Gas Exchange and Mass Transport in Plants Flashcards
(99 cards)
1
Q
Properties of Water
A
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
2
Q
How much of a cells content in water?
A
80%
3
Q
What is the structure of water?
A
- Because the electrons in the hydrogen are pulled towards the oxygen the opposite side of the hydrogen atoms are left slightly positive.
- The unshared electrons on the other side of the oxygen atom cause that side to have a slight negative charge.
- Weakly negative
- This difference in charge across the molecule makes it polar.
- The polarity of the molecule allows hydrogen onds to form between the slightly negative oxygen of one molecule and slightly positive hydrogen of another. * These hydrogen bonds give water some of its most useful properties.
4
Q
What are the properties of water?
A
- It is an important metabolite
- It’s a good solvent
- It has a relatively large latent heat of vaporisation - high boiling point
- It has a relatively high heat capacity - the amount of energy needed to raise the temperature of 1g of a substance by 1°C
- Has strong cohesion between molecules
5
Q
How is water being an important metabolite important?
A
Is apart of many metabolic reactions, including condensation and hydrolysis reactions.
6
Q
How is water being a good solvent important?
A
- The polar nature of water molecules mean they can be attracted to and totally surround ionic compounds allowing them to dissolve and form a solution.
- This allows useful substances to be dissolved and transported around the organism.
- Other polar molecules (like glucose) can dissolve in water because hydrogen bonds form between them and water molecules
7
Q
How is water having a relatively large latent heat of vaporasation important?
A
- Needs a lot of energy to break the hydrogen bonds between water molecules to allow them to become a gas.
- Lots of heat energy is absorbed by the hydrogen bonds before they break which allows small amounts of water to have a large cooling effect with little loss of water through evaporation e.g sweating.
8
Q
How is water having a relatively high heat capacity important?
A
- Lots of heat energy is absorbed by the hydrogen bonds before they break which means that there is less heat energy available to increase the temperature of the water so it takes a lot of heat energy to actually increase water temperature specifically.
- This is useful because it prevents water from having rapid temperature changes. Water bodies such as ponds, lakes, rivers and the ocean have more stable temperatures than on land and water inside organisms also remains fairly stable.
- This helps organisms to regulate their own body temperature.
9
Q
How is water having a strong cohesion between molecules important?
A
- Molecules stick together because they are polar.
- This allows water to flow easily which supports columns of water in the tube-like transport cells of plants and produces surface tension where water meets air - allowing some insects to ‘walk on water’.
10
Q
Plant structure
A
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
11
Q
What is the function of the waxy cuticle?
A
- Is a thick waxy layer
- It acts as a barrier against water evaporation, helps to maintain water balance within the plant, and shields the leaf from UV radiation, pathogens, and insects
12
Q
What is the function of the upper epidermis?
A
- To protect the leaf and allow light to penetrate for photosynthesis
- It acts as a barrier between the leaf and the external environment, preventing excessive water loss through transpiration
13
Q
What is the function of the palisade mesophyll?
A
- Photosynthesis, where light energy is converted into chemical energy in the form of glucose.
- This tissue is specifically adapted to maximise light absorption and CO2 uptake, making it the main site of photosynthesis within the leaf.
- Contains lots of chloroplasts
14
Q
What is the function of the spongy mesophyll?
A
- Facilitates gas exchange, particularly the diffusion of carbon dioxide and oxygen, which are essential for photosynthesis and respiration.
- It’s characterised by loosely packed cells with large air spaces, providing a surface for gases to move within the leaf.
- Contains chloroplasts for photosynthesis, contributing to the overall energy production of the plant.
15
Q
What is the function of the xylem?
A
- Xylem is a crucial vascular tissue in plants responsible for transporting water and dissolved mineral ions from the roots to the rest of the plant.
- It also plays a vital role in providing structural support and can store food.
- This transport is facilitated by the process of transpiration, where water moves upwards through the xylem due to a difference in water potential between the roots and the leaves.
16
Q
What is the function of the phloem?
A
- Transport sugars and other organic molecules, primarily sucrose, from the leaves (source) to other parts of the plant where they are needed (sink).
- This transport is known as translocation.
- Phloem is a living tissue, unlike xylem, and is composed of sieve tube elements and companion cells.
