Transport in Plants

Question 1

Why do plants need transport systems?

Answer 1

• Plants need substances like water, minerals, and sugars to live as well as get rid of waste substances
• Plants are multicellular so they have a small surface area to volume ratio. They are also relatively big with a high metabolic rate.
• Exchanging substances by direct diffusion would be too slow to meet their metabolic needs. Therefore plants need transport systems to move substances to and from individual cells quickly.

Question 2

What is xylem tissue?

Answer 2

Xylem tissue transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves. The Xylem is also used in support.

Question 3

What is Phloem tissue?

Answer 3

Phloem tissue mainly transports sugars both up and down the plant.

Question 4

What do the Xylem and Phloem look like in roots, leaves, and stem cross sections?

Answer 4

In roots, the Xylem is an X with the Phloem around to form a circle
In leaves, the Xylem is on top of the Phloem in little circles
In stems the Xylem is in the center with the phloem on the outside in little circles around the stem

Question 5

What is the structure of the Xylem?

Answer 5

• Xylem vessels are very long, tube-like structures formed from cells (vessel elements) joined end to end.
• There are no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily
• The cells are dead, so they contain no cytoplasm
• Their walls are thickened with a woody substance called lignin, which helps to support the xylem vessels and stops them from collapsing inwards. Lignin can be deposited in Xylem walls in different ways, e.g. in a spiral or as distinct rings.
• The amount of lignin increases as the cell gets older
• water and ions move into and out of the vessels through small pits in the walls where there is no lignin.

Question 6

What is the structure of the Phloem?

Answer 6

• Phloem tissue transports solutes, mainly sugars like sucrose, round plants.
• Phloem is formed from cells arranged in tubes and is only used as a transport vessel
• Phloem tissue contains phloem fibers, phloem parenchyma, sieve tube elements and companion cells.

Question 7

What are sieve tube elements and how do they work?

Answer 7

• They are living cells that form the tube for transporting solutes through the plant.
• They are joined end to end to form sieve tubes
• The sieve parts are the end walls, which have lots of holes in them to allow solutes to pass through
• Unusually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm, and a few organelles.
• The cytoplasm of adjacent cells is connected through the holes in the sieve plates

Question 8

What are companion cells and how do they work?

Answer 8

• The lack of a nucleus and other organelles in sieve tube elements in sieve tube elements means that they cannot survive on their own. So there is a companion cell for every sieve tube element
• Companion cells carry out the living function for both themselves and their sieve cells. For example, they provide energy for the active transport of solutes.

Question 9

What is the process of dissecting plant stems?

Answer 9

• Use a scalpel to cut a cross-section of the stem. Cut the sections as thinly as possible- thin sections are better for viewing under a microscope.
• Use tweezers to gently place the cut sections in water until you come to use them. This prevents them from drying out.
• Transfer each section to a dish containing a stain, e.g. toluidine blue O, and leave for one minute. TBO stains the lignin in the walls of the Xylem vessels blue-green. This will let you see the position of the Xylem vessels and examine their structure.
• Rinse off the sections in water and mount each one onto a slide.

Question 10

How does water enter the plant?

Answer 10

Water has to get from the soil, through the root, and into the xylem to be transported around the plant. Water enters the root hair cells and then passes through the root cortex, including the endodermis, to reach the Xylem. Water is from into the roots via osmosis.

Question 11

How do the root hair cells create a water potential gradient?

Answer 11

The soil around the roots generally has a high water potential and leaves have a lower water potential (as water constantly evaporates from them). This creates a water potential gradient that keeps water moving through the plant in the right direction.

Question 12

What is the symplast pathway?

Answer 12

The symplast pathway goes through the living parts of cells- the cytoplasm. The cytoplasm of neighboring cells connects through plasmodesmata. Water moves through the symplast pathway via osmosis.

Question 13

What is the apoplast pathway?

Answer 13

The apoplast pathway goes through the non-living parts of the cells- the cell walls. The walls are very absorbent and water can simply diffuse through them., as well as pass through the spaces between them. The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. This is an example of mass flow. (least resistant pathway)

Question 14

How does water going through the apoplast pathway end up in the Xylem?

Answer 14

When the water in the apoplast pathway gets to the epidermis cells in the root, its path is blocked by a waxy strip in the cell walls called the Casparian strip. Now the water has to take the symplast pathway. This is useful because it means the water has to go through a cell membrane. Cell membranes are partially permeable and can control whether or not substances in the water get through. Once past this barrier, the water moves into the xylem.

Question 15

How does water leave the xylem at the leaves?

Answer 15

• At the leaves, water leaves the xylem and moves into the cells mainly by the apoplast pathway.
• Water evaporates from the cell walls into the spaces between cells in the leaf
• When the stomata open, the water diffuses out of the leaf into the surrounding air.
• The loss of water from a plant’s surface is called transpiration.

Question 16

How do cohesion and tension help water move up plants?

Answer 16

• Water evaporates from the leaves at the top of the xylem
• This creates a tension which pulls more water into the lead
• Water molecules are cohesive and stick together. So when some are pulled into the leaf others follow. This means the whole column of water in the Xylem, from the leaves down to the roots, moves upwards.
• Water enters the stem through the root cortex cells.

Question 17

How does adhesion help water move up plants?

