GENERAL STRUCTURE OF A CELL CELL-SURFACE STRUCTURES Here belongs the cell wall and the cell membrane. 1)Cell wall – we can find it in the cells of plants, fungi and bacteria. Cell wall holds the shape of a cell, it is important for protection of a cell, it makes the cell strong and it is permeable. Cell wall sometimes store other substances as well. If it stores inorganic substances it is called incrustation and if organic it is called impregnation. Structure of the cell wall: -polysaccharides – In the cell wall of plants there is polysaccharide called cellulose. In the cell walls of fungi cells, there is chitin. - proteins 2)Cell membrane – every cell has a cell membrane. Cell membrane consists of a lipid bilayer with molecules of proteins. Cell membrane is semipermeable. CYTOPLASM It's a jelly-like material located between the cell membrane and the nuclear envelope of a cell. It is important because it creates the environment for the cell organelles and it enables various reactions and transport of substances in a cell. Cytoplasm contains: 1)membrane organelles 2)ribosomes – they are small granules which consist of RNA and proteins and they produce proteins 3)cytoskeleton 4)inclusions – they are droplets, granules or crystals which store nutrients, secretory products or waste material. NUCLEUS (KARYON) According to the structure of the nucleus we distinguish two types of cells: a)PROKARYOTIC CELLS – the structure of their nucleus is very simple and they have only a few membrane cell organelles. Prokaryotic cells are for example bacteria. b)EUKARYOTIC CELLS – the structure of their nucleus is complicated and they have many membrane cell organelles. Eukaryotic cells are for example the cells of plants, fungi, animals. STRUCTURE OF THE PROKARYTOTIC CELLS CELL SURFACE STRUCTURE Cell surface structure consist of cell wall, cell membrane and capsule. The cell wall, cell membrane and the capsule together are called cell envelope. Prokaryotic cells may have flagella or pilli which are the protein fibres that are important for the movement of a cell. CYTOPLASM Contains: a) membrane organelles – a typical prokaryotic cell doesn't have any membrane organelles. Only those prokaryotic cells in which photosynthesis takes place contain thylakoids which contain pigments for photosynthesis. b) ribosomes – there are many ribosomes and they can create chains c) cytoskeleton – it doesn't have any d) inclusions – there are many inclusions in a prokaryotic cell NUCLEUS The nucleus of prokaryotic cells consists of one circular molecule of DNA which is called a NUCLEOID. Nucleus in the prokaryotic cells doesn't have any membrane around it. STRUCTURE OF EUKARYOTIC CELLS CELL SURFACE STRUCTURES a) cell wall – in plants, bacteria, fungi. There is no cell wall around animal cells. b) cell membrane CYTOPLASM membrane organelles – there are various membrane organelles in eukaryotic cells. They have one or two membranes on the surface. ENDOPLASMATIC RETICULUM – it is a system of flat sacs that are interconnected. It contains enzymes that participate in reactions in which the cell creates substances that are secreted – for example hormones, enzymes. There are two types of endoplasmatic reticulum: rough – it has ribosomes on its surface and these ribosomes produce proteins smooth – there are no ribosomes on the surface GOLGI APPARATUS – it is a group of sacs. It contains enzymes which modify the substances produced by the endoplasmatic reticulum (for example hormones, enzymes) ENDOPLASMATIC RETICULUM and GOLGI APPARATUS are synthetic centers of a cell. LYSOSOMES – they are typical for animal cells. They are small sacs which contain digestive enzymes. Their function is to change the substances which entered the cell or which the cell doesn't need anymore. In plant cells, there are no lysosomes but they have vacuoles. VACUOLES – there is one membrane on their surface and it is called tonoplast. Inside of a vacuole there are chemical substances which the cell stores or doesn't need anymore. Young cells have many small vacuoles, old cells have only one big vacuoles. MITOCHONDRIA – they have two membranes on the surface. The inner space fills matrix. A mitochondrion is important for the cell, because of the processes by which energy is released. Therefore they are called energetic centers of a cell. b) RIBOSOMES – there is no membrane around them, they are small granules and they consist of RNA and proteins. c) cytoskeleton – it is important for the shape of cells. It consists of microfilaments and microtubules. Groups of microfilaments have the ability to change their length that's why they are important for movement of cytoplasm, but also for the movement of a cell. Groups of microtubules create flagella or pili which are important for the movement of cells. d) inclusions 3)Nucleous Nucleus is the biggest organelle of cells. There are two membranes around it. There are small openings = pores on the surface. Inside of the nucleus there is: a) chromatin which consists of DNA and proteins. It creates chromosomes. b)nucleolus – they are small granules which consist of RNA and proteins. Nucleus is important for regulation of all the processes inside the cells and also because there is the genetic information in it. Cell metabolism Metabolism means change of substances, which were absorbed into the cell, into substances which the cell needs for it’s growth, reproduction…etc. There are two ways of commiting metabolism: 1)anabolism – in this set of processes synthetic processes prevail. In this process energy-rich substances are made from energy-poor substances. 2)catabolism - In catabolism, large molecules such as polysaccharides, fatty acids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides and amino acids. It is a process in which energy is released. Flow of energy, substances and information Flow of energy- In flow of energy two laws of thermodynamics prevail. Energy cannot be created or destroyed, but it can be changed from one form to another. Energy cannot be changed from one form to another without a loss of usable energy. Also other rules exist about flow of energy. Energy is changed by all the chemical reactions. If some substances are created, energy is used. Flow of substances- Flow of substances includes many actions. Which are as folows: 1)absorption of substances 2)secretion of substances 2)chemical changes of substances 1)Absorption of substances is very important for several reasons. Cell gets substances which are needed for its growth and for building cell structures. Cell also needs the energy which is contained in the substances. Some substances are very important for chemical reactions i.e. water. 2)Secretion of substances Cell secretes substances for two reasons. It secretes waste materials made during chemical reactions which are toxic for the cell. The other reason is that it secretes substances which are needed for other cells. 3)Chemical changes are done to some of the substances which enter the cell. The change of substances which has entered the cell into substances which are needed for the cell is called metabolism. Flow of information- It is all regulative processes. A good example is when nucleus controls the order in which the processes should happen. If any of the processes is disturbed the cell dies.