Module 3: Section 3 - Transport in Plants

Question 1

Why do multicellular plants need transport systems?

Answer 1

1) plants need substances like water, minerals and sugars to live. They also need to get rid of waste substances
2) like animals, plants are multicellular - so they have a small surface area : volume ratio. They’re also relatively big with a relatively high metabolic rate
3) exchanging substances by direct diffusion (from the outer surface to the cells) would be too slow to meet their metabolic needs
4) so plants need transport systems to move substances to and from individual cells quickly

Question 2

What do xylem and phloem tissue do?

Answer 2

1) xylem tissue transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves. Phloem tissue mainly transports sugars (also in solution) both up and down the plant
2) xylem and phloem make up the plants vascular system. They are found throughout a plant and transport material to all parts. Where they’re found in each part is connected to the xylem’s other function: support

Question 3

How does xylem and phloem provide support?

Answer 3

1) in a root, the xylem is in the centre surrounded by phloem to provide support for the root as it pushes through the soil
2) in the stems, the xylem and phloem are near the outside to provide a sort of ‘scaffolding’ that reduces bending
3) in a leaf, xylem and phloem make up a network of veins which support the thin leaves

Question 4

Xylem vessels are adapted for transporting water and mineral ions. How are xylem vessels adapted for their function?

Answer 4

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

Question 5

How is phloem tissue adapted for transporting solutes?

Answer 5

1) like xylem, phloem is formed from cells arranged in tubes. But, unlike xylem, it’s purely transport tissue - it’s not used for support as well
2) phloem tissue contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
3) sieve tube elements and companion cells are the most important cell types in phloem for transport

Question 6

How do sieve tube elements help phloem transport?

Answer 6

Sieve tube elements

1) these are living cells that form the tube for transporting solutes through the plant
2) they are joined end to end to form sieve tubes
3) the ‘sieve’ parts are the end walls, which have lots of holes in them to allow solutes to pass through
4) unusually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles
5) the cytoplasm of adjacent cells is connected through the holes in the sieve plates

Question 7

How do companion cells help phloem transport?

Answer 7

1) the lack of a nucleus and other organelles in sieve tube elements means that they can’t survive on their own. So there’s a companion cell for every sieve tube element
2) companion cells carry out the living functions for both themselves and their sieve cells. For example, they provide the energy for the active transport of solutes

Question 8

Give the four steps in which you would dissect a plant stem

Answer 8

1) use a scapel to cut a cross-section of the stem (transverse or longitudinal). Cut the sections as thinly as possible - thin sections are better for viewing under a microscope
2) use tweezers to gently place the cut sections in water until you come to use them. This stops them from drying out
3) transfer each section to a dish containing a stain, e.g. toluidine blue O (TBO), 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
4) rinse off the sections in water and mount each one onto a slide

Question 9

Water has to get from the soil, through the root and into the xylem to be transported around the plant

Water enters through root hair cells and then passes through the root cortex, including the endodermis, to reach the xylem

Water is drawn into the roots via osmosis. This means it moves down a water potential gradient - please explain how?

Answer 9

1) water always moves away from areas of higher water potential to areas of lower water potential - it goes down a water potential gradient
2) the soil around roots generally has a high water potential (i.e. there’s lots of water there) and leaves have a lower water potential (because water constantly evaporates from them)
3) this creates a water potential gradient that keeps water moving through the plant in the right direction, from roots (high) to leaves (low)

Question 10

Water travels through the roots (via the root cortex) into the xylem by two different paths: the symplast pathway and the apoplast pathway. Please explain the symplast pathway to me

Answer 10

The symplast pathway - goes through the living parts of cells - the cytoplasm. The cytoplasms of neighbouring cells connect through plasmodesmata (small channels in the cell walls). Water moves through the symplast pathway via osmosis

Question 11

Water travels through the roots (via the root cortex) into the xylem by two different paths: the symplast pathway and the apoplast pathway. Please explain the apoplast pathway to me

Answer 11

The apoplast pathway - goes through 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 (e.g. along a pressure gradient). This is an example of mass flow

Question 12

What is the Casparian strip and why is it useful?

