Licht an! 1 In der Schule

Hinter den Bergen Zwei Bilder sagen mehr als tausend Worte Wolfgang Ullrich Wer über Erfahrung mit Bildern verfügt, weiss, wie unerwartet und wie stark sie aufeinander reagieren können. So uninteressant ein Bild für sich alleine sein mag, so komisch, zynisch, erhellend oder erhebend wird es oft, wenn man es mit anderen Bildern kombiniert. Ähnliches kennt man sonst am ehesten vom Chemieunterricht: Zuerst harmlose Stoffe entfalten ungeahnte Energien und Effekte, sobald sie zusammengebracht werden. Besonders verblüffend ist immer wieder, was alles durch die Verbindung von nur zwei Bildern passieren kann. Einmal werden sie zum Gegensatzpaar, in einem anderen Fall bestätigen und verstärken sie sich gegenseitig – oder sie variieren ein Thema, erzählen auf knappste Weise eine Geschichte, geben in ihrer Verschiedenheit ein Rätsel auf. In der Geschichte der Kunst, aber ebenso in der Propaganda oder Werbung nutzte man die Aussagekraft von Bildpaaren häufig: vom Diptychon in der christlichen Tradition des Andachtsbilds bis hin zu plakativen Vorher-nachher-Bildern, mit denen ein Haarwuchsmittel angepriesen oder ein kultureller Niedergang beklagt wird. In der Moderne, in der Bilder allgemein verfügbar geworden sind, ist das Kombinieren von Bildern sogar mindestens so wichtig geworden wie das Machen neuer Bilder. Die wohl subtilste Form des Bildpaars taucht aber vor allem in früheren Jahrhunderten auf. Es handelt sich dabei um Pendants. Sie ergeben sich daraus, dass man zwei Bilder zusammenfügt, die ein verwandtes Sujet zeigen, es aber unterschiedlich behandeln, also aus jeweils anderer Perspektive, in verschiedenen Stimmungen oder mit wechselnden Konnotationen darstellen. Die Blütezeit der Pendants war das 18. Jahrhundert, sie wurden aber bereits von Claude Lorrain und auch noch von Caspar David Friedrich virtuos eingesetzt.[1] Viele Pendants stellten aber nicht einmal die Künstler selbst zusammen; vielmehr wurden sie in den Werkstätten und Verkaufsräumen von Druckgraphikern arrangiert. Diese erkannten nämlich, dass sich Kupferstiche oft besser paarweise als alleine verkaufen ließen, weil sie dem Publikum dann mehr Stoff zum Räsonieren boten. So wurde es sogar üblich, Stiche von Werken verschiedener Künstler zu kombinieren, und selbst Bilder, die zu unterschiedlichen Zeiten entstanden waren, ließen sich als Pendants vereint wiederfinden.[2] Der Tiroler Fotograf Lois Hechenblaikner hat diese Praxis nicht nur neu aufgegriffen, sondern sie um eine zusätzliche – entscheidende – Dimension bereichert. In Gegenüberstellungen, an denen er seit dem Jahr 2000 arbeitet, präsentiert er jeweils ein Bild in Schwarz-Weiß – meist ein Foto aus dem reichen Nachlass des Agraringenieurs Armin Kniely (1907-1998) – neben einem Farbbild, das er selbst fotografiert hat. Die unterschiedlichen Entstehungszeiten der Bilder sind damit, anders als bei den Pendants des 18. Jahrhunderts, bereits auf den ersten Blick erkennbar und auch Thema: Lois Hechenblaikner geht es darum, sichtbar zu machen, wie sich eine Region – fast alle Motive stammen aus Tirol – innerhalb von nur zwei Generationen verändert hat. Wo man in den 1930er und selbst noch in den 1960er Jahren Bergbauern bei ihrer mühsamen Feldarbeit aufnehmen sowie ein ritualisiertes Brauchtum dokumentierten konnte, sind mittlerweile der Tourismus und die Formate der Eventkultur am prägendsten geworden. Doch statt nur einen Strukturwandel festzuhalten, besteht Hechenblaikners besondere Fähigkeit darin, in jeder Gegenüberstellung zwei formal analoge Szenerien aufeinander zu beziehen. Erst so werden aus seinen Bildpaaren auch Pendants – zwei Fotos, die trotz aller Unterschiede jeweils genügend Gemeinsames haben, um zum Vergleich herauszufordern. Die Bildpaare gehen bei Hechenblaikner auch immer über das bloß Assoziative hinaus, das in der Kunst sonst beliebt ist, weil das Arrangement dann eine Vieldeutigkeit verheißt und verrätselt wirkt (oft aber nur beliebig ist). Vielmehr empfindet man seine Pendants als geradezu unheimlich – als unglaublich und grotesk –, weil die von ihm entdeckten Ähnlichkeiten so stark sind. Daher überlegt man auch, ob es sich bei den Farbfotos nicht vielleicht um direkte Remakes der Schwarz-weiß-Motive handelt. Doch ist schnell zu erkennen, dass Lois Hechenblaikner seine Bilder nicht eigens inszeniert: Da fast immer Menschen, nicht selten sogar größere Gruppen darauf zu sehen sind, wäre es zu aufwändig, für einen einzelnen Fotografen gar unmöglich, die Szenen zu stellen. Vielmehr ist es Hechenblaikners hervorragendem Bildgedächtnis zu verdanken, wenn er bei seinen Streifzügen durch das Land immer wieder Situationen entdeckt, die denen auf alten Fotos genau entsprechen: Wie einst ein Bauer Jauche auf die Felder spritzte, so schießt man jetzt Schnee aus Kanonen, die Stecken, um die man früher das Heu zum Trocknen wickelte, ähneln verblüffend Handy-Masten, und wo die Landwirte einst stolz ihre Schafe präsentierten, posieren heute Golfspieler mit ihren Trolleys. Die Botschaft der Pendants ist somit eine doppelte: Sie künden sowohl von den gewaltigen Umbrüchen in den Alpen während der letzten Jahrzehnte als auch davon, dass alle Veränderungen doch nur scheinbar sind, ja dass sich lediglich die Hüllen und Akzidenzien gewandelt haben, in denen dieselben Muster geradezu zeitlos zur Geltung kommen. Man könnte darin eine Bestätigung für das sehen, was Aby Warburg als Pathosformeln bezeichnet hat, nämlich Gesten und Konstellationen, die über Epochen und sogar über Kulturen hinweg immer wieder auftauchen und die damit den Status anthropologischer Konstanten besitzen. Wollte Warburg diese Formeln mithilfe eines Atlasses erforschen, der Bilder unterschiedlichster Herkunft mit jeweils verwandten Sujets auf Tableaus versammelte[3], so läßt sich das vorliegende Buch von Lois Hechenblaikner als Tiroler Variante eines ähnlichen Projekts würdigen. Doch während Warburg, getrieben von metaphysischem Interesse und auf der Suche nach so etwas wie Archetypen menschlicher Existenz, auf den einzelnen Tafeln seines Atlasses weit ausholt und auch Bilder zusammenbringt, die zumindest auf den ersten Blick nur wenig gemeinsam haben, ist Hechenblaikners Arbeit ungleich konzentrierter. Da er immer nur Bildpaare zeigt, denen zudem allen dasselbe Schema zugrunde liegt, wird der Betrachter viel stärker geführt. Jede Doppelseite gerät bei ihm zu einer neuen Pointe. Doch verhindert die Form des Pendants, dass mit dem Erfassen der Pointe schon alles vorbei ist. Im Gegenteil fordert sie zur weiteren Reflexion heraus, und das Spiel von Identität und Differenz, das Lois Hechenblaikner mit jeder Gegenüberstellung auf die Spitze treibt, eröffnet dafür den Raum. Viel mehr als einzelne Bilder, mehr aber auch als andere Formen von Bildpaaren verlangen Pendants sogar ausdrücklich aktive Rezipienten, die diesen Raum mit ihren Gedanken füllen. Sie können dann entweder darüber sinnieren, wieso sich zwei vermeintlich so unterschiedliche Lebenswelten wie Landwirtschaft und Tourismus, Religion und Sport, Brauchtum und Kommerz in so ähnlichen Bildformeln ausdrücken, oder sie können der Überlegung nachgehen, wie sich eine Region innerhalb kurzer Zeit so stark verändern konnte, dass alle Tätigkeiten und Situationen komplett