Transport in Plants

Question 1

Plants require transport systems as:

Answer 1

• Low SA:V ratio
• High metabolic demands (some parts of plant do not photosynthesis and require materials to be transported and removed-e.g., mineral ions and hormones)
• Size (large perennial plants e.g., trees require transport system to move substances along entire organism)

Need water, minerals and sugars to live and also need to get rid of waste substances.

Multicellular plants have a low SA:V, however they are relatively big with a high metabolic rate.

Exchanging substances by direct diffusion would be too slow to meet their metabolic needs.

So plants need transport systems to move substances to and from individual cells quickly

Question 2

Herbaceous Dicotyledonous plants

Answer 2

Herbaceous Dicotyledonous plants (dicots)

Dicots have vascular bundle (phloem and xylem) arranged in a ring and 2 cotyledons (organs that act as food stores for developing embryo plant and form the first leaves when the seed germinates)

Herbaceous plants have soft tissue and short life cycle, while woody plant have hard, lignified tissue and a long-life cycle.

Question 3

Role of the xylem

Answer 3

Transports water and mineral ions up the plant (transpiration pull)
Supports the plant and keeps it upright

Question 4

Features of the xylem

Answer 4

• Lignin allows xylem to withstand high pressure from transpiration pull, as well as support the plant
• Non-lignified pits allow water and mineral ions to leave xylem to cells
• Xylem parenchyma cells store food and contain tannin (bitter taste prevents attack)
• No end walls for continuous flow

Question 5

Role of phloem

Answer 5

Used for translocation- transports assimilates (sugars e.g., sucrose) from source to sink of plant. (It’s sucrose not glucose, as glucose is converted to sucrose as it is less reactive, so not used up in respiration)

Question 6

Phloem features

Answer 6

Sieve plates- aid uptake of food particles

Companion cells- have lots of mitochondria, a large nucleus and a dense cytoplasm.

Companion cells carry out the living functions of both themselves and their sieve cells. Act as a life support system as they contain the nucleus and organelles of the live sieve tube cells and also provide energy for active transport of translocation.

Live surrounding cells- perform active transport for energy for translocation

Question 7

Why do plants require water

Answer 7

• Minerals and ions are transported in water
• Turgor pressure (support)
• Turgor drives cell expansion
• Loss of water allows plant to cool
• Raw material for photosynthesis

Question 8

Movement of water into root hair cell

Answer 8

Root hair cells: Large SA:V ratio, can penetrate between soil particles, high concentration of solutes in root hair cytoplasm.
Lower water potential of soil water than inside root, due to high concentration of solutes in root cytoplasm and vacuole, but low concentration in root water. Moves by osmosis.

Question 9

Movement of eater through root hair cell

Answer 9

Symplast
- Moves through the symplast- the continuous cytoplasm and plasmodesmata.
- Through diffusion, as root cell contains higher water potential than neighbouring cell, so water travels through, until xylem is reached.
Apoplast
- Moves through the apoplast- the cell walls
- As water molecules reach the xylem, more water molecules are pulled through the apoplast behind them due to water’s cohesive properties.
- The pull from the water creates tension, so there is a continuous flow of water.

At the endodermis, each endodermal cell contains waxy suberin, forming a waterproof layer called the casparian strip. Water can no longer move through the apoplast pathway so is forced through the cell membrane and into the cytoplasm, now into the symplast pathway.
The process of forcing water through the partially permeable cell membrane into the cytoplasm allows the water to be filtered and control what enters xylem

Question 10

Water into xylem

Answer 10

Endodermis uses active transport to move mineral ions into the xylem first, which decreases water potential in xylem. Water moves by osmosis from endodermis into xylem down concentration gradient.
The water transport into the bottom of the xylem creates root pressure, which pushes water up the xylem.

Question 11

What is transpiration
What is transpiration stream

Answer 11

Transpiration is the loss of water vapour out the stomata of a leaf, through evaporating by diffusion.

Transpiration stream is the continuous absorption of water from the plants roots by osmosis, through the apoplast and symplast pathway, into the xylem to the leaves, where it evaporates by diffusion through the stomata.

Question 12

Transpiration

Answer 12

When water enters the leaves from the xylem, it enters surrounding cells (mesophyll cells) and travels between them through apoplast and symplast pathways (through osmosis). The water then evaporates in the large air spaces between the cells and leaves stomata through diffusion gradient.
The transpiration creates a transpiration stream as the evaporation of water causes a lower water potential of the cells, so water moves in from adjacent cells.

Question 13

Cohesion-tension theory

Answer 13

Capillary action- movement of water along a tube against force of gravity due to water’s cohesive and adhesive properties (due to hydrogen bonds).
Transpiration pull- water is continuously drawn up the xylem in a continuous stream to replace the water lost by evaporation.
Root pressure- when water moves by osmosis into the root from endodermis, forcing water up the xylem.