TRANSPORT IN PLANTS

Keywords

• Active transport
• Root hair
• Stomata
• Concentration gradient
• Diffusion Flaccid Osmosis
• Phloem
• Plasmolysis
• Translocation
• Transpiration
• Turgid
• Turgidity
• Xylem

By the end of this chapter, you will be able to:

1. Understand the importance and key methods of movement of materials into and out of the cells.
2. Investigate the different ways in which materials move into; through and out of the cells
3. Know how the root hair is adapted for absorption of water and mineral salts.
4. Understand the processes of transpiration and translocation.
5. Conduct experiments and understand factors that affect transpiration

Introduction

In this Chapter you are going to study how substances enter and exit from cells, the transport process in plants, the structures involved and their functions. During your study you will discover that substances such as how nutrients and excretions move in and out of the cell by diffusion, osmosis and active transport. Important to know is that movement of substances depends on the permeability of the cell wall and cell membrane.

 Plants require water and mineral salts which they acquire from the soil for their cells to use in the process of photosynthesis and their metabolism. Manufactured food produced in their cells or their leaves also has to move to areas of use and excess to areas of storage. In order to achieve this, plants have a transport system consisting of vessels known as the xylem and phloem, Water and mineral salts are transported in the xylem while manufactured food is transported in the phloem.

6.1 Diffusion

 Diffusion is the movement of liquids or gases from a region where they are more concentrated to a region where they are less concentrated. Diffusion occurs through a medium like air or water. Our sense smell depends on molecules in air reaching our nose by diffusion. You can detect a bad smell or pleasant one because of diffusion. In the activity below you are going to investigate how substances move by the process of diffusion.

Activity 6.1 investigating the process of diffusion

Key question

How does diffusion occur?

 What you need

• Tea bags
• Clear glass container
• Warm water.
• Notebooks

What to do

1. In pairs, place warm water in a clear glass container to cover at least % of its total volume.
2. Insert a tea bag in the warm water and observe.
3. Record your observations.
4. Discuss and explain your observations with the rest of the class.

Factors affecting the rate of diffusion

1. Diffusion is a slow process. However it can be sped up by:
2. Increasing the concentration gradient between two regions. Increasing the temperature of the medium.
3. Reducing the distance over which the diffusion is to take place.
4. Increasing the surface area over which the diffusion is to take place e.g. by folding like in our lungs
5. The size and nature of the diffusing agent by being small and uncharged (not positive or negative ions)

Examples of diffusion in plants and animals

1. Oxygen diffuses out and carbon dioxide diffuses in through the stomata (pores) in leaves during the day.
2. Single celled organisms take in oxygen and get rid of carbon dioxide by diffusion across the cell surfaces
3. Oxygen diffuses out of the blood and carbon dioxide diffuses into the blood in the tissues of animals.
4. Sensitivity Sense of smell in animals depends on diffusion of chemical substances towards it.
5. Absorption of glucose in the ileum occurs by diffusion.
6. Movement of manufactured substances in plants from leaves to other parts of the plant occurs in several ways one of which is by diffusion.
7. Removal of waste products of metabolism from cells in animals across the cell surfaces by diffusion into blood.

6.2 Osmosis

 Water molecules tend to move from areas where they are highly concentrated to areas where they are few and not present at all. In biological systems you will find that substances are moving across barriers in form of membranes. Most of these membranes are semipermeable that is, they are selective they do not allow everything to pass through them. They are also called selectively permeable membranes. The difference in the concentration of water molecules on both sides of the membrane is called a concentration gradient.

 The movement of water molecules from a region where they are more concentrated to one where they are less concentrated is known as osmosis. In the next activities 6.2 and 6.3 you are going to investigate how water moves by osmosis in plants and animal cells.

Activity 6.2 Investigating osmosis basing on the effect of different sucrose concentrations on raw unshelled eggs

Key question

 What is the effect of solutions of different concentrations on raw unshelled eggs?

 What you need

• Sucrose concentrations of different concentrations 0.0M, 0.5M and 1.0M
• Raw unshelled eggs
• 4 beakers / plastic cups.  
• Markers
• Notebooks

What to do

In your groups,

1. Label three beakers 0.0M, 0.5M and 1.0M solution respectively.
2. Place one egg in each of the beakers.
3. In the beaker labeled 0.0M pour distilled water to completely cover the egg.
4. In the beaker labeled 0.5M pour sucrose solution of 0.5M to completely cover the egg.
5. In the beaker labeled 1.0M pour sucrose solution of 1.0M to completely cover the egg.
6. Leave the beakers for two to three days.
7. Observe and carefully draw the eggs in their solutions. Label the drawings.
8. Pour off all the solutions and carefully remove each egg. Examine and record any differences.

Analyzing the results

1. Identify the egg which has

(i) Increased in volume

 (ii) Decreased in volume.

2. Which egg membrane feels firmer?

3. Explain your observation in (i) and (ii).

4. Which egg became plasmolysis?

Activity 6.3. Investigating osmosis basing on the effect of different sucrose concentrations on raw potatoes

Key question

What is the effect of solutions of different sucrose concentrations on raw potatoes?

