Plants have developed transport systems to move nutrients, water, and minerals throughout their multicellular structures. Water and minerals are absorbed by root hairs and enter the xylem tissue in roots. They then travel upwards through the xylem vessels of stems towards leaves. Leaves release water to the atmosphere through transpiration, creating tension that pulls water up from the roots. Transport systems allow for the distribution of necessities to all plant cells.
Translocation of food in plants
1. Source and sink
2. Pathway of translocation
3. Source-sink relationship/interaction
4. Source-sink pathways follow patterns
5. Materials transported
6. The mechanism of phloem transport
7. The Pressure -Flow Model
8. Phloem loading and unloading
9. Summary
Translocation of food in plants
1. Source and sink
2. Pathway of translocation
3. Source-sink relationship/interaction
4. Source-sink pathways follow patterns
5. Materials transported
6. The mechanism of phloem transport
7. The Pressure -Flow Model
8. Phloem loading and unloading
9. Summary
• PRIMARY PIT FIELD
• PITS
• STRUCTURE OF PITS
• TYPES OF PITS
• COMBINATION IN PITS
• STRUCTURE OF BORDERED PITS
• COMBINATION IN BORDERED PITS
• PLASMODESMATA
• STRUCTURE OF PLASMODESMATA
• CLASSIFICATION OF PLASMODESMATA
• FUNCTION OF PLASMODESMATA
Biology GCE O level syllabus: Transport system in Plants
Include: Xylem, Phloem, Entry of water into plant and so forth...
NOTE: PLEASE DOWNLOAD BECAUSE THERE ARE MANY ANIMATIONS THAT HIDE SOME OF THE CONTENTS
• PRIMARY PIT FIELD
• PITS
• STRUCTURE OF PITS
• TYPES OF PITS
• COMBINATION IN PITS
• STRUCTURE OF BORDERED PITS
• COMBINATION IN BORDERED PITS
• PLASMODESMATA
• STRUCTURE OF PLASMODESMATA
• CLASSIFICATION OF PLASMODESMATA
• FUNCTION OF PLASMODESMATA
Biology GCE O level syllabus: Transport system in Plants
Include: Xylem, Phloem, Entry of water into plant and so forth...
NOTE: PLEASE DOWNLOAD BECAUSE THERE ARE MANY ANIMATIONS THAT HIDE SOME OF THE CONTENTS
Part I
Explain the need for transport systems in multicellular plants
Describe the distribution of xylem and phloem tissue in roots, stems and leaves
Explain the absorption process in roots
Describe transport mechanisms
Part II
List factors that affects rate transpiration
Describe xerophyte properties
List the series of events that leads to translocation
The development of Vascular plant allows the kingdom of plant to not only spread but conquer the world. The fascinating efficiency of the plant transport system is one that should be a joy for anyone to study,
Hello readers,
This PPT is about the chapter:- Tissue which is in science class IX
Question Are In The Book Of NCERT
I Hope this will help You...
Thanks....
Based on ncert and will be suitable for notes. Covers all the domains. In this PPT nutrition is covered and it also describes the structure of chloroplast which is not described in ncert. All the questions whether objective or descriptive are covered. Might be appear lengthy but is suitable for notes.
All the best 👍
Only green plants possess the amazing ability of trapping solar energy to produce food.
Photosynthesis is the process in which green plants absorb solar energy to make food from carbon dioxide and water.
More for teachers who do not have much science background than for students. Discusses the ideas of cycles and systems and goes into some detail about some representative sample cycles.
This could be followed by the water cycle slide show:
http://www.slideshare.net/MMoiraWhitehouse/teach-water-cycle-copy
Essentials of Automations: Optimizing FME Workflows with ParametersSafe Software
Are you looking to streamline your workflows and boost your projects’ efficiency? Do you find yourself searching for ways to add flexibility and control over your FME workflows? If so, you’re in the right place.
