6b. Plant Systems: Plant Structures & Funcitons

Question 1

Cuticle

Structure & Function

Answer 1

Structure:

• Waxy and transparent
• No chloroplast
• Composed of cutin (lipid)

Function:

• Waxy to reduce water loss through evaporation from the leaf and prevents the invasion of bacteria or viruses
• Transparent for light to enter the leaf

Question 2

Epidermis (Upper & Lower)

Structure & Function

Answer 2

Structure:

• Single layer of closely packed cells (horizontal)
• Covered on the outside with cuticle
• No chloroplast

Function:

• Protects the inner cells
• Allows light to pass through to the mesophyll

Question 3

Palisade Mesophyll

Structure & Function

Answer 3

Structure:

• Closely packed, long and cylindrical cells nearest to the upper epidermis (vertical)
• Contain the largest number of chloroplasts

Function:

• To absorb light energy for photosynthesis

Question 4

Spongy Mesophyll

Structure & Function

Answer 4

Structure:

• Irregularly shaped cells
• Contain chloroplasts for photosynthesis
• Loosely packed with large intercellular air spaces among the cells to increase surface area for gaseous exchange
• Cells are covered with a thin film of moisture to allow gases to dissolve and diffuse more easily
• Contains vascular bundles

Function:

• Photosynthesis
• Gaseous exchange

Question 5

Vascular Bundle

Structure & Function

Answer 5

Structure:

• Xylem (dead structure)
• Phloem
• Cambium (consists of undifferentiated cells which can divide to form new xylem and phloem)
• Network is extensive

Function:

• Extensive network provides effective transport
• Xylem transports water and mineral salts from roots to mesophyll cells in leaf
• Phloem transports sucrose and amino acids away from the leaf to other parts of the plant

Question 6

Large Intercellular Air Spaces

Structure & Function

Answer 6

Structure:

• Interconnecting system of air spaces in the spongy mesophyll

Function:

• Allow circulation of air inside leaf for photosynthesis and respiration
• Allow for rapid diffusion of carbon dioixde and oxygen into and out of the cells

Question 7

Guard cells

Structure & Function

Answer 7

Structure:

• Between epidermal cells
• Contains chloroplasts
• Cell wall near the stoma is thicker than elsewhere in the cell

Function:

• Regulate the size of the stomata for gaseous exchange and transpiration

Question 8

Stomata

Structure & Function

Answer 8

Structure:

• Opening in the middle of two guard cells

Function:

• Allow carbon dioxide and oxygen to diffuse in and out of the leaf (gaseous exchange)
• Allow water vapour to diffuse out of the leaf (transpiration)

Question 9

6 Ways a Leaf is Adapted for Photosynthesis

Answer 9

Adaptations of Leaf:

1. Waxy and transparent cuticle on upper and lower epidermis: reduces water loss through evaporation and allows light to enter the leaf.
2. Stomata present in epidermal layers: open in the presence of light, allowing carbon dioxide to diffuse in and oxygen to diffuse out of the leaf.
3. Chloroplasts containing chlorophyll in all mesophyll cells: absorbs energy from sunlight and transfers it to chemical stores of energy in glucose molecules.
4. More chloroplasts in upper palisade tissue: can absorb more light near the leaf surface for photosynthesis.
5. Interconnecting system of air spaces in the spongy mesophyll: Allow rapid diffusion of carbon dioxide into and oxygen out of mesophyll cells.
6. Veins containing xylem and phloem situated close to mesophyll cells: transports water and mineral salts to mesophyll cells, transports sucrose and amino acids away from the leaf.

Question 10

What happens to guard cells in the day?

Answer 10

1. Guard cells photosynthesize
2. Chemical energy is used to pump potassium ions (K⁺) into the guard cells from the neighbouring epidermal cells.
3. Concentration of K⁺ increases in the guard cells.
4. Water potential in guard cells is lowered.
5. Water from neighbouring cells enter guard cells by osmosis.
6. Guard cells swell and become turgid.
7. Due to the difference in thickness of cell wall in guard cells, one side expands more than the other.
8. Stoma opens.

Question 11

What happens to guard cells in the night?

Answer 11

1. Potassium ions move out of the guard cells via diffusion.
2. Water potential in guard cells increases.
3. Water moves out of the guard cells by osmosis.
4. Guard cells become flaccid.
5. Stoma closes.

Question 12

What are some significant features of a plant for photosynthesis, gaseous exchange, & transport?

Answer 12

1. Distribution of chloroplasts in photosynthesis:

• Mainly in the palisade and spongy mesophyll.
• Chlorophyll in chloroplasts traps light energy and convert it into chemical energy to form glucose.

2. Stomata and mesophyll cells in gaseous exchange:

• Spongy mesophyll cells are loosely packed with large, interconnecting air spaces among the cells to increase surface area for a faster rate of diffusion of gases in and out of the mesophyll cells
• Cells and are covered with a thin film of moisture, to alllow gases to dissolve for a faster rate of diffusion of gases
• Stomata allow the exchange of carbon dioxide and oxygen with the atmosphere.

