Golgi bodies: Shape, size, structure and function.

Key points: Golgi complex, history, introduction, shape, size, structure, function.

History

In 1890 Camillo Golgi described apparato reticulate interno i.e. internal reticular apparatus in the nerve cells. In 1900 Holmgren described a system of clear canals, which he called trophospongium. Baker (1951,1953) referred to the Golgi apparatus as lipochodria because of the presumed lipid content. Structure similar to golgi complex have been found in plants. Botanists refer to them as dictyosomes.

What are Golgi bodies?

The Golgi complex is generally a single large structure. Nerve cells, liver cells and most of the plant cells have multiple Golgi complexes, there are about 50 in liver cells. Cells with dispersed Golgi complexes may have hundreds scattered throughout the cytoplasm of the cell.

Shape and size of Golgi complex

The shape is variable in different somatic cell types of animals. Even in the same cells there are variation in different functional stages. It varies from a compact mass to disperse filamentous network. The position of Golgi complex is also variable. They are ectodermal in origin and is polarized between the nucleus and periphery. In exocrine cells it lies between nucleus and secretory pole. The size of Golgi complex is large in nerve and gland cells, but small in muscles cells. The size is linked to the functional state.

Structure of Golgi complex under electron microscope

Electron microscope observations of thin sections reveal the presence of three membranous components:

  1. Flattened sacs or cisternae
  2. Small tubules and vesicles
  3. Large vacuoles and filled with amorphous or granular substance.

These membranous structures are characterized by the absence of ribosomes i.e. they are smooth membranes.

Functions of Golgi complex

There are several functions that have been observed attributed to Golgi complex

  • Role in protein secretion (migration, transport and packaging)

In pancreatic exocrine cells there are distributed proteins (digestive enzymes). Now proteins are formed at ribosomes which are attached to endoplasmic reticulum. The synthesized proteins are then transferred to ER. From there they move towards Golgi apparatus. In the Golgi complex the proteins are contracted and transformed into zymogen granules. These zymogen granules released from Golgi complex and migrate to the surface of the cell. Here the limiting membrane of the zymogen granule fuses with the plasmalemma thus discharging its contents. 

  • Secretion of polysaccharides

Precursors enter the goblet cells from the capillaries of the vascular system. The amino acids are synthesized into proteins on the ribosome of ER. The proteins are then transferred to the cisternae of Golgi complex. The simple sugar molecules go directly from blood stream to cisternae where they are complexed with protein to form a glycoprotein.

Glycosylation

In many cells the protein released from the ER is combined with the carbohydrate to produce complex carbohydrates like glycoprotein, mucopolysaccharide, glycogen and glycolipids. Addition of carbohydrate components to the protein occurs in the Golgi complex as well as in ER.

After completion of glycosylation the glycoprotein is released into the lumen of Golgi complex cisternae.

  • Sulphation

Golgi bodies take part in the sulphate metabolism. Compounds containing active Sulphur are formed in two steps process. Sulphate is first activated by ATP in two stages, the process requires two separate enzymes. The process is carried out by enzyme suplotransferases.

  • Plasma membrane formation

Secretory granules originating from Golgi complex fuse with the plasma membrane during the process of exocytosis. The membrane of the granules become incorporated with the plasma membrane and contributes to the renewal of plasma membrane components. Golgi complex plays an important part in the synthesis of carbohydrate components in the plasma membrane.

  • Lipid packaging and secretion

The epithelial cells produce chylomicrons which contains lipids in the form of lipoproteins. The Golgi complex may also be involved in adding of carbohydrates of chylomicrons. Therefore the overall role of Golgi complex is the concentration and modification of secretory material. These changes convert lipid to chylomicrons. The Golgi complex provides the membrane for the envelopment of lipid so that it can be released from the cell.

  • Acrosome formation

In early stages of mammalian development the spermatid of cell has Golgi apparatus which was spherical in shape with parallel flattened cisternae. Later the complex becomes irregular and the cisternae dilate to form sacs. proacrosomic granules appear in the centre of Golgi complex and fuse to form acrosome.