Answer 12

when water in the apoplast pathway gets to the endodermis 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 are able to control whether or not substances in the water get through

once past this barrier, the water moves into the xylem

Question 13

Both pathways are used, but which pathway is the main pathway and why?

Answer 13

The main one is the apoplast pathway because it provides the least resistance

Question 14

How do plants lose water from the leaves and what is this process called?

Answer 14

1) xylem vessels transport the water all around the plant
2) at the leaves, water leaves the xylem and moves into the cells mainly by apoplast pathway
3) water evaporates from the cell walls into the spaces between cells in the leaf
4) when the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the leaf (down the water potential gradient) into the surrounding air
5) the loss of water from a plant’s surface is called transpiration

Question 15

How does cohesion and tension help water move up plants, from roots to leaves, against the force of gravity?

Answer 15

1) water evaporates from the leaves at the ‘top’ of the xylem (transpiration)
2) this creates a tension (suction), which pulls more water into the leaf
3) water molecules are cohesive 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
4) water enters the stem through the root cortex cells

Question 16

How does adhesion help water to move up plants, from roots to leaves, against the force of gravity?

Answer 16

1) as well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels
2) this helps water rise up through the xylem vessels

Question 17

How is transpiration a side effect of the gas exchange needed for photosynthesis?

Answer 17

1) a plant needs to open its stomata to let in carbon dioxide so that it can produce glucose (by photosynthesis)
2) but this also lets water out - there’s a higher concentration of water inside the leaf than in the air outside, so water moves out of the leaf down the water potential gradient when the stomata open
3) so transpiration is really a side effect of the gas exchange needed for photosynthesis

Question 18

How does light affect transpiration rate?

Answer 18

Light - 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. When it’s dark the stomata are usually closed, so there’s little transpiration

Question 19

How does temperature affect transpiration rate?

Answer 19

Temperature - 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 20

How does humidity affect transpiration rate?

Answer 20

Humidity - the lower the humidity, the higher 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 21

How does wind affect transpiration rate?

Answer 21

Wind - 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 22

A potometer is a special piece of apparatus used to estimate transpiration rates. It actually measures water uptake by a plant, but its assumed that water uptake by plant is directly related to water loss by the leaves. How can you use it to estimate how different factors affect transpiration rate?

Answer 22

Here’s what you’d do:

1) cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the surface area available for water uptake
2) assemble the potometer in water and insert the shoot underwater, so no air can enter
3) remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water
4) check that the apparatus is watertight and airtight
5) dry the leaves, allow time for the shoot to acclimatise, and then shut the tap
6) remove the end of the capillary tube from the beaker water until one air bubble has formed, then put the end of the tube back into the water
7) record the starting position of the air bubble
8) start a stopwatch and record the distance moved by the bubble per unit time, e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate
9) remember, only change one variable at a time. All other conditions must be kept constant

Question 23

Xerophytes are plants like cacti and marram grass (which grows on sand dunes). They’re adapted to live in dry climates. What adaptions do they have to prevent them losing too much water by transpiration?

Answer 23

1) marram grass has stomata that are sunk in pits, so they’re sheltered from the wind. This helps to slow transpiration down
2) it also has a layer of ‘hairs’ on the epidermis - this traps moist air round the stomata, which reduces the water potential gradient between the leaf and the air, slowing transpiration down
3) in hot or windy conditions marram grass plants roll their leaves - again this traps moist air, slowing down transpiration. It also reduces the exposed surface area for losing water and protects the stomata from wind
4) both marram grass and cacti have a thick, waxy layer on the epidermis - this reduces water loss by evaporation because the layer is waterproof
5) cacti have spines instead of leaves - this reduces the surface area for water loss
6) cacti also close their stomata at the hottest times of the day when transpiration rates are the highest

