Unit 8 Plant Transport

Describe transpiration.

A loss of water vapour from plant leaves by evaporation of water at the surface of the mesophyll cells followed by diffusion of water vapour through the stomata.

Describe what happens to chemicals during the process of translocation.

Chemicals like pesticides are absorbed by leaves and translocated to regions with infection/lots of pests and the pests are then killed.

Which properties of water does mass flow of water in thoughout the plant rely on?

Cohesion; water molecules stick to each other. Adhesion; water molecules stick to the inside of xylem vessels.

Describe the adaptations of hydrophytes.

Common adaptations to increase the rate of gaseous exchange: Leaf Shape: The submerged leaves are often highly dissected or divided to create a very large surface area for absorption and photosynthesis. It also minimises water resistance and hence potential damage to the leaves. In many cases, the submerged leaves are totally different to floating or emergent leaves on the same plant. The emergent leaves are usually much less divided, if not entire and have a more similar internal structure to those of land plants. Lack of protective layer: The epidermal layer shows very little, if any, sign of cuticle formation, as water loss is not a problem. All the surface cells appear to be able to absorb water, nutrients and dissolved gases directly from the surrounding water. As a result, the xylem tubes is often greatly reduced, if not absent. *There are also no stomata on the underside of the leaves. Location of Stomata: The green pigment-containing chloroplasts important for photosynthesis are restricted to the upper surface of the leaves which are the only surface to be well lit. Stomata are also found only on the upper surface of the leaf. This upper surface often has a thick waxy cuticle to repel water and help to keep the stomata open and clear. Presence of Aerenchyma: They allow diffusion of oxygen from the aerial portions of the plant into the roots. Thus the roots don't have to depend on getting oxygen from the soil. "Regular" plants may have porosity (% air space in roots and stems) of 2-7% of their volume, while a wetland plant may be up to 60% pore space by volume. For a typical hydrophytic plant, air moves into the internal gas spaces of young leaves on the water surface and is forced down through the aerenchyma of the stem to the roots by the slight pressure caused by the heating of the leaves. The older leaves lose their capacity to support pressure gradients so gas from the roots returns out through the old leaves. Stem and Lenticel Hypertrophy: The presence of little or no mechanical strengthening tissue in stems and leaf petioles. If these plants are removed from the water, they hang limply. They are normally supported by water all around them and so have no need of mechanical strengthening. Mechanical support would be disadvantage as it would limit flexibility in the event of changes in water level or water movements. Roots: Roots are often also reduced and their main function is anchorage. The root hairs which function in absorption are often absent and roots themselves may be entirely gone. Rapid Early Shoot Growth: Under flooded conditions, several herbaceous and woody species exhibit this which gets the shoot above the surface of the water quickly to facilitate gas exchange.

What controls the opening and closing of stomata?

Guard cells.

Describe the function of guard cells.

In light, guard cells take up water by osmosis and become turgid. Because their inner walls are rigid they are pulled apart, opening the pore. In darkness water is lost and the inner walls move together closing the pore. Because of this, the transpiration rate is increased by an increase in light intensity.

What affects the rate of transpiration?

Increased Temperature: This increases the kinetic (movement) energy of water molecules which leads to them diffusing faster. Increased air movement (wind): Removes water molecules as they pass out of the leaf. This leads to maintaining a steep concentration gradient for diffusion. Decreased humidity; this reduces the concentration of water molecules outside the leaf which leads to steeper concentration gradient for diffusion. Increased light intensity; stomata open to allow gas exchange for photosynthesis. Water vapour can then diffuse out of the leaf. Note: Most of the factors that result in a change in transpiration rate are linked to diffusion. When writing explanation, try to include references to the concentration gradient caused by a change in the factor.

Describe what mass flow is.

It is the movement of fluid through a tube. All fluid travels in the same direction.

Why is wilting a good thing?

Plant bends down therefore light intensity is reduced. Stomata closes which means less water vapour is lost. Lower light intensity can lead to lower temperature which means less KE for water vapour which means that water won't diffuse out as fast.

What are xerophytes?

Plants that are able to exist in conditions where water is scarce. They survive in very dry regions, such as a desert or an ice- or snow-covered region in the Alps or the Arctic. Xerophytes can live in these environments because they contain specialized features that help them prevent water loss.

What are hydrophytes?

Plants that grow submerged or partially submerged in water.

Describe the pathway taken by water absorbed by roots.

Root hair cell to root cortex cell to xylem to mesophyl cell.

Describe stomata.