Conclusion

Golgi bodies are ectodermal in origin. They consist of cisternae and large vacuoles. They involve in many processes such as protein synthesis, polysaccharides, lysosomes, acrosome formation etc.

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Pancreas: Exocrine or endocrine gland?

key points: pancreas, exocrine gland, endocrine gland, pancreatic enzymes, islets of Langerhans, glucose, glucagon etc.

Do you remember studying different types of glands? Some of them were endocrine while some of them were exocrine. Before we go to Pancreas, let us first understand what are endocrine and exocrine glands?

What are endocrine glands?

Our nervous system controls and co ordinates with different organs as well as try to responds with the external environment. But human body carries several activities in various organ systems at the same time, Therefore nervous system has to maintain a balance and regulates each activity at certain time. Endocrine system brings all the co ordination and balance the activities with the help of chemical messengers called as ‘hormones’. But what does the endocrine system do? (Endo- inside and crine- separate) The system literally means secrete internally. The endocrine system consists of certain endocrine glands which perform the function of secreting hormones. They are also known as ductless glands since they release the hormones into bloodstream and not via any specialized ducts. For example, Pituitary gland- oxytocin, ADH and thyroid gland- thyroxine etc.

What are exocrine glands?

Exocrine glands require the ducts to transport the secreted substance at target site. They can not release the substance directly into bloodstream and also the substance which are produced by them shows the action at nearby areas. There are several exocrine glands such as lacrimal glands which produce tears or sweat gland for production of sweat or salivary gland for producing saliva. If you notice their function of these glands their action is restricted to certain area and shows effect around the area where they are located.

Pancreas as endocrine and exocrine glands

Liver and pancreas are the only glands in human body which act as both endocrine and exocrine glands. Let us first understand the endocrine function of pancreas.

Pancreas are located in the abdominal cavity close to duodenum and behind the stomach. They are mainly responsible for two functions are as follows:

  1. Secreting of hormones for controlling glucose (Insulin and Glucagon).
  2. Secreting digestive enzymes.

Pancreas as an endocrine gland:

Pancreas has a region called as “Islets of Langerhans” which typically consists of four cells

  1. Alpha cells
  2. Beta cells
  3. Delta cells
  4. F cells

Beta cells are responsible for producing insulin whereas alpha cells produce glucagon. Both the hormones carry the function of maintaining glucose balance inside the blood stream. The delta cells produce a hormone known as somatostatin which restrains the secretion of insulin and glucagon.

Brief mechanism of insulin and glucagon

Case I: Consider that a person had carbohydrate diet which means he is now having a lots of sugar inside the body. This is sensed by pancreas and it stimulates the beta cells to produce glucose controlling hormone called as “Insulin”.  This insulin gets released into the blood stream and bind to the glucose. This glucose is stored in the liver in the form of glycogen.

Case II: Now consider that the same person is starving and has no food to eat. In this case the pancreas stimulate alpha cells to produce “Glucagon”. Many times people get confused between glycogen and glucagon. Let’s fix this mind, glycogen is a complex form of sugar while glucagon is a hormone to break that sugar into glucose i.e. simple sugar. Hence the stored glycogen is now broken by glucagon and free glucose is released into the blood stream.

Pancreas as exocrine gland

As a duct gland it secrets digestive juices which break down the nutrients. These juices are then poured to the duodenum which is the beginning of small intestine. These enzymes travel through series of ducts and meets the pancreatic duct. The pancreatic duct meets the common bile duct and it carries bile to duodenum.

Conclusion

Pancreas secrete several enzymes which helps in the digestion. The enzymes are transported via certain ducts. Therefore Pancreas are said to have an exocrine function. Also to maintain blood sugar level it secretes hormones (insulin and glucagon) which defines its endocrine function. Hence pancreas act as both endocrine and exocrine system.

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Photosynthesis: Process, steps and reactions

What is photosynthesis?

