The cell membrane (fig 5) consists of a double layer of phospholipids these are the main part of the cell membrane, it is made up of a head and two fatty acid tails. They spontaneously fold up to form a double-layered molecule. Proteins make up around half of the cell membrane. Some of these are embedded in the membrane. Some are receptors and others are ion channels. “Scientists use the fluid mosaic model to describe the structure of the cell”. (dictionary, 2017) The cell membrane is selectively permeable meaning the membrane selects certain materials to enter and exit. Oxygen, which cells need to perform metabolic functions such as cellular respiration, and carbon dioxide, which is a waste product, can easily enter and exit through the membrane. Water can also freely cross the membrane in a process known as osmosis. However, highly charged molecules, like ions, cannot enter through, neither can a large macromolecules like carbohydrates or amino acids. Instead, these molecules must enter through proteins that are fixed in the cell membrane. (dictionary, 2017)
Molecules do this by:

• Diffusion ( fig6)
Is the movement of molecules or ions from high concentration to low concentration, until equilibrium is reached, this process does not require energy.

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• Osmosis (fig7)
Osmosis is the diffusion of water molecules, from an area of high concentration to low concentration. An example of osmosis includes an experimental lesson that Walsall college students completed, it showed the concentration levels of salt on three potato cells. The higher the concentration of salt the less the potato weighed due to there being a low concentration of water inside the cell; the water was drawn out to the higher concentration causing the potato to decrease in size. This is called hypertonic, when the concentration levels were roughly the same levels inside and out, there was no change, and this is called isotonic. When there was no salt added to the solution around the potato cell, the potato cell increased in mass. This happened because the net water amount moved into the cell to a low concentration, therefore increasing the mass of the potato.

The chart below shows the differences the effects of the level of salt concentration in percentages:

Active transport
(Fig 8) Is a transport method where the cell membrane absorbs larger molecules by engulfing them. The cell membrane surrounds the molecule and pinches off, creating a vesicle which travels through the cell membrane. This is also done by:

• Phagocytosis (cell eating)
• Pinocytosis (cell drinking)
• Receptor-mediated endocytosis
Cell surface receptors specifically capture a certain molecule.

This process is the opposite of endocytosis, the vesicle floats up to the cell membrane and releases the captured waste products to the exterior of the cell membrane.

Once the cell has received the nutrients through the transport methods discussed previously, it uses these nutrients to convert to energy so the cell can keep the organism alive. One of the processes used to turn fuel into energy is through aerobic respiration.
The equation for aerobic respiration is:

Glucose + Oxygen Carbon Dioxide + Water + Energy

This equation shows when glucose is broken down it produces, carbon dioxide, water and energy. When the bonds of the glucose are broken, energy is released. The cell stores the energy in chemical bonds called adenosine triphosphate (ATP), as well as this, it can then be used as a backup for whenever the cell needs it. “This occurs when a molecule of adenosine diphosphate (ADP) uses the energy released during respiration to bond with a third phosphate which then becomes ATP. When the cell needs energy to work, the energy is released by splitting off one or more of its phosphate groups, therefore giving the cell energy”. (m, 2018)


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