Mitochondrion: Power Generator of Cell

A mitochondrion (plural mitochondria) is a membrane-enclosed organelle found in most eukaryotic cell. These organelles range from 0.5–10 micrometers (μm) in diameter. Mitochondria are also known as "cellular power plants" because they generate most of the cell's energy in the form of adenosine triphosphate (ATP), which is used as chemical energy.

The word mitochondrion comes from the Greek μίτος or mitos, thread + χονδρίον or khondrion, granule. The number of mitochondria in a cell varies widely by organism and tissue type. Many cells have only a single mitochondrion, whereas others can contain several thousand mitochondria. The organelle is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, and the cristae and matrix.

Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome. Further, its DNA shows substantial similarity to bacterial genomes.

Structure

A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and proteins. The two membranes, however, have different properties. Because of this double-membraned organization, there are five distinct compartments within the mitochondrion. There is the outer mitochondrial membrane, the intermembrane space (the space between the outer and inner membranes), the inner mitochondrial membrane, the cristae space (formed by infoldings of the inner membrane), and the matrix (space within the inner membrane).

Outer mitochondrial membrane

The outer mitochondrial membrane, which encloses the entire organelle, has a protein-to-phospholipid ratio similar to that of the eukaryotic plasma membrane (about 1:1 by weight). It contains large numbers of integral proteins called porins. These porins form channels that allow molecules 5000 Daltons or less in molecular weight to freely diffuse from one side of the membrane to the other. Larger proteins can also enter the mitochondrion if a signaling sequence at their N-terminus binds to a large multisubunit protein called translocase of the outer membrane, which then actively moves them across the membrane. Disruption of the outer membrane permits proteins in the intermembrane space to leak into the cytosol, leading to certain cell death.

Intermembrane space

The intermembrane space is basically the space between the outer membrane and the inner membrane. Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules such as ions and sugars in the intermembrane space is the same as the cytosol. However, as large proteins must have a specific signaling sequence to be transported across the outer membrane, the protein composition of this space is different than the protein composition of the cytosol. One protein that is localized to the intermembrane space in this way is cytochrome c.

Inner mitochondrial membrane

The inner mitochondrial membrane contains proteins with four types of functions:

1. Those that perform the redox reactions of oxidative phosphorylation
2. ATP synthase, which generates ATP in the matrix
3. Specific transport proteins that regulate metabolite passage into and out of the matrix
4. Protein import machinery.

It contains more than 100 different polypeptides, and has a very high protein-to-phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). The inner membrane is home to around 1/5 of the total protein in a mitochondrion. In addition, the inner membrane is rich in an unusual phospholipid, cardiolipin. Cardiolipin contains four fatty acids rather than two and may help to make the inner membrane impermeable. Unlike the outer membrane, the inner membrane does not contain porins and is highly impermeable to all molecules. Almost all ions and molecules require special membrane transporters to enter or exit the matrix. Proteins are ferried into the matrix via the translocase of the inner membrane (TIM) complex or via Oxa1. In addition, there is a membrane potential across the inner membrane formed by the action of the enzymes of the electron transport chain.

Cristae

The inner mitochondrial membrane is compartmentalized into numerous cristae, which expand the surface area of the inner mitochondrial membrane, enhancing its ability to produce ATP. These are not simple random folds but rather invaginations of the inner membrane, which can affect overall chemiosmotic function. In typical liver mitochondria, for example, the surface area, including cristae, is about five times that of the outer membrane. Mitochondria of cells that have greater demand for ATP, such as muscle cells, contain more cristae than typical liver mitochondria.

Matrix

The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total protein in a mitochondrion. The matrix is important in the production of ATP with the aid of the ATP synthase contained in the inner membrane. The matrix contains a highly-concentrated mixture of hundreds of enzymes, special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. Of the enzymes, the major functions include oxidation of pyruvate and fatty acids, and the citric acid cycle.

Mitochondria have their own genetic material, and the machinery to manufacture their own RNAs and proteins. A published human mitochondrial DNA sequence revealed 16,569 base pairs encoding 37 total genes: 22 tRNA, 2 rRNA, and 13 peptide genes. The 13 mitochondrial peptides in humans are integrated into the inner mitochondrial membrane, along with proteins encoded by genes that reside in the host cell's nucleus.