Photosynthesis is the process in which most of the green plants use carbon dioxide and water to produce sugar (simple form of carbohydrates) in the presence of sunlight. The term photosynthesis means ‘synthesis using sunlight’. But why photosynthesis is so important? If you think it is useful for the production of oxygen and ultimately for the survival of most of the species on the earth, then you might be partly right. With the help of photosynthesis plants produce sugar which is necessary for most of the herbivorous animals. It also helps to maintain the level of carbon dioxide in the atmosphere. Lastly it produces about 170 million tons of dry matter of which 90% is produced in the ocean.

The overall chemical reaction of photosynthesis is represented as follows:

6CO2 + 12 H2O    ————>    sugar (glucose) + 6O2 + 6H2O

Site of photosynthesis

The process of photosynthesis takes place in the chloroplast. But where is this chloroplast present in plants? They are present in the middle layers (mesophyll cells) of leaf. If you carefully observe the structure of leaf, you will see the upper epidermis and below that there is a palisade mesophyll followed by spongy mesophylls. Now most of the chloroplasts are present in the palisade mesophylls. The chloroplast is surrounded by two membranes. It is filled with the fluid inside it known as stroma. There are flattened fluid filled sacs called as thylakoids or lamellae. When these thylakoids are arranged in a stack like pile of coins they are known as grana. Various grana are joined by intergranal or stroma lamellae. One should note that the process of photosynthesis occurs in two phases or two reactions. First is light reaction that occurs in grana region while second is dark reaction which occurs in the stroma of chloroplast.  

Layers of leaf
Mesophyll layer in leaf

Pigments involved in photosynthesis

Different pigments of photosynthesis are present in the chloroplast. The basic function of these pigments is to absorb different wavelengths of light. They belong to two main groups:

  • Chlorophylls

These are green pigments with two principle types as chlorophyll a (bluish green) and chlorophyll b (olive green). Chlorophylls absorb mainly violet-blue and red region of visible light (VIBGYOR region).

  • Carotenoids

These are orange-yellow pigments with two principle types as carotene (beta carotene) that is orange in color and xanthophyll that is yellow in color.

NOTE: Chlorophyll a mainly absorbs violet and red light

             Chlorophyll b mainly absorbs blue and red light

Raw material for photosynthesis

  • As per the overall reaction of photosynthesis you can observe that water and carbon dioxide act as reactants, hence they can be said as raw material for the process.
  • Since carbon dioxide is available in the atmosphere, plants utilize that during photosynthesis and released it during respiration.
  • Similarly when water is used as raw material it undergoes it gives H+ and OH- ions during photolysis. The obtained hydrogen ions are then used to synthesize energy rich molecules.

Mechanism of photosynthesis

As we discussed earlier that the process of photosynthesis occurs in two phases

  • Light reaction or photochemical phase
  • Dark reaction or biosynthetic phase

NOTE: It is important to understand that in the light phase the plant is making certain chemical or energy rich molecules such as NADPH and ATP which can be later used to synthesize the sugar in the bio-synthetic phase. Hence the goal of photo-chemical phase is to prepare energy rich molecule and in bio-synthetic phase it is important to synthesize sugar i.e. glucose.  

  1. Light phase or photo-chemical phase

As the name suggest it is clear that this phase requires light or specifically presence of photons. It occurs in the grana of thylakoid membrane. It involves several steps such as

  • Activation of chlorophyll

When the chlorophyll molecule receives the light energy or photons (smallest unit of light), it gets excited and emits electron. This emitted electron then travel through ETC (electron transport chain). The reaction can be represented as

Chlorophyll a  +  hv (photons of light) ————->  Chlorophyll a  + e (emitted electron).

Light and dark reaction in photosynthesis
  • Photolysis of water

This step involves splitting of water molecule with the help of light energy. The water molecule splits into it H+ and OH- ions.

  • Formation of NADPH

The hydrogen ions released by splitting of water are used to reduce the NADP molecule to NADPH since it is an energy rich molecule. This energy rich molecule is used to reduce carbon dioxide to produce sugars (glucose).

