Lecture 15: Transport in Plants Flashcards
Describe the transpiration-cohesion-tension mechanism. What is its significance?
long distance transport of water
Define passive transport and identify the 3 types
Passive transport is movement of molecules across short distances or across membranes without the addition of energy
- simple diffusion: net movement of a population of molecules down its concentration gradient
- osmosis: net diffusion of water against the solute concentration gradient
Define active transport
uses energy to move solutes against their concentration gradient across a membrane through transport proteins
T or F: Diffusion is the primary means for transporting all materials through non-vascular plants. Why/why not
True because they only need transfer of molecules across short distances
When does diffusion occur in vascular plants?
to transport CO2 and O2 in and out of the stomata and across cell membranes in the mesophyll
Define cytoplasmic streaming
cytoplasm circulates within and between cells through the plasmodesmata so that large molecules and organelles (like chloroplasts) can be moved from cell to cell.
What are the two mechanisms of short distance transport?
- diffusion
2. cytoplasmic streaming
T or F: gases are transported in plants - why/why not?
FALSE. all living cells do gas exchange
Define transpiration
the process of water movement through a plant body
involves the uptake, transport, and evaporation of water
Why does cytoplasmic streaming occur?
so chloroplasts can move from shaded areas of the leaf to sunnier spots to maximize photosynthesis
Describe the net diffusion of O2 in plants
O2 diffuses through plant roots from the soil and is released out leaves during cellular respiration
Describe the net diffusion of CO2 in plants
CO2 is taken in/diffused through leaves/green areas through photosynthesis and released by the roots
Which materials are transported from roots to shoots?
water and minerals are taken in through plant roots and released from leaves as water vapour
What materials are produced by the plant and transported throughout the plant body?
Sugars are made by the plant during photosynthesis and are transported to other areas of the body
Which tissue system do the sugar and water (and mineral) transporting cells belong to? what are the tissues and the cells for each?
VASCULAR TISSUE SYSTEM transports sugars and water (and minerals) throughout the plant body.
PHLOEM TISSUES:
transport sugars
- companion cell
- sieve tube element
XYLEM TISSUES:
transports water and minerals
- vessel element
- tracheids
To get from the roots to the shoots, is water pushed up by the roots or pulled up by the leaves?
Water is PULLED up by the leaves through the process of transpiration.
How much water is lost to evaporation during transpiration?
over 95%
Why are plants so inefficient with water use compared to animals?
Unlike in animals where water is constantly being re-circulated through the vertebrate body via the circulatory system, plants do not have a circulatory system.
Instead, in plants, water is taken up and transported BY evaporative water loss from leaves (that is what pulls the water up from the roots)
Briefly describe water’s journey through a plant body
- uptake by roots
- pulled up through plant body
- evaporates from leaf surface
T or F: there is a film of water coating the surface of mesophyll cells inside leaves?
TRUE.
T or F: the concentration of water molecules inside a leaf is much higher than the concentration in the atmosphere outside the leaf
TRUE
Describe the function of stomata
open to allow for gas exchange and water to exit the leaf as water vapour
What is the direction of the water potential gradient?
Water potential gradient moves from highest (roots) to lowest (leaves) concentrations
Describe the steps of the Transpiration-Cohesion-Tension Model
- water vapour diffuses out stomata (down concentration gradient)
- water vapour replaced from the water film on the mesophyll cell surfaces
- air-water interface retreats
- increase surface tension pulls water from cells and air spaces
- water from xylem is pulled into cells and air spaces
Describe the steps of the Transpiration-Cohesion-Tension Model
- water vapour diffuses out stomata when it opens (down concentration gradient)
- water vapour replaced from the water film on the mesophyll cell surfaces
- air-water interface retreats
- increase surface tension pulls water from cells and air spaces
- water from xylem is pulled into cells and air spaces
How does surface tension created by evaporation have an effect on the movement of water?
Surface tension created by the evaporation of water PULLS water from one mesophyll cell to another mesophyll cell
water then gets PULLED from the xylem tube to nearby mesophyll cells
Water is then PULLED up the entire xylem tube from the roots
Why does water get pulled and the chains of H2O molecules don’t break?
the forces of adhesion and cohesion
Define adhesion
Adhesion is the bond/attraction between water molecules and another polar substance (e.g. cellulose walls of xylem)
Define cohesion
Cohesion is the strong bond/attraction between water molecules and other water molecules
What 2 properties keep water moving up the xylem?
