More Basic Plant Cytology

The vacuole is a specialized and complex organelle

The vacuole is essentially a very large smooth vesicle, but this appearance is deceiving! From a membrane point of view the vacuole is just a big bag, the tonoplast. The vacuole contents have little to offer visually, hence the name "little vacuum:" vacuole. Of course pigments are sometimes stored here, for example anthocyanins, and thus they were known even a century ago not to be completely empty or just water.

Indeed the vacuole is mostly water, as is the cytosol. This water supply of course serves to provide an available supply of water, makes a large cell out of a small amount of metabolically-expensive cytosol. The water in the vacuole helps provide the turgor pressure for cell growth and tissue support. To keep water moving into the vacuole by osmosis, the vacuole accumulates a range of solutes: minerals, ions, organic acids, polyphenols, sugars, enzymes, and a diverse array of relatively-toxic substances.

The solutes in the vacuole are toxic to herbivores

The solutes of the vacuole do more than provide an osmolarity to bring water into the cell. They do more than paint flowers red and blue. The toxins are more than just stored away here. In fact animals eating plants get doses of these toxins.

Functionally the vacuole is a recycling center

But what is more important from the plant cell's point of view, are the enzymes located here. The enzyme array of vacuoles is impressive. These include lipases, glycosidases, nucleases, and proteases: the range of enzymes needed to digest just about any cellular component. These make the vacuole the plant equivalent of a lysosome in animal cell. In recent years we have learned much about the recycling capacity of the vacuole. Materials are delivered into the vacuole, are digested into raw materials, and those are returned to the cytosol for reuse. Obviously our thinking about vacuoles has changed greatly over the past century: from an empty bag of water, to toxic waste storage pool, to component recycling center.

Plants have mitochondria for respiration

Yes, plants have mitochondria. Many people do not realize that plants have mitochondria and carry out respiration. This misconception needs to be repaired immediately. Surely most plants obtain most of their energy through photosynthesis, but this does not mean that respiration is unimportant. Quite the opposite is true! All living plant cells survive the night-time and cloudy days by respiration, most of which takes place in the cell's mitochondria. Consider root cells...under the soil there is no light penetrating to drive photosynthesis; its entire energy needs must be satisfied by respiration! Indeed all living cells of plants (except for sieve tube members which use companion cell contributions instead) have mitochondria that assist with respiration.

The mitochondrion is probably derived from an endosymbiont

The mitochondrion consists of a smooth outer membrane and a much larger inner membrane; the inner membrane is about 70% protein and is highly convoluted to fit inside the outer membrane. The space between the two membranes is called the intermembrane space. The fluid inside the inner membrane is called the matrix. This matrix is believed to be the cytosol of an ancient prokaryotic endosymbiont. It contains prokaryotic-style 70S ribosomes and its own genome, the chondriome, in the form of a single, circular DNA molecule which is not associated with histones. The genes in this DNA code for some of the critical enzymes and electron transfer proteins needed for the Kreb's cycle and the electron transfer chain which are translated on mitochondrial ribosomes. The key word here is "some;" indeed many of the components in the mitochondrion are coded by DNA genes which are now housed and transcribed in the eukaryotic nucleus and are translated on cytosolic ribosomes. Mitochondria replicate their naked chromosome in a process similar to fission in baceria; they divide by a process similar to cytokinesis by furrowing.

Mitochondria host the Kreb's cycle, electron transfer chain, and oxidative phosphorylation

Indeed plant mitochondria are not much different from those in animals. While they may be more round than oval in shape, plant mitochondria carry out the Kreb's cycle in the matrix and operate the electron transfer chain in the infolded cristae of the inner membrane. The electron transfer chain also pumps protons (H+) into the intermembrane space. The pumping and diffusional gradient between the space and the matrix represents a conservation of energy from respiration. As protons leak back into the matrix from the intermembrane space through an ATP synthase membrane protein complex, this potential energy is trapped in the terminal phosphate bond of ATP. These structural and functional ideas are partially shown here.

Chloroplasts are also likely to be derived endosymbionts

Chloroplasts are likely derived from endosymbiotic cyanobacteria. They have two smooth membranes (inner and outer) surrounding the stroma. The membranes are likely derived from the cell membrane of the endosymbiont and the stroma is its cytosol. The membranes are made of mostly of glycosylglycerides rather than phospholipids.

