Most ribosomes are attached to the surface of the rough endoplasmic reticulum. The endoplasmic reticulum is a series of interconnecting flattened tubular tunnels, which are continuous with the outer membrane of the nucleus. It runs through the cytoplasm of all eukaryotic cells. The ER of a cell often takes up more than a tenth of the total cell volume. There are two types of endoplasmic reticulum- rough and smooth. Smooth ER contains no ribosomes so appears smooth, rough ER appears rough due to the presence of ribosomes on the membrane. Smooth ER and rough ER also have different functions but they both help to provide a structural skeleton to maintain the cellular shape.
The rough ER is concerned with the transportation of proteins, which are made by the ribosomes on the surface of the membranes. The membranes form a series of sheets known as lamellae of reticulum, and these enclose flattened sacs called cisternae. The purpose of this is to form a maze-like structure so that there is a very large surface area for chemical reactions to take place. Information coded in DNA sequences in the nucleus is transcribed as messenger RNA. Messenger RNA exits the nucleus through pores in the nuclear envelope to enter the cytoplasm. At the ribosomes on the rough ER, the messenger RNA is translated into proteins. The newly made proteins are taken into the tubes of the rough ER so that they cannot escape into the cytoplasm, and are threaded through pores in the membrane to accumulate in the cisternal space where they can fold into their normal three-dimensional shape. Proteins made by the rough ER are then either secreted or used where they are needed within the cell.
The smooth endoplasmic reticulum is very similar to the rough endoplasmic reticulum in structure, but is used for the synthesis of lipids and steroids. The network of tubes and sheets help to provide a very large surface area for this to occur. Some of the most important substances produced are phospholipids, which are the main component of the membrane round the cell and around many of the organelles within it. Phospholipids are made up of fatty acids, glycerol and phosphate, and make fresh membrane by attaching themselves to the inner side of the membrane and flipping inwards. They provide a waterproof layer as they are made out of lipids, which are hydrophobic. Smooth ER also contains enzymes for detoxifying chemicals, including drugs and pesticides and it acts as a storage site for calcium in skeletal muscle cells. It is extensive in cells that secrete steroid hormones and is involved in the detoxification of urea in the liver. When smooth ER is broken into small pieces, it reseals into vesicles, called microsomes.
Once lipids and proteins have been made in the endoplasmic reticulum, they pass to the golgi apparatus in ‘transport vesicles.’ Golgi apparatus are only found in animal cells. The golgi apparatus is a stack of flattened, membranous sacs that are important in packaging macromolecules for transport elsewhere in the cell. Numerous smaller vesicles containing these packaged macromolecules surround the stack. The enzymatic or hormonal contents of lysosomes, peroxisomes and secretory vesicles are packaged in membrane-bound vesicles at the periphery of the golgi apparatus. When proteins and lipids reach the golgi body they first pass through the cis-golgi network, which returns any wrongly exported proteins or lipids back to the endoplasmic reticulum. Proteins and lipids that aren’t returned pass through the stack of cisternae. This stack of cisternae has a very large surface area for chemical reactions. Here molecules are modified by, for example, sticking sugar molecules onto them to form glycoproteins and glycolipids. Macromolecules are then chemically labelled and sent to their final destinations by being secreted using vesicles or lysosomes.
Lysosomes are common in animal cells but rare in plant cells. They consist of a membrane surrounding a small sphere of liquid. This fluid contains powerful protein digesting enzymes (lysozymes). The enzymes work best at a low pH so are in an acid solution. The membrane surrounding the enzymes is there in order to prevent them from leaking into the cytoplasm and damaging the cell. Each lysosome contains about forty enzymes and these can be used in a highly controlled way to break down foreign bodies or worn out organelles in a cell. Lysosomes are especially important in white blood cells that are used to destroy bacteria. The contents are carefully released into the vacuole around the bacteria to digest it. Lysosomes are sometimes referred to as suicide sacks because in some circumstances they can burst open and kill the cell in which they are contained in a process known as autolysis.
Other membrane bound organelles are mitochondria. These are vitally important for producing energy for the cell. Mitochondria are usually long thin structures, 2-5 µm long, and are contained within a double membrane. The outer membrane is a smooth surface layer, but the inner membrane is highly convoluted, forming folds called cristae. This gives the mitochondria a very large internal surface area for attachment of some of the enzymes involved in respiration. It is on these cristae that food (sugar) is combined with oxygen in aerobic respiration to produce adenosine triphosphate (ATP)-the primary energy source for the cell. Energy is needed for a cell to move, divide, produce secretory products, and for active transport (taking molecules into a cell against a concentration gradient). Within the inner membrane there is a fluid matrix, this is the site of fatty acid oxidation. Mitochondria also contain their own DNA, phosphate granules and ribosomes, to make proteins. Eukaryotic cells usually contain many of these organelles and without them a cell would not have the energy to function.
Chloroplasts are an organelle found in all higher plant cells. They have a double outer membrane, similarly to mitochondria and this is called the chloroplast envelope. A substance called chlorophyll is contained within these organelles, giving the cell a green colour. The function of chlorophyll is to absorb light energy to use in photosynthesis, the process in which carbon dioxide and water are converted into oxygen and glucose. Energy from the glucose is then used by the mitochondria to produce ATP. There are several types of chlorophyll and each type is most efficient in different wavelengths of light. Within a chloroplast there are two distinct regions. The stoma is a colourless, gelatinous matrix. This contains DNA, ribosomes, oil droplets and starch grain stores, which temporarily store the products of photosynthesis before they are needed in respiration. There are also granum which resemble stacks of coins, and these are made up of thylakoids. There may be as many as fifty grana in each chloroplast. Larger thylakoids connect the grana and are known as intergranal lamellae. It is the thylakoids that contain the chlorophyll. The way they are stacked increases the surface area for photosynthetic reactions to take place, so the organelle can produce as much glucose as possible.
Some cells, especially epithelial cells try to increase the surface area of their plasma membrane by using tiny finger like projections called microvilli. These are about 1mm high and 0.08mm in diameter. Some cells have only a few microvilli on their surface but others such as the absorptive epithelial cells in the gut and cells lining nephrons in the kidney can have several thousand on their surface. A greater surface area allows the increased passage of nutrients across the cell membrane so absorption can take place at a faster rate. The core of microvilli contains a group of actin microfilaments that run along their length. Actin is a type of protein and it helps provide a skeletal core to maintain the structure of the microvillus. The filaments can also contract to help speed up absorption. On the outside surface of the microvillus is a glycoprotein containing coat which is known as the glycocalyx. The glycoproteins of the glycocalyx include enzymes which are important for the final breakdown of nutrients prior to their absorption by the cell.
Each organelle has a very specific structure, which enables it to carry out its function. Although each organelle has a different structure, there are several common features. Most organelles are enclosed within a membrane to control exchange of material between it and the cytoplasm. Organelles that are designed to carry out chemical reactions try to ensure that they have the maximum surface area for these reactions to take place on. Although each organelle in a eukaryotic cell has a separate function, organelles are very interdependent and work together so that many tasks can be performed at once. If organelles start failing to function then a cell will no longer be able to carry out the processes it needs to survive.
Bibliography
Books used: -
- AS Biology textbook- ‘A New Introduction to Biology’
- Philip Allan Updates- ‘Exam Revision notes’
- Toole and toole textbook
The following websites: -