3. Vascular bundles in transport:

• Network is extensive to provide effective transport of water and mineral salts from the roots to the leaves through the xylem, and sucrose and amino acids away from the leaves to other parts of the plant through the phloem.

Question 13

Xylem

Structure & Function

Answer 13

Structure:

• Long, continuous, hollow tubes
• Consisting of dead cells
• Empty lumen (no cellular contents or cell membrane)
• Inner walls are strengthened by lignin deposits

Function:

• Transport water and mineral salts from the roots to the stems and leaves (one direction).
• Flow of water along them is a passive process.
• Provide mechanical support to the plant

Question 14

Adaptations of Xylem

Answer 14

1. Empty lumen, consisting of dead cells: reduces resistance to the flow of water.
2. Inner walls are strengthened with deposits of lignin: prevents the collapse of the vessel and provides mechanical support to the plant. (Lignin can deposit in different patterns)

Question 15

Phloem

Structure & Function

Answer 15

Structure:

• Phloem is composed of columns of sieve tube cells, specialised cells.
• Individual sieve tube cells are seperated by perforated walls called sieve plates.
• Sieve tube cells are closely associated with companion cells which lie next to them.

Function:

• Transport manufactured food substances such as sucrose and amino acids throughout the plant (either direction).
• Process is known as translocation, an active process.

Question 16

Adaptations of Phloem

Answer 16

1. Sieve tube cells are living: translocation is an active process.
2. Sieve tube cells have reduced quantities of cytoplasm and no nucleus, in combination with the presence of sieve plates: reduces resistance to the flow of food substances.
3. Presence of companion cells: which perform genetic and metabolic functions of the sieve tube cells, keeping them alive.
4. Companion cells next to the sieve tube cells contain many mitochondria: provide the energy needed by the sieve tube cells for translocation.

Question 17

Xylem vs Phloem

Compare

Answer 17

Differences:

1. Substances transported: Xylem transports water and mineral salts while phloem transports manufactured food substances, e.g. sucrose and amino acids.
2. Direction of transport: Xylem transports substances in one direction, from roots to leaves, while phloem transports substances in either direction, from leaves to the whole plant.
3. Energy required: Flow of water along xylem is a passive process which does not require energy while flow of food substances along phloem is an active process which requires energy (translocation).

Question 18

Chloroplast

Structure & Function

Answer 18

Structure:

• Double membrane (outer & inner)
• Stroma: dense fluid within the membrane of chloroplast
• Granum: stack of thylakoids
• Thylakoids: a system of interconnected membranous sacs
• Chlorophyll: in the membrane of the thylakoids

Function:

• Site of photosynthesis

Question 19

Chlorophyll

Structure & Function

Answer 19

Structure:

• Green chemical which contains magnesium
• Absorbs light in violet blue and red region and reflects green
• Found in the membrane of thylakoids

Function:

• Traps light energy and converts it into ATP for the formation of glucose and their subsequent uses.

Question 20

Describe the intake of carbon dioxide in a leaf

Process

Answer 20

1. Due to photosynthesis, carbon dioxide in the leaf is used up.
2. Carbon dioxide concentration in the leaf becomes lower than carbon dioxide concentration in the atmosphere.
3. Carbon dioxide diffuses into the intercellular air spaces in the leaf from the surrounding air through the stomata.
4. Carbon dioxide dissolves in the thin film of water covering all mesophyll cells.
5. Dissolved carbon dioxide diffuses into the mesophyll cells.

Question 21

Describe the intake of water through the roots

Process

Answer 21

1. Each root hair cell grows between the soil particles, coming into close contact with the soil solution surrounding them.
2. The cell sap in the root hair cell is relatively concentrated with sugars and mineral salts. Thus, it has a lower water potential than the soil solution.
3. Water enters the root hair cells by osmosis.
4. The entry of water dilutes the sap. The root hair cell now has a higher water potential than the next cell in the cortex.
5. Water moves from the root hair cell to the next cell by osmosis.
6. Water then travels from cell to cell by osmosis until it reaches the xylem.

Question 22

Root Hair Cell’s Adaptation to its Function

Function + Adaptation

Answer 22

1. Function: To absorb water by osmosis

• The cells have a long and narrow extension to increase its surface area to volume ratio, thus increasing the rate of absorption of water by osmosis and mineral salts by active transport.
• The cell sap has a lower water potential than the soil solution, thus water will continuously enter the root hair cell by osmosis.

2. Function: To absorb mineral salts by active transport

• The root hair cell contains mitochondria to provide energy for the active transport of mineral salts into the cell.

Question 23

Transpiration

Definition & Why does it occur?

Answer 23

Transpiration is the loss of water vapour from the leaves through the stomata.

Water evaporates from the thin film of moisture surrounding the mesophyll cells.
The water vapour then diffuses out of the leaf via the stomata.

Transpiration will always occur when the stomata are open.
It is a consequence of gaseous exchange in plants.

