4.2.3.2 Plant Organ System

Question 1

What is a plant organ system made up of?

Answer 1

• Roots, stems, and leaves for transport of substances (sugars, water, and minerals) around the plant
• Specialist tissues include root hair cells, phloem and xylem

Question 2

What is the purpose of water in plants?

Answer 2

• Water is necessary for photosynthesis (how plants use E from Sun to create food)
• To maintain cell turgidity for structure and growth
• Helps in germination of seeds
• Helps in transport of nutrients and minerals from soil to plants
• Water is an essential nutrient for plants and comprises 95% of a plants tissue
• Transpiration from leaves stops plants from overheating
• Warmer temps, wind and dry air increases rate of transpiration
• As water evaporates through the leaves, more water is pulled up through the roots of the plant

Question 3

Function of root hair cells:

Answer 3

• Efficient uptake of water and mineral ions
• Root hair cells absorb water by osmosis
• Root hair cells absorb minerals (nitrate ions) by active transport

Question 4

Adaptations of root hair cells:

Answer 4

• Large surface area with a projection/root hair
• Thin cellulose walls
• Many mitochondria
• Adapted for efficient uptake of water by osmosis, and mineral ions by active transport

Question 5

Function of xylem:

Answer 5

• Xylem tissue transports water and mineral ions from the roots to the stems and leaves.
• They are involved in a process that carries water and mineral ions from the roots to the leaves, called the transpiration stream
• Transport of water in transpiration stream

Question 6

Adaptations of xylem:

Answer 6

• Mature xylem consists of elongated dead cells arranged end to end to form continuous vessels (tubes)
• Mature xylem vessels contain no cytoplasm
• Have tough walls containing a woody material called lignin that builds up in spirals in cell walls - lignin makes spirals very strong, to withstand the pressure from the water + provide support. - adapted for transport of water in transpiration stream
• The contents and end walls break down to form a hollow centre (lumen)

Question 7

Function of phloem:

Answer 7

Phloem tissue transports dissolved sugars and amino acids from the leaves to the rest of the plant for immediate use or storage - this movement of food molecules through phloem tissue is called translocation

Question 8

Adaptations of phloem:

Answer 8

• composed of tubes and elongated cells, made of living cells lined end to end, found in the vascular bundle
• Phloem vessels are made of sieve elements
• The cell walls between phloem cells break down to form perforated sieve plates. These allow water carrying dissolved food (sugars and amino acids) to move freely along the tubes from one phloem cell to the next through pores in the end walls
• Supported by companion cells (which contain nucleus and mitochondria) that keep them alive. The mitochondria of the companion cells transfer the energy needed for translocation to occur
• Cells that have lost most of their internal structure – e.g. no vacuole or nucleus. This helps to reduce any resistance during translocation and allows efficient movement of substances
• Vessels contain cytoplasm
• Two-way flow allows substances to be transported all around the plant

Question 9

How are xylem and phloem grouped in plants?

Answer 9

• Xylem and phloem are found grouped in vascular bundles - the position of these vary depending on the part of the plant
• The phloem vessels are found on the outside of xylem vessels

Question 9

Where are xylem and phloem vessels found in stems?

Answer 9

• The stem has to resist compression (squashing) and bending forces caused by the plants weight and the wind
• The vascular bundles are arranged near the edge of the stem, with the phloem in the outside and the xylem on the inside of the vascular bundle

Question 10

Where are xylem and phloem vessels found in leaves?

Answer 10

• The vascular bundle are located among the spongey mesophyll
• Phloem nearer lower surface

Question 11

How does water move through a plant (transpiration stream)?

Answer 11

1. Root hair cells have lots of mitochondria for releasing energy for the uptake of mineral ions.
2. Root hair cells have a large surface area for rapid water uptake.
3. Water enters root hairs by osmosis.
4. Water moves across the root one cell at a time.
5. Water enters the xylem of the stem.
6. Xylem is composed of hollow vessel/lumen strengthened by lignin.
7. Water moves up the xylem in the stem.
8. Water leaves the xylem and enters the leaf cells by osmosis.
9. Water moves across the leaf one cell at a time by osmosis.
10. Some water is used in the cells for photosynthesis.
11. The remaining water evaporates into the air spaces of the spongy mesophyll.
12. In the air spaces the humidity is now high.
13. From the air spaces water diffuses out of the stomata and evaporated off the surface of the leaf.

Question 12

Transpiration stream:

Answer 12

• The flow of water through a plant, from the roots to the leaves, via the xylem vessels
• Water will eventually be lost by evaporation out of the stomata

Question 13

Transpiration:

Answer 13

The loss of water from leaves by evaporation

Question 14

What happens in the transpiration stream?

