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IB HL Biology > 9.3 Growth in plants > Flashcards

9.3 Growth in plants Flashcards

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1
Q

How is growth different in animals and plants?

A

-A few animals continue to grow as long as they live, for example, sharks, some fish and some reptiles.

-Most animals go through distinct phases: embryonic, juvenile and then adult.

-Once adulthood is reached, growth stops.

-Many plants, however, show indeterminate growth, which is the ability to grow throughout their lives when conditions permit it.

-That is one reason why some plants can reach massive heights.

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2
Q

Explain how plants grow and where this growth is concentrated

A

-Growth in plants is concentrated in the meristem at the tip of roots and shoots, which are made up of undifferentiated (unspecialised) cells that can go through mitosis and cell division rapidly.

-Growth is concentrated in the root tips and shoot tips.

-As new cells are formed, the cells at the very tip, or apex, remain meristematic.

-The other cells first grow longer in the zone of elongation and then begin to specialise in the zone of differentiation.

-This allows the plant to grow and produce the types of cells needed for each function of life.

-These growth areas are called apical meristems (apical referring to the apex or tip of root or shoot).

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3
Q

What causes the indeterminate growth of plants throughout their lives?

A

The undifferentiated cells in the meristem that can continuously produce new cells

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4
Q

Explain how the root apical meristem works

A

-The root apical meristem elongates the root and allows it to grow deeper into the ground and around obstacles as needed.

-The root cap protects the meristem and sheds cells as the root grows through the soil.

-Half of the cells produced in the meristem remain undifferentiated while the other half specialises and contributes to growth and development.

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5
Q

Picture of the apical meristem in the root

A

.

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6
Q

What part of the plant (other than the stem) also has an apical meristem?

A

-The root

-Mitosis and cell division in the shoot apical meristem produces cells to increase the length of the stem, and to develop leaves, buds and any other above-ground structures.

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7
Q

Micrograph of the apical meristem from a Coleus plant

A
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8
Q

What does the apical meristem give rise to?

A

Other, partially differentiated tissues that can further differentiate.

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9
Q

What does the apical meristem give rise to?

A

Other, partially differentiated tissues that can further differentiate.

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10
Q

What does the procambium give rise to?

A

The xylem and phloem.

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11
Q

What will the protoderm become? (when it differentiates?)

A

The epidermis

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12
Q

What will the ground meristem become?

A

The cortex and mesophyll.

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13
Q

Describe the parts of a plant that can be seen in the leaf primordial that will develop into fully functional and differentiated leaves

A

-The apical meristem gives rise to other, partially differentiated tissues that can further differentiate.

-The procambium will give rise to the xylem and phloem, the protoderm will become the epidermis, and the ground meristem will become the cortex and mesophyll.

-All of these can be seen above in the leaf primordia that will develop into fully functional and differentiated leaves.

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14
Q

The apical meristem creates dormant meristems in the ___

A

Axillary buds, where the leaf joins the stem, that have the potential to grow into new shoots or branches.

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15
Q

How do plants respond to environmental changes?

A

They can sense environmental conditions and respond by changing the number of cells produced by the meristem and by altering the pattern of growth and specialisation to produce, for example, flowers in the correct season or chemicals to ward off an insect attack.

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16
Q

What is the purpose of mitosis and cell division at the meristem in the shoot apex?

A

They provide cells needed for extension of the stem and development of leaves.

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17
Q

Continuous growth of plants is due to ___

A

Undifferentiated cells in the meristems of plants.

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18
Q

What are hormones?

A

Chemical messengers that carry information from one cell of an organism to another.

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19
Q

What is the difference between hormones in animals and plants?

A

-Animal hormones can be proteins or lipid compounds.

-In plants, they are usually small organic molecules that are simpler than animal hormones.

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20
Q

What is a similarity between hormones in animals and plants?

A

In both plants and animals, only very low concentrations are needed to trigger a change in the organism.

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21
Q

How can hormones move throughout a plant?

A

They can be transmitted from one part of the plant to another carried in the sap of the phloem or xylem.

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22
Q

What is auxin?

A

A class of plant hormones, all of which have a similar chemical structure and influence plant growth and behaviour.

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23
Q

What is IAA?

A

Indole-3-acetic acid, the most common and well-studied of the auxins.

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24
Q

Where is auxin synthesized and where does it move to?

A

It is synthesised in the apical meristem and travels down the stem.

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25
Q

What does the movement of auxin down the stem cause?

A

Cell elongation and inhibits the growth of axillary (side) buds in nodes.

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26
Q

The further auxin travels from the apical meristem, ___

A

The lower its concentration becomes.

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27
Q

Picture of plant nodes containing axillary buds

A
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28
Q

What effect does auxin have on plants?

A

-It inhibits the growth of the axillary buds, causing the plant to grow vertically upwards to trap more light for photosynthesis.

-This is known as apical dominance.