Join us for an insightful dive into the world of FME parameters, a critical element in optimizing workflow efficiency. This webinar marks the beginning of our three-part “Essentials of Automation” series. This first webinar is designed to equip you with the knowledge and skills to utilize parameters effectively: enhancing the flexibility, maintainability, and user control of your FME projects.
Here’s what you’ll gain:
- Essentials of FME Parameters: Understand the pivotal role of parameters, including Reader/Writer, Transformer, User, and FME Flow categories. Discover how they are the key to unlocking automation and optimization within your workflows.
- Practical Applications in FME Form: Delve into key user parameter types including choice, connections, and file URLs. Allow users to control how a workflow runs, making your workflows more reusable. Learn to import values and deliver the best user experience for your workflows while enhancing accuracy.
- Optimization Strategies in FME Flow: Explore the creation and strategic deployment of parameters in FME Flow, including the use of deployment and geometry parameters, to maximize workflow efficiency.
- Pro Tips for Success: Gain insights on parameterizing connections and leveraging new features like Conditional Visibility for clarity and simplicity.
We’ll wrap up with a glimpse into future webinars, followed by a Q&A session to address your specific questions surrounding this topic.
Don’t miss this opportunity to elevate your FME expertise and drive your projects to new heights of efficiency.
GDG Cloud Southlake #33: Boule & Rebala: Effective AppSec in SDLC using Deplo...James Anderson
Effective Application Security in Software Delivery lifecycle using Deployment Firewall and DBOM
The modern software delivery process (or the CI/CD process) includes many tools, distributed teams, open-source code, and cloud platforms. Constant focus on speed to release software to market, along with the traditional slow and manual security checks has caused gaps in continuous security as an important piece in the software supply chain. Today organizations feel more susceptible to external and internal cyber threats due to the vast attack surface in their applications supply chain and the lack of end-to-end governance and risk management.
The software team must secure its software delivery process to avoid vulnerability and security breaches. This needs to be achieved with existing tool chains and without extensive rework of the delivery processes. This talk will present strategies and techniques for providing visibility into the true risk of the existing vulnerabilities, preventing the introduction of security issues in the software, resolving vulnerabilities in production environments quickly, and capturing the deployment bill of materials (DBOM).
Speakers:
Bob Boule
Robert Boule is a technology enthusiast with PASSION for technology and making things work along with a knack for helping others understand how things work. He comes with around 20 years of solution engineering experience in application security, software continuous delivery, and SaaS platforms. He is known for his dynamic presentations in CI/CD and application security integrated in software delivery lifecycle.
Gopinath Rebala
Gopinath Rebala is the CTO of OpsMx, where he has overall responsibility for the machine learning and data processing architectures for Secure Software Delivery. Gopi also has a strong connection with our customers, leading design and architecture for strategic implementations. Gopi is a frequent speaker and well-known leader in continuous delivery and integrating security into software delivery.
The Art of the Pitch: WordPress Relationships and SalesLaura Byrne
Clients don’t know what they don’t know. What web solutions are right for them? How does WordPress come into the picture? How do you make sure you understand scope and timeline? What do you do if sometime changes?
All these questions and more will be explored as we talk about matching clients’ needs with what your agency offers without pulling teeth or pulling your hair out. Practical tips, and strategies for successful relationship building that leads to closing the deal.
Connector Corner: Automate dynamic content and events by pushing a buttonDianaGray10
Here is something new! In our next Connector Corner webinar, we will demonstrate how you can use a single workflow to:
Create a campaign using Mailchimp with merge tags/fields
Send an interactive Slack channel message (using buttons)
Have the message received by managers and peers along with a test email for review
But there’s more:
In a second workflow supporting the same use case, you’ll see:
Your campaign sent to target colleagues for approval
If the “Approve” button is clicked, a Jira/Zendesk ticket is created for the marketing design team
But—if the “Reject” button is pushed, colleagues will be alerted via Slack message
Join us to learn more about this new, human-in-the-loop capability, brought to you by Integration Service connectors.
And...
