Transport in Plants

Question 1

epidermis

Answer 1

A continuous layer on the outside of the plant, its one cell thick and provides protection. In stems and leaves it’s covered with waxy cuticle which is waterproof so protects it from drying out and infection. In leaves it has stomata which allows exchange of gas. In roots it has extensions called root hairs which increase surface area.

Question 2

Parenchyma

Answer 2

Made up of thin walled cells, are very metabolically active, has many functions. Can be used for storage of food like starch. When turgid they help support the cell. Air spaces between the cells allow gas exchange. They form the cortex in roots and stems. The pith is made of similar cells.

Question 3

Collenchyma

Answer 3

A modified from of parenchyma with extra cellulose deposited in the corners of the cell. Provides extra strength. The midriff of leaves contain collenchyma

Question 4

Endodermis

Answer 4

One cell thick, surrounds the vascular tissue in stems and roots

Question 5

Mesophyll

Answer 5

Made up of specialised parenchyma cells found between the upper and lower epidermis in leaves. Contains chloroplasts. Spongey mesophyll has lots of air spaces. Paliside mesophyll are near the upper surface of the leaf where they receive more sunlight. More chloroplasts in paliside then mesophyll.

Question 6

Pericycle

Answer 6

One to several cells thick, just inside the endodermis and next to the vascular tissue. In roots it is one cell thick and new roots grow from it. In stems it is formed from sclerenchyma which is a dead lignified cell that provides extra strength.

Question 7

Vascular tissue

Answer 7

In roots xylem is in an X shape with the phloem in the gaps. In stems xylem and phloem are found in vascular bundles. The outside of the bundles have caps made of sclerenhyma fibres which provides extra support. Sclerenchyma are long, dead empty cells with lignified walls.

Question 8

Transpiration definition and gas exchange

Answer 8

The energy of the sun causes water to evaporate from the leaves. This is a side effect of gas exchange as the stomata are open to allow gas’s to be exchanged with the environment for photosynthesis and water vapour diffuses out of the stomata in transpiration.

Question 9

Transpiration

Answer 9

Water evaporates from the walls of the mesophyll into the air spaces. If there is a water potential gradient between the air inside the leaf (higher water potential) and the air outside the leaf (lower water potential) then water vapour will diffuse out of the leaf through the stomata.

Question 10

Water potential water transport overview

Answer 10

The energy from the sun causes water to evaporate from the leaves, this reduces the water potential in the leaves and sets up a water potential gradient throughout the plant. Water moves across the root into the xylem vessel, then upwards through the root to the stem and into the leaves.

Question 11

Water movement through a leaf

Answer 11

Water diffuses from an air space through an open stomata, this is transpiration. It is carried away from the leaf by air movement. This reduces the water potential outside the leaf. Water evaporates from the mesophyll cell wall into the air space. Water moves through the mesophyll cell wall. Water leaves a xylem vessel through a pit. It may enter the cytoplasm or cell wall of a mesophyll cell. Water moves up the xylem vessel to replace water lost from the leaf.

Question 12

How does humidity affect transpiration

Answer 12

If the water potential gradient between the air spaces in the leaf and the air outside becomes steeper, the rate of transpiration will increase, in low humidity the gradient is steep so there is more transpiration.

Question 13

How does wind speed affect transpiration

Answer 13

It blows water away from the leaf lowering the water potential

Question 14

How does heat affect transpiration

Answer 14

Gives water on the mesophyll walls the energy it needs to evaporate, which increases the water potential in the leaf.

Question 15

How does light intensity affect transpiration

Answer 15

Causes stomata to open, as most transpiration takes place through the stomata. Stomata are open in the day time to allow CO2 to diffuse in for photosynthesis. Closing at night stops unnecessary water loss.

Question 16

Very dry conditions

Answer 16

A plant may have to close all or some of its stomata to prevent its leaves from dying out. It will close its stomata when more water is being lost from the leaves then taken up.

Question 17

xerophyte

Answer 17

Xerophytes are plants adapted to living in dry conditions, they try to minimise water loss. They have smaller leaves which reduce the surface area for water loss. Have a thick waxy cuticle. May have sunken stomata in a pit, which prevents water vapour from being blown away so gets rid of the water potential gradient. Xerophytes respond to low water availability by closing the stomata to prevent water loss. They contain hairs and pits which serve as a means of trapping moist air, thus reducing the water vapour potential. Xerophytes also roll the leaves to reduce the exposure of lower epidermis to the atmosphere, thus trapping air.

Question 18

Movement of water into the root

Answer 18

Water enters through root hair cells and moves into the xylem tissue located in the centre of the root. This movement occurs as a result of a water potential gradient, as the water potential is higher inside the soil than inside the root hair cells, due to the dissolved substances in the cell sap. Therefore, the purpose of root hair cells is to provide a large surface area for the movement of water to occur.

Question 19

Movement of minerals into the root

Answer 19

Minerals are also absorbed through the root hair cells by active transport, as they need to be pumped against the concentration gradient.

Question 20

Movement of water from root to xylem

Answer 20

Water taken up by the root hairs cross the cortex of the root and enter the xylem in the centre. The water potential in the xylem is lower then in the root hairs. It then moves through the cortex by the synplastic or apoplastic pathway. It moves through the cells in the root till its gets to the pit in the xylem vessel.

Question 21

Symplastic pathway

Answer 21

Where water enters the cytoplasm through the plasma membrane and passes from one cell to the next through plasmodesmata.