durch andere Tätigkeiten und Situationen ersetzt wurden. Ist also einmal die Ähnlichkeit des Unterschiedlichen das Faszinosum, so das andere Mal die Andersheit des Gleichen. Jeweils aber muss der Rezipient beides – Identität und Differenz – zusammendenken. Er kann sich dabei auf ein einzelnes Beispiel konzentrieren und Überlegungen zur jeweiligen Rolle etwa von Schafen und Trolleys – zu ihrer Eignung als Symbole für eine Lebensform – anstellen. Er kann sich aber auch von der Summe der Bildpaare anregen lassen und so zu Theoriebildung animiert werden, um für das ihm Dargebotene eine Erklärung zu finden. In einem Text spricht Lois Hechenblaikner selbst davon, durch die Bildpaare solle „im Kopf des Betrachters ein so starkes drittes Bild entstehen, dass ihm förmlich ‚das Licht aufgeht’“. Doch sind es – je nach Interesse, Haltung und Vorbildung – durchaus unterschiedliche Lichter, die aufgehen können. Während Hechenblaikner die Gegenüberstellungen zuerst mit dem Ziel anlegte, die Veränderung Tirols und des Alpenraumes als Verlustgeschichte zu erzählen, ist es mit ihnen genauso möglich, die verschwundene Welt nachträglich zu dekonstruieren. Läßt der Wechsel vom Schwarz-weiß zur Farbe einerseits den Schluss zu, dass sich in der Gegenwart eine brutale Marktlogik durchgesetzt hat, derzufolge alles mit möglichst starken Reizen – mit Grellheit und Obszönität – auf sich aufmerksam macht, so kann man daraus andererseits also auch die Mahnung ableiten, auf das Vergangene nicht nur deshalb sentimental zu blicken, weil es durch die einheitliche Tönung verfremdet erscheint. Obwohl das Leben der Bergbauern, ihre familiäre Bindung, ihr Gottvertrauen und ihre dem Rhythmus der Jahreszeiten gehorchende Arbeit idyllisch wirken mag, könnte das alles vielmehr ähnlich – genauso? – derb, banal und kalt gewesen sein wie die heutige von Unterhaltungsindustrie, Animiergewerbe und Konsum bestimmte Welt des Massentourismus. Oder muss es sich doch eindeutig um einen Verfall handeln, wenn allenthalben natürliche Materialien durch Kunststoffe ausgetauscht, freie Blicke durch Werbetafeln verstellt, beschauliche Szenerien von Massenevents abgelöst, Ernst und Strenge durch Albernheit und Ausgelassenheit ersetzt wurden? Aber ist es nicht auch ein Zeichen von Freiheit und Wohlstand – und damit sogar ein Fortschritt –, wenn die Menschen ihre Launen heute ausleben können und nicht länger ein enges, allein von Armut und Notwendigkeiten bestimmtes Leben führen müssen? Selbst wenn man klar für eine Deutung optiert und sich entweder als Kulturpessimist oder als Fürsprecher einer grundsätzlichen Gleichwertigkeit verschiedener Epochen oder aber als Fortschrittseuphoriker bekennt, kann man die jeweils alternativen Interpretationen wohl nie ganz ausblenden. Je besser ein Bildpaar ist, desto stärker sind vielmehr zugleich die jeweils anderen Sichtweisen präsent. Es gehört sogar geradezu zum Wesen von Pendants (wie ihr Name, abgeleitet vom lateinischen ‚pendere‘, bereits verrät), zu einem Abwägen zu verleiten und feste Positionen in die Schwebe zu bringen. Pendants sind also die ideale Bildform für Skeptiker, die sich ungern festlegen und immer gerne mindestens eine zweite Sichtweise parat haben. Zwei Bilder, so wie Lois Hechenblaikner sie aufeinander bezieht, sagen daher tatsächlich mehr als tausend Worte, mit denen nämlich jeweils nur eine einzige Position formuliert werden kann: Satz und Gegensatz in sich vereinend, verführen Pendants zum philosophischen Denken – zu einem nie abschließbaren Nachdenken über Geschichte, Menschen, Kultur. Das Schwanken zum Prinzip erhebend, sind sie die besten Stimulanzien des Geistes.

