CELL BIOLOGY


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Location of oraganelles in the cell!

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a.
 
 

Did you know cells are enclosed by a semi-permeable membrane???

 Plasma Memebrane
 

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phospholipid bilayer

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The cell is highly organized with many functional units or organelles. Most of these units are limited by one or more membranes. To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle.  In essence membranes are essential for the integrity and function of the cell.

Membrane components may:
be protective
regulate transport in and out of cell or subcellular domain
allow selective receptivity and signal transduction by providing transmembrane receptors that bind signaling molecules
allow cell recognition
provide anchoring sites for cytoskeletal filaments or components of the extracellular matrix. This allows the cell to maintain its shape and perhaps move to distant sites.
help compartmentalize subcellular domains or microdomains
provide a stable site for the binding and catalysis of enzymes.
regulate the fusion of the membrane with other membranes in the cell via specialized junctions
provide a passageway across the membrane for certain molecules, such as in gap junctions.
allow directed cell or organelle motility

A cell is the smallest unit that can carry out life processes.They can vary in size, shape and structure. A microscope is used for their study. Every cell has a selectively permeable cell membrane. This means that some substances can pass through the cell membrane very easil, while some cannot. this is the permeability of the cell membrane and semipermeable membranes just allow few substances to pass through them.these semipermiable membranes regulate their interaction with their surroundings by letting few substances pass through them easily. by this activity the cell membrane regulates the chemical composition of the cell. the semipermeable membranes are as a result of the chemical and electrical properties of the membranes molecules. the example of the substance which can pass through them are lipid molecules, such as alchohol, ether and chloroform. small molecules like water, glucose,amino acids, carbon dioxide and oxygen can also pass through membranes ver easily. but large molecules like starch and proteins cannot. even in the same cell, the permeability may vary from one moment to another.
 
A plasma membrane is the flexible boundry between the cell and its environment. It allows a steady supply of nutrients such as amino acids, glucose adn lipids to come into the cell no matter what the external conditions are. If a phosphate group replaces a fatty acid, a phospholipid is formed. It has a glycerol backbone, two fatty acidchains and a phosphate group. The plasma memebrane is composed of a phospholipid bilayer, which has two layers of phospholipids back-to-back.
diffusion: when a substance moves from an area of high concentration to an area of low concentration.Active transport is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. Unlike passive transport, this process requires chemical energy. In this form of transport, molecules move against either an electrical or concentration gradient (collectively termed an electrochemical gradient). This is achieved by either altering the affinity of the binding site or altering the rate at which the protein changes conformations.

Glucose, sodium ions and choride ions are just a few examples of molecules and ions that must efficently get across the plasma membrane but to which the lipid bilayer of the membrane is virtually impermiable. Their transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass. Facilitated diffusion is the name given this process. It is similar to simple diffusion in the sense that it does not require expenditure of metabolic energy and transport is again down an electrochemical gradient.

Two major groups of integral membrane proteins are involved in facilitated diffusion:

  1. Carrier proteins(also known as permeases or transporters) bind a specific type of solute and are thereby induced to undergo a series of conformational changes which has the effect of carrying the solute to the other side of the membrane. The carrier then discharges the solute and, through another conformational change, reorients in the membrane to its original state. Typically, a given carrier will transport only a small group of related molecules.

    Some important and illustrative groups of transporters are:
    • Certain of the hexose transporters, which transport glucose and similar monosaccharides into and out of cells
    • Band 3, the anion transporter, which facilitates transport of bicarbonate and chloride ions

  2. Ion Channels do not really bind the solute, but are like hydrophilic pores through the membrane that open and allow certain types of solutes, usually inorganic ions, to pass through. In general, channels are quite specific for the type of solute they will transport and transport through channels is quite a bit faster than by carrier proteins. Additionally, many channels contain a "gate" which is functions to control the channel's permiability. When the gate is open, the channel transports, and when the gate is closed, the channel is closed. Such gates can be controlled either by voltage across the membrane (voltage-gated channels) or have a binding site for a ligand which, when bound, causes the channels to open (ligand-gated channels).

Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration. Cell membranes are completely permeable to water, therefore, the environment the cell is exposed to can have a dramatic effect on the cell. Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel. Hypotonic Solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode. Isotonic Solutions: contain the same concentration of solute as an another solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic.
 