Question 22

Apoplastic pathway

Answer 22

Where the water moves through the cell walls, water moves from cell wall to cell wall through the intercellular spaces. In this pathway, water doesn’t pass through any plasma membranes therefore it can carry dissolved mineral ions and salts.

Question 23

Casparian strip

Answer 23

When the water reaches a part of the root called the endodermis, it encounters a layer of suberin which is known as the Casparian strip, which cannot be penetrated by water. Therefore, in order for the water to cross the endodermis, the water that has been moving through the cell walls must now enter the symplastic pathway.

Question 24

From root to stem

Answer 24

The water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient. The push of water upwards is aided by the root pressure which is where the endodermis moves minerals into the xylem by active transport, which drives water into the xylem by osmosis, thus pushing it upwards.

Question 25

Water moving up the stem

Answer 25

The flow of water is also maintained with the help of surface tension of water and the attractive forces between water molecules known as cohesion, this is due to hydrogen bonding between the water molecules. The action of these two forces in combination is known as the tension-cohesion theory, there is capillary action where the water molecule adhere to the walls of xylem, thus pulling water up (adhesion). This forms the transpiration stream.

Question 26

Translocation

Answer 26

Translocation is an energy requiring process which serves as a means of transporting assimilates such as sucrose in the phloem between sources which release sucrose such as leaves and sinks e.g. roots which remove sucrose from the phloem.

Question 27

How is sucrose loaded into phloem sieve tubes

Answer 27

Sucrose enters the phloem in a process known as active loading where companion cells use ATP to transport hydrogen ions into the surrounding tissue, thus creating a diffusion gradient, which causes the H+ ions to diffuse back into the companion cells. It is a form of facilitated diffusion involving cotransporter proteins which allows the returning H+ ions to bring sucrose molecules into the companion cells, thus causing the concentration of sucrose in the companion cells to increase. As a result of that, the sucrose diffuses out of the companion cells down the concentration gradient into the sieve tube elements through links known as plasmodesmata.

Question 28

What is the mass flow in phloem

Answer 28

As sucrose enters the sieve tube elements, the water potential inside the tube is reduced, therefore causing water to enter via osmosis, as a result increasing the hydrostatic pressure of the sieve tube. Therefore, water moves down the sieve tube from an area of higher pressure to an area of lower pressure. Eventually, in the sink sucrose is removed from the sieve tube elements by diffusion or active transport into the surrounding cells, thus the water potential in the sieve tube increases. This in turn means that water leaves the sieve tube by osmosis, as a result reducing the hydrostatic pressure in the phloem at the sink. Movement from source to sink is a gradient in hydrostatic pressure.

Question 29

Summary of phloem transport

Answer 29

Therefore, in summary, the mass flow of water from the source to the sink down the hydrostatic pressure gradient is a means of supplying assimilates such as sucrose to where they are needed.

Question 30

Structure of phloem

Answer 30

Made of sieve tube elements and companion cells, are living cells. A sieve tube is made of elongated sieve elements joined end to end to form a continuous tube. There is no nucleus or ribosome and the cytoplasm is very thin and forms a thin layer lining the wall. Where the end walls of two sieve elements meet is a sieve plate, it has large pores to allow liquid to flow. The sieve plate stops the liquid pouring out when it is cut as the phloem sap is at high pressure. The sieve plate is sealed with a carbohydrate called callose.

Question 31

Companion cells

Answer 31

Has the same structure as normal plant cells but more ribosomes and mitochondria as they are metabolically very active. Numerous plasmodesmata connect the companion cells and sieve tube elements.

Question 32

Xylem vessels

Answer 32

Are made of vessel elements arranged end to end. They are long cylinders made of dead tissue with open ends, therefore they can form a continuous column. Xylem vessels also contain pits which enable water to move sideways between the vessels. They are thickened with a tough substance called lignin, which is deposited in spiral patterns to enable the plant to remain flexible

Question 33

Potometer

Answer 33

Used to measure the rate of water uptake and a high proportion of this water is used in transpiration so can be used to measure the rate of transpiration. Must be water tight and air tight so that there is no air bubbles which break the continuous water stream. You insert the plant when the apparatus is under water, and cut the stem under water. Petroleum jelly around the joints keep the apparatus air tight. Measuring the movement of the meniscus can be used to determine the rate of transpiration.

Question 34

Epidermal Peel

Answer 34

You strip the epidermis from the underlying tissue of leaves by grasping it with a pair of tweezers and striping it away. It can be placed on a slide and viewed with a microscope. You can compare whether stomata are open or closed under different conditions or estimate the number of stomata per unit area.

Question 35

How to find the number of stomata per unit area

Answer 35

You count the number of stomata in the field of view, you then find the area of the field of view by using a calibrated eyepiece graticule and the formula pie r squared

Question 36

Epidermal impression

Answer 36

The surface of the leaf is covered with a layer of clear nail varnish which is allowed to dry, you should avoid hairy epidermis. It can be peeled off using fine forceps and then mounted on a slide, you can then see whether the stomata are open or closed.

Question 37

Measuring surface area off leaf using a grid

Answer 37

It is used to measure the rate of transpiration per unit area which allows valid comparison to be made. To measure the area off a leaf you can use grid paper and draw the outline off the leaf on the grid. You count whole squares and count part of the square where there is a bit off the leaf.