In this article, you will be able to compare and contrast prokaryotic and eukaryotic cells, define an organelle, explain the function of various organelles, and describe cell fractionation.
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All living organisms, from the simplest bacterium to the most complex avian, are composed of cells. Cells are the most basic unit of life, which arise from the division and the differentiation of pre-existing cells. While cells are complex in and of themselves, we can simplify this by dividing them into two types: prokaryotic cells and eukaryotic cells.
We can create this distinction in terms of cell count, where their DNA (genetic material) is located, and the presence or absence of membrane-bound organelles. Prokaryotic cells are exclusively unicellular, contain DNA is a region known as a nucleoid, and do not contain membrane-bound organelles. In contrast, eukaryotic cells may be either unicellular or multicellular, contain DNA in a region known as a nucleus, and do contain membrane-bound organelles.
Organelles may be thought of as the “organs” of cells. While organs, such as the heart or the liver, each play distinct roles in an organism, organelles play distinct roles in the cell they are in. Organelles may be either non-membrane-bound organelles or membrane-bound organelles.
- Plasma Membrane: A selectively permeable membrane composed of lipids, carbohydrates, and proteins which permit the entrance and exit of materials from the cell.
- Cytosol: The fluid filled suspension in which all organelles are suspended in.
- Nucleolus: Stores genetic information in the form of DNA, which contains a multitude of genes.
- Cytoskeleton: Maintains the shape and the overall structure of the cell.
- Ribosomes: Involved in the synthesis of polypeptides or proteins for inside the cell.
- Mitochondrion: Involved in the production of energy in the form of ATP.
- Chloroplast: Carries out photosynthesis exclusively in photosynthetic cells, such as plant cells.
- Lysosome: Utilizes hydrolytic enzymes to degrade food particles and recycle worn out organelles.
- Peroxisome: Involved in the oxidation of fatty acids and synthesis of hydrogen peroxide.
- Smooth Endoplasmic Reticulum (SER): Involved in the synthesis of lipids, storage of calcium, and the detoxification of alcohol.
- Rough Endoplasmic Reticulum (RER): Involved in the synthesis of polypeptides or proteins for outside the cell.
- Golgi Apparatus: Involved in the post-translational modification of polypeptides or proteins, such as phosphorylation (adding a phosphate group) and glycosylation (adding a carbohydrate group).
- Nucleus: Stores genetic information in the form of DNA, which contains a multitude of genes.
Separation of Organelles
While we can distinguish between prokaryotic cells and eukaryotic cells, how can we be sure these organelles exist in the conditions we have set? Experimentally, we perform a technique known as cell fractionation, which is a popular research tool in laboratories. Initially, we would need to collect a sample of cells for analysis, destroy the membrane artificially with a solvent, and create a mixture of various materials known as a homogenate.
Following homogenization, we would carry out what is known as differential centrifugation. Through a centrifuge, a student would accelerate the sample for a given amount of time in order to isolate an organelle of interest, such as nucleus or a ribosome. Generally, the denser organelle (e.g. mitochondrion, chloroplast) will sediment out in a pellet first in comparison to an organelle of lower density (e.g. ribosome).
Cells: Prokaryotic and Eukaryotic Practice Problems
Describe three differences between prokaryotic cells and eukaryotic cells.
A researcher is investigating β-cells of the pancreas, which are responsible for synthesizing the peptide hormone insulin. Insulin is a water-soluble peptide hormone which lowers the blood glucose levels in cells beyond the pancreas. Based on this information, explain whether insulin would be synthesized in a cytosolic ribosome or a ribosome attached to the RER.
A student is performing a cell fractionation experiment to separate organelles from an unknown cell. At an applied acceleration of 20,000g (g is the force of gravity) for 15 minutes, a pellet forms enriched in mitochondria. If the student continued separating, would the student expect a pellet enriched in ribosomes before or after isolating mitochondria?
Cells: Prokaryotic and Eukaryotic Practice Problem Solutions
Prokaryotic cells are exclusively unicellular, store DNA in a nucleoid, and have non-membrane-bound organelles. Eukaryotic cells may be either unicellular or multicellular, store DNA in a nucleus, and have membrane-bound organelles.
A protein is being synthesized for export. Thus, it must be synthesized on a ribosome attached to the RER.
A pellet enriched in ribosomes would be expected after isolating mitochondria due to their lower density.
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