chapter 7: nutrition in plants

Question 1

what is photosynthesis?

Answer 1

• the process in which light energy absorbed by chlorophyll is transformed into chemical energy
• the chemical energy is used to synthesise glucose from water and carbon dioxide
• water and carbon dioxide are the raw materials for photosynthesis
• oxygen is released during he process

Question 2

what is a limiting factor? and what are the limiting factors of photosynthesis?

Answer 2

• any factor that directly affects a process if its quantity is changed
  > limiting factor
  photosynthesis is affected by external factors such as:
• light intensity
• the concentration of carbon dioxide
• temperature

Question 3

what are the three reasons of why photosynthesis is important?

Answer 3

1. photosynthesis makes chemical energy available to animals
2. photosynthesis removes carbon dioxide and provides oxygen
3. energy is stored in fossil fuels through photosynthesis

Question 4

how does photosynthesis make chemical energy available to animals and other organisms?

Answer 4

• sunlight is the ultimate source of energy for living organisms
• during photosynthesis, light energy is converted into chemical energy
  > which is stored within carbohydrate molecules
• fat, proteins and other organic compounds can be formed from carbohydrates
• all these other substances eventually become the food of other organisms
• they thus obtain chemical energy directly or indirectly from plants
  > because plants are producers from the food chain

Question 5

what happens to glucose that is formed during photosynthesis?

Answer 5

GLUCOSE IS EITHER USED IMMEDIATELY OR STORED!
1: it is used immediately
> for cellular respiration to provide energy for cellular activities
> also to form cellular cell walls

2:
(a) in daylight, rate of photosynthesis is so great that sugars are formed faster than can be removed
> excess glucose is converted to starch
(b) in darkness, photosynthesis stops and starch is converted by enzymes back into glucose

3: glucose is converted to sucrose
> it is transported to other parts of the plants or to storage organs like:
seeds, stem tubers, or root tubers via phloem

4: glucose reacts with nitrates and other mineral salts absorbed from the soil
- to form amino acids in leaves
> excess is transported to other parts of the plant for synthesis of new protoplasm and for storage as proteins
- amino acids are used to form proteins which is used for the synthesis of protoplasm in leaves

5: glucose is also used to form fats
> used for storage
> used in cellular respiration
> for synthesis of new protoplasm

Question 6

how does photosynthesis remove carbon dioxide and provides oxygen?
( and what is oxygen used for)

Answer 6

• photosynthesis removes carbon dioxide from the air and at the same time > produces oxygen
• oxygen released is used by living organisms in respiration to release energy for cell activities
• this purifies the air
  > maintaining a constant level of oxygen and carbon dioxide in the atmosphere

Question 7

how is energy stored in fossil fuels through photosynthesis?

Answer 7

• all the energy in fossil fuels like coal, oil and gas come from sun
  > captured by photosynthesis
• burning of fossil fuels releases energy, which we use in our daily lives such as cooking and generating electricity in our homes

Question 8

what are the external features of a leaf?
(there are 4)

Answer 8

• lamina
• network of veins
• leaf arrangement
  petiole

Question 9

what does a lamina have?

Answer 9

• it has a large flat surface compared to its volume
  > enables it to obtain the maximum amount of sunlight for photosynthesis
• it is large and thin
  > carbon dioxide can rapidly reach the inner cells

Question 10

what is the function of the network of veins in a leaf?

Answer 10

• veins carry water and mineral salts to the cells in lamina
  -& carry manufactured food from these cells to other parts of the plant
  -eg: hibiscus has a main vein (mid-rib) giving off branches repeatedly > forming a network of fine veins
• leaves of grasses usually have parallel veins
  *leaves of trees and bushes usually have a network of veins

Question 11

what are the internal structures of a lamina?

Answer 11

1. upper epidermis
2. palisade mesophyll
3. spongy mesophyll
4. lower epidermis
5. stomata

Question 12

what is mesophyll and where is it located?

