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How Substances Are Transported From One Organ To Another In Plants And Animals.

This is a past A Level required essay. The essay is basic, yet informative, providing the highest amount of marks!

Date : 22/11/2020

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Aaryan

Uploaded by : Aaryan
Uploaded on : 22/11/2020
Subject : Biology

Hemoglobins are a group of chemically similar molecules found in a wide variety of organisms. They are protein molecules with a quaternary structure that has evolved to make it efficient at loading oxygen under one set of conditions, however unloading under a different set of conditions. The process by which haemoglobin binds with oxygen is called loading, or associating. In humans this takes place in the lungs. The process by which haemoglobin releases its oxygen is called unloading, or dissociating. In humans this takes place in the tissues.

The role of haemoglobin is to transport oxygen around the body. To do this efficiently, haemoglobin must be able to readily associate with oxygen at the surface of gas exchange, however readily dissociate at the tissues which require oxygen. It is able to do this by changing its chemical attraction to oxygen under the presence of certain substances, such as carbon dioxide. When a Carbon Dioxide molecule is present, the haemoglobin molecule is able to bind loosely to oxygen, releasing it, as a result. However, in a high oxygen concentration area, such as a gas exchange surface, haemoglobin binds to oxygen more readily. This property of haemoglobin allows it to transport oxygen around the body efficiently, to all the different organs of the heart, while being able to bring carbon dioxide back to be released by the body.

The human heart pumps blood around the body. The heart itself it made of two separate pumps, which lye side by side to each other. The heart has two sides to ensure there is not a pressure drop. This is because when the blood leaves the heart, it must be oxygenated at the lungs. Once oxygenated, they lose pressure, so the blood returns to the left side of the heart, where is is pushed out of the heart, under high pressure.

Blood initially returns to the atria of the heart via the pulmonary vein and the vena cava. When the vena cava fills, its pressure increases, causing the AV valve to open, allowing blood into the ventricle. Again, blood fills the right ventricle. Once this occurs, the ventricle contracts, increasing their pressure. As the pressure is higher in the ventricle than the atria, the AV valve is closed. The high pressure in the ventricle causes the semi lunar valve to open, forcing blood into the arteries. This blood travels to the lungs, where gas exchange takes place. The blood takes in oxygen, and gives out carbon dioxide. The blood returns to the left side of the heart, where the atria fills up, followed by the ventricle. However, this time, when the ventricle contracts, it pushes blood out at a high pressure, pushing blood out towards all the different organs and tissues of the body which need oxygen. Once the oxygen dissociates from the haemoglobin, the blood returns to the right side of the heart to repeat the process. This tends to happen around 70 times in the human body.

In plants, water is absorbed by the roots through extensions called root hairs. In flowering plants, this water is transported around the plant via xylem vessels. The force that pulls the water is called transpiration. The energy for this is provided by the sun, and is therefore passive.

The main factor that is responsible for the movement of water up the xylem, from the roots to the leaves is cohesion-tension. Initially, water evaporates from mesophyll cells due to heat, and this leads to transpiration. Water molecules form hydrogen bonds between one another, and this is cohesion. Water forms a continuous, unbroken column across the mesophyll cells and down the xylem. As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion. A column of this water it therefore pulled up the xylem as a result of transpiration, and this is called the transpiration stream. This water is used by cells in photosynthesis.

Translocation is the movement of solutes to where they re needed in a plant. It is an energy requiring process that happens in the phloem. Translocation moves solutes from sources to sinks . The source is where the solute is made, and the sink is where it s used up.

Scientists aren t certain exactly how the solute are transported from source to sink by translocation. The best supported theory is the mass flow hypothesis:

1. Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells. This creates a high pressure inside the sieve tubes at the source end of the phloem.

2. At the sink end, solutes are removed from the phloem to be used up. This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes.

3. The result is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes towards the sink. When they reach the sink the solutes will be used in respiration or stored e.g. starch.

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