Seeds and Germination
Table of Contents
Learning Intentions - Seeds and Germination
Learning Intentions
1. Identify and explain the structure and functions of a seed
Draw and label a diagram of a seed including the:
plumule, radicle, cotyledon, micropyle and testa
Describe the structure and function of all of the parts of a seed
Describe the features of the seed and relate them to their role in the stages of germination.
Explain how the structure of seeds and leaves aid life processes (especially - cellular respiration)
2. Describe and discuss the life processes of germination
Define germination
Identify where the processes of germination fit in to the life cycle of plants
Explain the significance of germination to the life cycle of plants
Label a diagram with the root (radicle) and shoot (plumule).
List all the conditions needed for germination to occur
Describe how the environmental conditions of water availability and temperature affect germination of a plant
Describe the function of enzymes
Explain how the environmental factors of water and temperature affects germination and link this to enzyme activity.
Carry out an investigation to determine the effects of environmental factors on germination of a plant
Comprehensive investigation and discussion, which includes the implications of environmental factors for the germination and survival of the plant
Learning Intention 1:
Seeds - Structure and Function
1. Identify and explain the structure and functions of a seed
Draw and label a diagram of a seed including the:
plumule, radicle, cotyledon, micropyle and testa
Describe the structure and function of all of the parts of a seed
Describe the features of the seed and relate them to their role in the stages of germination.
Explain how the structure of seeds and leaves aid life processes
Seed Parts
The seed has several parts:
Testa - Seed Coat
Embryo - consisting of: Cotyledon, Plumule, Radicle
Endosperm - The food for the seed (except in dicots, where the cotyledon becomes the food)
Microphyle - small hole that lets water in
Hilum - scar where the seed was attached to the parent plant
Monocotyledons vs Dicotyledons
Monocots
Monocots are called monocotyledons because they have only one cotyledon
Monocot seeds include corn and wheat
In monocots the Endosperm Develops into a Starch filled food reserve for the plant to use
In monocots the single Cotyledon is thin
In monocots the single thin Cotyledon acts as a conduit to transfer nutrients from the Endosperm to the Embryo (like a placenta)
The Endosperm is the largest part of a mature monocot seed
Dicots
Dicots are called dicotyledons because they have two cotyledons
Dicots are also called Eudicot
Dicot Seeds include Beans
In Dicots the Endosperm is used up by the developing Cotyledon while the seed is still immature - so in the mature seed the endosperm is too small to see a
In Dicots the middle of the embryo's cotyledons (seed leaves) grow and swell as they fill with starch and other nutrients. They swell until they fill the whole seed.
The two Cotyledons are the largest part of a mature dicot seed
A Bean Seed - A Dicot
A photo taken of the bean seeds dissected in class
Seed Formation
Double Fertilization
Two sperm cells released from pollen tube
One sperm cell connects with Egg cell to create zygote, which then becomes the Embryo
Other sperm cell combines with the two nuclei in the large center cell of the Ovule creating the Endosperm nucleus
Immature Seed
The Endosperm grows occupying most of what will become the seed
The Embryo grows in 3 parts:
Epicotyl = above the Cotyledons = top half of embryo
this has the plumule - miniature leaves
this is the embryotic shoot
Cotyledons - seed leaves - one = monocot, two = dicot
Hypocotyl = below the Cotyledon = bottom half of embryo
this ends with the radicle - the embryotic root
The Seed Matures
The seed absorbs nutrients such as starch, protein and oils from the parent plant - storing it in the Endosperm (Monocot) or the Cotyledon (Dicot)
The outside of the ovary starts to form a strong layer this becomes the seed coat (Testa)
The Final stage of Maturation
The seed loses water - desiccation
The embryo stops growing as the water is lost
Cellular respiration is so slow that is is undetectable
The seed is now dormant
Seeds and Flowers = An R in MRSGREN = Reproduction
How do plants disperse their seeds?
State the purpose of seed dispersal
State the purpose of fruit
Describe the structure of a seed with regards to its dispersal method
Name at least 5 seed dispersal methods and give examples of plants that use these methods
For the plant to colonize the land, the seed must be dispersed to new locations. This is Seed dispersal.