Organization and distribution

Mitochondria are found in nearly all eukaryotes. They vary in number and location according to cell type. Substantial numbers of mitochondria are in the liver, with about 1000–2000 mitochondria per cell making up 1/5th of the cell volume. The mitochondria can be found nestled between myofibrils of muscle or wrapped around the sperm flagellum. Often they form a complex 3D branching network inside the cell with the cytoskeleton. The association with the cytoskeleton determines mitochondrial shape, which can affect the function as well.

Function

The most prominent roles of the mitochondrion are its production of ATP and regulation of cellular metabolism. The central set of reactions involved in ATP production are collectively known as the citric acid cycle, or the Krebs Cycle. However, the mitochondrion has many other functions in addition to the production of ATP. Glucose and Oxygen are used to produce ATP, carbon dioxide and water. Collectively these reactions are called aerobic respiration.

Useful website to read: http://www.johnkyrk.com/mitochondrion.html

Cell Membrane

Structure of cell membrane

The cell membrane (also called the plasma membrane, plasmalemma, or phospholipid bilayer) is the interface between the cellular machinery inside the cell and the fluid outside. It surrounds all living cells. It is a semipermeable lipid bilayer found in all cells. It controls how substances can move in and out of the cell and is responsible for many other properties of the cell as well. The membranes that surround the nucleus and other organelles are almost identical to the cell membrane. Membranes are composed of phospholipids, proteins and carbohydrates arranged in a fluid mosaic structure, as shown in this diagram. The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton and, if present, the extracellular cell wall.

The phospholipids form a thin, flexible sheet, while the proteins "float" in the phospholipid sheet like icebergs, and the carbohydrates extend out from the proteins.

The phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facing outwards, and their non-polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer. This hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the two sides of the membrane. Different kinds of membranes can contain phospholipids with different fatty acids, affecting the strength and flexibility of the membrane, and animal cell membranes also contain cholesterol linking the fatty acids together and so stabilising and strengthening the membrane.

The proteins usually span from one side of the phospholipid bilayer to the other (integral proteins), but can also sit on one of the surfaces (peripheral proteins). They can slide around the membrane very quickly and collide with each other, but can never flip from one side to the other. The proteins have hydrophilic amino acids in contact with the water on the outside of membranes, and hydrophobic amino acids in contact with the fatty chains inside the membrane. Proteins comprise about 50% of the mass of membranes, and are responsible for most of the membrane's properties.

• Proteins that span the membrane are usually involved in transporting substances across the membrane.
• Proteins on the inside surface of cell membranes are often attached to the cytoskeleton and are involved in maintaining the cell's shape, or in cell motility. They may also be enzymes, catalysing reactions in the cytoplasm.
• Proteins on the outside surface of cell membranes can act as receptors by having a specific binding site where hormones or other chemicals can bind. This binding then triggers other events in the cell. They may also be involved in cell signalling and cell recognition, or they may be enzymes, such as maltase in the small intestine.

The carbohydrates are found on the outer surface of all eukaryotic cell membranes, and are attached to the membrane proteins or sometimes to the phospholipids. Proteins with carbohydrates attached are called glycoproteins, while phospholipids with carbohydrates attached are called glycolipids. The carbohydrates are short polysaccharides composed of a variety of different monosaccharides, and form a cell coat or glycocalyx outside the cell membrane. The glycocalyx is involved in protection and cell recognition, and antigens such as the ABO antigens on blood cells are usually cell-surface glycoproteins.

Remember that a membrane is not just a lipid bilayer, but comprises the lipid, protein and carbohydrate parts.

