Plant Anatomy

The cell

The plant cell

The term “cell” goes back to the English naturalist Robert Hooke, who examined plants with a simple self-made microscope and found that cork was divided into small chambers, which he called “cellulae” because they reminded him of the cells of the monks in the monastery. He published these studies in 1665 in his book “Micrographia”. In general, the structure of all living things from cells was only recognized 150 years later.

Evolution of the eukaryotic cell: endosymbiotic theory

Early evolution of marine life, possibly associated with minerals that can bind organic molecules and catalyze the first synthesis reactions.
Experimentally it was proven that under reducing conditions (CO2, methane, ammonia) complex organic molecules can form de novo from simple compounds under suitable environmental conditions (“primordial soup”) (ATP, amino acids, lipids, purine and pyrimidine bases up to RNA , Polypeptides). What is important is the property of lipids to form bilayers on water and to form micelles in water, droplets delimited by a lipid membrane.
First of all, simple reaction spaces bounded by a membrane and from which bacteria later developed; Prokaryotes without further internal organelles delimited by membranes. The only membrane is the plasma membrane. The DNA consists of a ring, is free in the cytoplasm and has only one membrane attachment point. Membranes are extremely important, among other things for the delimitation of internal cell compartments, control of the uptake and release of substances, cell communication or the development of electrical potentials.
By phagocytosis, hypothetical ureukaryotes ingested procaryotes, which were not digested afterwards, but continued to exist as internal organelles and took on important functions for the cell. Mitochondria and plastids emerged from these. Other important compartments are the nucleus, vacuole, Golgi apparatus, dictyosomes and endoplasmic reticulum, which may already have been present in ureukaryotes and can be traced back to invaginations of the plasma membrane.
The process of ingestion of free-living organisms without digesting them can also be observed regularly in species living today. Photosynthetic endosymbionts can be found e.g. in sea cucumber. The endosymbiotic theory is supported by so much data that it can actually be granted the status of a theory.

Cell structure

Cell wall: The cell wall is a kind of shaping exoskeleton that resists the turgor pressure (5-10 bar) and thus helps to stabilize individual organs or the entire plant. It is normally saturated with water, i.e. it is swollen and consists of different polysaccharides. The cell wall is formed by the cell from the outside to the inside and is a composite material made of an amorphous base substance with embedded fibers.

Middle lamella: It emerges from the cell plate that is created immediately after cell division and does not contain any fibrils. It belongs equally to both daughter cells and can be easily broken down by enzymes (pectinases). Then the tissue breaks down into individual cells.

Primary cell wall: The primary wall consists of the middle lamella and the wall substances superimposed by both daughter cells, so it has three layers. The most important wall substances are pectins and hemicelluloses (matrix substances), which are secreted by the Golgi apparatus via the dictyosomes, as well as glycoproteins. Cellulose in contrast, it only represents a small proportion. Therefore, the wall is very flexible and allows a considerable increase in size. The later increase in the cellulose content stops growth.

Plasmodesmata: Plasmodesmata are recesses in the cell wall through which neighboring cells stay in contact. Not only is the cytoplasm connected in this way, but also the intracellular membrane system. Membrane tubes of the endoplasmic reticulum or the dictyosomes can run through from one cell to the next. The cell wall can also later be broken down by enzymes and thus secondary plasmodesmata can develop.

Cell wall polysaccharides

Pectins: a mixture of acidic, strongly negatively charged sugar molecules (galacturonans, rhamnogalacturonans) or short-chain polysaccharides (arabinans, galactans, arabinogalactans), which are highly hydrophilic and therefore easily soluble in water and have an extreme swelling capacity. The individual molecules are linked by divalent ions (Ca2 +, Mg2 +) and are sometimes formed in bulk in fruits (plant mucilage).

Hemicellulose: less hydrophilic, longer-chain polysaccharides with enormous structural diversity. Essentially glucans [ß (13 and ß (14) linked glucose)] and xyloglucans [ß (14) linked glucose with mostly α (16) linked xylose chains.
They cover cellulose chains and thus increase the mechanical strength of the cell walls. Oligosaccharides from the group of hemicelluloses play an important role in cell communication (receptor and recognition structures) or as a trigger for defense reactions when pathogens penetrate.