CHARACTERISTICS OF LIFE The world is filled with familiar objects, such as tables, rocks, plants, pets, and automobiles. Which of these objects are living or were once living? What are the criteria for assigning something to the living world or the nonliving world? Biologists have established that living things share seven characteristics of life. These characteristics are organization and the presence of one or more cells, response to a stimulus (plural, stimuli), homeostasis, metabolism, growth and development, reproduction, and change through time. Organization and Cells Organization is the high degree of order within an organism’s internal and external parts and in its interactions with the living world. For example, compare an owl to a rock. The rock has a spe- cific shape, but that shape is usually irregular. Furthermore, differ- ent rocks, even rocks of the same type, are likely to have different shapes and sizes. In contrast, the owl is an amazingly organized individual, as shown in Figure 1-2. Owls of the same species have the same body parts arranged in nearly the same way and interact with the environment in the same way. Copyright © by Holt, Rinehart and Winston. All rights reserved. ORGANISM (Barn Owl) ORGAN (Owl’s Ear) TISSUE (Nervous Tissue Within the Ear) CELL (Nerve Cell) Every living organism has a level of organization. The different levels of organization for a complex multicellular organism, such as an owl, are shown in the figure below. FIGURE 1-2 THE SCIENCE OF LIFE 7 All living organisms, whether made up of one cell or many cells, have some degree of organization. A cell is the smallest unit that can perform all life’s processes. Some organisms, such as bacteria, are made up of one cell and are called unicellular (YOON-uh-SEL-yoo-luhr) organisms. Other organisms, such as humans or trees, are made up of multiple cells and are called multicellular (MUHL-ti-SEL-yoo-luhr) organisms. Complex multicellular organisms have the level of orga- nization shown in Figure 1-2. In the highest level, the organism is made up of organ systems, or groups of specialized parts that carry out a certain function in the organism. For example, an owl’s ner- vous system is made up of a brain, sense organs, nerve cells, and other parts that sense and respond to the owl’s surroundings. Organ systems are made up of organs. Organs are structures that carry out specialized jobs within an organ system. An owl’s ear is an organ that allows the owl to hear. All organs are made up of tissues. Tissues are groups of cells that have similar abilities and that allow the organ to function. For example, nervous tissue in the ear allows the ear to detect sound. Tissues are made up of cells. A cell must be covered by a membrane, contain all genetic information necessary for replication, and be able to carry out all cell functions. Within each cell are organelles. Organelles are tiny structures that carry out functions necessary for the cell to stay alive. Organelles contain biological molecules, the chemical compounds that provide physical structure and that bring about movement, energy use, and other cellular functions. All biological molecules are made up of atoms. Atoms are the simplest particle of an ele- ment that retains all the properties of a certain element. Response to Stimuli Another characteristic of life is that an organism can respond to a stimulus—a physical or chemical change in the internal or external environment. For example, an owl dilates its pupils to keep the level of light entering the eye constant. Organisms must be able to respond and react to changes in their environment to stay alive. ORGANELLE (Mitochondrion) BIOLOGICAL MOLECULE (Phospholipid) ATOM (Oxygen) cell from the Latin, cella meaning “small room,” or “hut” Word Roots and Origins www.scilinks.org Topic: Characteristics of Life Keyword: HM60257 mb06se_bios01.qxd 5/18/07 10:37 AM Page 7 8 CHAPTER 1 Homeostasis All living things, from single cells to entire organisms, have mecha- nisms that allow them to maintain stable internal conditions. Without these mechanisms, organisms can die. For example, a cell’s water content is closely controlled by the taking in or releas- ing of water. A cell that takes in too much water will rupture and die. A cell that doesn’t get enough water will also shrivel and die. Homeostasis (HOH-mee-OH-STAY-sis) is the maintenance of a stable level of internal conditions even though environmental conditions are constantly changing. Organisms have regulatory systems that maintain internal conditions, such as temperature, water content, and uptake of nutrients by the cell. In fact, multi- cellular organisms usually have more than one way of maintain- ing important aspects of their internal environment. For example, an owl’s temperature is maintained at about 40°C (104°F). To keep a constant temperature, an owl’s cells burn fuel to produce body heat. In