CORVINUS UNIVERSITY OF BUDAPEST FACULTY OF HORTICULTURAL SCIENCE MODERN HORTICULTURE BOTANY Authors: Zsolt Erős-Honti (Chapter 1, 2, 3, 4) Mária Höhn (Chapter 5, 6, 7, 8, 9, 10) Chapter I.: Plant cell (Citology) 1.1. The concept of the cell: elaboration and changes of the cell theory Today, generally accepted is the fact that all living creatures consist of cells. However, till the middle of the 17th century, researchers were not aware that all organisms would be composed of such units. In the 1650‘s, Jan Swammerdam, Dutch naturalist observed oval bodies in the blood and later he also discovered that the frog embryo consisted of small orbicles. For the very first time, plant cells were found by an English polyhistor, Robert Hooke in 1663 when examining the cork of woody plants. In his work Micrographia he named the observed structures (resembling the cells of the honeycomb) ‗cellula‘ (―cubicle‖, ―compartment‖), thus the denomination of cells themselves also dates back to him. (Although he had thought that the described ‗cellulae‘ were small water conducting tubes of the plant, and later it turned out that he only had observed the mere cell walls of dead cells in his microscope, his naming has remained ever since.) After the first observations, 175 more years passed until the cell concept was generalized to all the known living beings. In 1838, Mathias Jakob Schleiden, a German botanist discussed his observations with Theodor Schwann, a German physiologist in 1838 after that Schleiden determined cellular organization in plants, while Schwann did the same for animals. They published their common observations one year later (1839) in Schwann‘s book, where they also gave the points of the classic cell theory: 1. The anatomical, organizational and physiological units of all living creatures is the cell; 2. the cell is a dual entity: it is a distinct, living unit and also the building block of the organism, at the same time; 3. cell assembles from inorganic material (similar to crystals). According to the last point, Schleiden and Schwann thought that the nucleus of the living cell precipitates and gets outside the protoplasm where later it extends to form new living cells. Consequently, living and non-living stages interchange and thus form a continuity. This can be considered as the survival of the theory of spontaneous generation (‗natura non facit saltus‘, i.e. ‗nature does not leap‘) accepted since as early as the antiquity. Later, the 3rd point of the classic cell theory was refuted by Robert Remak, Albert Kölliker and Rudolf Virchow who carried out researches on the reproductive processes of the cells. Virchow was the first one to state the fact that ‗all cells come from cells‘ (i.e. ‗omnis cellula e cellula‘) that is accepted ever since. 1.2. Cell evolution: prokaryotic and eukaryotic cell, different types of the eukaryotic cell Only indirect evidences do we have on the emergence of the first living cell. According to them, the appearance of the first cell is estimated to have occurred cca. 3.8-4 billion years ago. Though evolution biologists elaborated several theories on the main evolutionary stages of evolving of the first living cells out of the dissolved organic compounds in the ancient oceans, we still have mere assumptions on how the borderline between prebiotic (i.e. the evolution of molecules) and biotic evolution was overstepped. The structure of the first cells was rather simple. Among recent organisms, bacterial cells have similar composition of such ancient features. The most important peculiarity of these cells is that their genetic material is free in the cytoplasm, and not bound in a real nucleus (i.e. it is not enclose by a nuclear envelope). Due to this characteristic, these organisms are called prokaryotes (‗karyon‘ = ‗nucleus‘) in contrast with the later (2-3 bya) evolved eukaryotes having an enveloped nucleus containing the DNA. Beside the lack of nucleus, several further distinguishing features can be observed between the two cell types (listed in Table 1): prokaryotic cells lack extended inner membrane system and large organelles; their size is around the magnitude of a micrometre in contrast with the eukaryotic cells of 10-100 microns. The organisation of the DNA in the two cell types is also differing. Table 1. Comparison of prokaryotic and eukaryotic cell. PROKARYOTIC CELL EUKARYOTIC CELL Origin 3,8 Bya 2,7 Bya Size ~ 1 μm 10-100 μm Nucleus Missing Present single circular DNA always several, molecule linear DNA molecules DNA naked (or associated DNA associated to Genetic material to non-histon proteins) histones 7 9 6 6 1,5×10 ― 5×10 base 1×10 ― 5×10 base pairs pairs Initiated at one single Replication origo Initiated at several origos (Θ-replication) Cell cycle Not observed Observed