Organisms are composed of cells, and these cells have specific structures within in them that allow them to carry out their functions. These structures are called Organelles. The fine detail of the cell (which may be revealed by an electron microscope) is called the cell's ultrastructure. Organelles perform different functions within a cell, and this is called the Division of Labour.
Membrane Bound Organelles
The Nucleus is the largest organelle in a cell. It contains a dense structure called the Nucleolus and is surrounded by the Nuclear Envelope, a structure composed of two membranes, seperated by fluid, which contain a number of nuclear pores that can allow relatively large molecules through. The nucleus contains nearly all of the cell's genetic material. The Nucleolus creates Ribonucleic Acid and Ribosomes, which then travel out of the nucleus, through the nuclear pores, to the cytoplasm where they are involced in Protein Synthesis.
The Endoplasmic Reticulum (ER) is found near the Nucleas and is made up of a number of flattened sacs called Cisternae, which are continuous with the Nuclear Envelope. The Rough Endoplasmic Reticulum is called so because it has a lot of Ribosomes on its outer surface. The Smooth Endoplasmic Reticulum however, does not have Ribosomes. The Rough transports proteins that are synthesised in the Ribosomes, and the Smooth synthesises Lipids.
The Golgi Apparatus is a stack of membrane bound flattened sacs, and are responsible for the modification of proteins received from the ER. These proteins are then transported in vesicles around the cell.
Lysosomes are membrane bound spherical sacs which contain digestive enzymes used to break down materials, such as non-self microorganisms engulfed by Phagocytes.
Mitochondria are round double membrane bound organelles responsible for Aerobic Respiration. Their inner membrane is folded inside to form Cristae, which are folded in the Matrix - the central part of a Mitochondrion. During Aerobic Respiration, ATP is produced in the Mitochondria.
Found only in plants and some protoctists, Chloroplasts are responsible for Photosynthesis. Chloroplasts contain two fluid separated membranes and the inner membrane is folded into a network of flattened sacs called Thylakoids that are stacked into Grana (one Granum, two Grana). The Thylakoids contain Chlorophyll in which the process of Photosynthesis occurs.
Vesicles are membrane bound sacs that are used to store or transport substances around the cell. Lysosomes are actually Vesicles.
Vacuoles are essentially larger Vesicles, and they are formed by the joining together of many Vesicles. They are membrane bound organelles that have no specific shape and contain water with a number of different compounds within it. Their function varies greatly depending on the type of cell they are part of. In plant cells they are important in maintaining Turgor Pressure.
Non Membrane Bound Organelles
Ribosomes are small spherical organelles, composed of two subunits, which can be found on the Rough Endoplasmic Reticulum (and also in the cytoplasm and in mitochondria, and other places). Ribosomes translate genetic information in the form of mRNA into proteins.
Centrioles are microtubules found next to the nucleus of animal cells and some protoctists. They move Chromosomes around by forming fibres called Spindle, during cell division.