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Home , Biochemistry, Cell (biology)

CHAPTER-I

INTRODUCTION TO

CELL AND IT’S ORGANIZATION

The is the basic structural and functional unit of all known living . It is the smallest unit of that is classified as a living thing, and is often called the building block of life. Organisms can be classified as unicellular (consisting of a single cell; including most ) or multicellular (including and ). contain about 10 trillion (1013) cells. Most and cells are between 1 and 100 µm and therefore are visible only under the . The cell was discovered by in 1665. The , first developed in 1839 by and , states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

TYPES OF CELLS

There are two types of cells: eukaryotic and prokaryotic The cell is simpler, and therefore smaller, than a cell, lacking a nucleus and most of the other of .

SUBCELLULAR COMPONENTS

All cells, whether prokaryotic or eukaryotic, have a that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a salty takes up most of the cell volume. All cells possess DNA, the hereditary material of , and RNA, containing the information necessary to build various such as , the cell's primary machinery. There are also other kinds of in cells. This article lists these primary components of the cell, then briefly describe their .

MEMBRANE

The cytoplasm of a cell is surrounded by a or plasma membrane. The plasma membrane in plants and is usually covered by a . This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of (hydrophobic -like ) and hydrophilic molecules. Hence, the layer is called a bilayer. It may also be called a fluid membrane. Embedded within this membrane is a variety of molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi- permeable', in that it can either let a substance ( or ) pass through freely, pass through to a limited extent or not pass through at all. Cell surface also contain proteins that allow cells to detect external signaling molecules such as

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during , the uptake of external materials by a cell, and , the separation of daughter cells after ; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of , intermediate filaments and . There is a great number of proteins associated with them, each controlling a cell's by directing, bundling, and aligning filaments. The is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.

GENETIC MATERIAL

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long-term information storage, but some (e.g., ) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA) in organisms that use DNA for the itself. Transfer RNA (tRNA) molecules are used to add amino acids during protein .

Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial ) in the region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and .

A cell has genetic material contained in the (the nuclear ) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 23 pairs of linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called . This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome

ORGANELLES

The contains many different organs, such as the heart, lung, and kidney, with each performing a different function. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized for carrying out one or more vital functions. Both eukaryotic and prokaryotic cells have organelles but organelles in eukaryotes are generally more complex and may be membrane bound. There are several types of organelles in a cell. Some (such as the nucleus and ) are typically solitary, while others (such as mitochondria, and ) can be numerous (hundreds to thousands). The is the gelatinous fluid that fills the cell and surrounds the organelles.

Diagram of a cell nucleus

• Cell nucleus – eukaryotes only - A cell's information center, the cell nucleus is the most conspicuous found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis () occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the . The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The is a specialized region within the nucleus where subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm.

• Mitochondria and – eukaryotes only - the power generators: Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy by oxidative , using to release energy stored in cellular (typically pertaining to ) to generate ATP. Mitochondria multiply by splitting in two. Respiration occurs in the cell mitochondria.

Diagram of an

• – eukaryotes only: The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in sequestration and release.

• Golgi apparatus – eukaryotes only : The primary function of the Golgi apparatus is to process and package the such as proteins and lipids that are synthesized byy the cell.

• Ribosomes: The ribosome is a large complex of RNA and protein molecules. They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).

• LLysosomes and Peroxisomes – eukaryotes only: Lysosomes contain digestive enzymes (acid ). They digest excess or worn-out organelles, particles, and engulfed viruses or baacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane- bound system.

• – the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. are composed of two , which separate during cell division and help in the formation of the mitotic . A single centrosome is present in the animal cells. They are also found in some fungi and cells. • : Vacuoles store food and waste. Some vacuoles store extra . They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably , have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of eukaryotic cells are usually larger in those of plants than animals.

THE

Cross section view of the that can be formed by in aqueous solutions

A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selective barrier, within or around a cell. It consists of a bilayer with embedded proteins that may constitute close to 50% of membrane content. The cellular membranes should not be confused with isolating tissues formed by layers of cells, such as mucous and basement membranes.

Function

Membranes in cells typically define enclosed spaces or compartments in which cells may maintain a chemical or biochemical environment that differs from the outside. For example, the membrane around peroxisomes shields the rest of the cell from peroxides, and the cell membrane separates a cell from its surrounding medium. Most organelles are defined by such membranes, and are called "membrane-bound" organelles.

