Study of Plant Cell Organelles with Emphasis on Cell Wall, Chloroplast, Mitochondria and Nucleus

Unit I: Cell and Organelles: Study of plant cell organelles with emphasis on cell wall, Chloroplast, Mitochondria and Nucleus.

Cell wall is outermost, rigid, protective and semitransparent covering of plant cells and cell of fungi, bacteria and some protists. It was discovered by Robert Hooke in 1665. The cell wall is the rigid, semi permeable protective layer in some cell types.

This outer covering is positioned next to the cell membrane (plasma membrane) in most plant cells, fungi, bacteria, algae, and some archaea. Animal cells however, do not have a cell wall. The cell wall conducts many important functions in a cell including protection, structure, and support.

Cell wall composition varies depending on the organism. In plants, the cell wall is composed mainly of strong fibers of the carbohydrate polymer cellulose. Cellulose is the major component of cotton fiber and wood and is used in paper production.  Fungal cell wall – of chitin  Bacterial cell wall – of peptidoglycan or mucopeptide or murein  Plant cell wall – of cellulose, hemicelluloses and protein.

Cell wall has the following different layers:

1. Middle Lamella . It is outermost thin cementing layer between adjacent cells . Formed during cytokinesis . This outer cell wall layer that contains polysaccharides called pectins. Pectins aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another. This is also composed of magnesium and calcium. 2. Primary Wall . More or less elastic layer formed after the formation of middle lamella. . Composed of protein, hemicelluloses and loose network of microfibril of cellulose. . Primary wall and middle lamella are found in all types of plant cells. The primary cell wall provides the strength and flexibility needed to allow for cell growth. 3. Secondary wall . Much thicker, rigid and inelastic layer formed inner to primary wall. . Is formed only when cells attain maximum size . Is common in aclerenchyma, collenchymas, tracheid and vessels. . Composed of cellulose, hemicellular, pectin and lignin. . S1, S2, and S3 are its sublayers. . It may contain wax, cutin and suberin and silica. . This rigid layer strengthens and supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant vascular tissue cells. 1

Plasmodesmata: Primary and secondary walls are not formed continuously. They form gaps, known as plasmodesmata. It is a cytoplasmic bridge between two cells. Plasmodesmata are known for long time playing a passive role in permitting free movements of small metabolites and growth hormones between plant cells.

Desmosomes: It is the protoplasmic bridge between two cells of animal cells.

FUNCTIONS OF CELL WALL

A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the cell. Additional functions of the cell wall include:

1. Support the cell wall provides mechanical strength and support. It also controls the direction of cell growth.

2. Withstand turgor pressure turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture.

3. Regulate growth sends signals for the cell to enter the cell cycle in order to divide and grow.

4. Regulate diffusion the cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out.

5. Communication cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells).

6. Protection provides a barrier to protect against plant viruses and other pathogens. It also helps to prevent water loss.

7. Storage stores carbohydrates for use in plant growth, especially in seeds.

8. Shape It provides definite shape to cell due to its rigidity. It protects the protoplasm against mechanical injury.Cell wall of root hairs absorbs water. It has some enzymatic activity.

9. Transpiration Cutin and suberin of cell wall of stem and leaves help to reduce the rate of transpiration. The permeable nature of cell wall allows the exchange of any substance through it.

Primary cell wall

Primary cell wall is the first cell wall laid down by the protoplast inner to the middle lamella. The primary wall is thin and elastic and composed of cellulose, hemicellulose, pectic substances, lipids, proteins, some minerals and water. During development of middle lamella and primary wall, certain openings are left at places between the adjacent cells. These are called plasmodesmata (Strasberger 1901). Through these pores cytoplasmic continuity is maintained between the neighbouring cells. It is also made of cellulose, hemicellulose, proteins and polysaccharides.

