lecture2

Physical basis of heredity: Structure and function of cell and cell organelles

Physical basis of Heredity:

     Mendel had no knowledge of chromosome or genes and he was able to postulate that the inheritance in particulate and that the elements of factors controls the particular character and is transmitted from one generation to the next. His conclusions also gave the fact that;

• Each of the two parents has two elements for a character and

•  Only one these transmitted to the next generation through gametes.

     In 1900, Sutton studies the chromosomal behavior during meiosis and found that likeness between segregation of Mendel’s factor determines during gametogenesis. It was therefore concluded that chromosomes are the carriers of heredity particles and the Mendel’s factor are physically located in the chromosomes. In other words, he suggested that, the chromosomes constitute the physical basis of heredity. Johanson applied the term ‘gene’ to represent the hereditary factors. There are handed down from parent to progeny through successive generation.

Structure and function of cell organelles:

• Cell is the basic unit of organization or structure of all living matter. It was first discovered by Robert Hook in 1665 in cork tissue. Loewy and Sickevitz in 1963, defined a cell as “a unit of biological activity de-limited by a semi-permeable membrane and capable of self-reproduction in a medium free of other living system”.

• The cell has also been defined as “a unit of life that is the smallest unit which can carry on the activities indispensable to life to grow, to synthesize new living material and to produce new cells”.

• The cell theory or cell doctrine was formulated by independently by M.J. Schleiden and Theodor Schwann in 1838-39. It states that the animals and plants differ from each other superficially. But they have same pattern of organization and construction.

• It further states that the bodies of both animals and plants are composed of cells and that each cell can not only act independently but also function as an integral part of complete organism.

• Thus, the cell is considered as morphological and physiological unit of living organisms or in words of Schwann and Schleiden as “functional and biological unit”. Cells reproduce, assimilate, respire, respond to changes in the environment and absorb water and other materials from internal or external courses.

• Purkinje in 1840 coined the term protoplasm. (Greek words, protos = first; plasm = organization) for the juicy living substance of animals.

• The protoplasm theory states that all living matter, out of which animals and plants are formed is protoplasm. Further, the cell is an accumulation of living substance or protoplasm, which is limited in space by another membrane and possesses a nucleus.

• The protoplasm occurs everywhere in the cell i.e. the plasma membrane, the nucleus and the portion in between the plasma membrane and the nucleus. The portion of the protoplasm which occurs between the plasma membrane and the nucleus is named as cytoplasm and the portion of the protoplasm occurring in the nucleus is named as nucleoplasm.

Shape:

• The plant and animal cells exhibit various forms and shapes. But the shape of the cell may be irregular, triangular, tubular, cuboidal, polygonal, cylindrical, oval, rounded or elongated in different animals and vary from organ to organ.

• Even the cells of the same organ may display variations in the shape. Generally, the shape of the cell remains correlated with its functions. For example, the epithelial cells have flat shape and the muscle cells are elongated. Moreover, external or internal environment may also cause shape variations in the cell due to internal or mechanical stress or pressure, surface tension etc.

Size:

      Mostly the eukaryotic cells are microscopic in size, but definitely they are larger in size than the bacterial cells. The size of cells varies from 1µ to 175 mm. The ostrich egg cell is usually considered as largest cell (with 175 mm diameter). But certain longest nurve cells have been found to have a length of 3 to 3.5 feet.

Number:

      The body of unicellular or acellular organisms (Protozoa and Protophyta) consists of single cell. Most of the animals and plants are multicellular and may have many cells. The number of cells in the multicellular organisms usually remains correlated with the size of the organisms. Small-sized organisms have less number of cells in comparison to large-sized organisms. The differences between Animal and Plant Cell are as follows:

Cell wall: 

• In plants (including bacteria) a cell is always surrounded by a cell wall lined throughout with plasma lemma. The cell wall is found in plants and is absent in animals. In case of animal cells, the outermost layer of cell is plasma lemma, which is also occasionally called ‘cell membrane’ or ‘plasma membrane’.

• Cell wall is the outermost part of the cell and is always non-living, though produced and maintained by living protoplasm. It is a rigid structure and protects the inner parts of a cell. It maintains the shape of the cell and provides mechanical support to the tissues. It originates from the phragmoplast (phragma = fence, separation).

