Describe the structural compartmentation of mammalian cells and the differing functions of these compartments

by wanghj (student)

All mammalian cells are eukaryotic cells. They have a true nucleus and they are normally enclosed by a plasma membrane. In a typical eukaryotic cell, one would expect to have, along with a plasma membrane and nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, cytoskeleton etc.[3] These organelles are all membrane-bound structures, each have a unique role to play in the functioning of the cell. All these organelles are specific proteins and they all interact with each other to support the cell. However cells are different from each other. Cells differ from species to species and they also differ from different organs.[5] Every cell type has its own function and its function determines the quantity of each organelle. For example, we expect to find more mitochondria in a muscle tissue cell than in a skin cell. A muscle tissue cell needs to have extra production of ATP to allow for contraction whereas the skin cell does not need to do so. This assignment will give a brief overview of the structural compartmentation and their function of a typical mammalian cell with special focus on the plasma membrane and mitochondria.

Figure 1, a cartoon picture of a typical eukaryotic cell depicting all the compartments[2]

Plasma membrane contains the content of the cell. It holds the structural integrity of the cell but at the same time it is also very flexible to allow for movement. The membrane has three other important functions: it has ability to identity molecules and other cells on the surface; it has a selective barrier which allows the membrane to control the exchange of molecules in and out of the cell and it can communicate between cells with its receptor on the surface.[5] The plasma membrane is composed of a bilayer of phospholipids. A phospholipid molecule has a hydrophilic end and a hydrophobic end. When submersed in a watery environment, like the cytoplasm and the external environment, the phospholipid molecules coalesce and organise themselves into neat bilayer in which the hydrophilic end of the molecule is closest to the water and the hydrophobic end is facing 90° away from the water. The hydrophobic ends of the phospholipid stick together due to the hydrophobic interaction in which the non-polar molecules cannot form hydrogen bonds with water. Furthermore, the hydrophobic end also repels water and along with the hydrophobic interaction, the phospholipid line up in a bilayer fashion. Due to the fact that the hydrophobic interactions are very weak forces, the phospholipid bilayer is very flexible. On the other hand, due to the number of the hydrophobic interactions, it is relatively strong and is a barrier to other solvents making it suitable to maintain the structural integrity.[4,6] The plasma membrane’s recognition ability is due to the glycoproteins present on the surface. Glycoproteins are a sequence of glucose and other complex sugars on the protein peptide chain.[5] The sequence of glucose on the protein can be matched by a complementary sequence on another glycoprotein on the surface of cell plasma. If the sequences are complementary, the cells stick together and make contact allowing the formation of tissues and them organs. If the sequences are not complementary, the cells do not attach. This is important for growing organisms as there is a rapid production of new cells.

Figure 2, a picture of a phospholipid bilayer with all the proteins[1]

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Figure 2, a picture of a phospholipid bilayer with all the proteins[1]

One of the main functions of a plasma membrane is to allow molecule to be exchanged. There are several ways it can happen and it is helped by the structure of the membrane. The first type of diffusion is passive diffusion. The plasma membrane is fluid and gaps form in the phospholipid bilayer (due to the weak hydrophobic bonds), these gaps allows for small and/or non-polar molecules to pass through. This type of diffusion requires no ATP; it depends on the concentration gradient where molecules will move from a highly concentrated area to a lowly concentrated area until equilibrium is achieved. The second form is facilitated diffusion, which also does not require the use of energy. There are integral proteins located through the cell membrane and these integral proteins carry polar molecules across the membrane. Moreover, cells are polarised; the polarisation of the cell is regulated by ion pump which pump Na+ and K+ across the cell membrane in and out of the cell.[5] The pumping of the ions across the cell membrane is important especially for nerve cells that utilises the polarisation and depolarisation of the cell to generate an action potential and move nerve impulse along the cell. The third method, active transport, does require energy. Active transport is the movement of molecules against the concentration gradient. The transport proteins are, in nature, glycoproteins.[6] They recognise the molecule (they have complementary sequence of glucose, mentioned above) and attaches the molecule to itself. The carrier protein goes conformation change and expels the molecule to the other side of the membrane. Again the process is vastly critical for secretory cells.

In addition, the membrane has receptors on the surface which allow for communication.[6] The receptor enables cells to respond to external environment. The receptors are glycolipids. Like glycoproteins, they attach to the target molecules and goes conformational change. However instead of trying to transport it across the membrane, it starts a series of internal cellular processes. This may alert the cell that a foreign particle has made contact with the cell.

All the functions above are essential for a mammalian cell. The plasma membrane is a fluid structure which does not restrict the movement of the cell but at the same time fairly strong to maintain its content. It is the first point of contact between the cell and the environment and that is what makes it an important compartment of the cell.

All living cells in require ATP. ATP is vital for protein synthesis, movement and active transport. ATP is produced in the mitochondria. Mitochondria have a double membrane; the outer membrane is smooth and holds the content of the organelle whereas the inner membrane is folded (the folds are referred to as cristae).[3] The inner membrane is crucial to respiration as it is the location of oxidative phosphorylation which is the process that generates ATP.

Figure 3, a mitochondrion and its structures[7]

The process of respiration starts with glucose being turned into pyruvate. The pyruvate then is absorbed by the matrix of the mitochondrion. Enzymes in the matrix remove hydrogen and carbon dioxide from the pyruvate to turn it to acetyl CoA. The hydrogen is accepted by NAD+. The acetyl CoA remains in the matrix and enters the Krebs cycle, where it is first converted to citrate and then decarboxylated. During the process of decarboxylation, hydrogen atoms are also removed from the citrate. The removal of hydrogen atoms are oxidation reactions. The hydrogen is accepted by either NAD+ or FAD. The oxidation reactions release energy which is stored by carriers. The carriers then enter the electron transport chain located on the inner membrane. The electrons move down the series of electron carriers releasing the stored energy as they pass down the chain. The energy released by the electrons pumps hydrogen protons across the inner membrane from the matrix to the inter-membrane space. The hydrogen protons build up in the inter-membrane space and create a concentration gradient. ATP synthase, also located on the inner membrane, allows the protons back into the matrix and as they move across the membrane, the energy is released and used by ATP synthase to produce ATP. The whole process is called chemiosmosis.[4,5,6] It is this that produces ATP and allows for the basis of life to exist.

The nucleus is the largest organelle in a typical mammalian cell.[5] Like mitochondria, it has a double membrane. However the nuclear envelop has pores that allow for the transportation of molecules from the nucleus most notably mRNA and protein peptides derived from transcription and translation. The main function of the nucleus is to store DNA information. A nucleolus may be found in many of the mammalian cells.

The site of protein synthesis occurs in the endoplasmic reticulum. There are two types of endoplasmic reticulum: rough and smooth. The rough endoplasmic reticulum is embedded with ribosomes which make the surface look rough. The ribosomes on the surface are responsible for protein synthesis. They can also be found floating freely in the cytoplasm. Smooth endoplasmic reticulum however controls phospholipid synthesis and detoxification of toxins.[6]

One other organelle that is important with regard to proteins is the Golgi apparatus. Golgi apparatus is a series of flattened discs. Its main function is to modify proteins after their synthesis. They then package the protein into other vesicles to other parts of the cell.[5]

To conclude, the compartmentation of the cell allows it to many differing functions. The plasma membrane allows the cell to be flexible and change shape but at the same time containing the integrity of the structure. The nucleus contains all the DNA information that allows for protein synthesis. Furthermore, there are mitochondria that produce the ATP that all living organism depend for existence. The cell is the smallest unit of an organism, but its differing compartments support all aspects of life.