3.1.3 Transport in Plants - transport systems in multicellular plants

Question 1

What is the need for plant transport systems?

Answer 1

Due to their metabolic demands, size and SA:V.

• Reactants for and products of metabolism need transporting to and from their cells of synthesis.
• Continued growth leads to a size that means substances need to be moved to and from the roots and shoots.
• The SA:V of plants means they cannot rely upon diffusion alone to supply their cells with everything they need.

Question 2

What is the structure and function of epidermal tissues?

Answer 2

Waxy cuticle – reduces water loss from the leaf

Guard cells and stomata – Guard cells open and close the stomata to control water loss and allow for gas exchange

Question 3

What is the structure and function of palisade mesophyll?

Answer 3

Palisade cells – cells near the top surface of the leaf that are packed with chloroplasts that contain chlorophyll. Both adaptions maximise photosynthesis.

Question 4

What is the structure and function of spongy mesophyll?

Answer 4

Air spaces in the leaf between cells – increases surface are for gas exchange so that carbon dioxide can diffuse into photosynthesising cells.

Question 5

What is the structure and function of xylem?

Answer 5

The structure of xylem is hollow tubes strengthened by lignified dead cells adapted for the transportation of mineral ions through the plant in the transpiration stream.
Xylem allows transport of water and mineral ions from the roots to the stem and the leaves. One-way flow.

Question 6

What is the structure and function of phloem?

Answer 6

Cell sap moves from one phloem cell to the next through pores in the end walls called sieve plates.
Phloem transports dissolved sugars from the leaves to the rest of the plant for immediate use of storage (translocation).

Question 7

What is the difference between Monocots and Dicots (Eudicots)?

Answer 7

Monocots:

• 1 cotyledon
• Leaf venation is mostly parallel
• Fibrous root system (adventitious roots
• Flower parts usually in 3 or multiples of 3
• Usually herbaceous, never woody
• Pollen grain has 1 furrow or aperture
• Vascular bundles in stem are scattered (cross sectionally)
• Vascular tissue in root arranged in a ring (cross sectionally)
• 22% of angiosperms

Dicots:

• 2 cotyledon
• Leaf venation is mostly netlike or reticulate
• Typically taproot system (underground roots arise form radicle)
• Flower parts usually in 4 or 5 or in multiples of 4 or 5
• Woody or herbaceous
• Tricolpate pollen grain (pollen grain has 3 furrows or aperture)
• Vascular bundles in stem are arranged in a ring (cross sectionally)
• Root xylem is usually star shaped, the phloem between arms of the star (cross sectionally)
• 75% of all angiosperms

Question 8

What is the structure of xylem vessels?

Answer 8

Xylem vessels are made from dead xylem cells (lignified). There are no ends, forming supporting tubules.
Water is absorbed from the soil through root hair cells. Water is transported through xylem vessels up the stem to the leaves. Water evaporates from the leaves (transpiration).
Xylem vessels are involved in the movement of water and minerals for a plant from its roots to its leaves.

Question 9

What is the structure of phloem vessels?

Answer 9

Phloem vessels are living tissue involved in the movement of food substances from the stems to the rest of the plant. Sieve tube elements form the main transporting vessel of the phloem. The tonoplast, nucleus and other organelles breakdown leaving a fluid filled tube. Companion cells actively support the sieve tube elements linked to the adjacent cells by many plasmodesmata.

Question 10

What are Parenchyma cells?

Answer 10

Perform the primary metabolic functions in the plant, and include the palisade cells. They have thin and flexible walls, and generally retain the ability to divide and differentiate.

Question 11

What are Collenchyma cells?

Answer 11

Grouped in strands, these cell support young parts of the plant shoot. Cells are often elongated with unevenly thickened cell wall. Celery ‘strings’ are petioles which are strings of collenchyma.

Question 12

What are sclerenchyma cells?

Answer 12

Function as supporting elements, with thickened secondary walls that contain a large amount of lignin. Mature cells cannot elongate, and occur in regions that have stopped growing.

Question 13

Define Xerophytes

Answer 13

Plants in dry habitats that have evolved to enable them to live and reproduce in places where water availability is very low.

Question 14

What is an example of a xerophyte?

Answer 14

Conifers, marram grass, and Cacti are examples of xerophytes.

Question 15

What ways are there to conserve water?

Answer 15

• Thick waxy cuticle – minimises water loss.
• Sunken stomata – reduce air movement producing a microclimate of still, humid (moist) air that reduces water vapour potential gradient and so reduces transpiration.
• Reduced number of stomata – reduce water loss by transpiration but also reduce their gas exchange capabilities.
• Hairy leaves – creates a microclimate of still, humid air reducing water potential gradient and minimises the water loss by transpiration on the surface of the leaf.
• Curled leaves – reduces water loss by transpiration since it confines all of the stomata within a microenvironment of still, humid air to reduce diffusion of water vapour from the stomata.
• Succulents – store water in specialised parenchyma tissue in their stems and roots.
• Leaf loss – prevents water loss through their leaves by simply losing their leaves when water is not available.
• Root adaptations – long tap roots growing deep into the ground can penetrate several metres, so that they can access water well below the surface. A mass of widespread, shallow roots with a large surface area able to absorb any available water
• Avoiding the problems – some plants are adapted to cope with their problems of low water availability by avoiding the situation completely:
• Loose leaves
• Become dormant
  Leave seeds behind to germinate and grow rapidly when rain falls again
• Others survive as storage organs
• Some plants can withstand dehydration and recover.

Question 16

Define Hydrophytes

Answer 16

Plants that live in water (submerged, on the surface or at the edges of bodies of water)

Question 17

What is an example of a hydrophyte?

Answer 17

Water lilies, water cress, duckweeds, bulrushes, and yellow iris are examples of Hydrophytes.

Question 18

What is a major problem of hydrophytes?

Answer 18

Water-logging is a major problem for all hydrophytes since the air spaces in the plant need to be full of air not water, for the plant to survive.

Question 19

What are the adaptations of hydrophytes?

Answer 19

• Very thin or no waxy cuticle – hydrophytes do not need to conserve water as there is plenty available so water loss by transpiration is not an issue.
• Many always-open stomata on the supper surfaces – maximising the number of stomata maximises gaseous exchange. In water lilies the stomata need to be on the upper surface of the leaf so that they are in contact with the air.
• Reduced structure of the plant – water supports the leaves and flowers so there is no need for strong supporting structures.
• Wide, flat leaves – in order to capture as much light as possible.
• Small roots – water can diffuse directly into stem and leaf tissue so there is less need for uptake by roots.
• Large surface areas of stems and roots under water – maximises the area for photosynthesis and for oxygen to diffuse into submerged plants.
• Air sacs – to enable the leaves and/or flowers to float to the surface of the water.
• Aerenchyma – specialised parenchyma forms in the leaves, stems, and roots of hydrophytes. It has many large air spaces. It has several different functions within the plants, including:
• Making the leaves and stems more buoyant
• Forming a low-resistance internal pathway for the movement of substances such as oxygen to tissues below the water.