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Effect of Temperature on pigment release by Beetroot

GCSE: Biology

Title:  Effect of Temperature on pigment release by Beetroot
Description  Effect of Temperature on pigment release by Beetroot
Word Count:  1800


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Before I explain my results, I am briefly going to consider the structure of the beetroot cell. The outermost layer is the cell wall, which is present only in plant cells and is made up of a carbohydrate called cellulose and also has other protein substances embedded within it. The cell wall is a rigid layer and gives structural stability to the cell and also limits the permeability of large substances into and out of the cell. Within the cell wall, surrounding the cytoplasm is the cell membrane which is a semi-permeable membrane consisting of a phospholipid bilayer. The bilayer consists of phospholipids which arrange themselves so that the hydrophobic (‘water hating’) tails are shielded from the surrounding water. The heads of the molecules are hydrophilic (‘water loving’) and face the water. Overall, the cell membrane acts to selectively allow substances to move into and out of the cell and maintains the cell potential. Proteins within the membrane act as molecular signals allowing the cells to communicate with each other and other substances outside the cell. About 70% of the cell membrane is actually protein. The cytoplasm of the cell has a number of organelles, although the one that I will focus on is the vacuole. Vacuoles act to store food for the plant and also assist in structural stability of the plant along with the cell wall. The vacuoles in plant cells are normally larger than those found in animal cells and contain a fluid called, cell sap. This fluid is rich in nutrients and other substances and is surrounded by a membrane called the tonoplast, separating it from the cytoplasm. The tonoplast is similar in composition to the cell membrane.

In Beetroot cells, the vacuoles also contain pigments called, ‘betalain’ which are red pigments, whose function is not entirely clear. They replace pigments called ‘anthocyanins’ in higher plant species such as the beetroot. When the beetroot cylinders are heated, it is this pigment that leaks out of the cell giving an indication of the cell’s permeability in relation to temperature, as measured by the colorimeter. In the experiment, at low temperatures of below 40 C, the cell membrane is intact and effectively carries out its task of controlling the entry and exit of molecules into and out of the cell. On the graph, this is shown by the small gradient of the line below 40 C. As the temperature increase however, the proteins in the membrane begun to denature and no longer function appropriately. The protein is held together by hydrogen bonds and disulphide bridges between the amino acids. Excess heat causes an increase in the kinetic energy given to these molecules making them vibrate and eventually breaking the bonds. The rise in temperature also causes the water within the membrane to expand putting pressure on the membrane from within. As a result, the lipid component of the membrane liquefies. The denaturation of the proteins and liquefaction of the phospholipid bilayer greatly increases the permeability of the cell membrane. A similar effect on the tonoplast of the vacuole causes a leakage of the ‘betalain’ pigments out of the vacuole and eventually out of the cell itself through the cell membrane into the surrounding water used in the experiment. One explanation for the movement of betalain pigments out of the cell is diffusion. The higher concentration of betalain pigments inside the cell means that they will diffuse into the surrounding water where their concentration is low. The kinetic energy generated by heat will also facilitate this process as the betalain pigments will have a higher energy, making them move faster.


Figure 1: Diffusion of pigments from an area of their higher concentration (left of diagram) to an area of lower concentration (right of diagram), an analogy that can be applied to the ‘betalain’ pigments.

Diffusion is not however, the only method of movement since it will reach an equilibrium when the concentration on both sides is the same. The continual increase in permeability indicates that other factors also come into play. One explanation for the steep gradient between 45 C and 75 C is that, the concentration of betalain pigments within the cell are greatest at this range, and thereafter the number of pigments moving out decrease due to the lower concentrations within the cell.
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