# 8: From single cell to supracell - cell structure - function  relationships

 

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Index

Anthony van Leeuwenhoek Cell Structure extracellular intracellular land plant emergence
cytoplasm Max Schulze plasmamembrane autotrophic heterotrophic
cell theory Prokaryotic evolution Robert Hooke Schwann  
cell organnelles, eukaryotes Eukaryotic evolution Prokyaryotic definition Eukaryotic definition Schleiden

 

One can start by asking a question: Can we define life at the cellular level? There is not an easy answer to this question, as many aspects of the definition of life have to be taken into account in our answer. The ability to regenerate through sexual or asexual means, the ability to synthesize food by either breaking down complex molecules and then re-assembling them into useful energy forms, the ability to synthesize essential food resources from relatively simple inorganic and organic material are important facets of plant cell life. Clearly, an understanding of the structure of cells, will enhance our understanding of cell structure, and the evolution of the more complex forms that exist today. Inquisitiveness and inventiveness were keys in the past and remain keys to future breakthroughs in cell culture, genetic engineering and many of the other exciting fields of structural and molecular biology. However, the most important ‘key’ of all, remains a thorough understanding of cell structure.

In the 17th century Galileo Galilei arranged two glass lenses and with this instrument, Galilei looked at an insect. As a result of his curiosity he was able to describe the stunning geometrical patterns within its tiny eye. Galilei thus became the first person to record a biological observation through a rudimentary microscope.

Robert Hooke was soon at the forefront of these studies. When Hooke turned his attention to a thinly sliced piece of cork, he observed tiny empty compartments, to which he gave the name cellulae, which means small rooms, and hence, the origin of the biological term "cell". What Robert Hooke was observing at the time, were the walls of dead cells - he did not think of them as being dead, simply as he did not know that they could be alive. Hooke also noticed that other plant materials contained what he described as "juices", but did not speculate as to what the juice-filled structures might represent.

Anthony van Leeuwenhoek, a Dutch shopkeeper, is a accredited as being the first person to observe a single bacterium - even though this small organism would not be seen again for another 200 years. This was an age of exploration and not of interpretation and once the limits of these rudimentary microscopes had been reached, biologists gave up their interests in cell structure, without having been able to explain what they had seen.

By 1820 improved lens design permitted closer views of the cell. The botanist, Robert Brown, observed a sphere-like structure in nearly all the cells that he examined. This structure was called the "nucleus". By 1839, the zoologist Theodor Schwann reported the presence of cells in animal tissues. Collaborative work with the botanist, Matthias Schleiden, concluded that cells were present in all plants and animals and that the nucleus was somehow of paramount importance in cell reproduction. Schleiden and Schwann proposed that each living cell had the potential to exist independently.

Schwann summarized the meaning of these observations that have become known as the first two principles of the cell theory.

All organisms are composed of one or more cells.

The cells are the basic living units of organization.

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As yet, there was a basic question that remained unanswered - where do cells come from? In 1859, Rudolf Virchow continued with, and completed studies of cell growth and reproduction, reaching the third of the principles of the cell theory, in his pronouncement ’omnis cellula e cellula’ that,

All cells arise from pre-existing cells.

 

The Beginning of our understanding of Cell Theory In 1861, Max Schultze defined a cell as a mass of protoplasm, which contained a nucleus. Many biologists have added to these observations, including Strassburger who described the wreathlike structures surrounding the vascular bundles in Zea mays, which, more than 100 years later, was shown to be associated with C4 photosynthesis.
From these simple beginnings it is clear now that the cell may be considered the smallest basic unit of biological life, and further, that continuity was seen to arise from the division and growth of single cells. These cell biologists thus realized that within each tiny cell, events were going on that had profound implications for all levels of biological organization.

 

Basic Aspects of Cell Structure and Function

 To describe a single cell is not an easy task - in animal and plant cells there are three shared features - a nucleus, surrounded by cytoplasm, which in turn is surrounded by a cell membrane called the plasmalemma.

1. Plasmamembrane (or plasmalemma). The boundary between the cell’s living content and its immediate environment.

2. Nucleus. A membrane-bound compartment that contains the hereditary instructions (in DNA) and other molecules that control the reading of instructions passed on from the DNA via t-RNA and m-RNA to the cytoplasm of the cell. Bacterial DNA is concentrated in a structure called a nucleoid, which has no surrounding nuclear membrane.

3. Cytoplasm. The cytoplasm includes all components of the protoplast, but excludes the plasmalemma, the nucleus and the vacuole (only in plant cells). It consists of internal membrane complexes, called Endoplasmic reticulum, mitochondria, ribosomes, Golgi apparatus (dictyosomes in plants), and in plant cells, a unique structure called the chloroplast.

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There are, however, fundamental differences between a plant and an animal cell. For example, with few exceptions, plant cells are associated with cell walls, the presence of which results in the formation of a structural and physiological barrier between the extracellular (outside) and intracellular (inside) compartments associated with the day to day life of the plant cell. Animal cells on the other hand, lack a rigid to semi-rigid wall structure and are encased by a cell membrane that separates the extracellular from the intracellular compartments.

