Phloem 9.2

What do you remember about Xylem?

Multiple Choice

Which of the following are responsible for transpiration?
I. adhesion
II. cohesion
III. evaporation
IV. active transport

• I and II only

• I, II and III only

• I, and III only

• I, II, III and IV

Multiple Choice

Which factor will decrease the rate of transpiration?

• Wind
• Temperature
• Light intensity
• Humidity

Multiple Choice

Which cells are found in xylem?

• Tracheids

• Sieve Tubes

• Companion cells

• Cambium cells

Multiple Choice

What is a halophyte?

• Aquatic Plant

• Heat tolerant Plant

• Salt tolerant plant

• Cold tolerant plant

Multiple Choice

What BEST defines adhesion.

• Water molecules bonding to each other due to the hydrophobic nature of water.

• Water molecules bonded to other other surfaces due to the transparency of water.

• Water molecules sticking to each other due to the polarity of water.

• Water molecules sticking to other surfaces due to the polarity of water.

Multiple Choice

Which of the following is an adaptation of a xerophyte?

• reduced roots

• hairy stomata

• many stomata

• thin cuticle

Multiple Choice

Why do plants need to open stomata?
I. To get rid of excess CO₂

II. To allow transpiration
III. To exchange gases with the  environment

• I only
• I and II only
• II and III only
• I, II and III

Multiple Choice

Plant absorbs water through the _________. 

• stem

• roots 

• leaves 

• all of the above

Multiple Choice

What is the function of root hair?

• it is a place to store the food and gas

• it increases surface area to absorb more water

Multiple Choice

Which of these is TRUE?

• Xylem carries water and dissolved minerals up the plant

• Phloem carries water and dissolved minerals up the plant

• Stomata carry water and dissolved minerals up the plant

Multiple Choice

Why is transpiration an inevitable consequence of gas exchange in the leaf?

• Water creates a hypertonic environment in the leaf, but hypotonic environment everywhere else in the plant

• When stomata open, only molecules containing oxygen may enter or leave the leaf

• When stomata open for carbon dioxide absorption, water vapor leaves simultaneously

• When stomata open for oxygen absorption, water vapor leaves simultaneously

Multiple Choice

When photosynthesis takes place during the day, stomata will...

• open

• close

Multiple Choice

What is the name of the part of the root that allows water and minerals to enter?

• epidermis

• root hairs

• phloem

• phyla

Multiple Choice

Water and minerals move in which direction within the plant

• Upward

• Down

Multiple Choice

What part of the leaf prevents water loss?

• Spongy mesophyll

• Palisade mesophyll

• Waxy cuticle

• Xylem

Multiple Choice

Within a leaf, chloroplasts are found in abundance within the

• Waxy cuticle

• Guard cells

• Xylem tissue

• Palisade mesophyll

Multiple Choice

Which of the following is not an adaptation of a xerophyte?

• Reduced leaves

• Rolled leaves

• Stomata in pits

• Salt excretion

Multiple Choice

Within a stem, the vascular bundle is made up of

• pith and cortex

• guard cells

• xylem and phloem

• mesophyll cells

Multiple Choice

Guard cells are responsible for

• protecting the roots from water loss

• producing glucose for plant growth and metabolism

• controlling the opening and closing of the stomata

• producing xylem

Structure and function are correlated in the phloem of plants

​PHLOEM

Phloem Structure

Structure-function relationships of phloem sieve tubes

Companion Cell

Sieve Cell

Phloem comprise of sieve elements and companion cells

• Sieve elements

• have no nuclei (to maximise space) and have thick and rigid cell walls (to withstand pressure)

• connect to form a tube with porous plates at their transverse ends (allows material flow)

• Companion cells

possess a highly folded membrane to maximize SA:Vol ratio (more material exchange)

Active transport is used to load organic compounds into phloem sieve tubes at the source

Companion cells support phloem transport by:

•Providing metabolic support for sieve elements

•Facilitate loading and unloading at source (leaves) and sink (roots)

•Via interconnecting plasmodesmata (symplastic)

•By actively pumping materials from within the cell wall space of the companion cells (apoplastic)

Companion Cells

Companion cells move materials in two ways:

Phloem versus Xylem

Transports organic

molecules

End walls (sieve plates)

exist between cells

Two-way movement

(bidirectional)

PHLOEM

Cells are living but need
support (companion cells)

Transports water

and minerals

No end walls between cells

(continuous tube)

One-way movement

(unidirectional)

Outer cells are not living
(xylem vessel is hollow)

XYLEM

Multiple Select

The main types of phloem cells are listed below. Check the boxes that apply.