addition, an owl’s feathers can fluff up in cold weather. In this way, they trap an insulating layer of air next to the bird’s body to maintain its body temperature. Metabolism Living organisms use energy to power all the life processes, such as repair, movement, and growth. This energy use depends on metabolism (muh-TAB-uh-LIZ-uhm). Metabolism is the sum of all the chemical reactions that take in and transform energy and materials from the environment. For example, plants, algae, and some bacteria use the sun’s energy to generate sugar molecules during a process called photosynthesis. Some organisms depend on obtaining food energy from other organisms. For instance, an owl’s metabolism allows the owl to extract and modify the chemi- cals trapped in its nightly prey and use them as energy to fuel activities and growth. Growth and Development All living things grow and increase in size. Some nonliving things, such as crystals or icicles, grow by accumulating more of the same material of which they are made. In contrast, the growth of living things results from the division and enlargement of cells. Cell division is the formation of two new cells from an existing cell, as shown in Figure 1-3. In unicellular organisms, the primary change that occurs following cell division is cell enlargement. In multi- cellular life, however, organisms mature through cell division, cell enlargement, and development. Development is the process by which an organism becomes a mature adult. Development involves cell division and cell differen- tiation, or specialization. As a result of development, an adult organism is composed of many cells specialized for different func- tions, such as carrying oxygen in the blood or hearing. In fact, the human body is composed of trillions of specialized cells, all of which originated from a single cell, the fertilized egg. This unicellular organism, Escherichia coli, inhabits the human intestines. E. coli reproduces by means of cell division, during which the original cell splits into two identical offspring cells. FIGURE 1-3 Observing Homeostasis Materials 500 mL beakers (3), wax pen, tap water, thermometer, ice, hot water, goldfish, small dip net, watch or clock with a second hand Procedure 1. Use a wax pen to label three 500 mL beakers as follows: 27°C (80°F), 20°C (68°F), 10°C (50°F). Put 250 mL of tap water in each beaker. Use hot water or ice to adjust the tem- perature of the water in each beaker to match the temperature on the label. 2. Put the goldfish in the beaker of 27°C water. Record the number of times the gills move in 1 minute. 3. Move the goldfish to the beaker of 20°C water. Repeat observations. Move the goldfish to the beaker of 10°C. Repeat observations. Analysis What happens to the rate at which gills move when the temp- erature changes? Why? How do gills help fish maintain homeostasis? Quick Lab mb06se_bios01.qxd 5/18/07 10:37 AM Page 8 THE SCIENCE OF LIFE 9 Reproduction All organisms produce new organisms like themselves in a process called reproduction. Reproduction, unlike other characteristics, is not essential to the survival of an individual organism. However, because no organism lives forever, reproduction is essential for the continuation of a species. Glass frogs, as shown in Figure 1-4, lay many eggs in their lifetime. However, only a few of the frogs’ off- spring reach adulthood and successfully reproduce. During reproduction, organisms transmit hereditary informa- tion to their offspring. Hereditary information is encoded in a large molecule called deoxyribonucleic acid, or DNA. A short segment of DNA that contains the instructions for a single trait of an organism is called a gene. DNA is like a large library. It contains all the books—genes—that the cell will ever need for making all the struc- tures and chemicals necessary for life. Hereditary information is transferred to offspring during two kinds of reproduction. In sexual reproduction, hereditary information recombines from two organisms of the same species. The resulting offspring are similar but not identical to their parents. For example, a male frog’s sperm can fertilize a female’s egg and form a single fer- tilized egg cell. The fertilized egg then develops into a new frog. In asexual reproduction, hereditary information from different organisms is not combined; thus the original organism and the new organism are genetically the same. A bacterium, for example, reproduces asexually when it splits into two identical cells. Change Through Time Although individual organisms experience many changes during their lifetime, their basic genetic characteristics do not change. However, populations of living organisms evolve or change through time. The ability of populations of organisms to change over time is important for survival in a changing world. This factor is also impor- tant in explaining the diversity of life-forms we see on Earth today.