Today, the study of cell structure involves the sue of light and electron microscopes. Indeed, it is impossible to see the fine detail of the typical animal or plant cell without the aid of an instrument as powerful as the transmission electron or scanning electron microscope.

Whilst cell size may vary enormously - a bird’s egg is an example of an exceptionally. The red cells within a watermelon may be seen with the unaided eye, as can the cells within the segments of an orange. Fro the most part cells are small, ranging from about 5 to 20 micrometers in diameter and can thus not be seen without the aid of a microscope.

Cells vary, not only in size and function, but also in their level of specialization. This specialization is based on the absence or presence of a nucleus. Cells without a recognizable nucleus are termed prokaryotes, those with a recognizable nucleus, eukaryotes.

The Evolution of Prokaryotic cells.

prokaryotes

Prokaryote from the Greek: pro = before, karyon + "nut or nucleus.

Some examples of prokaryotes include the bacteria and the cyanobacteria (previously called the blue-green algae). Bacteria and cyanobacteria are typically about ten times smaller than other cells and function very efficiently biochemically, without many of the more complex structures that are present in higher (eukaryotic) cells. Enzymes and other molecules necessary for the cell’s activities are found within the plasmalemma - ribosomes (construction sites for protein synthesis are simply dispersed within the bacteria’s small cell volume, whilst the DNA is concentrated in a structure called the nucleoid. Most bacterial and cyanobacterial cells share another common feature, they are surrounded by a rigid, or semi-rigid cell wall, which is composed of secretions from the cell itself. This wall sometimes has a protective coating on the outside. Until the atmosphere became oxygen-rich, respiration by cellular life was anaerobic (without, or in low O2), As a result, the low efficiency of the respiratory process in terms of energy production, cellular life remained prokaryotic and comparably simple. Molecular oxygen levels increased slowly, due to the photosynthesis by autotrophic organisms in primordial times.

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The Development of Eukaryotic Cells

(Eukaryote from the Greek:- eu + after, karyon + "nut", a true nucleus)

All cells other than the bacteria and cyanobacteria are classed as eukaryotic, due to the presence of a well-defined nucleus, within which DNA is found. These cellular types began to appear as higher atmospheric CO2 concentrations were augmented. These organisms were much more specialized with time, also much more efficient energy utilizers and producers. All higher living plant systems are eukaryotic today, except for bacteria viruses and blue green algae. So as the relatively simple biochemistry of these prokaryotic cells and biochemical processes that they undertook improved and the cellular entities become more efficient, this led to increasing complexity of the cellular structure and its contained substructures. As metabolism become more efficient, so filamentous and then multicellular life form evolved. As examples, the Volvocine line of algae, where some cells deal with mobility, others with reproduction. Here we have the first glimmer of organization in plants, that terminates today, in the higher land plants, where again, some cells are involved in reproduction, others in support and anchorage (roots) others in aerial support (stems) and others as primary energy producers (leaves). Cap this with the need to establish lines of communication, and transport systems and one soon realizes that the modern higher plant, is more than a multicellular system. Indeed, the system involved in rapid intercellular transport (plasmodesmata) whilst small (40-120 nm in tot diameter) is so fine, that plasmodesmata correctly could be termed ‘molecular sieves’, which regulate molecular traffic between adjacent cells within our supracellular organism. Intercellular communication is a particularly taxing research area, involving studies at the electron microscope level, together with studies relating to the movement of substances in living cells at the light microscope level, by injecting fluorescent-labeled molecules of know molecular mass, directly into the living protoplasts. The process itself (the injection) is not new, it is commonly used for dermal (skin infusion) in the pharmaceutics industry, what is new, is the application of this technique to living cells.

Eukaryotic cells cannot be duplicated by artificially sustained chemical process under laboratory conditions because of the great number and complexity of mostly antagonistic chemical reactions which occur within such a restricted space. Many of these reactions are totally incompatible, yet they proceed smoothly. This could never have been the case, unless organelles had evolved which would effectively compartmentalize these reactions. Thus,

Organelles physically separate the different (often incompatible) biochemical reactions within the cytoplasm and - Organelles also separate the different biochemical reactions in time, as molecules being used and produced are passed from one organelle to another.

As an example, the process of photosynthesis requires the presence of chloroplasts, which are capable of capturing light energy, and converting this into a more useful energy form, such as ATP (adenosine triphosphate) it further requires an enzyme-mediated system, which can incorporate CO2 into more useful carbohydrates via the following simplified and unbalanced equations

Chlorophyll + ADP + Pi + sunlight -> ATP (energy) + O2

CO2 + NADPH2 + ATP -> glucose + NADP + ADP + H2O

Respiration on the other hand, require the presence of organelles called mitochondria in order to utilize the carbohydrates formed by the photosynthetic process. Increase in the oxygen concentration also favoured the evolution of aerobic respiratory pathways, and the evolution of the mitochondrion, which resulted in the development of a more efficient process of energy utilization -

CH2ON + O2 + -> CO2 + H2O + energy (as ATP)

Organisms can be classified further, on the basis of their ability (or lack of it) to synthesize their own food requirements from essentially simple organic and inorganic resources. Organisms are thus classified as being either heterotrophic or autotrophic. Interestingly, heterotrophic organisms also tend to be prokaryotic (lacking a nucleus) as well.