Speakers:
Akshay Agnihotri, Product Manager
Charlie Greenberg, Host
Neuro-symbolic is not enough, we need neuro-*semantic*Frank van Harmelen
Neuro-symbolic (NeSy) AI is on the rise. However, simply machine learning on just any symbolic structure is not sufficient to really harvest the gains of NeSy. These will only be gained when the symbolic structures have an actual semantics. I give an operational definition of semantics as “predictable inference”.
All of this illustrated with link prediction over knowledge graphs, but the argument is general.
LF Energy Webinar: Electrical Grid Modelling and Simulation Through PowSyBl -...DanBrown980551
Do you want to learn how to model and simulate an electrical network from scratch in under an hour?
Then welcome to this PowSyBl workshop, hosted by Rte, the French Transmission System Operator (TSO)!
During the webinar, you will discover the PowSyBl ecosystem as well as handle and study an electrical network through an interactive Python notebook.
PowSyBl is an open source project hosted by LF Energy, which offers a comprehensive set of features for electrical grid modelling and simulation. Among other advanced features, PowSyBl provides:
- A fully editable and extendable library for grid component modelling;
- Visualization tools to display your network;
- Grid simulation tools, such as power flows, security analyses (with or without remedial actions) and sensitivity analyses;
The framework is mostly written in Java, with a Python binding so that Python developers can access PowSyBl functionalities as well.
What you will learn during the webinar:
- For beginners: discover PowSyBl's functionalities through a quick general presentation and the notebook, without needing any expert coding skills;
- For advanced developers: master the skills to efficiently apply PowSyBl functionalities to your real-world scenarios.
Search and Society: Reimagining Information Access for Radical FuturesBhaskar Mitra
The field of Information retrieval (IR) is currently undergoing a transformative shift, at least partly due to the emerging applications of generative AI to information access. In this talk, we will deliberate on the sociotechnical implications of generative AI for information access. We will argue that there is both a critical necessity and an exciting opportunity for the IR community to re-center our research agendas on societal needs while dismantling the artificial separation between the work on fairness, accountability, transparency, and ethics in IR and the rest of IR research. Instead of adopting a reactionary strategy of trying to mitigate potential social harms from emerging technologies, the community should aim to proactively set the research agenda for the kinds of systems we should build inspired by diverse explicitly stated sociotechnical imaginaries. The sociotechnical imaginaries that underpin the design and development of information access technologies needs to be explicitly articulated, and we need to develop theories of change in context of these diverse perspectives. Our guiding future imaginaries must be informed by other academic fields, such as democratic theory and critical theory, and should be co-developed with social science scholars, legal scholars, civil rights and social justice activists, and artists, among others.
Epistemic Interaction - tuning interfaces to provide information for AI supportAlan Dix
Paper presented at SYNERGY workshop at AVI 2024, Genoa, Italy. 3rd June 2024
https://alandix.com/academic/papers/synergy2024-epistemic/
As machine learning integrates deeper into human-computer interactions, the concept of epistemic interaction emerges, aiming to refine these interactions to enhance system adaptability. This approach encourages minor, intentional adjustments in user behaviour to enrich the data available for system learning. This paper introduces epistemic interaction within the context of human-system communication, illustrating how deliberate interaction design can improve system understanding and adaptation. Through concrete examples, we demonstrate the potential of epistemic interaction to significantly advance human-computer interaction by leveraging intuitive human communication strategies to inform system design and functionality, offering a novel pathway for enriching user-system engagements.
GraphRAG is All You need? LLM & Knowledge GraphGuy Korland
Guy Korland, CEO and Co-founder of FalkorDB, will review two articles on the integration of language models with knowledge graphs.
1. Unifying Large Language Models and Knowledge Graphs: A Roadmap.
https://arxiv.org/abs/2306.08302
2. Microsoft Research's GraphRAG paper and a review paper on various uses of knowledge graphs:
https://www.microsoft.com/en-us/research/blog/graphrag-unlocking-llm-discovery-on-narrative-private-data/
Dev Dives: Train smarter, not harder – active learning and UiPath LLMs for do...UiPathCommunity
💥 Speed, accuracy, and scaling – discover the superpowers of GenAI in action with UiPath Document Understanding and Communications Mining™:
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Learn about the latest enhancements to out-of-the-box document processing – with little to no training required
Get an exclusive demo of the new family of UiPath LLMs – GenAI models specialized for processing different types of documents and messages
This is a hands-on session specifically designed for automation developers and AI enthusiasts seeking to enhance their knowledge in leveraging the latest intelligent document processing capabilities offered by UiPath.