Answer 17

• As well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels.
• This helps water to rise through the xylem vessels

Question 18

How is transpiration a consequence of gas exchange?

Answer 18

• A plant needs to open its stomata to let in carbon dioxide so that it can produce glucose
• but this also lets water out as there is a higher conc of water inside than outside of the leaf so water moves out down the water potential gradient
• therefore transpiration can be argued as a side effect of gas exchange needed for photosynthesis.

Question 19

How does light affect transpiration rate?

Answer 19

The lighter it is the faster the transpiration rate. This is because the stomata open when it gets light, so CO2 can diffuse into the leaf for photosynthesis.

Question 20

How does temperature affect transpiration rate?

Answer 20

The higher the temperature the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.

Question 21

How does Humidity affect transpiration rate?

Answer 21

The lower the humidity, the faster the transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration.

Question 22

How does wind affect transpiration rate?

Answer 22

The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases the rate of transpiration.

Question 23

How can a potometer be used to measure the transpiration rate

Answer 23

1. Cut a shoot underwater to prevent air from entering the xylem and at a slant for an increased surface area
2. Assemble the potometer in water and insert the shoot underwater so no air can enter
3. keep the end of the capillary tube in the water
4. make sure the apparatus is watertight and airtight
5. Dry the leaves, allow time for the shoot to acclimatise, and then shut the tap.
6. Make an air bubble in the capillary tube
7. record where the bubble starts
8. Start a stopwatch and measure the distance moved by the bubble per minute. the rate of bubble movement is an estimate of the transpiration rate.

Question 24

What are Xerophytic plants?

Answer 24

Xerophytic plants are plants that are adapted to reduce water loss.

Question 25

How is marram grass adapted for water loss?

Answer 25

• Marram grass has stomata that are sunk in pits, so they’re sheltered from the wind. This helps to slow transpiration down.
• It also has a layer of hairs on the epidermis which traps moist air around the stomata, reducing transpiration as there is a lower water potential gradient
• In hot or windy environments marram grass rolls its leaves- this traps moist air, slowing down transpiration. It also reduces the exposed surface area for losing water and protects the stomata from wind.
• has a thick waxy cuticle which reduces water loss by evaporation because the layer is waterproof.

Question 26

How are cacti adapted for water loss?

Answer 26

• has a thick waxy cuticle which reduces water loss by evaporation because the layer is waterproof.
• Cacti have spines instead of leaves- this reduces the surface area for water loss
• Cacti also have spines instead of leaves which reduces their surface area for water loss
• Cacti also close their stomata at the hottest times of the day when transpiration rates are at their highest

Question 27

What is translocation and what are assimilates?

Answer 27

Translocation is the movement of dissolved substances to where they are needed in a plant. Dissolved substances are sometimes called assimilates. It is an energy-required process that happens in the phloem.

Question 27

What are hydrophilic plants?

Answer 27

They are plants that are adapted to survive in water. This is vital as water has low oxygen rates.

Question 27

How does the mass flow hypothesis link with phloem transport?

Answer 27

• Active transport is used to actively load the solutes into the sieve tubes of the phloem at the source.
• This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.
• This creates a high pressure in the sieve tubes at the source end of the phloem
• At the sink end, solutes are removed from the phloem to be used up.
• This increases the water potential inside the sieve tubes so water also leaves the tubes by osmosis
• this lowers the pressure inside the sieve tubes
• the result is a pressure gradient from the source end to the sink end
• this gradient pushes solutes along the sieve tubes to where they’re needed

Question 27

How are water lilies adapted for water survival?

Answer 27

• They have air spaces in the tissues which help the plants to float and can act as a store of oxygen for use in respiration. For example, water lilies have large air spaces in their leaves THis allows the leaves to float on the surface of the water, increasing the amount t of light they receive. Ait spaces in the roots and stems allow oxygen to move from the floating leaves down to the underwater parts.
• Stomata are usually only present on the upper surface of floating leaves. This helps maximize gas exchange.
• Hydrophytes often have flexible leaves and stems- these plants are supported by the water around them, so they don’t need rigid stems for support. Flexibility helps to prevent damage by water currents.

Question 27

What is the function of enzymes in translocation?

Answer 27

Enzymes maintain a concentration gradient from the source to the sink by changing the dissolved substances at the sink. This makes sure there’s always a lower concentration at the sink than at the source

Question 28

What are sources and sinks and how is translocation related?

Answer 28

Translocation moves substances from sources to sinks. The source of a substance is where it is made. The sink is the area where it is used. Some areas of a plant can be both a sink and a source e.g. roots.

Question 28

What is active loading and how is it used?

Answer 28

Active loading is used to move substances into the companion cells, from surrounding tissues, and from the companion cells into the sieve tubes, against a concentration gradient. The concentration of sucrose is usually higher in the companion cells than the surrounding tissue cells, and higher in the sieve tube cells than the companion cells.

Question 28

How do the co-transporter proteins work?

Answer 28

• In the companion cells ATP is used to actively transport hydrogen ions out of the cell and into surrounding tissue cells.
• This sets up a concentration gradient- there are more H+ ions in the surrounding tissue than in the companion cell.
• An H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell
• A sucrose molecule binds to the co-transport protein at the same time. The movement of the H+ ion is used to move the sucrose molecule into the cell, against its concentration gradient.
• Sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process.