Answer 12

• lies just between the upper and lower epidermis
  > main site of photosynthesis
  1) palisade mesophyll
• packed with the most chloroplast
• 1 or 2 layers of closely packed, long, cylindrical cells

2) spongy mesophyll
- cells are irregular in shape
- numerous large intercellular air spaces among the cells
- cells carry out photosynthesis but has fewer chloroplasts than the palisade
- cells are covered with a thin film of moisture
- and contains transport tissues, xylem and phloem ( vascular bundle)

Question 13

what is the lower epidermis?

Answer 13

• it is beneath the mesophyll
• consists of a single layer of closely packed cells covered by an outer layer of cuticle
  > reduces water loss through epidermis cells

Question 14

what is the stomata?

Answer 14

• lower epidermis contains minute openings called the stomata

Question 15

how has the petiole adapted for photosynthesis?

Answer 15

holds leaf in position to absorb maximum light energy

Question 16

how has the thin broad lamina adapted for photosynthesis?

Answer 16

• the thin lamina provides a short diffusion distance for gases and enables light to reach all mesophyll cells
• the broad lamina provides a large surface area to absorb maximum sunlight

Question 17

how has the waxy cuticle on the upper and lower epidermis adapted for photosynthesis?

Answer 17

• it reduces water loss through evaporation from the leaf
  >is transparent for light to enter the leaf

Question 18

how has the stomata present in epidermal layers adapted for photosynthesis?

Answer 18

• they open in the presence of light, allowing carbon dioxide to diffuse in and oxygen to diffuse out of the leaf

Question 19

how has chloroplasts containing chlorophyll in all mesophyll cells the adapted for photosynthesis?

Answer 19

• chlorophyll absorbs and transforms light energy to chemical energy used in the manufacture of sugars

Question 20

how has more chloroplasts in upper palisade tissue adapted for photosynthesis?

Answer 20

• more light can be absorbed near the leaf surface

Question 21

how has the interconnecting system of air spaces in the spongy mesophyll adapted for photosynthesis?

Answer 21

• it allows rapid diffusion of carbon dioxide and oxygen in and out of the mesophyll cell

Question 22

how has the veins containing xylem and phloem situated close to mesophyll cells adapted for photosynthesis?

Answer 22

• xylems transports water and mineral salts to mesophyll cells
• phloem transports sugars away from the leaf

Question 23

how do guard cells control the size of stomata in sunlight?

Answer 23

in sunlight:
- the guard cells photosynthesise, converting light energy to chemical energy
- the chemical energy is used to pump potassium ions into the guard cells
> from the neighbouring epidermal cells
> water potential of guard cells is lowered
- water from neighbouring epidermal cells then enter guard cells by osmosis
> increases the turgidity of the guard cells and causes them to be swollen
- guard cells haves a thicker cell wall on one side of the cell
> causes the swollen guard cells to become more curved and pull the stoma open
- hence the swollen guard cells become more curved and pulls the stoma open

Question 24

how do the guard cells control the size of stomata at night?

Answer 24

• the potassium ions that have accumulated in the guard cells during the day diffuses out of the guard cells
• water potential in the guard cells is increased
  > water leaves the cells by osmosis
  -the guard cells become flaccid and the stoma closes

Question 25

how does carbon dioxide and water enter the leaf?

Answer 25

1. in daylight, when photosynthesis occurs
   > carbon dioxide in leaf is rapidly used up
   - carbon dioxide concentration in leaf becomes lower than the atmospheric air
   > the diffusion gradient exists
   - carbon dioxide diffuses from the surrounding air through the stomata into the air spaces in leaf
2. the surfaces of mesophyll cells are always covered by a thin film of water
   > so carbon dioxide can dissolve in it
3. the dissolves carbon dioxide then diffuses into the cells

Question 26

how does the xylem transports water and dissolved mineral salts to the leaf?

Answer 26

• xylem transports water and mineral salts from the roots to the leaf
• once out of the veins, water and mineral salts move from cell to cell
  > right through the mesophyll of the cell