Cells

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1.- The cellular theory

The cellular theory is one of the basic theories in the field of Biology. It basically enunciates that all living beings are composed of cells and that these cells come from other pre-existing cells. The precursor of this theory was the British scientist Robert Hooke, who coined the term "cell", referring to tiny that composed a sheet of cork that had observed the microscope . Later, the German scholar Theodor Schwann first postulated the concept of cell theory (all living things are composed of cells) and also German Matthias Schleiden enunciated the other basic principle (cells derived from other pre-existing cells).

2.- Cell types and their characteristics

There are two basic types of cells:

• Prokaryotes: they are very simple. These cells scarcely have any organelles , and their genetic material floats freely in the cytosol . They have got a rigid layer of variable composition called cellular wall around the cytosol that comfers them rigidity. The cell itlesf usually composes the complete organism. Their cytoplasm usually contains rybosomes, whose function is to synthesize proteins, and small vesicles.

• Eukaryotes: they are more complex than prokaryotes, and have many more components. One of the most important differences between prokaryote and eukaryote cells is the presence of a nucleus, which separates the genetic material from the cytoplasm. Eukaryotic cells don't have a cellular wall. Instead, they have a cellular membrane. The lack of rigidity that this involves confers them a spherical shape more or less deformed. vegetal cells, however, have a wall made up of cellulose, which allows them to adopt polygonal shapes. This gives rigidity to the structures they form.

3.- Cellular components and parts of the eukaryotic cell

Components:

• Plasmatic Membrane: thin covering that surrounds all known cell types and is responsible for regulating the exchange of molecules and substances between the exterior and interior of the cell. It consists of a lipid bilayer with proteins embedded or just stuck in it. It is a fluid substance. 
• Cell wall: rigid layer outside the plasma membrane that confers rigidity to the cell. Only found in prokaryotes and vegetal eukaryotes. It composition varies, but in vegetal cells it is made up mostly of cellulose and lignin. 
• Cytoplasm: it is s the material that occupies the space between the plasmatic membrane and the nucleus. It is made up of a liquid substance called cytosol, the cell organelles and the cytoskeleton. 

Organelles:

• Centrosome: is an exclusive of animal organelle. It consists of two tubular structures arranged perpendicularly, called diplosome, and a "crown" of tubules. It form and organizes the cytoskeleton. 
• Ribosome: small organelles that are not observable by light microscopies. They consist of two subunits (proteins and RNA ), responsible for decoding the information contained in the cell's genetic code. 
• Endoplasmic reticulum: a system made up of tiny membranous sacs and tubes of similar composition to that of the plasmatic membrane. Its function is to synthesize lipids and proteins. Some areas of the reticulum have ribosomes on their outer face, so they are called rough endoplasmic reticulum. The parts without ribosomes are called smooth endoplasmic reticulum. 
• Golgi apparatus: the excretory system of the cell, which expels waste substances manufactured in the endoplasmic reticulum. It is situated near the nucleus. 
• Lysosomes: vesicles from the Golgi apparatus containing enzymes responsible for digesting acid absorbed substances or waste. 
• Vacuoles: are water tanks and other substances. In vegetal cells are very large, occupying between 60% and 90% of the cytosol. 
• Mitochondria: these organelles are the cytoplasm of all eukaryotic cells. They consist of a double membrane, the outer face is smooth and the internal presents some rough ridges called mitochondria. Mitochondria carry out cellular respiration, which oxidize nutrients in order to obtain energy. 
• Chloroplasts: it is only found in vegetal cells, because its function is to perform photosynthesis. They are delimited by a double membrane and contain a liquid called stroma, most of which is occupied by a kind of flattened sacs called thylakoids, some of which are stacked and form structures called grana. There are photosynthetic pigments in the thylakoids, together with other components form systems whose mission is to transform light energy into chemical energy, useful to the cell. 
• Nucleus: is the organelle responsible for monitoring the functions of the cell. It is involved in the transmission of characters from parent to descendants. It is formed by a double porous membrane with ribosomes on its outer face which separates the genetic material from the cytoplasm. The nucleus may contain some bodies, called nucleoli, which produce ribosomes. 
  

4.- Cell Nutrition

The incorporation of matter

• Diffusion: is the displacement of molecules from areas of high concentration (outside the cell) to other with lower concentration (inside the cell) through a permeable membrane (plasma membrane). It is the method the cell uses to capture CO2 and O2. 
• Osmosis: using this method, the cell transfers the dissolvent (but not the solute) of a solution through a semipermeable membrane. The water enters the cell by osmosis. 
• Endocytosis: it is the process by which the cell forms a fold of its plasmatic membrane that traps a molecule. Then fold fuses with a lysosome, which sheds digestive enzymes on the molecule captured. Depending on if the captured molecules are solid or liquid, this method is classified in: 
• Phagocytosis: capture of solid material. It is the method used by white blood cells to capture bacteria. 
• Pinocytosis: capture of liquid molecules. 

Obtaining energy

• Chemical energy: it is related to chemical reactions in which the bonds in the reactants that merge break to make products. In this process energy can be absorbed or released. In the first case, the reaction is called endothermic, and the second exothermic. This energy is used by all living beings. 
• Light energy: it is associated with electromagnetic radiation emitted by the Sun, which is transformed into chemical energy through photosynthesis. 

Depending on the type of energy and matter (inorganic / organic) used, we distinguish two types of nutrition: 

• Heterotrophic: it uses chemical energy and organic matter. It is used by animals, protozoa, fungi and many bacteria. 
• Autotrophic: it uses light or chemical energy and inorganic materials. it is used by algae, plants and some bacteria. 

Metabolism

• Anabolism: in this phase complex molecules are produced from other simpler molecules. It is an endothermic process. 
• Catabolism: during this stage the complex molecules are divided into simpler ones. It is an exothermic process. The resulting energy is stored in molecules of ATP. 

5.- Relations between cells

• Cell encystment: is the reaction to an unfavorable external stimulus. The cell produces an insulating layer around itself that leaves it in a dormant state until conditions are favorable again. 
• Tactisms and tropisms: are biological responses to stimuli such as light, water or some chemicals. Tropisms imply changes in growth direction. Tactisms also imply movement, either towards the stimulus or fleeing from it. 
• Nastic movements: non permanent reactions con to changes in cell volume. Are characteristic of plants. An example are the flowers that open and close. 
• Amoeboid-Stroke: This occurs because pseudopods (extensions) of the plasma membrane emitted by the cell through the cytoskeleton. They can only carry out cells without cell wall, due to lack of rigidity required. 
• Vibrating-Movement: is produced by vibration of elongated structures present in the cell membrane (cilia or undulipodia) 
• Contractile-Movement: movement consisting of shortening and stretching of the cell.

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