9.2 Transport in the phloem of plants

Question 1

What is translocation?

Answer 1

Translocation is the movement of organic compounds from sources to sinks

Question 2

What is a source?

Answer 2

Where the organic compounds are synthesised

Question 3

What is a sink?

Answer 3

Where the compounds are delivered to for use or storage

Question 4

What are the sources in plants?

Answer 4

Photosynthetic tissue aka leaves

Question 5

What are the sinks in plants?

Answer 5

Roots
Fruits
Seeds

Question 6

How are organic compounds transported from sources to sinks?

Answer 6

Via the phloem

Question 7

What is the phloem?

Answer 7

A vascular tube system

It is responsible for transporting organic compounds from sources to sink

Question 8

How are sugars transported and why?

Answer 8

They are transported as sucrose because it is soluble but metabolically inert

Question 9

What is nutrient- rich viscous fluid of the phloem?

Answer 9

Plant sap

Question 10

What are the two types of cells phloem sieve tubes composed of?

Answer 10

Sieve element cells and companion cells

Question 11

In addition to sieve element cells and companion cells, what cells does the phloem also have and what do they do?

Answer 11

Sclerenchymal and parenchymal cells which fil additional spaces and provide support

Question 12

What are sieve element cells?

Answer 12

They are long and narrow cells that are connected together to form the sieve tube

Question 13

How are sieve elements connected?

Answer 13

By sieve plates at their transverse ends which are porous to enable flow between cells

Question 14

What do sieve elements lack and how is it helpful?

Answer 14

They lack a nuclei and have reduced organelles to increase flow rate

Question 15

What helps sieve elements withstand hydrostatic pressure?

Answer 15

Thick and rigid cell walls

Question 16

What do companion cells do?

Answer 16

Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink

Question 17

Why do companion cells have an infolding plasma membrane?

Answer 17

It increases surface area to volume ratio to allow more material exchange

Question 18

Why do companion cells have many mitochondria?

Answer 18

To fuel the active transport of materials between the source and sink and the sieve tube

Question 19

What do companion cells have to move materials in and out of the sieve tube?

Answer 19

Transport proteins

Question 20

If the sieve element did not have the support of a companion cell what would it not be able to do and why?

Answer 20

Sustain independent metabolic activity because sieve elements have no nuclei and fewer organelles

Question 21

What exists between sieve elements and companion cells in relatively large numbers?

Answer 21

Plasmodesmata

Question 22

What does plasmodesmata do?

Answer 22

Connect the cytoplasm of companion and sieve element cells and mediate the symplastic exchange of metabolites

Question 23

What does the phloem transport?

Answer 23

Transports food and water

Question 24

How many ways is the movement in phloem?

Answer 24

Two ways

Question 25

What do connecting sieve cells share?

Answer 25

A highly perforated sieve plate

Question 26

What mediates the movement of sap?

Answer 26

Hydrostatic pressure from the xylem

Question 27

How can you usually differentiate xylem and phloem cells in a microscopic image?

Answer 27

By the diameter of their cavity

Question 28

Does the xylem or the phloem have a larger cavity?

Answer 28

The xylem

Question 29

What are xylem and phloem grouped into?

Answer 29

Bundles that extend from the roots to the shoots in vascular plants

Question 30

Where can you find differences in between plant types?

Answer 30

Distribution and arrangement

Question 31

What are two different plant types?

Answer 31

Monocotyledones and Dicotyledons

Question 32

In monocotyledons roots how does the stele present itself?

Answer 32

The stele is large and vessels will form a radiating circle around the central pith

Question 33

When talking about the vessels surrounding the stele in roots, where are xylem vessels located?

Answer 33

More internally

Question 34

When talking about the vessels surrounding the stele in roots, where are phloem vessels located?

Answer 34

More externally

Question 35

In dicotyledons roots how does the stele present itself?

Answer 35

The stele is very small and the xylem is located centrally with the phloem surrounding it

Question 36

In dicotyledons roots how do xylem vessels form?

Answer 36

They form a cross shape

Question 37

In dicotyledons roots where are phloem situated?

Answer 37

In the surrounding gaps

Question 38

In monocotyledons stems, where are the vascular bundles found?

Answer 38

In a scattered arrangement throughout the stem

Question 39

In monocotyledons stems, where are the phloem vessels positioned?

Answer 39

Positioned externally towards the outside of the stem

Question 40

In dicotyledons stems, how are vascular bundles arranged?

Answer 40

In a circle around the centre of the stem

Question 41

In dicotyledon stems what separates phloem and xylem vessels and in which order are they separated?

Answer 41

The cambium Xylem on the inside and phloem on the outside

Question 42

How does the organic compounds produced at the source loaded into phloem sieve tubes?

