Bio402 Phloem slides

What is the main function of the phloem?

To conduct food (photosynthates) such as sugars, amino acids, lipids, and signaling molecules throughout the plant

What are the two broad categories of vascular conducting tissues?

Xylem (water/minerals) and Phloem (food/sugars).

What does “Sesuvium – Go with the Flow-em” refer to in your slides?

It’s a mnemonic that emphasizes that phloem transports sugars passively along the pressure flow — sugars “go with the flow”

Phloem is considered what kind of tissue?

A complex tissue (contains multiple cell types: sieve elements, companion/albuminous cells, parenchyma, and sclerenchyma)

What are sieve elements?

Specialized conducting cells of the phloem that transport sugars and signaling molecules; they remain alive at maturity but lose the nucleus and vacuole

What type of sieve elements exist in gymnosperms?

Sieve cells

What type of sieve elements exist in angiosperms?

Sieve tube elements

Describe the shape and arrangement of sieve cells.

Long, narrow cells with tapering, overlapping end walls

What are sieve areas, and where are they located in sieve cells?

Regions of clustered pores located along lateral and end walls allowing cytoplasmic continuity between cells

Do sieve cells have distinct end walls like sieve tube elements?

No — their lateral and end walls are indistinguishable

What is the approximate diameter of pores in sieve areas?

About 0.3–2 µm, much larger than plasmodesmata (~0.02 µm)

What type of associated cell do sieve cells have?

Albuminous cells — non-sister cells that maintain metabolism for sieve cells

In what plant groups do sieve cells occur?

Non-flowering plants, especially gymnosperms and ferns

How do sieve tube elements differ from sieve cells?

They are shorter and wider, arranged end-to-end to form sieve tubes with continuous cytoplasm

What distinguishes the end walls of sieve tube elements?

The end walls are distinct from lateral walls and contain sieve plates

How large are pores in sieve plates?

About 15 µm — much larger than those in sieve cell

What are the two types of sieve plates?

Simple (one large sieve area) and compound (two or more sieve areas)

What structural features allow sieve tubes to remain open for flow?

They have little or no secondary wall, and their protoplasts maintain a parietal position with limited organelles

What are companion cells?

Specialized living cells closely associated with sieve tube elements that maintain the sieve element’s metabolism

How do companion cells and sieve tube elements form?

They develop from the same mother cell, forming a metabolic unit connected by plasmodesmata

What is the role of companion cells in phloem loading?

They actively load sugars and signaling molecules into sieve tube elements and help maintain the pressure flow

What are albuminous cells, and what do they do?

Metabolically active cells in gymnosperms that function like companion cells but are not derived from the same mother cell as the sieve cell

How are albuminous cells connected to sieve cells?

By numerous plasmodesmata that allow transfer of nutrients and signals

What are phloem parenchyma cells?

Living parenchyma cells that store nutrients and may contain crystals or other inclusions

What types of substances are stored in phloem parenchyma?

Sugars, amino acids, and sometimes calcium oxalate crystals

What two types of sclerenchyma occur in phloem?

Phloem fibers and phloem sclereids

What is the function of phloem fibers?

Provide mechanical strength and sometimes persist after the phloem elements

What are phloem sclereids?

Heavily lignified support cells that reinforce mature phloem tissue

Are sieve elements living or dead at maturity?

Living, but they lose the nucleus and vacuole; only essential organelles remain

Which organelles remain functional in mature sieve elements?

Plasma membrane, ER, mitochondria, and plastids (in peripheral position)

What happens to the nucleus and vacuole in developing sieve elements?

They undergo selective autophagy and disintegrate

What is the term for the area of large pores at the end walls of sieve tube elements?

Sieve plates

What material plugs sieve plates when phloem is damaged?

P-protein (phloem protein) forms a slimy plug to prevent sap loss

What is the overall mechanism of phloem transport?

Osmotically generated pressure flow from sources (leaves) to sinks (roots, fruits, young tissues)

How is turgor pressure involved in phloem transport?

Water enters phloem from xylem at the source, generating turgor pressure that pushes sugars toward sinks

What happens at the sink end of phloem transport?

Sugars are unloaded, water exits back to xylem, and pressure decreases

What is phloem loading?

The process of transferring sugars into sieve tube elements from photosynthetic tissues

What are the two main types of phloem loading?

Type I (open/symplastic) and Type II (closed/apoplastic)

Describe Type I (open/symplastic) loading.

High density of plasmodesmata; sugars enter companion cells and are polymerized (“polymer trap” mechanism)

Describe Type II (closed/apoplastic) loading.

Few plasmodesmata; sucrose diffuses into the apoplast and enters sieve tubes by active transport via sucrose–proton co-transporters

What are the two types of primary phloem?

Protophloem and metaphloem

Where and when does protophloem mature?

In actively elongating regions of stems and roots

What happens to protophloem sieve tubes during elongation?

They often become stretched and crushed, ceasing function

What remains after protophloem crushes in some plants?

Protophloem fibers that provide temporary support

Where does metaphloem develop, and what is its structure?

