Design an experiment to investigate the effect of temperature on the movement of a pigment through a membrane
Vickie Hayton 12 FRE
Design an experiment to investigate the effect of temperature on the movement of a pigment through a membrane
Hypothesis
The tonoplast is the membrane that separates the vacuole from the rest of the cell. The membrane is selectively permeable and a phospholipid bilayer. The membrane is made up of phospholipids, which have a phosphate group and two fatty acid tails. The phosphate group is polar and hydrophilic, whereas the fatty acid tails are non-polar and hydrophobic. The fatty acid tails therefore try to get as far away as they can from the watery fluid in the vacuole and the watery cytoplasm, so the fatty acid tails point inwards and the phosphate heads point outwards. Also in the bilayer there are proteins, which can be intrinsic or extrinsic. Intrinsic proteins are proteins that span the full width of the membrane, whereas extrinsic proteins only go a small way into the membrane. The proteins provide structural support, act as carriers for water-soluble substances, can act as enzymes, form ion channels and they can act as receptors for hormones. Carbohydrate chains can join to the extrinsic proteins forming glycoproteins. These act as recognition sites. Carbohydrate chains can also join to the phospholipids forming glycolipids, this act as recognition sites and helps the stability of the membrane. Also there is cholesterol in the membrane this prevents leakage of water and ions from the cell, and therefore reduces the movement of the phospholipids, giving the membrane stability.
This whole structure of the tonoplast is called the fluid mosaic model. This is because the phospholipids and proteins in the membrane are continuously moving and the membrane resembles a mosaic in appearance.
Below is a diagram of the phospholipid membrane.
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As the temperature increases the membrane becomes more fluid because the individual molecules have extra energy so they move around more. Furthermore at I think that the cholesterol molecules would also be affected by temperature, this would mean that the membrane would become even more fluid at high temperatures, because of the affect of cholesterol on the membrane. As a result the pigment would be able to get through the membrane at higher temperatures because there would be much larger gaps between the molecules.
Also the proteins in the membranes start to denature as the temperature increases. This is because of their structure. Proteins are comprised of amino acids. Amino acids have an –NH2 group, which is a base; a carboxyl group, which is an acid; and an R group, which is different for every amino acid. When two amino acids bond they form a peptide bond, which is a condensation reaction, where the –OH part of one of the amino acids bonds with a hydrogen of the other amino acid and produces water. The molecule that is produced is called a dipeptide bond; if there are more peptides bonded together they are called a polypeptide. The primary structure of the protein is the type of amino acids contained in the polypeptide chain and the sequence in which they are joined.
The secondary structure of proteins is the shape of the polypeptide chains. This is because of hydrogen bonding between the oxygen in the –CO group of one amino acid and the hydrogen in the –NH group of another amino acid. This shape can be an alpha-helix or it could be a beta-pleated sheet shape, or they can have no regular arrangement at all, this depends on the R-groups of the amino acids. Hydrogen bonds occur because the –CO part of the amino acid is polar, as is the –NH part this means that the charges from the electrons in the molecule are unevenly distributed, called a dipole. In the –CO group there are more electrons towards the oxygen atom, whereas in the –NH group there are fewer electrons towards the hydrogen atom. This means that the oxygen atom is more negative, whereas the hydrogen atom in the bond is more positive. Therefore the oxygen and hydrogen attract because they have opposite charges, causing a hydrogen bond. Hydrogen bonds are very weak, however if there are lots of them they can have a significant effect, as in the secondary structure of proteins. As the temperature increases the hydrogen bonds break, this causes the proteins to change shape, so they denature. This means that the pigment should be able to fit through the proteins, whereas normally it cannot.
Below is a diagram of an alpha-helix secondary structure
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Below is a diagram showing the beta-pleated sheet secondary structure
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The tertiary structure of proteins is how the α-helixes and β-pleated sheet structure coil and fold. That is the way in which a protein coils up to from a precise three-dimensional shape. There are four bonds that are used to keep the tertiary structure. These are disulphide bridges, which are covalent and occur between cysteine molecules; ionic bonds, which are between carboxyl and amine groups; hydrogen bonds, between hydrogen and oxygen atoms; and there are hydrophobic interactions between non-polar R groups, which move away from water and therefore alter the shape of the molecule. Temperature also affects the hydrogen bonds in the tertiary structure, also causing the proteins to denature, allowing the pigment to get through the tonoplast.
Proteins also have a quaternary structure this is when many polypeptide chains come together to become a protein. Also there are prosthetic groups within the proteins.
Anthocyanin does not normally move through the membrane.
I think that the pigment anthocyanin would move through the membrane via diffusion at high temperatures. Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, until the concentrations of both regions are equal, creating a dynamic equilibrium. The pigment would be transported through the membrane by diffusion because the concentration of the pigment in the vacuole would be greater than the concentration in the cytoplasm, as there is no pigment there. Only lipid soluble non-polar molecules or very small molecules can diffuse directly through the membrane because the fatty acid tails are hydrophobic and non-polar, therefore they repel any substances that are hydrophilic and polar. As a result I think that the pigment would have to be transported across the membrane by facilitated diffusion. This is when the molecules of pigment would have to travel through the membrane in the intrinsic proteins.
Below is a graph showing how diffusion means that the molecules move to the side with the lower concentration.
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Below is a diagram showing facilitated diffusion with channel proteins and carrier proteins.
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The factors that affect the rate of diffusion are the steepness of the concentration gradient because the larger the surface area the faster the rate of diffusion. Also the thickness of the membrane, the thicker the membrane is the longer it takes the substances to diffuse through. Finally the temperature affects the rate of diffusion, as the temperature increases the rate of diffusion increases proportionally. This is because when heat is given to the molecules this is converted into kinetic energy so the molecules move around faster. This causes the rate of diffusion to also increase. The relationship is proportional because an increase in temperature will then cause an equal increase in the rate. However the rate can only increase so much because there are a limiting number of intrinsic proteins that the pigment can get through.
I, therefore, think that as the temperature increases the pigment, anthocyanin, will start to move through the tonoplast because the membrane will become more fluid, also the proteins will become denatured and the cholesterol will not work as well. Therefore the pigment should be able to get through the tonoplast. Moreover I think that the pigment will be transported by diffusion, and the rate of diffusion also increases with temperature. Therefore I predict that the temperature at which the pigment will start move is about 40°C. However I think that at 60°C the membrane will have completely broken, therefore all of the pigment will have got through the membrane.
