Living Organisms• A living thing is composed of one or more cells. These units, generally too small to be seen with the unaided eye, are organized into tissues. A tissue is a series of cells that accomplish a shared function. Tissues, in turn, form organs, such as the stomach and kidney. A number of organs working together compose an organ system. An organism is a complex series of various organ systems.
Cell Structure• All cells are similar in that they contain a gelatinous substance called protoplasm. Protoplasm is the viscid, translucent, polyphasic colloid with water as the continuous phase that makes up the essential material of all cells. It is composed mainly of nucleic acids, proteins, lipids, carbohydrates, and inorganic salts. The protoplasm surrounding the nucleus is known as the cytoplasm and that composing the nucleus is the nucleoplasm (also called karyoplasm).• The cell contains an outer membrane, the plasma (cell) membrane, a nucleus (a spherical or oval organelle often near the center) and cytoplasm, in which are cell organelles (little organs) suspended in a fluid, the cytosol, and inclusion bodies containing secretion and storage substances.
Plasma Membrane (1)• The plasma membrane is formed from a double layer of lipids and proteins and composes the cells outer membrane. It surrounds the protoplasm of the cell and helps maintain its shape and structure. The plasma membrane separates each cell from its environment. The function of the plasma membrane is to regulate active transport. This process selectively allows certain substances to enter and leave the cell while barring others. It also sends and receives chemical and electrical messages, including signals for the cell to manufacture proteins or to divide.
Plasma Membrane (2)• The moving of substances in and out of cells is either a passive process or an active process. Passive transport requires no energy and can occur by simple diffusion (the spreading or scattering of particles), which is the passage of molecules of one substance from a region where it is present in high concentration to the molecules of another substance to form a uniform mixture of the two. Oxygen, nitrogen, and other small molecules that can dissolve easily in lipids move readily back and forth across the bilayer. Some molecules such as glucose, or sodium and potassium ions, cannot cross the membrane on their own by simple diffusion. They need selective transport proteins or special channels to allow them in and out of a cell. In active transport the cell works to bring molecules in and push them out. The energy needed for active transport is derived from molecules of a higher energy phosphate known as ATP (adenosine triphosphate).
Nucleus (1)• The nucleus is a small sphere in the central portion of a cell. It is the biggest, densest and most obvious structure within the cell and is the control center of the cell. The nucleus is enclosed in a double layer of nuclear membrane. At numerous points these membranes are joined, forming the rims of circular openings, the water filled nuclear pores, through which large molecules e.g. ribonucleic acid (RNA) can pass in and out of the nucleus. Within the nuclear membrane is nucleoplasm. It contains one or two small spherical bodies called nucleoli.
Nucleus (2)• The nucleolus contains the DNA coding. The function of DNA (Deoxyribonucleic acid) is to pass on the characteristics of one generation of cells to the next. The DNA is contained by a number of chromosomes, which consist of long strands of DNA tightly wound into coils with proteins called histones. The combination of DNA and histone proteins is known as chromation. The nucleolus also controls the synthesis of some of the cells RNA.
Nucleus (3)• DNA molecules are too large to pass out of the nucleus. Hence part of the DNA molecule assembles (by a process called transcription) a nucleic acid which is smaller than DNA, called messenger ribonucleic acid (mRNA) which can pass into the cytoplasm. mRNA carries the code for polypeptide and protein assembly to the ribosomes. Amino acids are also carried to the ribosomes attached to other, smaller RNA molecules, called transfer RNA (tRNA). Polypeptides and proteins can then be assembled on the ribosomes from the amino acids according to the mRNA code (a process of translation).
Chromosomes (1)• A chromosome is a rod-shaped structure containing genes that is found in the cell nucleus. It is composed of long strands of DNA with many proteins attached. In general, all chromosomes have similar structures with slight variations. The DNA and protein is found in the arms of the chromosome and genes are located at specific sites. Genes are small segments of the DNA molecule. Each gene provides information required to determine a proteins amino acid sequence. The two arms of the chromosome can usually be identified as the short arm and the long arm.