The options are
Wind - like a dandelion or a swan-plant - the seed can float on the wind
Water - like a coconut or a mangrove - the seed can float on the water
Explosive - the seed can be thrown away from the plant by the force of an exploding seed pod - gorse
Animal - by Hook - the seed can be hooked onto the coat or foot pads of an animal and transported until the hook breaks - like prickles in the grass when you run in bare feet (Onehunga weed)
Animal - by Digestive system - the fruit containing the seed can be eaten, and the seed deposited in a lump of manure - like apples, tomatoes, oranges
Fruit - Seed dispersal by Animal droppings
Whist the ovule becomes the seed, the ovary becomes the fruit
The fruit grows as the seed develops
The fruit ripens as the seed reaches maturity and dormancy
The ripening of the fruit is caused by enzymes that digest the cell walls of within the fruit making it softer. Enzymes also turn starches into sugars making the fruit tastier. As much as 20% of a the mass of a ripe fruit is sugar. The fruit also changes colour to indicate to animals that it is ripe.
Interestingly, for grains such as wheat, rice and corn. The outside layer is actually dry pericarp (wall of the ovary) adhering tightly to the seed.
A ripened edible fruit uses animals to disperse the seeds of the fruit through their droppings
Learning Intention 2:
Germination
Describe and discuss the life processes of germination
Define germination
Identify where the processes of germination fit in to the life cycle of plants
Explain the significance of germination to the life cycle of plants
Label a diagram with the root (radicle) and shoot (plumule).
List all the conditions needed for germination to occur
Describe how the environmental conditions of water availability and temperature affect germination of a plant
Describe the function of enzymes
Explain how the environmental factors of water and temperature affects germination and link this to enzyme activity.
Carry out an investigation to determine the effects of environmental factors on germination of a plant
Comprehensive investigation and discussion, which includes the implications of environmental factors for the germination and survival of the plant
Dormancy
The Dry State
Much like the Aral Sea, inside the dry seed, nothing happens.
With no water, the enzymes cannot float around and do their work.
With no water the nutrients and other chemicals of the seed stay still.
Imbibition
Wet
Water is drawn into the seed by osmosis
The water enters through the Micropyle, as small microscopic hole next to the embryo
As water enters the enzymes can start to move again - like flooding a dry dock
The seed swells with water
If sufficient water enters, then the seed ends its dormancy, and starts to germinate
Germination
Grow
Germination begins with the release of the hormones called gibberellins.
These gibberellins float through the seed till they get to the cells surrounding the endosperm or the condyleton - here they signal the cells to produce the enzyme alpha amylase
alpha amylase starts to turn the starch stored in the condyleton or endosperm into sugar
Hormones are chemical messengers used by plants and animals to send a message from one part of the body to another
Enzymes are proteins that do things
Amylase is an enzyme that converts starch into maltose - this makes it ready to be used as a fuel for cellular respiration
Enzymes
What are enzymes
Enzymes do things
They either:
break down things - catabolic
build things up - anabolic
Enzymes hold chemicals momentarily allowing them to react with other things this causes the building or the breaking
The chemical being held is the substrate. The shape of the substrate matches the active site on the enzyme
The active site is where the chemical is held
Each type of enzyme has a specific active site shape, this will fit only the targeted substrate
Once the substrate has bonded to the active site, the enzyme usually changes shape slightly, this forces a change on the substrate increasing the chance of the chemical reaction occurring
Once the reaction has occurred the enzyme changes shape again, releasing the products
In other words:
The enzymes active site binds to its target substrate, a chemical reaction occurs, turning the substrate into a product, the product is then released
Enzymes are important for the R in MRSGREN = Respiration
The Key Enzymes in Germination are:
Alpha Amylase
this turns the stored starch from a giant structure of branching chains with thousands of glucose molecules ( a polysaccharide) into a just tiny bits with just two glucose molecules
This makes Maltose - two glucoses (a disaccharide)
These maltose molecules can then diffuse across the cell, into the embryo and then into the cells of the embryo
Maltase
This turns Maltose into two glucose molecules - ready for their use as a fuel for cellular respiration
Hexokinase
the entry point for Glucose entering into Cellular Respiration
Alpha Amylase - main example (only talk about this one)
Water entry into the seed triggers the release of gibberellins which signal the cells around the cotyledon or endosperm to produce and release alpha Amylase
Starch consists of thousands of glucose molecules stuck together
Alpha Amylase cuts the huge starch molecule up into small molecules called Maltose, these have just two glucose molecules each
This is much like an arborist on a tree. The arborist is Alpha-amylase, the tree is starch and the logs would then be maltose
Maltose is tiny compared with Starch. As such, it can float on the water that the seed has absorbed.