Speakers:
👨🏫 Andras Palfi, Senior Product Manager, UiPath
👩🏫 Lenka Dulovicova, Product Program Manager, UiPath
2. Why is a transport system needed?
• Plant cells need a regular supply of nutrients
and raw materials just like animal cells.
• They however need these at a lower rate and
their needs also differ.
• Multicellular plants have a small surface area
to volume ratio.
• Consequently, a transport system is needed to
supply some materials to them.
3. Materials required by the plant
1. Oxygen – all the cells of a plant need oxygen
for respiration. Actively photosynthesizing
cells however produce more oxygen than they
need. Other cells take in oxygen from the
environment. Their rate of respiration is
however much less than that of mammals and
so do not need a rapid supply.
4. Materials required by the plant cont'd
2. Carbon dioxide – photosynthetic cells require
carbon dioxide during daylight.
3. Organic nutrients – cells that do not make
their own food require organic nutrients
from photosynthetic cells.
4. Inorganic ions and water – all the cells of a
plant require inorganic ions and water.
These are transported from the soil by roots.
5. Other needs
• One of the greatest requirements of a plant is
sunlight.
• The leaves are adapted to ensure that the
cells receive maximum sunlight.
6. The nature of the transport system in
plants
• Plants have a much slower transport system
than mammals because their energy
requirements are much less than that of
mammals.
• Carbon dioxide and oxygen diffuse into and
out of a leaf easily and readily.
• Plants have two transport systems; one for
inorganic ions and water and another for
organic materials.
13. Transport of water and inorganic ions
• Water enters the plant through the root hairs
and moves across the root to the xylem tissue
in the centre.
• It then rises up to the leaves through the
stem.
14. Structure of xylem tissue
• Xylem has dual function of support and
transport. This is made possible because of
their structure.
• In angiosperms, xylem tissue contains:
1.Vessel elements
2.Tracheids
3.Fibres
4.Parenchyma cells
15. Xylem vessels
• Xylem vessels are made up of several vessel
elements arranged end to end.
• Vessel elements are elongated.
• Vessel elements were originally normal plant
cells in whose walls lignin was laid down.
• Lignin is impermeable to water, and as more is
laid down in the wall of a cell, the cell
eventually dies.
16. Xylem vessels cont'd
• The empty space that is left is called a lumen.
• Wherever plasmodesmata were in the original
cell wall, no lignin was laid there.
• The non-lignified areas form pits which are
gaps in the cell wall.
• The pits are crossed by permeable,
unthickened cellulose cell wall.
17. Xylem vessels cont'd
• The end walls separating neighbouring xylem
elements break down and a continuous tube
is formed.
• The long, non-living tube is a xylem vessel.
18. Tracheids
• These are also dead cells with lignified walls.
• Their ends are not open and so do not form
vessels.
• The ends taper off.
• They have pits and so they can help in the
transport of water.
• They are the main conducting tissue in
gymnosperms (primitive)
19. Fibres
• These cells are also elongated and dead.
• Their function is to support the plant.
21. Parenchyma cells
• These are like regular plant cells.
• They have unthickened cell walls and the
organelles that would be found in a plant cell.
• However, because they are not exposed to
light, they have no chloroplasts.
• They have a variety of shapes but are usually
isodiametric (same size in all directions).
22. Movement from soil to root hair
• Some of the epidermal cells just behind the
root tip are drawn out to form long extensions
called root hairs.
• These root hairs extend into spaces between
soil particles.
• There they absorb water.
• Many root hairs are formed and these provide
a large surface area in contact with soil water.
