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What do plants need to get rid of?

Waste substances?


How are plants like animals?

They're multicellular
Small surface area to volume ratio
Relatively big
Relatively high metabolic rate


Why can't plants just diffuse everything they need to get and get rid of into and out of the plant?

Exchanging substances by direct diffusion by direct diffusion from the outer surface to the cells would be too slow to meet their metabolic needs


What do plants need due to this issue?

Transport systems to move substances to and from individual cells quickly


What does xylem tissue do?

Transport water and mineral ions in solution.
Substances move up plant from roots to leaves
Support the plant


What does phloem tissue do?

Mainly transports sugars (also in sugars) both up and down the plant


What do xylem and pholem make up?

A plants vascular system


Where are xylem and phloem found?

Throughout a plant as they transport materials to all parts
Where they are found in each part is connected to the xylem's function of support


Describe where xylem is in a root?

In the centre and surrounded by phloem to provide support as it pushes through the soil


Where are xylem and phloem found in the stems?

Near the outside to provide a sort of scaffolding that reduces bending


Where are xylem and pholem found in a leaf?

Make up a network of veins which supports the thin leaves


What are all the diagrams that you need to learn about xylem and phloem?

Transverse cross-sections
Herbaceous dicotyledonous plants (flowering plants without woody stem)


What does transverse mean?

Sections cut through each structure at a right angle to its length


Longitudinal cross-sections

Taken along length of structure
Shows where xylem and phloem are located in a typical stem


What is xylem tissue made from?

Several different cell types


Describe xylem vessels

Very long, tube-like structures formed from vessel elements
No end walls making uninterupted tube allowing water to pass through middle
Dead cells so no cytoplasm
Thickened walls with lignin (increases as plant ages)



Woody substances
Supports xylem vessels and stops them collapsing inwards
Can be deposited in xylem walls in different ways in spiral or distinct rings
Water and ions move into and out of vessel through small pits in wall where's no lignin


What does phloem transport?

Solutes (dissolved substances) mainly sugars like sucrose round plants


How is phloem formed?

Like xylem from cells arranged in tube


Function of phloem tube?

Transport tube not support


What does phloem tissue contain?

Phloem fibres
Phloem parenchyma
Sieve tube elements
Companion cells


What are the most important cells in phloem for transport?

Sieve tube elements
Companion cells


What are sieve tube elements?

Living cells that form the tube for transporting solutes through the plants


How are sieve tube elements joined?

End to end to form sieve tubes


What are the sieve part of sieve tube elements?

End walls which have lots of holes in them allowing solutes to pass through them


What is unusual about sieve tube elements for living cells?

No nucleus
Very thin layer of cytoplasm
Few organelles


How are cytoplasms between sieve tube elements cells connected?

Cytoplasm of adjacent cells is connected through holes in sieve played


Companion cells purpose?

Sieve tube elements lack of nucleus and other organelles means they can't survive in their own so 1 companion cell for 1 sieve tube element
Carry out living functions for both themselves and sieve cells e.g. Provide energy for active transport of solutes


How do you look at plant tissue?

Dissect and prepare section of tissue
Look under microscope
Draw it


How to dissect and prepare a section of a tissue? Step 1

Use scalpel/ razor blade to cut cross-section of stem transverse or longitudinal. Cut sections thinly as possible thin sections are better for viewing under a microscope


How to dissect and prepare a section of a tissue? Step 2

Use tweezers to gently place cut sections in ester until you come to use them stopped my them from drying out


How to dissect and prepare a section of a tissue? Step 3

Transfer each section to dish containing stain e.g. Toluidine blue O(TBO) and leave for 1 minute. TBO stains lignin in walls of xylem vessels blue-green. See position of xylem vessels and examine their structure


How to dissect and prepare a section of a tissue? Step 4

Rinse off section in water and mount each one onto a slide


What do plants need?

