0610 · 8.2Water uptake

Root hair cells and surface area

0:00When you water a plant, you pour water onto the soil, not directly into its leaves. Yet the leaves need that water for photosynthesis. So how does water get from the soil into the plant, and then all the way to a leaf? The journey begins with cells so small that you need a microscope to see them clearly. These are root hair cells. Their shape helps the plant collect water from the soil, and they connect to a pathway leading through the root, up the stem and into the leaves.

0:25By the end of this video, you will be able to recognise root hair cells, explain why their large surface area matters, trace the pathway of water through a plant, and describe an investigation that makes part of that normally invisible pathway visible. First, picture a young root growing through soil. The soil contains particles, air spaces and water. Some of that water forms thin films around soil particles, with mineral ions dissolved in it.

0:49Just behind the growing root tip, many cells on the outside of the root have long, narrow projections. These are root hairs. They reach between soil particles and make close contact with the water around them. A root hair is not a tiny root, and it is not a separate structure attached to a cell. It is an extension of one root hair cell. That distinction helps you understand both its appearance and its function.

1:13In a diagram, look for a cell with a long, thin projection extending from its main body. That projection is the most useful identifying feature. The cell has a cell wall, a cell membrane, cytoplasm, a nucleus and a large vacuole. The projection is enclosed by the same cell wall and cell membrane as the rest of the cell. It is all one continuous cell. In a microscope image, you may not see every internal structure clearly.

1:39Instead, look for fine projections growing from the outside of a young root. Do not expect a root hair cell to look like a green leaf cell. Its job is absorption, not collecting light. Root hair cells absorb water and mineral ions from the soil. Water is needed for processes such as photosynthesis, while mineral ions supply substances the plant needs for healthy growth. For this topic, the important point is that root hairs increase the uptake of both.

2:04Why does the long projection help? Imagine comparing two cells with similar main bodies. One has a fairly smooth outer surface. The other has a long, narrow extension. The extension adds extra surface that can contact soil water. Across a root, thousands of these extensions provide a large total surface area. There is more contact between the absorbing cells and the water containing dissolved mineral ions. That is the connection you need to understand.

2:31The large surface area of root hairs increases the uptake of water and mineral ions. The advantage is not simply that the hairs make the root look bigger. They increase the surface available for absorption.

Osmosis and the water pathway

2:42Now we need one idea from the topic on movement into and out of cells. Water enters root hair cells by osmosis. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Water potential describes the tendency of water to move. For the usual situation in this example, the soil water has a higher water potential than the contents of the root hair cell.

3:09Water therefore moves into the cell through its partially permeable cell membrane. The cell does not actively pump the water in. Osmosis is a passive process. Also, mineral ions do not enter by osmosis. Osmosis refers specifically to the movement of water. Once water has entered the root hair cell, it is still near the outside of the root. The plant’s long-distance water transport tissue lies farther inside. Between the outer surface and that transport tissue are the root cortex cells.

3:39The cortex is a region of cells inside the root. Water passes through this region towards the xylem. For this syllabus, the pathway through the root is root hair cells, then root cortex cells, then xylem. You do not need to divide this journey into more detailed microscopic routes. Xylem is the tissue that carries water and mineral ions through the plant. Here, think of it as a connected transport route.

4:01We are following where the water goes, rather than studying the detailed structure of that tissue. Water enters the xylem in the root, travels up through the xylem in the stem, and reaches the xylem in the leaf veins. A leaf vein is therefore not just a line on the leaf surface. It contains transport tissue that delivers water into the leaf. From the xylem in the leaf, water passes to the mesophyll cells.

4:24These are the cells inside the leaf that carry out photosynthesis. So the complete pathway you need is root hair cells, root cortex cells, xylem, mesophyll cells. Notice how the pathway matches the organisation of the plant. First, water enters through an absorbing surface. Next, it crosses the root towards the transport tissue. Then it travels along that tissue to the leaf. Finally, it reaches the cells that need it.

4:50What drives the continued movement of water up the plant is explained in the video on transpiration. Here, our focus is identifying the route.

