Plastids are only found in which of the following types of cells or organisms?

C. Plant Cells.

The members of Kingdom Plantae are all eukaryotic and multi-cellular. They exhibit many different cell types. We will discuss some of these but let's begin by reviewing the features of a typical plant cell.

Plant cells are similar to those of other eukaryotic organisms. They possess a cell membrane (plasma membrane), nucleus, ribosomes, endoplasmic reticulum, Golgi bodies, mitochondria, and cytoskeleton that function more or less as their counterparts do in animal or fungal cells. The distinguishing feature of plant cells is that many of them possess plastids, a large central vacuole, and a cell wall. We will focus on these three special features in our review of plant cells.

1. Plastids

Figure 1.4 of your text diagrams a plant cell of the sort that would be found in a plant leaf. The plastids in this cell are chloroplasts, the green, photosynthetic plastids common in cells of the plant shoot (the above-ground part of the plant). All plant cells have plastids but not all types of plastids are green. The features that all plastids do share include the following:

•  All plastids are surrounded by two membranes, similar to mitochondria.

•  Plastid membranes consist of glycosylglycerides rather than phospholipids. In other words, the membrane lipids of plastids have a sugar as the polar head group rather than a phosphate (see Figure 1.5 in your text).

•  All plastids have some degree of internal membrane structure. The most striking case is the thylakoid membranes of chloroplasts (see Figure 1.16 of your text).

•  Plastids have a small, circular chromosome encoding about 120 genes. This chromosome is similar to those of eubacteria in that it is circular, not associated with histone proteins (histones of nuclear chromosomes are shown in Figure 1.9 of your text), and contains very little non-coding DNA. Plastids also contain 70s ribosomes that translate the mRNA produced from these genes. (Mitochondria also have their own chromosome and ribosomes, similar to plastids, except that the mitochondrial genome contains only about 13 genes).

a. Types of plastids

There are many kinds of plastids in the different cell types of the plant body. The most common and important include the following:

•  Proplastids - These are the undeveloped plastids in the cells of meristems. Proplastids are small, colorless, and have little internal membrane structure. They develop into different plastid types as their host cell matures. The type of plastid they develop into depends on where they are in the plant body and other factors.

•  Etioplasts - Etioplasts are present in plant shoots that have been grown in the dark. They are proplastids that have begun to develop into chloroplasts but are arrested at an early stage by lack of a light signal from the environment. Etioplasts are pale yellow-green because they contain a precursor of chlorophyll called protochlorophyllide. They have an internal membrane structure that looks like packed tubes in cross section (see figure 1.18 in your text). When exposed to light, etioplasts quickly develop into chloroplasts.

•  Chloroplasts - These are the most familiar plastids, present in the green leaves and stems of most plants. The elaborate internal membrane system of chloroplasts is known as the thylakoid membranes (see Figure 1.16). Chloroplasts are grass-green, contain chlorophyll and carotenoid pigments, and participate in photosynthesis, which is the harvesting of solar energy and its use to make carbon dioxide into simple carbohydrates. Chloroplasts and other plastids are the site of much other metabolism. Starch synthesis, amino acid synthesis, fatty acid synthesis, and other synthetic pathways all occur inside plastids.

•  Amyloplasts - Amyloplasts are colorless plastids found in roots, tubers, and the endosperm of seeds. They make and store starch.

•  Chromoplasts - These are the orange, yellow, or red plastids that give color to flower petals, many fruits, and turning leaves. Chromoplasts develop from chloroplasts during fruit ripening, flower development, or leaf senescence. During this development, chlorophyll is degraded, the thylakoid membranes disappear, and droplets of oily carotenoid pigments accumulate. Figure 1.17 in your text shows a chloroplast that is converting to a chromoplast during the ripening of a tomato fruit.

As noted above, all plastid types develop from proplastids. Some can also interconvert, for example an amyloplast can become a chromoplast or a chloroplast.

b. The Endosymbiont Hypothesis

Possibly the greatest hypothesis ever proposed in cell biology is the endosymbiont hypothesis, which is the idea that chloroplasts, mitochondria, and other eukaryotic cell organelles were once free-living prokaryotic cells that were engulfed by an archebacterial host and retained as intra-cellular symbionts. According to the endosymbiont hypothesis, eukaryote cells are conglomerations of prokaryote cells that were acquired sequentially in great leaps of single-celled PreCambrian evolution. The endosymbiont hypothesis had been discussed lightly for many years but Lynn Margulis pressed the point in the 1970's. She was initially ridiculed but since then the accumulating evidence has caused the endosymbiont hypothesis to be widely accepted, especially for the origins of chloroplasts and mitochondria. Some of this evidence includes the following:

•  Chloroplasts and mitochondria have their own small chromosomes. These are eubacterial in nature, being circular, lacking histone proteins, and having very little non-coding DNA.

•  Mitochondria and chloroplasts have their own ribosomes and RNA polymerase encoded by genes on their chromosomes. These are also eubacterial in nature and clearly different than the RNA polymerases and ribosomes produced by genes in the nucleus. In general, the genes found in the chloroplast and mitochondrial genomes have much greater sequence similarity to homologues in bacteria than to homologues in their own nucleus.

The mitochondrial ancestor is not agreed upon but the preponderance of evidence (mostly gene sequence analysis) suggests that chloroplasts are descended from cyanobacteria. Modern, free-living cyanobacteria have about 1,000 genes, many more than the 120 present in chloroplasts. It is proposed that over eons of symbiosis, many of the original genes present in early chloroplasts were either lost or migrated to the nucleus. Studies of genome sequences support the idea that gene transfer between organelles and even between species is common over evolutionary time scales.

