Plant Systems

Transport of Substances in Flowering Plants

Transport in flowering plants involves the movement of water, mineral salts and organic compounds through the vascular tissues (phloem and xylem).

Figure. 4.13: Vascular tissue of a monocot and dicot stem The movement of water from the soil to the leaves is termed the transpiration stream, while the movement of food and other organic materials from the leaves to all parts of the plant is called translocation.

Water and mineral transport occur in the xylem tissue. The main cells of the xylem are the xylem vessels, tracheids, xylem parenchyma and xylem fibres. These cells have various adaptations and function together to facilitate the efficient transport of water and minerals.

Figure. 4.14: Cells of the xylem

Table 4.3: Parts of a xylem cell Xylem cell type Description or adaptation Functions Tracheids Elongated shape, thick lignified walls with pits Water and mineral transport, structural support Vessels Short and wide with perforation plates, lignified walls Efficient water transport, structural support Xylem Parenchyma Living cells with thin cell walls Storage of nutrients, lateral transport Xylem Fibers Thick lignified walls, elongated shape Structural support The process by which plants lose water through their leaves is called transpiration.

This occurs through a mechanism called transpiration pull, which refers to the continuous movement of water through the xylem that is drawn upwards from roots to the leaves. The processes of water and mineral transport are aided by:

a. evaporation of water from the leaves

b. Cohesion: the attraction among water molecules helps the molecules to be pulled along in the xylem.

c. Adhesion: the attraction between water molecules and the xylem cell walls

d. Tension: negative pressure in the xylem Dissolved minerals (minerals dissolved in water) from the soil are generally transported by diffusion through the root hairs. However, the movement of dissolved minerals from the xylem to the leaves is by mass flow. Mass flow is the bulk movement of substances through a medium by factors such as pressure gradient and concentration gradient.

Use this link to watch a video on transpiration:

https://youtube.com/@fuseschool?si=ttVZLL02C2RKLT87 The phloem tissue is responsible for transporting food and other organic compounds in plants. Organic food materials (usually sucrose, a carbohydrate) are translocated from the leaves to all growing areas such as shoot tips, roots, flowers and fruits.

Other substances transported are amino acids in dissolved form, small quantities of plant hormones (e.g. auxins, gibberellins and cytokinins) and organic acids, including malic and citric acids. These substances are transported in the phloem mainly by mass flow (which ensures long-distance transport of materials in the phloem), diffusion and active transport. The cells that form the phloem tissue are sieve tubes, companion cells, phloem parenchyma and phloem fibres.

Table 4.4: Parts of a phloem cell Phloem cell type Description or adaptations Functions Sieve Tube Elements Elongated shape, sieve plates, lack of organelles Transport of organic nutrients, e.g. glucose Companion Cells Close association with sieve tube elements, abundant organelles Support and metabolic assistance to sieve tube elements Phloem Parenchyma Living cells, thin cell walls Storage of nutrients, lateral transport Phloem Fibers Thick lignified walls, elongated shape Structural support Use this link to watch a video on translocation in phloem:

https://youtube.com/@fuseschool?si=ttVZLL02C2RKLT87

Figure 4.15: Phloem.

The main difference between the transport of materials in phloem and xylem is that the phloem transports food in bidirectional paths to all parts of the plant, mostly by pressure flow, while the xylem transports water and dissolved minerals upward from the roots to the leaves, generally by transpiration pull.

Table 4.5: Table of differences in xylem and phloem tissue Xylem Phloem It consists of vessels, tracheids, xylem fibres and xylem parenchyma It consists of sieve tubes, companion cells, phloem fibres and phloem parenchyma Absence of any cross wall Presence of sieve plates at the end of sieve tube cells.

Xylem fibres are smaller when compared with phloem Phloem fibres are larger when compared with xylem No companion cells Companion cells control the activities of the sieve tubes, Transports water and dissolved minerals in an upward direction from the soil, through the roots to the leaves.

Transports food substances in all directions to all parts of the plant All cells in the xylem are dead except the xylem parenchyma All cells are living except phloem fibres Factors Affecting the Transport System in Flowering Plants Several factors affect the transport system in flowering plants. These can broadly be considered as physiological, morphological and environmental factors.

