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.