Fission Mitosis, meiosis Chromosomes are Chromosomes are moved Cell division anchored by microtubules of the to the cell membrane spindle apparatus Endomembrane system Simple Complex (ER, Golgi etc.) Cytoskeleton Missing (simple) Present Organelles Few Always present Protein filaments Organelles of locomotion (bacterial cilium, Complex organelles bacterial flagellum) Ribosome size 70S (30S + 50S) 80S (40S + 60S) Transcription and In the same compartment Spatially separated translation Intercellular junctions Missing Present Differentiation Limited Complex Cytosis Missing Observable Usually multicellular Multicellularity Rarely (e.g. cyanobacteria) organisms Apoptosis Not observed (ambiguous) Present Despite their basically similar structures, the cells of the different eukaryotic kingdoms (Regnum) may considerably differ from each other (Table 2). The most obvious difference is that animal cells lack cell wall that determines the shape and volume of plant and fungal cells. However, the composition of this organelle is not the same in the later two groups, either: the main polysaccharide of the plant cell wall is the cellulose, while in fungi it is substituted by chitin-like compounds (a molecule basically occurring in animals). Animal cells are usually capable of locomotion: they can move with pseudopodia, cilia or flagella. Though less frequently, but fungal cells sometimes possess flagella (e.g. the zoospores, some gametes or the vegetative cells of the chytrids). Conversely, only the sperm cells of some certain plant groups are motile. (Nevertheless, the sperm cells of angiosperms cannot move actively.) All eukaryotic cells contain smaller or larger vesicles filled with different fluids that are necessary for the transport of previously synthesized or modified macromolecules between the membrane organelles or during the cytotic processes (endo- and exocytosis). An important derivative of this vesicular system is the lysosomal system in animal cells, where digestive processes and the recycling of degraded material take place within (Quite recently, the presence of lytic vacuoles with a similar role was also proven in plant and fungal cells.) Nevertheless, the main functions of larger vacuoles are space filling and fluid storage in both plants and fungi. The metabolism of the organisms of the three kingdoms is fundamentally different. Plants are photoautotrophic, i.e. they are capable of producing their own organic compounds from simple inorganic molecules (using light energy). To the contrary, fungi and animals can produce their own organic matter only from other organic compounds, i.e. they are heterotrophic1. The 1 It is worth to note that although fungi cannot produce organic matter from inorganic carbon compounds (e.g. + - - from CO2), they can build inorganic nitrogen forms (e.g. NH4 , NO2 , NO3 ) into nitrogen containing macromolecules. organelles responsible for autotrophy in the plant cells are the chloroplasts that are missing from the other types of the eukaryotic cells. Table 2. Comparison of plant, fungal and animal cells. Plant cell Fungal cell Animal cell Metabolism autotrophic heterotrophic heterotrophic present, present, Cell wall containing containing chitin- missing cellulose like compounds Organelles of cilia, flagella cilia, flagella, sometimes flagella locomotion (only on gametes) pseudopods Characteristic type of cell vacuoles vacuoles lysosomal system cavities Specific Spitzenkörper, plastids centriole organelles lomasome 1.3. Cytoplasm and cytoskeleton The basic substance of all living cells is the cytoplasm, an aqueous solution of different phases2. It contains dissolved ions, intermediary metabolic compounds (sugars, amino acids, carbonic acids) and macromolecules (enzymes, nucleic acids). At certain regions (usually close to the cell membrane) the hydration shells of the colloidal particles connect to each other and thus the cytoplasm becomes a jelly-like substance (cytogel). Enzymes of certain basic metabolic processes are dissolved in the cytoplasm. This is the site of the first stage of monosaccharide degradation (biological oxidation), i.e. glycolysis, as well as that of anaerob fermentation. The most important process of macromolecular metabolism happening here is translation (protein synthesis). It is catalyzed by the ribosomes that are supramolecular complexes consisting of proteins and RNAs (rRNA). The ribosome is composed of a small and a large subunit and its role is to translate the information encoded in the mRNA (i.e. its nucleotide sequence) 2 Cytological literature distinguishes the term ‘cytoplasm’ from ‘cytosol’. In this interpretation, ‘cytosol’ means the total protoplasm of the cell composing of the watery ground substance, the ‘cytosol’, together with the organelles