All living things are made up of one or more cells. A cell is the smallest unit that can carry on all of the processes of life. Beginning in the 17th century, curious naturalists were able to use microscopes to study objects too small to be seen with the unaided eye. Their studies led them to propose the cellular basis of life. Hooke In 1665, English scientist Robert Hooke studied nature by using an early light microscope, such as the one in Figure 4-1a. A light micro- scope is an instrument that uses optical lenses to magnify objects by bending light rays. Hooke looked at a thin slice of cork from the bark of a cork oak tree. “I could exceedingly plainly perceive it to be all perforated and porous,” Hooke wrote. He described “a great many little boxes” that reminded him of the cubicles or “cells” where monks live. When Hooke focused his microscope on the cells of tree stems, roots, and ferns, he found that each had similar little boxes. The drawings that Hooke made of the cells he saw are shown in Figure 4-1b. The “little boxes” that Hooke observed were the remains of dead plant cells, such as the cork cells shown in Figure 4-1c. SECTION 1 OBJECTIVES ● Name the scientists who first observed living and nonliving cells. ● Summarize the research that led to the development of the cell theory. ● State the three principles of the cell theory. ● Explain why the cell is considered to be the basic unit of life. VOCABULARY cell cell theory Robert Hooke used an early microscope (a) to see cells in thin slices of cork. His drawings of what he saw (b) indicate that he had clearly observed the remains of cork cells (300) (c). FIGURE 4-1 (a) (b) (c) Copyright © by Holt, Rinehart and Winston. All rights reserved. 70 CHAPTER 4 Leeuwenhoek The first person to observe living cells was a Dutch trader named Anton van Leeuwenhoek. Leeuwenhoek made microscopes that were simple and tiny, but he ground lenses so precisely that the magnification was 10 times that of Hooke’s instruments. In 1673, Leeuwenhoek, shown in Figure 4-2a, was able to observe a previ- ously unseen world of microorganisms. He observed cells with green stripes from an alga of the genus Spirogyra, as shown in Figure 4-2b, and bell-shaped cells on stalks of a protist of the genus Vorticella, as shown in Figure 4-2c. Leeuwenhoek called these organisms animalcules. We now call them protists. THE CELL THEORY Although Hooke and Leeuwenhoek were the first to report observ- ing cells, the importance of this observation was not realized until about 150 years later. At this time, biologists began to organize information about cells into a unified understanding. In 1838, the German botanist Matthias Schleiden concluded that all plants were composed of cells. The next year, the German zoologist Theodor Schwann concluded the same thing for animals. And finally, in his study of human diseases, the German physician Rudolf Virchow (1821–1902) noted that all cells come from other cells. These three observations were combined to form a basic theory about the cel- lular nature of life. The cell theory has three essential parts, which are summarized in Table 4-1. Anton van Leeuwenhoek (1632–1723) is shown here with one of his hand-held lenses (a). Leeuwenhoek observed an alga of the genus Spirogyra (b) and a protist of the genus Vorticella (c). FIGURE 4-2 TABLE 4-1 The Cell Theory All living organisms are composed of one or more cells. Cells are the basic units of structure and function in an organism. Cells come only from the reproduction of existing cells. (a) (b) (c) www.scilinks.org Topic: Cell Theory Keyword: HM60241 mb06se_csfs01.qxd 5/18/07 10:54 AM Page 70

What is a Plant Cell? Plant cells are eukaryotic cells that vary in several fundamental factors from other eukaryotic organisms. Both plant and animal cells contain a nucleus along with similar organelles. One of the distinctive aspects of a plant cell is the presence of a cell wall outside the cell membrane. Plant Cell Structure Just like different organs within the body, plant cell structure includes various components known as cell organelles that perform different functions to sustain itself. These organelles include: Cell Wall It is a rigid layer which is composed of polysaccharides cellulose, pectin and hemicellulose. It is located outside the cell membrane. It also comprises glycoproteins and polymers such as lignin, cutin, or suberin. The primary function of the cell wall is to protect and provide structural support to the cell. The plant cell wall is also involved in protecting the cell against mechanical stress and providing form and structure to the cell. It also filters the molecules passing in and out of it. The