Chromosomes (2)• There is a narrow area on each chromosome called centromere. The centromere is the point that appears to attach the spindle fibers during mitosis. Chromosome movement occurs about centromere. Chromosomes normally occur in pairs. There are 23 pairs of chromosomes in human cells, for a total of 46. One of each pair is contributed by the mother and one by the father. Chromosomes are also responsible for the determination of sex. Normally, females have two identical X-chromosomes, whereas males have one X and Y chromosome.
Cytoplasm• Cytoplasm is a jelly-like substance of protoplasm which occupies the region between the plasma membrane and the nucleus. It is composed of all the structures inside a cell, excluding the nucleus. The cytoplasm is the site of most of the intermediary metabolism of the cell: food is converted into forms that can be used to build cell parts; chemical energy is released from food and transferred to an area where energy is needed in chemical reactions; specific compounds, such as proteins, are synthesized to be used within the cell itself or to be exported to other parts of the organism. In general, it carries on the work of the cell. The cytoplasm contains:
Endoplasmic Reticulum (1)• The endoplasmic reticulum is a network of interconnected tubular and sac-like channels. The space between their walls is continuous with the space of the nuclear membrane and can thus transport substances from one part of the cell to another. One form of ER, rough or granular endoplasmic reticulum, has ribosomes attached to its outer surface and the other form, smooth or agranular, has no ribosomes. The spaces between both types are connected. Ribosomes on rough ER synthesize proteins while smooth ER is involved in carbohydrate metabolism. Specialized types of ER are present in some cells e.g. in skeletal muscle cells smooth ER stores calcium ions which are liberated to initiate contraction of muscle cells.
Endoplasmic Reticulum (2)• The endoplasmic reticulum appears to serve several functions. Its membranes provide an increase in surface area where chemical reactions can occur. The channels of the reticulum provide both storage space for products synthesized by the cell and transportation routes through which material can travel to other parts of the cell. The endoplasmic reticulum is also a cells membrane factory. Phospholipids and cholesterol, the main components of membranes throughout the cell, are synthesized in the smooth portion of ER. These compounds form the coating of protein filled sacs, called vesicles, that break off from ER, migrate to another organelle, fuse with it, and then deposit the protein cargo. Most of the proteins leaving ER are still not mature. They must undergo further processing in another organelle, the Golgi apparatus, before they are ready to perform their functions within or outside the cell.
Ribosomes (1)• Granules, called ribosomes, shaped somewhat like balls, are sites of protein synthesis. Ribosomes are extremely tiny. A single cell may contain thousands of ribosomes. Each ribosome is made of two unequally sized subunits, which are composed of at least 40 different proteins and a form of RNA called ribosomal RNA. Within these ribosomes, various chemicals called amino acids, guided by signals from the nucleus are assembled in precisely the right arrangement to form proteins, the major part of the organic matter in living cells. Proteins perform most of the significant chemical reactions that occur in cells. They are also important in maintaining its structure.
Ribosomes (2)• Proteins are long strings of amino acids attached to one another like beads in a necklace. Different proteins have different sequences of amino acids, which are determined, or coded, by the DNA. In protein synthesis, an RNA copy of the DNA of a gene is transported to the cytoplasm, where ribosomes, other RNAs and enzymes come together to translate the RNA structure into a specific amino acid sequence, or protein. Protein synthesis occurs through the interaction of three kinds of RNA molecules. During translation, a strand of messenger RNA moves between the two parts of a ribosome. It provides the coded message for the amino acid sequence. The ribosome "reads" the message of the mRNA in groups of three, rather than one nucleotide at a time. These groups are called codons.
Ribosomes (3)• Each codon specifies one of twenty different amino acids or is a signal to start or stop making protein. The amino acids called for by the mRNA are brought from the cytoplasm to the ribosome by tRNA. This small molecule is a connector: one end carries three nucleotides, known as anticodon, which will join to a codon in the mRNA according to the rules of base pairing. The molecules other end carries an amino acid. As the mRNA passes through the ribosome, tRNA brings the correct amino acids in and they are linked together by peptide bonds to form a polypeptide chain. When all amino acids for a protein chain are formed, the chain is released.
