V Biology - Plant Transport

Root hair cell adaptations

• Elongated: large surface area

• Short diffusion distance because the wall is one cell thick

• Lots of mitochondria for respiration to release ATP for active transport

Absorption of water into plants by root hair cells

The water in the soil diffuses into the roots by osmosis from an area of high water concentration to a low water concentration across a partially permeable membrane and travels up the roots into the stem and travels up the stem into the leaves.

Absorption of mineral ions into plants by root hair cells

The mineral ions move from an area of low concentration to an area of high concentration across a partially permeable membrane using ATP from respiration into the roots via active transport and travels through the roots and stem into the leaves.

Role of the Xylem

Water and dissolved mineral ions travel in these vessels from root to shoots and leaves in one direction.

Xylem Vessels

Have thick cellulose cell walls, strengthened by lignin. Once xylem cells have formed the xylem, they die making long, thin, hollow vessels for water to move through. The thick walls also help support plants.

Role of the Phloem

Carries dissolved sucrose and amino acids from the leaves to the growing and storage parts of the plants. Transports sucrose up the plants from stores of starch e.g. in root tubes.

Phloem cells

These cells are alive - if they are damaged they cannot work properly. Phloem are made of companion cells and sieves. Cells are joined by small tubes in the cell wall at the end of each cell, forming a continuous system. The end walls are called sieve plates.

Sieve tubes

Nearly empty - allow sap (sucrose) to move easily

Companion cells

Have normal cell contents including lots of mitochondria

Transpiration

The loss of water by evaporation from the plants - they lose water when they open the stomata in the leaves for gas exchange.

Stomata

Small holes located on the underside of the leaves to allow for the exchange of gases. Water also evaporates through the stomata.

Guard cells

Each stoma is surrounded by two guard cells, which control the opening and closing of the stoma. The guard cells gain water and become more turgid. They curve out opening the stoma and allowing gases in and out.

Stage 1 of the Opening of the Stomata

Accumulate solutes in their vacuoles which lowers the water potential. Water moves in by osmosis. The guard cells swell up which changes their shape - opening the stomata.

Stage 2 of the Opening of the Stomata

The stomata are open for gas exchange. During this time water is lost as water vapour moves out of the leaf down the water potential gradient.

Stage 3 of the Opening of the Stomata

At night the guard cells lose water so becoming flaccid and close the stomata.

Closing at Night is a Useful Adaptation because:

The guard cells are the only cells in the lower epidermis to contain chloroplasts and so the opening and closing of  the stomata is caused by light intensity.

• There is no light so no need for photosynthesis

• No need to cool the plant

Stage 1 of the Transpiration Stream

Water leaves the cells of the mesophyll and evaporates into the air spaces. This water vapour diffuses out through the stomata down the water vapour potential gradient

Stage 2 of the Transpiration Stream

The loss of water from the mesophyll cells reduces the water potential of these cells so water moves by osmosis from the surrounding cells into these cells down the water potential gradient.

Stage 3 of the Transpiration Stream

Water from the xylem moves into the mesophyll cells down the water potential gradient.

Stage 4 of the Transpiration Stream

The loss of water from the xylem causes water to be pulled up the xylem in the stem and the roots in a continuous flow, called the ‘transpiration stream’

Transpiration stream functions

• Supplies water to the palisade mesophyll for photosynthesis

• Carries mineral ions dissolved in the water to cells in the plants

• Provides water to cells to keep cells turgid

• Allows evaporation from the leaf surface, which cools the leaf in a similar way to sweating cooling the skin

Effect of Light Intensity on the Rate of Transpiration

The rate of transpiration increases as light intensity increases because of the opening of the stomata for gas exchange for photosynthesis

Effect of Temperature on the Rate of Transpiration

Higher temperatures increase the rate of transpiration by increasing the rate of evaporation from the mesophyll cells

Effect of Wind Speed on the Rate of Transpiration

The rate of transpiration increases with faster air movements across the surface of the leaf as the moving air removes any water vapour which might have been near the stomata so increasing the water vapour potential gradient

Effect of Humidity on the Rate of Transpiration

The rate of transpiration is higher when the air is less humid as there is an increased water vapour potential gradient.