7 Transport in flowering plants (7.1 & 7.2)

Root hair is a fine tubular outgrowth of the root epidermal cell. They are long and narrow to increase the surface area to volume ratio, increasing the rate of absorption of water and ions. The cell sap of the root hair cells is concentrated to maintain a steep diffusion gradient and allow water to enter the cell through osmosis easily.
Root hair cells are living, so they can provide the energy needed for the active transport of mineral ions

2. Outline the pathway taken by water through the root, stem and leaf, limited to: root hair cells, root cortex cells, xylem and mesophyll cells

This sap in the root hair cell is a concentrated solution of various salts, hence it has a low water potential. Water enters the root hair cell through osmosis through the cell membrane of the root hair cell, which is partially permeable. The entry of the water dilutes the cell sap, hence, water moves into the next since it has a lower water potential. Hence, water passes from root hair cells to the inner root cortex cells until the water enters the xylem vessels and is carried up to the mesophyll cells in the leaves.

3. Investigate, using a suitable stain, the pathway of water in a cut stem

The pathway can be investigated by placing a plant(like celery)into a beaker of water that has had a stain added to it(food colouring will work well).
After a few hours, you can see the leaves of the celery turning the same colour as the dyed water, proving that water is being taken up by the celery.
If a cross-section of the celery is cut, only certain areas of the stalk are stained with the dye, showing that the water is being carried in specific vessels through the stem – these are the xylem vessels

7.2 Transpiration and translocation

1. Describe transpiration as the loss of water vapour from leaves

Importance of transpiration
The suction force due to transpiration is the main factor in lifting water and dissolved mineral salts from the roots to the leaves. In this way, the leaves are supplied with sufficient water and salts for photosynthesis
As water evaporates from the leaves, it removes latent heat, allowing the plant to cool down and preventing it from being scorched by the hot sun
Providing water to keep cells turgid to support the structure of the plant

2. Understand that water evaporates from the surfaces of the mesophyll cells into air spaces and then diffuses out of the leaves through the stomata as water vapour

The mesophyll of the leaf has numerous intercellular spaces among the mesophyll cells
Water continuously moves out of the mesophyll cells to form a thin film of moisture over their surfaces
From the wet cell walls, water evaporates from the intercellular spaces and diffuses out of the leaf through the stomata
As water evaporates from the mesophyll cells, the cell sap becomes more concentrated
The cells, therefore, draw water from the nearest vein
The veins, in turn, remove water from the xylem vessels
The evaporation from the leaves results in a suction force[transpiration pull], and water is pulled through the xylem vessels and up the stem from the roots

3. Explain:

(a) The effects of wind speed, and the variation of temperature, humidity and light intensity on transpiration rate

Temperature: The Rate of transpiration increases
If temperature increases, the water molecules will have more kinetic energy, causing them to move faster, which means they will evaporate more easily
Humidity: The Rate of transpiration decreases
In a humid environment, air is almost saturated with water vapour
This means there is hardly any concentration gradient between the airspaces inside the leaf and the air outside the leaf, and the rate of evaporation is slow
Wind Speed: The Rate of transpiration increases
In still air, the region around a transpiring leaf will become saturated with water vapour so that no more can escape from the leaf. In these conditions, transpiration would slow down. In moving air, the water vapour will be swept away from the leaf as fast as it diffuses out. This will speed up transpiration.
Light intensity: The Rate of transpiration increases.
Light intensity affects the size of the stomata of the leaf. In sunlight [increasing light intensity], the stomata remain open; hence, the rate of transpiration increases. In darkness, stomata close hence, the rate of transpiration decreases

(b) How wilting occurs

Turgor pressure in the mesophyll cells in the leaf helps to keep the leaf firm and widely spread out to absorb light for photosynthesis. In strong sunlight, water loss from the cell vacuoles results in the cells losing their turgor and becoming flaccid. A leaf with flaccid cells will be limp, and the stem will droop. A plant that loses water to this extent is said to be wilting

4. Investigate the effects of wind speed, light intensity and temperature variation on transpiration rate

Cut a shoot underwater to prevent entering the xylem and place it in a tube.
Set up the apparatus as shown in the diagram below and make sure it is airtight, using vaseline to seal any gaps
Dry the leaves of the shoot (wet leaves will affect the results)
Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
Set up the environmental factor you are investigating
Allow the plant to adapt to the new environment for 5 minutes.
Record the starting location of the air bubble. Leave for a set period.
Record the end location of the air bubble.
Change the light intensity or wind speed or level of humidity, or temperature(only one – whichever factor is being investigated).
Reset the bubble by opening the tap below the reservoir. Repeat the experiment.
The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa

5. Explain the mechanism by which water moves upwards in the xylem in terms of the transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules

Water molecules are attracted to each other by cohesion, creating a continuous column of water up the plant
Water moves through the xylem vessels in a continuous transpiration stream from roots to leaves via the stem
Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves.
As water molecules are held together by cohesive forces (each molecule ' pulls’ on the one below it), so water is pulled up through the plant.
If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quickly

6. Describe translocation as the movement of sucrose and amino acids in the phloem from parts of plants that produce or release them (sources)to parts of plants that use or store them (sinks)

7. Identify the positions of tissues as seen in transverse sections of non-woody dicotyledonous roots and stems, limited to: xylem, phloem and cortex

Root: xylem and phloem lie on different radii
Stem: In a dicotyledonous stem, the xylem and phloem are grouped to form vascular bundles. The vascular bundles are arranged in a ring around a central region called the pith. In each vascular bundle, the xylem and phloem lie along the same radius. The phloem lies outside the xylem, with a tissue called the cambium between them. The cambium cells can divide and differentiate to form new xylem and phloem tissue, giving rise to a thickening of the stem
The stem is covered by a layer of cells called the epidermis. The Epidermal cells are protected by the acuticle, which prevents evaporation of water from the stem. The region between the vascular bundles and the epidermis is the cortex. Both the cortex and the pith serve to store up food substances,e.g. starch

8. State the functions of xylem as the transport of water and mineral ions, and support

Conducting water and dissolved mineral salts from the roots to the stems and the leaves
Providing mechanical support within the plant

9. Relate the structure of xylem vessels to their function, limited to:

(a) thick walls with lignin (details of lignification are not required)

They have thickened cell walls containing a material called lignin. Lignin is deposited in the form of rings or spirals. The whole wall of vessels is lignified, except in regions called pits, to prevent the collapse of the vessel

(b) No cell contents

The hollow space[lumen] is continuous with no cell contents [cytoplasm, nucleus, vacuole. A continuous lumen without any partition wall or protoplasm within to hinder the passage of water and mineral salts

(c) cells joined end-to-end with no cross walls to form along continuous tube

Xylem vessels also contain many long lignified supporting cells called fibres
Xylem vessels are long, thin hollow tubes stretching from the root to the leaf. A vessel is made up of a series of long cells joined end to end.

Credits: Notes compiled by Manahil Naeem of Karachi Grammar School

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