Bio Plant water transport

Why do simple unicellular organisms rely on diffusion?

They have a high SA:V allowing fast exchange of nutrients and gases directly across their cell membrane

Why do multicellular organisms require a transport system?

Larger size, smaller SA:V, diffusion is too slow to supply nutrients to cells further from the surface and remove waste products efficiently. Transport systems are essential for maintaining a high metabolic rate and ensuring survival

What and where to does the phloem tissue carry?

The phloem tissue carries dissolved sucrose, amino acids and mineral ions from the leaves to the growing and storage parts of the plant. They also transport sucrose up the plant from starch stores eg root tubers

Describe the structure of phloem tissues

Phloem tissues are alive, and are made of companion cells and sieve tubes. Sieve tubes are joined by small holes in the cell wall at the end of each cell, forming a continuous system. The end cell walls are called sieve plates.

Describe the contents of phloem tissue cells

Phloem tissues still have cell organelles and cytoplasm. Sieve tubes are nearly empty but have cytoplasm to allow sap (sucrose) to move easily. Companion cells have normal cell contents, including lots of mitochondria

What and where to do the xylem vessels carry?

The xylem carries water and dissolved mineral ions from the root to the shoots and leaves in one direction.

Describe the structure of xylem vessels

Xylem vessels have thick waterproof cellulose cell walls, strengthened by lignin. Once xylem cells have been lignified, they die and the cell walls of the ends of the cells get broken down. The xylem vessels formed are long, thin, hollow vessels for water to move through with little resistance.

Explain how root hair cells are adapted for efficient uptake of water and mineral ions through osmosis and active transport

Elongated projection, larger SA:V, thin wall, shorter diffusion distances, concentrated cell sap, steep concentration gradient, leads to more collisions

What is the transpiration stream?

The transpiration stream is the continuous, upwards movement of water and dissolved mineral ions from the roots through the xylem to the leaves, by the loss of water vapour from leaves (transpiration)

By which process does water move into the roots hairs?

Water moves into the root hairs by osmosis

Describe how water moves from the root hair cells to the xylem

Water moves by osmosis from cell to cell across the root to the xylem. Each cell has a higher salt (and other solutes) concentration to encourage osmosis.

Why is there lower pressure at the top of the xylem which pulls water up?

The lower pressure is due to transpiration, the evaporation of water from the leaves into the atmosphere

How does water get from the xylem to the palisade mesophyll?

Water enters the leaf cells from the xylem by osmosis

How does water leave the cell?

Water evaporates from the spongey mesophyll cells into the air spaces which are humid, allowing it to dissolve. Water vapour as a gas diffuses out of the leaf through the stomata (transpiration)

Explain why increased light intensity increases the rate of transpiration?

Increased light intensity causes more stomata to open which results in increased loss of water vapour by evaporation until all stomata are open

Explain why increased air movement increases the rate of transpiration?

Increased air movement removes more water vapour from the air immediately surrounding the leaf leading to a steeper water potential gradient between inside and outside leaf

Explain why increased temperature increases the rate of transpiration?

Increased kinetic energy and movement of water molecules increasing the rate of evaporation from mesophyll cells and diffusion out of the stomata

Explain why decreased humidity increases the rate of transpiration

Makes water potential gradient steeper so water vapour diffuses from inside leaf to outside leaf at a higher rate

What is water potential?

Measure of potential energy of water per unit volume, suggests its tendency to move from one area to another through osmosis

Why is measuring the distance an air bubble moves in a potometer not entirely accurate for measuring rate of transpiration?

The air bubble represents water but in a real plant the water is used for photosynthesis in cells and some water is also used to keep the cells turgid. Also some water is produced internally by the plant through respiration, so the water uptake measured is slightly lower than the actual water loss

Is a potometer likely to over or under measure?

A potometer is generally considered to overmeasure the actual rate of transpiration as it measures water uptake and not all water absorbed by the plant is transpired

Why does the stem need to be cut under the water?

Prevents blockage of xylem by air bubbles

Why does the stem need to be cut at an angle?

larger SA:V for water uptake

Why does the potometer need to be set up under the water?

Prevents blockage of xylem by air bubbles

Why does the potometer need to be watertight?

To ensure that water isn’t lost in the experiment and entry of air bubbles

Why do the leaves need to be dry?

Wet leaves leads to a lower rate of evaporation and therefore transpiration

Why do we wait 10 mins before taking measurements?

To ensure that the plant has started to transpire and has acclimatised to the conditions