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Day 3

1. Plant Mitochondria

Mitochondria in plants are younger than chloroplasts (in evolutionary terms), and they have similar functions to animal mitochondria. They are the powerhouse of the cell, the cellular respiration takes place here and they are also semi-autonomous organelles (they have a 200k bp DNA, significantly more than the animal mitochondria). There are more mitochondrial genome copies in the meristematic cells than in the differentiated cells, as the mitochondria continue to replicate and to be passed on, without actually duplicating the DNA.

2. Peroxysomes

Peroxysomes, as glyoxysomes, are known as microbodies. They are responsible in fighting oxidative stress, as they carry on an oxidation reaction:

$RH_{2} + O_{2} \rightarrow H_{2}O_{2} + R$

R is the organic substrate.

Hydrogen peroxide is then easy to eliminate inside the peroxysome itself, as it contains a wide crystal of catalase, an enzyme that produces water and oxygen:

$H_{2}O_{2} \rightarrow H_{2}O + \frac{1}{2}O_{2}$

As you can see, we reduced the number of oxygen molecules by half.

Against oxidative stress we have also other non-enzymatic weapons, such as vitamins, ascorbic acid and polyphenols, as well as other enzymatic ones: catalase, peroxidase, superoxide reductase and superoxide dismutase.

3. Glyossysomes

Glyossysomes are more common in seeds were big oily reservoirs are stored: they are responsible for the glyossylic acid cycle, which turns lipids into carbohydrates, which are more directly employable by the embryo in its growth. These organelles were probably fully responsible for the respiratory metabolism of the plant cells before mitochondria entered the scene.

4. Oleosomes

Oleasomes are microbodies that contain lipids reservoirs (generally an oily drop). They are sorrounded by a single layer of phospholipids (if they had the double layer like a normal membrane, the oil would pass through it), and they are stabilized by fork-like proteins known as oleosins. Oleosins stabilize the single layer avoiding ruptures, but they are also responsible for preventing fusion of adjacent oleosomes.

5. Vacuoles

Vacuoles are the biggest organelles in the plant cell: at maturity, they can take up to 80-90% of the cell volume. Most of them are filled with a water-y matrix, and surrounded by a double-layered membrane known as tonoplast. Vacuoles are responsible for maintaining the internal pressure inside the plant cell: the so-called turgor pressure of the cell. If the cell loses water, the vacuole collapses and the cell withers.

There are two types of specialized vacuoles:

  • Protein bodies, that contain proteins and are generally found in seeds, where the proteins are demolished during growth
  • Lytic vacuoles, which contain hydrolytic enzymes: they are released during senescense, programmed cellular death (aka apoptosis), or in response to damage. They cause a controlled demolition in space and time of portions of the plant cell. The acid pH inside lytic vacuoles is maintained thanks to the vacuolar ATPase, that moves protons from the cytosol to the vacuole.

Vacuoles can be used also as a stocking for primary and secondary metabolites, and they also contain substances for protection and detoxification. This detoxification potential can be also useful in the bioremediation of polluted soils.