Nutrition & Transport in Plants

Question 1

Define photosynthesis

Answer 1

• A process in which light energy is absorbed by chlorophyll & converted into chemical energy.
• The chemical energy is used to synthesise carbohydrates from water and carbon dioxide.
• Water & carbon dioxide are the raw materials for photosynthesis.
• Oxygen is released during the process.

Question 2

Conditions for photosynthesis to occur

Answer 2

• Chlorophyll - absorbs sunlight during the day
• Sunlight
• CO₂
• Suitable temperature
• Water

Question 3

Reactions in photosynthesis & overall equation

Answer 3

Light-dependent/Light stage
- light energy ⟶ chemical energy (Condition: chlorophyll)
- 12 H₂O ⟶ 6O₂ + 24H (Condition: photolysis of water)

Light-independent/Dark stage
- 6CO₂ +24H ⟶ C₆ H₁₂O₆ + 6H₂O (Condition: enzyme-controlled reactions)

Overall eqn
6CO₂ + 6H₂O ⟶ C₆ H₁₂O₆ + 6O₂ (Conditions: (a) light energy, (b) chlorophyll)

Question 4

What is a limiting factor

Answer 4

Factor that directly affects or limits a process if its quantity or conc is altered

Question 5

Factors affecting rate of photosynthesis

Answer 5

• Light intensity
• Temperature (effect on enzymes, the graph will be quadratic)
• CO₂ concentration

Question 6

Fate of glucose in leaves

Answer 6

• Used for cellular respiration
• Form cellulose cell wall
• Excess: converted to sucrose - transported to storage organs, or
• Excess: converted to starch - temporary storage in leaves
• Converted into AA by reacting w/ nitrates & mineral salts, combined to form proteins - synthesis of new protoplasm in leaf
• Converted into fats - storage of energy, cellular resp or synthesis of new protoplasm (eg, inn seed of plants)

Question 7

Importance of photosynthesis

Answer 7

• Chemical energy stored in plants - transferred to other organism, feeding
• Purify air - remove CO₂ from atmosphere, release O₂ into atmosphere as by-product
• Coal formed from trees provide source of fuel

Question 8

External features of a leaf

Answer 8

Lamina: Large SA - max absorption of sunlight, thin - allows rapid diffusion of CO₂ to reach inner cells of leaf

Petiole: Positions lamina for max absorption of sunlight & gaseous exchange
- w/o: leaves have long lamina, bends at an angle where it is away from the stem - maximise absorption of sunlight

Veins: Network of veins branching out from vein in mid-rib, allow transport of water & mineral salts to cells in lamina, transport manufactured food

Question 9

Internal structure of leaf + characteristics

Answer 9

Cuticle:
- Waxy layer - prevents excessive water loss
- Transparent - allow sunlight to penetrate to mesophyll

Upper epidermis:
- Single layer of closely packed cells
- Less stomata compared to lower epi. - minimises loss of water at the upper surface of a leaf

Palisade mesophyll (Main site of photosynthesis):
- Densely packed - maximise exposure to sunlight
- Cells: long, cylindrical, contain numerous(most) chloroplasts - for max absorption of sunlight

Spongy mesophyll:
- Cells: Irregularly shaped & more loosely packed
- Numerous large intercellular air spaces - allows rapid diffusion of air from stomata to interior of leaf for gaseous exchange

Guard cells & stomata:
- Each stoma - surrounded by guard cells - control size
- Stomata open in the light & close in the dark
- Stoma - found on the underside of leaf: More shaded(cooler), less evaporation

Question 10

Explain how does the stoma work in sunlight

Answer 10

1. GC photosynthesise - produce chemical energy which pumps ions into cells
2. Ions lower WP in GC hence, water moves into GC
3. GC swell & become turgid - become curved & pull stoma open which allows CO₂ to enter & O₂ to be released