  • Photophosphorylation

This step involves addition of a phosphate group. When ADP is phosphorylated (addition of phosphate/ IP) and convert into ATP (another energy rich molecule) in the presence of sunlight is called as photophosphorylation.

The overall light reaction can be summarized as,

2H2O  +  2NADP  +  3ADP +  3IP  —————>    2NADPH   +    3ATP

  1. Dark reaction or biosynthetic phase

This phase does not require light and occurs in the stroma of chloroplast. Previously synthesized NADPH and ATP molecules are now utilized in this phase to make glucose.

These molecules helps to convert the carbon dioxide (raw material) into carbohydrates (sugar). This process is called as carbon fixation.

This process was discovered by Melvin Calvin and Andy Benson, it is called as Calvin cycle or Calvin Benson cycle.

This process takes place by series of reactions where RuBP (ribulose biphosphate) act as an acceptor for CO2. The hydrogen that is released by NADPH is combined with CO2 by using energy from ATP molecule glucose is formed. It is important to note that Calvin cycle should repeat 6 times to form 1 glucose molecule.  Refer the overall light reaction and calculate the number of NADPH and ATP as multiple of 6.

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Plant hormones: structure & function- The Biology Bug

Key terms: Plant hormones, chemical nature of hormones, Biological function of hormones, Auxins, Gibberlins, Ethylene, Cytokinin, Abscisic acid

Most of the physiological activities in plants are controlled by certain chemical substances which are called as plant hormones or phytohormones or growth hormones. They are the signalling molecules which act either as growth promoters or growth inhibitors. Plant hormones are simple molecules but have diverse chemical composition for growth and development of plant. Before going into details of phytohormones let us first understand how animal hormones differ from plant hormones.

  • Animal hormones are made by specific glands in endocrine system but in plants the phytohormones are not made by any specialized tissue.
  • Plant hormones have very short life and gets quickly vanished after their immediate action.
  • Unlike animal hormones, plant hormones are made in a small quantity.

There are several types of plant hormones such as auxins, gibberlins, cytokinins, ethylene, abscisic acid, flowering hormones, synthetic growth retardants etc. However, we will discuss first five basic hormones.

Auxins (Cell elongation)

The meaning of auxin is ‘to increase’ or ‘to grow’. The auxin was discovered while studying the bending of coleoptile of canary grass towards light. They observed that when unilateral source of light was provided to the growing coleoptile, it bended in the direction of light. This was because of the presence of auxins.

Chemical nature of Auxin

Indole-3-acetaldehyde, Indole-3-acetonitrile, Indole-3-ethanol, 4-chloro-IAA are found in plants. Certain substances identical to the properties of auxins are IBA (Indole butyric acid), 2,4-dichloro-phenoxy-acetic acid, Indole propionic acid (IPA) etc.

Biological functions in plant

  1. Cell elongation: The primary function of auxins is to stimulate the elongation of cells in shoot. They promote elongation and growth of stem and roots. Similarly the dormancy of buds can be broken for certain plants in temperate region with the help of gibberlin.
  2. Cell division: They help in cell division and promote the growth of certain tissue. For example, when auxin is added during the tissue culture, callus (mass of undifferentiated cells) is formed.
  3. Apical dominance: In most of the vascular plants it is observed that especially the plants which are tall and sparsely branched, if the terminal bud shows higher growth whereas lateral buds remained suppressed. Removal of the apical bud results in rapid growth of lateral buds. This phenomenon is caused by auxins and is known as ‘apical dominance’.
  4. Root initiation: Increase in auxin can promote the lateral branch roots i.e. higher the concentration of auxin, initiates more lateral branch roots. For example, application of IAA in lanolin paste to the cut end of young stem results in an early rooting.

Gibberlins (Cell differentiation)

They are essential for various processes such as seed germination, stem elongation, leaf expansion, pollen maturation etc. Gibberlins were discovered in Japan when rice plants were found to suffer from a disease, bakane (foolish seedling) disease. The diseased rice plants were found to be longer than the healthier ones. This was caused by Gibberella fujikori  and later the substance was isolated and named as ‘gibberlins’.