What you need 

• Medium sized Irish potato
• Sucrose solutions of 0.0M, 0.5M and 1.0M concentration
• 4 beakers / plastic cups
• Notebooks
• Rulers
• Knife, corn borer
• Tissue papers

 What to do

1. In your groups, prepare 4 potato cylinders using a cork borer by pushing it into the potato until it goes through the potato, then using the blunt end of your pen or stopper provided push out the potato tissue. Repeat until you have 4 cylinders. Alternatively cut out 4 strips of potato tissue of the same size
2. Place the cylinder / strip next to a ruler. Cut the cylinders / strips flat at both ends keeping the knife vertical to a length of 8cm. Label three beakers 0.0M, 0.5M and 1.0M respectively.
3. Place a potato cylinder / strip in each one of them.
4. In the beaker labeled 0.0M pour distilled water to cover the potato cylinder / strip.
5. In the beaker labeled 0.5M pour sucrose solution of 0.5M.  In the beaker labeled 1.0 M pour sucrose solution of 1.0M. Leave the beakers undisturbed for 12 to 24 hours.
6. Remove the potato cylinders after 12 to 24 hours. Mop them dry then accurately measure their lengths one at a time.
7. Record you results in the Table 6.1.

Table 6.1: Record of results

10.Also observe and record their flexibility or firmness

 11.Write a report following the format below; title, question, prediction, materials, procedure, record / analysis of results and conclusion

  Questions

1. Which cylinder had?

a) The greatest change in length?

b) the least change in length?

2. Explain the results in question 1 .

3. Which cylinder is;

 a) hard and firm?

b) soft and flexible?

4. Explain the firmness and flexibility of the cylinders to the changes in their length.

5. Share your results with rest of class.

6.3 Active transport

During your study on movement of molecules you will also discover that molecules can also move from areas of low concentration to areas of high concentrations across semipermeable membranes. This process requires energy to occur and is called active transport. The energy released is used to transport ions against a concentration gradient. The energy is obtained from the process of respiration. Anything that affects respiration also affects active transport.

Factors affecting active transport

Active transport requires energy and is therefore affected by energy production and availability. Any factor that affects the production of energy (respiration) also affects active transport.

The factors include:

(i) Glucose concentration

 (ii) Temperature

 (iii) Oxygen concentration

 (iv) pH

Examples of active transport in plant and animal cells

1. Mineral ions are absorbed from the soil where they are less concentrated then across the cell walls of root hair cells by active transport.
2. Plants living in salty environment like salt marshes take in salts by active transport so that their cells are highly concentrated with salt so that water enters them by osmosis.
3. Digested food is absorbed across the ileum by active transport.
4. Fish that live in salty water in the sea take in salts by active transport. This is how they prevent loss of water from their bodies to the sea.
5. Waste products of metabolism move from cells to the tissue fluid outside the cells against a concentration gradient by active transport. This ensures no wastes are left in the cells.

Activity 6.4 Finding out the circumstances in which diffusion, osmosis and active transport are involved in the entry and exit of substances to and from a cell

 Key question

 (i) What is the meaning of the terms diffusion, osmosis and active transport?

 (ii) What are the circumstances in which each is involved in the entry and exit of substances to and from cells?

What you need

• Biology textbooks
• Computer with internet

What to do

 1 In your groups, research and discuss the meaning of diffusion, osmosis and active transport, as well circumstances in which each is involved in the entry and exit of substances to and from cells. Pens Notebooks

2. Record your results in a table.

3. Make a presentation to the class.

6.4 Adaptations of root hairs for water and mineral salts absorption

A root hair is an extension that develops on the piliferous layer cells (the outer layer of the root) in the root. It is thin walled and it provides a large surface area for the absorption of water and mineral salts. After absorption, water moves across the root through the cortex up to the xylem as shown in Figure 6.3.

 Its structure is made in such a way that it can carry out the above functions properly. In the activity that follows make a thorough study of the structure of the root hair and explain how it is adapted to its functions.

Activity 6.5 investigating the adaptations of root hairs for water absorption

 Key question

 What are the adaptations of root hair for water absorption?

What you need.

• Germinating seedling of maize or bean
• Pencils, rubber
• Hand lens
• Notebooks

 What to do

1 In your groups, observe the root system of the germinating seedling provided.

 2 In both cases note the structures on the root and explain how the root hair is adapted for absorption of water and mineral salts.

3. Prepare a report and share it with the class

4. Discuss with the class and note down the accepted explanation using notes or diagrams.

So far you have learnt how molecules move as a result of diffusion, osmosis and active transport across cells. In the course of your study of Biology you will find many examples of the processes in organisms. Now let us turn to a process involving movement of water in the whole plant.

Activity 6.6 investigating the process of transpiration in plants

Key question

 How does the process of transpiration occur in plants?

 What you need.