-When the shoot apex has grown far enough above an axillary bud, the auxin concentration becomes too low to inhibit growth and the buds begin to develop.

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29
Q

Picture showing apical dominance in a pine tree

A
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30
Q

What is the purpose of mitosis and cell division in the shoot apex meristem?

A

-They provide cells needed for extension of the stem and development of leaves.

-This growth and differentiation are under the direct control of plant hormones such as auxin, which contributes to the elongation of cells.

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31
Q

Define a tropism

A

-Directional growth in response to an external stimulus, such as light, gravity, touch, water, or chemicals.

-Plant shoots respond to their environment through tropisms.

32
Q

What is phototropism?

A

The response to light

33
Q

What is gravitropism (or geotropism)?

A

The response to gravity

34
Q

Phototropism and gravitropism (or geotropism) can be ___ or ___.

A

Positive, which is growth towards the stimulus

Negative, which is growth away from the stimulus

35
Q

Shoots usually show positive ___ and negative ___

A

Phototropism

Gravitropism

36
Q

Explain why shoots usually show positive phototropism and negative gravitropism

A

-This means that they grow directly towards the source of light and away from gravity, which generally brings them towards the light.

-Thus, the shoot receives maximum light energy for photosynthesis and the plant is better able to survive and reproduce.

37
Q

Roots generally show positive ___ and negative ___.

A

Gravitropism

Phototropism

38
Q

Explain why plant roots generally show positive gravitropism and negative phototropism

A

-By moving away from light and towards gravity, roots grow into the soil.

-This allows them to absorb water and necessary minerals and to provide firm support for the plant.

39
Q

Give examples of other tropisms (other than photo and gravi)

A

-Other tropisms include hydrotropism (water), chemotropism (chemicals), and thigmotropism (growth in response to touch).

-Many roots show negative thigmotropism, allowing them to grow around obstacles.

-Unequal distribution of auxin along the growing roots and stems allows the cells to grow unevenly, turning them in a particular direction.

40
Q

What is produced when light is above a plant?

A

When light is overhead, as with sunlight, auxin is produced at the shoot apex and diffuses evenly down the stem.

41
Q

What happens when auxin is produced at the shoot apex and diffuses evenly down the stem?

A

All the cells in this area grow at the same rate, causing the shoot to extend vertically upwards, with no bending.

42
Q

How will a plant react if it senses that a light source is to the side of a stem?

A

-It uses auxin transporters to redistribute auxin so that it accumulates on the side with the least light.

-The cells on the shaded side of the stem respond to the increased concentration of auxin by growing faster than the side of the stem in the light.

-This causes the shoot to bend towards the light source.

43
Q

Diagram of phototropism in a shoot

A
44
Q

What is the effect of auxin different in shoot cells and root cells?

A

-The concentration of auxin that elongates shoot cells in the phototropic response above inhibits the growth of cells in the root.

-(These responses are also mediated by other hormones such as cytokinin, which is produced in root meristems.)

45
Q

Explain what happens to auxin in a root placed horizontally

A

-Light from above and gravity will both cause redistribution of auxin so it accumulates on the lower shaded side.

-This will cause inhibition of cell elongation in the shaded root cells while cells on the upper side elongate normally.

-The overall effect is the bending of the root in the direction of gravity (away from light), showing positive gravitropism and negative phototropism.

46
Q

What does auxin control in the shoot?

A

-Stem elongation

-Apical dominance

-Tropisms

47
Q

How does auxin control stem elongation in the shoot?

A

By increasing elongation of new cells formed by the apical meristem.

48
Q

How does auxin control apical dominance in the shoot?

A

By allowing the shoot to elongate before the axillary buds begin growing.

49
Q

How does auxin control tropisms in the shoot?

A

It allows directional growth by causing different rates of cell elongation on different sides of the stem.

50
Q

What is the main mechanism of positive phototropism in a shoot at the cellular level?

A

Increased cell elongation on the shaded side of the shoot.

51
Q

How is auxin moved throughout plants?

A

Much of the auxin is transported in the bulk flow of the phloem, but it is also actively moved from cell to cell through several methods.

52
Q

What are phototropins?

A

-Pigments/proteins that detect light intensity.

-When light is equally bright on all sides of the shoot tip, auxin moves symmetrically downward, being pumped into and out of successive layers of cells through specialized protein pumps.

53
Q

Where do plant cells have auxin influx carriers and auxin efflux carriers and what does this allow?

A

-Plant cells have auxin influx carriers in their apical (top) cell membranes and auxin efflux carriers in their bottom (basal) cell membranes.

-This allows the cells to move auxin continually downward, pumped into the top of a cell and out of the bottom, then into the cell below.

-This even distribution of auxin leads to even cell growth and vertical extension of the stem.

54
Q

What do phototropins do when there is a difference in brightness on different sides of the shoot and what happens as a result?

A

-They cause some types of auxin efflux carriers to increase on the internal lateral (side) membrane.