The Maltose can float, or diffuse, through the seed, to the embryo.
In the cells of the Embryo the Maltose is hydrolyzed (broken) into just two Glucose molecules
Maltase
Maltase breaks, or hydrolyses, the Maltose into glucose so that it can be used in Cellular respiration
Hexokinase
Hexokinase is an enzyme that does not break down molecules, rather it adds an atom onto its substrate
Hexokinase adds a phosphate to glucose, making Glucose-6-phosphate.
This is the very first step in cellular respiration
Enzymes and Activation Energy
For a chemical reaction to occur the reactants must bang into each other fast enough (with enough energy). This is the activation energy - the energy required for the reaction to be activated
Also, the reactants need to hit in the correct orientation - like to react they need to hit the correct parts - like spider man going through a window or slamming into a wall
Enzymes lower the energy required for the chemical reaction to occur by grabbing hold of the molecule and then holding it and even bending it in the correct way for the other reactant to collide with it
So, by holding the reactant the right way the collisions with the other reactant are more likely to result in a reaction
The hotter it is the more energy both reactants have, so the more likely it is that the reactants will collide whilst one is bound in the enzyme, and the more likely it is to collide with enough force for the reaction to occur
At the optimum temperature for an enzyme it will be conducting its reaction at its fastest rate. This means that the molecules are colliding with sufficient energy for the reaction to occur, whilst the enzyme holds the reactant in place. In other words, the molecules are colliding with sufficient kinetic energy for the activation energy to be overcome.
Factors effecting Enzyme activity
Enzymes work by having chemical substrates land in their active sites
Temperature
Chemical substrates must diffuse through water to land on the enzymes active site.
The warmer the water is, the faster the rate of diffusion, because the water molecules themselves are moving faster
Colder = slower water = slower diffusion = slower collisions = slower enzyme action
Hotter = faster movement within the water = faster collisions = faster enzyme activity
The enzyme works the fastest at its optimum temperature
Past the optimum temperature, the atoms of the enzyme itself start to move to much and the active site changes shape. If the active site changes shape, then the substrate cannot fit. So, that enzyme can no longer participate in the reaction - this is why the graph drops so quickly after the optimum temperature.
This change in shape is called denaturation, we say the protein has denatured. After this it will not work.
Heat Collisions and Activation Energy
Hotter = more successful collisions
Enzyme holds onto the substrate so that the site for the reaction is exposed - increasing the likelihood of a reaction should the other reactant collide
This way the enzyme reduces the ACTIVATION ENERGY required for a reaction
The HOTTER it is, the more collisions will occur, and the more energy the collisions will have, so more collisions will have sufficient energy for the reaction to occur
Denatured
Proteins are held in their shape with a range of chemical bonds
The proteins shape is its nature
Heat causes atoms to move
When it is too hot, the atoms in the proteins move too much and the weaker chemical bonds (like the hydrogen bonds) will break
This causes the protein to change its shape
This new shape is not its natural shape, so it is called denatured
Cellular Respiration
Cellular respiration is how cells turn food into energy
Cellular respiration need oxygen and a fuel, it then releases chemical potential energy, carbon dioxide and water
The food used is Glucose
To get to glucose, the seeds starch needs to be digested
Starch = alpha amylase = maltose = maltase = glucose = hexokinase = glucose-6-phosphate
The overall chemical reaction that is Cellular Respiration is:
Glucose + Oxygen = Carbon Dioxide + Water + Energy
For more detail:
1 Glucose + 6 Oxygen molecules = 6 Carbon Dioxide + 6 Water + 32 ATP
Cellular Respiration is an R in MRSGREN = Respiration
When the seed is dormant, the level of Respiration is undetectably low.