Probably the most important feature of a biomembrane is that it is a selectively permeable structure. This means that the size, charge, and other chemical properties of the and molecules attempting to cross it will determine whether they succeed in doing so. Selective permeability is essential for effective separation of a cell or organelle from its surroundings. Biological membranes also have certain mechanical or elastic properties.

Particles that are required for cellular function but are unable to diffuse freely across a membrane enter through a protein or are taken in by means of endocytosis. TRANSPORT ACROSS THE BIOLOGICAL MEMBRANE

• Passive diffusion-movement of solute along the concentration gradient • -movement of solute along the conc gradient with the help of carrier proteins • Active transport-movement of solute against the conc gradient with the utilization of energy in the form of ATP

TRANSPORT MODES

• Uniport • Symport • Cotransport • Antiport

TRANSPORT OF MACROMOECULES

• Endocytosis • Exocytosis

HIGH ENERGY COMPOUNDS

High-energy can mean one of two things:

• The phosphate-phosphate bonds formed when compounds such as and are created.

• The compounds that contain these bonds, which include the diphosphates and nucleoside triphosphates, and the high-energy storage compounds of the muscle, the phosphagens. When people speak of a high-energy phosphate pool, they speak of the total concentration of these compounds with these high-energy bonds.

High-energy phosphate bonds are bonds, acid anhydride linkages, formed by taking phosphoric acid derivatives and dehydrating them. As a consequence, the of these bonds is exergonic under physiological conditions, releasing energy.

Energy released by high energy phosphate reactions Reaction ΔG [kJ/mol]

ATP + H2O → ADP + Pi -36.8

ADP + H2O → AMP + Pi -36.0

ATP + H2O → AMP + PPi -40.6 PPi + H2O → 2 Pi -31.8

AMP + H2O → A + Pi -12.6

Except for PPi → 2 Pi, these reactions are, in general, not allowed to go uncontrolled in the human cell but are instead coupled to other processes needing energy to drive them to completion. Thus, high-energy phosphate reactions can:

• provide energy to cellular processes, allowing them to run • couple processes to a particular nucleoside, allowing for regulatory control of the process • drive the reaction to the right, by taking a reversible process and making it irreversible.

The one exception is of value because it allows a single hydrolysis, ATP + 2H2O → AMP + PPi, to effectively supply the energy of hydrolysis of two high-energy bonds, with the hydrolysis of PPi being allowed to go to completion in a separate reaction. The AMP is regenerated to ATP in two steps, with the equilibrium reaction ATP + AMP ↔ 2ADP, followed by of ATP by the usual means, oxidative phosphorylation or other energy-producing pathways such as .

Often, high-energy phosphate bonds are denoted by the character '~'. In this "squiggle" notation, ATP becomes A-P~P~P. The squiggle notation was invented by Fritz Albert Lipmann, who first proposed ATP as the main energy transfer molecule of the cell, in 1941. It emphasizes the special of these bonds.

ATP is often called a high energy compound and its phosphoanhydride bonds are referred to as high-energy bonds. There is nothing special about the bonds themselves. They are high-energy bonds in the sense that free energy is released when they are hydrolyzed, for the reasons given above. Lipmann’s term “high-energy bond” and his symbol ~P (squiggle P) for a compound having a high phosphate group transfer potential are vivid, concise, and useful notations. In fact Lipmann’s squiggle did much to stimulate interest in .The term 'high energy' with respect to these bonds can be misleading, because the negative free energy change is not due directly to the breaking of the bonds themselves. The breaking of these bonds, as with the breaking of any bond, is an endergonic step (i.e., it absorbs energy, not releases it). The negative free energy change comes instead from the increased resonance stabilization and solvation of the products relative to the reactants.

Adenosine triphosphate

Adenosine-5'-triphosphate (ATP) is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for . It is one of the end products of photophosphorylation and and used by enzymes and structural proteins in many cellular processes, including biosynthetic reactions, , and cell division. One molecule of ATP contains three phosphate groups, and it is produced by ATP from inorganic phosphate and adenosine diphosphate (ADP) or adenosine monophosphate (AMP). The three main ways of ATP synthesis are level phosphorylation, oxidative phosphorylation in cellular respiration, and photophosphorylation in .

Cyclic adenosine monophosphate (cAMP, cyclicAMP or 3'-5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is derived from adenosine triphosphate (ATP) and used for intracellular in many different organisms, conveying the cAMP-dependent pathway

K.ANITA PRIYADHARSHINI

LECTURER

DEPT.OF PHARMACEUTICAL

SRM COLLEGE OF