Chemical nature of Gibberlins

Gibberlins are weakly acidic growth hormones and contain a gibbane ring structure. The structural feature that all gibberlins have in common and defines them as a family of molecules, is that they are derived from ent-kaurene ring.

Biological functions of Gibberlins

  1. Seed germination: Germination starts vigorously if certain seeds are exposed to light or red light. This requirement of light has overcome with the help of gibberlic acid. For example, light sensitive seeds such as tobacco shows poor germination in dark. Therefore treatment of gibberlic acid helps in seed germination.
  2. Bolting effect: In many herbaceous plants the early period of growth shows rosette- habit with a short stem and cauline leaves (arrangement of leaves on stem). Gibberlin triggers the growth of sub-apical meristem which causes the rosette plant to grow faster. In these plants the internodal length is short. Gibberlin promotes the internodal elongation causing rise in the stem height. Such type of growth in stem of rosette plant is known as ‘bolting’.
  3. Leaf expansion: Gibberlins help in making the leaves broader as well as elongate them. This increase in the surface of area provides sufficient space for photosynthesis.

Cytokinins (Cell division)

Cytokinins mainly induce cell division. It either works individually or with auxins. It was discovered by Miller and Skoog, the first hormone they found was named as ‘kinetin‘.

Chemical nature of Cytokinins

Zeatin is the most abundant naturally occurring free cytokinin, but dihydrozeatin (DZ) and isopentyl adenine also commonly found in higher plants and bacteria.

Biological functions of Cytokinins

  1. Cell division: It induces cell division in non meristematic cells. Cytokinins along with the auxins induces root development, bud & shoot development.
  2. Cell enlargement: Cytokinins( specifically kinetin) induces cell enlargement. They promote the expansion of cells in the leaf disks.
  3. Apical dominance: They are antagonistic to auxins with respect to apical dominance which means cytokinins trigger the growth of lateral buds.
  4. Delay of senescence: The aging of leaves caused due to loss of chlorophyll and rapid breakdown of proteins. This process is known as senescence. Cytokinins delay the senescence by controlling protein synthesis and mobilization of resources.

Ethylene (Fruit ripening)

It is the only hormone in plants which exists in the gaseous state.

Chemical nature of Ethylene

It is the simplest olefinic gas which is highly volatile in nature, It is also flammable and undergoes oxidation to produce ethylene oxide.

Biological functions of Ethylene

  1. Fruit ripening: It is known as fruit ripening hormone. Ethylene lamps are used for ripening of certain fleshy fruits. There are different types of fruits that react differently with the exogenous application of ethylne. Hence it is more used in the food industry.
  2. Epinasty: When upper surface of leaf grows faster than the lower surface, the leaf curves downward and this phenomenon is called as ‘epinasty’. Ethylene causes epinasty in dicot plants. NOTE: Monocots do not exibit this response.
  3. Inhibition of geotropism(growth towards gravity): Ethylene can nullify the geotropic effect. Roots become negatively geotrpic while stem turns positively geotrpic.

Abscisic acid /ABA (Abscission)

It is also known as stress hormone because production of this hormone production of this hormone is stimulated by environmental conditions such as drought, water logging etc.

Biological function of Abscisic acid

  1. Abscission: It promotes the abscission of flowers and fruits.
  2. Stomatal closing: ABA controls closure of stomata when the plant undergoes water stress. It is observed that application of ABA to leaves of normal plants causes closing of stomata. It travels from mesophyll cells of chloroplast to the guard cells of stomata during the period of water stress.
  3. It is also known to inhibit the seed germination and causes seed dormancy.

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PROTEINS: Definition its Structures, Domains and Motifs.

Key terms: proteins, structure of proteins, alpha helix, Beta sheet, loops, domains, motifs, folding, unfolding.

