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Question Video: Recalling the Definition of a Semipermeable Membrane Biology

The cell membrane of the root hair cell allows water and dissolved nutrients to pass into the cell, but it stops other substances from doing so. What term is given to this type of membrane?

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Video Transcript

The cell membrane of the root hair cell allows water and dissolved nutrients to pass into the cell, but it stops other substances from doing so. What term is given to this type of membrane? (A) Partially selective, (B) semipermeable, (C) stratified squamous, or (D) double layered.

This question is asking about the membrane surrounding a root hair cell. To answer this question, let’s review the key facts about root hair cells and their role in plant transport.

The roots of a plant are essential for providing the plant with water and nutrients and for keeping them secured in the ground. Roots are organs, and on their outer surface are found root hair cells, which are specialized for their role of absorption. But how exactly do they perform this function? Root hair cells have a shape which gives them a large surface-area-to-volume ratio, which increases the rate at which substances can be taken up from the surroundings.

As well as its specialized shape, it has multiple components including a vacuole, a semipermeable membrane, and a cell wall, to help with its role. The cell wall is thin, allowing movement of water and nutrients, such as nitrates, between the soil and root whilst still providing structure. The semipermeable, also known as selectively permeable, membrane between the cell wall and cytoplasm allows small molecules through whilst blocking the passage of larger molecules.

Water molecules are small, so they can pass through the membrane into the root hair cell by osmosis. This is the random movement of water molecules from where they are in a higher concentration, higher water potential, to where they are in a lower concentration, lower water potential, across a semipermeable membrane. Mineral ions are also small enough to pass across the membrane. But many of them are in a lower concentration in the soil than in the cell, so they need to be moved against their concentration gradient by active transport.

This process is carried out by transport proteins in the cell membrane, and it requires energy. There are plenty of mitochondria in the cell to provide the energy required, through the process of cellular respiration.

Finally, the vacuole is a fluid-filled sac, where the outside of the sac is also a semipermeable membrane. It contains water and nutrients and helps the root hair cell maintain its shape. The vacuole contains a high concentration of solutes. A solute is a substance which is dissolved to make a solution. If there is a high concentration of solutes, it means the water concentration, or water potential, is low. Because the solutes are too large to move out of the vacuole through the membrane, water molecules move into the vacuole down their concentration gradient by osmosis.

Now that we have reviewed the structure and function of the root hair cell, we can return to the question. We now know the correct answer is (B). A structure that allows water, dissolved nutrients, and mineral ions to pass into the cell but stops other substances from doing so is called a semipermeable membrane.

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