Physiological factors These refer to the internal processes of the plant which affect the movement of water and nutrients. These include

a. Rate of transpiration: As transpiration increases, more water and minerals are absorbed from the soil to be transported in the xylem. Higher rate of occurrence of transpiration also means that movement of water with dissolved organic nutrients, such as sucrose, is enhanced in the phloem.

b. Root pressure: As the root pressure increases, more water is pushed up through the xylem, hence more water uptake. The action by the root pressure can be seen, where fluid emerges (exudation) from a cut stem from which the leaves have been removed. The action is also seen during guttation.

This is the exudation of water droplets, containing dissolved minerals and sugars, from the tips or edges of leaves, usually at night, through specialised pores called hydathodes, when stomata are closed, and root pressure is high.

c. Photosynthesis: A high rate of photosynthesis increases sugar concentration in photosynthetic cells, creating a concentration gradient, thereby causing the movement of these organic compounds through the phloem to other parts of the plant, such as the roots and fruits.

Morphological factors These are the factors that relate to plant structures which influence the absorption and transport of substances in plants. They include

a. Surface area and nature of the leaf: Leaves with a large surface area show an increased rate of transpiration, hence higher uptake of water and minerals in the xylem. The opening and closure of the stomata on the leaves control the movement of water through the leaves by transpiration. The stomata open to allow water vapour from the plant into the atmosphere and close to prevent water loss. Generally, the stomata are located on the lower side of the leaf or are concentrated at the lower surface more than on the upper surface. Again, hairs on the leaves reduce water loss. Some plants also roll their leaves to prevent excessive water loss in dry environments.

b. Structure of cells of xylem: Wider vessels have less resistance to the flow of water within the xylem tissue.

c. Density of root hair: The higher the density of root hair (the greater the number of root hairs), the higher the surface area of the roots and hence better contact with the soil to absorb water and dissolved minerals and transport them through the xylem.

Environmental factors These are the external conditions which affect the rate of transpiration.

Environmental factors affecting water movement in plants are

a. Temperature: The higher the temperature, the higher the rate of transpiration in the xylem. Movement of food materials within the phloem is increased as higher temperatures increase metabolic activity.

b. Light intensity: The higher the light intensity, the better the rate of photosynthesis to produce sugars, with increased transport of food from the leaves. Light also causes the stomata to open, as more sugar is produced in the guard cells to make them turgid. This increases the rate of transpiration from the leaves.

c. Humidity: Low humidity in the environment increases the rate of transpiration, which enhances water movement in the xylem.

d. Availability of water in the soil: Adequate water levels in the soil facilitate the movement of water through the xylem and movement of nutrients through the phloem.

Activity 4.7 Observation of xylem action in plant flowers Materials needed

• Plastic cup or beaker

• Food colour (red or blue)

• White flowers with their stalks, e.g. carnation, water lily, hibiscus, rose or periwinkle flowers

• Water Instructions

1. With your partner, gather the materials needed.

2. Half fill the beaker or cup with water

3. Add three to four drops of the food colour to the water

4. Put the flower in the coloured water

5. Observe for one to two hours the changes in the flower.

6. Record and discuss your observations with other teams in your class.

7. Predict the possible observations to be made if a transverse section of the stalk of the flower is placed under the microscope.

8. Predict the possible conditions that will affect the xylem action in the flowers used.

Activity 4.8 Ringing experiment or Girdling experiment Aim: Demonstrate the role of phloem in the transport of nutrients in plants.

Materials needed

• A healthy potted plant or a young tree

• Sharp knife or scalpel

• Ruler

• Marker

• Plastic wrap or aluminium foil

• Watering can Procedure

1. Select a healthy plant with a stem that is thick enough to handle the ringing process without breaking the stem.

2. Use the marker to draw two parallel lines around the stem, about 2-3 cm apart. This will be the area where the bark with the phloem will be removed.

3. Carefully use a sharp knife or scalpel to cut along the marked lines.

Remove the bark and the underlying phloem layer between the two lines, exposing the xylem. Be careful not to damage the xylem.

4. Wrap the exposed area with plastic wrap or aluminium foil to prevent it from drying out and to protect it from pests and diseases.

