Plant biology

Question 1

What is transpiration?

Answer 1

• water evaporation during gas exchange in plants
  
  • water is lost
  • in stomata
  • water potential is lower in air

Question 2

What are stomata?

Answer 2

• pores in epidermis of a plant
• gas exchange
  
  • CO2 can’t pass through waxy cuticle

Question 3

Where is water loss controlled?

Answer 3

• guard cells
  
  • in pairs (on each side of stoma)
  • control aperture of stoma
  • thin outer walls and thick inner walls

Question 4

How is the water loss controlled?

Answer 4

• guard cells
• potassium pumps
  
  • light activated
  • K+ from epidermis move to guard cells

Question 5

When does wilting occur?

Answer 5

• transpiration → water uptake
• stoma closes (flaccid)

Question 6

How is water replaced?

Answer 6

• transpiration = water loss
• plant uptakes water from soil through roots
  
  • water gradient created by active transport of minerals into roots
• water travels through cell walls and cytoplasm to xylem

Question 7

How does transport of water occur?

Answer 7

• xylem structure
  
  • thickened walls
  • low pressure (negative)
• water properties
  
  • adhesion (travels up the xylem) and cohesion (continuous stream)

Question 8

What is transpiration-pull?

Answer 8

• water is pulled up
  
  • adhesion
  • low pressure in xylem
    
    • the higher, the lower the pressure is
• energy comes from heat (transpiration)
• cavitation = liquid unable to resist water pressure, xylem vessels break

Question 9

How is water gradient in roots created?

Answer 9

• mineral ions go through protein pumps
  
  • active transport
• relationship with fungus
  
  • as it attaches to the roots it absorbs ions creating gradient

Question 10

What are xerophytes?

Answer 10

• plants growing on deserts and dry habitats

Question 11

How do xerophytes adapt to hot environment?

Answer 11

• reduced leaves
  
  • lower area of transpiration
• fleshy leaves
  
  • water storage
• silver or shiny surface
  
  • reflecting sun, low transpiration
• CAM metabolism

Question 12

What is CAM metabolism?

Answer 12

• CO2 is absorbed at night
  
  • low transpiration
• stored as malic acid (C4H8O5) in vacuoles
• stomata closed during day
  
  • photosynthesis still occurs

Question 13

How do xerophytes absorb water?

Answer 13

• deep root system
  
  • water from ground layers
• high concentration of ions in roots

Question 14

How does Marram grass adapt?

Answer 14

• rolled leaves
  
  • thick outer layer
• stomata in pits
  
  • water vapour saturation
    
    • more diffusion shells

Question 15

How do halophytes adapt to salty environment?

Answer 15

• reduced leaves
• leaves are shed
  
  • stems preform photosynthesis
• water storage in leaves
• thick cuticle and layered epidermis
• sunken stomata
• long roots
• structures removing salt build-up

Question 16

What are the factors affecting transpiration?

Answer 16

Environmental: relative humidity, temperature, air movement, atmospheric pressure, water supply, light intensity

Intrinsic: leaf surface, thickness of epidermis and cuticle, stomatal frequency, size and position

Question 17

What is a potometer?

Answer 17

• transpiration rate measured
• rate of water loss: uptake
• movement of water (air bubble) measured

Question 18

Where are carbohydrates transported in plants?

Answer 18

• from source to sink
• source (storage): leaves, green stems,
• sink (uses): roots, shoots

Question 19

What is translocation?

Answer 19

• transport of organic solutes in plants
• phloem links parts that need supply

Question 20

How is transport in phloem possible?

Answer 20

• due to pressure gradients solutes move

Question 21

What is the structure of phloem?

Answer 21

• sieve tubes
  
  • sieve tube cells and plates
  • no nucleus
  • huge vacuole
• companion cells
  
  • nucleus
  • support for sieve tube members
  • control active transport
  • mitochondria

Question 22

Apoplastic phloem loading

Answer 22

• carbohydrates into phloem
• sucrose = most prominent in phloem sap
  
  • not readily used for metabolism
  • good to transport
• active transport
  
  • H+ gradient created outside companion cells (apoplast)
    
    • as the H+ moves back it creates energy
  • sugar into sieve tube
    
    • sugar co-transporter

Question 23

Symplastic phloem loading

Answer 23

• through plasmodesmata
  
  • connections between cells
• converted to oligosaccharide in companion cells to maintain sucrose gradient

Question 24

How is osmotic pressure created in phloem?

Answer 24

• high concentration of solutes
  
  • osmotic pressure created
  • water taken from xylem
• solutes are unloaded into sink
  
  • water returns to xylem

Question 25

How is hydrostatic pressure created?

Answer 25

• water is incompressible
• uptake of water = increased volume
• build-up of pressure
  
  • water moves down (to the sink)

Question 26

How are phloem transport rates measured?