Question 11

Explain how does the stoma work at night

Answer 11

1. K⁺ ions diffuse out of GC
2. Water potential in GC increases. Water exit GC by osmosis.
3. GC becomes flaccid & stoma closes to minimise water loss

Question 12

Entry of CO₂ into leaf during the day

Answer 12

1. CO₂ - rapidly used up during P. Conc inside leaf - lower than atmospheric air.
2. CO₂ diffuses into the leaf via stomata
3. CO₂ dissolves into film of water surrounding mesophyll cells & diffuses into cells

Question 13

Functions of xylem

Answer 13

• Conduct water & dissolved MS from roots to stems & leaves
• provide mechanical support for plant

Question 14

Characteristics of xylem

Answer 14

• many dead cells
• long, hollow, empty tube (w/o protoplasm)
• lignified wall

Question 15

Function of phloem

Answer 15

Transport MF (sucrose, AA) from green parts of plant, esp leaves, to other parts of plant

Question 16

Adaptations of phloem

Answer 16

• Companion cells have many mitochondria - provide energy needed to load sugars from mesophyll cells into sieve tubes by active transport
• Holes in sieve tube - allow rapid flow of MF subs through sieve tubes

Question 17

Where and function of cambium

Answer 17

• between phloem & xylem
• cambium cells divide & differentiate to form new xylem & phloem tissues, giving rise to thickening of stem

Question 18

Function of pith & cortex

Answer 18

Store up food subs such as starch

Question 19

Adaptation of epidermis (layer of cells covering stem)

Answer 19

Epidermal cells are protected by a waxy, waterproof cuticle, greatly reduces evaporation of water from stem

Question 20

Define translocation

Answer 20

Transport of MF subs such as sugars & AA in plants

Question 21

During a translocation study using aphids, what should be done to the aphids while it is feeding and why?

Answer 21

• Anaesthesise
• To enable the body aphids to be cut off while it is feeding
• To ensure the promboscis remains in the plant

Question 22

Why is there swelling during a translocation study using a ringing experiment?

Answer 22

• Removal of phloem prevents the translocation of sugars to the region below the ring
• The accumulation of sugars in the region just above the ring lowers the WP of cells in that region
• Water enters the region, resulting in swelling

Question 23

How does water enter the root & moves between root hair cells?

Answer 23

• Each soil particle has a thin film of liquid surrounding it. The soil solution is a dilute solution of mineral salts.
• The sap in root hair cell is more concentrated due to the presence of sugars & MS; it has a lower WP than soil solution. Hence, water enters root hari by osmosis.
• Entry of water dilutes root hair’s cell sap. Sap now has a high WP than that of the next cell. Hence, water passes by osmosis from root hair cell into inner cell.
• Similarly, water pases from one cell to the other. This process continues until the water enters the xylem vessels.

Question 24

Explain why the absorption of water & MS decreased in water-logged soil

Answer 24

• Water-logged soil has a very diluted soil solution - root hair cell unable to aborb mineral ions by diffusion, this lowers conc of ions
• RHC expend energy to absorb mineral ions - affects the absorption of water, cell sap would not have as much mineral ions
• WP gradient would be less steep which means that less water will be absorbed by osmosis

Question 25

3 processes by which water flows from the roots to the aerial parts of a plant

Answer 25

Root pressure
Capillary action
Transpiration

Question 26

What is root pressure

Answer 26

The process resulting from the constant entry of water into the roots
- the cells around xylem vessels in the root pump ions into the root by active transport, lowering the water potential in the roots
- causes water to move into the xylem vessels and up the plant by osmosis

Question 27

What is capillary action

Answer 27

• tendency of water to move up inside very narrow tubes
• due to the interactions between water molecules and the surfaces of the tubes
  1. cohesion - the sticking together of particles of the same substance
  2. adhesion - sticking on a surface

Question 28

Define transpiration

Answer 28

The loss of water from the aerial parts of the plant, especially through the stomata of the leaves

Question 29

What is transpiration pull

Answer 29

suction force caused by transpiration which results in water to move up the xylem

Question 30

What results in transpiration pull?