• sieve tube elements

• tracheids

• companion cells

• phloem parenchyma

• phloem fibres

Multiple Select

The organelles not present in the sieve tube elements - check all that apply.

• mitochondria

• nucleus

• Golgi body

• ribosomes

• endoplasmic reticulum

Active Translocation

Plants transport organic compounds from sources to sinks

Translocation is the movement of organic compounds (e.g. sugars, amino acids) also called assimilates from sources to sinks

• The source is where the organic compounds are synthesized – this is the photosynthetic tissues (leaves)

• The sink is where the compounds are delivered for use or storage – this includes roots, fruits, and seeds

Organic compounds are transported from sources to sinks via a vascular tube system called the phloem

• Sugars are principally transported as sucrose (disaccharide), because it is soluble but metabolically inert

• The nutrient-rich, viscous fluid of the phloem is called plant sap

Source

Sink

PHLOEM

Source: Phloem Loading

High concentrations of solutes in the phloem at the source lead to water uptake by osmosis

Apoplastic loading is an active process that occurs

against a concentration gradient (and needs ATP)

•Protons are pumped out of phloem cells

•They passively return via a co-transport protein

which facilitates the joint movement of solutes

The build up of solutes in the phloem creates a

hypertonic solution that draws water via osmosis

H+

H+

S

S

ATP

H+

APOPLAST

PHLOEM

Active loading creates high
concentrations of solutes in
the phloem (hypertonicity)

Multiple Choice

Identify the source

• Potato tuber

• Fruit

• Leaf

• Seed

Multiple Choice

Identify the process used to load organic compounds into phloem sieve tubes at the source.

• Endocytosis

• Facilitated diffusion

• Osmosis

• Active transport

Phloem: Mass Flow

Incompressibility of water allows transport along hydrostatic pressure gradients

High concentrations of solute in the phloem draws water from the xylem (osmosis)

•The incompressibility of water causes the sap volume and pressure to increase

•High sap pressure at the source causes sap to move to the sink via mass flow

SOURCE

SINK

Xylem

Phloem

Multiple Choice

An increase in solute concentration in a phloem sieve tube causes water to be drawn in by what process?

• Osmosis

• Endocytosis

• Facilitated diffusion

• Simple diffusion

Multiple Choice

The active transport of solutes (such as sucrose) into the phloem by companion cells in the source makes the sap solution which of the following?

• Isotonic

• Hypotonic

• Hypertonic

• Dilute

Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks

Sink: Phloem Unloading

Organic molecules are actively unloaded at sinks

•This lowers solute concentration in the phloem,

causing water to return to the xylem (osmosis)

•The drop in sap pressure maintains a pressure

gradient relative to the source (allows mass flow)

Direction of mass flow can be reversed according to

need (e.g. growing regions need nutrients from sinks)

Solutes
+ ATP

Metabolites

ATP

ATP

Storage

Metabolism

Summary: Phloem Transport

Translocation of sucrose

Transpiration of water

Source
(leaf cell)

Sink

(root cell)

Companion Cells

Active Translocation:

• Source produces

organic nutrients

• Loaded to phloem

• Water from xylem

raises sap pressure

• Mass flow (phloem)

• Unloaded at sink

Multiple Choice

Identify the source

• Potato tuber

• Fruit

• Leaf

• Seed

Multiple Choice

Sugars are principally transported in what form?

• Glucose

• Fructose

• Sucrose

• Starch

Translocation Rate

Analysis of data from experiments measuring phloem transport using aphid stylets and radioactive CO2

Translocation rates can be measured using

aphid stylets and radioactive carbon dioxide

• Aphids are insects that feed on sap in

phloem via a stylet (protruding mouthpiece)

• Plants incorporate radioactive CO2 to form

radioactively labeled sugars within the sap

• Severing a stylet allows sap to be collected

Aphid feeding on sap via stylet

Measuring Translocation

The rate of translocation can be measured according to the time taken for the

radioactively labeled sugars to be detected at various points along the phloem

Starter
colony

Colony

#1

Colony

#2

Colony

#3

Glass chamber containing radioisotope (14CO2)

Distance (cm)

0

20

40

60

Time (minutes)

0

85

170

220

Analysis of data from experiments measuring phloem transport using aphid stylets and radioactive CO2