Secondary cell wall

Secondary cell wall is formed towards inner side primary wall and is made of several layers of cellulose, hemicellulose. Deposition of lignin and suberin takes place after the primary wall is fully formed. The wall is thick and nonelastic and provides additional strength.

Tertiary cell wall

In addition to primary and secondary cell wall, tertiary cell wall is deposited in a few cells. First reported by Butchen (1955) in tracheids of some gymnosperms, it is considered to be a dried residue of the protoplasm. It looks like swollen nodules on the inner side of the secondary wall. Besides cellulose and hemicellulose, xylan is also present in the tertiary cell wall.

Cell wall depositions

The cell wall is very thin and delicate in the beginning. As the cell grows, it is stretched and new substances are deposited on the primary cell wall. The various substances found on the cell wall are:

1. Lignin:

It is a complex chemical substance deposited in the secondary wall. With the lignin deposition, the protoplasm is lost and the cells become woody and hard. Lignin deposition is generally not uniform and may result in annular, spiral, scalariform, reticulate or pitted patterns. These thickenings provide mechanical support to the cell. Most of the vegetable fibres are lignified.

2. Cutin:

It is a waxy substance, forms a thin or thick layer called cuticle on the stem or leaf surfaces. Cutin is impervious to water and checks evaporation. Deposition of cutin is found to be more in xeric plants.

3. Suberin:

This fatty substance is also waxy in nature. This is deposited on cork cells and is impermeable to water and gases.

4. Mucilage:

It is a slimy substance made of complex carbohydrates. It absorbs water and stores it. It becomes hard when dry and viscous when moist. Mucilage is present in leaves of Aloe, flowers of Hibiscus, seeds of Linum, etc.

5. Mineral crystals:

Silica, calcium oxalate and calcium carbonate, etc. are deposited in the cell wall in the form of crystals. The main function of cell wall is to provide mechanical strength. It is also capable of imbibing water and thus, helps the movement of water and solutes inside the cell.

Chloroplast

Chloroplasts are organelles present in plant cells and some eukaryotic organisms. Chloroplasts are the most important plastids found in plant cells. It is the structure in a green plant cell in which photosynthesis occurs.

Chloroplast is one of the three types of plastids. The chloroplasts take part in the process of photosynthesis and it is of great biological importance. Animal cells do not have chloroplasts. All green plant take part in the process of photosynthesis which converts energy into sugars and the byproduct of the process is oxygen that all animals breathe. This process happens in chloroplasts.

The distribution of chloroplasts is homogeneous in the cytoplasm of the cells and in certain cells chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. A typical plant cell might contain about 50 chloroplasts per cell.

The chloroplasts are cellular organelles of green plants and some eukaryotic organisms. These organelles conduct photosynthesis. They absorb sunlight and convert it into sugar molecules and also produce free energy stored in the form of ATP and NADPH through photosynthesis. Chloroplasts are unique organelles and are said to have originated as endosymbiotic bacteria.

Chloroplast Structure

Chloroplasts found in higher plants are generally biconvex or planoconvex shaped. In different plants chloroplasts have different shapes, they vary from spheroid, filamentous saucershaped, discoid or ovoid shaped.

They are vesicular and have a colorless center. Some chloroplasts are in shape of club, they have a thin middle zone and the ends are filled with chlorophyll. In algae a single huge chloroplast is seen that appears as a network, a spiral band or a stellate plate.

The size of the chloroplast also varies from species to species and it is constant for a given cell type. In higher plants, the average size of chloroplast is 46 Âµ in diameter and 13 Âµ in thickness.

Structure of Chloroplast

The chloroplast are double membrane bound organelles and are the site of photosynthesis The chloroplasts have a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner membrane of the chloroplast enclose a semigellike fluid known as the stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma.

Outer membrane It is a semiporous membrane and is permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins.

Intermembrane Space It is usually a thin intermembrane space about 1020 nanometers and it is present between the outer and the inner membrane of the chloroplast.

Inner membrane The inner membrane of the chloroplast forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane.