• Endoplasmic reticulum, Golgi complex, mitochondria and microtubules play an important role in the formation of the cell wall. It is mainly composed of cellulose. However, it may also contain hemicellulose, pectin, chitin, cutin and lignin. The composition of these substances varies from cell to cell. The cell wall is complex in nature and is differentiated into middle lamella, primary cell wall and secondary cell wall.

1. Middle lamella: It is the outmost layer of plant cell wall and connects the two adjacent cells. It is composed of calcium and magnesium pectate and does not contain any cellulose. Some consider middle lamella as intercellular substance or intercellular matrix.

2. Primary cell wall: It is thin, elastic and lies between middle lamella and secondary cell wall. It is mainly composed of cellulose. It develops after middle lamella by deposition of hemicellulose, cellulose and pectin substances.

           3. Secondary cell wall: It is the inner most layer of cell wall and lies between primary cell wall and plasma membrane. It is relatively thick and is primarily composed of microfibrils of cellulose. In some tissues, besides cellulose, lignin and suberin are also found in the secondary cell wall. The cell wall has minute apertures through which the cells of a tissue are interconnected. These apertures of cell wall are known as plasmodesmata.

They are also referred to as canals of the cell wall. The main functions of cell wall are

• It determines the shape and size of a cell

• It provides protection to the inner parts of a cell from the attack by pathogens.

• It provides mechanical support to the tissues and act as a skeletal framework of plants.

• It helps in transport of substances between two cells.

Plasma lemma or plasma membrane: 

• The term was coined by J.Q. Plower in 1931. This membrane is present just beneath the cell wall in plant cells, while it is the outer membrane in animal cell. In plants, it lies between the cytoplasm and the cell wall.

• It is a living, ultra-thin, elastic, porous, semi-permeable membrane covering of cell. The plasma membrane is about 75-100 angstroms thick. In most of the cells, it is trilaminar (three layered) and made up of protein and lipids. The outer protein layer is 25 angstroms thick; the middle lipid layer is 25 to 30 angstroms thick and the inner protein layer is 25 to 30 angstroms thick. The three-layered protein-lipid-protein membrane is called a unit membrane.

• The outer and inner layers are made up of proteins and the middle layer is made up of lipids. Structure can be best explained by fluid mosaic theory. It is found to contain many pores through which exchange of molecules may occur.

The main functions of plasma membrane are:

• Primarily the plasma membrane provides mechanical support and external form to the protoplasm (cytoplasm and nucleus) and it also delimits the protoplasm from the exterior.

• It checks the entry and exit of undesirable substances.

• Due to its semi permeability, it transmits necessary materials to and from the cell (selective permeability).

• Moreover, it permits only one way passage for molecules like minerals into the cell and restricts their outward movement.

Cytoplasm:

The plasma membrane is followed by cytoplasm which is distinguished into (a) Cytoplasmic matrix / hyaloplasm and (b) Cytoplasmic structures

   a)     Cytoplasmic matrix:

• The space between the plasma membrane and the nucleus is filled by amorphous, translucent, homogeneous colloidal liquid known as hyaloplasm or cytoplasmic matrix. The portion of cytoplasm other than cell organelles is known as hyaloplasm. When the cell is active, the cytoplasm is in fluid state.

• The cytoplasm is in gel condition, when the cell is dormant. The cytoplasmic matrix consists of various inorganic molecules such as water, salts of sodium and other metals and various organic compounds viz., carbohydrates, lipids, nucleoproteins, nucleic acids (RNA and DNA) and variety of enzymes.

• The peripheral layer of cytoplasmic matrix is relatively nongranular, viscous, clear and rigid and is known as ectoplasm. The inner portion of cytoplasmic matrix is granular, less viscous and is known as endoplasm.

   b)     Cytoplasmic structures:

• In the cytoplasmic matrix certain non-living and living structures remain suspended. The living structures or cytoplasmic organoids are membrane bound and are called organelles or organoids.

• These living structures include plastids, mitochondria, endoplasmic reticulum, Golgi complex, lysosomes, ribosomes, microtubules, microfilaments, centrosome, basal granules, sphaerosomes, microbodies, cilia and flagella etc.

• The non-living structures or cytoplasmic inclusions called paraplasm or deutoplasm include ergastic substances, crystals, fats, oil droplets, starch granules glycogen granules, vacuole etc.

