topic 9 – plant science

TOPIC 9 – PLANT SCIENCE

Introduction to Plants Plants make up over 50% of the living

organisms on this planet They belong to the kingdom Plantae There are five phylum:

Bryophyta Filicinophyta Coniferophyta Angiospermophyta

Introduction to Plants (cont)

Angiosperms are the most dominant phylum

Angiosperms, or flowering plants, produce seeds enclosed inside fruits.

Angiosperm comes from the word angerion – a container sperma – a seed phyton – a plant.

Introduction to Plants (cont)

Angiosperms are divided into two large groups: Monocotyledons (Monocots) Dicotyledons (Dicots)

These names refer to the number of leaves contained in the embryo, called cotyledons.

Typical Monocot

Monocots vs. Dicots

Typical Dicot

Shoot Apical Meristems

Lateral Meristems

Comparison of Growth for Apical and Lateral Meristems

APICAL MERISTEMS LATERAL MERISTEMS Primary growth Allows plant to grow

longer (upwards) Forms leaves and

branches Increases

photosynthetic capacity

Found in both monocots and dicots

Secondary Growth Allows plant to grow

in width Widening of main

trunk for support and depositing of vascular tissue and bark

Found only in dicots

Stems Supports the leaves for photosynthesis Transports water and nutrients from roots

to leaves Support is achieved by:

Tugor Cellulose walls Lignin reinforcing the xylem

Stems Consist of an epidermis which surrounds

the vascular tissue, composed of xylem (water transport, up the stem) and phloem (mineral and sugar transport, up and down the stem to sinks for storage)

Meristems deposit secondary xylem and phloem, which will grow outwards to become primary xylem and phloem.

Plan Diagram of the Stem

Leaf Structure Consists of a:

Leaf Blade Leaf stalk

Leaves have a large surface area and a small space between layers

Designed for photosynthesis

Leaf Structure (cont) Leaves consist of: Outer structure - Epidermis

Tough, transparent layer Upper – waxy Cuticle Lower – specialized cells called guard cells, that

form openings in the bottom, called Stoma Inner Structure – Specialized Cells

Mesophyll cells

Leaf Structure (cont) Upper Surface – Palisade Mesophyll

Tightly packed Contain chloroplasts

Lower Surface – Spongy Mesophyll Loosely packed with air spaces

Vascular Bundles Consist of xylem and phloem Bring water to and transport sugars and

minerals away and to leaves Support the leaves along with cellulose and

turgor

Plan Diagram of the Leaf

Roots First stage of development for the seed

when it germinates Tap Roots Lateral Roots

Roles Absorption Anchors Support Storage

Roots (cont) Roots have an outer coat, called the

epidermis, and the inner portion is called the cortex

In the root, there is a vascular bundle, of xylem and phloem

Branching of roots allow for a greater surface area

Root hairs off of growing roots, increase the surface area as well.

Diagram of Roots and tissue Plan Diagram

Modifications of Stems, Roots and Leaves

Roots

Prop Roots

Storage Roots

Pneumatophores

Buttress Roots

Examples of Root Modifications


Stems

Bulbs

Tubers

Rhizomes

Stolons

Examples of Stem Modifications


Leaves

Tendrils

Reproductive Leaves

Bracts or floral leaves

Spines

Control of Plant Growth - Phototropism

Plant growth is controlled by gravity and light Plant grows against gravity Plants grow towards the light

Responses to the above stimuli, called tropisms

Growth towards light called phototropism Controlled by a hormone called auxin Produced in the tip of the shoot


Steps of phototropism Photoreceptors in the tip of the plant sense the light Stimulate the production of auxin Auxin will travel to the “shady side” of the plant, as

detected by the phototropins Promotes the elongation of cells in stems, by

loosening the connections between the cell walls and cellulose microfibrils

Promotes the stem to grow more on the shadier side and go towards the light.

Allows the leaves on the sunny side to get more light and photosynthesize at a greater rate.