17
Q
What is the function of the stoma?
A
- Stomata are tiny pores, primarily found on the underside of leaves, that regulate gas exchange and water vapour movement between the plant and its environment.
- Main function is to allow carbon dioxide to enter for photosynthesis and oxygen and water vapour to exit during transpiration.
- This is achieved through the opening and closing of the stomata, which is controlled by guard cells.
18
Q
What is the function of the air spaces?
A
- Air spaces within a leaf’s spongy mesophyll layer primarily facilitate gas exchange during photosynthesis and respiration.
- These spaces provide a pathway for gases like oxygen and carbon dioxide to diffuse into and out of the mesophyll cells, where photosynthesis occurs.
- The air spaces also contribute to the overall structure and flexibility of the leaf.
19
Q
What is the function of the lower epidermis?
A
- Facilitates gas exchange.
- It contains numerous tiny pores called stomata, which are controlled by guard cells to regulate the movement of carbon dioxide, oxygen, and water vapour.
- These pores are crucial for photosynthesis and transpiration.
20
Q
Gas exchange in plants
A
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
21
Q
What do plants need CO2 and O2 for?
A
- Photosynthesis (CO2)
- Respiration (O2)
- Both produce the other as a waste product
22
Q
How does the balance of photosynthesis to respiration change?
A
Depending on the time of day the balance of photosynthesis to respiration will create different concentration gradients which cause gases to diffuse in or out.
23
Q
What layer are the gases exchanged in the leaf?
A
Gases are exchanged with the atmosphere in the mesophyll layer of the leaf
24
Q
How is the spongy mesophyll adapted for gas exchange?
A
Has large surface area due to large air spaces
25
Where do gases move in and out of in the leaf?
* Leaves are thin to reduce the distance gases have to diffuse in and out of the leaf.
* Gases have to move in and out of the leaf through the stomata - pores at the bottom of the leaf
26
What is transpiration and what does the stomata do?
Transpiration is the loss of water from the plant,
controls how much water can leave the leaves
27
How do plants in dry enviroments prevent excessive water loss?
Plants in dry environments will have fewer stomata to help reduce water loss
28
What to guard cells do?
* The guard cells open and close the stomata depending on how much water is in the leaf
* When plants have enough water guard cells are turgid which keep the pores open.
* When plants are dehydrated the guard cells become flaccid causing the hole to close.
29
Water transport in plants
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
30
What do plants need water for?
* Photosynthesis
* To transport minerals
* Maintain structural rigidity (maintain turgidity of cells)
* Regulate temperature
31
How is water absorbed by the plant?
Water is absorbed from the soil by osmosis and moves up the stem of the plant from the roots to the leaves
32
How are mineral ions absorbed by plants?
Mineral ions absorbed by active transport from the soil (including nitrates and phosphates) are dissolved in the water
33
Why do plants require nitrates?
To produce:
* DNA
* Amino acids (proteins)
* Chlorophyll
Phosphates are also required to make DNA and ATP without these essential minerals, plants would not be able to grow, produce fruit or seeds
34
What is the structure of the xylem?
• Elongated tubes formed from dead cells (called vessel elements) joined together in bundles
• No end walls between cells
• Thick walls made of lignin
• Gaps in the cell wall
35
What is the function of the xylem?
The xylem vessels transport water and mineral ions up the plant and provides support
36
How does the xylem having a hollow lumen benefit the plant?
Their hollow lumen (no cytoplasm and no end walls) makes an uninterrupted tube allowing water and mineral ions to pass through the middle easily
37
How does the xylem having a walls made of lignin benefit the plant?
The lignin that forms the walls is strong and prevents the tube collapsing, gaps in the lignin allow water to leave the vessels and pass between them
38
How does the xylem having a waterproof lignin benefit the plant?
The lignin is also waterproof to prevent water from adhering too much to the surface
39
What is step one in the Cohesion - tension theory?
* Water evaporates from the mesophyll due to heat from the sun (Transpiration)
* This leaves the cells with a negativ water potential which causes more water to diffuse in through osmosis
40
What is step two in the Cohesion - tension theory?
* This increase in water tension pulls more water into the leaf
* This is known as transpiration pull
41
What is step three in the Cohesion - tension theory?
