The Mechanism of Cellular Respiration

Cellular respiration:

It is divided into few stages

Glycolysis

Pyruvic acid oxidation

Krebs’s cycle /citric acid cycle

Respiratory chain

• 1.       Glycolysis:

Glycol means “Glucose” & lysis means “breakdown”. So this is the process of glucose break down & formation of Pyruvic acid. It can take place in absence/ presence of oxygen, both resulting in same product. A series of steps involved in Glycolysis require specific enzymes.

Glycolysis can be divided into two phases:

        i.            Preparatory Phase

      ii.            Oxidative Phase

Preparatory phase

The first step in Glycolysis in the transfer of phosphate group from ATP to the 6^th carbon atom of glucose; as a result a molecule of glucose 6 phosphate is formed. An enzyme catalyzes the conversion of glucose 6 phosphate to its isomer, fructose 6 phosphate. At this stage another ATP molecule transfer a second phosphate group at 1^st carbon atom of the glucose. The product is fructose-1, 6-biphosphate. The next step in Glycolysis is the enzymatic splitting of fructose-1, 6-diphosphate into 3-phospoglyceraldehyeand dihydroxy acetone phosphate. These two molecules are isomers and are readily interconnected by enzymes.

Oxidative Phase

In this phase two electrons or two hydrogen atoms are removed from the molecule of 3-phosphoglyceraldehde (PGAL) and transferred to a molecule of NAD. During this reaction of second phosphate group is donated to the molecule, which resulted in the formation of 1, 3-diphosphoglyceric acid (DPGA). The oxidation of PGAL is an energy yielding process. At the very next in Glycolysis ATP is formed. The end product of this reaction is 3-phosphoglyceric acid. In the next step 3-phosphoglyceric acid is converted to 2phosphoglyceric acid. From 2-phosphoglyceric acid a molecule of water is removed and the product is phosphoenol Pyruvic acid (PEP). PEP then gives up its high-energy phosphate to convert a second molecule of ADP to ATP. The product is Pyruvic acid (C₃H₄O₃).

• 2.       Pyruvic acid Oxidation:

Pyruvic acid, the end product of Glycolysis does not enter the Krebs cycle directly. The Pyruvic acid is first changed into 2-carbon acetic acid molecule. One carbon is released as Co₂. Acetic acid on entering the mitochondrion unites with coenzymes A (CoA) to form acetyl CoA. In addition more hydrogen is transferred to NAD.

• 3.       Krebs’s Cycle:

Sir Hans Krebs discovered Krebs’s cycle. It starts after the formation of Acetyl CO-A. Krebs cycle takes place in mitochondrial membrane & comprises of following steps:

         i.            The union of acetyl CoA with oxaloacetate to form citrate. In this process molecule of CoA is regenerated and one molecule of water is used. Oxaloacetate is a 4-carbon acid with two carboxyl groups. Citrate thus has 6-carbon atoms and three carboxyl groups.

       ii.            In the next reactions NAD mediated oxidation takes place and citrate is changed into Ketoglutarate.

      iii.            Ketoglutarate is then oxidized & decarboxylated simultaneously. Thus a new product Succinate is formed. One ATP molecule is also synthesized.

     iv.            The next step in the Krebs cycle is the oxidation of Succinate to fumarate. Once again, two hydrogen atoms are moved, but this time the oxidizing agent is a coenzyme called flavin adenine dinucloetide (FAD), which is reduced to FADH₂.

       v.            With the addition of another molecule of water, fumigate is converted to malate.

     vi.            Anther NAD mediated oxidation of malate produces oxaloacetate, the original 4-carbon molecule.

• 4.       Respiratory Chain

NADH formed in Krebs’s cycle transfers its hydrogen atom to the electron carriers of respiratory chains. This transfer brings about transport of electron down to all carriers resulting in a series of reduction oxidation process & ultimately releasing O₂ & water is formed. Electron carriers are

         i.            CoenzymeQ

       ii.            Series of cytochromes enzymes

      iii.            Molecular oxygen

There are two types of Aerobic Respiration

• 1.       External Respiration

In this stage the organisms take the air (containing oxygen) into their bodies. This stage includes the transport of oxygen obtained from the inhaled oxygen to each cell of the body.