Investigating xylem with eosin

4:58How could we investigate that route rather than just accept a diagram? Water is transparent, so it is difficult to see where it travels inside a stem. A suitable stain makes the pathway visible. A common school investigation uses a leafy celery stalk standing in dilute eosin solution. Eosin is a coloured stain. As the coloured solution travels through the water transport pathway, it stains the tissue along that pathway.

5:23Under teacher supervision, prepare a fresh cut at the lower end of the celery stalk and place that end in the solution. Keep the leaves attached and leave the stalk for a suitable period so that the stain can travel. Record the starting appearance and the time allowed. An otherwise similar stalk standing in plain water provides a useful comparison, helping you distinguish staining from the tissue’s natural colour. Then examine the stalk and its leaves.

5:46A teacher can cut thin transverse sections at different positions along the stalk. A transverse section is a cut across the stalk, rather than along its length. Use a hand lens or microscope to inspect the cut surfaces and record where the colour appears. Wear eye protection when handling the stain, and leave sharp cutting to the teacher or follow the laboratory’s supervised cutting procedure. Let us work through an example of interpreting this investigation.

6:11Suppose a question shows red-stained regions in a cut celery stalk, and red colour along some leaf veins. It asks what these observations suggest about the pathway of water. First, identify what the stain is tracing. The stain was supplied in water at the cut lower end, so its position helps reveal where that water travelled. Second, examine the pattern. The staining is concentrated in particular regions, rather than spread evenly through every part of the stalk.

6:38That suggests water travels through specialised transport tissue. Third, identify the tissue. The stained regions are the xylem. Staining in the leaf veins shows that this pathway continues from the stalk into the leaves. The conclusion is that water travels through xylem in the above-ground parts of the plant. There is an important limit to this conclusion. Because the celery stalk was cut and placed directly in the stain, this investigation does not demonstrate uptake by root hair cells.

7:05It investigates the above-ground pathway, not the entire journey from soil. Also, avoid saying that every cell must turn red. The useful observation is where the stain becomes concentrated.

Answering water uptake questions

7:16Now let us connect this understanding to examination questions. Cambridge may ask you to identify a root hair cell, state its functions, state the effect of its large surface area, or outline the pathway of water. If the command is identify, use the diagram or image to name the structure. Look for the long projection from a cell on the root surface. If the command is outline, give the main stages in the correct order.

7:41You do not need a long account of every process occurring along the way. Try this exam-style question. Identify cell A. State its functions. State how its large surface area affects uptake. Outline the pathway taken by water from this cell to the mesophyll cells. First, the long projection tells us that cell A is a root hair cell. Then give the functions and the surface area statement precisely. Finally, trace the pathway in order.

8:07Cell A is a root hair cell. Root hair cells absorb water from the soil. Root hair cells absorb mineral ions from the soil. The large surface area of root hairs increases the uptake of water and mineral ions. Water passes from root hair cells through root cortex cells into the xylem. Water travels through the xylem in the root, stem and leaf. Water passes from the leaf xylem to the mesophyll cells.

8:35Each sentence makes a clear biological point. The answer is short because the reasoning has already helped us choose the correct structures and order.

Common mistakes and recap

8:44Before we finish, check three common mistakes. Do not describe a root hair as a separate root. It is a projection of one cell. Do not say that mineral ions enter by osmosis. Osmosis is the movement of water through a partially permeable membrane. And do not jump straight from root hairs to the stem. Water first crosses the root cortex to reach the xylem. Let us recap the whole journey.

9:09Root hair cells have long projections that provide a large surface area for absorbing water and mineral ions. Water enters these cells by osmosis. The pathway is root hair cells, root cortex cells, xylem, mesophyll cells. Xylem connects the water supply in the root to the stem and leaf veins. A suitable stain, such as eosin, can reveal the pathway through above-ground parts. By examining stained sections, we can connect an observation to the conclusion that water travels through xylem.

9:39The key idea is a connected journey. The absorbing surface collects water, the root provides access to the transport tissue, and that tissue delivers water towards the leaf cells that need it.