2.4 growth and reproduction in plants

growth is a process where one or more of ____ occur

1. increased cell size by accumulating resources

2. cell proliferation aka more cells are produced by mitotic cell division (this is only applicable to multicellular organisms)

determinate growth

Determinate growth is a biological pattern where an organism (plant or animal) grows to a genetically fixed size and then stops

indeterminate growth

plant growth occurs in what kind of tissue?

meristematic tissue

meristematic plant cells

• totipotent (can give rise to any type of cell) and undifferentiated

• meristematic plant cells are similar to stem cells in animal tissue

apical meristems

lateral meristems

apical meristems: causes lengthening of plant (primary growth)

• occurs at tips of shoots and roots

lateral meristems: widening of plant (secondary growth)

• occurs at cambium

phytohormones

these are plant hormones

• they are specifically signalling chemicals that control growth, development and response to stimuli in plants

name 5 phytohormones

• auxins

• ctyokinin

• abscisic acid

• cibberellin

• ethylene

what are auxins

auxins are a group of hormones that have a wide range of functions in plants such as

• root and shoot growth/flowering (but levels can activate or inhibit growth)

• fruit and leaf development

• wound response

name a naturall occuring auxin

indole-3-acetic acid (IAA)

how do auxins work

auxins control the growth of plants by promoting cell division and causing some parts of the plants to elongate by making those cells get longer

stems and roots respond differently to high concentrations of auxins

how do roots respond to auxins

auxin in the roots inhibits cell growth, which causes the cells to be shorter

• if the lower layer of cells is shorter the root will turn down and away from the light

how does the stem respond to auxins

auxins in the stem stimulates cell growth so lower cell layer is longer and turns towards the light

what is a plant topism

plant topisms are mechanisms by which plants adapt to environmental changes

• basically a tropism is a growth in response to an external stimulus

• some common stimuli that can influence plant growth include light, gravity, water and touch’

• phototropism, gravitropism, hydrotropism, thigmotropism

positive vs negative tropism

positive tropism: when a plant moves towards a stimulus

negative tropism is when a plant moves away from a stimulus

positive phototropism

basically this combines the concepts of positive tropism (grows towards a stimulus) and the fact that this stimulus is light

• its the directional growth response towards lateral light in plant shoots

if light isnt distributed laterally, will the plant still display positive phototropism?

no, if the light is directly above it, the plant will just grow upward because auxin is distributed evenly

only if the light is at an angle, will the auxin move to the darker side in order to make the cells in that area grow faster

describe how auxin works in response to light

when sun is overhead, auxins get evenly distributed because theres no dark side. but if there is a more shaded side, auxins will move there and it will cause those cells to elongate.

what is a coleoptile

how are they related to the function of auxins

a protective sheath around the emerging shoot or root

• they contain specialized receptor cells that detect a stimulus and cause auxins to move

• an example of this specialized receptor cells are phototropins which are receptor protiens involved in phototropism

• The cells in the area with the higher auxin concentration elongate more than the cells with lower auxin concentration.

• elongation pushes the growth to the other side

are auxins only found in the coleoptile

no, covering the tip of the shoot prevents light detection by phototropins, which prevents phototropism

explain how auxins are transported between cells

1. first auxins must enter into the cell by diffusing across the phospholipid bilayer at the top of the coleoptile via auxin influx faciliated diffusion channels (travels along concentration gradient but must travel through a carrier protien, hence facilitated)

2. now that the auxins are in a cell, they need to move out into another cell. this is done through auxin efflux pumps also known as PIN protiens. these protiens move the auxins from one cell to the next

Now whats interesting is that the position of these efflux pumps controls where the auxins go and the concentration gradients made.

• normally auxin efflux pumps are found in the basal part of the cells, but they can relocated based on stimuli, for example they can move into the side of the membrane if light is on the other side

• by moving, efflux carriers can set up concentration gradients in different areas of the apical meristem

how do auxins affect gene expression?

1. auxin enters the cell and binds to an auxin receptor

2. the auxin-receptor complex activates specific genes, in particular the proton pump gene which is the DNA that encodes for building proton pumps

3. DNA is transcribed into RNA and its used to synthesize proton pumps

4. these proton pumps are inserted into the cell membrane to do their job and pump protons into the apoplast

whats the purpose of pumping protons into the apoplast?

pumping protons into the apoplast decreases the pH which activates enzymes called expansins. expansins loosen the cellulose in the cell wall by disrupting hydrogen bonds and this allows water to flow into the vacuole, expanding and elongating the cell.

explain the interaction between auxins and cytokinins

• root tips produce ctyokinins which travel to the shoots

• shoot tips produce auxins which are transported to the roots

• purpose of auxins: drives root growth, root hairs

• purpose of cytokinins: drives branching, leaves and flowers/seeds

• their interaction forms a positive feedback loop, the interaction between these phytohormones help ensure that the root and shoot growth is integrated

plant life cycle

btw, ethylene = ethene

angiosperms will produce fruit as part of their sexual reproduction

• fruits are the mature ovaries of plants

• the purpose of fruits is to help disperse seeds to allow future germination of new plants

• fruits respond to ethylene via a positive feedback loop that stimulates ripening

what is an angiosperm

a flowering plant that produces seeds enclosed in fruit

structure and anotomy of fruit

outside peel/skin = exocarp

meat part of the fruit = mesocarp

lining around the seed = endocarp

seed

positive feedback of ethylene on ripening

micropropagation

cloning a plant from cells taken from an adult plant

• cloning plants are easier than cloning animals because plants can undo specialisation in their cells, a process called dedifferentiation

in micropropagation its best to use the ____ because the cells are ____

in micropropagation use the shoot apex because cells are totipotent

name 3 benefits of micropropagation

• When a new variety of plant is created, micropropagation allows rapid increase in numbers of plants, known as 'bulking up' a new variety. 