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Organelle content and composition in Eukaryotic Cells.

 

 

Organelles in eukaryotic cells

NUCLEUS Control and storage of hereditary material
ENDOPLASMIC RETICULUM Manufacture of proteins, lipid components of cellular membranes, modification of proteins destined for secretion
GOLGI BODIES Protein transport and other substances - involved in secretion and excretion
DICTYOSOMES (in plants) Involved in cell wall synthesis, secretion of substances (including proteins) through the plasmalemma and into the extra-plasmalemma bound continuum (cell wall and intercellular spaces).
LYSOSOMES Degradation, and recycling of basic materials
MITOCHONDRIA Energy synthesis from carbohydrates; ATP formation from ADP
PLASTIDS Structure involved in storage of starch (amyloplasts), pigments (chromoplasts), oil droplets (elaioplasts), colourless substances (leucoplasts), and chlorophyll-containing pigments (chloroplasts).
VACUOLES Membrane-bound (by the tonoplast) - sites of accumulation of ergastic (waste) products of metabolism - often as calcium carbonate crystals as well as pigments (such as anthocyanin, which gives the beetroot its characteristic colour).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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The ability of plant cells to make use of the energy of the sun, in as far as trapping it and its utilization as a form of usable energy was a very great step forward in the evolution of species. A more efficient energy trapping system, would result in a plant being able to compete more easily with others, which were less efficient. Thus the ability to trap this energy in a more useful chemical form rather than simply heat, is a further criterion separating plant from animal cell.

Comparison of Heterotrophs and Autotrophs 

 

 

Heterotrophs

Greek: hetros = different, trophos = feeder

Autotrophs

Greek: autos = self, trophos = feeder

Organism which is dependent on others for an outside supply of organic energy Independent of others for a source of organic energy - self feeders
For example: the fungi (bacteria) today are mostly e.g. many of the higher algae; and
heterotrophs also some blue green algae. Angiosperms and gymnosperms more successful especially those which develop a highly efficient system for the manufacture of food from simple inorganic; organic molecules and sunlight.

 

 

 

 

 

 

 

 

Remember early plants (and for that matter, animals as well) were confined to a watery environment. This was due to the lack of Ozone in the stratosphere, and the resultant high levels of UV-B radiation.

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 The Emergence of Land Plants

The first land plants: Were simple thalloid structures, very like the liverworts today. As greater specialization took place, plants became more complex. Perhaps the most significant change was the fact that less and less of their life cycle was dependent on the presence of a watery matrix. In the Primitive plants, reproduction is either asexual or sexual - the latter phase resulting in the production of motile sperm, requiring H2O medium. Asexual reproduction still takes place by means of spores in these plants. The more advanced plants ultimately evolved a means of protecting the embryo without the necessity of water and hence the development of the seed, in gymnosperms and to more advanced levels, the angiosperms.

Morphological and anatomical specialization's resulted in a change from a prostrate to erect life form, thus evolving and separating these altered plants from their common but more primitive ancestral prokaryotic life forms. In order to achieve a greater autonomy from a water environment the following sequence had to be initiated:

(1) Formation of an efficient rooting system for the transport of water and minerals from the soil to the aerial parts of the organism. In order to achieve this -

(2) An efficient conducting system, consisting of xylem and phloem had to evolve The xylem for the transport of water and minerals from soil: the phloem for carbohydrate transport.

(3) Development of stems, roots and leaf-like structures - which are in themselves specialized organs.

(4) Specialization of reproductive structures, the "seed" in Selaginella - rudimentary covering of the egg cell, followed by the naked seed in Gymnosperms and ultimately, the enclosed seed in Angiosperms.

A similar process of evolution must have taken place in animal cells - from a simple, single-celled organism such as Paramecium, which was entirely dependent on water for its existence, to the large and extremely diverse multicellular organisms of the animal kingdom, each with varying levels of specialization in transport, reproductive, locomotory and sense organs.

Whether the organism is an animal or a plant, clearly, significant changes took place that resulted in the formation first of multicellular organisms, each with their own specialized often multi-functional cell types and tissues, then the supracellular organisms evolved, with highly developed symplasmic and symplasmic (phloem and xylem respectively) transport systems.

It is important though, to remember that presented earlier that all living cells contain basically similar structures. The concept of differentiation into highly specialized cells - often accompanied by anatomical specialization can in most instances, be directly related to increased specialization of the physiological and/or biochemical processes involved in the life of cells within the supracellular organism.

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