• 2.       Internal Respiration

The second stage, which is called internal respiration, consists of the oxidation of glucose, amino acid and fatty acids etc., with molecular oxygen. This respiration is also known as cellular respiration is also known cellular respiration as it occurs within cells.

In the internal or cellular respiration glucose and other compounds are passed through such enzymatic reactions, which release the chemical energy gradually in small amounts, with the help of which ATP molecules are.

There are two method of respiration in the organisms.

Anaerobic Respiration

Some organisms oxidize their food without using any molecular oxygen. This is known as anaerobic respiration.

In this type of respiration considerably less amount of energy is produced as compared with the other type of respiration. It is also called fermentation.

In anaerobic respiration, a glucose molecule is broken down into two molecules of lactic acid with a release of only 47,000 calories of energy.

Glucose→2Lactic acid+ Energy (47,000 calories)

        i.            Holic Fermentation

In primitive cells and in some eukaryotic cells such as yeast, Pyruvic acid is further broken down by alcoholic fermentation into alcohol (C2H5 OH) and

CO₂

2(C₃H₄O₃)       2(C₂H₅OH)  +    2CO₂

Pyruvic acid       Alcohol

      ii.            Actic acid fermentation

In lactic acid fermentation, each Pyruvic acid molecule is converted in to lactic acid C₃ H₆ O₃ in the absence of oxygen gas.

2(C₃H₄O₃)     +      4H      2(C₃H₄O₃)

Pyruvic acid                          Lactic acid

Both alcoholic and lactic acid fermentation yield about 2% of the energy present within the chemical bonds of glucose, which is converted into adenosine triphosphate (ATP).

Respiration

Definition: Living organisms need energy, which is provided by the phenomenon of respiration. It is the process by which organism’s breakdown complex compounds containing carbon to get a maximum of usable energy. Generally respiration means the exchange of respiratory gases (CO2 and O2) between the organism and its environment. This exchange is called external respiration, which is followed by cellular respiration. Cellular respiration is the process by which energy is made available to cells in a step-be-step breakdown of C-chain molecules in the cells.

Aerobic Respiration

In most of the higher and larger organisms, the glucose etc. is oxidized by using molecular oxygen. This type of respiration is known as aerobic respiration.

In aerobic respiration a mole of glucose is oxidized completely into carbon dioxide and water releasing enormous amount of energy. One glucose molecule in this respiration produces 686, 000 calories of energy. Aerobic respiration thus produces 20 times more energy than the anaerobic respiration.

Gaseous Exchange between Organisms and Environment

In aerobic respiration the organisms utilize the environment oxygen to oxidize their organic compounds as a result of which carbon dioxide is produced. The carbon dioxide is toxic to the organism and it is, therefore, necessary that the organism should expel the carbon dioxide out of their bodies in some way.

The aerobic organisms in the process of respiration take up oxygen from their environment and eliminate carbon dioxide from their bodies to the environment. The exchange of gases of between the organisms and their environment from the first stage of aerobic respiration.

The Limiting Factors of Photosynthesis

Limiting factor can be any environment factor e.g., absence of some metabolic reaction or deficiency of light or in avail ability of suitable temperature, CO2, water etc.

Effect of Absence/ Deficiency of Light

The rate of photosynthesis is proportional to light intensity up to a certain limit. As the light intensity increases the rate of reaction also increases but at very high light intensity the rate of reaction doesn’t change.

Effect of Suitable temperature Availability

The process of photosynthesis goes well certain range of temperature. If temperature exceeds this range the reaction retards / stops. Decrease in temperature decreases the rate of reaction.

Effect of CO2 Amount Provided

CO2 is the major reactant of photosynthesis. So, its high amount will induce rate of photosynthesis to increase only when factors are ideally present / available.

Rubiso

Rubiso is an enzyme, which catalyzes the first step of photosynthesis. It has affinity both for Co2 & O2. So in the presence of large amount of CO2, it binds O2 when large concentration of CO2 is available (to bring about respiration). That mean in presence of large amount of oxygen, Rubisco will not be available to catalyze fixation of CO2.

In contrast, if concentration of available CO2 exceeds the threshold level, the stomata get closed & thus rate of photosynthesis declines.

