Essay: How materials are exchanged across the cells membrane

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Within this assignment, I will be looking at how materials are exchanged across the cells membrane. I will be explaining the processes, diffusion, osmosis, active transport and phagocytosis. My assignment will also explain how these exchanges are related to the cell membranes structure and, explain the movement of water from cells using the concept of water potential.

Diffusion is the process by which molecules spread from areas of high concentration, to areas of low concentration. This happens without any outside force as it is a spontaneous process. The process is a result of the intrinsic thermal energy (heat) found in all molecules at temperatures above absolute zero. When the molecules become uniform throughout a space it is called equilibrium (, 2015).
Passive Transport

There is also passive transport, this is diffusion over a cell membrane. This is also a spontaneous diffusion where molecules move from a concentrated area, to a less concentrated area. Due to the cell membrane only allowing certain molecules through, there will be different rates of diffusion for different molecules. Some molecules may need help to cross the cell membrane, this is called facilitated diffusion.


Osmosis is the diffusion of water across the semipermeable membrane, from an area of low solute concentration (hypotonic solution), to an area of high solute concentration (hypertonic solution). Hypotonic is when the solution outside of the red blood cells has a lower osmotic pressure than the cytoplasm of the red blood cells. The cells take in water in an attempt to equalize the osmotic pressure, causing them to swell and potentially burst (, 2015). Hypertonic is when the osmotic pressure of the solution outside the blood cells is higher than the osmotic pressure inside the red blood cells. The water inside the blood cells then exit the cell in an attempt to lower the pressure, causing the cell to then shrink. Isotonic solution is when the osmotic pressure is the same inside and outside of the cell. This means the cells are normal. This is a spontaneous process where a substance moves from the less concentrated area, to the more concentrated area. The direction of water flow is determined by the solute concentration and not by the nature of the solute molecules themselves(Bailey,2015).

Active Transport

Active transport is the movement of a substance across the membrane, from a lower concentration to a higher concentration by using energy. This goes against a concentration gradient and the usual form on energy used is ATP. Carrier proteins that are found in the cells membrane, use energy to transport molecules or ions across the membrane. Active transport is used when the concentration of a cell is higher than the outside of the cell or, when a molecule is too large to diffuse through the membrane.


Phagocytosis is the process of where a cell ingests foreign bodies and cell debris, such as, bacteria, parasites and dead host cells. It does this by hunting out the debris, stretching itself around it and then engulfing it. This is how vesicles are formed, vesicles are small sacks within a cell containing fluid.

The cell membrane is composed of a phospholipid bilayer, in which proteins are imbedded. The membrane forms a solubility barrier which protects its contents from the outside world. The phospholipid layer contains a phosphate group in place of a third fatty acid. These then ionise and form a polar head, while the two fatty acids form a nonpolar tail. The heads that face outwards within the phospholipid layer as they are hydrophilic (water loving), where as the tails that face inwards are hydrophobic (water repelling). A cell membrane can also be described as selectively permeable, as it regulates the exit and entrance of substances in and out of the cell. It also facilitates the transportation of materials needed for survival, this is because it has a lipid bilayer. A lipid bilayer determines the basic structure and the proteins carry out the various functions (, 2015).The movements that occur across the membrane are either passive, occurring without energy, or active, occurring with energy.
Water potential is defined as, the measure of the tendency of water molecules to move from one place to another (, 2015). The movement of water in and out of cells, are influenced by the concentration of the solute on either side of the cell membrane. If water moves out of the cell it will shrink, when it moves into the cell it swell and can even burst. Water always moves from a higher water potential to a lower water potential, or down the concentration gradient. So the higher the pressure within a cell, the more likely water will leave it. This relates to pressure potential, this contributes to water potential.

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