Stroma Stroma is a alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma. The chloroplast DNA chlroplast ribosomes and the thylakoid sytem, starch granules and many proteins are found floating around the stroma.

Thylakoid System The thylakoid system is suspended in the stroma. The thylakoid system is a collection of membranous sacks called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as grana. Each granum contains around 1020 thylakoids.

Thylakoids are interconnected small sacks, the membranes of these thylakoids is the site for the light reactions of the photosynthesis to take place. The word 'thylakoid' is derived from the Greek word "thylakos" which means 'sack'.

Important protein complexes which carry out light reaction of photosynthesis are embedded in the membranes of the thylakoids. The Photosystem I and the Photosystem II are complexes that harvest light with chlorophyll and carotenoids, they absorb the light energy and use it to energize the electrons.

The molecules present in the thylakoid membrane use the electrons that are energized to pump hydrogen ions into the thylakoid space, this decrease the pH and become acidic in nature. A large protein complex known as the ATP synthase controls the concentration gradient of the hydrogen ions in the thylakoid space to generate ATP energy and the hydrogen ions flow back into the stroma.

Thylakoids are of two types granal thylakoids and stromal thylakoids. Granal thylakoids are arranged in the grana are pancake shaped circular discs, which are about 300600 nanometers in diameter. The stromal thylakoids are in contact with the stroma and are in the form of helicoid sheets.

The granal thylakoids contain only photosystem II protein complex, this allows them to stack tightly and form many granal layers wiht granal membrane. This structure increases stability and surface area for the capture of light.

The photosystem I and ATP synthase protein complexes are present in the stroma. These protein complexes acts as spacers between the sheets of stromal thylakoids.

Functions of chloroplast

 In plants all the cells participate in plant immune response as they lack specialized immune cells. The chloroplasts with the nucleus and cell membrane and ER are the key organelles of pathogen defense.  The most important function of chloroplast is to make food by the process of photosynthesis. Food is prepared in the form of sugars. During the process of photosynthesis sugar and oxygen are made using light energy, water, and carbon dioxide.  Light reactions takes place on the membranes of the thylakoids.

 Chloroplasts, like the mitochondria use the potential energy of the H+ ions or the hydrogen ion gradient to generate energy in the form of ATP.  The dark reactions also known as the Calvin cycle takes place in the stroma of chloroplast.  Production of NADPH2 molecules and oxygen as a result of photolysis of water.  BY the utilization of assimilatory powers the 6carbon atom is broken into two molecules of phosphoglyceric acid.

MITOCHONDRIA INTRODUCTION

Mitochondria is a membrane bound cellular structure and is found in most of the eukaryotic cells. The mitochondria ranges from 0.5 to 1.0 micrometer in diameter. The mitochondria are sometimes described as power plants of the cells. These organelles generate most of the energy of the cell in the form of adenosine triphosphate (ATP) and it is used a source of chemical energy.

The mitochondria also involved in other cellular activities like signaling, cellular differentiation, cell senescence and also control of cell cycle and cell growth. Mitochondria also affect human health, like mitochondrial disorder and cardiac dysfunction and they also play important role in the aging process.

The mitochondria in plants play an important role in the production of ATP via the process of oxidative phosphorylation. Mitochondria also play essential roles in other aspects of plant development and performance. It also has various properties which allow the mitochondria to interact with special features of metabolism in plant cell.

STRUCTURE OF MITOCHONDRIA

Mitochondria are rod shaped structure found in both animal and plant cells. It is a double membrane bound organelle. It has the outer membrane and the inner membrane. The membranes are made up of phospholipids and proteins.

Mitochondria are bound by a double membrane the outer membrane and the inner membrane. The outer membrane is smooth and covers the organelle. The inner membrane has many folds known as cristae. The cristae increases the available surface are to enhance the productivity of cellular respiration.