The Features Which Terrestrial Plants Have Adopted

Adaptations

• 1.       The arrangement of leaves is accurate to allow maximum of their surface to sunlight.
• 2.       The flat surface of leaves also provides maximum surface area for absorption of sunlight.
• 3.       The epidermis of leaves is made up of single cell & covered by cuticle. The cuticle avoids water loss & thin epidermis contain tiny pores i.e., stomata for the exchange if gases with surrounding.
• 4.       Different types of mesophyll cells are present in the epidermis. Palisade cells: which are compact. Spongy cells: present in lower layer & contain intercellular air spaces.
• 5.       Terrestrial plants have adapted for gaseous exchange by having more stomata in lower epidermis of leaves as compared to aquatic plants.
• 6.       Aquatic plants have more stomata on upper leaf epidermis as compared to terrestrial plants
• 7.       Xylem vessels have affinity for sap to facilitate their transport to leaf by various mechanisms. Phloem cells are adapted to transport food from leaves to all plant body efficiently.
• 8.       Stomata are so controlled that they provide entry of air into leaf & it leads to intercellular spaces after diffusing in to water (present around mesophyll cell in a thin layer).
• 9.       Chloroplast is present in mesophyll cells, where photosynthesis takes place.

The Role of Chloroplast and Light in Photosynthesis

It is the driving energy of photosynthesis Light is visible part of solar radiations. Light behaves as waves as well as short of energy called photons. The visible light ranges from about 389 to 750nm in wavelength. The amount of energy of a photon is inversely related to the wavelength of the light. Thus, a photon of violet light has nearly twice as much energy as a photon of red light. However in photosynthesis, number of quanta (photons) is more effective than the energy of quanta.

As the sunlight comprises of wide range of wavelengths. Only the rays of suitable wavelengths are absorbed by the chlorophylls.

Absorption spectrum of chlorophylls indicates that absorption is maximum in blue and red parts of the spectrum.

On absorption of light the electrons of chlorophyll get excited. The electron carries of ETC (Electron Transport Chain) then transport them & during their transport Chemiosmosis or formation of ATP & reduction of NADP to NADPH takes place

Chlorophyll

These are different kinds of chlorophylls. The chlorophyll a, b, c and d are found in eukaryotic photosynthetic plants and algae while the other are found in photosynthetic bacteria and are known as bacterial chlorophylls.

Chlorophylls absorb mainly violet-blue and orange red wavelengths. Green and yellow wave lengths are least absorbed by chlorophylls and are transmitted or reflected, although the yellow are often masked by dark green colour, hence plants appear green.

Action Spectrum

Chlorophyll a is the must abundant and the must important photosynthetic pigment as it takes part directly in the light all photosynthetic organisms except photosynthetic bacteria.

Chlorophyll b is found along with chlorophyll a in all green plants and green algae. Chlorophylls are insoluble in water but soluble in organic solvents.

Carotenoids-accessory pigments

Carotenoids are yellow and red to orange pigments that absorb strongly the blue violet range different wavelengths than the chlorophyll absorbs. So they broaden the spectrum of light that provides energy for photosynthesis.

Thus chlorophyll b is called accessory pigment because it absorbs light and transfers the energy to chlorophyll a, which then initiates the light reaction.

Some carotenoids protect chlorophyll from intense light by absorbing and dissipating excessive light energy, rather than transmitting energy to chlorophyll.

Cyclic Phosphorylation

Under certain condition, photo excited electron takes an alternative path called cyclic electron flow.

This path used Photosystem I but not Photosystem II. The electron cycle back from primary electron acceptor to ferredoxin (Fd) then to the cytochromes complex and from there continues on the P700 chlorophyll. The coupling of ETC by Chemiosmosis generates ATP. There is no production of NADPH and no release of oxygen. This results due to the accumulation of NADPH in the chloroplasts when Calvin cycle shows down.

Chemiosmosis

In both cyclic and non-cycle phosphorylation electron transport chain pumps protons (H+) across the membrane into thylakoids space. The energy used for the pumping comes from the electrons moving through the electron transport chain. Next the hydrogen ions move down their gradient through special complex called ATP syntheses, which are built in the theylakoid during this diffusion of electrons the energy of electrons is used to make ATP.

