In and Out of cells

Water is the main component of all cells. We find water in the cytoplasm and in cell sap. This water contains many dissolved substances and these substances plus the water enters and leaves the cells through the cell membrane.  The cell membrane allows certain particles through it but it blocks the passage of others. Because of this nature, it is described as a partially (or selectively) permeable membrane. Particles enter and leave cells by three processes:

• Diffusion
• Osmosis
• Active transport

Particles in liquids and gases have kinetic energy, therefore they move about at speed in all directions. These particles move in a random motion. Where there is an area of high concentration some of these particles collide into one another, lose energy and slow down. Others will escape from the area of high concentration to an area of low concentration elsewhere. Very few particles travel the opposite way. The result is a concentration gradient with particles diffusing from an area of high concentration to an area of low concentration. Diffusion occurs in gases and with any substance in a solution. Diffusion: The movement of particles from an area of high concentration to an area of low concentration until they spread out evenly. 

There are two important rules to remember:

• The larger the particles the slower the rate of diffusion.
• The greater the difference in concentration the greater the rate of diffusion. This difference is known as the concentration gradient.

In order to carry out photosynthesis, a plant requires carbon dioxide. On the underside of leaves, there are small holes known as stomata, carbon dioxide diffuse into the leaves via these. Leaves produce oxygen and water vapour and these, in turn, diffuse out via the stomata.

The cells in our body need glucose and oxygen for respiration. Both these are carried in the blood. When blood reaches the cells the molecules of glucose and oxygen diffuse out of the blood and into the cells. 

As cells use up the glucose and oxygen they produce waste chemicals and carbon dioxide. If these were to build up in the cells they would poison them, therefore they diffuse out of the cells into the blood.

Osmosis is a special kind of diffusion involving water molecules. It occurs when two solutions are separated by a partially permeable membrane.

The definition of osmosis is as follows:

Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/concentrated solution) through a partially permeable membrane.

Animal cells have a partially permeable cell membrane. This means that if they are placed in pure water because their cytoplasm is a stronger solution than the pure water, water will pass into the cells by osmosis. The cells will, therefore, swell up. However, animal cells have no cell wall to stop them from swelling too much so they keep swelling until they burst. We call this haemolysis.

Osmosis is the way in which plants take up water. This is how. Root hairs of the plant take in water from the soil by osmosis. The cell membrane of the root hair cell acts a partially permeable membrane (the cell wall is fully permeable) and because the cell sap inside the vacuole is a strong solution (low water concentration) water passes from the soil (high water concentration) into the root hair cell by osmosis. The concentration of the sap in the vacuole is now weaker as there is a high concentration of water. Water will now pass from this area of high concentration to the next cell which has a low water concentration by osmosis. In this way, water continues to move along the cells of the root up the xylem to the leaf. all the time water is moving to areas of lower water concentration.

As water enters plant cells it makes the cell swell up. The water moves into the plant cell vacuole and pushes against the cell wall. Eventually, the cell contains as much water as it can hold. The strong cell wall stops the cell bursting. We say that cell is turgid. Turgid cells are useful implants as the give the plant support as they keep the stems of plants upright.

When plants are placed into a strong sugar or salts solution water will pass out of the cells by osmosis. As water passes out, the sap vacuole starts to shrink. These cells are no longer firm they are limp. We say that they are flaccid and the plant will wilt.

If a lot of water leaves the cells then the cytoplasm starts to peel away from the cell wall. We say that the cell has undergone plasmolysis.

Sometimes organisms require certain substances and therefore need to move dissolved substances from a region of low concentration to a region where they are at a higher concentration. This process is, of course, the opposite to the direction in which particles would normally move in diffusion. This is active transport. Inactive transport particles move against a concentration gradient and therefore require energy which must be supplied by the cell. Carrier proteins that are found in the cell membrane of cells use energy to transport molecules or ions across the membrane, against the concentration gradient. When organisms utilise active transport the energy for the process comes from respiration. Due to this the cells capable of active transport usually have more mitochondria, in which respiration takes place than other cells. Definition of active Transport: The movement of particles from an area of low concentration to an area of high, against a concentration gradient.

Plants require mineral salts such as nitrates for growth. The concentration of nitrates is higher on plant root cell than it is in the soil solution surrounding it. The plant cannot rely on diffusion as the nitrates would diffuse out of root cell into the soil. Hence the cells utilise energy to actively transport nitrates across the cell membrane into the root cell, against the concentration gradient.

The process of active transport takes place in humans during digestion of food in the ileum (small intestine). Once food has been absorbed by the villi after some time the concentration of food molecules inside the villi increases at this point no more food can diffuse in. As more food is still required simple sugars, amino acids, vitamins and minerals are actively transported into the villiform an area of low concentration to an area of high concentration, against the concentration gradient.