Transport in Plants

Xylem	Phloem
Transports Water and Mineral  Ions	Transports Solutes (dissolved substances) which are mostly sugars such as sucrose around plants to where they need to be.
Substances move UP	Substances move UP AND DOWN
Also used for SUPPORT	Phloem is  formed from cells arranged in tubes.
Long and Tube-like made from cells (VESSEL ELEMENTS) joined end to end.	All tissue is purely transport tissue, no structural tissue present.
There are no end-walls on the cells to form one long continuous tube.	Contains Phloem fibres, Phloem Parenchyma, Sieve tube elements and companion cells.
These cells are actually dead, so contain no cytoplasm	Sieve tube elements are the most important cells for transport in the phloem.
Cell Walls are thickened with LIGNIN (a woody substance) which helps support the Xylem vessels and stops them collapsing inwards	Sieve Tube Elements Companion Cells The LIVING cells that form the tube for transporting solutes through the plant. The lack of a nucleus and other organelles meant that they can't survive on their own. This means there is a 1:1 ratio of Companion cells to sieve tube elements. They are joined end-to-end to form Sieve Tubes. Companion cells carry out the living functions for  both themselves and their sieve cells. They provide the energy for ACTIVE TRANSPORT of solutes The 'Sieve' part are the end walls, which have lots of holes in them to allow solutes to pass through. While alive, they usually have no nucleus, a thin layer of cytoplasm and very few organelles. The cytoplasm of adjacent cells is connected through the holes in the sieve plates.
Lignin can be supported in different ways - spiral, or distinct rings
The amount of lignin increases as the cells get older
Water and Mineral Ions move in and out of the vessels through small pits in the walls where lignin doesn't occur.

Water enters a plant through its ROOT HAIR CELLS. Water has to go through Soil --> Root --> Xylem to be transported around the plant.

Root Hair Cells --> Root Cortex + Endodermis -->Xylem.

Water is drawn into the roots through osmosis. It travels Down a WATER POTENTIAL GRADIENT.

• Water travels from areas of High water potential gradient to areas of LOW WPG. This means it travels down a water potential gradient.
• The soil usually has a high water potential (there's lots of water present).
• Leaves have a Low water potential because water always evaporates from them.
• This creates a water potential gradient that keeps moving water from roots (high) to leaves (low) constantly.

The Symplast Pathway	The Apoplast Pathway
Goes through the LIVING parts of cells - the cytoplasm.	Goes through the NON-LIVING parts of cells - The cell walls.
The cytoplasm of neighbouring cells connects through plasmodesmata (small channels in the cell walls).	The cell walls are very absorbent and water can diffuse through them as well as pass through  spaces between them.
Water moves through the Symplast pathway using osmosis.	The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. (i.e. along a pressure gradient). This is an example of mass flow.

In the apoplast pathway, when the water gets to the endodermis cells in the root, its path is blocked by a waxy strip in cell walls called the CASPARIAN STRIP. The water now has to take the symplast pathway. This is useful because the water has to go through a cell membrane, which controls what goes in and out of a cell, so it controls which substances in the water get through. Cell Membranes are PARTIALLY PERMEABLE. The water then moves into the XYLEM.

Both Pathways are used, but APOPLAST is the main one because there is less resistance.

• Xylem vessels transport water all around the plant.
• At the leaves, water leaves the Xylem and moves into the cells through the apoplast pathway.
• Water evaporates from the cell walls into the spaces between the cells in the leaf.
• When the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the leaf into the surrounding air.
• The loss of water from a plant's surface is called TRANSPIRATION.
• Water moves up a plant AGAINST THE FORCE OF GRAVITY.
• The movement from roots to leaves is called the TRANSPIRATION STREAM.
• The mechanisms that move the water include COHESION, TENSION and ADHESION

COHESION AND TENSION	ADHESION
Helps water move up plants against the force of gravity.	As well as being attracted to itself, water is attracted to the walls of the xylem vessels
Water evaporates from the leaves at the top of the xylem (transpiration)	This helps water rise up through the xylem vessels.
This creates tension (suction) which pulls more water into the leaf.
Water molecules are cohesive (they stick together) so when some are pulled into the leaf, others follow.
This means the whole column of water in the xylem moves up the plant.
Water enters the stem through the root cortex cell.

Cohesion and Tension allow the mass flow of water over long distances up the stem.

Transpiration

• Transpiration is a consequence of gas exchange.
• Transpiration is defined as The Evaporation of water from a plant's surface, specifically, the leaves.