24. Movement from soil to root hair
cont'd
• This therefore increases the rate at which
water is absorbed.
• The root hairs are very delicate and have to be
replaced very often.
• Water moves down a water potential
gradient.
• This gradient is created by the presence of
inorganic ions and organic materials like
sugars and proteins in the cell sap of the root
hairs.
25. Movement from soil to root hair
cont'd
• This concentration of inorganic ions inside the
root hairs is greater than the concentration of
inorganic ions in the soil water.
• Mineral ions also move into the root hair cells.
• These ions can either move by facilitated
diffusion or active transport.
• If the concentration of a particular ion is
greater outside the root hair cell than inside
26. Movement from soil to root hair
cont'd
the root hair cell, then movement is passive
by facilitated diffusion.
• If the concentration is greater inside the cell
than outside, then movement is by active
transport.
• Usually, the ions needed by the plant are in
low concentrations in the soil.
• Most movement is therefore by active
transport.
27. Movement from soil to root hair
cont'd
• Root hairs are important for the absorption of
minerals.
• In some plants, especially trees, there is a
symbiotic relationship with fungi called
mycorrhizas.
• These serve a similar function to root hairs by
helping with the absorption of nutrients from
the soil and transporting them.
28. Movement from soil to root hair
cont'd
• Both water and minerals move together even
though different processes are employed to
move them.
• Mineral salts cannot be absorbed if water is
not present.
29. Movement from root hair to xylem
• Water moves across the cortex into the xylem
because the water potential in the xylem
vessels is lower than that of the cortical cells.
• The water takes two possible routes across
the cortex:
1.Apoplast pathway
2.Symplast pathway
30. Apoplast pathway
• The cell wall is made of fibres of cellulose
which form a network.
• The water can soak into the fibres as they
would soak into paper.
• Because of this , water can pass from cell wall
to cell wall without entering the cytoplasm of
cortical cells.
• Movement may be through the intercellular
spaces.
31. Symplast pathway
• The water moves into the cytoplasm or
vacuole of the cortical cells.
• It then passes into adjacent cells through the
plasmodesmata.
33. Movement from root hair to xylem
cont'd
• The cells in the outer layer of the stele, the
endodermis, have suberin in their cell walls.
• This suberin forms the Casparian strip and
makes the endodermis impenetrable to water.
• The suberin is laid down as the plant gets
older.
• Some cells do not have suberin deposits.
These are called passage cells.
34. Movement from root hair to xylem
cont'd
• Water can then travel symplastically and then
pass through passage cells.
• The water then continues to move through
the pericycle and into the xylem.
• Symplastic movement of water gives a plant
control over what ions pass into its xylem
vessels since everything passes the plasma
membrane.
35. Movement of water up the xylem
vessel
• To fully explain how water moves up the
xylem vessels, transpiration must first be
considered.
36. Transpiration (movement from
leaves to atmosphere)
• Mesophyll cells are not tightly packed and
have air spaces among them.
• Water from the mesophyll cells evaporates
into the air spaces making the air spaces
saturated with water vapour.
• There is direct contact between the air inside
the leaf and the air outside the leaf through
the stomata.
37. Transpiration cont'd
• If there is a water potential gradient, then
water will move out of the leaf.
• This loss of water vapour from the leaf is
called transpiration.
• Transpiration rate will increase if a steep
gradient is created.
38. Transpiration cont'd
• Several things can increase the rate of
transpiration.
• These include:
1.Low humidity
2.Rise in temperature
3.Increased wind speed
4.Opening of the stomata
39. Movement of water up xylem
vessels cont'd
• Movement of water up the xylem is
dependent on the difference in pressure at
the top and bottom of the vessel.
• As water leaves the leaf by transpiration,
water constantly leaves the top of the xylem
vessel down a gradient.
• The water either moves into mesophyll cells
or along their cell walls.
40. Movement of water up xylem
vessels cont'd
• As water leaves the top of the xylem vessels,
hydrostatic pressure is reduced.
• The hydrostatic pressure is greater at the
bottom of the vessel than at the top.