Question 24

Hydrophytes are plants like water lilies, which live in aquatic habitats. As they grow in water, they don’t need adaptions to reduce water loss (like xerophytes), but they do need adaptations to help them cope with low oxygen level. Please give some adaptions of hydrophytes

Answer 24

1) air spaces in the tissue help the plants to float and can act as a store of oxygen for use in respiration. E.g., water lilies have large air spaces in their leaves. This allows the leaves to float on the surface of the water, increasing the amount of light they receive. Air spaces in the roots and stems allow oxygen to move from the floating leaves down to parts of the plant that are underwater
2) stomata are usually only present on the upper surface of floating leaves. This helps maximise gas exchange
3) 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 25

Scientists still aren’t certain exactly how the dissolved substances (solutes) are transported from source to sink by translocation. The best support theory is the mass flow hypothesis. Please explain mass flow hypothesis (jesus christ, sorry for this one)

Answer 25

STAGE 1:

1) active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) into the sieve tubes of the phloem at the source (e.g. the leaves).
2) this lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells
3) this creates a high pressure inside the sieve tubes at the source end of the phloem

STAGE 2

1) at the sink end, solutes are removed from the phloem to be used up
2) this increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis
3) this lowers the pressure inside the sieve tubes

STAGE 3

1) the result is a pressure gradient from the source end to the sink end
2) this gradient pushes solutes along the sieve tubes to where they’re needed

Question 26

What is translocation?

Explain what a ‘source’ and a ‘sink’ is in your definition

Answer 26

1) translocation is the movement of dissolved substances (e.g. sugars like sucrose, and amino acids) to where they’re needed in a plant. Dissolved substances are sometimes called assimilates
2) it’s an energy-requiring process that happens in the phloem
3) translocation moves substances from ‘sources’ to ‘sinks’. The source of a substance is where it’s made (so it’s at a high concentration there). The sink is the area where it’s used up (so it’s at a lower concentration there).

Question 27

Give an example of the source and the sink in a plant

Answer 27

The source for sucrose is usually the leaves (where it’s made), and sinks are the other parts of the plant, especially the food storage organs and the meristems (areas of growth) in the roots, stems and leaves

Question 28

Give an example of when some parts of the plant are both a sink and a source

Answer 28

Sucrose can be stored in the roots. During the growing season, sucrose is transported from the roots to the leaves to provide the leaves with energy for growth. In this case, the roots are the source and the leaves are a sink

Question 29

Enzymes maintain a concentration gradient from the source to the sink by changing the dissolved substances at the sink (e.g. by breaking them down or making them into something else). This makes sure there’s always a lower concentration at the sink than at the source. Give an example of this using potatoes

Answer 29

In potatoes, sucrose is converted to starch in the sink areas, so there’s always a lower concentration of sucrose at the sink than inside the phloem. This makes sure a constant supply of new sucrose reaches the sink from the phloem. In other sinks, enzymes such as invertase break down sucrose into glucose (and fructose) for use by the plant - again this makes sure there’s a lower concentration of sucrose at the sink

Question 30

What is active loading and why is it used?

Answer 30

1) 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
2) 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
3) so sucrose is moved to where it needs to go using active transport and co-transporter proteins

Question 31

How does active transport work?

Answer 31

1) in the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into surrounding tissue cells
2) this sets up a concentration gradient - there are more H+ ions in the surrounding tissue than in the companion cell
3) an H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell (down the concentration gradient)
4) a sucrose molecule binds to the co-transport protein at the same time. The movement of the H+ ions is used to move the sucrose molecule into the cell, against its concentration gradient
5) sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process

NB: ATP is one of the products of respiration. The breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions

Question 32

Have a 15 minute break

Answer 32

Go on, it’s productive. Take a shower, go running, cup o tea?