This research assignment will discuss why cells are usually microscopic in size, given that they need to be able to exchange material with their surrounding environment.
The cell is highly organised with many functional units or organelles (Spurger).A membrane is a fluid mosaic which consists of proteins, lipids and carbohydrates which seperates the cell from it’s surrounding environment or subdivides a cell into specialised regions or compartments (Watters). The membrane is specialised in that it contains the specific proteins and lipid components that it requires in order to fulfil the requirements needed by that organelle or cell. Membranes are vital for the integrity and function of a cell.
Lipids are the one class of large biological molecules that does not include polymers. They share one important chemical property: they have little or no affinity for water, so they are grouped together.
Proteins are the most structurally complex molecules known, they also account for on average 50% of the dry weight of most cells. (Membranes online)
Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units. Some of these are bonded to lipids covalently called glycolipids however the majority of them are covalently bonded to proteins called glycoproteins.
The cell membrane is a mixture of many different proteins, carbohydrates and lipids, embedded in the fluid matrix of the lipid bilayer. The lipid bilayer is the primary fabric of the membrane, and its structure creates a partially permeable membrane. (Hughes)`
THIS DIAGRAM SHOWS THE STRUCTURE OF A CELL MEMBRANE. IT REFERS TO THE GLYCOLIPIDS AND GLYCOPROTEINS WHICH ARE THE PRODUCT OF CARBOHYDRATES COVALENTLY BONDING WITH LIPIDS AND PROTEINS.
The cellular membrane has three basic properties, firstly a bilayer lipid membrane. Second, a lipid membrane and third, it’s dynamic nature. These three properties ensure that the cell is able to function correctly by obtaining the materials that it needs through the membrane and excreting the materials that it must excrete through the membrane.
Cell membranes act as barriers to most, but not all molecules (Transport in and out of cells). The development of a cell membrane that could allow some molecules to pass through but stop other from doing the same was a huge evolutionary step of the cell. Cell membranes are partially permeable barriers separating the cell from its surrounding environment.
One way of moving material across a partially permeable membrane is facilitated diffusion. Facilitated diffusion requires the use of a protein to facilitate the movement of a molecule across the membrane. (As can be seen below)
In other cases, the protein changes it’s shape, allowing the molecule to pass through.
The protein changes shape and releases the molecule to the side with the lower concentration.
(Diagrams sourced from _Membranes_)
Additional energy is not required for facilitated diffusion as the molecule is travelling from high to low concentration. The energy of movement comes from the concentration gradient. This process does not work for moving ions or molecules against the concentration gradient.
Active transport is used to move molecules or ions from low to high concentration which means moving it against the concentration gradient. One way of doing this is the sodium-potassium pump which uses active transport to move 3 sodium ions to the outside of the cell for every 2 potassium ions that move in (Membranes).
_Ions can only cross the membrane using integral membrane proteins called ion channels, or proteins. This figure shows ions moving in an ion channel. Ion channels can only allow ions to move passively, without the addition of energy. They are essentially water-filled pores, and ions pass through the interior of the pore. – (_ Zoology 448, 9 Jan 2003)
For an ion to be able to move through a membrane, two things are required. The forces acting on the ion must allow this movement energetically and there must be a pathway/open channel (Zoology 448).
Water molecules are able to pass through a permeable membrane via osmosis. Osmosis is definition is the movement of water molecules from an are of high concentration to an area of low concentration. Cell membranes are completely permeable to water. Osmosis does not require energy.
Cells are also fully permeable to oxygen. Oxygen moves into a cell in much the same way as water.
Glucose can move in or out of a cell via Facilitated diffusion or active transport. Much like ions, glucose requires a channel to move through a membrane.
As has been shown, the moving of materials in and out of a cell is vital. The reason a cell is microscopic is simply smaller is faster. This is true both in terms of diffusion and in terms of chemical and electrical movement. By decreasing the distance between a cells nucleus and the many proteins and organelles that it constantly has to regulate, the cell is maximising the speed in which intercellular communication can take place while providing the perfect conditions for diffusion. Like wise, the surface area and volume of a cell are directly influential in the efficiency of the cells nutrient absorption and waste excretion system. The membrane of a eukaryotic cell is it’s only form of absorbtion therefore it’s surface area must be large enough to allow the organelles in the cell to receive the materials they need. This is done by
maximising the surface area to volume ratio (Study World).