This arrangement of phospholipids would form a barrier to water and to water-soluble substances. Other molecules are scattered among the phospholipids, some of which include proteins and polysaccharides. The phospholipid bilayer that surrounds every living cell, helps keep the cell contents inside and the outside world outside. Without this no cell can exist. Therefore phospholipids have one end which is attracted to water and another which is repelled by it, this enables them to form cell membrane.
The cell membrane surrounding the outside of a cell, which is usually known as the cell surface membrane, has a few different functions.
-It controls the passage of substances into and out of the cell, in a number of ways. For substances to enter or leave a cell the molecules have to move through protein pores. Cell membranes have a variety of these, which all have the right size and structure to allow particular substances to pass through, this is known as ‘Facilitated Diffusion’.
Diffusion of water molecules across the cell membrane is known as Osmosis. Osmosis is best regarded as a special type of diffusion involving water molecules only. There would be a movement of solute molecules from B to A by diffusion (meaning more solute molecules would pass from B to A in a given time than from A to B). At the same time there would be a movement of water molecules from A to B by diffusion because solution A has a higher concentration of solute molecules than solution B has. Eventually, the concentrations of water molecules and solute molecules in A would equal that in B.
solute + solvent = solution.
Active transport is the process when cells move molecules or ions against their concentration gradient. The cell membrane contains protein molecules, which pushes ions into the cell, there are different types for a particular ion.
When the ions bump into the protein, the protein uses energy from ATP, to change shape and therefore pushes the ion into the cell.
-It receives information from the outside of the cell; the information is transmitted along neurons. The cell surface membrane of a neuron contains proteins. These are like channels for particular ions, and are sensitive for potential difference. So when an action potential sweeps down the cell surface membrane of the neuron, the change in potential difference opens ion channels in the membrane. This causes some of the vesicles of acetylcholine fuses with the presyneuptic membrane.
- Cell recognition. Dangerous organisms get into the human body even though there are a number of defence lines. So it’s important that the invading micro-organisms can be recognised, they are distinguished by chemicals on their surface, usually protein, carbohydrates, etc.. Your immune system is able to recognise any foreign bodies, as all cells have a particular surface chemical. So these are attacked by antigens. There are two types of white blood cells, which help destroy invading micro-organisms by actually eating them. (This is also known as Phagocytosis). They are called Granulocytes and Macrophages.
For Phagocytosis to take place, the micro-organisms needs to stick itself to the cell surface membrane of the Phagocyte.
The cell membrane inside the cell has a number of functions. The cell membrane provides attachment sites for molecules involved in the following:
-Capturing light energy. The Thylakoid membrane and Chlorophyll molecules, group together to form a Photosystem. There are hundreds of pigment molecules, including Chlorophylls a and b which make up part of the Photosystem complex. So when light falls onto a Photosystem the pigment molecules absorb energy, and pass it to a pair of chlorophyll a molecules, this emits electrons. When electrons have been emitted from chlorophyll molecules, it is carried along a chain of electron carriers, this is found in the phospholipid bilayer. The carriers pick up the hydrogen ions, as well as the electrons in the Thylakoid membrane. There are large protein molecules called ATP, these span the membrane allowing the hydrogen ions to pass through them, down their concentration gradient and electrical potential gradient. This movement enables enough energy for the ATP synthases to make ATP, and out of the Thylakoid space and into the stroma.
The cell membrane inside the cell doesn’t just provide attachment for molecules involved in Photosynthesis, etc.., in the inside of the cell the cell membrane also surrounds and separates the compartments/contents of it. The different parts of the cell are therefore separated and isolated from one another, because of this different processes are able to take place.
With a Mitochondria, there are two types of membranes present, an inner membrane and an outer membrane. The outer membrane has the same functions as most outer membranes have; it separates them from one another. Mitochondria have a highly folded membrane, to which it supports the proteins of the electron transport responsible for synthesising ATP, also because it increases the surface area.
The Nucleus is surrounded by a Nuclear envelope, it is also made up of two membranes, these have may gaps in them called Nuclear pores. These are large enough to allow partially assembled ribosome’s from the Nucleolus to pass through.
Overall. All the other organelles present in Eukaryotic cells are all surrounded by a cell membrane. Cell membranes can either be found inside the cell or it can be found surrounded around the cell. They have a number of different functions whether controlling the passages substance into and out of the cell, receiving information of involved in the process of Photosynthesis.
