Growth in plants

Question 1

What allows and is indeterminate growth?

Answer 1

• The ability to grow throughout their lives, this is due to the undifferentiated cells in the meristem than produce new cells
• Growth is concentrated in the meristem at the tip of roots
• Meristems are made out of undifferentiated cells that can grow through mitosis and cell division rapidly

Question 2

Where does growth occur?

Answer 2

• At the apex (top), where new cells are formed remain meristematic, other cells elongate and then begin to specialize in the zone of differentiation
• These areas of growth are called apical meristems (the apex or tip of the root/shoot)

Question 3

What does mitosis and cell division mean for growth?

Answer 3

• Mitosis and cell division in the shoot apical meristem produces cells to increase the length of the organ of the plant and develop them further

Question 4

What do apical meristems do?

Answer 4

• The apical meristem gives rise to other partially differentiated tissues that can further differentiate
• The apical meristem also creates dormant meristems in the axillary buds (where leaf and stem join)
• Response to stimuli (sensitivity) may lead to changing the number of cells produced by the meristem
• Growth of apical meristems is also called primary growth

Question 5

What do plant hormones do?

Answer 5

• Hormones are chemical messengers that carry information from one cell of an organism to another
• In plants they are transported in the sap

Question 6

What is auxin and what does it do?

Answer 6

• A hormone synthesised in the apical meristem and travels down the stem which causes cell elongation and inhibits the growth of axillary (side) buds
• The further it travels, the lower the concentration becomes

Question 7

What is apical dominance?

Answer 7

• When auxins inhibit the growth of the axillary buds which causes the plant to grow vertically upward to trap more light
• When the shoot apex is far away from the axillary bud and the auxin concentration becomes too low, the bud begins to develop again (branches)

Question 8

What is plant tropism?

Answer 8

• Tropism: directional growth in response to an external stimulus, plants respond to there stimuli by tropism
• Phototropism: response to light
• Gravitropism: response to gravity
• Positive is growth towards the stimuli and negative is growth away from it

Question 9

How do shoots and roots respond use tropism?

Answer 9

• Shoots: Positive phototropism and negative gravitropism for maximum light energy
• Roots: Positive gravitropism and negative phototropism for max water and mineral absorption

Other types of tropism: chemotropism, hydrotropism, thigotropism (touch)

Question 10

How to auxins respond to light?

Answer 10

• When light is overhead, auxins at the shoot apex diffuse evenly down the stem
• As the cells grow at the same rate, the shoot extends vertically upward
• When the position of the light changes, the auxins accumulate on the side with least light which means there is a higher concentration and therefore grows faster
• The shoot bends towards the light source
  Check book

Question 11

How do auxins respond to gravity?

Answer 11

• When a phototropic response is made, the growth of cells in the root is inhibited
• The auxins accumulate on the lower shaded side
• Cell elongation is inhibited in the shaded root cells, the cells on the upper side elongate and hence the root bends towards gravity (away from light)
• Positive gravitropism and negative phototropism

Question 12

How is auxin moved from cell to cell?

Answer 12

• Either through the sap in the phloem OR
• When the light intensity is equal, auxins move downward and are pumped into and out of successive layers of cells through specialized protein pumps
• The cells have auxin influx carriers in the apical membranes and auxin efflux carriers in the basal membranes
• This makes sure that auxin moves continually downward, leads to even cell growth and extension of the stem

Question 13

What happens when the light intensity is varied?

Answer 13

• Auxin efflux carriers increase on the internal side membrane and the auxins are transported to the shades side of the plant, creates concentration gradient
• This triggers responses such as cell elongation

Question 14

What responses might occur after auxins accumulate on the shaded side?

Answer 14

• Auxins alter gene expressions, for example transcription of genes that produce proton pumps may be changed
• Auxins stimulate cell elongation in the stem. The pattern of gene expression is altered

Question 15

How does cell elongation in the stem occur?

Answer 15

• A proton pump is stimulated by auxins, which moves H+ ions into the cell wall
• A higher concentration in the cell walls means it becomes more acidic which breaks the bonds between cellulose fibers in the cell wall
• Reduced number of bonds makes the cell wall more flexible and the turgor pressure slides the cellulose fibers further apart, thus cell elongation occurs

Question 16

What is micropropagation?

Answer 16

• A technology that uses plant’s flexible growth patterns to produce many clones from an original plant (plant cells are totipotent and can give rise to an entire organism)
• A tissue sample from the shoot apical meristem is sterilized to promote cell growth but not differentiation
• This gives rise to a callus (large mass of undifferentiated cells) which are separated and placed into different medium to trigger cell differentiation and development

Question 17

What are the advantages of micropropagation?

Answer 17

• Allows rapid increase in number of plants
• Production of virus-free individuals of existing varieties, newly produced cells don’t get affected by virus of parent cell (uninfected)
• Allows the production of orchids and other rare species, bypasses issues