* Water molecules are cohesive due to the fact they form hydrogen bonds
* This is known as cohesion so when some are pulled into the leaf others follow
* This pulls the whole column of water in the xylem up from roots to mesophyll tissue in the leaves
42
What is step four in the Cohesion - tension theory?
Water enters the stem through the roots
43
What evidence is there to support the cohesion theory?
• If a trunk or stem is damaged and a xylem cell is broken water does not leak out (which it would if the vessels were under pressure). Once air enters the tree can no longer draw up water because the continuous column of water has been broken.
• The trunks of trees reduce in diameter during the daytime when transpiration is at its greatest (increased photosynthesis and temperature). This is because adhesion of water molecules to walls of xylem results in a tension which pulls the xylem walls in. At night when transpiration is at its lowest there is less tension so the diameter increases.
44
Investigating transpiration practical
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
45
How does water movement increase?
Water movement increases as transpiration rate increases
46
Where does water evaporate from?
Water evaporates from the spaces between cells in the leaf through the stomata, it moves down the water potential gradient so for water to leave through stomata there has to be less water in the atmosphere than in the leaf
47
What factors affect the transpiration rate?
* Light intensity
* Temperature
* Humidity
* Air movement
48
How does light intensity affect transpiration rate?
* Stomata open when it is light to let in CO2 for photosynthesis, when it is dark no photosynthesis is happening so the stomata close
* Little transpiration happens at night/darkness.
49
How does temperature affect transpiration rate?
* In warmer conditions (including more sun) water particles have more kinetic energy so they are able to evaporate from the surface of the leaf faster
* This increases the water potential gradient between the inside and outside of the leaf so the transpiration rate is increased.
50
How does humidity affect transpiration rate?
* If the air around plants is dry the water potential gradient is increased so transpiration occurs faster
* In humid environments the water potential gradient is reduced as there is a lot more water in the atmosphere so the rate of transpiration decreases
51
How does air movement affect transpiration rate?
Lots of wind blows water molecules away from the stomata increasing the water potential gradient and therefore increasing the transpiration rate
52
What does a potometer measure?
A potometer can measure water uptake by the plant and can be used to estimate how different factors affect the transpiration rate e.g using a fan, creating a humid environment, using different light intensities
53
How should the shoot be cut?
* The shoot must be cut underwater to prevent air entering the xylem (so the water column remains broken)
* It should also be cut on an angle to increase the surface area available to uptake water
54
How can you make the apparatus water tight and what do you need to know about the capillary tube?
* The apparatus must be airtight and water-tight - Vaseline or grease can be placed around joints to help with this
* The end of the capillary tube must stay submerged in water, the diameter of the capillary tube should be known/measured
55
Why do the leaves need to be dry and acclimatised?
* Leaves must be dry and the plant allowed to acclimatise (start transpiring) before the experiment is started
* Only one variable can be changed at a time all others must be kept constant so that they do not affect the results
* If using different plants the surface area should be measured as it will vary and can affect the rate e.g more stomata
56
Why is only an estimate gained?
This is only an estimate as it assumes all water taken up is lost in transpiration but some water is used up in reactions
e.g photosynthesis, some is produced by respiration, and some is used to support the plant (stored in vacuoles)
57
What is step one for measuring transpiration?
Removing the capillary tube from the beaker of water for a short time allows an air bubble to form at the end
58
What is step two for measuring transpiration?
The start point of the air bubble must be recorded - its position can be pushed back by emptying water from the reservoir using the tap (this can be used to restart/repeat the experiment
59
What is step three for measuring transpiration?
* Using a stopwatch the time it takes for the bubble to move a certain distance can be recorded
* The rate (distance per unit time eg mm/min) is an estimate of the transpiration rate.
60
Adaptations in xerophytes
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
61
What are xerophytes?
* For plants in hot, dry or windy climates (exposed places such as deserts, mountains or tundra) they have to have extra adaptations to prevent too much water loss when their stomata are open
* These plants are known as xerophytes
* Most of their adaptations reduce the factors that can increase transpiration
62
How does the adaptation of having small leaf surface area help xerophytes?