Glycoproteins: main proteins of the cell wall, with a high proportion of hydroxylated proline. The polypeptide part represents a rod structure of approx. 80 nm, which is surrounded by arabinoside. It is believed that they serve to stiffen the cell wall. The amino acid sequence of the polypeptide is similar to that of the collagen of the intercellular matrix in animals.

Cellulose: long-chain, straight polysaccharides consisting of 2-15000 glucose units [ß (14) linked], which form the cell wall structure. Cellulose is a ribbon-shaped molecule because the type of linkage means that the monomers lie in one plane. Due to the formation of hydrogen bonds, they are placed close together and thus form elementary fibrils (approx. 3 nm thick), which in turn combine to form microfibrils (5-30 nm thick, up to 8 µm long).
They are also band-shaped and arranged like crystal lattices over large areas. Cellulose is therefore only flexible to a limited extent and breaks easily. On the other hand, it is extremely tear-resistant with around 80% of the tear strength of steel. In the plasma membrane are of
Rosette-shaped cellulose synthase complexes form several cellulose chains at the same time, which immediately crystallize into elementary fibrils. Thicker microfibrils result from several closely spaced complexes. While the synthesis of the microfibrils as once this has been clarified, there is still great uncertainty about the way in which the orientation of the fibrils is controlled. Despite intensive research, there is still no generally accepted hypothesis. Cellulose is the most common organic
Macromolecule, which is synthesized in the order of 1011 tons annually and represents one of the most important vegetable raw materials.

Cell wall crusts

Silicification: The storage of silicon (silica) makes cell walls very brittle and hard. Grasses in particular often have massive silica deposits in the epidemic cells, which are often formed in the form of teeth on the leaf margin and like one
Acting saw. On the other hand, within the leaf blade there are so-called short silica cells, which contain one or more crystals. Further examples are stinging hairs of the stinging nettles (Urticaceae) or the flower nennets (Loasaceae).

Cell 2

It consists of two different elements, microtubules, actin filaments, which can locally stiffen the cytoplasm. Both systems take on other important tasks within the cell. The shape of the cell can be influenced by the filaments and they are responsible for movement phenomena within the cell, e.g. Plasma flow, vesicle transport, chromosome separation and are necessary prerequisites for endo- and exocytosis.

Actin: is a thin, thread-like molecule made of globular monomers. Actin filaments serve as attachment points for myosin (an ATPase), which is capable of conformational changes when ATP is consumed and thus clings to an actin filament (10 nm per movement). So both filaments slide past each other: sliding fiber model.

Microtubules .: The basic unit is a dimer made up of α- and β-tubulin, which are lined up in a row and form a thin tube. On them are small motor proteins (Dynein or Kinesine) which can also let filaments slide past each other through ATP consumption.

Cell-internal compartments

Biomembranes: All internal compartments are delimited by biomembranes, which consist of a lipid bilayer. Their hydrophilic heads point towards the cytoplasm or the cell wall or towards the interior of vesicles. All membranes are rich in protein. Usually the inside and outside of the membranes are very different. In addition to the peripheral proteins that only sit on one side of the membrane (30-40%), others cross the membrane (integral or transmembrane proteins 60-70%). The most important include ion pumps or e.g. Cellulose synthase complexes. The proteins can move laterally in the membrane by diffusion but can hardly get from one side to the other (fluid mosaic model). Biomembranes are selectively permeable, that is, they allow water through practically unhindered, but not dissolved substances. In this way, substances are enriched and membrane potentials built up that are in the range of the breakdown voltage of the lipid bilayer (100 mV or 100,000 V cm-1). The internal membrane system is a continuum and is represented by the Golgi apparatus, which in turn represents the entirety of the dictyosomes, the endoplasmic reticulum and the nuclear membrane. The ER supplies the dictyosomes with material in the form of small vesicles (ER vesicles) on the cis side. On the trans side, Golgi vesicles or lysosomes leave the dictyosome. Dictyosomes consist of 3-12 flat stacks of membranes.
The ER runs through the entire cell in the form of thin tubes (cisterns) and merges seamlessly into the nuclear membrane (perinuclear cistern). Its surface is partially covered with ribosomes, the protein biosynthesis complexes (rough ER), if not, then one speaks of a smooth ER. The ribosomes can join together and are then referred to as polysomes.