Substances like water, minerals and sugars to live
Need carbon dioxide but it enters at leaves where it's needed


How does water enter a plant?

Through its root hair cells


What does a water get from and to to be transported around the plant?

From the soil
Through the roots
Into the xylem


Where does water go to reach the xylem?

Enters through root just cell
Through the root cortex including the endodermis to reach the xylem


How is water drawn into the roots?

Via osmosis
Travels down a water potential gradient


What's osmosis?

Diffusion of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential


How does water always move?

From areas of higher water potential to areas of lower water potential
Goes down a water potential gradient


What does the soil around the roots generally have?

A higher water potential (lots of water down there)


Leaves have?

Lower water potential (because water constantly evaporates from them)


What do these two things mean for water potential gradient?

Creates a water potential gradient keeping water moving through the plant in the right direction from roots (high) to leaves (low)


How many paths does water travel through the root via the root cortex into the xylem?



Two xylem paths?

Symplast pathway
Apoplast pathway
Vacoluar pathway


What does the symplast pathway go?

Through the living parts of the cell-the cytoplasm. The cytoplasm of neighbouring connect through plasmodesmata


What are plasmodesmata?

Small channels in the cell wall


How does water move through symplast pathway?

Via osmosis


Apoplast pathway goes where?

Through non-living parts of cells-the cell walls.


Why can the apoplast pathway go through the cell wall?

Walls are very absorbent and water can simply diffuse through them as well as pass through spaces between them


What can the water in the apoplast pathway carry?

Solutes and move from areas of high hydrostatic Pressure to areas of low hydrostatic pressure. (Along pressure gradient)
Example of mass flow


What happens when water in the apoplast pathway gets to endodermis cell?

It's path is blocked by a waxy strip in the cell walls (Casparian strip). Now water takes symplast pathway


Why is this useful?

Water has to go through a cell membrane
Cell membranes are partially permable and are able to control whether or not substances in the water get through


What happens once the water gets past the casparian strip?

Water moves into the xylem


Which of the two pathways is used?

Both pathways are used but apoplast pathway is the main one because it provides the least resistance


What do xylem vessels do?

Transport water all around the plant


What happens at the leaves with the xylem?

Water leaves the xylem and moves into the cells mainly by the apoplast pathway


Where does water evaporate?

From the cell walls into spaces between cells in the leaf


What happens when the stomata opens?

The water diffuses out of the leaf down the water potential gradient into surrounding air


What's the stomata called?

Tiny pores in the surface of the leaf


What's transpiration?

The loss of water from a plants surface


What is the movement of water from root to leaves called?

Transportation stream


What do the mechanisms that move water include?



What do cohesion and tension help to do?

Water move up plants from roots to leaves against the force of gravity


Where does water evaporate?

At the leaves at the top of the xylem (transpiration)


What does water evaporating from leaves do?

Create a tension (suction) which pulls more water up the leave


Water are molecules are?

Cohesive so when some are pulled into the leaf others follow meaning a whole column of water in the xylem from leaves down to roots moves upwards


Where does water enter?

The stem through the root cortex cells


Cohesive is?

Stick together


What is adhesion also partly responsible for?

The movement of water


How is adhesion responsible for the movement of water?

As well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels
Helping water to rise up through the xylem vessels


What does cohesion and tension allow?

The mass flow of water over long distances up the stem


Why does transpiration happen?

A result of gas exchange


Why does the plant open the stomata?

Needs to let in carbon dioxide so it can produce glucose (by photosynthesis)


What also happens when the stomata is opened?

Water is let out so there's a higher concentration of water inside the leaf than in the air outside so water moves out of the leaf down the water pore beak fragment


What's transpiration a side effect of?

Gas exchange needed for photosynthesis


How does water move?

From areas of higher water potential to areas of lower water potential
Down the water potential gradient


What are the four main factors affecting transpiration rate?



How does light affect rate of transpiration?

