Transport in plants

Question 1

why do organisms need to exchange substances with their environment?

Answer 1

Every organism, whatever its size, needs to exchange things with its environment.
1) Cells need to take in things like oxygen and glucose
for aerobic respiration and other metabolic reactions.
2) They also need to excrete waste products from these
reactions — like carbon dioxide and urea.
How easy the exchange of substances is depends on the
organism’s surface area to volume ratio (SA:V).

Question 2

How to work out surface area to volume ratio?

Answer 2

to calculate the surface area to volume ratio you just divide the surface area by the volume

for example-block measuring 2 cm × 4 cm × 4 cm.
Its volume is 2 × 4 × 4 = 32 cm3
Its surface area is 2 × 4 × 4 = 32 cm2 (top and bottom surfaces of cube)
2+ 4 × 2 × 4 = 32 cm2 (four sides of the cube)
Total surface area = 64 cm2
surface area : volume ratio of 64 : 32 or 2 : 1.

Question 3

Why do multicellular organisms need exchange surfaces?

Answer 3

Multicellular Organisms Need Exchange Surfaces
An organism needs to supply every one of its cells with substances like glucose and oxygen (for respiration).
It also needs to remove waste products from every cell to avoid damaging itself.
1) In single-celled organisms, these substances can diffuse directly into (or out of) the cell across
the cell surface membrane. The diffusion rate is quick because of the small distances the
substances have to travel (see p. 54).
2) In multicellular animals, diffusion across the outer membrane is too slow, for several reasons:
• Some cells are deep within the body — there’s a big distance between them and the
outside environment.
• Larger animals have a low surface area to volume ratio — it’s difficult to exchange enough
substances to supply a large volume of animal through a relatively small outer surface.
• Multicellular organisms have a higher metabolic rate than single-celled organisms,
so they use up oxygen and glucose faster.
So rather than using straightforward diffusion to absorb and excrete substances,
multicellular animals need specialised exchange surfaces — like the alveoli in the lungs…

Question 4

What special features do exchange surfaces have to improve efficiency?

Answer 4

They have a large surface area
thin
a good blood supply
good ventilation

Question 5

what’s an example of a large surface area?

Answer 5

Example — ROOT HAIR CELLS
1) The cells on plant roots grow into long ‘hairs’ which stick out into the soil.
Each branch of a root will be covered in millions of these microscopic hairs.
2) This gives the roots a large surface area, which helps to
increase the rate of absorption of water (by osmosis) and
mineral ions (by active transport) from the soil.

Question 6

an example of a thin exchange surface?

Answer 6

Example — the ALVEOLI
1) The alveoli are the gas exchange surface in the lungs.
2) Each alveolus is made from a single layer of thin,
flat cells called the alveolar epithelium.
3) O2 diffuses out of the alveolar space into the blood. CO2 diffuses in the opposite direction.
4) The thin alveolar epithelium helps to decrease the
distance over which O2 and CO2 diffusion takes place,
which increases the rate of diffusion

Question 7

What is examples of good blood supply and ventilation?

Answer 7

Example 1 — ALVEOLI
1) The alveoli are surrounded by a large capillary network, giving each alveolus its own blood supply.
The blood constantly takes oxygen away from the alveoli, and brings more carbon dioxide.
2) The lungs are also ventilated (you breathe in and out so the air in each alveolus is constantly replaced.
3) These features help to maintain concentration gradients of O2 and CO2

Example 2 — FISH GILLS
1) The gills are the gas exchange surface in fish. In the gills, O2 and CO2 are exchanged between the fish’s blood and the surrounding water.
2) Fish gills contain a large network of capillaries — this keeps them well-supplied with blood.
They’re also well-ventilated — fresh water constantly passes over them. These features help to
maintain a concentration gradient of O2 — increasing the rate at which O2 diffuses into the blood.

Question 8

Why do multicellular plants need Transport Systems?

Answer 8

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
(SA:V, see page 70). 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 9

What two types of tissue are involved in transport in plants?

Answer 9

Xylem tissue and phloem tissue

Question 10

what does xylem tissues transfer?

Answer 10

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.

Question 11

what do the xylem and phloem make up?

Answer 11

2) Xylem and phloem make up a plant’s vascular system. They are found throughout a plant and transport
materials to all parts. Where they’re found in each part is connected to the xylem’s other function — support:

Question 12

In the root where is the xylem and the phloem?

Answer 12

• In a root, the xylem is in the centre
surrounded by phloem to provide support
for the root as it pushes through the soil.

Question 13

in the stems where are the xylem and phloem?

Answer 13

• In the stems, the xylem and phloem
are near the outside to provide a sort
of ‘scaffolding’ that reduces bending.

Question 14

In the Leaf where are the xylem and phloem?

Answer 14

• In a leaf, xylem and phloem
make up a network of veins
which support the thin leaves.

Question 15

The position of the xylem and phloem in the root, leaf and stem
are shown in?

Answer 15

3) The position of the xylem and phloem in the root, leaf and stem
are shown in these transverse cross-sections.

Question 16

what does transverse mean?

Answer 16

Transverse means the sections cut through each structure at a right angle to its length.

Question 17

what are longitudinal cross-sections?

Answer 17

4) You can also get longitudinal cross-sections. These are taken
along the length of a structure. For example, this cross-section
shows where the xylem and phloem are located in a typical stem

Question 18

How is design and adapted for transporting water and mineral ions?