Dark or Light Independent Reaction

This reaction does not require light directly and can occur in the presence as well as absence of light provided ATP and NADPH are present.

In this pathway a number of cyclic events take place. Calvin used radioactive carbon c14 to prepare c14o2. The Calvin cycle can be divided into three phases: Carbon fixation, Reduction and Regeneration of CO2 acceptor (RuBP).

Carbon Fixation

In corporation of CO2 into organic material is called carbon fixation. A5- Carbon compound (RUBP) combines with CO2 to form a highly unstable 6- Carbon compound. This 6-Carbon compound then breaks into two molecules of PGA or Phosphoglyceric acid (3-carbon compound).

Since the product is a three-carbon compound, so the Calvin cycle is also known as C3 pathway.

Reduction

PGA is then reduced to a 3-Carbon Carbohydrate by the addition of a phosphate group (1, 3 biphosphoglycerate) in the presence of ATP. This is further reduced into G3P (Glyceraldehydes 3-phosphate) in presence of NADPH. This is then further processed to manufacture glucose.

Regeneration of CO2 acceptor (RUBP)

Through a complex series of reaction, the carbon skeleton of five molecules of three carbon G3P are rearranged into three molecules of five carbon ribulose phosphate (RuP).

Each RuP is phosphorylated to ribulose biphosphate (RuBP), the five-carbon CO2 acceptor with which the cycle started. Again more molecule of ATP of light reactions are used for this phosphorylation of three RUP molecules.

The Mechanism of Photosynthesis in Detail

Mechanism of Photosynthesis.

Light Reaction (takes place in the thylakoids membrane).

(Energy conversion phase: formation of ATP and NADPH2)

The sunlight energy, which is absorbed by photosynthesis pigments, drives the process of photosynthesis. Photosynthesis pigments are organized into clusters, called photo systems for efficient absorption and utilization of solar energy in the thylakoids membranes.

Each photosyntem consist of a light gathering antenna complex and a reaction center. The antenna complex has many molecules of chlorophyll a chlorophyll b and carotenoids. Reaction center has one or more molecules of chlorophyll along with a primary electron acceptor, and associated electron carries of electron transport system.

Photosystem I (PS I)

Photosystem II (PS II)

These are named so in order of their discovery. Photosystem I ha chlorophyll and molecule, which absorbs maximum light of 700 nm and is called P700the reaction center of Photosystem II is P680, the form of chlorophyll a, which absorbs best the best the light of 680 nm.

In predominant types of electron transport called non-cyclic electron flow, the electrons pass through the two Photosystem (non-cyclic phosphorylation). In less common type of path called cyclic electron flow only Photosystem I is involved (cycle phosphorylation).

NON-Cyclic Phosphorylation

• 1.       When Photosystem II absorbs light, an electron excited to a higher energy level in the reaction center and is captured by the primary electron acceptor of PS II. The oxidized chlorophyll is now a very strong oxidizing agent; its electron “hole” must be filled.
• 2.       This hole is filled by electrons, which are extracted by an enzyme from water. The spitting of water in photosynthesis that releases oxygen is called photolysis.
• 3.       Each photo excited electron passes from the electron acceptor of photosyntem II to photosyntem I via an electron transport chain. This chain consists of an electron carrier called plastoquinone (PQ), a complex of two cytochromes and a copper containing protein called plastocyanin (PC).
• 4.       As electrons move down the chain their energy goes on decreasing and is used by thylakoids membrane to produce ATP (photophosporylation).
• 5.       The electron reaches “bottom” of the electron transport chain and fills an electron “hole” in P700 which is created when light energy is absorbed by molecules of P700 and drives an electron from P700 to the primary acceptor of Photosystem I.
• 6.       The primary electron acceptor of Photosystem I passes the photo excited electrons to a second electron transport chain, which transmits them to ferredoxin (Fd) and then to form NADPH which provide redacting power for the synthesis of sugar in the Calvin Cycle.

This path of electrons through the two Photosystem during non-cyclic phosphorylation is known as Z- scheme from its shape.

