Structure and Function of Eukaryotic Cell Organelles
What are eukaryotic and prokaryotic cells?
Organelles in Eukaryotic Cells
Nearly all animal cells have a nucleus, with the only exception being the red blood cell. The nucleus has two major functions, which are housing the DNA and controlling the cell’s activities. In the centre of the nucleus is the nucleolus. This doesn’t have a membrane, but holds itself together. In the nucleolus, ribosomes are created through the mixture of RNA and proteins. These proteins are originally found in the cytoplasm, outside the nucleus, but they travel through the pores in the nuclear envelope, through the chromatin and into the nucleolus. The structure of the nucleolus allows easy access for the proteins, as well as an easy exit for the ribosome subunits. The chromatin that surrounds the nucleolus contains both DNA and proteins. When the cell is dividing the chromatin, which is a mess of DNA strands, start to curl. After they have finished curling you can see clear, organised chromosomes. The cell divides when each of these chromosomes replicates itself through mitosis. On the outside of the nucleus is the nuclear envelope. This is a double layered membrane which holds together the contents of the nucleus, and is attached to the rough endoplasmic reticulum. Within the envelope are nuclear pores which allow proteins to enter the nucleus and the sub units of the ribosomes to leave.
Endoplasmic Reticulum (ER) is made from a network of membranous tubules and flattened sacs, known as cisternae. Within the ER membrane is the cisternal space. There are two morphologies of ER, smooth (SER) and rough (RER). RER has many ribosomes on the exterior giving a rough appearance, and consists of flattened sacs. RER is attached to the nuclear envelope, and is the site of first stage protein modification. The ribosomes attached to the RER synthesise proteins, which then enter the cisternal space. Enzymes within the cisternal space then change these proteins into a 3D form. After modification the majority of proteins are transported via vesicles to other organelles such as the Golgi Body. The structure aids to the RER’s function as it has a great surface area, meaning that the maximal number of proteins can be synthesised at any one time. By having a direct link to the nucleus it allows proteins to enter through here, quickening transportation. SER is made from tubules and this shape prevents ribosomes from attaching. SER’s role is to synthesise carbohydrates and lipids and to store calcium. If ribosomes were present, these substances wouldn’t be able to pass through the SER’s membrane. Although having limited similarities RER and SER are interconnected, aiding with their functions as substances can pass between them through the cisternal space.
Ribosomes are tiny organelles. They are made within the nucleolus from 60% RNA and 40% protein, and consist of two subunits, one large and one small. When produced, the subunits stay separate to exit through the nuclear pores: it is only because of their miniscule size that they can fit through. Many ribosomes then bind themselves to the RER, although some stay free in the cytoplasm. Free and bound ribosomes both create polypeptide chains, which are used to make proteins. To create a polypeptide chain the large and small subunits must join together. Between the two, runs a strand of messenger RNA which holds the gene code for the ribosome to read. As the ribosome reads the mRNA, it collects amino acids from transfer RNA that match each codon. By having two separate units it keeps the mRNA secure which allows for accurate reading. New amino acids are brought into site A, and then joined via a peptide bond to the growing polypeptide chain held in site P. After the amino acid has joined, the tRNA leaves via the exit site (E). Once the mRNA strand has been read, the subunits separate again. The chain...
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