-This causes the transport of auxin to the shaded side of the plant and establishes a concentration gradient.

-The shaded side of the shoot experiences greater cell elongation, bending the stem toward the light.

55
Q

Diagram showing auxin transport and distribution in overhead versus side lighting

A
56
Q

What happens once auxin has accumulated on the shaded side?

A

It triggers several changes in the cells. These changes allow multiple responses, including the cell elongation that leads to phototropic (and other tropic) responses.

57
Q

What responses do the changes triggered by the accumulation of auxin on the shaded size allow?

A

-Auxin alters gene expression in several gene families

-Auxin stimulates cell elongation in the stem

58
Q

Explain how auxin alters gene expression in several gene families

A

-The expression of these genes is also linked to other factors in the cell, allowing auxin to trigger a wide variety of responses depending on the type of cell and environmental conditions.

-Auxin stimulates the transcription of genes that produce proton pumps.

59
Q

Explain how auxin stimulates cell elongation in the stem

A

-Auxin stimulates cell elongation at the shoot apex by initiating events that loosen cell wall fibers. These are the events:

1) Auxin activates proton pumps in the plasma membrane. These pump protons into the cell wall. A decrease in pH in the cell wall activates cell wall loosening proteins, disrupting hydrogen bonding between cellulose fibers.

2) Auxin causes a rapid increase in the expression of genes coding for the proton pump and the cell wall loosening proteins.

60
Q

Diagram summarizing how auxin leads to cell elongation through proton pumps

A
61
Q

The cell-to-cell transport of auxin down the stem is achieved through ___

A
62
Q

What kinds of stem cells are meristematic plant cells and what does this mean?

A

They are totipotent, which means that they can differentiate into any plant tissue.

63
Q

What is dedifferentiation and what does this allow?

A

-Plants have a much larger ability to undo specialization in their cells than animals, a process called differentiation.

-Therefore, with appropriate conditions and growth media, it is possible to grow a new, cloned plant from cells taken from an adult plant.

64
Q

What is micropropagation used for?

A

It is a technology that uses plants’ flexible growth patterns to produce very large numbers of clones from an original parent plant.

65
Q

Explain how micropropagation works

A

-A small tissue sample is taken, usually from the shoot apical meristem, and sterilized.

-It is grown in a sterile medium with concentrations of auxin that promote cell growth but not differentiation.

-This produces a large mass of undifferentiated cells called a callus.

-The callus can then be broken up to create many tiny cell samples that are grown in a different medium, this one with concentrations of hormones that trigger cell differentiation and plant development.

66
Q

Define micropropagation

A

-A method used to mass-produce clones of a parent plant.

-It involves the use of tissue culture techniques for meristematic tissue or somatic cells on nutrient media under controlled sterile conditions.

67
Q

Diagram of the basic micropropagation technique

A
68
Q

Why is a tissue culture technique widely used in the flower industry?

A

-Because it can be used to produce vast quantities of certain plants

-This technique is used especially for orchids

69
Q

What are the benefits of micropropagation?

A

-Rapid increase in the number of plants

-Production of virus-free individuals of existing varieties

-Production of orchids and other rare species

70
Q

Explain how a rapid increase in the number of plants is a benefit of micropropagation

A

-When a new variety of plants is created, micropropagation allows a rapid increase in numbers of plants, known as ‘bulking up’ a new variety.

-Other methods of plant cultivation would take much longer.

71
Q

Explain how the production of virus-free individuals of existing varieties

A

-Even when a parent plant is infected with a virus, the virus is usually not found in the newly produced cells of the apical meristem.

-The sterile micropropagation technique allows uninfected individuals to be grown from an infected parent.

72
Q

Explain how the production of orchids and other rare species is a benefit of micropropagation

A

-Orchids are delicate and difficult to breed, as well as difficult to grow from their tiny seeds.

-Micropropagation allows many individuals to be created from a callus, bypassing issues with breeding and germination.

73
Q

Totipotency is a term used to describe a cell’s ability to differentiate to give rise to all plant tissues.

In plants, this means that ___

A

Cell differentiation depends largely on the control of gene expression.

74
Q

In micropropagation, why do cells from the callus need to be transferred to a second, different growth medium?

A

To provide different concentrations of chemicals to trigger cell differentiation.

75
Q

Explain how auxin and efflux pumps work together

A

-Auxin is transported from cell to cell in one direction

-Auxin efflux pumps embedded in the plasma membrane transport auxin out of the cell by active transport

-The location of auxin efflux pumps determines the direction of transport

-When a shoot is exposed to light from above, auxin efflux pumps are located at the base of cells in the shoot apex

-When the shoot is exposed to light from one side, this causes auxin efflux pumps to change location inside the cell. They move to the side of the cell furthest away from light

-This causes auxin to be transported to the shady side of the shoot, generating a higher concentration of auxin on the shady side than on the side exposed to light.

IB HL Biology (68 decks)

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