Sec 2 Biology (Transport in plants)

What are the characteristics of sieve tube cells? What are companion cells and their functions?

A mature sieve tube cell only has a thin layer of cytoplasm and each one has lost their central vacuole, nucleus and most organelles. Thus, they need companion cells to survive. A companion cell is a narrow, thin-walled cell with many mitochondria, cytoplasm and a nucleus. They carry out metabolic processes needed to keep the sieve tube cell alive and provide energy, helping the sieve tube cell transport sucrose and amino acids.

What is a phloem and its function? What are its adaptations?

A phloem consists of columns of elongated, thin walled living cells called sieve tube cells. It conducts sucrose and amino acids from the leaves to other parts of the plant. Companion cells have many mitochondria to provide the energy from respiration needed for cells to load sugars into the sieve tubes by active transport. A phloem has sieve plates to allow the rapid flow of sucrose and amino acids through the sieve tubes.

What are the advantages and disadvantages of wilting?

Advantages: When the leaf folds up when the plant is wilting, the surface area exposed to sunlight decreases. Hence, the guard cells become flaccid and the stomata closes, reducing the rate of transpiration. Disadvantages: The rate of photosynthesis is reduced as water becomes a limiting factor. Since the stomata is closed during wilting, the amount of carbon dioxide absorbed by the leaves is reduced. Therefore, the rate of photosynthesis is reduced as water and carbon dioxide are limited and there is a reduced surface area of the leaf exposed to sunlight.

What are the adaptations of aquatic, tropical and xerophytic plants?

Aquatic Plants: Since the lower surface of the leaves are in contact with water, most gases would dissolve poorly. Hence, there is more stomata on the upper surface of the leaf for gaseous exchanges to take place. They do not have thick lignified walls as the water column supports the plant. Tropical Plants: Since the lower surface of the leaves is exposed to less sunlight, there will be more stomata on the lower surface as it will be cooler and there will be a reduced rate of transpiration so there is less water loss. Xerophytic Plants: Since xerophytic plants are adapted to a low water availability environment, they have reduced leaves which reduces surface area exposed to air, hence reducing the loss of water caused by evaporation. They have sunken stomata that is shielded from the wind, reducing the rate of transpiration. They have a layer of moist air trapped in the 'pit' which decreases the concentration gradient for water vapour to diffuse out of the leaf. They have hair on the surface of their leaves to trap moisture which reduces water vapour concentration gradient. Lastly, they have waxy leaves to reduce water loss via evaporation. ​

What is the function of cambium cells?

Cambium cells divide and differentiate to form new xylem and phloem tissues.

What are the factors affecting the rate of transpiration?

Humidity: The humidity affects the water vapour concentration gradient between the leaf and the atmosphere. Thus affecting the rate of diffusion of water vapour, and hence the rate of transpiration. Wind: As wind blows away the water vapour accumulating outside the stomata, it increases the water vapour concentration between the leaf and the atmosphere. Therefore, the stronger the wind, the faster the rate of diffusion of water vapour into the atmosphere, the faster the rate of transpiration. Temperature: When the temperature is higher, there is a higher rate of evaporation and diffusion. Hence there is a faster rate of diffusion of water vapour, increasing the rate of transpiration. Light: Since light increases the size of the stomata, there is a faster rate of diffusion of water vapour into the atmosphere, which increases the rate of transpiration

What is translocation?

It is the movement of sucrose and amino acids through the phloem.

Why do plants need a transport system? What are the functions of the xylem and phloem?

Plants need a transport system to supply water absorbed by the roots to the leaves and transport sugars produced by photosynthesis in the leaves to all parts of the plant. The xylem transport water and mineral salts to the leaves. The phloem transport sucrose produced by the leaves during photosynthesis to all parts of the plant.

What is the ringing experiment?

Ringing is the removal of the strip of bark around the entire circumference of a branch or trunk. The removal of the phloem prevents the translocation of sugars to the region below the ring. The accumulation of sugars in the region just above the ring results in swelling.

What is a xylem and its function? What are its adaptations?

The xylem consists of xylem vessels that are long hollow tubes made up of many dead cells. It stretches from the roots to the leaves and are positioned differently in different parts of the plant. The xylem conducts water and dissolved mineral salts from the roots to the stems and leaves. It also provides mechanical support for the plant. Xylem vessels have empty lumens without protoplasm or cross walls to reduce water resistance for water flowing through. They also have walls thickened with lignin to strengthen the wall and prevent the vessels from collapsing.

What is transpiration? What is a transpiration pull?

Transpiration is the loss of water from the aerial parts of the plant, especially through the stomata of the leaves. The transpiration pull is the suction force caused by transpiration, which is the main factor that causes movement of water up the xylem. Since water molecules attract each other by cohesive forces, they 'pull' up other water molecules up the xylem when water vapour is lost through transpiration.

Why is transpiration essential to photosynthesis? What is the importance of transpiration?

Transpiration pull draws and mineral salts from the roots to the stem and to the leaves for photosynthesis. Transpiration allows for the evaporation of water from the cells in the leaves which cools the plant as it is in a hot environment. The water pulled up by the transpiration pull transported to the leaves keep the cells turgid, allowing the leaves to be spread out wide to trap sunlight for photosynthesis.

How do root hairs absorb mineral salts? What are the adaptations of root hair cells?

When the concentration of mineral salts in the soil is higher than in the root hair cell, root hairs absorb the mineral salts through diffusion. When the concentration of mineral salts in the soil is lower than in the root hair cell, root hairs absorb mineral salts against the concentration gradient via active transport. They have an elongated shape to increase SA: V ratio which increases the rate of absorption of water and mineral salts. They have a cell sap which contains a high concentation of sugars, amino acids and salts. Thus, it will have a lower water potential than the soil, resulting in water entering the roots via osmosis. They have many mitochondria to provide more energy released from respiration for active transport of ions into the cell. ​

How does wilting occur?

When the rate of transpiration exceeds the rate of absorption of water by roots in strong sunlight, cells lose their tugor and become flaccid, hence the plant wilts.