Movement across Cell Membranes

Cell membranes are a barrier to most substances, and this property allows materials to be concentrated inside cells, or simply separated from the outside environment. This compartmentalisation is essential for life, as it enables reactions to take place that would otherwise be impossible. Eukaryotic cells can also compartmentalise materials inside organelles. Obviously materials need to be able to enter and leave cells, and there are five main methods by which substances can move across a cell membrane:

1. Lipid Diffusion
2. Osmosis
3. Passive Transport
4. Active Transport
5. Vesicles
   

1. Lipid Diffusion (or simple diffusion)

A few substances can diffuse directly through the lipid bilayer part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2. For these molecules the membrane is no barrier at all. Since lipid diffusion is (obviously) a passive diffusion process, no energy is involved and substances can only move down their concentration gradient. Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.

2. Osmosis

Osmosis is the diffusion of water across a membrane. It is in fact just normal lipid diffusion, but since water is so important and so abundant in cells (its concentration is about 50 M), the diffusion of water has its own name - osmosis. The contents of cells are essentially solutions of numerous different solutes, and the more concentrated the solution, the more solute molecules there are in a given volume, so the fewer water molecules there are. Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute to a concentrated solution.

Osmotic Pressure (OP). This is an older term used to describe osmosis. The more concentrated a solution, the higher the osmotic pressure. It; therefore, means the opposite to water potential, and so water move from a low to a high OP.
Cells and Osmosis. The concentration (or OP) of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider:

• Isotonic solution a solution of equal OP (or concentration) to a cell
• Hypertonic solution a solution of higher OP (or concentration) than a cell
• Hypotonic solution a solution of lower OP (or concentration) than a cell

The effects of these solutions on cells are shown in following diagram:

These are problems that living cells face all the time. For example:

• Simple animal cells (protozoans) in fresh water habitats are surrounded by a hypotonic solution and constantly need to expel water using contractile vacuoles to prevent swelling and lysis.
• Cells in marine environments are surrounded by a hypertonic solution, and must actively pump ions into their cells to reduce their water potential and so reduce water loss by osmosis.
• Young non-woody plants rely on cell turgor for their support, and without enough water they wilt. Plants take up water through their root hair cells by osmosis, and must actively pump ions into their cells to keep them hypertonic compared to the soil. This is particularly difficult for plants rooted in salt water.

3. Passive Transport (or facilitated diffusion)

Passive transport is the transport of substances across a membrane by a trans-membrane protein molecule. The transport proteins tend to be specific for one molecule (a bit like enzymes), so substances can only cross a membrane if it contains the appropriate protein. As the name suggests, this is a passive diffusion process, so no energy is involved and substances can only move down their concentration gradient. There are two kinds of transport protein:

• Channel Proteins form a water-filled pore or channel in the membrane. This allows charged substances (usually ions) to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions.


• Carrier Proteins have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane. The substance will bind on the side where it at a high concentration and be released where it is at a low concentration.

4. Active Transport (or Pumping)

Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule. The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side. The proteins are highly specific, so there is a different protein pump for each molecule to be transported. The protein pumps are also ATPase enzymes, since they catalyse the splitting of ATP to ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule. Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.

The Na+K+ Pump. This transport protein is present in the cell membranes of all animal cells and is the most abundant and important of all membrane pumps.

The Na+K+ pump is a complex pump, simultaneously pumping three sodium ions out of the cell and two potassium ions into the cell for each molecule of ATP split. This means that, apart from moving ions around, it also generates a potential difference across the cell membrane. This is called the membrane potential, and all animal cells have it.

The rate of diffusion of a substance across a membrane increases as its concentration gradient increases. Active transport stops if cellular respiration stops, since there is no energy.

5. Vesicles

The processes described so far only apply to small molecules. Large molecules (such as proteins, polysaccharides and nucleotides) and even whole cells are moved in and out of cells by using membrane vesicles.

• Endocytosis is the transport of materials into a cell. Materials are enclosed by a fold of the cell membrane, which then pinches shut to form a closed vesicle. Strictly speaking the material has not yet crossed the membrane, so it is usually digested and the small product molecules are absorbed by the methods above. When the materials and the vesicles are small (such as a protein molecule) the process is known as pinocytosis (cell drinking), and if the materials are large (such as a white blood cell ingesting a bacterial cell) the process is known as phagocytosis (cell eating).

• Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. Materials to be exported must first be enclosed in a membrane vesicle, usually from the RER and Golgi Body. Hormones and digestive enzymes are secreted by exocytosis from the secretory cells of the intestine and endocrine glands.

Sometimes materials can pass straight through cells without ever making contact with the cytoplasm by being taken in by endocytosis at one end of a cell and passing out by exocytosis at the other end.

For further reading, please go to:

http://hyperphysics.phy-astr.gsu.edu/HBASE/biology/celmem.html

http://www.wisc-online.com/objects/index_tj.asp?objID=AP1101

Cell Size and Shape

Shape

There exist cells which have a variable shape, such as the leukocytes, and some connective tissue cells and cells with a stable shape, such as the erythrocytes, epithelial cells, muscle cells and nerve cells. These stable cells always have a typical more or less fixed shape which is a specific characteristic of each cell type. Some cells are encased in a rigid wall, which constrains their shape, while others have a flexible cell membrane (and no rigid cell wall).
The shape of the cell depends partly on the surface tension and viscosity of the cytoplasm, the mechanical action which the adjoining cells exert, the rigidity of the membrane and the functional adaptation.

Many cells when isolated in a liquid medium tend to take a spherical form, obeying the laws of surface tension. This is the case with the leukocyte which in the circulating blood are spherical, but by the influence of adequate stimuli can emit pseudopodia (ameboid movement) and become completely irregular in shape.
The cells of many plant and animal tissues have a polyhedral shape, statistically more or less constant, determined principally by pressure from adjacent cells.

Figure 1: Cell shapes

Volume

The volume of the cell is variable and oscillates within broad limits. In plants and in animals, cells are found which are visible to the naked eye and which possess a very great volume. Thus, the eggs of certain birds may have a diameter of several centimeters and are composed, at least at first, of a single cell.

This is nevertheless the exception, the great majority of cells being visible only with the microscope since their diameter is only a few thousandths of a millimeter (micron). The smallest cells have a diameter of 4 microns (4 one-thousandth of a millimeter). In the tissues of the human body, with exception of the nerve cells, the volume varies between 200 cubic microns and 15,000 cubic microns. In general, the volume of the cell is fairly constant for any one cell type and independent of the size of the individual. For example, the renal or hepatic cells of a bull, of a horse, or of a mouse have an almost equal size. The differences in the total mass of the organ are due to the number and not to the volume of the cells.

Size
The size of cells is also related to their functions. Eggs (or to use the latin word, ova) are very large, often being the largest cells an organism produces. The large size of many eggs is related to the process of development that occurs after the egg is fertilized, when the contents of the egg (now termed a zygote) are used in a rapid series of cellular divisions, each requiring tremendous amounts of energy that is available in the zygote cells. Later in life the energy must be acquired, but at first a sort of inheritance/trust fund of energy is used.

Cells range in size from small bacteria to large, unfertilized eggs laid by birds and dinosaurs. The relative size ranges of biological things is shown in Figure 2. In science we use the metric system for measuring. Here are some measurements and convesrions that will aid your understanding of biology.
1 meter = 100 cm = 1,000 mm = 1,000,000 µm = 1,000,000,000 nm
1 centimenter (cm) = 1/100 meter = 10 mm
1 millimeter (mm) = 1/1000 meter = 1/10 cm
1 micrometer (µm) = 1/1,000,000 meter = 1/10,000 cm
1 nanometer (nm) = 1/1,000,000,000 meter = 1/10,000,000 cm

Figure 2: Size of cells

Cell Anatomy

Life is both wonderful and majestic. Yet for all of its majesty, all organisms are composed of the fundamental unit of life, the cell. Cells are the basic building blocks of all living creatures. However, they didn't always exist as cells. Each cell is formed from a variety of molecules that can be classified into several major families such as Nucleic Acids and Amino Acids. Through cycles of selection from environmental pressures, these molecules come together to form the first primitive cell, for the sole purpose of ultimate survival.

The cell is the basic unit of life. All organisms are made up of cells (or in some cases, a single cell). Most cells are very small; most are invisible without using a microscope. Cells are covered by a cell membrane and come in many different shapes. The contents of a cell are called the protoplasm.

