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The cell is the central resource of energy in biology. All organisms are made from one or more cells. All the life processes of metabolism and genetics are carried out within these cells. They are the smallest unit of life and are all direct descendants of the first cells that evolved on early earth. Although these first cells arose spontaneously from the conditions of the early earth, modern cells can only be made by the division of previously existing cells.
Organisms can be divided into two major groups, depending on the structure of their cells. These groups are eukaryotes (the animal and plants) and the prokaryotes (bacteria). Eukaryote means 'true nucleus' while prokaryote means 'before the nucleus'.
The prokaryote cell is a simpler cell than the eukaryote cell, and much slower, as it is lacking a nucleus and a lot of other organelles present in eukaryotes. There are two types of prokaryotes present: archaea and bacteria. Prokaryotic cells have three separate areas: outside of the cell, inside of the cell and enclosed in the cell. From the outside, flagellum comes out from the cells surface. These are structures made of proteins that facilitate movement and communication between cells. Enclosing the cell is the cell envelope - usually containing a cell wall covering a plasma membrane though some bacteria have another layer known as the bacterial capsule. The envelope gives firmness to the cell and acts as a protective layer. Prokaryotes usually have exceptions such as Mycoplasma and Thermoplasma. Inside the cell is the cytoplasmic region that holds the cell genome and ribosomes. Though not forming a nucleus, the DNA is condensed in a nucleoid. Prokaryotes have their genetic material as a single circular DNA molecule. This is condensed to form a nucleoid which is free in the cytoplasm - it does not have a nuclear membrane around it. The cytoplasm is the site of most of the cell's metabolism. It doesn't have any cytoskeleton or membrane bound organelles. The bacterial cell wall is made of peptidoglycan - a matrix of polysaccharides linked by short polypeptide chains. This protects the cell, maintains its shape and prevents excessive uptake of water. The Ribosomes found in prokaryotes are for protein synthesis therefore smaller than those found in eukaryotes.
Cell structure in eukaryotic cells is much more complex than in prokaryotic cells. They are also greater in size and volume. The big difference between eukaryotes and prokaryotes is that eukaryotic cells contain membrane-bound sections in which metabolic activities occur. Eukaryotic cells have a nucleus with nuclear membrane as a boundary which is a double layer of membranes with pores to let very big molecules pass through. The endomembranes form the endoplasmic reticulum and the Golgi apparatus. The genetic material is linear DNA organised as proper chromosomes. The DNA is wrapped around histone proteins to form nucleosomes. Mitochondria are present for aerobic respiration. These have a double layer of membranes with the inner layer folded to form cristae. Microbodies in eukaryotes only have a single membrane around them and are produced by budding for the endoplasmic reticulum or the Golgi apparatus. The cytosol has a cytoskeleton unlike those in the prokaryotes.
There are two types of eukaryotic cells - plant and animal cells. Animal cells have some structures that are unique to them.
Centrioles may be present. These are formed by the centrosome which is also known as microtubule organising centre (MTOC).
Microfilaments are membrane extensions that increase the surface area for absorption, for example in the small intestine.
Plant cells also have some structures unique to them. Plant cells have more than one MTOC and these are distributed close to the nucleus of the plant cell.
Central vacuole with a membrane boundary. The vacuole store sugars, ions and pigments and is important in maintaining the turgidity of the cell.
The plant cell wall is made of cellulose. The wall maintains the cell's shape, as well as preventing excessive uptake of water.
Plasmodesmata are present in walls. These are cytoplasmic have links between neighbouring cells.
The middle lamella is composed of pectin which glues neighbouring cells together at their cell walls.
Some cells may have chloroplasts. These carry out photosynthesis and have a double membrane around them.
Prokaryotes and Eukaryotes both have a membrane that envelops the cell, separating the inside from the rest of the environment, decides what moves in and out, and maintains the electrical potential of the cell. All cells contain DNA, genes and RNA containing the information required to build a variety of proteins such as enzymes. The cell's cytoplasm is surrounded by the cell membrane. The membrane in prokaryotes and plant cells are usually bordered by a cell wall. This membrane protects the cell with a double layered lipid.
The cytoskeleton is anchored to proteins in the cell membrane to both maintain its shape and allow it to move. It's a dynamic structure constantly breaking down and reforming. There are three major components of the cytoskeleton:
Actin filaments. Sometimes called microfilaments, these are long filaments of protein (actin) monomers about 7 nm in diameter. Each filament is composed of two protein chains loosely wrapped around each other like two strands of beads, with each bead being a molecule of actin. The formation of actin filaments is regulated by other proteins in the cell that switch on polymerisation when appropriate (during growth, for example).
Microtubules. Microtubules are hollow tubes approximately 25 nm in diameter consisting of 13 protofilaments arranged in a circular fashion. Each protofilament is composed of stacks of the protein tubulin. Microtubules form spontaneously, but will only form around a structure called an organising centre (centrosome) that provides a nucleus from which they can grow.
Intermediate filaments. The basic protein of an intermediate filament is a protein called vimentin, though some cells may have intermediate filaments constructed from other proteins. Skin cells, for example, contain intermediate filaments made of keratin. Intermediate filaments are tough protein fibres 8-10 nm in diameter. Each strand of the threadlike protein molecule is wrapped round each other like the strands of a cable. They are stable and once formed do not readily break down.