Question 24

Importance of Transpiration

Answer 24

1. Creates transpiration pull, which draws water and mineral salts from the roots to the stem and leaves, for photosynthesis, metabolic processses, and keeping cells turgid.
2. Removes latent heat which helps cool the plant, preventing it from being scorched.

Question 25

What forces bring water up the xylem?

Answer 25

Three forces bring water up the xylem:

• Root pressure
• Capillary action
• Transpiration pull (main force)

Question 26

Root Pressure

Process

Answer 26

Root pressure:

1. The respiring cells around the xylem vessels in the roots use active transport to pump mineral salts into the vessels.
2. This lowers the water potential in the xylem vessels, which causes water to move into the xylem vessels by osmosis.
3. This pushes water into the xylem vessels and upwards.

Question 27

Capillary Action

Process

Answer 27

Capillary action:

1. Cohesion: Water molecules are polar and are attracted to each other.
2. Adhesion: Water molecules are attracted to the hydrophillic parts of the walls of the xylem.
3. As a result of the cohesion between water molecules and adhesion between water and the walls of the xylem, water can be pulled up in a continuous stream.

Question 28

Transpiration Pull

Process

Answer 28

Transpiration pull:

1. Water evaporates from the film of moisture surrounding the mesophyll cells into the intercellular air space.
2. The water vapour then diffuses out of the leaf via the stomata by transpiration.
3. As water leaves the mesophyll cells to replenish the film of moisture, the water potential of the cell decreases, and water moves from the neighbouring cells to these mesophyll cells by osmosis. The neighbouring cells then draw water from cells deeper inside the leaf. These cells then draw water from the xylem.
4. This results in transpiration pull, a suction force which pulls the whole column of water up the xylem, against the force of gravity.

Question 29

Factors Affecting the Rate of Transpiration

Answer 29

1. Humidity
2. Wind
3. Temperature
4. Light intensity

Question 30

How does humidity affect the rate of transpiration?

Answer 30

As humidity increases, rate of transpiration decreases.

• Higher humidity indicates the presence of a lot of water vapour in the air surrounding the leaf.
• This decreases the water vapour concentration gradient.
• As such, water vapour diffuses out of the leaf at a lower rate, leading to a lower rate of transpiration.

As humidity decreases, rate of transpiration increases.

• Lower humidity indicates drier air, and lesser water vapour in the air surrounding the leaf.
• This increases the water vapour concentration gradient.
• As such, water vapour diffuses out at a higher rate, leading to a higher rate of transpiration.

Question 31

How does wind affect the rate of transpiration?

Answer 31

When wind speed increases, the rate of transpiration increases.

• Higher wind speed indicates that the water vapour in the air surrounding the leaf is blown away.
• This increases the water vapour concentration gradient.
• As such, water vapour diffuses out at a higher rate, leading to a higher rate of transpiration.

When wind speed decreases, the rate of transpiration decreases.

• Lower wind speed may result in water vapour accumulating in the air surrounding the leaf.
• This decreases the water vapour concentration gradient.
• As such, water vapour diffuses out at a lower rate, leading to a lower rate of transpiration.

Question 32

How does temperature affect the rate of transpiration?

Answer 32

When temperature increases, the rate of transpiration increases.

• Higher temperature increases the rate of evaporation, increasing the concentration of water vapour in the intercellular air spaces.
• This increases the water vapour concentration gradient.
• As such, water vapour diffuses out at a higher rate, leading to higher rate of transpiration.
• However, at very high temperatures, the stomata will close to prevent excessive transpiration.

When temperature decreases, the rate of transpiration decreases.

• Lower temperature decreases the rate of evaporation, decreasing the concentration of water vapour in the intercellular air spaces.
• This decreases the water vapour concentration gradient.
• As such, water vapour diffuses out at a lower rate, leading to a lower rate of transpiration.

Question 33

How does light intensity affect the rate of transpiration?

Answer 33

When light intensity increases, the rate of transpiration increases.

• A higher light intensity causes the stomata to open and become wider.
• This increases the rate of transpiration.

When light intensity decreases, the rate of transpiration decreases.

• A lower light intensity causes the stomata to close.
• This decreases the rate of transpiration.

Question 34

When does wilting occur?

Answer 34

Water lost by transpiration has to be replaced by absorption from the roots.

If the rate of transpiration is less than the rate of water absorption at the roots, plant cells become turgid, and plant becomes firm and upright.

If the rate of transpiration is more than the rate of water absorption at the roots, plant cells become flaccid, and plant wilts.

Question 35

Advantages and Disadvantages of Wilting

Answer 35

Advantage of wilting:

• When the leaves fold up, the surface area exposed to sunlight is reduced.
• This causes the guard cells to become flaccid and the stomata to close.
• Transpiration is thus reduced.

Disadvantage of wilting:

• The leaves droop and less leaf surface is exposed to the sun and thus the rate of photosynthesis decreases.
• Stomata close and less diffusion of carbon dioxide into leaves and thus the rate of photosynthesis decreases.