5. Water the plant regularly and ensure it receives adequate sunlight, if it is a potted plant.

6. Observe the plant within the first few days

7. Observe the plants after a few weeks.

8. Suggest reasons for the observations you have made.

Photosynthesis Photosynthesis is the biochemical process where green plants and other chlorophyll-containing organisms (e.g. algae and some bacteria) make their food using light energy. The conditions that are needed for photosynthesis to occur are;

1. Carbon dioxide from the atmosphere

2. Water from the soil

3. chlorophyll (mainly in the chloroplasts in the leaf)

4. Light energy (generally from the sun) absorbed by chlorophyll.

These conditions and factors can be verified with experiments in the laboratory.

In plants, photosynthesis occurs in the chloroplasts because chloroplasts contain the green pigment chlorophyll, which absorbs light.

Figure 4.16: Structure of a chloroplast.

Photosynthesis results in the conversion of light energy into chemical energy stored in glucose, a food molecule.

The chemical equation for photosynthesis is:

6CO₂+ 6H₂O chlorophyll light energy C₆H₁₂O₆+ 6H₂O + 6O₂ Where,

• 6CO₂ means that there are six molecules of carbon dioxide needed.

• 6H₂O means that there are six molecules of water needed.

• Carbon dioxide and water are reactants.

• Light energy is required.

• Chlorophyll is required.

• C₆H₁₂O₆ means that one molecule of glucose is manufactured, and it is the primary product of photosynthesis.

• 6O₂ means that there are six oxygen molecules formed in the reaction.

Oxygen is released as a by-product of photosynthesis.

Photosynthesis produces glucose and oxygen as by-products.

This is the simplest representation of photosynthesis, as it is a complex reaction that involves multiple numbers of intermediate steps and various enzymes, which are outlined below.

The two main stages in photosynthesis are ;

1. The light-dependent stage, or light reactions, or photochemical stage:

a. Occurs in the thylakoid membranes of the chloroplast.

b. Uses light energy to split water molecules to produce oxygen, protons (hydrogen ions) and electrons.

c. Produces ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate): energy-rich molecules needed for the dark reactions.

2. The light-independent stage, or Dark Reactions, or Calvin Cycle:

a. Occurs in the stroma of the chloroplast.

b. Uses NADPH (energy molecule) from the light-dependent stage to combine hydrogen with carbon dioxide to make glucose through a series of enzyme-catalysed reactions. The chemical energy needed for these processes to occur is from ATP (an energy molecule)

c. NADP+, ADP + P, and glucose are the end products of the reaction.

Figure. 4.17: The processes of photosynthesis.

Factors affecting photosynthesis Light intensity The range of light absorbed by plants for photosynthesis is between 400 nm and 700 nm. This range is called photosynthetically active radiation. The most important light wavelengths for photosynthesis are 430nm (blue light) and 660 nm (red light), absorbed by chlorophyll a, and 450nm (blue light) and 635nm (red light) absorbed by chlorophyll b. The rate of photosynthesis generally increases with increasing light intensity as light is required in the light-dependent stage.

However, after an optimum light intensity is reached, additional light does not affect the process.

Carbon dioxide concentration Generally, an increase in carbon dioxide concentration increases the rate of photosynthesis, as CO₂is required for fixing carbon at the light-independent stage. This happens when CO₂concentration is the limiting factor.

Temperature An increase in temperature generally increases the rate of photosynthesis, since this speeds up enzymatic reactions. However, beyond an optimum temperature, the rate of photosynthesis decreases because such high temperatures can denature/ destroy enzymes, thereby hindering the rate at which photosynthesis occurs.

Availability of water Insufficient water supply indirectly reduces the rate of photosynthesis. This is because when water is in short supply, the stomata close to prevent water loss.

Thus, the amount of carbon dioxide entering the leaves also reduces, thereby reducing the rate of photosynthesis.

Chlorophyll concentration Higher concentrations of chlorophyll increase the rate of photosynthesis due to efficient absorption of sunlight.

Watch this video on photosynthesis:

https://www.youtube.com/watch?v=qkRe_OMfwv4),

Activity 4.9 The photosynthesis maze.

Activity 4.10 Data analysis of light intensity and photosynthesis.

Given the following data collected after an experiment on the effect of light intensity on the rate of photosynthesis, analyse and interpret the results:

Light Intensity/nm 100 200 400 800 1600

Rate of Photosynthesis

(O₂ produced in ml/min) 2 5 8 12 12 Use the information in the table to plot a graph of the rate of photosynthesis at different light intensities.

1. What trend do you observe in the data?

2. Explain why the rate of photosynthesis levels off at higher light intensities.