Animal Cell Anatomy

1. Cell membrane - the thin layer of protein and fat that surrounds the cell. The cell membrane is semipermeable, allowing some substances to pass into the cell and blocking others.
2. Centrosome - (also called the "microtubule organizing center") a small body located near the nucleus - it has a dense center and radiating tubules. The centrosomes is where microtubules are made. During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell. The centriole is the dense center of the centrosome.
3. Cytoplasm - the jellylike material outside the cell nucleus in which the organelles are located.
4. Golgi body - (also called the Golgi apparatus or golgi complex) a flattened, layered, sac-like organelle that looks like a stack of pancakes and is located near the nucleus. It produces the membranes that surround the lysosomes. The Golgi body packages proteins and carbohydrates into membrane-bound vesicles for "export" from the cell.
5. Lysosome - (also called cell vesicles) round organelles surrounded by a membrane and containing digestive enzymes. This is where the digestion of cell nutrients takes place.
6. Mitochondrion - spherical to rod-shaped organelles with a double membrane. The inner membrane is infolded many times, forming a series of projections (called cristae). The mitochondrion converts the energy stored in glucose into ATP (adenosine triphosphate) for the cell.
7. Nuclear membrane - the membrane that surrounds the nucleus.
8. Nucleolus - an organelle within the nucleus - it is where ribosomal RNA is produced. Some cells have more than one nucleolus.
9. Nucleus - spherical body containing many organelles, including the nucleolus. The nucleus controls many of the functions of the cell (by controlling protein synthesis) and contains DNA (in chromosomes). The nucleus is surrounded by the nuclear membrane.
10. Ribosome - small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.
11. Rough endoplasmic reticulum - (rough ER) a vast system of interconnected, membranous, infolded and convoluted sacks that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). Rough ER is covered with ribosomes that give it a rough appearance. Rough ER transports materials through the cell and produces proteins in sacks called cisternae (which are sent to the Golgi body, or inserted into the cell membrane).
12. Smooth endoplasmic reticulum - (smooth ER) a vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). The space within the ER is called the ER lumen. Smooth ER transports materials through the cell. It contains enzymes and produces and digests lipids (fats) and membrane proteins; smooth ER buds off from rough ER, moving the newly-made proteins and lipids to the Golgi body, lysosomes, and membranes.
13. Vacuole - fluid-filled, membrane-surrounded cavities inside a cell. The vacuole fills with food being digested and waste material that is on its way out of the cell.
    

Plant Cell Anatomy

The cell is the basic unit of life. Plant cells (unlike animal cells) are surrounded by a thick, rigid cell wall.