Cell organelles

Vacuole: the largest compartment of the cell (90%), essentially filled with water and usually in an acidic environment (approx. PH 5). The vacuole membrane is called the tonoplast.
Through dissolved substances, water flows into the vacuole and thus creates an increased internal pressure (turgor, hypertonic compared to the external medium). The growth of a cell after division is essentially due to the uptake of water and the corresponding increase in volume. If the vacuole is hypotonic to the external medium, water flows out and plasmolysis occurs. The vacuole can shrink considerably. Depending on the amount of leaked water, one speaks of border or convulsive plasmolysis. The plasma membrane becomes detached from the cell wall, but remains attached to the plasmodesmata by thin plasma threads. The process is completely reversible. The vacuole stores substances such as water-soluble dyes (anthocyanins), alkaloids or acids (e.g. malate) or serves as
Repository for waste (e.g. calcium oxalate).

Mitochondria: Small organelles with a double membrane covering. The structure of the outer membrane corresponds to the eukaryotic membrane (occurrence of cholesterol), the inner one, however, differs greatly due to only 20% lipids and 80% protein content and the characteristic lipid cardiolipin (typical of bacteria). It is much less permeable than the outer membrane. It is strongly folded, sack-like or inversed in a tubular shape. The most important function is the oxidative phosphorylation (ATP production by the ATP synthesis complex). The energy for this is made available by the oxidation of organic substances such as carbohydrates and fats via the electron transport chain of cell respiration. In addition, calcium is stored under energy consumption.

Plastids: Plastids are organelles typical of plants, also with a double envelope membrane. The outside is more permeable than the inside. Therefore, the inner membrane is equipped with many translocators and the main place of lipid biosynthesis. Like mitochondria, plastids multiply according to the mode of division of bacteria. All plastids arise from proplastids in the meristems, which, like meristem cells, often divide and are in principle mutually convertible. Only the “gerontoplasts” in senescent tissues are end stages. In the dark, the proplastids develop into “etioplasts”. They do not turn green, but remain pale yellow and have a prolamellar body which, when exposed to light, becomes an internal membrane system
differentiated. The most important type of plastid is the chloroplast. The internal membranes are known as the thylakoids and are often layered in stacks called grana. The thylakoids are not in contact with the inner membrane and represent the main photosynthesis site with the membrane proteins and pigments necessary for this.

Cell nucleus: the nucleus is enclosed by a hollow spherical double membrane that emerges from the ER. This is interspersed with numerous pores that allow transport between the nucleus and cytoplasm. The nucleus contains most of the cell’s DNA, along with a number of auxiliary and structural proteins, chromatin. It is distributed over various linear strands of DNA that are more or less strongly condensed and are known as chromosomes. Size, shape and number of chromosomes denote the karyotype of a cell, an important characteristic in genetics, systematics
and phylogeny. Typically there is a double set of chromosomes in every cell (diploid cells), a single set (haploid) in sex cells or gametophytes (e.g. moss plants). In many plants there is a multiplication of chromosomes (polyploidy).

Mitosis and the cell cycle: Before a cell can divide, the DNA has to be doubled so that it can be distributed between two daughter cells. This happens in the interphase, the working phase of the cell. This is divided into three sections (G1, S, G2). The G1 (Gap) phase describes the area of ​​cell growth, in the S phase (synthesis) new DNA is synthesized and the G2 phase prepares the next cell division. The interphase is followed by the M phase (mitosis) in which the chromosomes slowly condense and migrate towards the middle of the cell (prophase). At the time it forms
first the preprophase tape from microtubules then the spindle apparatus. In the metaphase, all chromosomes are arranged in the plane of division, the sister chromatids separate from each other at the centromere and are pulled apart in the opposite direction by the microtubules of the spindle apparatus (anaphase). Then a new cell wall and nuclear envelope are built up and the chromosomes decondense (telophase).

Meiosis: with meiosis an additional step is inserted. First, the sister chromosomes arrange themselves in the division plane and are separated from one another.
This is followed by normal mitosis, in which the sister chromatids are separated. In this way, 4 haploid cells (e.g. spores or pollen) are created.

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