Transport in Angiosperms Root System

Transpiration Water uptake Factors affecting Transpiration

Translocation

Transport in Angiosperms Roots – Absorption and uptake

Provide large surface area for uptake of water and minerals

Water is absorbed by osmosis Amount of water absorbed is increased by root

hairs, on ends of growing roots Minerals absorbed by active transport

Water uptake Occurs by osmosis

Flows through epidermis, into cortex by mass flow, as the cells are interconnected

Three possible routes for uptake of water: Apoplast Pathway (Mass Flow) Symplast Pathway Vacuolar Pathway

Water uptake Apoplast Pathway (Mass Flow)

Most common way for water to move (faster) Water does not enter the cell Moves through the cell walls until it reached

the endodermis Cells of the endodermis have a Casparian

Strip around them that is impermeable to water

The water is diverted to the spaces of dead cells, eventually to the xylem

Water uptake Symplast Pathway

Water enters the cytoplasm but not the vacuole

It passes from cell to cell via connections between cellular cytoplasm of adjacent cells, called plasmodesmata

The organelles are packed together in cells, and as a result, block significant progress of water

It is not the major pathway for water. Minerals mainly move through this pathway.

Water uptake Vacuolar Pathway

Water enters the cell and move into the vacuole It can be stored in the cells It can also travel through the cytoplasm and the

cell wall to the next cell, to move into cortex

Once in the endodermis, water can move into the xylem and pulled via transpiration forces.

Pathways for water uptake

Uptake of Minerals Minerals are important to build cells

walls, carbohydrate storage and protein synthesis

Processes for mineral uptake: Active transport Mass flow (in water) Fungal hyphae

Transpiration Transpiration

the loss of water vapour from the leaves and stems of plants.

Like perspiration As water is lost, the amount of water in

the plant decreases. A pull is created in the plant to “pull” water up the plant. This is similar to maintaining homeostasis.

Transpiration (cont) Water moves from root to leaf by

transpiration pull

Water moves up the stem to leaves in the xylem Dead material Made of tracheids and xylem vessels

Xylem Tissue

Mechanism of Movement of Water – Transpiration Pull

Controlled by stomata Stomata open and close depending on

the amount of water in the plant If there is a lot of water – high turgor

pressure in guard cells and stomata are open

If there is a deficiency of water – low turgor pressure in guard cells and stomata close

If water drops, abscisic acid is released, overriding all variables and stomata close

Mechanism of Movement of Water – Transpiration Pull

When stomata are open, water vapour is lost to the external environment

Concentration gradient is created The lost water needs to be replaced Water moves from the high concentration

(roots) to lower concentration (leaves) and moves up the plant

Cohesive forces of water allow water to move in a continuous flow

Transpiration

Factors that affect Transpiration

Biotic Factors

Size of the plant The thickness of the cuticle How widely spaced the stomata are Whether the stomata are open or closed

Factors that affect Transpiration

Abiotic Factors Temperature

Humidity

Wind

Light

All of these can be over ridden by abscisic acid

Translocation Movement of manufactured food (sugars

and amino acids). Occurs in the phloem tissues of the

vascular bundles. Moves sugars from source to sink (leaves

to storage) and from source to areas of new growth, like ends of shoots and new leaves.

Phloem tissue allows movement up and down the stem of the plant

Translocation Phloem Tissue, is living tissue, and

consists of: Sieve tubes

Flow of sugars and minerals Companion cells

Control flow / Active transport

Theory of Translocation is by mass flow, from source to sink

Source and sink can change, depending on use and season

Translocation

Transpiration and Adaptations of Xerophytes

Small, thick leaves Reducing the number of stomata Stomata located in crypts or pits

on the leaf surface Thickened, waxy cuticle Hair-like cells on the surface to

trap water vapour Become dormant in the dry

months Store water in the fleshy stems

and restore the water in the rainy season

Using alternative photosynthetic processes called CAM photosynthesis (Crassulacean acid metabolism) and C4 photosynthesis

Reproduction in Flowering Plants

Parts of the flower Pollination Fertilization Seed formation and

dispersal Seed germination Control of flowering

- Photoperiodism

Typical Dicotyledonous Flower Sepal

enclose and protect the flower in the bud, and are small, green and leaf like.

Petals (together called the corolla) coloured and used to attract insects or other small

animals to pollinate the flower.

Stamen – male part of the flower, which consists of:

Anthers – produces the male sex cells house the pollen grains

Filament or stalk – holds up the anther

Typical Dicotyledonous Flower Carpels – female part of the flower, and

they may be on their own or fused together. Each carpel consists or:

Ovary – at the base of the carpel which contains the female sex cells (containing many ovules)

Stigma – sticky top of the carpel (to receive the pollen)

Connecting style – supports the stigma

Pollination and Fertilization

Pollination the transfer of pollen from a mature anther to

a receptive stigma.