Answer 30

• water continuously moves out of mesophyll cells to form a thin film of moisture
• water evaporates from the thin film of moisture and moves into the intercellular air space
• water vapour accumulates in the large air spaces near the stomata
• water vapour diffuses through stomata to the drier air outside the leaf. this is transpiration
• as water evaporates from mesophyll cells, the WP of cell sap decreases
• mesophyll cells begin to absorb water by osmosis from cells deeper inside the leaf
• these cells in turn remove water from the xylem vessels
• this results in transpiration pull which is a suction force which pulls the whole column of water up the xylem vessels

Question 31

What is the pathway of water from roots to the leaves?

Answer 31

1. The sap in the root hair cell has lower water potential than the soil solution. Water enters the root hair cell by osmosis.
2. Water flows across the root cortex, down a water potential gradient until it reaches the xylem.
3. Xylem conducts water upwards.
4. Water enters the mesophyll cells & forms a thin film of moisture around the cells.
5. Water evaporates from the surface of mesophyll cells into the intercellular air space.
6. Water vapour accumulates in the air spaces.
7. Water vapour diffuses out of the leaf through the stomata into the environment. This is transpiration.

Question 32

Factors affecting transpiration rate

Answer 32

1. Humidity (high/low):
   - high: WV conc grad b/w leaf & atmosphere decreases, rate of transp decreases
   - low: WV conc grad b/w leaf & atmosphere increases, rate of transp increases
2. Wind (faster/slower, windy/no): stronger/faster: accumulated WV outside stomata - brought away, increases WV conc grad b/w leaf & atmosphere, rate of T increases
3. Temperature of air (high/low): higher temp, higher rate of evaporation, higher rate of T
4. Light intensity (high/low): presence of light: stomata is open & wider, higher rate of T

Question 33

Importance of transpiration

Answer 33

• Transpiration results in a major suction force, transpirational pull, that draws water and mineral salts up the xylem vessels from roots to the stems and leaves
• Water is required at the leaves for photosynthesis
• To maintain turgidity: Water is needed in stems and leaves to replace water lost by mesophyll cells. Maintaining turgidity ensure leaves are well spread out to maximise the photosynthetic surface area exposed to sunlight
• Evaporation of water helps to cool the plant - removes latent heat of vaporisation

Question 34

Cause of wilting

Answer 34

Rate of water loss (transpiration) exceeds rate of water absorption

Question 35

Advantages of wilting

Answer 35

• Reduces rate of transpiration
• Prevents excessive water loss
• Cooling of plant

Question 36

Disadvantages of wilting

Answer 36

• Stomata close, decreasing intake of CO2 and rate of photosynthesis decreases
• Leaves droop and hence decrease absorption of sunlight, hence rate of photosynthesis decreases.

Question 37

On a hot day, there might be a slight drop in photosynthetic rate in afternoon. Suggest an explanation for such change.

Answer 37

• rate of transpiration may be very high
• cause plant to wilt
• wilting reduces surface area of the leaf
• amounts of water, CO2 & light available for plant cells become limited
• reduces the rate of photosynthesis

Question 38

Describe the conditions in which wilting is most likely to occur.

Answer 38

• Wilting occurs when there is low humidity, high temperatures, windy conditions, and high light intensity.
• A low level of humidity increases diffusion gradient between inside the leaf & outside. This will result in higher rate of water loss as transpiration rate increases.
• Higher temp leads to an increase in rate of evaporation of water in leaf as well as relative decrease in humidity of air outside the leaf.
• Windy conditions remove water vapour around leaves & result in a steeper diffusion gradient between inside leaf & outside air.
• When there is high light intensity, the stomata is turgid, open & wider. More water vapour will leave the leaf via stomata, thus there is high transpiration rate. As a result, wilting will occur due to excessive water loss.