1. Amyloplast - an organelle in some plant cells that stores starch. Amyloplasts are found in starchy plants like tubers and fruits.
2. Cell membrane - the thin layer of protein and fat that surrounds the cell, but is inside the cell wall. The cell membrane is semipermeable, allowing some substances to pass into the cell and blocking others.
3. Cell wall - a thick, rigid membrane that surrounds a plant cell. This layer of cellulose fiber gives the cell most of its support and structure. The cell wall also bonds with other cell walls to form the structure of the plant.
4. Centrosome - (also called the "microtubule organizing center") a small body located near the nucleus - it has a dense center and radiating tubules. The centrosomes is where microtubules are made. During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell.
5. Chlorophyll - chlorophyll is a molecule that can use light energy from sunlight to turn water and carbon dioxide gas into sugar and oxygen (this process is called photosynthesis). Chlorophyll is magnesium based and is usually green.
6. Chloroplast - an elongated or disc-shaped organelle containing chlorophyll. Photosynthesis (in which energy from sunlight is converted into chemical energy - food) takes place in the chloroplasts.
7. Christae - (singular crista) the multiply-folded inner membrane of a cell's mitochondrion that are finger-like projections. The walls of the cristae are the site of the cell's energy production (it is where ATP is generated).
8. Cytoplasm - the jellylike material outside the cell nucleus in which the organelles are located.
9. Golgi body - (also called the golgi apparatus or golgi complex) a flattened, layered, sac-like organelle that looks like a stack of pancakes and is located near the nucleus. The golgi body packages proteins and carbohydrates into membrane-bound vesicles for "export" from the cell.
10. Granum - (plural grana) A stack of thylakoid disks within the chloroplast is called a granum.
11. Mitochondrion - spherical to rod-shaped organelles with a double membrane. The inner membrane is infolded many times, forming a series of projections (called cristae). The mitochondrion converts the energy stored in glucose into ATP (adenosine triphosphate) for the cell.
12. Nuclear membrane - the membrane that surrounds the nucleus.
13. Nucleolus - an organelle within the nucleus - it is where ribosomal RNA is produced.
14. Nucleus - spherical body containing many organelles, including the nucleolus. The nucleus controls many of the functions of the cell (by controlling protein synthesis) and contains DNA (in chromosomes). The nucleus is surrounded by the nuclear membrane
15. Ribosome - small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.
16. Rough endoplasmic reticulum - (rough ER) a vast system of interconnected, membranous, infolded and convoluted sacks that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). Rough ER is covered with ribosomes that give it a rough appearance. Rough ER transport materials through the cell and produces proteins in sacks called cisternae (which are sent to the Golgi body, or inserted into the cell membrane).
17. Smooth endoplasmic reticulum - (smooth ER) a vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane). The space within the ER is called the ER lumen. Smooth ER transport materials through the cell. It contains enzymes and produces and digests lipids (fats) and membrane proteins; smooth ER buds off from rough ER, moving the newly-made proteins and lipids to the Golgi body and membranes.
18. Stroma - part of the chloroplasts in plant cells, located within the inner membrane of chloroplasts, between the grana.
19. Thylakoid disk - thylakoid disks are disk-shaped membrane structures in chloroplasts that contain chlorophyll. Chloroplasts are made up of stacks of thylakoid disks; a stack of thylakoid disks is called a granum. Photosynthesis (the production of ATP molecules from sunlight) takes place on thylakoid disks.
20. Vacuole - a large, membrane-bound space within a plant cell that is filled with fluid. Most plant cells have a single vacuole that takes up much of the cell. It helps maintain the shape of the cell.

Viruses

A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell.

Viruese are strange things that straddle the fence between living and non-living.Viruses are too small to be seen by the naked eye. On the one hand, if they're floating around in the air or sitting on a doorknob, they're inert. But if they come into contact with a suitable plant, animal or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship.

A virus is basically a tiny bundle of genetic material—either DNA or RNA—carried in a shell called the viral coat, or capsid, which is made up of bits of protein called capsomeres. Some viruses have an additional layer around this coat called an envelope. Viruses can’t metabolize nutrients, produce and excrete wastes, move around on their own, or even reproduce unless they are inside another organism’s cells. So, they have to invade a 'host' cell and take over its machinery in order to be able to make more virus particles.

They aren’t even cells.

The cells of the mucous membranes, such as those lining the respiratory passages that we breathe through, are particularly open to virus attacks because they are not covered by protective skin.


More than 5,000 types of virus have been discovered. Viruses vary in shape from simple helical and icosahedral shapes, to more complex structures. They are about 100 times smaller than bacteria. The origins of viruses are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—others may have evolved from bacteria.

Viruses spread in many ways; plant viruses are often transmitted from plant to plant by insects that feed on sap, such as aphids, while animal viruses can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing, and others such as norovirus, are transmitted by the faecal-oral route, when they contaminate hands, food or water. Rotaviruses are often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through sex.

Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer -- though definitely not all -- have been linked to viruses.

Cells

The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The word cell comes from the Latin cellula, meaning, a small room. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an unfertilized ostrich egg cell.

The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

Living cells are dived into two types - procaryote and eucaryote (sometimes spelled prokaryote and eukaryote). This division is based on internal complexity.