Permanent Tissues

Tissues

The cells of these tissues lack the ability to divide. They largely originate from primary meristematic tissue.

These are further divided into the following groups

(i)                  Epidermal Tissues (ii) Ground Tissues  (iii) Support Tissues.

• 1.       Epidermal Tissues

These are composed of single layer of cells which are found as the outermost protective covering of leaf, stem and roots.

Properties

(i)                  The cells in epidermal tissue are living having thick walls.

(ii)                They are flattened.

(iii)               They are closely packed with no inter-cellular spaces.

Functions

(i)                  They act as a barrier between the environment and the internal plant tissues. They are also responsible for the absorption of water and minerals primarily in the root region.

(ii)               They secrete cutin (the coating of cutin iscalled cuticle) on stem and leaves. They cutin prevents overheating of water.

(iii)               Epidermal tissues also have some specialized cells that perform specific functions. For Example:

(a)    Root Hairs: absorb water and minerals.

(b)   Leaf hairs: (1-2 cells) reflect light to protect against overheating and excessive water loss. The layer of leaf hair acts to hold in a layer of humidity ‘trapped’. This layer also prevents air moving directly against the stomata which would encourage water loss.

(c)    Stomata: are made by guard cells and are most abundant on underside of leaves. They regulates diffusion of Co2 into the leaf for photosynthesis as well as regulate loss of water from the leaf.

(d)   Salt glands: are the waste-bins for the excess salt absorbed from the soil. They form a crust of salt on leaves which reflects light to prevent overheating.

2.       Ground Tissues

Ground tissues are simple tissues made up of parenchyma cells.

Parenchyma cells are most abundant cells in plants. Overall they are spherical but flat at point of contact.

Properties of Parenchyma Cells

(i)                  These are most abundant cells in plants.

(ii)                These are large sized living cells.

(iii)               These have thin walls.

(iv)              Some times may develop the ability to divide.

(v)                These have large vacuoles storage of food.

Functions

(i)                  Sometimes perform the function for the storage of food.

(ii)                In leaves, these are sites for photosynthesis and are called mesophyll.

(iii)               In some parts they are the sites of respiration and protein synthesis.

3.       Support Tissues

These tissues provide strength and flexibility to the plant. These are of two types.

(i)                  Collenchymas Tissues.

(ii)                Sclerenchyma Tissues.

(i)                  Collenchymas Tissues

(i)                  These are living cells.

(ii)                These have angular thickening of cellulose in the primary cell walls, which become unevenly thicker.

Location

These are found just beneath the epidermis in the cortex of young herbaceous stems and in the midribs of leaves and plants of flowers

Properties

(iii)               These are made of elongated cells.

(iv)              These are flexible.

Function

(v)                These provide support to the young herbaceous parts of plant due to their flexibility.

Sclerenchyma Tissues

Properties

(i)                  These are thick walled cells.

(ii)                Cell walls are heavily impregnated with lignin which provides hardness and strength to the cell and is the main chemical component of wood.

(iii)               These are two types of Sclerenchyma Tissues.

(a)    Fibrous Cells

These are long and cylindrical.

These are found in xylem and phloem for strength and transport of water.

(b)   Stone Cells

These are shorter than fibers.

These have uniformly thick cell walls.

These are found in testa (seed coat) and shells of Nuts or endocarp of stone fruits to provide protection.

(ii)                Compound Tissues

These which are compound of different kinds of cells, performing a commom function are called compound or complex tissues.

Xylem Tissues

This vascular tissue provides strength and transports water and dissolved salts from the roots to the stem and leaves in plants.

Types

There are three types of cells in xylem tissues which help each other to perform the function of transport. These are:-

(i)                  Xylem Parenchyma Cells.  (ii)  Vessel Elements

These are thick walled and dead cells. These join end to end to form long pipelines for unidirectional transport of water from roots to leaves.

(i)                  Tracheids

These are also thick walled and dead cells. These are spindle shaped which in addition to being involved in the transport of water, provide strength to root, stem and branches.

Phloem Tissues

These tissues are responsible for the conduction of dissolved organic matter between different parts of the plant body.

Types

These types of cells are found which function together for conduction

These are:

(i)                  Phloem parenchyma Cells

These store the surplus food.