• This causes water to move up the vessel.
• The water in the xylem vessel is under great
tension and would cause the vessel to
collapse if the lignin wasn’t present.
41. Movement of water up xylem
vessels cont'd
• Movement through the vessels is by
massflow.
• This means that all the water molecules move
together as a body of liquid.
• This is due to cohesion and adhesion.
• Cohesion is the attraction of water molecules
to each other.
• Adhesion is the attraction of water molecules
to the lignin in the walls of the xylem vessels.
42. Movement of water up xylem
vessels cont'd
• If an air bubble becomes trapped (an air lock
develops)in the column then the difference in
pressure is not transmitted.
• This prevents the water moving upwards.
• Development of air locks is prevented by the
small diameter of the vessel.
• If however one develops, the pits allow the
water to move to another xylem vessel.
43. Movement of water up xylem
vessels cont'd
• The pressure difference between the top and
bottom of the plant is maintained by
increasing the pressure at the bottom of the
vessels (root pressure).
• Root pressure is increased when ions are
actively secreted into the root cells.
• This lowers the water potential and water
moves in in great amounts.
44. Movement of water up xylem
vessels cont'd
• The contribution of root pressure is very small
and not significant as water will continue to
move up the xylem of a dead plant.
• Water movement in plants is passive and is
fuelled largely by transpiration.
NB. As water is being moved, minerals salts
move with it also
45. Movement of water from xylem to
leaf cells
• Water diffuses from the top of the xylem
vessels to the cells of the leaf.
• This is down a concentration gradient the
creation of which is explained before.
46. Movement of organic materials
• Organic materials eg. Sugars are made by the
plant.
• These materials are also called assimilates.
• The method of transporting soluble organic
substances is called translocation.
• Phloem tissue is responsible for translocation.
48. Sieve elements
• A sieve element is a living cell
• It contains a cell wall, a plasma membrane,
cytoplasm, endoplasmic reticulum and
mitochondria.
• It doesn’t have a nucleus and ribosomes.
• The layer of cytoplasm is very thin.
• The end walls have a very special feature.
49. Sieve elements cont'd
• A sieve plate is formed where two sieve
elements meet.
• The plate is made up of the end walls of two
elements perforated by holes.
• Many elongated sieve elements, joined at
their end walls forms a sieve tube.
• A sieve tube is a continuous column.
50. Sieve elements cont'd
• If the elements are viewed under the
microscope, strands of protein can be seen
passing through the pores.
• These strands however are produced in
response to the damage caused when the
tissue was cut during slide preparation;
51. Companion cells
• A companion cell is found lying close to every
sieve tube element.
• Companion cells have the same structure as
regular plant cells.
• They however have a large number of
mitochondria and ribosomes.
• Many plasmodesmata make contact between
the cytoplasm of the companion cell and the
sieve element.
52. Contents of sieve tubes
• The liquid inside sieve tubes is called phloem
sap or just sap.
53. Collecting sap for analysis
• This is a difficult process.
• As soon as the tissue is cut, the elements
respond by immediately secreting protein.
• Within hours it secretes a carbohydrate called
callose.
• Castor oil plant is unusual and the sap
continues to flow even after the phloem is cut
for some time.
54. Collecting sap for analysis cont'd
• In other plants, aphids are used to collect the
sap.
• Aphids feed by sticking their stylet into the
phloem.
• If the stylet is cut near to the insect’s head the
sap continues to flow.
• The flow is slow and so the plant’s phloem
‘clotting’ mechanism is not employed.
55. Process of translocation
• For the food to be transported, it has to be
loaded from its site of manufacture to the
phloem.
• Triose sugars are converted into sucrose.
• The sucrose in solution moves across the cells
of the leaves into the phloem tissue.
• The sucrose is actively loaded into the phloem
element from the companion cell.
56. Process of translocation cont'd
• The sucrose moves in solution and can
therefore move by the apoplast or symplast
pathway.
• Both the companion cell and the sieve
elements work together.
• Sucrose is loaded into the companion cell by
active transport.