The lighter it is the faster the transpiration
This is because the stomata when it's light so CO2 can diffuse into leaf for photosynthesis. When dark the stomata are usually closed so little transpiration


How does temperature affect transpiration rate?

The higher the temperature the faster the transpiration rate
Warmer water molecules have more energy so evaporate from cells inside leaf faster increasing water potential gradient between inside and outside of leaf making water diffuse out of leaf faster


Humidity transpiration rate?

Lower humidity the faster the transpiration rate
If air around plant is dry the water potential gradient between leaf and air is increased increasing transpiration


Wind transpiration rate?

The windier it is the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata increasing water potential gradient increasing rate of transpiration


What's a potometer?

A special piece of appartus used to estimate transpiration rate


What does a potometer actually measure?

Water uptake by the plant but it's assumed that water uptake is directly related to water loss by leaves


How do you estimate how different factors affect transpiration rate? Step 1

Cut shoot underwater to prevent air from entering the xylem. Cut it at a slant increasing surface area available for water uptake


How do you estimate how different factors affect transpiration rate? Step 2

Assemble potometer in water and insert shoot underwater so no air can enter


How do you estimate how different factors affect transpiration rate? Step 3

Remove appartus from water but keep end of capillary tube submerged in a beaker of water


How do you estimate how different factors affect transpiration rate? Step 4

Check that the appartus is watertight and airtight


How do you estimate how different factors affect transpiration rate? Step 5

Dry leaves allow time for shoot to acclimatise and then shut the tap


How do you estimate how different factors affect transpiration rate? Step 6

Remove end of capillary tube from beaker of water until one air bubble was formed then put the end of the tube back into the water


How do you estimate how different factors affect transpiration rate? Step 7

Record starting position of the air bubble


How do you estimate how different factors affect transpiration rate? Step 8

Start a stopwatch and record distance moved by bubble per unit time e.g. Per hour. The rate of air bubble movement is an estimate of the transpiration rate


How do you estimate how different factors affect transpiration rate? Step 9

Remember only change one variable (temperature... at a time) all other conditions (light, humidity...) must be kept constant


Air bubble called?

Air-water meniscus


Potomotor things to remember

Rervoir of water used to return bubble to start for repeats
As plant takes up water air bubble moves along scale
Tap shut during experiment
Capillary tube with scale
bubble moves
Beaker of water underneath


What are xerophytes?

Plants like cacti and marram grass (grows on sand dunes)
Adapted to dry climates
Adaptations that prevent them losing too much water by transpiration


Marram grass adaptations

Stomata sunk into pits
A layer of hair on epidermis
In hot or windy conditions, marram grass plants roll their leaves
Thick waxy layer on epidermis


How is the marram grass having stomata that are sunk in pits a good adaptation?

They are sheltered from the wind
Helps slow transpiration down


How is the marram grass having a layer of hairs on the epidermis a good adaptation?

Traps moist air round stomata reducing water potential gradient between leaf and air slowing transpiration down


How is the marram grass rolling their leaves in hot or windy conditions a good adaptation?

Traps moist air
Slowing down transpiration
Reduces exposed surface area for losing water and protects stomata from wind


How is the marram grass having a thick waxy layer on epidermis a good adaptation?

Reduces water loss by evaporation because the layer is waterproof (water can't move through it)


Cacti adaptations

Thick, waxy layer on the epidermis reducing water loss by evaporation (waterproof layer)
Spines instead of leaves reducing surface area for water loss
Closes their stomata at the hottest times of the day when transpiration rates are the highest.


Hydrophytes are?

Plants like water lilies which live in aquatic habitats. As they grow in water they don't need adaptations to reduce water loss but they need adaptations to help them cope with low oxygen level


Hydrophytes adaptations

Air spaces in tissues help plant float and act as a store of oxygen for use in respiration
Stomata usually only present on upper surface of floating leaves
Often have flexible leaves and stems


Air spaces in tissues help plant float and act as a store of oxygen for use in respiration explain more?