(ii)                Sieve Tube Cells

These are elongated. Their end walls have small pores so the area at end is called sieve plate. These cells join end to end to form sieve tubes. They posses little protoplasm and lose their nuclei and Ribosomes etc. during development.

(iii)               Companion Cells

In some plants sieve tubes cells are accompanied with nucleated neighboring cells called companion cells. The companion cells contain functional DNA and Ribosomes and they make proteins for the sieve tube cells. Thee regulate or control the movement of food through sieve tubes.

Tissues

Tissue

Groups of cells associated together and performing a common function are called tissues.

Plant Tissues

Types

1.       Simple tissues

• 2.       Compound Tissues
•          i.            Simple Tissues

When single types of cells perform similar function then they form a simple tissue. These are further divided into two groups.......

(i)                  Meristematic Tissues   (ii)  Permanent Tissues

Meristematic Tissues

These are composed of cells, Which have power of division.

Properties

(i)                  They the ability of division.

(ii)                They have dense cytoplasm, large nuclei in the center with small or no vacuoles.

(iii)               They have thin and delicate cell walls, composed of cellulose.

(iv)              They are alike.

(v)                They do not have inter-cellular spaces among them.

Location

On the basis of location, meristematic cells are divided into three groups.

(i)                  Apical Meristems

It means the dividing cells are present at the shoot or root tip (apices).

These are also called growing points. These result in the increase in length of these shoots and roots. Such a growth is called primary growth.

(ii)                Lateral Meristems

It means the dividing cells are present on the lateral sides of shoot and roots. These results in the increase in diameter of these shoot and root.

Such a growth is called secondary growth.

There are two types of lateral Meristems

• a.       Vascular Cambium: This is present in between the center and phloem. This gives rise to new xylem cells towards xylem and new phloem cells towards on side.
• b.      Cork Cambium: This is present in the outer lateral sides and gives rise to a corky layer.

(iii)               Intercalary Meristems

Patches of this type of Meristems are present among the mature tissues. This helps in the regeneration of removed parts removed by herbivores. Mostly it is present in grasses.

The Relation Between Respiration and Photosynthesis in Plants

Respiration

The gaseous exchange in plant is not very evident during the daytime as the products of respiration.

I.e. carbon dioxide and water are the reactions in the process of photosynthesis......

So the carbon dioxide and water produced in the respiration are utilized in photosynthesis, occurring in the daylight. In the bright sunshine, because of high rate photosynthesis the carbon dioxide produced in respiration falls short and therefore, some carbon dioxide has to be taken into the plant form outside for photosynthesis.

In the day time the plant therefore, takes in carbon dioxide and expel out oxygen. The process of photosynthesis occurs in chloroplasts whereas the process of respiration takes place in cytoplasm and mitochondria.

G2 Phase (The second gap of DNA Synthesis)

The period after DNA synthesis, but before the next cell division is the G2 Phase. In this phase the cell prepares itself for mitosis by synthesizing essential proteins mainly for the production of spindle fibers......

Mitosis

A type of cell division in which a cell divides in to daughter cells each with the same number of chromosomes as was present in parent cell.

Mitosis, in eukaryotic cells, consists two phases majorly i.e. Karyokinesis Nuclear cell division) and cytokinesis (cytoplasmic division). These two process usually occur together.

Mitosis occurs exclusively in eukaryotes. For convince metaphase, anaphase and telephone.

Stryctre of Chromosomes

During most of the life of eukaryotic cell, the nucleus contains darkly coloured. Thread like (extended form) structure called chromatin (Greek word for colour). This chromatin is actually a long molecules of DNA, continuous from end to end, complexed with special protein called histones. But when cell divides, chromatins condense to form chromosomes (Greek cord coloured bodies). Most chromosomes consist of two arms (Jelomer with one arm)that extend out from a specialized region called the Centromere (meaning middle body).

Before a cell divides, it duplicates its chromosomes. The two remain attached at their Centromeres. As long as they are attached to one another, the copies are called sister Chromatids. Each Chromatid is a single DNA molecule identical to the DNA of the original chromosomes before replication. The whole structure (two sister Chromatids attach at their Centromere is still considered to be a single chromosomes.

