essential role in how plants sense gravity. This allows the plants to grow in the correct direction, with the roots facing down and the shoots facing up. This ability to sense the force of gravity and grow parallel to it is called gravitropism.
Gravitropism occurs differently in different tissue types, because shoot and root tissues must grow in opposite directions. In the stems, gravitropism manifests itself in the endodermal cells of the shoots and causes them to grow in the opposite direction to gravity (negative gravitropism) while, in the roots, it manifests itself at the tip of each root, causing them to grow downwards. , in the same direction of gravity (positive gravitropism).
These tissues contain statocytes (specialized cells that sense gravity), which contain a special class of amyloplasts called statoliths. These statoliths are characterized by accumulating very compact and dense starch granules , making them (to statocytes) more dense than cytosol. Due to this difference in density, these amyloplasts always tend to move downwards, accumulating at the bottom of the cell, regardless of its orientation.
Amyloplast-mediated mechanism of gravitropism
When a cell is moved or rotated, the amyloplasts are no longer at the bottom, so they begin to sediment towards the new bottom due to their higher density. On their way, they come into contact with the endoplasmic reticulum, which triggers a series of processes that include the release of calcium from the endoplasmic reticulum, and the release of a hormone called IAA (which is an auxin) at the bottom of the endoplasmic reticulum. cell.
This process is the same for both stems and roots. However, the effect of the IAA hormone is opposite in both cases. In stem buds, the IAA hormone has the effect of stimulating cell elongation and growth. Thus, the cells that are below the statocytes are stimulated, elongate and reproduce, pushing the bud upwards.
In root cells the effect of the hormone is just the opposite. The IAA in these cells inhibits growth rather than stimulates it. Therefore, the cells below the statocytes (and receiving the IAA hormone discharge) do not grow while those above them grow normally, pushing the root tip downwards.
There are still details of the process of synthesis and storage of starch in amyloplasts, as well as gravitropism, which have not yet been clarified. However, it is clear that amyloplasts are organelles of great importance.
References
Nelson, D.L., Cox, M.M. (2013). Lehninger-Principles of biochemistry. (6th edition). 818-821. W. H. Freeman and Company. New York
Clark, MA, Choi, J. & Douglas, M. (2018). Biology 2e . 938-939. OpenStax. Huston. Available at https://openstax.org/details/books/biology-2e