Develops after elongation stops; consists of wider and shorter sieve elements that remain functional longer

What happens to metaphloem during secondary growth?

It may be crushed as new phloem is produced by the vascular cambium

Which vascular tissue conducts sugars and signaling molecules?

Phloem (food-conducting vascular tissue)

Which vascular tissue originates from the procambium (primary) and vascular cambium (secondary)?

Both xylem and phloem, but specifically, primary phloem = procambium, secondary phloem = vascular cambium

Which phloem type is axial only, and which includes both axial and radial systems?

Primary phloem = axial only; secondary phloem = axial + radial

Sieve Cells → _______

Gymnosperms

Sieve Tube Elements → _______

Angiosperms

Which phloem element has sieve plates?

Sieve tube elements

Which phloem element has sieve areas but no sieve plates?

Sieve cells

Which are long, narrow, and tapering with overlapping end walls?

Sieve cells

Which are shorter, wider, and stacked end-to-end?

Sieve tube elements

Which conducting element has pores ~15 µm and simple or compound sieve plates?

Sieve tube elements

Which has pores 0.3–2 µm and occurs along both lateral and end walls?

Sieve cells

Which type of phloem cell remains alive at maturity but lacks a nucleus?

Sieve elements (both sieve cells and sieve tube elements)

Which cells are metabolically connected to sieve tube elements via plasmodesmata and control their metabolism?

Companion cells

Which type of associated cell develops from the same mother cell as the sieve tube element?

Companion cell

Which associated cell performs a similar role in gymnosperms but is not derived from the same mother cell?

Albuminous cell

Which associated cell type pairs with sieve cells?

Albuminous Cells

Which associated cell type pairs with sieve tube elements?

Companion Cells

Which phloem cell type stores starch, sugars, and sometimes crystals?

Phloem parenchyma

Which phloem cells are alive at maturity and help in lateral transport?

Phloem parenchyma

Which phloem cells provide mechanical support and are dead at maturity?

Phloem fibers

Which phloem cells are irregularly shaped and heavily lignified for additional reinforcement?

Phloem sclereids

Which phloem cell types are collectively considered sclerenchyma?

Phloem fibers and phloem sclereids

During sieve element development, what happens to the nucleus and vacuole?

They break down via selective autophagy

Which organelles remain in mature sieve elements?

Plasma membrane, ER, mitochondria, and plastids remain along the cell periphery

What substance quickly seals off sieve plates when phloem is damaged?

P-protein (Phloem Protein) forms a slime plug

Where are sieve plates located?

At the end walls of sieve tube elements

What forms the sieve plate pores?

Enlargement of plasmodesmata by callose deposition and ER breakdown during development

What drives the movement of sugars through the phloem?

Osmotically generated pressure flow

What are the source and sink in phloem transport?

Source: Photosynthetic tissues (leaves); Sink: Non-photosynthetic tissues (roots, fruits, developing organs)

How is turgor pressure involved in sugar movement?

Water enters phloem at the source → creates positive pressure → pushes sap toward sinks

Where does water return after unloading sugars at the sink?

It moves back to the xylem

What phrase summarizes the flow direction of sugars in phloem?

“Sugars go with the Flow-em”

What is phloem loading?

The process of moving sugars from photosynthetic cells into sieve tube elements

What are the two types of phloem loading?

Type I (symplastic/open) and Type II (apoplastic/closed)

What characterizes Type I (symplastic/open) loading?

Many plasmodesmata between companion cell and sieve tube; uses polymer trap mechanism

What characterizes Type II (apoplastic/closed) loading?

Few plasmodesmata; uses active transport (sucrose–proton co-transporters)

Which phloem loading type requires ATP-dependent transport across the plasma membrane?

Type II (apoplastic)

What are the two developmental stages of primary phloem?

Protophloem and Metaphloem

Where does protophloem mature, and what happens to it?

In elongating regions; sieve tubes become stretched and crushed as the organ grows

What remains after protophloem elements collapse?

Fibers or thick-walled collenchyma-like cells in some eudicot leaves

Where does metaphloem mature?

In regions that have stopped elongating

How does metaphloem differ from protophloem in structure?

It is wider, shorter, and remains functional longer

Which phloem cell type is unique to angiosperms?

Companion cells and sieve tube elements

Which phloem cell type is unique to gymnosperms?

Sieve cells and albuminous cells

Which phloem cells are alive at maturity?

Sieve elements, companion cells, albuminous cells, phloem parenchyma

Which phloem cells are dead at maturity?

Phloem fibers and sclereids

Which phloem cell type stores sugars and sometimes crystals?

Phloem parenchyma

Which phloem component provides structural strength?

Phloem fibers and sclereids

Which two features are found only in sieve tube elements?

Sieve plates and P-protein

Which two features are found only in sieve cells?

Sieve areas only and association with albuminous cells

Which cell type loads sucrose in the symplastic route?

Companion cells using the polymer trap mechanism

Which cell type loads sucrose in the apoplastic route?

Companion cells via sucrose–proton co-transporters