During cell division the whole sister Chromatids separate, with each Chromatid becoming an independent daughter chromosome.

Bioenergetics

The quantitative study of energy relationships & conversions in different forms is called Bioenergetics.

For example, the solar energy is converted into chemical energy by the plants in the process of photosynthesis. Similarly, this chemical energy of utilized in to some metabolic reaction producing heat of some other form of energy etc.

Thus these energy conversions obey the laws of thermodynamics.

How and Why Organisms Need Energy?

How?                                  

For any organism, the ultimate source of energy is its food obtained by any means. This food is when metabolized in the body release some energy which was previously present in the form of potential Energy in the bonds of food particles/ molecules. When the food is catabolised this potential energy is converted to kinetic energy. Some of this released energy is converted to chemical energy (ATP) so that it can be stored in the body. This chemical energy is actually a form of potential energy. So when body requires energy for performing some task/ activity, this potential energy is converted again to kinetic energy by the hydrolysis of ATP.

Why?

All organisms need free energy for performing different activities & running the various processes of metabolism.

For Example

•          i.            For the synthesis of proteins.
•        ii.            Active Transport.
•       iii.            Phagocytosis.
•      iv.            Pinocytosis.
•        v.            Mechanical work.
•      vi.            Maintaining temperature of body.
•     vii.            Nerve impulse etc.

Role of Redox Reactions

Redox

The term Redox is a combination of two processes Reduction. So, a Redox reaction involves both these processes simultaneously.

Oxidation Reduction

Oxidation-reduction can be defined as

•          i.            Gain of Oxygen.
•        ii.            Loss of Hydrogen.
•       iii.            Loss of electrons.

Whereas reduction is referred to

•          i.            Loss of Oxygen.
•        ii.            Gain of Hydrogen.
•       iii.            Gain of electrons.

These two processes are constantly involved in flow of energy. Is the energy generation & utilization go in cyclic manner by manner by these reactions.

Energy Exchange & electron Exchange

If the electrons are lost from an atom & gained by some other atom, they carry the energy by some other atom. This transfer of energy may in turn be utilized to make a stable association between two atoms.

Every atom possesses both proteins & electrons.

For example, Hydrogen atom has one proton & one electron and as already mentioned “loss or gain of hydrogen atoms is also called Redox Reaction”. So, when a molecule loses a hydrogen atom, it actually loss an electron & vice versa when a molecule gains a hydrogen atom.

CELL CYCLE

Cell Cycle is the sequence of events in the life from the time a cell is produced unit it completes mitosis and produces new cells. The eukaryotic cell cycle consists of major phase i.e. Interphase and Mitotic phase (M phase) also called cell division.

Interphase

It is the period between cell divisions, during which the cell acquires nutrients from its environment, grows and replicates its chromosomes. The cell biologist divided Interphase into three sub phase i.e. G1, S and G2.

G1 phase (first gap in DNA Synthesis)

This is the phase in which most of the growth and metabolic activity occurs. The protein supply increase in the cell and number of many of its organelles like mitochondria, ribosome’s, Golgi bodies and carries out its specialized functions e.g. hormone synthesis and recreation. On later in G1phase, at a time called restriction point, the cell undergoes a kind of internal evaluation of its ability to complete the cell cycle and produce two viable daughter cells. If evaluation is positive the cell committed to DNA replication and cell division and if evaluation is negative the cells temporarily or permanently atop dividing are said to have entered a state of quiescence also called go phase.

For Example

Neurons oftenly undergo long periods of quiescence. Liver and kindly cells undergo the period of quiescence temporally and other cells such as epithelial, cells do not enter Go phase and continue throughout organism life.

S phase (DNA Synthesis Phase)

Chromosomes replication occurs in this phase. Each chromosomes is duplicated as a result each chromosome consists of two sister chomratids, which contains identical genes. During S phase, animal cells also duplicate their Centrioles.

Vacuole

These are fluid filled single-membrane bound organelles.

Number

Animal cells have large number of small sized vacuoles in them but plant cells have single large vacuole in them.

Types of vacuole and their functions

Food Vacuole

Many cells take in materials from outside in the form of food vacuole.