The double membrane of the mitochondria divides the organelle into two distinct parts the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow space between the outer and the inner membrane. The mitochondrial matrix is the content enclosed by the inner membrane.

The fluid inside the mitochondria is called the matrix. Most of the chemical reactions of the cellular respiration process happen in the inner membrane of the mitochondria due to the high concentration of enzymes.

Mitochondria are independent organelles, they have their own DNA and ribosomes. They can replicate and multiply on their own and make their own proteins. They have circular DNA similar to bacteria and replicate by fission.

The components of mitochondria are as follows: Outer membrane  It is smooth and is composed of equal amounts of phospholipids and proteins.  It has a large number of special proteins known as the porins.  The porins are integral membrane proteins and they allow the movement of molecules that are of 5000 daltons or less in weight to pass through it.  The outer membrane is freely permeable to nutrient molecules, ions, energy molecules like the ATP and ADP molecules. Inner membrane  The inner membrane of mitochondria is more complex in structure.  It is folded into a number of folds many times and is known as the cristae.  This folding help to increase the surface are inside the organelle.  The cristae and the proteins of the inner membrane aids in the production of ATP molecules.  Various chemical reactions take place in the inner membrane of the mitochondria.  Unlike the outer membrane, the inner membrane is strictly permeable, it is permeable only to oxygen, ATP and it also helps in regulating transfer of metabolites across the membrane.

Intermembrane space  It is the space between the outer and inner membrane of the mitochondria, it has the same composition as that of the cell's cytoplasm.  There is a difference in the protein content in the inter membrane space.

Matrix  The matrix of the mitochondria is a complex mixture of proteins and enzymes. These enzymes are important for the synthesis of ATP, mitochondrial ribosomes, tRNAs and mitochondrial DNA.

FUNCTIONS OF MITOCHONDRIA

Mitochondria supply nearly all the required biological energy. Only the mitochondria are fully capable of converting pyruvic acid to carbon dioxide and water. They are the respiratory centres of the cell. The enzymes for Krebs cycle are found in the matrix of the mitochondrion. The enzymes for electron transport are located in the inner membrane of mitochondrion.

The ATP molecules produced as a result of cellular respiration accumulate in the mitochondria. A set of enzymes that control synthesis of lecithin and phosphatidyl ethanolamine from fatty acids, glycerol and nitrogenous bases is present in most mitochondria. Mitochondrial genes control some hereditary characters, e.g., male sterility in maize.

1. It is well known fact that mitochondria are responsible for cell energy supply. 2. They produce energy in the form of ATP (adenosine triphosphate) by breakdown of carbohydrate substrates in presence of oxygen. This is how the food in body is converted to energy. Once we consume food, it is digested and absorbed into blood. Then it supplied to each and cell and organ in the body.This is then used to generate the energy. 3. These are the organelles where the TCA cycle (tricarboxyllic acid cycle) and other respiratory cycles of the cell takes place. 4. They also produce biproducts like glutarate, glycooxalic acid during the TCA cycle. These can further form glutamine, glycine like amino acids required for normal physiology and metabolism of the cell. 5. They promote cell growth and also signal transmission. In especially nerve cells they are concentrated at nerve ending to promote nerve growth and also supply energy for nerve conduction.

6. Mitochondria also are partly responsible for cell death in case of trauma. The membrane of mitochondria releases apoptosis factors leading to programmed cell death. 7. They also generate oxidative radicals during energy formation. These oxidative radical are necessary for various other reactions. But in excess can be harmful to the cell itself. 8. Mitochondria provide important intermediates for the synthesis of several biochemicals like chlorophyll, cytochromes, pyrimidine’s, steroids, alkaloids, etc. Mitochondria may store and release Calcium when required. Nucleus

Nucleus a doublemembrane bound cell organelle present in eukaryotic cells. The nucleus constitutes most of the genetic material of the cell the DNA.