An outcome of the migration of genes from the chloroplast to the nuclear genome is that many or the proteins needed by chloroplasts (and mitochondria) are no longer made from the chloroplast genome but must be imported from the cytosol. There are chloroplast membrane proteins that do this and, not surprisingly, they are similar in sequence and structure to those in protein import systems of bacteria.

2. The Central Vacuole

Most plant cells contain a large central vacuole that occupies 30 to 90% of their volume. The central vacuole is bounded by a single membrane called the tonoplast. The tonoplast, like the plasma membrane, contains many transport proteins that govern what crosses it. The water solution in the vacuole typically contains nutrient ions, toxic metal ions, and secondary compounds such as poisons or anthocyanin pigments. The pH of the vacuole is acidic (about 5.5 or less, quite different from the cytosolic pH of about 7.4). The functions of the central vacuole are "cheap size" and storage of compounds that would tend to poison the cytosol.

a. Cheap size

As we shall discuss in detail later, plants grow by cell division but also by increasing the size of their cells. In a growing root or shoot tip, plant cells exhibit a phase of cell expansion shortly after they have been produced by cell division, increasing their length at least several-fold. Much of this cell expansion, and much of the size of a plant, results from inflating the central vacuole with water, which is energetically less costly than increasing the volume of the cell with new cytoplasm. This "cheap size" is a way that plants can compensate for their inability to move, for example by growing toward what they need. Cheap size is adaptive in several contexts:

•  Plant shoots grow upward toward light while their roots simultaneously grow downward toward water in the soil. Being long in both directions allows better access to these resources.

•  Plants need a large surface area of leaf and root to absorb the carbon dioxide and soil water they need for growth.

•  Plants recover from herbivory by simply growing back lost parts.

•  Pollen and seeds disperse better when they are distributed from a shoot that is high above the ground.

In all of these cases, increasing size by essentially inflating the vacuole with water allows plants to get more size for less resources expended.

b. Storage

The central vacuole is also a site for storage of ions and compounds that are incompatible with the chemistry of the cytosol.

i. Proteases

Proteases are proteins that break down proteins. In general, proteases are used by cells to recycle amino acids from proteins that have worn out with use. They may also be a deterrent to herbivores and pathogens, in some cases. The central vacuole in plant cells contains many proteases.

ii. Salt ions

The vacuole must be in osmotic equilibrium with the cytosol of the cell, which contains many proteins, RNAs, and other solutes. The solution in the vacuole has a relatively high salt concentration to balance the solutes in the cytosol.

iii. Toxic metals

Metals such as cadmium are extremely toxic and can enter plant cells via transporters for other ions. They denature proteins on contact. Plant roots often encounter such toxic metals in soils. There are several strategies for coping with these. One is to bind the metal ion and transport it through the cytoplasm to the vacuole for safe storage. Some plants accumulate high concentrations of toxic metals in their vacuoles, partly to allow growth on metal-contaminated soils and partly to make themselves toxic to herbivores.

iv. Secondary compounds

The term "secondary compounds" is a catch-all used to describe anything not known to be involved in the metabolism of major cell components, such as proteins, nucleic acids, or major lipids. A better way to describe them may be as compounds that benefit the organism but not the cell. For example, the alkaloid poison strychnine prevents a plant from being eaten but at the scale of the individual plant cell, it is as toxic as it is to animal cells and must be sequestered in the central vacuole. Many secondary compounds are found in the central vacuole. The blue anthocyanin pigments present in some leaves and flowers are in found in the central vacuole, as are the alkaloid poisons strychnine, cocaine, and caffeine. Many of these secondary compounds are valuable as pharmaceuticals.

3. The Cell Wall

The cell wall is a semi-rigid casing that surrounds all plant cells. The cell wall imposes several limitations on plants. It restricts shape changes of the cell, and thus limits plant movements. The cell wall also prevents phagocytosis by plant cells, which is the engulfing of large particles of external material (eating). On the positive side, the cell wall of plant cells allows them to develop turgor pressure, which is hydrostatic pressure inside the cell. This is seen most dramatically when herbaceous plants wilt. Without sufficient water, the turgor pressure of plant cells is lost and they are seen to lack sufficient structural reinforcement to stand up without it. The ability to sustain turgor pressure allows plants to use a minimum of materials for their growth (another case of cheap size). In contrast, animal cells would burst before they developed internal pressure because they lack cell walls for reinforcement. A cell from a fully hydrated plant has an internal pressure similar to that of a properly inflated truck tire (about 45 psi). Cell walls also provide protection from attack by pathogens and pests. For example, most plant pathogens must have a way to dissolve or digest the plant cell wall in order to successfully attack a plant.

a. Cell wall construction

The plant cell wall exhibits a fiber-matrix construction, similar to fiberglas or reinforced concrete. In other words, the plant cell wall consists of strands of a very strong fibrous material cemented together with an adhesive matrix.

i. Cellulose microfibrils

The cellulose microfibrils of plant cell walls are composed of 60 to 70 cellulose strands that are hydrogen bonded together. Recall that cellulose is a polymer of glucose with a beta 1-4 glycosidic linkage. This makes it a relatively straight chain of glucose molecules and cellulose molecules pack and hydrogen bond together very well. The cellulose microfibrils of the plant cell wall are extruded onto the surface of plant cells in a way that we will discuss inmore detail later.

ii. The matrix

The glue that binds cellulose microfibrils together ina cell wall is composed of a diversity of compounds that include pectin (a familiar gelling agent from the food industry), hemicelluloses, and glycoproteins. Some cell walls also contain a very strong and complex molecule called lignin.

What cells are plastid found in?

Are plastids found in animal cells?

Are plastids found in prokaryotic or eukaryotic?

Are plastids only found in plants?