Nucleus:

• Robert Brown first observed a cell nucleus in flowering plants in 1837. Generally, a cell contains single nucleus. However, there are a number of exceptions in which more than one nucleus is present. Plant cells with more than one nucleus are called coenocytes. E.g.: Certain algae, fungi, Vaucharia, Rhizopus, whereas animal cells with this character are called syncytia. E.g.: striated muscle cells of higher animals. 

• The position of the nucleus in the cell varies according to cell type, although it is often in the centre of the cell. The nucleus is surrounded on all sides by cytoplasm from which it is separated by the nuclear envelope or nuclear membrane.

Morphology: The shape of the nucleus varies according to the species or cell type. The range of variation is limited, although in addition to the common spherical nuclei, ellipsoid or flattened nuclei occur. In majority of cells, the margin of the nucleus is quite regular, but some cells like leukocytes contain nuclei with lobes or infoldings of the margins. Nuclear size is a function of chromosome number. Size of the nucleus varies with ploidy level. The size of the nucleus is also correlated with the DNA content. Variation in the nuclear size is observed at different times during the cycle of cellular activities. The nucleus includes:

• Nuclear envelop / membrane,

• Nucleoplasm or karyoplasm,

• Nucleolus

• Chromatin

   1. Nuclear envelop / nuclear membrane: It is a double membrane, semipermeable structure broken at numerous intervals by pores or openings. Under light microscope, it appears as a thin line between nucleus and cytoplasm. The space between the inner and outer membrane is known as the perinuclear space. In many places the nuclear membrane joins the membrane of endoplasmic reticulum. The main function of nuclear membrane is to provide a pathway for the transport of materials between the nucleus and cytoplasm

   2. Nucleoplasm / Karyolymph: It is a fluid substance which escapes, if the nucleus is punctured. It fills the nuclear space around the chromosomes and the nucleolus. The karyolymph is composed primarily of protein materials and is rich in acidic proteins and RNA rich in bases, adenine and uracil. It is the site of certain enzymes in the nucleus.

   3. Nucleolus:

• Fontana first described the nucleolus in 1871. it is a relatively large, generally spherical body present within the nucleus. The number of nucleoli present in each nucleus depends upon the species and the number of the chromosomes or sets of chromosomes.

• In many plant and animal cells there is one nucleolus for each haploid set of chromosomes. Heterochromatic portions of specific chromosomes are found to be in contact with the nucleolus during interphase. These are called nucleolar organizing regions of the chromosomes and are responsible for producing much of nucleolar RNA.

• Generally, the nucleolus disappears during cell division and reappears in daughter cells at the end of cell division in each daughter nucleus. However, a persistent nucleolus is found to present in Spirogyra and Euglena. The important functions of nucleolus are formation of ribosomes and synthesis of RNA.

   4. Chromatin: The nucleus contains a darkly stained material called chromatin (Greek word, chromatin = colour), which is a combination of DNA, histone and other proteins that make up chromosomes. During interphase, the chromatin material is organized into a number of long, loosely coiled, irregular strands or threads called chromatin reticulum. When the cell begins to divide, the chromatin bodies condense to form shorter and thicker threads, which were termed chromosomes (Greek word, soma = body) by W. Waldeyer. The main functions of chromatin are

• To package DNA into a smaller volume to fit in the cell,

• To strengthen the DNA to allow mitosis and meiosis and

• To control gene expression and DNA replication.

Plastids:

    Plastids are the cytoplasmic organelles of the cells of plants and some protozoans such as Euglena. Whereas the cells of the bacteria, fungi and animals contain chromatophores instead of plastids. Plastids perform most important biological activities such as the synthesis of food and storage of carbohydrates, lipids and proteins. The term plastid is derived from the Greek word “plastikas” means formed or moulded and was used by A.P.W. Schimper in 1885. He classified the plastids into the following types based on their structure, pigments and function:

• Chromoplasts (coloured)

• Leucoplasts (colourless)

1. Chromoplasts: (Greek words, chroma = colour; plast = living) These are the coloured plastids of plant cells. They contain a variety of pigments and synthesize the food through photosynthesis. Based on the type of pigment present in them, the chromoplasts of microorganisms and plant cells are as follows:

a) Chloroplasts: (Greek words, chlor = green; plast = living) These are most widely occurring chromoplasts of the plants. They occur mostly in the green algae and higher plants. The chloroplasts contain the pigments chlorophyll A and chlorophyll B. They also contain DNA and RNA.

b) Phaeoplasts: (Greek words, phaeo = dark brown; plast = living) These contain the pigment “Fucoxanthin”, which absorbs the light. They occur in the diatoms, dinoflagellates and brown algae.

c) Rhodoplast:(Greek words, rhodo = red; plast = living) The rhodoplast contains the pigment phycoerythrin which absorbs light. The rhodoplast occur in red algae.