The terms hypotonic and hypertonic are usually the big hang up.  Both these terms are opposite of each other.  The prefix "hypo" means less than.  What does a hypotonic solution have less of?   If you are thinking particles or solute, you are right!!   Note that in the middle diagram   - the hypotonic solution surrounding the cell - there are fewer particles in the solution surrounding the cell than in the cell (2 vs. 3).  Since the parts of the solution (solute + solvent) must add up to 100%,  this means there is more water in the solution surrounding the cell than there is in the solution inside the cell!  A hypotonic solution contains fewer particles (less solute) but more water (the solvent) than the solution on the other side of the membrane.  Since water can move freely across cell membranes, it moves from an area where it is in greater concentration to an area where the concentration is less.   Thus, if a cell is placed in a hypotonic environment, water will move into the cell (from an area of greater water concentration to an area of lesser water concentration).  Note the heavy arrows showing movement of water into the cell in the hypotonic solution - there is a net gain of water into the cell.

Possibly the confusion lies in the fact that the term hypotonic refers to the particle concentration (hypo = less than) whereas the process we are concerned with (water movement) is determined by the water concentration! 

Isotonic solution: A solution that has the same salt concentration as the normal cells of the body and the blood. As opposed to a hypertonic solution or a hypotonic solution. An isotonic beverage may be drunk to replace the fluid and minerals which the body uses during physical activity.

C.   What are eukaryotic cells?

There are two basic types of cells : Eukaryotic and prokaryotic. Eukaryotic cell are one type of cells which  are present in all living things except bacteria. Eukaryotic cells have many kinds of internal membrane-bound structures. The most important of these in the nucleus, the structure in which the cells heriditary material (DNA) is located. The term, "Eukaryotic" means "true nucleus". They are much more compartmentalized.  

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what are prokaryotic cells?
 

Prokaryotic cell lack internal membrane-bound structures. Within this type of cell, membranes do not seperate different areas of the cell from each other. They make up the smallest single-celled organisms, bacteria. It means "without nucleus".

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What are viruses?

Retrovirus

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Virus
 

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Viruses are tiny particles unlike any other living organisms.Infact ,scientist do not consider viruses to be living .They are described as somewhere between living cells and non living things.  Viruses lacks all other cell structures necessary for metabolism , reproduction and growth.Viruses are much smaller than cells and were not actually seen until the electron microscope was invented. Viruses vary in their structure and shape. Some are rod-shaped, such as Tobacco Mosaic Virus, more commonly known as TMV. The herpes virus is polyhedral or many-sided. Still others may be helical, symmetry and  icosahedral symmetry . 

B.   All about enzymes!

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An enzyme is an organic chemical that involves in chemical reactions and makes the process faster with out being changed itself.They are reusable .They are formed from long chains of proteins folded up in a certain way .There are tons of natural enzymes existing .There are different enzymes for different chemical reactions .For example in  the breaking of sugar into energy ,there are ten diiferent chemical reactions , therefore there will  be ten different enzymes for each of them .The activities of enzymes depend on the temperatures,ionic conditions and the pH of the surroundings . For example if an enzyme works at temperature of 37degree celcius, and then if it is put into hot boiling water (100 degree celcius) , it will denature (stop working).Also ,if an enzyme works at a pH of 7 and if it is then kept in a pH of 14 ,it will stop working.(note:- DENATURE is a term which means that an enzymes stops working ).

d.   CENTRAL DOGMA OF MOLECULER BIOLOGY: Transcription of ribonucleic acid

DNA-------RNA-------PROTEINS

The central dogma of molecular biology is
DNA------RNA-------Proteins. DNA resides in the nucleus and it is in the form of large molecules which cannot come out of pores of the nucleus. As we know proteins are synthesized only in ribosomes the ouestion is as to how can the DNA in the nucleus come to ribosomes in cytoplasm. The answer to this is that RNA connects them by the process called transcription for DNA to RNA and from RNA to Ribosomes is known as Translation . RNA are smaller molecules compared to DNA molecules and can pass through the pores of the nucleus, thus it goes inside the nucleus performs the transcription process by reacting with DNA and then it comes out of the nucleus and goes to Ribosomes where proteins are synthesized.This process is know as translation. These processes thus form the central dogma of molecular biology.

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DNA strands!!

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Translation of protiens on ribosomes.

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nucleic acids are compounds that contain phosphorous and nitrogen in addition to carbon, hydrogen and oxygen. There are two kinds of nucleic acids. One is  called DNA------------deoxyribonucleic acid. the other is RNA---------ribonucleic acid. These substances were first found in the part of the cell called nucleus. RNA and DNA direct and control the development and activities of all the cells in an organism.Small particles, called RIBOSOMES, are sites of protein synthesis in the cell. They are found lining the membranes of the endoplasmic reticulum and in the cytoplasm. In cells that synthesize proteins to be released from the cell, the ribosomes are usually attached to the outer membrane surface of the endoplasmic reticulum. The protien pass through these memebranes into the canals which carry the proteins to the cell membrane and out of the cell. proteins that are to be used within the cell are synthesized on ribosomes that are free in the cytoplasm . These proteins are usually enzymes that function in the cell's cytoplasm.  

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