The chains of amino acids are folded into shapes that are held in place by chemical bonds; strong, covalent bonds. However, most of bonds that maintain the shape are weak bonds, usually hydrogen bonds- which can be broken easily by chemicals or heat. Different amino acids have different residual groups (R groups). Polar R groups interact with water by forming hydrogen bonds that face outwards. Non-polar R groups tend to cluster towards the centre of the molecule; this gives a hydrophobic core to the molecule. Some of these bonds break when proteins are heated up, so the tertiary structure changes and the protein doesn’t function, this is known as denaturation. Cold temperatures affect proteins aswell; as there isn’t enough energy for collisions to make a reaction.
An example of this is enzymes. An enzyme works best at or near its optimum temperature and pH. Temperature extremes can denature an enzyme by causing the molecules to lose their tertiary structure. Bonds that hold the polypeptides in specific shapes are broken and the shape of the active site changes; as a result the substrate molecules no longer fit. So the enzyme is denatured and loses all activity.
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Describe and explain how changing temperature affects the cell membrane.
A cell membrane is a thin layer around the outside of every cell. Cell membranes are partially permeable; the distance across the membrane is about 7nm. The components of a cell membrane consist of phospholipids, cholesterol, proteins, glycolipids and glycoproteins. Membranes have two layers of phospholipid, known as a bilayer. The lipid molecules that make up the membrane have a polar hydrophilic head and two non-polar hydrocarbon tails that are hydrophobic. The structure of a membrane is described as ‘fluid mosaic’ because the phospholipids are liquid and the proteins are dispersed within the bilayer like moveable mosaic tiles.
The functions of cell surface membranes include controlling what substances enter and leave the cell, as they are partially permeable- they let some molecules through but not others. They allow recognition by other cells and cell communication. Functions of membranes within cells include dividing the cell into different compartments which increases their surface area and making chemical reactions more efficient. They can form vesicles to transport substances, they also control what substances enters and leaves the organelle.
Temperature of the membrane affects how much the phospholipid bilayer can move which affects the membrane structure. There are 3 ranges of temperature which can affect the membrane. The lowest range of the temperatures is below 0°C- the phospholipids don’t have much energy at all, they’re packed tightly as the membrane is rigid, the channel proteins denature and ice crystals may pierce the membrane making is permeable. The next range of temperatures is 0 °C - 45 °C. In this range, the phospholipids can move around and aren’t packed tightly, the membrane is partially permeable. As the temperature increases, so does the permeability of the cell membrane. The last range of temperatures has the most affect on the cell membrane. With temperatures above 45 °C the phospholipid bilayer melts and begins to break down, the membrane becomes more permeable, the water inside the cell expands which puts pressure of the membrane, and the channel proteins then denature which increases the permeability of the membrane. Overall, the higher the temperature, the more damaging effect it has on the membrane.
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