|[b]The cell and its ultrastructure[/b]
The cell is the basic unit of structure in prokaryotic and eukaryotic organisms and responsible for their complex processes. They contain different cell organelles which are present in different size and numbers, depending on the cell„Ss function. In the human body, cells transmit information, they produce energy in the form of ATP, but they also take up nutrients and release waste products which can be toxic when accumulating inside the cell. Cells also have the ability to adapt; a good example can be seen with muscle cells.
Regular exercises will increase the number of mitochondria, the power house of the cell. There, a respiratory substrate, such as glucose, is gradually broken down into simpler molecules. A number of electrons are produced and accepted by electron acceptors in the electron transport chain. When the electrons pass down electron carriers, they release energy in the form of ATP, an immediate energy source. The more mitochondria in the cell, the more energy can be produced. Increased levels of ATP can be used longer and for more vigorous exercises.
It can be seen that cells carry out millions of biochemical reactions in a short period of time. The size of a cell does change the rate of reaction in a considerable amount. Large cells are logically bigger than small cells and more reaction should take place, maybe producing more products as well. However, large cells have a small surface are per volume ratio. Waste products might be produced faster than they can actually diffuse out of the cell. This is also why the human body can maintain a high and constant body core temperature of 36-37.8¢XC. Small cells have a bigger surface are per volume and can exchange more materials across the plasma membrane.
Prokaryotic cells are small cells and about five micrometers in size. Bacteria, simple unicellular organisms, are often prokaryotes. They contain a capsule and a murein cell wall which protects the cell. A nucleus is absent, consequently, the DNA is circular and present in the cytoplasm. Furthermore, prokaryotes lack large cell organelles such as lysosomes, the golgi apparatus, the endoplasmic reticulum, and the mitochondria in the cytoplasm. Ribosomes are small and always free in the cytoplasm.
Eukaryotic cells on the other site are larger cells, about 50 micrometers large, therefore five times the size of a prokaryotic cell. They do not posses a capsule but larger cell organelles. While the chloroplast is only present in plant cells, the remaining large organelles (mitochondria, golgi apparatus, endoplasmic reticulum, lysosomes) are present in all eukaryotes. Fungi cells contain a chitin cell wall, plant cells a cellulose cell wall, and animal cells lack a cell wall. Large ribosomes are free in the cytoplasm or attached to the rough endoplasmic reticulum. They are responsible for the synthesis of proteins by using messenger RNA which is manufactured in the nucleus. Therefore, chromosomes are present and kept in the nucleus by a nuclear envelope.
The cell¡¦s ultrastructure, the detailed structure of the cell, has been studied with an electron microscope. Beam of electrons are focused by electromagnets so that a maximum magnification of about 500 000 times the real size of the specimen is possible. The resolution ¡V the ability of an optical system to distinguish between adjacent points ¡V is about 0.001 micrometer. Compared to an optical microscope, which has a resolution of about 2 micrometer, the resolving power of the electron power is enhanced due to the very small wavelength of electrons.
There are two types of electrons microscopes, the transmission and the scanning microscope. The transmission electron microscope is used for extremely thin specimens. Electrons can pass through them and the internal structure can be investigated. The scanning microscope creates a three dimensional view of the specimen made by a beam of electrons which are reflecting of the surface. In both microscopes, a vacuum within the microscope makes sure that air does not affect image production. The specimen that is viewed is therefore dead and maybe a substantial disadvantage for many scientists.
For exactly that reason optical microscopes are used, despite their low resolution power. In an optical microscope, a beam of light is focused by glass lenses and a maximum magnification of up to 1500 times the real size of the specimen can be observed. Air does not affect the image production, thus a vacuum is not necessary and absent. The specimen can be alive or dead and stained to distinguish between objects.
The most important feature of the cell which can be identified from a cell¡¦s ultrastructure is the nucleus, which is about five micrometers in size. The nucleus contains chromosomes (genes made of DNA which control cell activities), separated from the cytoplasm by a nuclear envelope. A double membrane within the envelope contains small holes called nuclear pores (100nm in diameter) which allow the transport of proteins into the nucleus. DNA in the nucleus is continuously used to manufacture proteins.
A messenger RNA strand is copied from the DNA template strand by transcription and leaves the nucleus through these pores. Later, mRNA (messenger ribonucleic acid) binds to ribosomes in the cytoplasm and tRNA assembles amino acids for protein synthesise. Each assembled amino acids is joined to the growing chain, until a STOP condon has been reached on the mRNA and protein synthesise stops. A protein has been manufactured. Ribosomes are about 20-30 nanometres in size and often associated or close to the endoplasmic reticulum. The proteins can go into solution in the cytoplasm of form important cytoplasmic structural or motile elements.