* Reduced surface area for evaporation/fewer stomata
* E.g Conifer needles, cactus spines
63
How does the adaptation of having sunken stomata help xerophytes?
* Maintains humid air around the stomata to reduce the water potential gradient
* E.g. Marram grass, cacti
64
How does the adaptation of having stomatal hairs (trichores) help xerophytes?
* Maintains humid air around the stomata to reduce the water potential gradient and reduce evaporation
* E.g. Marram grass, couch grass
65
How does the adaptation of having rolled leaves help xerophytes?
* Reduces the effects of wind to reduce the water potential gradient and reduce evaporation
* E.g. Marram grass
66
How does the adaptation of having extensive root systems help xerophytes?
* Maximises water uptake, helps to increase chances of contact with water, often shallow but wide area to absorb rainfall. They often have swollen stems to store the collected water (succulents)
* E.g. Cacti, agave, yucca
67
How does the adaptation of having a reduced number of stomata help xerophytes?
* Reduce the amount of places water can evaporate from
68
How does the adaptation of having a thicker, waxy cuticle help xerophytes?
* Waterproof leaves and stems to reduce evaporation
* E.g. Yucca, agave, citrus plants
69
How can you investigate the stomatal density?
* By taking thin layers of epidermis or "imprints" of the leaf surface and examining under the microscope the number of stomata in an area of leaf tissue (stomatal density per mm?) can be calculated
* Multiple fields of few or leaf sections should be examined, and a mean taken so the sample is representative
* It can then be scaled up to the whole leaf
70
Mass flow hypothesis
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
71
What is translocation?
Translocation is the process by which products of photosynthesis are transported from a source (where they are made - usually leaves) to a sink (a tissue that needs them)
72
Where does translocation occur?
Translocation is carried out by the phloem tissue which is made up of sieve-tube elements and their companion cells
73
What is the structure of the phloem?
* A tube formed from elongated living cells called sieve tubes
* They have no nucleus or organelles just a thin layer of cytoplasm around the edge of the cell this maximises space for mass flow
* Each sieve tube is supported by a companion cell which has the organelles including a nucleus and many mitochondria to produce ATP and help sieve tubes carry out living functions
* There are end walls between cells but these contain holes to let solutes pass through - this is why they are known as sieve plates
74
What are sources?
Sources are nearly always leaves as this is where the majority of photosynthesis takes place.
75
What are sinks?
Sinks can be any part of the plant that has a high energy demand.
76
What examples are there of sinks?
* Roots carrying out active transport of mineral ions
* Any actively growing/dividing tissue - the meristems of roots and shoots
* Where storage of nutrients is occurring e.g seeds, fruits, or tubers
77
Where are sinks found in a plant?
Some of these sinks will be above the sources and some will be below, this means that translocation has to be bidirectional (unlike water movement in the xylem)
78
Why is translocation an active process?
Translocation being bidirectional means it must also be an active process which requires energy as the solutes (sucrose and other organic molecules) are pushed around under high pressure
79
Why is sucrose used when testing the mass flow hypothesis?
Sucrose is used as it contains more energy and is less reactive and less likely to be oxidised and form products during transport
80
What does the mass flow hypothesis rely on?
Creating hydrostatic pressure and creating a concentration gradient
81
What is step one in the mass flow hypothesis?
1. Source - releases sucrose into phloem
* High sucrose concentration, sucrose moved into sieve tubes by active loading. Decreasing water potential so that water moves in by osmosis from xylem increase in volume increases turgor pressure.
82
What is step two in the mass flow hypothesis?
2. Sink- removes sucrose from phloem
* Low sucrose concentration as sucrose moves out of sieve tubes down a concentration gradient as cells use it or convert to glucose/starch.
* Increases water potential in sieve tubes so water moves back to xylem by osmosis, reduction in volume decreases turgor pressure
83
What is step three in the mass flow hypothesis?
3. Mass Flow Of Solutes
* Turgor pressure gradient created between source and sink causes solutes to be pushed in that direction.
* Concentration gradient of sucrose between source and sink created as sucrose constantly being used for respiration or converted for storage.
84
How does pumping H+ ions out of the companion cell affect the sucrose concentration?
Active Loading of sucrose into companion cells at source
* H+ ions are actively pumped out of the companion cell (using ATP) into cells of the source tissue.
