Fluid Movement in Cells

Students explore the phenomenon of diffusion, osmosis, and transpiration in cells.

Introduction

In this lesson we will be covering the different ways fluids move in cells, how fluids enter a cell and how they are dispersed. When you drink water, eventually your cells will use that water; similarly, when you eat or breathe those resources eventually make it to your cells.

Permeability

All cells have an outer membrane that is responsible for “checking” if specific elements may pass through, this outer layer is referred to as the cell membrane. A cell membrane is referred to as selectively permeable because it allows specific materials through and stops others from entering. When something is permeable they allow all materials to pass through, likewise, if something is impermeable, it will not allow anything to pass through.

Look at the picture below, the plastic bag is impermeable, allowing no water to leak out. Likewise, the cloth is permeable allowing the water to flow through.

Diffusion

Diffusion is the process in fluids where particles tend to move from higher concentration to lower concentration areas, over time, this process evenly distributes the particles in their container. In a cell, diffusion plays an important role, imagine an amoeba produces carbon dioxide as a waste product. In a world with no diffusion, carbon dioxide would build up in the amoeba with no way of being disbursed, however, because of diffusion as carbon dioxide in the amoeba is formed it becomes high concentration compared to its surrounding environment. Overtime, the carbon dioxide particles will pass through the amoeba’s cytoplasm and membrane to lower concentrations outside of the cell.

Osmosis

About 70 percent of a cell’s content is water and a cell will quickly die without a supply of water. Using diffusion water can move throughout the cell, however cells have a selectively permeable membrane that allows only water to pass through. This process is known as osmosis.

Now recall the idea introduced at the beginning of this Topic. It suggested that you drink water to help your cells carry out their functions. When you are very active, you lose moisture from your body in your breath and in sweat. Moisture is lost by the body through the skin’s surface and the surface of the lungs. Water is then drawn from other cells and structures of the body to replace the water lost from these surfaces. This happens partly by osmosis and partly as a result of the body’s circulatory system. At some point, you need a new supply of water to restore the cell water content in your body to its normal level.

Water is important to living things because it dissolves many of the substances involved in cell processes. For example, glucose (which cells use for energy) dissolves in water to form a glucose solution. When water moves out of a cell, the dissolved substances inside the cell become more concentrated. When water moves into a cell, the dissolved substances inside the cell become more diluted.

Water tends to move by osmosis from a diluted solution to a more concentrated solution. In other words, water moves from a region where it is in high concentration to one where it is in lower concentration. That is why water moves into dehydrated carrot cells.

Fluid Movement in Plants

Most plants need a large supply of water. Plants require water to make sugars in the process of photosynthesis. Plants obtain water from the soil. How does water get from the soil into the plants? Roots need the sugars made in the leaves. How do cells in the roots of plants obtain these sugars? Recall from Topic 1 that tissues are groups of cells that perform similar functions. The transport of nutrients is the role of the plant’s tissues. Inside the plant, two types of tissues, called vascular tissues, connect the roots to the leaves. Phloem tissue transports sugars manufactured in the leaves to the rest of the plant. Xylem tissue conducts water and minerals absorbed by the root cells to every cell in the plant.

Xylem and phloem tissue usually occur together, along the length of the plant stems and roots. Both types of tissue are surrounded and supported by other tissue that gives the plant strength. This other tissue has large vacuoles for storing food and water.

If you examine the structure of a root system, you will see that its growing tips are covered with fine root hairs. These “hairs” are, in fact, extensions of single epidermal cells. Epidermal cells form epidermal tissue, which protects the outside of a plant. When the concentration of water in the soil is greater than the concentration of water in the root cells, water enters these root hairs by osmosis.

From the root hairs, water passes from cell to cell by osmosis until it reaches the xylem tissue. The tube-shaped cells making up xylem tissue have thick walls with holes in their ends. Stacked end to end, they form bundles of hollow vessels similar to drinking straws. Water can flow easily through these vessels. As more water enters the root hairs, it creates pressure that pushes water up the plant through the xylem tissue.

Water is transported by xylem tissue into the stems and the leaves. Leaves are the plant’s food-producing organs. Recall that photosynthesis manufactures sugars from water, carbon dioxide, and sunlight. Most photosynthesis takes place in a layer of cells in the leaf that are filled with chloroplasts. These cells are called palisade cells. Why are many leaves typically flat and thin? This shape provides a large surface area to absorb sunlight. This shape also makes it easy for gases to diffuse into the leaf cells.

Notice the tiny openings on the underside of the leaf. These openings are called stomata (singular: stoma). They allow air to enter the leaf, supplying the oxygen the cells need for respiration and the carbon dioxide they need for photosynthesis. Spaces between leaf cells allow the air to flow around each cell. Surrounding each stoma are guard cells, which can expand to close off the stoma.

Transpiration

Why do the stomata in a leaf open and close? To answer this question, recall that water first enters a plant through its root system. Then it moves into its shoot system. What happens next? The water does not continually circulate like the blood in our bodies. It does not go back into the root system. Instead, it exits the plant — through the open stomata in the leaves.

This loss of water from a plant through evaporation is called transpiration. The loss of water is not a problem as long as it is replaced by more water that enters the plant through the roots. In periods of drought and in deserts, however, water loss from a plant can be a serious problem.

Pulling and Pushing

If all the tissues of a plant were to magically disappear, leaving only the water in them behind, you would see a ghostly outline of the plant in a weblike network of water. There is no break in this water system. Fine columns of water connect every cell, from the leaves to the roots. The network extends even beyond the root hairs — it connects root hairs to channels of water in the soil.

According to the particle model, individual water particles are held together by bonds of attraction, which make the plant’s water network behave as a single unit. Water drawn into the root hairs by osmosis pushes slender water columns up the plant. At the same time, water lost from the leaves by transpiration pulls water up the xylem tissues all the way from the roots. Both these actions — pushing and pulling — are necessary to raise the water up to the top of very tall trees. In this way, trees can transport water without having a pumping organ similar to the human heart.

The information on this page and it associated figures are from the Science Focus 8.
Science Focus 8, pp. 130-137
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