Unit 9 - Transport in Plants

Question 1

Why do plants need transport systems?

Answer 1

• metabolic demands
• large size
• low surface area to volume ratio

Question 2

Define cotyledon.

Answer 2

Organ found in seeds that acts as a food store for the developing embryo and forms the first leaves when the seed germinates.

Question 3

Define dicotyledonous plant.

Answer 3

A plant which produces seeds with two cotyledons.

Question 4

Define herbaceous dicot.

Answer 4

A plant with soft tissues and a short life cycle.

Question 5

Define arborescent dicot.

Answer 5

A plant with hard lignified tissues and a long life cycle.

Question 6

Define vascular bundle in plants.

Answer 6

The arrangement of the transport tissues in a plant. In herbaceous dicots, the xylem and phloem form vascular bundles, with cambium tissue in between them (the cells in this tissue can differentiate into xylem or phloem).

Question 7

Where are the xylem and phloem found in the roots of a plant?

Answer 7

In the middle - this helps withstand the pressure from the stems and leaves being blown by the wind.

Question 8

Where are the xylem and phloem found in the stem of a plant?

Answer 8

Around the edge with the phloem on the outside and the xylem facing inwards, surrounded by parenchyma tissue. This gives strength and support.

Question 9

Where are the xylem and phloem found in the leaves of the plant?

Answer 9

The main vein (midrib), which supports the leaf’s structure. Branching veins spread throughout the leaf.

Question 10

What is the role of xylem in the plant?

Answer 10

To transport water and mineral ions from the roots of the plant to the rest of the plant.

Question 11

What is the structure of xylem?

Answer 11

Xylem is a non-living tissue. The vessels are made of long cells fused together into a hollow column (the cells have died and the contents and end walls decayed). Extra mechanical strength is given by xylem fibres which don’t transport water - they are long cells with lignified secondary walls. Lignin is a polymer which is laid in the xylem walls as rings, spirals or as tubes with bordered pits and gives strength to the vessels.

Question 12

What is parenchyma?

Answer 12

A tissue which packs around the xylem vessels. It stores food and contains tannin - a bitter tasting chemical protecting the xylem from being eaten by herbivores.

Question 13

What is the role of phloem in the plant?

Answer 13

To transport sugars and amino acids made in photosynthesis from the leaves around the plant (both up and down).

Question 14

What are the sieve tube elements of the phloem?

Answer 14

Unlignified cells joined to form a long hollow vessel. As the plant grows, pores appear in the walls between the cells to form sieve plates and the tonoplast, nucleus and other organelles break down.

Question 15

What are the companion cells of the phloem?

Answer 15

Cells which form with the sieve tube elements and are linked to them by plasmodesmata. They function to provide energy for the sieve tube elements.

Question 16

Why is water transport important in multicellular plants?

Answer 16

• turgor pressure supports stems and leaves and drives cell expansion
• evaporation keeps plants cool
• forms aqueous solutions for transport
• raw material for photosynthesis

Question 17

What adaptations do root hair cells have so they can absorb water from the soil?

Answer 17

• large SA:V ratio
• thin cell-surface membrane and cell wall
• high solute concentration in cytoplasm to maintain the water potential gradient
• very small

Question 18

Why does water move into the root hair cells from the soil?

Answer 18

Soil water has a high water potential because it has a low concentration of dissolved minerals, while root hair cells have a high concentration of dissolved solutes in the cytoplasm and vacuolar sap, lowering the water potential. This means that water will move by osmosis into the root hair cells.

Question 19

What are the three ways that water can move across the cortex from the root hair cells to the xylem vessels?

Answer 19

By the apoplast pathway, the symplast pathway or the vacuolar pathway.

Question 20

What is the apoplast pathway?

Answer 20

When water moves through the intercellular spaces and the cell walls of the parenchyma to reach the endodermis.

Question 21

What is the endodermis in the root of a plant?

Answer 21

The layer of cells surrounding the vascular bundle.

Question 22

What is the symplast pathway?

Answer 22

When water moves across the parenchyma through the cytoplasm, which forms plasmodesmata to connect the cells.

Question 23

What is the vacuolar pathway?

Answer 23

When water moves across the parenchyma from vacuole to vacuole. The water passes via the plasmodesmata.