Answer 14

1. Water moves through the plant from the roots up tothe leaves.
2. When the guard cells are open, water evaporatesout of the leaf via the stomata.
3. This creates a shortage of water in the leaf, sowater is drawn up from the roots, via the xylem toreplace it.
4. Water molecules inside the xylem cells arestrongly attracted to each other. There is strongcohesion between the molecules becauseofhydrogen bonding. A continuous column ofwater is therefore pulled up the stem.
5. More water is drawn into the roots via osmosis toreplace water lost inside the roots.
6. This is a constant cycle called thetranspirationstream.

Question 15

Cohesion:

Answer 15

water molecules are held together by hydrogen bonds and act as a column which is why water molecules ‘stick together’ when travelling up the xylem

Question 16

What are potometre’s used for?

Answer 16

• Used to measure the uptake of water by plants in different conditions
• The movement of the air bubble in the tube shows how much water is taken up

Question 17

How is a potometer is used to measure the uptake of water to measure the rate of transpiration?

Answer 17

1. Fill a beaker with water.
2. Place a capillary tube in the beaker of water and attach it to the bottom of a test tube.
3. Place a plant in the test tube sealed with an air tight bung.
4. Add a ruler beneath the capillary tube.
5. Inside capillary tube there is an air bubble - water from test tube absorbed through xylem and then evaporate from stomata - transpiration.
6. Use a ruler to measure the distance moved by air bubble.
7. Air bubble in capillary tube and water is going to move up from the beaker.

Question 18

How accurate is a potometer at measuring transpiration rate?

Answer 18

It measures water uptake, not the water lost, as some of the water will be used in photosynthesis so it isn’t very accurate

Question 19

Why is water uptake not the same as transpiration uptake?

Answer 19

• It is only a rough gauge as some water taken up by the roots is used inphotosynthesis (5-10%)
• Transpiration will still occur in a water shortage, which will ultimately lead to wilting

Question 20

Describe how bright light affects the rate of transpiration:

Answer 20

In bright light transpiration increases

Question 21

Explain how light affects the rate of transpiration:

Answer 21

The stomata (openings in the leaf) open wider to allow more CO2 into the leaf for photosynthesis

Question 22

Describe how temperature affects the rate of transpiration:

Answer 22

Transpiration is faster in higher temps

Question 23

Explain how temperature affects the rate of transpiration:

Answer 23

Evaporation and diffusion are faster at higher temps

Question 24

Describe how wind affects the rate of transpiration:

Answer 24

Transpiration is faster in windy conditions

Question 25

Explain how wind affects the rate of transpiration:

Answer 25

Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf

Question 26

Describe how humidity affects the rate of transpiration:

Answer 26

Transpiration is slower in humid conditions

Question 27

Explain how humidity affects the rate of transpiration:

Answer 27

Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air

Question 28

Disadvantage of having a large number of stomata per mm squared on a leaf:

Answer 28

• Means a high rate of transpiration
• Lots of water vapour evaporates from the surface of the leaf
• Plant may wilt and possibly die

Question 29

Why would very little water be lost if grease blocked stomata on the lower surface?

Answer 29

• Stomata found on lower surface oftheleaf
• Grease blockedstomata
• Water vapour could no longerevaporate out of stomata

Question 30

Why might stomata be called a ‘necessary evil’?

Answer 30

Stomata need to open to let carbon dioxide in for photosynthesis but it does allow for water to be lostby transpiration and if too much water is lost it could cause the plant to wilt

Question 31

Translocation:

Answer 31

movement of dissolved sugars and amino acids up and down (around) the plant

Question 32

What does the phloem transport?

Answer 32

• Phloem transports sucrose and amino acids up anddown the plant
• This is calledtranslocation
• Chemicalse.g.pesticides will also move throughthe phloem by translocation

Question 33

Why is translocation important?

Answer 33

Translocation is important as sugar is made in theleaves via photosynthesis (source) and needs tomove location to be used for either respiration orstorage (sink)

Question 34

Function of the stomata:

Answer 34

Allow gases to move in and out (carbon dioxide in, oxygen out) so to control gas exchange and to control water loss of the plant

Question 35

Adaptations of stomata:

Answer 35

• Inner cell wall is thicker tocause the cell to curve tomake a hole
• Gas exchange and water is loss occurs out of the stomata
• Surrounding each stoma are 2 guard cells - guard cells open and close to control gas exchange and water loss

Question 36

Guard cells function:

Answer 36

• Surrounding each stoma are 2guard cells
• Guard cells open and close the stoma to control gas exchange andwater loss of the plant

Question 37

How can the volume of water lost be controlled?

Answer 37

Opening and closing the stomata using guard cells

Question 38

What do stomata do in bright light? - (need more carbon dioxide for photosynthesis):

Answer 38

1. In bright light, potassium ions move into the guard cells
2. This makes the guard cells more concentrated (less dilute) than surrounding tissue
3. Water moves into the guard cell by osmosis (across a partially permeable membrane)
4. Cell swells unevenly because the thicker inner cell wall is less flexible than the thinner outer wall
   - the opposite happens when in dark light

Question 39

Why might stomata close in environments with very high CO2?