Fertilization occurs after the pollen grain has landed on a

stigma, and germinated there. It is the fusion of the male and female gametes.

Pollination and Fertilization

Process of Fertilization

The pollen produces a tube, which grows down between the cells of the style, and through the ovule.

The pollen tube delivers two male

nuclei. One of these male nuclei then fuses with the egg

nucleus in the embryo sac, forming a diploid zygote.

The other fuses with the other nucleus, which triggers formation of the food store for the developing embryo.

Seed Formation and Dispersal

Seed contains the developing embryo and the food store

The zygote grows by mitosis, forming the embryonic plant, consisting of an embryo root and stem.

A seed leaf or cotyledon forms. The seed leaf has two

forms, as angiosperms have two classes. Monocotyledons – have a single seed leaf Dicotyledons – have two seed leaves

The formation of stored food reserves is triggered. In many seeds the food store is absorbed into the cotyledons.


The outer layers of the ovule become the protective seed coat, or testa.

The micropyle is a small hole through the testa,

where it was attached to the parent plant.

The whole ovary develops into the fruit.

The water content decreases and the seed moves into a dormancy period, assisted by the formation of abscisic acid.


Seeds are dispersed when “fruit” ripens Seeds are dispersed in such a way as to

eliminate many seeds in one place and around the base of the parent plant (population dynamics)

Dispersed by: Wind Animals Explosive

Seed Germination Seeds are in suspended animation When metabolic activity starts, this is

germination

Seeds are dormant because: Incomplete seed development Presence of a plant growth regulator – abscisic

acid Impervious seed coat

Seed Germination In order for germination to occur, the

proper conditions are needed

Water – hydrates plant and activates amylase and removes the abscisic acid

Oxygen – for Cellular respiration Period of warm temperatures as this is

important for enzyme production.

Seed Germination The metabolic processes during the germination of a seed

are as follows: The seed absorbs water.

Gibberellin, or gibberellic acid, is released after the uptake of water and is a plant hormone

Gibberellin triggers the production of amylase.

Amylase causes the hydrolysis of starch into maltose. The starch is present in the seed’s endosperm or food reserve.

Maltose is then further hydrolysed into glucose that can be used for cellular respiration or may be converted into cellulose by condensation reactions.

Cellulose is used to produce the cell walls of new cells.

The seed coat cracks and out comes the plant.

Control of Flowering of Angiosperms - Photoperiodism

Photoperiodism Plant’s response to light involving the lengths of day

and night. It is the length of day and night that controls flowers

Plants that respond to large amounts of sunlight, and short periods of darkness are called long day plants (late spring, summer)

Plants that respond to small amounts of sunlight, and long periods of darkness are called short day plants (early spring, late fall)


It is actually the length of night that controls the flowering process

The control by light is brought about by a special blue-green pigment called phytochrome.

Phytochrome is a large protein that is

not a plant growth hormone, but a photoreceptor pigment.


There are two forms of phytochrome: inactive form Pr active form Pfr

In light, the Pr is converted to Pfr.

In darkness, the active form (Pfr) slowly converts back to Pr

The slow conversion allows the plant to time the dark period and controls the flowering in short-day and long-day plants.

Process of Photoperiodism A long day in the summer:

A lot of Pr is made into Pfr during the day.

In the night, because the night is short, little Pfr is converted back to Pr, and when the sun rises, there is still a lot of active phytochrome (Pfr) left

This signals a long day, short night, and promotes flowering in long day plants

This does not signal a short day, long night and inhibits flowering in short day plants

Process of Photoperiodism A short day in the spring or fall:

A small amount of Pr is made into Pfr during the day.

In the night, because the night is long, almost all Pfr is converted back to Pr, and when the sun rises, there is minimal active phytochrome (Pfr) left and lots of inactive (Pr)

This does not signal a long day, short night, and inhibits flowering in long day plants

This does signal a short day, long night and promotes flowering in short day plants

Summary of Photoperiodism

Long day plants need active phytochrome (Pfr) Pfr acts as a promoter Need a short night

Short day plants do not need active phytochrome (Pfr) Pfr acts as an inhibitor Need a long night

Experiment for Photoperiodism

topic 9 – plant science

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