* This creates a hydrogen concentration gradient across the companion cell membrane
* H+ ions can diffuse back down the gradient through special transport proteins which require a sucrose molecule to be co-transported.
* This increases the concentration of sucrose in the companion cells which can then diffuse into the sieve tube.
85
What evidence is there around the sieve tubes that supports the mass flow hypothesis?
When sieve tubes are cut sap is released - this demonstrates that unlike xylem sap is under positive pressure within the phloem.
* This can also be seen in aphids which tap into phloem to consume sap - the high pressure forces it through their digestive system resulting in the excretion of honeydew!
* Honeydew tends to flow faster out of aphids higher up the plant (near leaves) than lower down the stem - evidence for a pressure gradient.
86
What evidence is there around the sucrose concentration that supports the mass flow hypothesis?
* Sucrose concentration higher in leaves than roots (this supports the presence of sources and sinks)
* Increases in sucrose in leaf are followed by increases in sucrose in phloem
87
What evidence is there around the phloem that supports the mass flow hypothesis?
Downward flow in phloem occurs in daylight when photosynthesis is occurring but not at night - greater respiration at night and lack of photosynthesis would mean there is no longer a concentration/pressure gradient
88
What evidence is there around ATP that supports the mass flow hypothesis?
* Preventing ATP production stops translocation but not water movement
* Lack of ATP for active transport in companion cells - companion cells would die which would prevent mass flow
89
What evidence is there around the rings of bark that supports the mass flow hypothesis?
* If a ring of bark (containing phloem) is removed from a woody stem then a bulge forms above the ring - it is a build up of fluid that cannot continue
* There is more sugar in the fluid above the bulge than below which gives evidence for downward flow of sugars.
90
What evidence is there against the mass flow hypothesis?
* Sugar travels to many different sinks and does not travel first or faster to the one with the highest water potential which it should according to the mass flow hypothesis.
* The end plates of the sieve tubes would create a barrier to mass flow, or slow it down so why are they present?
91
Mass flow experiment
✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼✼
92
What are the two types of experiment that can be conducted to get evidence for the mass flow hypothesis?
* Ringing Experiments
* Tracing Experiments
93
What is the ringing experiment?
* Removal of a ring of bark which includes the phloem but leaves the xylem intact inside.
* Shows swelling above and below the ring
94
What does the ringing experiment show?
* This then shows swelling above the ring and samples of fluid above and below the ring can be sampled to show the difference in concentration of sugars.
* High concentration of sugars building up in cells above the ring cause water to move into the cells from the xylem which contributes to the swelling. * After a while non-photosynthetic tissues below the ring will begin to be affected e.g lack of growth and eventually death while everything above the ring will continue to grow perfectly fine.
95
What benefits are there of ringing?
* Growers and farmers sometimes use ringing - or a gentler form of scoring to break the flow of nutrients from the leaves to the roots for a short time.
* This could be done to reduce length of new branches, but it also enhances fruit growth and increases flower bud formation.
* This is because more nutrients are transported to the upper parts of the tree (above the score) than to the roots.
96
What is the tracing experiment?
* These rely on the plant taking up a substance that has a radioactive label - this could be in the form of CO2 which has C14 present.
* This would then be used by the plant in photosynthesis to produce sugars which can then be tracked around the plant as they are translocated.
* To compare phloem and xylem radioactive mineral ions could be dissolved in water and taken up from the roots into the xylem.
97
What does tracing rely on?
* Tracing relies on autoradiography - the plant needs to be killed either freezing with liquid nitrogen or drying at 100 °C so that all water is evaporated but the plant does not burn!
* Then the plant (or thin cross-sections) is placed onto a photographic film.
* Like an X-ray areas of radioactivity will change the colour of the film and turn it black (this can be reversed so the radioactivity appears white).
98
What is correlation for data?
* Correlation - where one variable being changed effects another.
* This can be positive (as one increases so does the other or negative (as one increases the other decreases).
99
What do you need to be careful of when interperting data?
* Correlation does not always equal causation - be careful when interpreting results!
* Most data will support an idea or hypothesis and you can try to explain it but be careful of saying something has proven something has happened, especially when there is little