Question 24

What is the Casparian strip?

Answer 24

A waterproof layer of suberin (a waxy material) which covers the endodermal cells.

Question 25

How does the Casparian strip affect the movement of water across the root?

Answer 25

It acts as an apoplast block by forcing water in the apoplast pathway into the cytoplasm or vacuoles.

Question 26

What is an advantage of the apoplast block created by the Casparian strip?

Answer 26

Water is forced to pass through the selectively permeable cell-surface membranes. This prevents potentially toxic solutes in the soil water from reaching the xylem.

Question 27

How do the endodermal cells increase the rate of osmosis into the xylem?

Answer 27

They move mineral ions into the xylem by active transport, which decreases the water potential in the xylem vessels, increasing the water potential gradient.

Question 28

How does the active transport of mineral ions into the xylem by the endodermis help with transpiration?

Answer 28

It creates root pressure, a minor factor for pushing water up the xylem vessel.

Question 29

Define transpiration.

Answer 29

The loss of water vapour through the leaves and stem of a plant

Question 30

Define the transpiration stream.

Answer 30

The process moving water from the roots of a plant through the xylem to the leaves and stem, culminating in transpiration.

Question 31

What is the cohesion-tension theory?

Answer 31

The combined effects of cohesion between water molecules and adhesion of the water molecules to carbohydrates on the xylem walls leads to capillary action, helping the water rise up the xylem.

Question 32

What is the transpiration pull?

Answer 32

Water being drawn up the xylem in a continuous stream to replace that lost in evaporation. This results in tension in the xylem, helping move water through the roots.

Question 33

What evidence is there for the cohesion-tension theory?

Answer 33

- changes in the diameter of trees - when a xylem vessel is broken air moves in

Question 34

What are stomata?

Answer 34

Pores created by the change in shape of guard cells on the surface of a leaf. When they open, water can leave the leaf and carbon dioxide can enter the leaf.

Question 35

How do guard cells open the stomata?

Answer 35

When environmental conditions are favourable, the guard cells actively transport potassium ions into the cells, which lowers the water potential, allowing water to move in by osmosis. This increases the turgor of the guard cells so they change shape, opening the stomata.

Question 36

How do the guard cells close the stomata?

Answer 36

When water is scarce, hormonal signals from the roots trigger turgor loss by causing a drop in levels of potassium ions (which increases the water potential), so water will diffuse out of the guard cells, closing the stomata. This allows the plant to conserve water.

Question 37

What effect does an increase in light intensity have on the rate of transpiration and why?

Answer 37

It increases the rate. This is because light is needed for photosynthesis, so as light intensity increases, the number of open stomata increases.

Question 38

What effect does an increase in relative humidity have on the rate of transpiration and why?

Answer 38

It will decrease the rate of transpiration because the atmosphere’s water potential will increase, decreasing the water potential gradient.

Question 39

What effect will an increase in temperature have on the rate of transpiration and why?

Answer 39

It will increase the rate of transpiration because the relative humidity will decrease and the kinetic energy of the water molecules will increase.

Question 40

What effect will an increase in air movement have on the rate of transpiration and why?

Answer 40

It will increase the rate of transpiration because water vapour will be moved away from the leaf more quickly, maintaining the water potential gradient.

Question 41

What is translocation?

Answer 41

Transport of organic solutes in the phloem.

Question 42

What are assimilates?

Answer 42

Products of photosynthesis which are transported through translocation - mainly sucrose and amino acids.

Question 43

What is a source?

Answer 43

The part of a plant where assimilates are made and transported from.

Question 44

Give three sources of assimilates.

Answer 44

- green parts of leaves and stem - storage organs e.g. tubers and tap roots at the start of the growing period - food stores in seeds at germination

Question 45

What is a sink?

Answer 45

The part of a plant where assimilates are transported to, to be stored or used in respiration.

Question 46

Give five main sinks for assimilates.

Answer 46

- roots - meristems - developing seeds - fruits - storage organs

Question 47

How are assimilates actively loaded into the phloem?

Answer 47

Hydrogen ions are actively transported into the source. They bind to sucrose and diffuse through a co-transporter protein, increasing the sucrose concentration in the companion cells. Sucrose then diffuses into the sieve tube elements.