Answer 26

• aphid stylets experiment
• aphids get to phloem by stylets (mouth parts)
• stylets cut off
  
  • phloem sap pumped by plant
  • phloem sap content and flow rate investigated
    
    • slow rate = close to sink

Question 27

What is radiotracking?

Answer 27

• plant leaf exposed to C-14
• spread of radioactive tracker observed
  
  • sink and source leaves distinguished
  • photosynthate moves to younger plants above source
• potential source leaves removed
  
  • photosynthate moves to younger leaves on the other side

Question 28

What are meristems?

Answer 28

• plants have indeterminate growth
• plant tissues responsible for growth
• undifferentiated cells
  
  • active cell division
• primary meristems = apical meristems
  
  • tips of stems and roots
  • length and thickness

Question 29

What is the role of shoot apical meristems?

Answer 29

• division through mitosis and cytokinesis
• shoot apical meristems
  
  • growth of stem
  • group of cells that develop into leaves and flowers
• division = 2 cells
  
  • one cell remains meristem
  • the other increases and differentiates
• meristems produce additional meristems
  
  • protoderm (epidermis)
  • procambium (vascular tissue)
  • ground (pith)
• young leaves at sides of shoot meristems
  
  • small bumps = leaf primordia

Question 30

How is growth in plants controlled?

Answer 30

• by hormones
• auxins
  
  • initiation of growth of roots
  • influencing the development of fruits
  • regulating leaf development
• most abundant auxin = indole-3-acetic acid (IAA)
  
  • elongation of stem
  • stimulation of mitosis
  • synthesised in apical meristem

Question 31

What is apical dominance?

Answer 31

• axillary buds = shoots formed at junction (node) of base of a leaf
• regions of meristems are left in node
  
  • no growth → auxins produced by shoot apical meristems
  • the further from apical meristem, the more likely it is to grow

Question 32

What is the role of cytokinins?

Answer 32

• hormones produced in root
• promotion of axillary bud growth
• relative ratio of cytokinins to auxins determines the growth
  
  • gibberellins also contribute to growth

Question 33

What are tropisms?

Answer 33

• directional growth
  
  • towards light or gravity
  • controlled by hormones
• towards light (phototropism) and opposite to gravity (gravitropism)

Question 34

How does phototropism work?

Answer 34

• photoreceptors (phototropins) absorb wavelengths
  
  • binding to receptors which control gene transcription
• genes involved are coding for auxin transporters (PIN3)
  
  • more auxin on shadowed side → plant cells grow faster
    
    • plant bends towards light

Question 35

How does location of auxins influence growth?

Answer 35

• in roots auxins are at the bottom (gravity)
  
  • prohibition of cell elongation
  • top cells elongate faster
  • root bends
  • positive geotropism
• in stems auxins are at the top
  
  • promotion of elongation
  • stem grows

Question 36

How does micropropagation of plants work?

Answer 36

• tissues from stock plants taken
  
  • pieces = explants
  • usually meristem
• explant placed in growth media
  
  • plant hormones inside
  • auxins to cytokinin ratio equal = undifferentiated mass (callus)
    
    • auxin : cytokinin = 10:1 → root development
    • less → stem development
• plant transferred to soil

Question 37

Why is micropropagation used?

Answer 37

• transporting plants without risk of viruses
  
  • usually in plasmodesmata
  • apical meristem free of viruses
• preservation of species (orchids)
  
  • difficult to germinate
  • more efficient
• production of flowers with desirable features (artificial selections)

Question 38

What are vegetative plants?

Answer 38

• seeds germinate
• young plants formed
• ends when reproductive phase begins
  
  • meristems produce flowers instead of leaves

Question 39

On what does reproductive phase depend?

Answer 39

Length of dark periods (night)

Question 40

How do plants measure length of dark periods?

Answer 40

• photosensitive pigment: phytochrome
  
  • two forms: PR and PFR
• mechanism
  
  • PR absorbs red light (660nm) → PFR
  • PFR absorbs far-red light (730nm) → PR
    
    • in sunlight PR → PFR
  • PR is more stable so at night PFR → PR
• PFR is the active form
  
  • binds to receptor proteins in cytoplasm
    
    • (long-day plants) at the end of short nights a lot of PFR remains = binding = transcription of genes needed for flowering
    • (short-day plants) receptor inhibits transcription so at the end of long night
      
      • high PFR = inhibition = no flowering

Question 41

How is gene expression used for flowering?

Answer 41

• genes in shoot apex
• products trigger cell differentiation leading to flower production

Question 42

How are plants induced to flower?

Answer 42

• manipulating amount of light a plant receives

Question 43

What are the stages of plant reproduction?

Answer 43

• pollination
  
  • delivery of pollen to the stigma of flower
  • by pollinators (birds, bees, butterflies), wind or water
    
    • mutualism –> pollinators get nectar
• fertilisation
  
  • fusion of male gametes / nuclei (from pollen grain) with ovule
  • zygote produced
• seed dispersal
  
  • transport of seed in fruit to spread them
    
    • wind, animals