2. Leucoplasts: (Greek words, leuco = white; plast = living) These are the colourless plastids which store the food material such as carbohydrates, lipids and proteins. The leucoplasts are rod like or spheroid in shape and occur in the embryonic cells, sex cells and meristematic cells. The most common leucoplasts of the plant’s cells are as follows:

   a) Amyloplasts: (Greek word, amyl = starch) These synthesize and store starch and occur in those cells which store starch.

   b) Elaioplasts: These store lipids and occur in seeds of monocotyledons and dicotyledons.

   c) Proteinoplasts or proteoplasts: These are the protein storing plastids which mostly occur in seed and contain few thylakoids.

Chloroplasts:

    These are the most common plastids of many plant cells and perform the function of photosynthesis. Distribution: The chloroplasts remain distributed homogeneously by in the cytoplasm of plant cells. But in certain cells, the chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. Shape: Higher plant chloroplasts are generally biconvex or plano-convex. However, in different plant cells, chloroplasts may have various shapes viz., filamentous, saucer shape, spheroid, ovoid, discoid or club-shaped. They are vesicular and have a colourless centre.

   Size: Generally, 2-3 µ in thickness and 5-10 µ in diameter. Polyploid plant cells have larger chloroplasts than diploid plant cells.

   Number: The number of chloroplasts varies from cell to cell and from species to species and is related with the physiological state of the cell. But it usually remains constant for a particular plant cell. The algae usually have a single huge chloroplast. The cells of higher plants have 20 to 40 chloroplasts.

   Ultra structure: Chloroplasts are bound by two-unit membranes. Each membrane is trilaminar, lipoproteinaceous. Both unit membranes are separated from each other by a distinct space known as periplastidial space.

The inner contents of the chloroplasts are heterogeneous and composed of:

• Matrix or stroma

• Grana

   (a) Matrix or stroma: The inner periplastidial space of the chloroplasts is filled with a watery, proteinaceous and transparent substance known as the matrix or stroma. The dark reaction of photosynthesis occurs in the matrix or stroma of chloroplasts and the stroma contains the multienzyme complex for the dark reactions. The grana and intergrana connecting membrane remain embedded in the matrix of chloroplasts.

   (b) Grana:

• Chloroplasts consists of many lamellar or membranous, granular and chlorophyll bearing bodies known as the grana, where the light reaction of photosynthesis takes place. The size of the grana may range from 0.3 to 2.7 µ.

• The chloroplasts may contain 40 to 60 grana in their matrix. Each granum of the chloroplast of a higher plant cell is composed of 10-100 disc like super imposed, membranous compartments known as thylakoids. In a granum, these thylakoids are arranged parallelly to form a stack. Each thylakoid is separated from the stroma by its unit membrane. Within the thylakoid membrane, 4 sub units appear to be arranged as a functional entity called quantasomes.

• Mechanism of energy transfer known as photophosphorylation occurs within quantasomes. Grana are interconnected by network of membranous tubules called stroma lamella or Fret’s channels. The chloroplasts contain the ribosomes which are smaller than the cytoplasmic ribosomes. The ribosomes of the chloroplasts are 70s type and resemble the bacterial ribosomes.

• Woodcock and Fernandez (1968) isolated segments of the DNA molecules from the chloroplasts. DNA of chloroplasts is present in stroma and plays an important role in cytoplasmic inheritance. The DNA of chloroplasts differs from the nuclear DNA in many aspects and it resembles closely the bacterial DNA. The most important and fundamental function of chloroplasts is the photosynthesis.

Mitochondria: (Greek word s, mitos = thread; chondrion = granule)

• These are first observed by Kolliker in 1880 who named them as granules. In 1882, Flemming named them as files, while in 1898, C. Benda gave the name mitochondria. In the cytoplasm of most cells occur many large-sized, round or rod-like structures called mitochondria. The mitochondria occur singly or in groups and their shape and size vary from cell to cell. They are filamentous in shape and bound by two membranes composed of lipids and proteins.