Question 48

How do assimilates move from source to sink?

Answer 48

Mass flow: A hydrostatic pressure gradient between the source and the sink is created by osmosis into the sieve tube elements near the source and osmosis out of the sieve tube elements near the sink into the sink cells. This causes water to move down the gradient towards the sink, carrying dissolved assimilates which then diffuse into the sink cells.

Question 49

What evidence is there for mass flow, specifically active transport?

Answer 49

- Scientists can see adaptations of the companion cells due to advances in microscopy - The flow is much faster than if by diffusion alone, suggesting an active process is involved - If the mitochondria in the companion cells are poisoned, translocation stops

Question 50

Define habitat.

Answer 50

Habitat = the place in which an organism lives

Question 51

Define adaptation.

Answer 51

Adaptation = a positive characteristic of an organism that has been favoured by natural selection

Question 52

Define mesophyte.

Answer 52

Mesophyte = a plant which is able to take up sufficient water to replace that lost in transpiration

Question 53

Define xerophyte.

Answer 53

Xerophyte = a plant living in an area where loss of water from transpiration is greater than water uptake by the roots

Question 54

Define hydrophyte.

Answer 54

Hydrophyte = a plant living either partially or completely submerged in water - these plants have problems with obtaining oxygen

Question 55

What adaptations do xerophytes have?

Answer 55

- thick waxy cuticle - reduced SA/SA:V ratio of leaves - reduced number of stomata - sunken stomata - hairy leaves - curled leaves - leaf loss in winter - either long, deep roots or widespread, shallow roots with a large SA

Question 56

How does a thick waxy cuticle help xerophytes conserve water?

Answer 56

The waxy cuticle is lipid-based and so acts as a barrier to evaporation. The shiny surface also reflects heat, lowering the temperature to reduce transpiration.

Question 57

How do sunken stomata help xerophytes conserve water?

Answer 57

Sunken stomata create humidity around the stomata, decreasing the water potential gradient to reduce transpiration.

Question 58

How do hairy leaves help xerophytes conserve water?

Answer 58

Hairy leaves trap layers of air, reducing air movement. This reduces the water potential gradient because water is carried away more slowly, decreasing transpiration.

Question 59

How do curled leaves help xerophytes conserve water?

Answer 59

Curled leaves trap the stomata in still, humid air, which reduces the water potential gradient and therefore transpiration.

Question 60

What is a succulent?

Answer 60

A succulent is a xerophyte which stores water in the parenchyma tissue in stems and roots.

Question 61

How do long, deep roots help xerophytes conserve water?

Answer 61

Long, deep roots can access water a long way below the ground's surface.

Question 62

How do widespread, shallow roots with a large surface area help xerophytes conserve water?

Answer 62

Widespread, shallow roots allow the plant to absorb as much water as possible after a rain shower before it evaporates.

Question 63

What adaptations do hydrophytes have?

Answer 63

- no waxy cuticle (or very thin) - wide, flat leaves - high number of always open stomata - reduced structure - arenchyma - large SA of stems and roots - air sacs - small roots - hydathodes

Question 64

Why do hydrophytes have wide, flat leaves?

Answer 64

Wide, flat leaves spread across the surface of the water to absorb as much light as possible.

Question 65

In hydrophytes, what is arenchyma?

Answer 65

Arenchyma is a specialised parenchyma tissue which contains many air spaces. It makes the plant's leaves and stems more buoyant and provides a pathway for oxygen to travel to underwater tissues.

Question 66

Why do hydrophytes have air sacs?

Answer 66

Air sacs allow leaves and flowers to float to the surface of the water.

Question 67

In hydrophytes, what are hydathodes?

Answer 67

Hydathodes are structures which release water droplets. These may then evaporate from the leaf surface.

Question 68

Where are cacti found?

Answer 68

Cacti are found in deserts in the western US.

Question 69

What adaptations do cacti have?

Answer 69

- hairy leaves - spines - sunken stomata - fleshy body - grow slowly - few flowers - thick stems - wide, extensive and deep roots

Question 70

Where is marram grass found?

Answer 70

Marram grass is found in coastal sand dunes.

Question 71

What adaptations does marram grass have?

Answer 71

- curled leaves - sunken stomata - hairy leaves - thick waxy cuticle