• Production of virus-free individuals of existing varieties. Viruses travel via vascular bundle but meristem tissue doesn’t have vascular tissue.

• Production of orchids and other rare species. Orchids are delicate and difficult to breed sexually, as well as difficult to grow from their tiny seeds. 

what is pollination

pollination is the transfer of pollen from an anther to a stigma

• self pollination: anther and stigma of same plant

• cross polination: anther and stigma come from different plants

is flowering affected only by abiotic factors

no, flowering is influenced by abiotic factors and biotic factors

• abiotic factors: temperature, sunlight, humidity

• biotic factors: pollination, seed dispersal

• all these factors introduce selective pressure which influences the genetics of a population

asexual reproduction

when plants reproduce asexually they produce genetically identitcal offspring

sexual reproduction

sexual reproduction leads to offspring with new genetic variations

• seuxal reproduction requires the delivery of sperm to the egg for fertilisation

  • moving the gametes requires different methods like via water, air or animals

  • ex. sexual reproduction in bryophytes like mosses and ferns, motile sperm swim through the water droplets or are carried in the wind to reach the egs

explain the technique of micropropagation

method used to mass produce clones of a parent plant. uses tissues from meristematic issues or somatic cells from parent plants to clone them because their cells can undo specialization in a process called dedifferentiation

what part of the plant should you use for micropropagation

use the shoot apex because cells are totipotent

angiosperm pollination is an example of what kind of relationship

mutualism where both organisms benefit from the relationship

benefits of micropropagation (3)

1. bulk up quanitity of a new variety, so we make a lot of a type of plant

2. plants are grown in sterile environment, so plants are virus free since viruses cant travel via vascular bundles since meristem tissue doesnt have it

3. make rare and difficult to grow plants

what is pollination

the transfer of pollen from an anther to a stigma, can be from the same plant or other plants

• same plant is self pollination

• different plant is cross pollination

what follows successful pollination

fertilization inside the ovule when sperm and egg fuse to make a diploid embryo

this ovule becomes a seed containing the embryonic plant and it has a supply of food for the plant embryo during germination so it dont die

what is fruit

fruit is the ovary that contains the ovule

fruits are any structure containing ___

seeds

veggies dont have seeds and often dont develop from the ovule

A plant seed is a ______ that has the potential to grow into a new plant. 

It contains an _____ as well as_____ the embryo.

A plant seed is a multicellular ripened ovule that has the potential to grow into a new plant. 

It contains an embryonic plant as well as structures to protect and nourish the embryo.

what do monocot seeds do? main difference from dicots?

monocot seeds like corn do not store food *unlike dicots

the singular cotyledon has a role in producing hormons and protecting the embryo

monocot means single seed leaf

the foot reserves are in a tissue called endosperm

what are the components of an embryonic dicot plant

• Radicle – an embryonic root

• Plumule – an embryonic shoot

• Hypocotyl – a shoot above the root and below the cotyledons

• Cotyledons – modified leaves that store food for the embryo

• Testa – a seed coat that protects the embryo and food stores

• Hilum – a scar where the seed was attached to the ovary

• Micropyle – a small pore above the hilum where the pollen tube entered to allow fertilisation.

anotated diagram of the reproductive structures of an angiosperm

angiosperms are flowering plants that use sperm bearing pollen, flowers and seeds to produce offspring and disperse them to new areas

male part of the flow is called the stamen that contains the sperm/pollen

the flower has a female part called the pistil that recieves the sperm in the ovary

how does a plant know when to flower

angiosperms use enviornmental signals to produce flowers

gene expression is affected by these conditions

what are the factors affecting flowering

internal: plant characteristics like age and health

external: temperature, photoperiod

what is floral initiation

• Floral initiation is the first visible step in the transition from vegetative to reproductive growth in plants.

• this is caused by the growth and differentiation of apical cells. so cells in the shoot apex change how they divide and differentiate bc of changes in gene expression

• 1.3: The SAM switches from producing leaves and stem tissue to forming the structures of a flower.

• 1.4: The new floral meristem produces floral organs: sepals, petals, stamens, and carpels.

• 1.5: The switch is triggered by specific environmental signals such as changes in day length (photoperiodism).

• 1.6: The process is irreversible and commits the meristem to flower production.

what plant needs fire to germinate

lodgepole pine

pinus contorta

what plant is poisonous to horses but must be digested by cows to germinate

kleingrass

steps of germination in starchy seeds