Eucaryote:

Animals, plants, fungi, and protists are eucaryotes, organisms whose cells are organized into complex structures enclosed within membranes. The presence of a nucleus gives these organisms their name, which comes from the Greek (eu), meaning "good/true," and karyon, "nut." Many eukaryotic cells contain other membrane-bound organelles such as mitochondria, chloroplasts and Golgi bodies. These cells tend to be larger than cells of bacteria, and have developed specialized packaging and transport mechanisms that may be necessary to support their larger size. Use the interactive animation of plant and animal cells to learn about their respective organelles.
http://www.cellsalive.com/cells/cell_model.htm


Procaryote:

The prokaryotes are a group of organisms that lack a cell nucleus, or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles.The word prokaryote comes from the Greek (pro-) "before" + karyon "nut or kernel", referring to the cell nucleus.

The prokaryotes are divided into two domains: the bacteria and the archaea. Archaea were recognized as a domain of life in 1990. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.

The Origin of Life on Earth

How did life begin on Earth? Though no one is ever likely to know the whole story, virtually everyone has wondered at one time or another, how life on Earth began.

There are at least three types of hypotheses which attempt to explain the origin of life on Earth. The first and oldest of these hypotheses suggest that life was created by a supreme being or spiritual force. Most cultures and religions have their own explanations of creation that are passed down from generation to generation. Because these ideas cannot be proved nor disproved, we consider them outside the boundaries of science. For that reason, they will not be pursued here and are left to each individual to decide.

The second set of hypotheses suggests that life began in another part of the universe and arrived on Earth by chance, such as with the crash of a comet or meteor.

The third, and most common hypothesis in the scientific community, is that life began approximately 3.5 billion years ago as the result of a complex sequence of chemical reactions that took place spontaneously in Earth's atmosphere.

The Sun and its planets formed between 5 and 4.6 billion years ago as matter in our solar system began to coalesce because of gravity. By about 3.9 billion years ago, the Earth had an atmosphere that contained the right mix of hydrogen, oxygen, carbon, and nitrogen to allow for the creation of life. Scientists believe that the energy from heat, lightning, or radioactive elements caused the formation of complex proteins and nucleic acids into strands of replicating genetic code. These molecules then organized and evolved to form the first simple forms of life. At 3.8 billion years ago, conditions became right for the fossilization of the Earth's early cellular life forms. These fossilized cells resemble present day cyanobacteria. Such cells are known as prokaryotes. Prokaryote cells are very simple, containing few specialized cellular structures and their DNA is not surrounded by a membranous envelope. The more complex cells of animals and plants, known as eukaryotes, first showed up about 2.1 billion year ago. Eukaryotes have a membrane-bound nucleus and many specialized structures located within their cell boundary. By 680 million years ago, eukaryotic cells were beginning to organize themselves into multicellular organisms. Starting at about 570 million years ago an enormous diversification of multicellular life occurred known as the Cambrian explosion. During this period all but one modern phylum of animal life made its first appearance on the Earth. Figure below describes the approximate time of origin of the Earth' s major groups of plants and animals.

What Is Life?

What is life? Does this sound like a strange question to you? Of course we all know what is meant by the word "life", but how would you define it?

Do all living things move? Do they all eat and breathe? Even though we all seem to know what is meant by saying something is "alive", it's not very easy to describe what "life" is. It's almost as hard as describing where life came from.

Even the biologists (people who study life) have a tough time describing what life is! But after many years of studying living things, from the mold on your old tuna sandwich to monkeys in the rainforest, biologists have determined that all living things do share some things in common:

1. Living things must be able to move by using their own energy
2. Living things need to take in energy
3. Living things get rid of waste
4. Living things grow and develop
5. Living things respond to their environment (e.g., toward food, away from heat, etc.)
6. Living things reproduce and pass their traits onto their offspring
7. Over time, living things evolve (change slowly) in response to their environment

Therefore, in order for something to be considered to "have life" as we know it, it must possess these characteristics.

Nitrogen Cycle

Please click on the URL below and then click on the play button to see the process of nitrogen cycle.

http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp29/animations/ch29/1_nitrogen_cycle.swf

Recombinant DNA Technology Video Clip

The flollowing URL shows you how recombinant DNA is taking place. It would be better if you have headset so that you can listen to it while watching. Click on the URL below and Enjoy.

http://present.smith.udel.edu/biotech/rDNA.html