Answer 18

Xylem is a tissue made from several different cell types (see page 66). You need to learn about xylem vessels —
the part of xylem tissue that actually transports the water and ions. Xylem vessels are adapted for their function:
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 19

how is phloem adapted for transporting solutes?

Answer 19

1) Phloem tissue transports solutes (dissolved substances), mainly sugars like sucrose, round plants.
2) Like xylem, phloem is formed from cells arranged in tubes.
But, unlike xylem, it’s purely a transport tissue — it isn’t used for support as well.
3) Phloem tissue contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells.
4) Sieve tube elements and companion cells are the most important cell types in phloem for transport:

Question 20

What’s a sieve tube element?

Answer 20

1 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 21

What are companion cells?

Answer 21

2 Companion cells
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 22

How to dissect plant stems?

Answer 22

You can look at plant tissue (e.g. part of a plant stem) under a microscope, and then draw it. But first you
need to dissect the plant and prepare a section of the tissue. You can do this using the following method:
1) Use a scalpel (or razor blade) 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 23

How does water enter a plant through root hair cells?

Answer 23

1) Water has to get from the soil, through the root and into the xylem
to be transported around the plant.
2) Water enters through root hair cells and then passes through the root cortex,
including the endodermis, to reach the xylem (see below).
3) Water is drawn into the roots via osmosis. This means it travels down a water potential gradient:
• Water always moves from areas of higher water potential to areas of
lower water potential — it goes down a water potential gradient.
• 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).
• This creates a water potential gradient that keeps water moving through the plant in the right direction, from roots (high) to leaves ( low).

Question 24

How does water move through the root into the xylem?

Answer 24

Water travels through the roots (via the root cortex) into the xylem by two different paths:
1) 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.
2) 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 (i.e. along a pressure gradient). This is an example of mass flow
• 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.
3) Both pathways are used, but the main one is the apoplast pathway because it provides the least resistance.

Question 25

How does water travel up the xylem and out the leaves?

Answer 25

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 the 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 26

how does water move up a plant against the force of gravity?

Answer 26

The movement of water from roots to leaves is called the transpiration stream. The mechanisms that move the water include cohesion, tension and adhesion. Cohesion and tension help water move up plants,
from roots to leaves, against the force of gravity 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 (they 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.
4) Water enters the stem through the root cortex cells.

Question 27

Why is adhesion partially responsible for the movement of water?

Answer 27

Adhesion is also partly responsible for the movement of water.
1) As well as being attracted to each other, water molecules
are attracted to the walls of the xylem vessels.
2) This helps water to rise up through the xylem vessels.

Question 28

water transport Simple Times

Answer 28

Water Enters a Plant through its Root Hair Cells
Water Enters a Plant through its Root Hair Cells
then Up the Xylem and Out at the Leaves
Water Moves Up a Plant Against the Force of Gravity

Question 29

Transpiration is the consequence of what?

Answer 29

gas exchange

Question 30

what is transpiration?

Answer 30

Transpiration is the evaporation of water from a plant surface especially the leaves

Question 31

why is transpiration a consequence of gas exchange?

Answer 31

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’s really a side effect of the gas exchange needed for photosynthesis.

Question 32

What four main factors affect the transpiration rate?

Answer 32

Light temperature humidity and wind

Question 33

how does light affect transpiration rate?

Answer 33

1) 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 34

How does temperature affect transpiration rate?

Answer 34

2) 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 35

How does humidity affect transpiration rate?

Answer 35

3) Humidity — 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 36

how does wind affect transpiration?

Answer 36

4) 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 37

what is a potometer?

Answer 37

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

Question 38

How to use a potometer to estimate how different factors affect the transpiration rate.

Answer 38

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 of
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 (e.g. temperature)
at a time. All other conditions (e.g. light, humidity)
must be kept constant.

Question 39

What are Xerophytes?

Answer 39

Xerophytes are plants like cacti and marram grass (which grows on sand dunes). They’re adapted to live in
dry climates. Their adaptations prevent them losing too much water by transpiration…

Question 40

How are Xerophytes adapted to reduce water loss?

Answer 40

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 (water can’t move through it).
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 41

hydrophilic plants adapted to survive in water

Answer 41

Hydrophytes are plants like water lilies, which live in aquatic habitats. As they
grow in water, they don’t need adaptations to reduce water loss (like xerophytes),
but they do need adaptations to help them cope with a low oxygen level.
Here are some adaptations of hydrophytes…
1) Air spaces in the tissues 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 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 42

What is translocation?

Answer 42

Translocation is the Movement of Dissolved Substances
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). 4) Some parts of a plant can be both a sink and a source. 5) 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.

Question 43

Why is the mass flow hypothesis the best explanation for phloem transport?

Answer 43

Scientists still aren’t certain exactly how the dissolved substances (solutes) are transported from source to sink by translocation. The best supported theory is the mass flow hypothesis:
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). There’s more on this on the next page. 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. 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. 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 44

What is active loading?

Answer 44

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.

Question 45

What is active loading?

Answer 45

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. Here’s how it works:

Question 46

How do substances get into the phloem by active loading?

Answer 46

• In the companion cell, ATP is used to actively transport hydrogen
ions (H+) 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 (down the concentration gradient).
• 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.