Cells - Session 2

Do you know how the plants are able to maintain their shape? Plants can maintain their shape under extreme conditions due to presence of the cell wall. The cell wall is a rigid, protective layer that surrounds the cell membrane in the plant cells. The main component of the plant cell wall is cellulose. Cellulose is a type of sugar. It provides strength to the plant’s cell wall.

Chloroplasts are green colored organelles found in the plant cells. They contain a pigment called chlorophyll. The chlorophyll gives green color to the plants. Chloroplasts are like food factories for plants. They use sunlight, water, and carbon dioxide to create glucose. Glucose is the food of the plant. This process is called the photosynthesis.

A sap vacuole is like a tiny storage bag inside the plant cells. It holds a watery fluid called the cell sap. Cell sap contains important stuff like water, nutrients, and the waste products. Sap vacuole helps the plant cell stay steady and upright. It also keeps the harmful waste products away from the important parts of the cell.

Water is very essential for living organisms. When we eat food, water in our mouth mixes with the saliva to help break-down the food. It also helps our stomach and the intestine to absorb nutrients from the food. Water helps our body get rid of the waste materials. Our kidneys use water to make urine. Urine carries waste out of our body. When we sweat, the water helps us cool-down and gets rid of some waste materials.

Water is like a transportation system in our body. Different substances move into and out of the cell with the help of the water. In plants, water transports the minerals and nutrients from the roots to the leaves. Water is a universal solvent. A solvent is a substance that can dissolves other substances. Water has the ability to dissolve most of the substances.

Diffusion is a process in which molecules move from an area of higher concentration to an area of lower concentration. This movement occurs naturally. It continues until the concentration becomes equal throughout the given space. An example of diffusion is the movement of molecules across the cell membrane. Molecules move from an area of higher concentration to an area of lower concentration through the cell membrane.

The concentration gradient affects the rate of diffusion. Concentration gradient refers to the difference in concentration of a substance between two regions. The steeper the difference in concentration between these regions the faster the diffusion rate. When there is a big difference, the molecules move more quickly to in order to try to equate the concentrations.

Temperature plays a significant role in the diffusion. When the temperature is high, the molecules increase in their energy. These high energy molecules move fast. They collide with the other molecules more frequently. As a result, the diffusion happens faster. Molecules have low energy at low temperature. They move slowly. As a result the rate of diffusion is also decreased.

A larger surface-area allows for faster diffusion. When there is more surface area available, there are more places for the molecules to interact with the surrounding medium. This speeds up the diffusion process. Imagine you have two sugar cubes. One sugar cube is intact. Other sugar cube is crushed into powder. Which sugar will dissolve faster in water? The powdered sugar will dissolve faster. This is because the surface-area of powdered sugar is greater. The small particles in the powdered sugar provide more points of contacts as compared to large sugar cube.

A solute is the substance that gets dissolved in the solvent. A solvent is the substance that dissolves the solute. For example, you might have dissolved the sugar in water. Sugar is a solute because it gets dissolved in the water. Water is a solvent because it dissolves the sugar.

Osmosis is a process by which water molecules move from an area of lower solute concentration to an area of higher solute concentration. This movement of water molecules takes place across the selectively-permeable-membrane. The selectively-permeable-membrane allows particular substances, like water, to pass through. It blocks the passage of the other substances such as salt or sugar.

Water molecules move into and out of the cells through osmosis. Cell membrane acts as the semipermeable membrane. The water molecules move from an area of lower solute concentration to an area of higher solute concentration. Let us say that the concentration of solutes is high inside the cell as compared to outside of the cell. In this case water molecules will move into the cell.

The solution that contains high concentration of solutes is called the hypertonic solution. If a cell is placed in a hypertonic solution, the water will move out of the cell. This is because the concentration of solutes are low inside the cell as compared to the hypertonic solution. As a result the cell will shrink.

We know the cell wall is a rigid structure seen in plant cells. When a plant cell is placed in the hypertonic solution it also loses water. When the cell loses water, the cell membrane contracts and moves away from the cell wall. This is called the plasmolysis. The cell that undergoes the plasmolysis is called the plasmolyzed cell.

The solution that contains very low concentration of solutes is called the hypotonic solution. When a cell is placed in the hypotonic solution, it becomes turgid. The water moves from the hypotonic solution into the cell. This is because the concentration of solute is high inside the cell as compared to the hypotonic solution. A turgid cell is similar to balloon filled with water. A turgid cell might burst.