Shoot, Architecture & Light Capture

Question 1

Describe the anatomy of typical dicot and monocot leaves

Answer 1

• In most vascular plants, leaves are the main photosynthetic organs
• The exchange of gases occurs through stomata in the upper and lower epidermis
• Epidermis is usually a single cell layer
• In between the upper and lower epidermis, there’s a ground tissue called the mesophyll
• This tissue consists of parenchyma cells specialized for photosynthesis

Question 2

What are the main photosynthetic organs of most vascular plants?

Answer 2

Leaves

Question 3

How does the gas exchange of typical monocot and dicot leaves occur?

Answer 3

Stomata in upper and lower epidermis

Question 4

Where is the mesophyll located?

Answer 4

In-between the upper and lower epidermis

Question 5

What’s the functions of the cells of the mesophyll?

Answer 5

This tissue consists of parenchyma cells specialized for photosynthesis

Question 6

Describe dicot leaves

Answer 6

• In dicot leaves, stomata are mainly found in the lower epidermis
• The mesophyll consists of 2 distinct layers of cells called palisade and spongy
• Palisade mesophyll consists of elongated cells arranged in one or more layers. This can be found in the upper part of the leaf, just beneath the upper epidermis
• Spongy mesophyll can be found between the palisade layer and the lower epidermis
• They’re loosely arranged cells with many air spaces
• Spongy mesophyll cells have less chloroplasts than in palisade mesophyll cells
• The vascular tissue of the leaf is continuous with vascular tissue of the stem
• Veins in the leaf are highly branched (net like venation) in the mesophyll layer
• Each vein is protected by a bundle sheath layer

Question 7

Where are stomata found in dicot leaves?

Answer 7

Mainly on the lower epidermis

Question 8

Describe the cells present in dicot leaves

Answer 8

• The mesophyll consists of 2 distinct layers of cells called palisade and spongy
• Palisade mesophyll consists of elongated cells arranged in one or more layers. This can be found in the upper part of the leaf, just beneath the upper epidermis
• Spongy mesophyll can be found between the palisade layer and the lower epidermis
• They’re loosely arranged cells with many air spaces
• Spongy mesophyll cells have less chloroplasts than in palisade mesophyll cells

Question 9

What are the 2 distinct cell layers of the mesophyll?

Answer 9

1. Spongy mesophyll
2. Palisade mesophyll

Question 10

What’s the structure of palisade mesophyll?

Answer 10

• Palisade mesophyll consists of elongated cells arranged in one or more layers.

Question 11

Where are palisade mesophylls found?

Answer 11

in the upper part of the leaf, just beneath the upper epidermis

Question 12

Where are spongy mesophylls found?

Answer 12

Spongy mesophyll can be found between the palisade layer and the lower epidermis

Question 13

What’s the structure of spongy mesophylls?

Answer 13

• They’re loosely arranged cells with many air spaces

Question 14

What’s the vascular tissue of the leaf continuous with?

Answer 14

Vascular tissue of the stem

Question 15

Describe the vein arrangement in the mesophyll layer of dicot leaves

Answer 15

Veins in the leaf are highly branched (net-like venation) in the mesophyll layer

Each vein is protected by a bundle sheath layer

Question 16

What is each vein protected by?

Answer 16

A bundle sheath layer

Question 17

Write the difference between palisade mesophylls and spongy mesophylls

Answer 17

Location: Palisade mesophylls are found in the upper part of the leaf, beneath the upper epidermis. Spongy mesophyll can be found between the palisade layer and the lower epidermis

Structure: Palisade mesophylls consist of elongated cells arranged in one or more layers. Spongy mesophylls are loosely arranged cells with many air spaces

Chloroplast Content: Spongy mesophylls cells have less chloroplasts than palisade mesophyll cells

Question 18

Describe the structure of monocot leaf

Answer 18

• In monocot leaves, stomata are present in both the lower and upper epidermis
• Mesophyll are not differentiated into palisade and spongy layers
• Chloroplast are abundant in all mesophyll cells
• Vascular tissue of the leaf is continuous with vascular tissue of the stem
• Veins in the leaf are pararally arranged
• Each vein is protected by a bundle sheath layer

Question 19

Write the functions of each part of the leaf

Answer 19

1. Cuticle: Prevents water loss and for Protection
2. Epidermis: Protection against pathogens and physical damage
3. Stomata: Gaseous exchange and transpiration
4. Palisade and Spongy mesophylls: Involving in efficient photosynthesis
5. Spongy parenchyma/ air spaces: Photosynthesis and storage of gases
6. Vein/ Vascular bundle/ xylem/ phloem: Involving in transport
7. Sclerenchyma: Support
8. Guard cells: Controlling gaseous exchange and transpiration

Question 20

Write the differences between dicot and monocot leaf structures

Answer 20

Stomata: In dicot leaves, stomata are mainly found in lower epidermis. In monocot leaves, stomata are present in both the lower and upper epidermis

Mesophylls: In monocot leaves, mesophyll is not differentiated into palisade and spongy. In dicot leaves, there are 2 distinct cell layers called palisade and spongy

Vein arrangement: In dicot leaves, veins are highly branched, with a net like venation in the mesophyll layer. In monocot leaves, veins are parallelly arranged (pararall venation)

Question 21

Define stomata

Answer 21

Pores surrounded by guard cells which can open and close, found in the epidermis of leaves and stems of plants

Question 22

Define guard cells

Answer 22

Guard cells are modified epidermal cells which have a distinct shape, and are the only epidermal cells that contain chloroplasts

Question 23

Describe guard cells

Answer 23

• Guard cells are modified epidermal cells which have a distinct shape, and are the only epidermal cells that contain chloroplasts
• Guard cells are typically bean shaped in angiosperms
• The cell walls of guard cells are unevenly thickened
• The inner cellulose wall is thicker and less elastic than the outer wall
• Some cellulose microfibrils are radially arranged to form inelastic hoops around guard cells
• Guard cells regulate the diameter of stomata by changing shape, widening or narrowing the gap between the pair of guard cells

Question 24

In angiosperms, what is the shape of guard cells?

