organelles in cells

• All eukaryotic cells contain membrane-bound internal structures called organelles, each with a specific function and structure. Different organelles share the common feature of having internal structures that are enclosed by their membrane. Membranes can be either single or double in structure.
• Although each organelle has a different structure and function, they all work together to contribute to the effective functioning of a cell as a unit.
• Many organelles have maximised the amount of surface area they have available to carry out their particular functions.
• One of the differences between organelles is their size.
• The arrangement of organelles also varies depending on whether the cell is a plant or animal. Plant and animal cells contain many of the same organelles however a plant cell possesses a cell wall, chloroplasts, and a large vacuole which animal cells do not.
• The cell membrane (plasma membrane, cytoplasmic membrane, and plasmalemma) surrounds cell contents in all cells and separates cell contents from surroundings. It controls the passage of water and other chemical substances into and out of the cell.
• A selective barrier permits the passage of certain molecules into or out of cells and gives the cell membrane the feature of being selectively permeable (semipermeable).
• In protoplasm, functions essential to life, such as making cellular products and respiration are carried out.
• The living contents of a cell are surrounded by a cell membrane, composed of a nucleus and cytoplasm. The cytoplasm consists of a liquid called cytosol in which there are dissolved chemical substances (for example, ions), suspended organelles and insoluble granules. Approximately 90% of the cytoplasm is water - a medium in which cell chemicals are dissolved or suspended.
• The nucleus (plural: nuclei) appears as a large, spherical or oval structure in the cytoplasm. It is colourless, transparent and slightly more jelly-like than the rest of the cell. Most organisms have one nucleus per cell. The nucleus stores information needed to control cell activities so it communicates with the surrounding cytoplasm. Electron micrographs reveal that the nucleus is surrounded by a double nuclear membrane or nuclear envelope, pierced by tiny pores. These pores regulate the passage of substances between the nucleus and cytoplasm.
• Common features of plant and animal cells are cell membranes, cytoplasm, cytoskeleton, nucleus, mitochondria, endoplasmic reticulum, Golgi body and ribosomes. Plant cells also have cell walls, chloroplasts and a large permanent vacuole and animal cells have centrioles and lysosomes.
• The nucleoplasm (nuclear sap) is the liquid part of the nucleus in which chromatin is found. Chromatin is made up of protein and nucleic acid.
• Deoxyribonucleic acid, DNA, is stored in nucleus. It's the chemical that holds (in coded form) genetic information controlling the cell's function. It contains hereditary information of an organism that is passed down from one generation to the next.
• Within one organism, the information stored in the DNA of each cell is the same. Before a cell divides, the information on the DNA must be copied so that it can be transmitted to newly formed cells. The chromatin separates into short, thick separate rod-shaped structures called chromosomes, visible in dividing cells.
• The nucleolus is a dense, granular region commonly seen within nucleoplasm. It contains a large amount of nucleic acid, some DNA, but mostly RNA, and is responsible for manufacturing organelles called ribosomes ('machinery' of cell)
• The outer nuclear membrane is usually continuous with a network of flattened, interconnected membranes - the endoplasmic reticulum (ER). The ER provides a connection of pathways between the nucleus and the cell's environment, allowing intracellular transport. Folding of sheets of membrane increases its surface area.
• The ER may or may not have ribosomes attached. The main function of the ER is transport but it also plays a role in processing cell products. Rough endoplasmic reticulum folds and processes proteins made by the cell and it can also synthesise lipids. Smooth endoplasmic reticulum is the main site of lipid production, essential for membrane repair and manufacture. ER may also transport substances from one cell to another in plant cells, passing through channels called cell pits in the cell wall.
• Ribosomes are small organelles that appear as dense, rounded granules in electron micrographs. Their small size increases surface area.
• Each ribosome is made of chemicals RNA and protein - the 'machinery' carrying out the genetically coded instructions of DNA to produce any proteins necessary for cell functioning and structure. Amino acids are joined in order to form polypeptides (structural unit of protein)
• Ribosomes may be found free in the cytoplasm or scattered over the surface of the ER. Newly synthesised proteins pass from the ribosome into the ER, where protein is folded.
• The Golgi body (Golgi apparatus) is also made of folding membranes but differs from the ER - does not have ribosomes attached and its membranes are arranged in stacks of four to ten. It is easily recognisable by its curved shape on one surface, where vesicles are seen budding off.
• The surface of the Golgi body is called the forming face and vesicles are evident of the secretory function of the Goldi body.
• Golgi bodies process, package and sort cell products. It is involved in adding proteins and carbohydrates to cell products and provides a membrane around cell products to package them. Features of this membrane are used to sort these products, determining where they cill end up. They may be transported within the cell to wherever they are required or they may be secreted from the cel.
• • Organelles within the cytoplasm of cells will eventually reach their 'use-by-date' and wear out. Instead of wasting the raw materials that make up these organelles, the cell has a clever method of recycling and reusing. This job is carried out by lysosomes (lysis = to break apart), special organelles found within the cytoplasm of animal cells.
• Lysosomes are formed in the Golgi body.
• Lysosomes contain digestive enzymes that are responsible for splitting complex chemical compounds into simpler ones, such as when proteins are broken down into amino acids.
• The simpler molecules from lysosomes can be used as building blocks for new compounds and organelles.
• Lysosomes may destroy the entire cell, a process known as apoptosis or programmed cell death.
• Apoptosis is a deliberate action by the cell to destroy old or damaged cells.
• Under the direction of the cell, the lysosome membrane ruptures, releasing the enzyme which then digests the contents of the cell, killing it in the process.
• • Mitochondria are the 'powerhouse' of the cell, producing energy in the form of energy-rich molecules in the process of cellular respiration. They are usually rod shaped but can be round. They vary in size and shape.
• • Mitochondria are smaller than the nucleus and chloroplasts but larger than the ribosomes.