The rough endoplasmic reticulum (ER) is an extensive network of branching tubes that is connected to the membrane of the nucleus. Its surface is covered with ribosomes at the cytosolic site of the membrane where translation takes place, the synthesis of proteins. Enzymes within the rough ER modify polypeptides by addition of carbohydrates or by the removal of signal sequences. Proteins may then move to the Golgi apparatus for export out of the cell or they may be transported within the cell for important biochemical reactions. Exported proteins may include digestive enzymes, hormones, antibodies, or structural proteins. White blood cells, for example, contain abundant rough ER for their important role in the immune system by secreting antibodies. The rough ER is also responsible for the synthesis of phospholipids and the assembly of polypeptides.
Similar in structure but different in function is the smooth ER. It lacks ribosomes on its outer surface, which is the reason for its smoother appearance, and it is also more tubular than the rough ER. The smooth ER is necessary for the synthesis of steroids, phospholipids, and lipids, but also for the lipid and carbohydrate metabolism in brain and muscle cells, for detoxification in liver cells, and for the excitation-contraction coupling in skeletal muscle cells. These cells which are specialised to their function often have more and larger smooth ER than cells with other functions.
The Golgi apparatus, a stack of flattened sacs surrounded by a membrane, receives protein-filled vesicles from the rough ER which fuse with the Golgi membrane by endocytosis. Enzymes modify these proteins, for example, by adding a sugar chain making a glycoprotein. The Golgi apparatus also adds directions to the protein so that the protein package can be transported to its destination. They proceed to different locations in the cell or move to the plasma membrane for secretion by exocytosis. Overall we can say that the Golgi apparatus is involved in PROCESSING, PACKAGING, and SECRETION.
Other vesicles that leave the Golgi apparatus are lysosomes. Vacuoles and vesicles are membranous sacs of liquid which store substances, whereby vacuoles are storage areas. Lysosomes are 0.05 to 0.5 micrometers small and responsible for intracellular digestion. They are more numerous in cells that perform phagocytosis and contain digestive enzymes. These enzymes are separated from the cytoplasm by a limiting membrane. Why the enzymes do not digest the membrane is not known up to now. Lysosomal enzymes digest particles and function optimally at a pH of 5. All enzymes are proteins, hence, lysosomal enzymes are synthesized on the rough ER and transferred to the golgi for modification and packaging.
Primary lysosomes are sacs with a small concentration of enzymes, digestion is not possible yet. Digestion begins by the formation of secondary lysosomes. This happens when a primary lysosome fuses with a phagocytic vacuole. Nutrients diffuse through the lysosomal membrane to the cytosol where digestion can take place.
The power house of the cell is called a mitochondrion and is about one micrometer in diameter and seven micrometers in length. It mainly consists of protein, but lipids, DNA and RNA are also present. Mitochondria are the only organelles in the cell which posses DNA and make them unusual as well. The DNA is used to synthesis proteins with the help of ribosomes, also present inside the mitochondrion. These proteins are used as enzymes or other functional molecules which help to manufacture ATP from aerobic or anaerobic respiration. As the DNA is a circular strand, not inside a nucleus, it is thought that mitochondria were formed during evolution by the penetration of prokaryotic organelles which also have circular DNA and similar ribosomes to the ones found in the mitochondria. Similar to the division of prokaryotes, mitochondria multiply by binary fission and not by mitosis. The main process of the mitochondria is to convert energy from the breakdown of glucose into adenosine triphosphate (ATP). Energy stored in the high energy phosphate bonds of ATP is available to power cellular functions. The metabolic activity of the cell is related to the number of cristae (larger surface area) and of mitochondria within the cell. Cells with a lot of muscle activity, such as the heart muscle, have many well developed mitochondria.
A chloroplast is only found in photosynthesising cells and is four to six micrometers in diameter and one to five micrometers in length. A good example for photosynthesising organelles would be plant cells which use light energy, carbon dioxide and water to produce carbohydrates and oxygen. Carbohydrates are then used in respiration to produce ATP. The inner membrane has folds called lamellae, where chlorophyll is found, which is surrounded by a fluid called stroma.
Also found in plant cells is a cell wall, made of cellulose fibres. They provide strength and protection and stop the cell from bursting in dilute solutions.