• Potted plant / leafy plant
• Pot with watered soil
• Soil- anhydrous copper sulphate Vaseline
• Glass plate (optional)
• Bell jar / clear polythene bag

What to do

1. In groups of two, make a research on the process of transpiration
2. You should be able to answer the question. What is transpiration?
3. Carry out an experiment to prove that transpiration occurs in plants.
4. Check the procedure you would follow to set up an experiment to investigate transpiration in a plant.
5. What do you observe after two hours of setting the experiment?
6. How can you prove that what you have observed is that substance?

• What conclusion would you make?
• Give an explanation of your observation.

Activity 6.7 investigating the process of transpiration

Key question

What is the various plant structures involved in the process of transpiration?

 What you need

• Biology textbooks
• Notebooks

What to do 1

1. In a pair reflect on what you learnt earlier on leaf structure, transport vessels in stems, roots and root hairs
2. Using this information explain how the process of transpiration takes place.
3. As individuals, draw diagrams and add short notes to explain the process of movement of the water from its absorption in pots to its loss in leaves.
4. Present to the class your diagrams and discuss with the class.

Factors affecting the rate of transpiration

The rate of transpiration in a plant changes from time to time. This is because the rate of transpiration depends on the existing conditions of the plant environment. These are mainly temperature, light intensity, wind movement and humidity (moisture content or the air). Farmers need to understand how these factors change in the time of day and seasons in order to ensure their plants do not dry as a result of high loss of water by transpiration. In the next activities you are going to investigate how these factors affect transpiration.

The temperature of the environment is a result of the heat from the sun and therefore varies with time of day. Temperature affects the evaporation of water and therefore transpiration. Thus rate of transpiration is low in cold and high on a hot day. Cloud cover cools the air by cutting off the sun rays and heat lowering transpiration rate.

 Activity 6.8 Finding out the effect of temperature on the rate of transpiration

 Key question

 How does temperature affect the rate of transpiration?

 What you need

• Potometer / capillary tubes
• Plant with leafy shoot
• Biology textbooks.
• Beakers.
• Water

What to do

 In your groups;

1. Use the science process skills to design and carry out experiments to investigate how temperature affects the rate of transpiration…
2. Discuss the significance of your findings for farmers and growers.
3. Report your findings and conclusions to the class.
4. Key question
5. What is the effect of light intensity on the rate of transpiration?

Light is one of the conditions influencing the opening of stomata? Generally stomata open during light and close during dark at night. In this way light affects the rate of escape of water vapor from the plant through the stomata and hence transpiration.

Activity 6.9 Finding out the effect of light intensity on the rate of transpiration

What you need

• Beakers
• Potometer / capillary tubes
• Waters
• Plant with leafy shoot
• Biology textbooks

What to do

 In your groups;

1 Use the science process skills to design and carry out experiments to investigate how light intensity affects the rate of transpiration.

2. Discuss the significance of your findings for farmers and growers

3. Report your findings and conclusions to the class.

Wind blows away the water vapour above the plant leaves. In this way the moisture content of the air is kept low resulting in a steep diffusion distance. Water vapor therefore diffuses at a faster rate increasing the rate of transpiration. In still air the water vapor accumulates in the air around the leaf surfaces resulting in a low diffusion gradient hence little water diffuses out of the leaves through the stomata which hence lowers the rate of transpiration.

Activity 6.10 Finding out the effect of wind on the rate of transpiration

 Key question

 What is the effect of wind on the rate of transpiration?

What you need

Potometer / capillary tubes

• Beakers
• Water
• Plant with leafy shoot
• Biology textbooks
• What to do

In your groups;

1 Use the science process skills to design and carry out experiments to investigate how wind affects the rate of transpiration.

2. Discuss the significance of your findings for farmers and growers.

3 Report your findings and conclusions to the class.

6.7 Translocation in plants

After food has been manufactured in plant leaves, it is moved to areas of the plant where it is used for respiration, growth or repair and excess is moved to storage areas in the stem or roots for storage. This movement of manufactured food takes place in the phloem which is a system of vessels distributed in the whole plant alongside the xylem. The movement of manufactured food in the phloem is known as translocation. In this Activity 6.11, you are going to find the meaning and importance of translocation.

Activity 6.11 Finding out the importance of translocation in plants

 Key question

 What is translocation and why is it necessary in plants?

What you need.

• Notebooks
• Biology textbooks
• Computer connected to the internet

 What to do

1 In a pair research on the need for and the meaning of translocation in plants.

2. Share your findings with the class.

3. Explain to the class your findings using notes and diagrams.

Chapter summary

In this chapter, you have learnt that:

• Transport in plants involves the process of diffusion, osmosis and active transport.
• Diffusion and osmosis are passive processes whereas active transport requires energy.
• Water is the transport medium. Water is absorbed from the soil by roots using root hairs.
• Root hairs are adapted for absorption of water and mineral salts by being thin walled, elongated and permeable.
• Transpiration is the loss of water vapor from plants surfaces especially leaves to the atmosphere.
• The rate of transpiration is affected by external factors such as wind, temperature and light intensity.
• Plants have a vascular system to conduct water, mineral salts and food substances.
• The phloem translocates manufactured food through the plant.