- adhesion by hydrogen bonding
2. cohesion by hydrogen bonding
Which substances do water molecules adhere to?
Cellulose walls of the xylem tissues
T or F: water has a weak attraction to cellulose?
FALSE. Water has a STRONG attraction to cellulose
Describe the vessel elements and tracheids of the xylem tissues
- have no cell membrane, so water is in DIRECT contact with the CELLULOSE CELL WALLS (benefits adhesion forces)
- water moves from cell to cell through pits or perforation plates (in vessel elements only)
- cells are dead at functional maturity to prevent obstructing water flow
Is water pushed or pulled into roots from the soil?
PULLED
How does water diffuse into roots?
Tension produced by transpiration passively pulls water from the soil, which means plants don’t need to use energy to uptake water or transport it from roots to shoots
Is the uptake/osmosis of water a form of active or passive transport? Then, do plants expend energy on transporting water?
PASSIVE transport.
the tension of transpiration pulls water from the soil PASSIVELY = NO ENERGY REQUIRED for transport
When are minerals transported into the xylem? is this active or passive transport?
Minerals are ACTIVELY transported when transpiration slows (eg. at night)
What follows minerals into the xylem to create what kind of pressure?
Water follows minerals into the xylem tissues by osmosis and creates POSITIVE ROOT PRESSURE
Is root pressure positive or negative pressure? What does this mean?
POSITIVE PRESSURE FORCES/PUSHES water and mineral ions UP the xylem
What are the 2 major differences between transpiration and root pressure?
- When stomata are open (during the day), TRANSPIRATION is the driver of bulk transport and water transport is PASSIVE
Whereas, ROOT PRESSURE transportation is ACTIVE and requires ATP. Also root pressure transportation occurs at night, when the stomata are closed and transpiration slows down.
- During transpiration, water and mineral ions are pulled up RAPIDLY by bulk flow from the roots due to evaporation in the shoots.
Whereas, root pressure has no measurable effect on transport during the day as it cannot overcome the force of gravity over long distances
Where is sugar produced?
in leaves, but mostly mature leaves
Where is sugar needed?
Every living cell requires sugar
How does sugar get where it needs to go?
Sugar is transported through the phloem tissues with the help of sieve-tube elements and companion cells.
What is phloem sap?
Sugars dissolved in water
How does phloem sap travel from cell to cell?
through the connections between the sieve plates (perforations) of sieve tube elements
Describe sieve tube elements
- alive at functional maturity because they are hollow and do not contain a nucleus, ribosomes, or vacuole
- plasma membrane is functional as plasmodesmata because the companion cell (per sieve tube element) is attached there and transports the necessary nutrients etc. for the sieve tube element to survive
Define sugar sink and sugar sources
Sugar SOURCE: an organ where photosynthesis occurs (and sugar is produced) or stored starch is broken down
Sugar SINK: an organ that consumes or stores sugar
T or F: Sugars are transported from sugar sources to sugar sinks?
TRUE.
In the example of a potato in early spring (planting time) vs late summer (harvest time), where are the sugar sources and sinks?
Early spring/planting time:
SOURCE: potatoes
SINK: leaves/green shoot
Late summer/harvest time:
SOURCE: mature plant/leaves
SINK: potatoes
Describe the steps of the Pressure Flow Model for sugar transport
- sugar moves from source cell to sieve tube element using ACTIVE TRANSPORT
- high solute concentration in sieve tube elements cause water to move in from nearby xylem (through osmosis)
Water uptake creates POSITIVE PRESSURE that PUSHES sugar down the sieve tube elements - Sugar moves from sieve-tube element to SINK using PASSIVE TRANSPORT
- Water recycles back into xylem
Why is the movement of sugar from source cells to sieve tube elements an active transport?
Because the sugar molecules are moving against their concentration gradient (from the source cells to the phloem tissues) which requires energy input and a transport protein to do so.
Why is the movement of sugar molecules from sieve tube elements to sink cells a passive transport?
Because the concentration of sugar molecules in the sink is lower than in the sieve tube elements
Describe short distance transport and identify which mechanisms are included
Short distance transport occurs as membrane transport as either:
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
- cytoplasmic streaming
Describe long distance transport and identify which mechanisms are included
- Water and nutrients are PASSIVELY PULLED from roots to shoots in xylem
- Transpiration-Cohesion-Tension model - Sugar is ACTIVELY transported from sugar sources to sugar sinks in phloem
- Pressure Flow Model