formation of the cell wall is guided by microtubules. It consists of three layers, namely, primary, secondary and the middle lamella. The primary cell wall is formed by cellulose laid down by enzymes. Cell membrane It is the semi-permeable membrane that is present within the cell wall. It is composed of a thin layer of protein and fat. The cell membrane plays an important role in regulating the entry and exit of specific substances within the cell. For instance, cell membrane keeps toxins from entering inside, while nutrients and essential minerals are transported across. Nucleus The nucleus is a membrane-bound structure that is present only in eukaryotic cells. The vital function of a nucleus is to store DNA or hereditary information required for cell division, metabolism and growth. 1. Nucleolus: It manufactures cells’ protein-producing structures and ribosomes. 2. Nucleopore: Nuclear membrane is perforated with holes called nucleopore that allow proteins and nucleic acids to pass through. Plastids They are membrane-bound organelles that have their own DNA. They are necessary to store starch and to carry out the process of photosynthesis. It is also used in the synthesis of many molecules, which form the building blocks of the cell. Some of the vital types of plastids and their functions are stated below: Leucoplasts They are found in the non-photosynthetic tissue of plants. They are used for the storage of protein, lipid and starch. Chromoplasts They are heterogeneous, colored plastid which is responsible for pigment synthesis and for storage in photosynthetic eukaryotic organisms. Chromoplasts have red-, orange- and yellow-colored pigments which provide color to all ripe fruits and flowers. Central Vacuole It occupies around 30% of the cell’s volume in a mature plant cell. Tonoplast is a membrane that surrounds the central vacuole. The vital function of the central vacuole apart from storage is to sustain turgor pressure against the cell wall. The central vacuole consists of cell sap. It is a mixture of salts, enzymes and other substances. Golgi Apparatus They are found in all eukaryotic cells, which are involved in distributing synthesized macromolecules to various parts of the cell. Ribosomes They are the smallest membrane-bound organelles which comprise RNA and protein. They are the sites for protein synthesis, hence, also referred to as the protein factories of the cell. Mitochondria They are the double-membraned organelles found in the cytoplasm of all eukaryotic cells. They provide energy by breaking down carbohydrate and sugar molecules, hence they are also referred to as the “Powerhouse of the cell.” Lysosome Lysosomes are called suicidal bags as they hold digestive enzymes in an enclosed membrane. They perform the function of cellular waste disposal by digesting worn-out organelles, food particles and foreign bodies in the cell. In plants, the role of lysosomes is undertaken by the vacuoles. Chloroplasts It is an elongated organelle enclosed by phospholipid membrane. The chloroplast is shaped like a disc and the stroma is the fluid within the chloroplast that comprises a circular DNA. Each chloroplast contains a green colored pigment called chlorophyll required for the process of photosynthesis. The chlorophyll absorbs light energy from the sun and uses it to transform carbon dioxide and water into glucose. Structure of Chloroplast Chloroplasts are found in all higher plants. It is oval or biconvex, found within the mesophyll of the plant cell. The size of the chloroplast usually varies between 4-6 µm in diameter and 1-3 µm in thickness. They are double-membrane organelle with the presence of outer, inner and intermembrane space. There are two distinct regions present inside a chloroplast known as the grana and stroma. • Grana are made up of stacks of disc-shaped structures known as thylakoids or lamellae. The granum of the chloroplast consists of chlorophyll pigments and are the functional units of chloroplasts. • Stroma is the homogenous matrix which contains grana and is similar to the cytoplasm in cells in which all the organelles are embedded. Stroma also contains various enzymes, DNA, ribosomes, and other substances. Stroma lamellae function by connecting the stacks of thylakoid sacs or grana. The chloroplast structure consists of the following parts: Membrane Envelope It comprises inner and outer lipid bilayer membranes. The inner membrane separates the stroma from the intermembrane space. Intermembrane Space The space between inner and outer membranes. Thylakoid System (Lamellae) The system is suspended in the stroma. It is a collection of membranous sacs called thylakoids or lamellae. The green