• Each membrane Tri lamellar in nature with two protein layers sandwiching a bimolecular layer of lipid. The outer membrane forms a bag like structure around the inner membrane, which gives out many finger-like folds into the lumen of the mitochondria. These folds are known as cristae. The space between the outer and inner mitochondrial membrane as well as the central space is filled up by a viscous mitochondrial matrix. The matrix, outer and inner membranes are found to contain many oxidative enzymes and coenzymes.

    Functions: Mitochondria are the sites of cell respiration · Oxidation of carbohydrates, lipids and proteins occurs in mitochondria. · Dehydrogenation · Oxidative phosphorylation · The mitochondria also contain some amount of DNA within the mitochondrial matrix and are thus associated with cytoplasmic inheritance. · Mitochondria contain ribosomes and are capable of synthesis of certain proteins · Oxidative decarboxylation and kerbs’ cycle takes place in the matrix of mitochondria, while respiratory chain and oxidative phosphorylation occurs in cristae of mitochondria. Since the major function of mitochondria is energy metabolism, during which ATP is synthesized, the mitochondria are also called power houses of the cell.

Endoplasmic Reticulum (ER):

• The endoplasmic reticulum was first observed by Porter in 1945 in liver cells of rats. Cytoplasmic matrix is transversed by vast reticulum or network of interconnecting tubules and vesicles known as endoplasmic reticulum.

• The endoplasmic reticulum is having a single vast interconnecting cavity which remains bound by a single unit membrane. The membrane of endoplasmic reticulum is supposed to have originated by impushing of plasma membrane in the matrix because it has an outer and inner layer of protein molecules sandwiching the middle layer of lipid molecules similar to plasma membrane.

• The membrane of endoplasmic reticulum may be either smooth when they do not have attached ribosomes or rough when they have ribosomes attached with it. Rough endoplasmic reticulum is present abundantly in pancreatic cells. One of the important functions of smooth endoplasmic reticulum is the synthesis of lipids and glycogen. Rough endoplasmic reticulum is associated with the synthesis of proteins. The membrane of endoplasmic reticulum is found to be continuous with the nuclear membrane and plasma membrane. The three principal forms of endoplasmic reticulum are:

• Cisternae: Long, flattened, sac like, unbranched tubules arranged parallelly in bundles 40-50 mµ in diameter.

• Vesicles: Oval, membrane bound, vascular structures, 25-230 mµ in diameter.

• Tubules: Branched structures forming reticulum system along with cisternae and vesicles 50-190 mµ in diameter. All three forms of endoplasmic reticulum are bound by a 50 A0 thick single unit membrane of lipoproteinaceous nature.

    Functions:

• The endoplasmic reticulum forms the ultra-structural skeletal frame work of cytoplasmic matrix and it provides mechanical support to it.

• It also acts as an intracellular circulatory system and it circulates various substances into and out of the cell by the membrane flow mechanisms.

• The endoplasmic reticulum acts as a storage and synthetic organ. For example: It synthesizes lipids, glycogen, cholesterol, glyserides, hormones etc.

• It acts as a source of nuclear membrane’s material during cell division.

• It protects cell from toxic effects by de-toxification.

• In certain cases, it transmits impulses intracellularly. In such cases it is known as sarcoplasmic reticulum. Endoplasmic reticulum which is of specialized nature and present in muscle cells is known as sarcoplasmic reticulum.

Golgi complex:

    It occurs in all cells except prokaryotic cells. In plant cells, they are called dictyosomes, which secrete necessary material for the formation of new cell wall during cell division. First reported by C. Golgi in 1898. It is a polymorphic structure having cisternae, vesicles and vacuoles. It is disc shaped and consists of central flattened platelike compartments / cisternae with a peripheral network of interconnecting tubules and peripherally occurring vesicles and golgian vacuoles. The membranes of Golgi complex are lipoproteinaceous and originate from membranes of endoplasmic reticulum.

    Functions:

• Storage of proteins and enzymes which are secreted by ribosomes and transported by endoplasmic reticulum.

• Secretory in function.

• The dictyosomes secretes necessary material for cell wall formation during cell division.

• It has a role in the formation of plasma membrane.

• It activates mitochondria to produce ATP, which is later utilized in respiratory cycle.