57. Loading of sucrose into phloem
• Hydrogen ions are actively moved out of the
companion cell, using ATP as an energy
source.
• The hydrogen ions then move back into the
companion cell down the concentration
gradient.
• As the hydrogen ions move back in, sucrose
moves into the companion cell as well.
58. Loading of sucrose into phloem
cont'd
• This is possible because the same protein that
acts as a carrier for hydrogen ions is also a
carrier for sucrose molecules (co-transporter
molecule).
• Even though sucrose is moving against a
concentration gradient, no ATP is required
because of the presence and movement of
hydrogen ions.
59. Loading of sucrose into phloem
cont'd
• The sucrose molecules then move through the
plasmodesmata into the sieve tube.
60. Movement in the phloem
• Movement in the phloem is by mass flow.
• The pressure difference is actively created (in
the xylem, the pressure difference is passively
created).
• When sucrose molecules are actively loaded
into the phloem, the water potential in the
phloem is reduced.
• Water thus moves into the phloem by
osmosis.
61. Movement in the phloem cont'd
• When the sucrose is removed from the
phloem by other cells like those of a fruit,
water again follows by osmosis.
• The movement of the water out of the
phloem creates a pressure difference.
• Hydrostatic pressure is low in the lower part
of the leaf and high in the area of the leaf
where the sieve tube is found.
62. Movement in the phloem cont'd
• The pressure difference causes water to flow
from a high pressure area to a low pressure
area taking solutes with it.
63. Unloading of sucrose from the
phloem
• It is still unclear as to how sucrose is unloaded
from the phloem.
• It is believed that it is unloaded by diffusion.
• As soon as it enters the cells an enzyme
converts it into something else thus
maintaining a concentration gradient. Eg.
Invertase hydrolyses sucrose to glucose and
fructose.
64. Translocation cont'd
• Wherever in the plant sucrose is loaded into
the phloem, that area is called a source.
• Wherever in the plant sucrose is unloaded
from the phloem, that area is called a sink.
65. Benefits of having a sieve plate
• They probably act as support preventing the
phloem from collapsing. The support for the
xylem comes from its lignified walls.
• They allow damage to be rapidly overcome
because of the ‘clotting mechanism’.
• The ‘clotting’ prevents loss of important
nutrients. It also prevents entry of pathogens.
66. Evidence for mass flow
• Initially there was much controversy about
whether movement in the phloem was by
mass flow.
• One reason for the controversy is the phloem
protein blocking the pores that is observed
when phloem is looked at.
• Now it is known that these proteins only form
when the phloem is damaged.
67. Evidence for mass flow cont'd
1. The rate of movement in phloem is
approximately 10000 times faster than it
would be if movement was by diffusion.
2. The rate of transport measured match
closely with the pressure differences
measured at source and sink (providing the
pores are unobstructed).
68. Evidence for mass flow cont'd
• There is also much evidence for the active
loading of sucrose into sieve elements.
• The work done so far has been tedious and
inconclusive.
• There is circumstantial evidence to support
active loading however.
69. Evidence for mass flow cont'd
• These include:
1.The pH of phloem sap is approximately 8. This
is what is expected if Hydrogen ions are
actively transported out of the companion
cell.
2.The difference in electrical potential across
the plasma membrane is approximately
-150mV.
70. Evidence for mass flow cont'd
• This is consistent with an excess of ions
outside of the companion cell compared with
inside.
3. ATP is in large amounts in phloem companion
cells. This would be needed if active transport
is taking place.
71. Similarities between movement of
manufactured food and movement of water.
• Both move by mass flow along a pressure
gradient.
• Both move through tubes formed by cells
stacked end to end.
72. Differences between xylem and
phloem
Xylem
• Vessels are made of dead
cells
• Vessels have lignified cell
walls
• The end walls disappear
completely
• Have pits
Phloem
• Elements are made from
living cells
• Phloem tubes do not have
lignified cell walls
• The end walls form sieve
plates. They do not
disappear completely
• Do not have pits. They have
plasmodesmata instead