Structure and Function of Organelles

Organelles are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of organelles commonly found in eukaryotic cells. In this course we will focus our attentions on only a handful of organelles. We will examine these organelles with an eye to their role at a molecular level in the cell, and leave descriptive cell biology for another course. For example, our interest in the mitochondrion lies in its ability to generate energy in a form that is useful to the cell, and to that end we are interested in how the membrane structure allows the energy generation to proceed, but we will not spend much time naming mitochondrial structures or discussing their distribution in different cell types.

It is important to know the basic facts about the following organelles. There are very useful drawings and photographs of these organelles in Chapter 4 of Purves, and we recommend examining the images to help comprehend the organelles.

Follow this link to view an animal cell while reading about the organelles.
Follow this link to view a plant cell while reading about these organelles.

Nucleus

This is where the DNA is kept and RNA is transcribed. RNA is transported out of the nucleus through the nuclear pores. Proteins needed inside the nucleus are transported in through the nuclear pores. The nucleolus is usually visible as a dark spot in the nucleus (note the dark nucleolus in this electron microscope photo of a nucleus), and is the site of ribosome formation.

Ribosomes

Ribosomes are the sites of protein synthesis , where RNA is translated into protein. Protein synthesis is extremely important to cells, and so large numbers of ribosomes are found throughout cells (often numbering in the hundreds or thousands). Ribosomes exist floating freely in the cytoplasm, and also bound to the endoplasmic reticulum (ER). ER bound to ribosomes is called rough ER because the ribosomes appear as black dots on the ER in electron microscope photos, giving the ER a rough texture. These organelles are quite small, made up of 50 proteins and several long RNAs intricately bound together. Ribosomes have no membrane. Ribosomes disassemble into two subunits when not actively synthesizing protein.

Mitochondria

Mitochondria (singular: mitochondrion) are the sites of aerobic respiration, and generally are the major energy production center in eukaryotes. Mitochondria have two membranes, an inner and an outer, clearly visible in this electron microscope photo of a mitochondrion. Note the reticulations, or many infoldings, of the inner membrane, This serves to increase the surface area of membrane on which membrane-bound reactions can take place. The existence of this double membrane has led many biologists to theorize that mitochondria are the descendants of some bacteria that was endocytosed by a larger cell billions of years ago, but not digested. This fascinating theory of symbiosis, which might lend an explanation to the development of eukaryotic cells, has additional supporting evidence. Mitochondria have their own DNA and their own ribosomes; and those ribosomes are more similar to bacterial ribosomes than to eukaryotic ribosomes.

Chloroplasts

These organelles are the site of photosynthesis in plants and other photosynthesizing organisms. They also have a double membrane. There is a more complete description of the chloroplast here, in the chapter on photosynthesis.

Endoplasmic Reticulum (ER)

The ER is the transport network for molecules targeted for certain modifications and specific final destinations, as opposed to molecules that are destined to float freely in the cytoplasm. There are two types of ER, rough and smooth. Rough ER has ribosomes attached to it, and smooth ER does not.

Golgi apparatus

This organelle modifies molecules and packages them into small membrane bound sacs called vesicles. These sacs can be targetted at various locations in the cell and even to its exterior.

Lysosome

This organelle digests waste materials and food within the cell, breaking down molecules into their base components with strong digestive enzymes . Here we can see an advantage of the compartmentalization of the eukaryotic cell: the cell could not support such destructive enzymes if they were not contained in a membrane-bound lysosome .