I am sure that you all must have heard this term ‘protein’ during your high school or college. But when it comes to study of proteins, sometimes it gets difficult for some people to clearly get that idea and basic concepts behind it. So have you ever imagined how exactly the protein looks like or how does it function at molecular level? Well then this article is for you. Let’s make it simple.

  • What are Proteins?

They are the organic compounds which contains a long chain of amino acids (polypeptide chain) and maintains the structural components of body. They are mainly present in the muscles (eg. myosin), nails, hair (eg. keratin), immune system (eg. antibody) etc. They are also present in the food and dairy products such as egg, milk, pulses, fish etc. These proteins can be extracted and used as supplement or therapeutic purposes.

  • Structure of Proteins

The structure depends on the nature of protein i. e. whether the protein is globular or fibrous in nature. But they are classified in the following four types:

  1. Primary structure
  2. Secondary structure
  3. Tertiary structure
  4. Quaternary structure

1.Primary structure

The primary structure of protein contains a simple chain of amino acids without any loops or turns inside it. They involve formation of peptide bond. The peptide bond is covalent bond formed between carboxyl group and amino group of two amino acids.

2. Secondary structure

This involves the folding of structure with respect to polypeptide chain due to the reaction between atoms. The folding results into two most common structures, alpha helix and beta pleated sheet.

A. alpha helix

The polypeptide chain is coiled spirally in this structure. The backbone forms inner part of the coil while the side chains extend outward from the coil. In this structure the carbonyl group of amino acid is bonded with the hydrogen of amino group via hydrogen bonding. Therefore it gives the appearance of ribbon to the secondary structure.

B. Beta sheet

The linear extended zigzag pleated sheet is formed by the hydrogen bonds which is either intramolecular or intermolecular. The sheet like structure is formed when two or more segments of polypeptide chain are present next to each other and connected via hydrogen bonds. Individual segments in beta sheets are known as beta strands and they are rarely found in proteins because the structure is not stable. When two adjacent beta strands line up, they can form bridges of hydrogen bonds. This stable structure is known as Beta sheet. However there are further two types of beta sheets: a) Parellel and b) Anti-parallel.

a) Parallel beta sheet:When beta strands line up edge to edge in the same direction, it forms highly stable sheet.

b) Anti-parallel beta sheet: Beta strands runs into opposite direction with each other. The anti-parallel conformation is more stable and more common than the parallel one.

  • Loops and turns in the secondary structure
  1. Loops and turns connect alpha helices and beta strands.
  2. The more common cause for a polypeptide chain to contain more loops is to make the structure more compact and stable.
  3. Loops that have only 4-5 amino acid residues are called as turns.
  4. When they have internal hydrogen bonds, loops generally have hydrophillic residues and are found on the surface of the protein. While turns and bends refer to short segments of amino acids that connect the ends of two adjacent segments of anti-parallel beta sheets.

Before moving to the tertiary structure, it is important to understand some basic concepts for the functions of protein such as domains and motifs.

  • Domains, motifs and folds

The polypeptide chains with more than 100 amino acid residues often fold into two or more stable globular units are called domains. The domain is a functional area of protein which performs certain physical or chemical activity. Along with the domains there are certain motifs that represent only the structural characteristics of protein. Motif does not perform functional activity, instead they are a part of domain. In many cases a domain from a large protein still retain to its 3-D structure even if it is separated from polypeptide chain. While it is not in the case of motif because it contains short sequences of amino acids.

3. Tertiary structure

It represents the entire three dimensional conformation of protein. It indicates all the secondary structure helices, loops, turns, bends, sheets and how are they assembled to form a domain in space. It basically explains the 3-D structure of single protein(unlike quaternary structure) and how do all the small components contribute to coiling and compacting the structure to make it stable.

4. Quaternary structure

The quaternary structure of protein involves the clustering of more than one protein chains into a specific shape. This complex structure of protein is formed via various reactions and interactions such as hydrogen bonding, salt bridge formation, disulphide bonds, van der waals forces, covalent bonds etc. It contains many sub units and give rise to a complex structure held by various bonds.