Cellular Organelles: Six Answers | Assessment Answer

The plasma membrane is the outermost covering of an animal cell. It is selectively permeable which means that some and not all molecules can pass through the plasma membrane (Kukulski et al. 2012). The diagram given in the question shows the phospholipids as molecule A and the Trans-membrane protein as molecule B. The head of the phospholipid consist of glycerol and phosphate whereas the tail consists of a fatty acid chain. Due to the presence of glycerol and phosphate group the head region becomes polar and hence it is hydrophilic (Zhou et al. 2015). The tail region, on the other hand, contains only hydrocarbons due to which it remains non-polar and hence becomes hydrophobic. This property gives help in the creation of a lipid bi-layer that ultimately forms the plasma membrane. When charged particles like ions or any polar substance try to enter the cell, they get attached to the head region of the plasma membrane but they cannot enter the cell as the middle of the lipid bi-layer is hydrophobic. The only way for them to enter the cell is by the help of channel proteins. These are proteins runs across the lipid bi-layer and has a specialized structure. The trans-membrane protein is an alfa-helix folded protein which has a central hydrophilic core due to which it can remain fully embedded in the tail part of the phospholipid and through them the polar molecule may enter the cell.  To enter the alfa-helix protein, there are some conditions that must be considered such as a size of the molecule and its polarity. If all are in the favorable region, then the molecule may enter the cell otherwise not. Non-polar molecules need to reach a typical type of proteins known as integral proteins that runs across the lipid bi-layer as the non-polar molecules get repelled from the surface of the cell membrane because of the presence of polar heads of the phospholipids. These proteins do not run completely through the membrane but is half embedded in the membrane, and they can be found on both sides of the cell. The non-polar molecules get attached to these proteins and reach the hydrophobic part of the cell membrane by diffusion. Once it reaches there, it starts to search for another integral protein that opens in the inner part of the cell or the outer part depending on its target point. Once that protein is found the non-polar protein gets transported to its required destination (Chang et al. 2013).

A cheek cell in hypotonic (weak) salt solution

A hypotonic solution is a solution where the amount of water is more outside the cell than inside the cell. When a cheek cell is placed inside a hypotonic salt solution due to difference in concentration of water across the cell membrane water will start rushing into the cell. This process of movement of water from a region of higher concentration to a region of lower concentration across a semipermeable membrane is known as osmosis. Ultimately the cell will start swelling up and then burst to release its DNA if the difference of water concentration is very high ( Rickard et al. 2014).

Glucose moving into a cell after a meal

Glucose molecule moves into a cell after a meal by the process of active transport. It is the movement of the glucoce molecule across the cell membrane from the region of lower concentration to a region of higher concentration. Cellular energy is used up for moving the glucose molecule against the gradient. Accumulation of high concentrations of glucose molecules occurs that the cell needs. The glucose molecules moves against the concentration gradient in order to enter the cell. Specific transmemebrane carrier proteins are involved in this process (Kaback 2012).

White blood cells taking in streptococcus bacteria

Bulk transport is the process by which large or bulk quantities of materials move out or into the cells. Bulk transport can be of two types, namely, enodcytosis and exocytosis. Endocytosis is the process by which cells engulf materials from outside in bulk quantity and forms a sac inside of the cell. The common example is white blood cells taking in bacteria like streptococcus  (Jeon, 2013).

High oxygen level in lungs

High oxygen level in lungs are due to the process of diffusion. Oxygen diffuses into the lungs and carbon dioxide diffuses out of the lungs. The exchange surfaces on the lungs helps in this process. Diffusion works down a concentration gradient and a steeper concentration gradient helps in faster diffusion. The concentration between the blood and the exchange transport must be steep for better diffusion. Carbon dioxide is breathed out and deoxygenated blood comes to the exchange surface and this means that oxygen is diffused inside in a  rapid fashion. This gives rise to high oxygen level in the lungs  (Phillips et al. 2012).

Salivary glands secreting saliva

Salivary glands secrets saliva by the process of exocytosis of the acinar cell protein storage granules (Jeon 2013).

References

Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P., 2013. Essential cell biology. Garland Science.

Chang, C.L., Hsieh, T.S., Yang, T.T., Rothberg, K.G., Azizoglu, D.B., Volk, E., Liao, J.C. and Liou, J., 2013. Feedback Regulation of Receptor-Induced Ca 2+ Signaling Mediated by E-Syt1 and Nir2 at Endoplasmic Reticulum-Plasma Membrane Junctions. Cell reports, 5(3), pp.813-825.

Davis, L., 2012. Basic methods in molecular biology. Elsevier.

Jeon, K. (2013). International review of cell and molecular biology. Waltham, Mass.: Academic Press.

Kaback, H.R., 2012. Active transport: Membrane vesicles, bioenergetics, molecules, and mechanisms. Bacterial Energetics: A Treatise on Structure and Function, 12, p.151.

Kierszenbaum, A.L. and Tres, L., 2015. Histology and cell biology: an introduction to pathology. Elsevier Health Sciences.

Kukulski, W., Schorb, M., Kaksonen, M. and Briggs, J.A., 2012. Plasma membrane reshaping during endocytosis is revealed by time-resolved electron tomography. Cell, 150(3), pp.508-520.

Lee, E., Koskimaki, J.E., Pandey, N.B. and Popel, A.S., 2013. Inhibition of lymphangiogenesis and angiogenesis in breast tumor xenografts and lymph nodes by a peptide derived from transmembrane protein 45A. Neoplasia,15(2), pp.112-IN6.

Phillips, R., Kondev, J., Theriot, J. and Garcia, H., 2012. Physical biology of the cell. Garland Science.

Rickard, A.C., Smith, J.E., Newell, P., Bailey, A., Kehoe, A. and Mann, C., 2014. Salt or sugar for your injured brain? A meta-analysis of randomised controlled trials of mannitol versus hypertonic sodium solutions to manage raised intracranial pressure in traumatic brain injury. Emergency Medicine Journal, 31(8), pp.679-683.

s-cool.co.uk, (2016). [online] Available at: https://www.s-cool.co.uk/a-level/biology/cells-and-organelles/test-it/exam-style-questions [Accessed 13 Jan. 2016].

Zhou, Y., Wong, C.O., Cho, K.J., Van Der Hoeven, D., Liang, H., Thakur, D.P., Luo, J., Babic, M., Zinsmaier, K.E., Zhu, M.X. and Hu, H., 2015. Membrane potential modulates plasma membrane phospholipid dynamics and K-Ras signaling. Science, 349(6250), pp.873-876.