Ribosomes (4)• Some ribosomes move freely in the cytoplasm and some are attached to the surface of the endoplasmic reticulum. The two kinds of ribosomes play similar roles in the manufacture of proteins. But while free ribosomes leave the proteins free to float in the cytoplasm, the bound ribosomes transfer their finished proteins into the endoplasmic reticulum. Proteins synthesized by endoplasmic reticulum ribosomes pass into the ER lumen then to the Golgi apparatus where they are processed. Proteins manufactured by free ribosomes perform their functions in the cytosol.
The Golgi Apparatus• The Golgi apparatus consists of a collection of membrane-enclosed sacs. Proteins from the endoplasmic reticulum have their structures altered here. This alteration is a kind of label which determines whether the protein will be (a) passed into lysosomes, (b) stored in secretory granules or (c) inserted into plasma membrane. Once final processing of the protein is complete, the proteins are removed from the Golgi apparatus and are moved to their final destinations in vesicles.
Lysosomes• Lysosomes are large single-membrane structures with no dividing membrane inside. Lysosomes are manufactured by the endoplasmic reticulum and Golgi apparatus. They vary in shape and size because they fuse with other vesicles to carry out their functions. Lysosomes contain digestive enzymes that break down large molecules, such as proteins, fats, and nucleic acids, into smaller constituents that can be oxidized by the mitochondria. Lysosomes also digest bacteria. When a bacterium enters the cell, lysosomes fuse with the vesicle of engulfed material and release digestive enzymes to break up the material. Lysosomes are known to contain over 40 different enzymes that can digest almost anything in the cell, including proteins, RNA, DNA, and carbohydrates.
Peroxisomes• Peroxisomes are also single-membrane organelles. They contain (a) enzymes which combine oxygen and hydrogen to form hydrogen peroxide (H2O2) and (b) an enzyme (catalase) which converts H2O2 to water.
Mitochondria (1)• Mitochondria are the largest organelles in an animal cell, after the nucleus. They are sausage or oval shaped structures surrounded by a double-layered membrane. The inner and outer membranes are separated by a fluid-filled gap. Mitochondria can change shape quite readily. They swell or contract in response to various hormones and drugs and during ATP (adenosine triphosphate) manufacture.• Mitochondria are now sometimes referred to as the powerhouse of cells because these organelles release the majority of the energy obtained from food and make it available to the energy-consuming process of the cell. Energy is generated from sugars and fatty acids. Specialized enzymes that trap energy from the breakdown of sugar are imbedded in the inner layer. Besides supplying energy, mitochondria also help the concentration of water, calcium, and other charged particles (ions) in the cytoplasm.
Mitochondria (2)• Mitochondria use oxygen to release the chemical energy stored in food. This process is called cellular respiration or catabolism. The biochemical reactions of cellular respiration fall into two groups: the carbon pathway, in which sugar is broken down into carbon dioxide and hydrogen; and the hydrogen pathway, which transfers hydrogen to oxygen in stages, forming water and releasing energy. In the hydrogen pathway, the hydrogens electrons pass through an "electron transport chain" made up of enzymes. The electrons give up part of their energy as they move from enzyme to enzyme. This energy is then stored in molecules of ATP (adenosine triphosphate). In the end, 38 molecules of ATP are formed for every ever molecule of sugar that is used up in respiration.
Mitochondria (3)• Mitochondria have some of their own DNA molecules and ribosomes and are self-replicating. They "reproduce" by splitting in half.• An interesting characteristic of human mitochondria is the fact that all of a persons mitochondria are descendants of those of his or her mother; no paternal mitochondria are present. This is unlike nuclear DNA which is equally derived from both parents.
Centrosome• The centrosome consists of two rod-like structures called centrioles arranged at right angles to one another. It is concerned with the synthesis of microtubules, e.g. the spindle and aster microtubules present during cell division.
Secretory Vesicles• All secretory substances are formed by the endoplasmic reticulum - Golgi apparatus system. They are then released from Golgi apparatus into the cytoplasm inside storage vesicles called secretory vesicles or secretory granules.• In addition to the above-mentioned organelles the cytoplasm may contain any of a variety of rod-like filaments, microfilaments and microtubular structures, depending on the function of the cell.