Contractile Vacuole

Sometimes, vacuoles are associated with elimination of wastes from the cells; such vacuoles are called contractile vacuoles.

The problem faced by plant cells to maintain their water quantity.

Environment can never be same as the environment inside the cell. So plant cells have a lot of problems to maintain the water quantity.

Water balance problems are somewhat different for plant cells because of the presence of rigid cell walls. In a hypotonic environment, water enters inside the cell and then into vacuole. When vacuole increases in size the cytoplasm presses firmly against the interior of cell wall, which expands a little. Plant cell becomes rigid. The internal pressure of such a rigid cell is known as Turgor pressure and process is called Turgor.

In isotonic environment the plant cell is flaccid (loose / not firm), because the net uptake of water is not enough to make it turgid.

In hypertonic solution, Water is released from the cell and because shrinkage of the cytoplasm but not of cell wall. This shrinkage of cytoplasm is called Plasmolysis.

The cell as unit of life in terms of its structure and function

Various cells have various structures but when they perform functions in coordination with each other then they become unit of life. It means there is a division of labour among the cells of an organism. The phenomenon is common to both animal and plant cells.

Plant Cells

In plants variety of cells are present which perform different functions e.g.

•         i.            Xylem cells conduct water and mineral salts soil from to the aerial parts of the plant.
•       ii.            Phloem cells transport food.
•     iii.            Sclerenchymatous cells give support to the plants
•      iv.            Parenchymatous cells store surplus food.

As these cells perform different functions they show great variation in shapes and sizes. Despite the structural and functional diversity, the plant cells have s common plant of organization.

Centrioles

Location

Animal cells and cells of some microorganisms and lower plants contain two Centrioles located near the exterior surface of the nucleus. They are absent in higher plants.

Structure

In cross section each Centrioles consists of a cylindrical array of nine microtubules. However, each of the nine microtubules is further composed of tree tubules. The two Centrioles are usually placed at right angle to each other.

Functions

Just before a cell divides, its Centrioles duplicate and one pair migrates to the opposite side of the nucleus. The spindle then forms between them.

Lysosomes

Lysosomes are cytoplasmic organelles and are different from others due to their morphology. Lysosomes (Lyso = splitting; soma = body) are found in most eukaryotic cells. They are most abundant in those animal cells, which exhibit phagocytic activity. They are bound by a single membrane and are simple sacs rich in acid phosphatise and several other hydrolytic enzymes.

These enzymes are synthesized on RER and are further processed in the Golgi apparatus. The processed enzymes are budded off as Golgi vesicles and are called as primary Lysosomes.

Discovery

These were discovered in the mid-twentieth century by a Belgian Scientist “de Duve”.

Functions

Because lysosomes contain strong digestive enzymes so these work for the break-down of food and waste materials inside the cell.

Golgi Apparatus

Golgi apparatus or Golgi complex is an assemblage of flat or curved sacs (cisternae) lying one above the other.

Discovery

These were discovered by “Camillo Golgi” in 1898 in the neural tissue of the animals.

Functions

•          i.            These connected with cell secretions, which are products formed within the cell on Ribosomes and then passed to the outside through endoplasmic reticulum and Golgi apparatus. These secretions are converted into finished product.
•        ii.            These also modify the proteins and lipids by adding carbohydrates and convert them into glycoprotein or glycolipids.

•       iii.            These are also involved in lysosome synthesis.

Endoplasmic Reticulum (ER)

This is a complex membranous system, which consists of two unit-membranes enclosing a narrow apace about 4mm wide. These form a network of interconnected channels that extends from cell membrane to the nuclear membrane.

Types

         i.            Rough Endoplasmic Reticulum.

       ii.            Smooth Endoplasmic Reticulum.

1.       Rough Endoplasmic Reticulum

Numerous Ribosomes are present on this type of Endoplasmic Reticulum giving it a rough texture.

2.       Smooth Endoplasmic Reticulum

These are free from Ribosomes.

Functions

a.       Rough Endoplasmic Reticulum helps in protein synthesis.

b.      Smooth Endoplasmic Reticulum is involved in lipid metabolism.

c.       These also help in transport of materials from one part of the cell to the other part.

d.      These also help to detoxify harmful chemicals that have entered the cell.