The stroma contains 70S ribosomes, developing starch grains, oil bodies, and the naked, circular DNA chromosomes. Also present in the stroma is an endomembrane system called thylakoid lamellae. Some of the lamellae are stacked as grana; other lamellae, called stroma lamellae, interconnect the grana. The chloroplast genome (the plastome) codes for many, but not all, of the proteins required for photosynthesis. The transcripted RNA is translated by the 70S (prokaryotic style) ribosomes. Other proteins are now coded in the nucleus and translated in the cytosol; these notably include the large subunit of RuBP carboxylase/oxygenase. The chloroplast genome is 145 kbp compared to 200 kbp in the mitochondrion and together these genomes represent perhaps 1/4 of the genes in the cell.

The chloroplast carries out photosynthesis

The stroma of the chloroplast hosts enzymes which carries out the Calvin cycle reactions to convert carbon dioxide to carbohydrate. The endomembrane system hosts the electron transfer proteins and pigments for the light reactions which split water and produce oxygen gas. These two reaction systems are coupled; each reaction system needs some products of the other. The electron transfer proteins of the light reactions pump protons into the thylakoid lamellae; the protons pass back through ATP synthases of the thylakoid membrane on their way back to the stroma; this results in ATP synthesis as in mitochondria. The ATP is used by the Calvin Cycle. Thus both systems require environmental light and carbon dioxide. There is no such thing as a "dark reaction." The carbohydrate product of photosynthesis can be accumulated in the chloroplast in the form of a developing starch grain or as lipid bodies.

The chloroplast is one of a family of organelles called plastids

The plastids are a family of organelles derived from the proplastid which has primitive lamellae, if any, and no storage material. From this colorless proplastid a plant grown in the dark will develop etioplasts which contain prolamellar bodies where are tubular arrays of raw materials for endomembrane synthesis and protochlorophyll. Upon exposure to light the prolamellar bodies are rapidly converted to thylakoids and the protochlorophyll is converted to chlorophyll as the etioplast becomes a chloroplast. These relationships are shown below. As chloroplasts mature and produce much starch, the chloroplast may significantly degrade its thylakoids to make more room for starch and thus become an amyloplast. Proplastids or chloroplasts accumulate carotenoid pigments they can become chromoplasts. Chromoplasts are responsible for the red color of tomatoes and peppers and certain kinds of flowers (Ripsalidopsis).

Peroxisomes participate in photorespiration

Microbodies were the original name for small single-membrane bound organelles. In plants these have been renamed peroxisomes and glyoxysomes. The peroxisome is shown with its crystalline matrix of catalase below. This organelle mostly degrades glycolate, a 2-C acid produced in chloroplasts as the result of RuBisCO combining with oxygen rather than carbon dioxide, in the process of photorespiration. The glycolate is transferred into the peroxisome. In degrading glycolate, oxygen is consumed and peroxide (H2O2) is produced. This toxic material is enzymatically degraded to water and oxygen by the enzyme catalase. This critical enzyme may comprise 40% of the protein in the peroxisome; little wonder catalase appears in crystalline form in peroxisomes. Mitochondria are a third-partner in photorespiration.

Oleosomes store oils

The partner for glyoxysomes are oleosomes. These organelles are spherical and are bounded by a phospholipid monolayer. The hydrophilic polar phosphate "head" of the phosopholipid faces the cytosol and the hydrophobic fatty-acid "tails" face the interior of the oleosome which is loaded with hydrophobic oils from the ER. There are peripheral proteins on the hydrophilic side of the monolayer. These proteins include oleosins which may help attach the enzyme lipase to this monolayer to initiate fat digestion for the glyoxylate cycle.

Glyoxysomes carry out the glyoxylate cycle

Glyoxysomes are abundant in oil storage tissues. This microbody contains the enzymes needed to degrade fatty acids into 4-C acids to drive mitochondrial respiration and reverse glycolysis to make sugars (gluconeogenesis) for growth and development. This pathway is called the glyoxylate cycle. This organelle is active as the oils are removed from storage and put into use or exported for transport in phloem. You will notice how, just like photorespiration, this pathway involves three organelles and the enzymes in the cytosol to complete the conversion of oils back into carbohydrates.

In the next lecture we will continue with the cytoskeleton, the process of mitosis, and the cell cycle.