The nucleus maintains the integrity of the genes which regulate the gene expression, inturn regulating the activities of the cell. Therefore, the nucleus is known as the control center of the cell. The nucleus is the largest organelle of the cell. The nucleus appears to be dense, spherical organelle.

It occupies about 10% of the total volume of the cell. In mammalian cells the average diameter of the nucleus is approximately 6 micrometers. Eukaryotes usually have a single nucleus, but a few cell types have no nuclei (e.g. absent in few cells like the mammalian RBCs) and a few others have many.

PROPERTIES The nucleus is a membranebound structure that contains the cell’s hereditary information and controls the cell’s growth and reproduction.

It is generally the most prominent organelle in the cell. It is surrounded by a structure called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm.

The cell’s chromosomes are also enclosed within it. Chromosomes contain DNA which provides the genetic information necessary for the production of other cell components and for reproduction of life.

STRUCTURE OF NUCLEUS The nucleus is the largest organelle of the cell. The nucleus appears to be dense, spherical organelle. It occupies about 10% of the total volume of the cell.

In mammalian cells the average diameter of the nucleus is approximately 6 micrometers. A semifluid matrix nucleoplasm is seen inside the nucleus which is a viscous fluid and is similar to the composition of the cytoplasm.

Nuclear Envelope 1. The nuclear envelope is also known as the nuclear membrane. 2. It is made up of two membranes the outer membrane and the inner membrane. 3. The outer membrane of the nucleus is continuous with the membrane of the rough endoplasmic reticulum. 4. The space between these layers is known as the perinuclear space.

5. The nuclear envelope encloses the nucleus and separates the genetic material of the cell from the cytoplasm of the cell. 6. It also serves as a barrier to prevent passage of macromolecules freely between the nucleoplasm and the cytoplasm. Nuclear Pore

1. The nuclear envelope is perforated with numerous pores called nuclear pores. 2. The nuclear pores are composed of many proteins known as nucleoproteins. 3. The nuclear pores regulate the passage of the molecules between the nucleus and cytoplasm.

4. The pores allow the passage of molecules of only about 9nm wide. The larger molecules are transferred through active transport. 5. Molecules like of DNA and RNA are allowed into the nucleus. But energy molecules (ATP), water and ions are permitted freely.

Chromosomes

1. The nucleus of the cell contains majority of the cells genetic material in the form of multiple linear DNA molecules. 2. These DNA molecules are organized into structures called chromosomes. 3. The DNA molecules are in complex with a large variety of proteins (histones) which form the chromosome. 4. In the cell they are organized in a DNAprotein complex known as chromatin. 5. During celldivision the chromatin forms welldefined chromosomes. 6. The genes within the chromosomes consists of the cells nuclear genome. 7. Mitochondria of the cell also contains a small fraction of genes. 8. Human cells has nearly 6 feet of DNA, which is divided into 46 individual molecules.

Nucleolus

1. The nucleolus is not surrounded by a membrane, it is a densely stained structure found in the nucleus. 2. The nucleoli are formed around the nuclear organizer regions. 3. It synthesizes and assembles ribosomes and r RNA. 4. The number of nucleoli is different from species to species but within a species the number is fixed. During cell division, the nucleolus disappears. 5. Studies suggest that nucleolus may be involved in cellular aging and senescence.

FUNCTIONS OF NUCLEUS

1. It controls the heredity characteristics of an organism. 2. It main cellular metabolism through controlling synthesis of particular enzymes. 3. It is responsible for protein synthesis, cell division, growth and differentiation. 4. Stores heredity material in the form of deoxyribonucleic acid (DNA) strands. Also stores proteins and ribonucleic acid (RNA) in the nucleolus. 5. It is a site for transcription process in which messenger RNA (mRNA) are produced for protein synthesis. 6. It helps in exchange of DNA and RNA (heredity materials) between the nucleus and the rest of the cell. 7. Nucleoli produce ribosomes and are known as protein factories. 8. It also regulates the integrity of genes and gene expression.