Answer 24

Typically bean shaped

Question 25

Describe the cell wall of guard cells

Answer 25

• The cell walls of guard cells are unevenly thickened
• The inner cellulose wall is thicker and less elastic than the outer wall

Question 26

How are inelastic hoops formed in guard cells?

Answer 26

• Some cellulose microfibrils are radially arranged to form inelastic hoops around guard cells

Question 27

What’s the function of guard cells?

Answer 27

• Guard cells regulate the diameter of stomata by changing shape, widening or narrowing the gap between the pair of guard cells

Question 28

Describe the structure of stomata

Answer 28

• Stomata are pores surrounded by guard cells which can open and close, found in the epidermis of leaves and stems of plants
• Guard cells are modified epidermal cells which have a distinct shape, and are the only epidermal cells that contain chloroplasts
• Guard cells are typically bean shaped in angiosperms
• The cell walls of guard cells are unevenly thickened
• The inner cellulose wall is thicker and less elastic than the outer wall
• Some cellulose microfibrils are radially arranged to form inelastic hoops around guard cells
• Guard cells regulate the diameter of stomata by changing shape, widening or narrowing the gap between the pair of guard cells

Question 29

Describe gaseous exchange

Answer 29

• Gaseous exchange is the exchange of gases between the cells of the organism and the environment
• In plants, gaseous exchange is possible via stomata and lenticels
• In addition to these, a small amount of gases can be exchanged via the cuticle
• There’s no special system within plants to transport O2 and CO2. These gases move entirely by diffusion

Question 30

Define gaseous exchange

Answer 30

Gaseous exchange is the exchange of gases between the cells of the organism and the environment

Question 31

What are the main modes of gaseous exchange in plants?

Answer 31

• In plants, gaseous exchange is possible via stomata and lenticels
• In addition to these, a small amount of gases can be exchanged via the cuticle

Question 32

Explain the mechanism of opening and closing of stomata

Answer 32

• Stomatal opening and closing depends on changes in turgor of guard cells
• If water flows into the cells by osmosis, their turgor increases and they expand, but they do not expand uniformly in all directions
• The relatively inelastic inner walls make them bend and draw away from each other other
• The result is that the pore opens
• If the guard cells lose water, the reverse happens: their turgor decreases and their inner walls become straighter, thus closing the pore.
• The K+ influx explains the mechanism

Question 33

Explain the K+ influx hypothesis

Answer 33

• During daytime, guard cells actively accumulate K+ from neighboring epidermal cells, thus lowering their water potential.
• That leads to the inflow of water by osmosis from the surrounding epidermal cells
• As a result, the turgor pressure in guard cells increase, opening stomata
• The accumulate of K+ in guard cells require energy which is provided by the transfer of electrons during photosynthesis of guard cells
• Stomatal closing occurs by losing K+ from guard cells to neighboring epidermal cells
• It leads to exosmosis of water from guard cells
• As a result, the turgor pressure of guard cells decrease, closing stomata.
• Abscisic Acid (ABA) plays a role in K+ influx mechanism as well

Question 34

Explain stomatal opening in K+ influx mechanism

Answer 34

• During daytime, guard cells actively accumulate K+ from neighboring epidermal cells, thus lowering their water potential.
• That leads to the inflow of water by osmosis from the surrounding epidermal cells
• As a result, the turgor pressure in guard cells increase, opening stomata
• The accumulate of K+ in guard cells require energy which is provided by the transfer of electrons during photosynthesis of guard cells

Question 35

Explain stomatal closure in K+ influx mechanism

Answer 35

• Stomatal closing occurs by losing K+ from guard cells to neighboring epidermal cells
• It leads to exosmosis of water from guard cells
• As a result, the turgor pressure of guard cells decrease, closing stomata.

Question 36

What’s the role of ABA in K+ influx mechanism?

Answer 36

• ABA is produced in roots and leaves in response to water deficiency
• Production of ABA leads to closure of stomata in drought by the removal of K+ in guard cells
• This prevents wilting of the plant

Question 37

Where is ABA produced?

Answer 37

In roots and leaves

Question 38

What is ABA produced in response to?

Answer 38

Water deficiency

Question 39

How does ABA prevent the wilting of the plant during a drought?

Answer 39

Production of ABA leads to closure of stomata in drought by the removal of K+ in guard cells. This prevents wilting of the plant

Question 40

What are the factors affecting stomatal action?

Answer 40

• Light stimulates the accumulation of K+ in guard cells. Stomata open during the day and mostly close at night
• Decrease in CO2 concentration in substomatal cavity leads to opening of stomata
• Internal clock of guard cells controlling their daily rhythm of opening and closing stomata
• Environmental stresses such as drought, wind and high temperature can cause stomata to close during day time.

Question 41

What are the environmental stresses that can cause stomata to close during day time?

Answer 41

Wind
Drought
High temperature

Question 42

How does light affect stomatal action?

Answer 42

Light stimulates accumulation of K+ ions in guard cells.

So, stomata open during the day and close during the night

Question 43

How does CO2 concentration affect stomatal action?

Answer 43

Decrease of CO2 concentration in substomatal cavity leads to opening of stomata