Cell membranes are involved in the transport of various substances within cells, and use several processes by which they carry out their functions. All cells acquire the molecules and ions they need through their plasma membranes. In eukaryotic cells, there is also transport in and out of membrane-bounded organelles such as the nucleus, ER, and mitochondria.

One of these types of cellular transport is osmosis. Osmosis is the diffusion of water from a region of high water concentration to a low water concentration through a semi-permeable cell membrane without the use of energy. This cell membrane is not able to allow large particles such as starch pass it, yet the fluidity of the membrane allows water to pass through due to its size and structure. Water moves toward areas of lower concentration until its water potential is zero, which the cell membrane regulates. Osmosis is the transport of water from high to low osmotic potential, low to high solute concentration, and low to high osmotic pressure. Solutions can be isotonic (where the solution has the same solute concentration as the cell), hypertonic (where the solution has more solute concentration than the cell), or hypotonic (where the solution has a lower concentration of solute than the cell). Through the water potential of each of these solutions, the cell membrane is able to control osmosis and osmotic pressure. This type of movement is quite significant to cells, as without osmosis, water would not enter or leave the cell, and could result in several possible consequences. In animal cells, a hypertonic solution may cause the cell to lose water and shrivel up; yet a hypotonic solution would result in the cell bursting and cytolysis occurring. In plants cells, a hypertonic solution would cause plasmolysis, or the process by which the vacuole and membrane of the cell shrink and it loses water; yet a hypotonic solution could trigger turgor pressure and the cell membrane would be pushed against the cell wall due to the expansion of the vacuole. Since water is a vital compound to life, it is completely necessary to transport it in and out of the cell, thus the great significant of osmosis. In plant cells, osmosis proves water for photosynthesis.

A second type of cellular transport is active transport that involves the use of energy. Proteins act as pumps or carriers. Ions such as sodium and potassium are pumped through active transport, where the membrane maintains very specific levels of these ions in and out of the cell. For example, to leave a cell, sodium and/or potassium ions bind to carner proteins such as permease, and through the use of ATP for energy (ATP is converted into ADP and an extra phosphate group), a phosphate group binds to the protein in a process called phosphorylation, and causes a change in protein shape so that the ion exits the cell. To enter into the cell, ions must go through the reverse process, as the phosphate group is released and the ion is released into the cell. Active transport is vital to a cell since the accumulation of ions outside of the cell draws water out of the cell and enables it to maintain osmotic balance, otherwise it would swell and burst from the diffusion of water. Amino acids can also be transported in this manner, and the proteins that activate active transport must provide both energy and a pathway across the lipid bi-layer. Active transport is important because it creates charge gradients, it is used to concentrate ions, minerals and nutrients inside the cell that are in low concentration outside, and it keeps unwanted ions or other molecules out of the cell that are able to diffuse through the cell membrane. In animal cells, active transport assists in glucose transport, while in plant cells, it regulates turgor. In both cells, it helps generate the hydrogen ion gradient.

A third type of cellular transport is endocytosis/exocytosis, both requiring energy. These processes are involved in the movement of large particles that are too big to fit through the membrane. Exocyotosis moves substances out of the cell, such as waste. The golgi body packages the waste particle, and forms a vesicle.

This vesicle fuses to the cell membrane due to the membrane’s fluidity and the membrane is broken to allow the waste to be extinguished from the cell. Endocytosis involves bringing large substances into the cell, and can be divided into three different processes. Phagocytosis literally is “cell eating” and it is for the ingestion of large solid substances such as food. An organism such as an ameba wraps around the desired food particle and engulfs it until a vesicle is formed and joins with the lysosomes (sacs of digestive enzymes) into the food vacuole to be digested. Pinocytosis can be acknowledged as “cell drinking” and it is used for solutions of larger particles. A similar process occurs as that of phagocytosis; to circle around the solution, form a vesicle, and then make a food vacuole. Finally, the Receptor Mediated Endocytosis involves a substance binding to a cell’s receptor protein to trigger endocytosis and allow the cell to engulf the substance. Hormones and cholesterol are two examples exemplifying this process. In protests, this process is used for nutrition, and in any cell, it controls the entering and exiting of cell products and certain substances.

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