Plant tissues, organs and systems

2.3.1 Plant tissues

Waxy cuticle	Reduce water loss by evaporation So it doesn’t wilt
Upper epidermis	Cover the leaf Root hair cell Adapt to absorb water by osmosis & mineral ions by active transport from soil efficiently
Palisade mesophyll	Chloroplast – absorb light for photosynthesis Close towards upper surface of leaf, packed with chloroplasts & are arranged closely together
Spongy mesophyll	Air space – CO2 in, O2 out (gas exchange) Cells packed loosely to allow gaps between cells Cells are covered by a thin layer of water which gases dissolve in
Xylem	Transport water & mineral ions from root to stems & leaves Have strong lignin spirals which allow them to withstand water pressure to transport water in transpiration stream & support plant stem Cell die & form long hallow tube which is strengthened by lignin spirals allow water & mineral ions to move up easily
Phloem	Transport dissolved sugar through translocation from leaves to rest of plant for immediate use or storage Cell walls between cells break down to form sieve plates – allow water carrying dissolved food move freely up & down tube to where it’s needed Companion cells keep them alive & contain mitochondria to provide energy to move dissolved food up & down plant
Guard cells / Stomata	Small pores on the underside of the leaf that control gas exchange & water loss by diffusion
	High light intensity	High temp
	Guard cells swell & cause stomata to open CO2 diffuse into leaf for photosynthesis	Close stomata to reduce water loss by transpiration
Lower epidermis
Root	Anchorage & absorption of water & mineral ions

Why more stomata on lower surface of leaf? (3)

• Cooler & more humid around lower surface
• Less water evapourate so won’t wilt

Meristem tissue

Define meristems in plants.

• Plant stem cells that can differentiate into specialised cells throughout the life of the plant
• Found at growing tips of shoots & roots

What can plant stem cells be used for?

• They can be used to make clones of plants quickly & economically

Describe and explain the functions of stem cells in meristem tissue in plants. (4)

• Differentiate into any type of plant cell, throughout life of plant
• Used to produce clones of plant quickly & economically
• Protect rare species from extinction
• To produce large no of identical plants for farmers eg disease resistance crops

2.3.2 Plant organ system

Root hair cells	Function To absorb water by osmosis & mineral ions by active transport from soil efficiently Adaptation Have large SA to absorb water & mineral ions Have large permanent vacuole to speed up movement of water by osmosis Contain mitochondria to provide energy to transport mineral ions into cell
Xylem cells	Function Transport water & mineral ions from root to stems & leaves Adaptation Have strong lignin spirals which allow them to withstand water pressure to transport water in transpiration stream & support plant stem Cell die & form long hallow tube which is strengthened by lignin spirals allow water & mineral ions to move up easily Few cell structures & so they are dead for more space & supported by lignin
Phloem cells	Function Transport dissolved sugar through translocation from leaves to rest of plant for immediate use or storage Adaptation Cell walls between cells break down to form sieve plates – allow water carrying dissolved food move freely up & down tube to where it’s needed Companion cells keep them alive & contain mitochondria to provide energy to move dissolved food up & down plant

Transpiration

Define transpiration (2)

• Water loss from plant by evaporation
• The transport of water through the xylem from root to leaf, doesn’t require energy

Define transpiration stream (2)

• Movement of water from roots to leaves through xylem

How do water move from roots to leaves? (2)

• By transpiration stream in xylem

Describe the process of transpiration (4)

1. Water evaporates from leaves through stomata, causing a pull
2. Water from soil moves up through roots → stem → leaves by osmosis as transpiration stream in xylem to replace water loss

What factors & explain how these factors affect the rate of transpiration (4)

• ↑ temp & air movement, ↓ humidity – ↑ evaporation from cell surfaces
• ↑ light intensity – ↑ rate of photosynthesis
• ↑ rate

Potometer

• Estimates transpiration rate by measuring water uptake
• Assume water uptake is directly related to water loss of leaves

 Method

1. Cut a shoot underwater – to prevent air entering xylem
2. Fill potometer with water & make sure there’s no air bubbles
3. Insert shoot to potometer using rubber tube under water
4. Remove potometer from water & seal joints with Vaseline
5. Dry leaves coz moisture on leaves will affect transpiration rate
6. Remove capillary tube from beaker of water to allow an air bubble introduced into capillary tube & place tube back into water
7. Allow plant to adapt to new environment for 5 mins
8. Record starting location of air bubble
9. Leave for set period of time eg 1 min
10. Record end location of air bubble
11. Calculate rate of transpiration r = radius of capillary tubel = distance moved by air bubble
12. Measure rate for 3 times & calculate mean
13. Once air bubble near junction of reservoir, open tap to add water from reservoir to push air bubble to start of capillary tube
14. Repeat experiment with one different variable eg temp / species

Further bubble travels in same time period, faster transpiration rate

Describe how student return air bubble to start of capillary tube? (1)

• Open tap to add water from reservoir

Give 2 precautions when setting up potometer to obtain reliable measurements of water uptake by plant shoot (2)

• Ensure airtight & watertight
• Cut shoot under water – prevent air entering xylem
• Cut shoot at a slant

Student assumed water uptake was equivalent to transpiration rate. Why this might not be a valid assumption? (2)

• Water used in photosynthesis
• Water used to provide support
• Apparatus not sealed

Why repeat experiment? (1)

• Improve reliability / identify anomalous result

Translocation

• Transport of dissolved sugar from leaves to rest of plant through phloem both upwards & downwards for immediate use or storage, requires energy

Why is it important?

• Sugar made in leaves need to move to other part of plant for growth