colored pigments called chlorophyll are found in the thylakoid membranes. It is the sight for the process of light-dependent reactions of the photosynthesis process. The thylakoids are arranged in stacks known as grana and each granum contains around 10-20 thylakoids. Stroma It is a colorless, alkaline, aqueous, protein-rich fluid present within the inner membrane of the chloroplast present surrounding the grana. Grana Stack of lamellae in plastids is known as grana. These are the sites of conversion of light energy into chemical energy. Chlorophyll It is a green photosynthetic pigment that helps in the process of photosynthesis. Functions of Chloroplast Following are the important chloroplast functions: • The most important function of the chloroplast is to synthesize food by the process of photosynthesis. • Absorbs light energy and converts it into chemical energy. • Chloroplast has a structure called chlorophyll which functions by trapping the solar energy and is used for the synthesis of food in all green plants. • Produces NADPH and molecular oxygen (O 2 ) by photolysis of water. • Produces ATP – Adenosine triphosphate by the process of photosynthesis. • The carbon dioxide (CO2) obtained from the air is used to generate carbon and sugar during the Calvin Cycle or dark reaction of photosynthesis. Mitochondria “Mitochondria are membrane-bound organelles present in the cytoplasm of all eukaryotic cells, that produce adenosine triphosphate (ATP), the main energy molecule used by the cell.” What are Mitochondria? Popularly known as the “Powerhouse of the cell,” mitochondria (singular: mitochondrion) are a double membrane-bound organelle found in most eukaryotic organisms. They are found inside the cytoplasm and essentially function as the cell’s “digestive system.” They play a major role in breaking down nutrients and generating energy-rich molecules for the cell. Many of the biochemical reactions involved in cellular respiration take place within the mitochondria. The term ‘mitochondrion’ is derived from the Greek words “mitos” and “chondrion” which means “thread” and “granules-like”, respectively. It was first described by a German pathologist named Richard Altmann in the year 1890. Structure of Mitochondria • The mitochondrion is a double-membraned, rod-shaped structure found in both plant and animal cell. • Its size ranges from 0.5 to 1.0 micrometers in diameter. • The structure comprises an outer membrane, an inner membrane, and a gel-like material called the matrix. • The outer membrane and the inner membrane are made of proteins and phospholipid layers separated by the intermembrane space. • The outer membrane covers the surface of the mitochondrion and has a large number of special proteins known as porins. Cristae The inner membrane of mitochondria is rather complex in structure. It has many folds that form a layered structure called cristae, and this helps in increasing the surface area inside the organelle. The cristae and the proteins of the inner membrane aid in the production of ATP molecules. The inner mitochondrial membrane is strictly permeable only to oxygen and ATP molecules. A number of chemical reactions take place within the inner membrane of mitochondria. Mitochondrial Matrix The mitochondrial matrix is a viscous fluid that contains a mixture of enzymes and proteins. It also comprises ribosomes, inorganic ions, mitochondrial DNA, nucleotide cofactors, and organic molecules. The enzymes present in the matrix play an important role in the synthesis of ATP molecules. Functions of Mitochondria The most important function of mitochondria is to produce energy through the process of oxidative phosphorylation. It is also involved in the following process: 1. Regulates the metabolic activity of the cell 2. Promotes the growth of new cells and cell multiplication 3. Helps in detoxifying ammonia in the liver cells 4. Plays an important role in apoptosis or programmed cell death 5. Responsible for building certain parts of the blood and various hormones like testosterone and estrogen 6. Helps in maintaining an adequate concentration of calcium ions within the compartments of the cell 7. It is also involved in various cellular activities like cellular differentiation, cell signaling, cell senescence, controlling the cell cycle and in cell growth. Disorders Associated with Mitochondria Any irregularity in the way mitochondria function can directly affect human health, but often, it is difficult to identify because symptoms differ from person to person. Disorders of the mitochondria can be quite severe; in some cases, they can even cause an organ to fail.

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