Lysosomes:

• The cytoplasm of animal cells contains many speroid or irregular shaped membrane bound vesicles known as lysosomes. The lysosomes originate from golgi complex and contain many digestive enzymes. Their function is the digestion of food material which comes into the cell by pinocytosis and phagocytosis.

• The lysosomes of plant cells are membrane bound storage organs containing hydrolytic digestive enzymes and are comprised of sphaerosomes, aleuron grains and vacuoles. Lysosomes are useful in the process of fertilization. They are also useful in autodissolution of cells.

Ribosomes:

• Robinson and Brown in 1953 first observed ribosomes in plant cells, while Palade in 1955 first observed them in animal cells. They are small, dense, round and granular particles occurring either freely in mitochondrial matrix, cytoplasm, chloroplasts or remain attached to membrane of endoplasmic reticulum forming the rough endoplasmic reticulum.

• They occur in all prokaryotic and eukaryotic cells and are hence called “universal components of all biological organisms”. They originate in the nucleus and consist of mainly RNA and proteins. Each ribosome is composed of two structural sub units viz., larger sub unit and smaller sub unit. The ribosomes are 70 S type in prokaryotes containing 50 S and 30 S subunits, while in eukryotes, they are 80 S type consisting of 60 S and 40 S subunits.

• The ribosome remains attached with the membranes of endoplasmic reticulum by larger subunit. The smaller subunit of ribosome is placed onto the larger subunit like a cap on the head. The ribosomes are essential for protein synthesis.

Micro tubules:

     The cytoplasm of plant and animal cells is transversed by numerous ultrafine tubules composed of tubulin protein and are called microtubules. The main function of microtubules is transportation of water, cytoplasmic streaming, formation of fibres or asters of the mitotic or meotic spindle during cell division. They form the structural units of centrioles, basal granules, cilia and flagella. They determine the shape of the cell.

Microfilaments (or Micro fibrils):

      The cytoplasm of most animal cells also contains many ultrafine, proteinaceous, solid microfilaments which maintain the structure of cell and form contractile components of muscle cells.

Centrosome:

      The centrosomes contain dense cytoplasm and is located near the nucleus of animal cells. During the cell division, the centrosome is found to contain two rod shaped granules known as centrioles. Each centriole consists of nine microfibrillar units and each microfibrillar unit is found to contain three microtubules. During cell division, microtubules help in the separation and movement of chromosomes.

Basal granules:

     The animal or plant cells which are having locomotory organelles such as cilia or flagella contain spherical bodies known as basal granules at the base of the cilia and are composed of nine fibrils. Each fibril consists of three microtubules, out of which two enter into the cilia or flagella. The basal granules may contain both DNA and RNA.

Sphaerosomes:

     These are organelles having a single membrane and a matrix which contains triglycerides. These are abundant in cell sin which lipids are stored and contain the hydrolytic enzyme, lipase, which probably has a role in mobilization of stored lipids when required in cell metabolism.

Microbodies:

      The cytoplasmic matrix of many kinds of cells viz., yeast, protozoa, higher plant cells, hepatocytes (liver cells) and kidney cells contain certain rough spherical membrane bound particles. They have a central granular crystalloid core containing some enzymes and occur in intimate relation with endoplasmic reticulum, mitochondria and chloroplasts.

 The main functions of microbodies are:

• Utilization of molecular oxygen

• They contain enzymes for hydrogen peroxide metabolism, purine metabolism, gluconogenesis (conversion of fat into carbohydrates) and photorespiration.

Cilia and flagella: 

      These are the cytoplasmic projections which are hair like and present on the outer surface of the cells. They help in locomotion of the cells. The cilia and flagella consist of nine outer fibrils around two large central fibrils. Each outer fibril consists of two microtubules. The cilia and flagella are originated from the basal granules and chemically consist of tubulin and dynein proteins and ATP (Adenosine Tri Phosphate).

Cytoplasmic vacuoles: 

      The cytoplasm of many plant cells and some animal cells contain numerous small or large sized, hollow liquid filled structures known as vacuoles. The vacuoles of plant cells are bound by a single semi-permeable membrane known as tonoplast. These vacuoles contain water, phenols, anthocyanins, alkaloids and storage products such as sugars and proteins. The cytoplasm without mitochondria and chloroplasts is known as cytosol.