Stomata are found on the underside of leaves, light stimulates them to open, they are open during the day to let CO2 diffuse in for photosynthesis and close during the night to reduce loss of water.

Describe the adaptations possessed by xerotypes.

Stomata are the microscopic openings in leaves that permit gas exchange. Think of them as pores for plants. And just like human pores release sweat, plant stomata release water in the form of water vapor. Certain xerophytes have a waxy covering over their stomata, thus curbing water loss. Others contain very few stomata, or stomata that only open at night when it's cooler. Each of these adaptations limits water loss and allows the plant to survive in dry environments. An example of this type of plant is Adam's Needle (Yucca filamentosa). This plant can survive in harsh dry desert environments because of its waxy covering. The second type of adaptation is focused on storing water instead of just limiting water loss. To do this plants have developed succulent leaves, plant stems, or tubers that can store water when it cannot be obtained directly from the environment.

What re mesophytes?

Terrestial plants that usually have enough water and do not need extreme adaptation.

What effect does the large surface area of root hair cells have on osmosis and active transport?

The large surface area of root hair cells increases the rate of absorption of water by osmosis and ions by active transport.

Give a description of a Phloem tissue.

This is a long tube that runs alongside the xylem tissue. They are made of long narrow tubes with perforated sieve plates along the thin length. Unlike the xylem, the phloem tissue is made of columns of living cells, which contains a cytoplasm but no nucleus, and its activities are controlled by a companion cell next to it which has a nucleus, but companion cells have no function in translocation.

What is translocation?

This is the movement of sucrose and amino acids in phloem from regions of production to regions of storage or regions for use in respiration or growth.

What is the difference between translocation and transpiration?

Transpiration is the loss of water through the leaves of the plant. As the temperature increases, the rate of transpiration increases as the plant tries to cool itself and meet the demands of water use for photosynthesis and metabolism. The movement of water out the stomata causes transpiration pull. Transpiration pull is internal pressure, in this case suction, that is created as water moves through the plant and out of the leaves during transpiration. Translocation is the movement of the liquids within the plant system. As the plant absorbs water and dissolved nutrients in the roots, the vascular system carries these substances to where the plant needs them. Translocation also refers to the movement of dissolved sugars in the phloem. The relationship between translocation and transpiration is recognized as a direct relationship. For example, as the rate of transpiration increases the rate of translocation of fluids will also increase.

What is the function of Phloem?

Transports food (sugar and other organic nutrients, e.g. amino acids) from leaves to the rest of the plant. This is called translocation.

What is the function of xylem?

Transports water and solutes from roots to leaves. It also to helps supporting the stem and strengthen it.

Explain what a transpiration stream is.

Water molecules are attracted to each other (cohesion) à water vapour evaporating from a leaf crates a kind of suction, pressure of water at the top of the vessels is lower than that of the bottom à water move up the stem in the xylem, more water is drawn into the leaf from the xylem. This creates a traspiration stream, pulling water up from the root.

Explain why water vapour can be lost through transpiration.

Water vapour can be lost through transpiration because of the amount of stomata on the leaves when the diffusion of co2 takes place. Water in the leaf cells forms a thin layer on their surface. The water evaporates into the air spaces in the spongy mesophyll. This creates a high concentration of water molecules. They diffuse out of the leaf into the surrounding air, through the stomata, by diffusion.

In the transport systems of plants, what are the continuous tubes called.

Xylem and phloem.

Give a description of a xylem vessel.

Xylem vessels consist of dead cells. They have a thick, strengthened cellulose cell wall with a hollow lumen. The end walls of the cells have disappeared, so a long, open tube is formed. The walls of the xylem vessel contains holes called pits which water enters through.

How do you find out the pathway of water?

You supply plants with water that has been dyed. This coloured water is then pulled up from the leaves and you can see the pathway after a few hours.

Why does wilting occur?

Young plant stems and leaves rely on their cells being turgid to keep them rigid. If the amount of water lost from the leaves of a plant is > than the amount taken into the roots à the plant will have a water shortage à cells become flaccid (soft) and will no longer press against each other à Stems and leaves lose their rigidity, and wilt. An increase of temperature and a decrease in humidity causes more water to diffuse out of the plant and vice versa.

How do you measure the rate of water uptake?

use a potometer. Fill potometer with water but leave an air bubble in it's capllary tube. Put one end of the potometer into the water, put a plant into the other end, the pulling water also pulls the air bubble, the distance that the air bubble travels per period of time is the rate of water uptake.