1. A plant opens its stomata to let in CO2 so it can produce glucose by photosynthesis.
2. This also lets water out because there is a higher concentration of water inside the leaf than the air outside.
3. Transpiration is technically a side effect of gas exchange, needed for photosynthesis.

What factors effect TRANSPIRATION RATE?

• Light - More Light, Faster Rate - this is because the stomata open when it is light. when it's dark, the stomata close, so there is less transpiration happening.
• Temperature - Higher Temperature, Faster Rate - warmer water molecules have more energy, so they evaporate from cells faster, this increases water potential gradient, increasing the rate of transpiration.
• Humidity - Lower Humidity, Faster Rate. - if the air around the plant is dry, water potential gradient increases, so they transpire at a faster rate.
• Wind - More Wind, Faster Rate - Lots of air movement blows away water molecules around the stomata, increasing water potential gradient Blah Blah Blah…..

Potometer

A potometer is a special piece of apparatus which ESTIMATES TRANSPIRATION RATES.

It measures water uptake by the plant, and it is ASSUMED that water uptake is directly related to water loss by the leaves.

Xerophytic Plants

Xerophytic plants are those like cacti and marram grass, They're adapted to live in dry climates. These adaptations prevent them losing too much water during transpiration.

• Marram grass has its stomata sunk in pits so they're sheltered from the wind, slowing transpiration
• It also ha a layer of hairs on the epidermis, this traps moist air reducing the water potential gradient between the leaf and the air.
• In hot or windy conditions, marram grass rolls up their leaves trapping moist air and reducing the exposed surface area and protects the stomata from the wind.
• Both marram grass and cacti have a thick waxy layer on the epidermis - reducing water loss by evaporation because this layer is waterproof.
• Cacti have spines instead of leaves reducing the surface area for water loss.
• Cacti also close their stomata at the hottest times of the day when transpiration rates are highest.

Hydrophilic Plants

Plants that are adapted to survive in water, such as water lilies, live in aquatic habitats. 

As they grow in water, they don't need adaptations to reduce water loss. However, they do need adaptations to help them cope with low oxygen levels...

• Air spaces in the tissues help the plants to float and can act as a store of oxygen for use in respiration. Water lilies have air spaces in the leaves to allow them to float on the surface of the water, increasing the amount of light they receive. Air spaces in the stem and roots allow oxygen to move from the floating leaves down to parts of the plant that are underwater.
• Stomata are usually only present on he upper surface of floating leaves. This maximises gas exchange.
• Hydrophytes have flexible leaves and stems. The plants are supported by the water around them, so they don't need rigid stems for support. Flexibility helps prevent damage by the currents of the water.

Translocation.

Translocation is the movement of dissolved substances (sugars like sucrose, and amino acids) to where it's needed in a plant. Dissolved substances are sometimes called ASSIMILATES.

• It's an energy-requiring process that happens in the phloem.
• Translocation moves substances from SOURCES to SINKS. Sources are where the substances are made, sinks are where substances are used up.
• Some parts of plants can be both a sink and a source.
• Enzymes maintain a concentration gradient by changing dissolved substances at the sink by breaking them down or making them into something else. This means there's always a lower concentration at the sink than at the source.

The Mass Flow Hypothesis

No-one actually knows exactly how dissolved substances are transported from source to sink. This is the best they can come up with...

1. Active transport is used to actively load the solutes into the sieve tubes of the phloem at the source. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.  this creates high pressure at the source end of the phloem.
2. At the sink end of the phloem, solutes are removed from the sieve tubes to be used up. this increases the water potential inside the sieve tubes, so water leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes.
3. The reduces the pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes to where they're needed.

Active Loading.

• Active loading is used to move substances into the companion cells from surrounding tissues, and from companion cells into the  sieve tubes, against a concentration gradient.
• The concentration of sucrose is usually higher in the companion cells than in the surrounding tissue cells, and higher in the sieve tube cells than in the companion cells.
• So, sucrose is moved to where it needs to be using active transport and cotransporter proteins. This is how it works:

1. In the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into surrounding tissue cells. This creates a concentration gradient of Hydrogen ions. There's more in the surrounding tissue than in the companion cells.
2. An H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell down the concentration gradient.
3. At the same time,  a sucrose molecule binds to the cotransport protein. the movement of H+ ions is used to movethe sucrose back into the cell against its concentration gradient.
4. Sucrose molecules are then transported out of the companion cells and into sieve tubes bythe same process.

ATP is a product of respiration. The breakdown of ATP supplies the initial energy needed for active transport of the H + ions.