The basic structure of eukaryotic cells is taught in all schools, with most students being aware of the fact that cells have a nucleus, and that there are differences between plant cells and animal cells. In reality eukaryotic cells are complex and dynamic places, made up of smaller sub-cellular organelles, constantly working to keep the cell alive and maintain its function.
The Nucleus – Controlling Cell Function
The nucleus contains the genetic information of the cell, encoded within strands of Deoxyribonucleic Acid (DNA). This information needs to get out of the nucleus, so that it can be used to make the proteins that the cell needs.
In order for this to happen, the information in the DNA is converted into a corresponding message on a strand of messenger Ribonucleic Acid (mRNA) through a process called transcription.
The nucleus is surrounded by a double membrane called the nuclear envelope. Pores, or gaps, are present within this structure to allow mRNA to pass out of the nucleus.
Within the nucleus is a smaller organelle called the Nucleolus (some cells may have more than one), which produces ribosomes.
Organelles and Protein Production – Ribosomes, Endoplasmic Reticulum and Golgi Apparatus
Once the mRNA has passed out of the nucleus, the message which it carries must be translated into a corresponding chain of amino acids (the building blocks of proteins). The process of translation is carried out by organelles called ribosomes.
Ribosomes are made from a combination of ribosomal RNA (rRNA) and protein. Some are free, floating around in the cytoplasm (traditionally thought of as a jelly like substance which fills the cell), while others are bound to the rough Endoplasmic Reticulum (ER).
The rough ER is a system of fluid filled sacks, and appears rough when viewed with an electron microscope due to the attached ribosomes. It is the main site of protein production in the cell.
The smooth ER is similar, but has no attached ribosomes. Its function within the cell is to produce and store non-protein based components such as carbohydrates and lipids.
Another organelle involved in the packaging and secretion of enzymes (a sub-group of proteins) is the Golgi apparatus, or Golgi body. The Golgi body is similar in structure to the endoplasmic reticulum, being a series of flattened membrane sacks, or cisternae. The function of the Golgi body is to process and package enzymes so that they can be secreted out of the cell.
This occurs through the production of small sacks called vesicles, containing enzymes, which bud off from the main membranes of the Golgi body at its edges. Vesicles can move to the edge of the cell and fuse with the cell membrane in order to release their contents to the outside environment.
Mitochondria – The Organelles of Energy Production
Mitochondria are the cellular organelles responsible for the generation of ATP (Adenosine Tri-Phosphate), which cells use as an energy source. ATP is produced through the process of aerobic respiration, which means that mitochondria need oxygen to function. Certain tissues, such as muscle, contain high numbers of mitochondria as they have high energy requirements.
Mitochondria are surrounded by a double membrane. The inner membrane is folded so that it has an increased surface are, enabling more chemical reactions (respiration) to take place. The folds within the membrane are called cristae.
Unlike the other organelles within animal cells, mitochondria contain their own DNA, and it has been suggested that they were once free living independent organisms.
Additional Plant Cell Organelles - Chloroplasts
The most obvious organelles which are present within some, but not all, plant cells, are chloroplasts. The function of these organelles is to carry out photosynthesis – harvesting energy from sunlight. For this reason, leaf cells contain many chloroplasts, while root cells do not.
Chloroplasts contain the green photosynthetic pigment, chlorophyll, within the membranes of stack like structures called grana. As with mitochondria, chloroplasts also contain their own DNA.
Further information about cell structure and related interactive activities can be found here.
Sources
Advanced Biology, Michael Kent, Oxford University Press (2000)
Molecular Expressions, Animal Cell Structure http://micro.magnet.fsu.edu/cells/animalcell.html
Molecular Expressions, Plant Cell Structure http://micro.magnet.fsu.edu/cells/plantcell.html
Accessed January 2012
Vicki Dennison - Vicki has a Ph.D, is a qualified Science teacher, and has a passion for inspiring young people.