Answer 42

Actively by companion cells

Question 43

How can materials pass into the sieve tube?

Answer 43

Via interconnecting plasmodesmata

Question 44

How can materials travel across the intervening cell wall?

Answer 44

They can be pumped by membrane proteins

Question 45

What describes the transport of apoplastic loading of sucrose into the phloem sieve tubes?

Answer 45

Active transport that requires ATP expenditure

Question 46

What are actively transported out of phloem cells and how?

Answer 46

Hydrogen ions are actively transported out of phloem cells by proton pumps

Question 47

How is a proton gradient created?

Answer 47

The concentration of hydrogen ions consequently builds up outside of the cell

Question 48

How do hydrogen ions diffuse back into the phloem cell?

Answer 48

Via a co transport protein which requires sucrose movement

Question 49

What does the diffusion of hydrogen ions back into the cell result in?

Answer 49

A build up of sucrose within the phloem sieve tube for subsequent transport from the source

Question 50

What makes the sap solution at the source to become hypertonic?

Answer 50

The active transport of solutes into the phloem by companion cells

Question 51

What does hypertonic sap solution at the source result in?

Answer 51

Water to be drawn from the xylem via osmosis

Question 52

Why does hydrostatic pressure at the source increase?

Answer 52

The incompressibility of the water that has built up in the phloem

Question 53

What does the increase in hydrostatic pressure force?

Answer 53

It forces the phloem sap to move towards areas of lower pressure

Question 54

What does the phloem sap moving towards areas of lower pressure cause?

Answer 54

Phloem transports solutes away from the source and to the sink

Question 55

What happens to the solutes that are unloaded from the phloem?

Answer 55

They are transported into sinks

Question 56

What do companion cells unload from the phloem?

Answer 56

Solutes

Question 57

What is the sap solution at the sink?

Answer 57

Hypotonic

Question 58

What does the sap solution being hypotonic then cause?

Answer 58

Water being drawn out of the phloem and back into the xylem via osmosis

Question 59

What does water being drawn out of the phloem and back into the xylem via osmosis ensure?

Answer 59

That the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source

Question 60

What does a low hydrostatic pressure at the sink mean?

Answer 60

Phloem sap will always move from the source towards the sink

Question 61

What are organic molecules when they are transported into the sink?

Answer 61

Either metabolised or stored with the tonoplast of vacuoles

Question 62

What are aphids?

Answer 62

A group of insects which feed mainly on sap from phloem

Question 63

What is a stylet?

Answer 63

A mouthpiece of aphids which pierces the plants sieve tube to allow sap to be extracted

Question 64

What aids the penetration of the stylet into the sieve tube?

Answer 64

Digestive enzymes that soften the intervening tissue layers

Question 65

What happens if the stylet is severed and why?

Answer 65

Sap will continue to flow from the plant due to the hydrostatic pressure within the sieve tube

Question 66

Why are aphids good for providing a measure of phloem transport rates?

Answer 66

Because aphids collect sap at various sites along a plants length

Question 67

When measuring phloem transport using aphids, where is the plant grown?

Answer 67

Within a lab with the leaves sealed within a glass chamber containing radioactively labelled CO2

Question 68

When measuring phloem transport using aphids, what will the leaves convert the CO2 into?

Answer 68

Into radioactively labelled sugars which are transported by the phloem

Question 69

When measuring phloem transport using aphids, where are the aphids placed and then what happens?

Answer 69

Along the plants length and are then encouraged to feed on the phloem sap

Question 70

When measuring phloem transport using aphids, what happens once the feeding has commenced?

Answer 70

The aphid stylet is severed and sap continues to flow from the plant at the selected positions

Question 71

When measuring phloem transport using aphids, why is the sap analysed?

Answer 71

For the presence of radioactively labelled sugars

Question 72

When measuring phloem transport using aphids, how can it be calculated?

Answer 72

Based on the time taken for the radioisotope to be detected at different positions along the plants length

Question 73

What is phloem transport also known as?

Answer 73

Translocation rate

Question 74

What determines the translocation rate?

Answer 74

The concentration of dissolved sugars in the phloem

Question 75

What affects the concentration of dissolved sugars in the phloem sap?

Answer 75

The rate of photosynthesis The rate of cellular respiration The rate of transpiration The diameter of the sieve tubes

Question 76

What affects the rate of photosynthesis?

Answer 76

Light intensity CO2 concentration Temperature

Question 77

What affects the rate of cellular respiration?

Answer 77

Any factor which physically stresses the plant

Question 78

What does the rate of transpiration determine?

Answer 78

How much water enters the phloem

Question 79

What does the diameter of sieve tubes affect?

Answer 79

Hydrostatic pressure but it may differ between plant species