Answer 39

• High carbon dioxide in the air, less stomata need to be open for a high rate ofphotosynthesis
• Having less stomata open means less water is lost – advantage

Question 40

Why is there a difference between the number of stomata on the top and bottom surface of a leaf?

Answer 40

• Fewer / no stomata on top surface of leaf as it is in direct sunlight so would be very warm and bright
• Rate of transpiration would be very high
• Plant would loose water due to evaporation of water vapour
• Concentration gradient would be steep
• Plant would wilt and possibly die
• The surface of the leaf is brighter and warmer – there would be a very high rate of transpiration solots of water lost out of the stomata
• (Most) plants have evolved to have stomata only on the lower surface as carbon dioxide can stillmove in and out of the stomata – but less water is likely to be lost via transpiration
• The plant is less likely to wilt due to excessive water loss

Question 41

Translocation:

Answer 41

movement of dissolved sugars and amino acids around the plant (up and down the plant)

Question 42

What is the process of translocation?

Answer 42

1. Sugars are made in the leaf cell (source) by photosynthesis.
2. The sugars (sucrose) are actively moved into the companion cell from the leaf.
3. From the companion cell the sugars diffuse into the phloem vessel.
4. The high concentration of sugars at the top of the phloem.
5. Causes water to also enter the phloem vessel by osmosis.
6. The movement of water into the phloem vessel causes the translocation of sugars to the root (sink).
7. In the root sucrose leaves the Phloem vessel and is stored as starch.

Question 43

Why is translocation important?

Answer 43

Translocation is important as sugar is made in theleaves via photosynthesis (source) and needs tomove location to be used for either respiration orstorage (sink)

Question 44

How is sucrose transported in Summer through a plant?

Answer 44

1. In Summer there is lots of photosynthesis so lots of sugar is made.
2. In the summer the source is in in the leaves as the leaves (photosynthesising leaves) produces sucrose.
3. Sucrose is (mostly) transported to the roots from the leaves and is stored as starch.
4. The sink is in the roots, the sucrose is converted into starch in the roots and it is for storage of excess sucrose.

Question 45

How is sucrose transported in Spring through a plant?

Answer 45

1. In Spring, the sugar is sent to the aid the new shoots’ growth.
2. The sink is in the leaves, the sucrose is used by the new shoots to grow (the sink is in the growing areas e.g. the new shoots and stem)
3. Sucrose is sent from the roots to the new shoots.
4. The source is in the roots (storage organs), starch is converted into sucrose in the roots.

Question 46

How is sucrose transported in Winter through a plant?

Answer 46

1. In the Winter, there is no light for photosynthesis (no leaves for photosynthesising).
2. Therefore the only source of sugar can be the storage organs in the roots.
3. The sink is any other part of the plant that needs sugar for respiration.

Question 47

Sources and sinks:

Answer 47

• Source: where the sucrose is made
• Sink: where the sucrose is stored or used

Question 48

What is the difference between transpiration and evaporation?

Answer 48

• Water vapour leaves out bottom of leaf for transpiration
• Water leaves out top of leaf for evaporation

Question 49

Xerophyte: + example

Answer 49

A plant adapted to live in dry conditions. e.g. Marram Grass

Question 50

Adaptations of a Marram Grass leaf (xerophyte):

Answer 50

• Thicker waxy cuticle - prevents evaporation of water
• Rolled leaf - reduces leaf surface area, reducing water loss
• Reduced leaf size - reduces leaf surface area
• Few air spaces - less evaporation of water
• Long tap root - during rainfall/condensation allows max absorption of water
• Reduced number of stomata - less evaporation of water from leaf
• Stomata in sunken pits or grooves - traps water vapour humidity, reduces water gradient, increased vapour
• Stomata surrounded by hairs - traps water vapour, increased humidity, reduces water vapour gradient
• Stomata close - prevents evaporation of water
• Widespread roots - can reach deep water sources
• High salt concentration found in root cells - water moves down a conc gradient therefore not using any E

Question 51

What are root hair cells adapted for?

Answer 51

Root hair cells are adapted for the efficient uptake of water and mineral ions

Question 52

Where are phloem and xylem vessels found in roots?

Answer 52

• Xylem Vessels are tough and strong so the vascular bundles are in the centre of the root to resist forces that could pull the plant out of the ground
• Phloem vessels are on the outside of xylem vessels

Question 53

Diagram of xylem and phloem vessels in the root:

Answer 53

Question 54

Diagram of xylem and phloem vessels in the leaf:

Answer 54

Question 55

Diagram of xylem and phloem vessels in the stem:

Answer 55

Question 56

Diagram of stomata and guard cells:

Answer 56