E.g. Water lilies have large air spaces in their leaves allowing the leaves to float on the surface of the water increasing the amount of light they receive. Air spaces in roots and stems allow oxygen to move from floating leaves down to parts of plant underwater


Stomata usually only present on upper surface of floating leaves why is this a good adaptation?

Helps maximise gas exchange


Often have flexible leaves and stems a good feature for hydrophytes?

Supported by water around them so don't need rigid stems for support
Flexibility helps prevent damage by water currents


What's translocation?

Movement of assimilates (dissolved substances e.g. Sugars like sucrose, and amino acids) to where they're needed in a plant.
Energy requiring process that happens in the phloem


What does translocation do?

Move substances from sources to sinks.



Substance where it's made at high concentration



Area where used up
At low concentration there


Why are sugars transported as sucrose?

It's not soluble and metabolically inactive so doesn't get used up during transpiration


Source for sucrose?

Leaves (where it's made)


Sink for sucrose?

Other parts of the plant especially the food storage organs and the meristem (areas of growth) in the roots, stem and leaves


What are roots sources or sink?

Sucrose can be stored in the roots
During growing season, sucrose is transported from roots to leaves to provide the leaves with energy for growth. In this case, roots are the source and the leaves are a sink.
So both as some parts of the plant can be


What do enzymes do to do with concentration gradient?

Enzymes maintain a concentration gradient from source to sink by changing the dissolved substances at the sink (by breaking them down or making them into something else) this makes sure that's always a lower concentration at the sink than at the source


What happens in potatoes?

Sucrose is converted to starch in sink areas, so there's always a lower concentration of sucrose at sink than inside the phloem. Making sure there's a constant supply of new sucrose reacted the sink from the phloem. In other sinks, enzymes such as invertase break down sucrose concentration into glucose (and frutose) for use by plants. Again make sure there's a lower concentration of sucrose at the sink


How are scientists thinking translocation happens?

Scientists aren't certain exactly how dissolved substances (solutes) are transported from source to sink by translocation. Best supported theory is mass flow hypothesis


Step 1 of the mass flow hypothesis

Active transport is used to actively load solutes (sucrose from photosynthesis) into sieve tubes of phloem at source (leaves)


Step 2 of translocation

This lowers the water potential inside the sieve tubes so water enters the tubes by osmosis from xylem and companion cells


Step 3 translocation

This created a high pressure inside the sieve tube at the source end of the phloem


Step 4 of translocation

At sink end, solutes are removed from the phloem to be used up increasing water potential inside the sieve tubes so water also leaves the tube by osmosis lowering the pressure inside the sieve tubes


Step 5 of translocation

The result is a pressure gradient from source end to sink end.
Gradient pushes solutes along sieve tubes to where they're needed


Active loading is used for what?

To move substances into companion cells from surrounding tissues and from companion cells into sieve tubes against a concentration gradient


What is the concentration of sucrose higher?

In the companion cells then the surrounding tissue cells and higher in the sieve tube cells than companion cells


What happens as a result?

Sucrose is moved to where it needs to go using active transport and co-transporter proteins.


Active transport

Uses energy to move substances against their concentration gradient


Step 1 active loading

In companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into surrounding tissue cells


Step 2 active loading

This sets up a concentration gradient- there are more H+ ions in the surrounding tissue than in the companion cell


Step 3 active loading

An H+ ion bonds to a co-transport protein in the companion cell membrane and re-enters the cell (down the concentration gradient)


Step 4 of scuba loading

A sucrose molecule binds to the co-transport protein at the same time. The movement of the H+ ions is used to move the sucrose molecule into the cell against its concentration gradient


Step 5 active loading

Sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process


What is ATP?

One of the products of respiration


What does the breakdown of ATP supply?

Initial energy needed for active transport of H+ ions