Imbibition, or the influx of water, results in increased cellular respiration - this is the key first step in germination
Dormant = Low Cellular Respiration (like a car engine in Park)
Imbibition (influx of water) = Increased cellular respiration (car engine in first gear) = starch converted to glucose by enzymes = glucose used by embryo for cellular respiration = High Cellular Respiration (car engine overtaking in sports mode) = radicle grows down to find more water = plumule grows upwards to find the light for photosynthesis = photosynthesis needed as starch supply in cotyledon might run out = photosynthesis is how the plant makes its food
Germination
The seed is dormant = sleeping
very low metabolic rate
Seed dormancy increases the window for the seed to land in a favorable place
The first steps in germination are:
Water enters seed through the micropyle = imbibition
Water causes seed to swell, the enzyme amylase begins to be produced
The enzyme alpha amylase digests the starch in the cotyledon and turns it into glucose
The glucose diffuses in the watery seed, into the embryo, providing the fuel for cellular respiration
As the embryo increases cellular respiration, it increases its available energy and it starts to grow
Note: cellular Respiration is a Life process and Growth is a Life process (MRSGREN)
The radicle emerges from the seed
The radicle senses gravity (positively geotropic) and grows downwards
The radicle puts out root hairs - to absorb more water and to secure the seed
The Epicotyl starts to grow away from gravity - this becomes the stem
The Epicotyl grows with a bent, or hooked, shape. This allows it to pull the delicate leaves of the plumule through the soil, rather than pushing them (think about the shape of the leaves and how pulling them rather than pulling them will reduce damage)
As the embryo uses up the starch store in the cotyledons they start to shrink
As the starch is used up, the dry weight of the plant decreases
Once the plant erupts from the soil it will sense light and start to produce high levels of chloroplasts, these turn the plumule green. These leaves are now called 'foliage' leaves
Once the foliage leaves are out the plant can photosynthesis
Once photosynthesizing, the plant is now making its own food by using energy from the sun to combine carbon dioxide and water - the mass of the plant now increases
In response to water, the embryo releases a hormones called gibberellins. Gibberellins are the first hormone to be released following imbibition and signals the end of dormancy and the start of germination. This hormone stimulates the release of the enzyme alpha amylase from the cells surrounding the food store (endosperm or cotyledon). The amylase starts to cut the starch into simpler sugars (maltose and glucose) for use as the fuel for cellular respiration by the embryo.
As the starch is used up the dry mass of the seedling decreases.
However, as the radicle absorbs more water the water content increases so the wet weight of the seedling increases.
However, if the shoot does not reach light (if it is planted too deep) then the food stores (endosperm or cotyledon) may run out, then the seedling will die.
Germination said a different way
The lifecycle of plants starts with Germination.
Germination is the development of a plant from a seed or a spore
This process starts with water
Water enters the seed through the microphyle
The presence of water allows the enzyme alpha amylase to digest the starch contained in the Cotyledon (or Endosperm), turning it first into maltose, then into glucose - this is food for the embryo
The availability of food and water allows the Embryo to increase cellular respiration. This provides energy for the embryo to grow
The warmer it is the faster the enzyme alpha amylase will work, so the more glucose will be available for cellular respiration. More cellular respiration there is, the more growth can occur. However, the enzyme has an optimal range, beyond this it stops working as their is too much movement of atoms within the enzyme and it denatures
The Radicle is the first bit to grow - it becomes the root for the plant
As the radicle follows gravity, it grows downwards, sending out sideways roots called root hairs to anchor the plant and to absorb water. (positive geotropic response - following gravity)
The Epicotyl begins to grow in the opposite direction of gravity, pulling the plumule along with it.
The cotyledon becomes smaller as the starch is used up
If the plumule finds sunlight it will start to create chlorophyll and turn green
The leaves of the plumule become the foliage leaves and begin to photosynthesis
The cotyledons are no longer needed, so they fall away.
Requirements for Germination
Water - so things can move
Oxygen - for cellular respiration
NOT Light - light is only needed when leaves develop from the plumule
Enzymes - alpha amylase, maltase, hexokinase
alpha amylase - starch into maltose (the example enzyme)
maltase - maltose into glucose
hexokinase - glucose into glucose-6-phosphate - ready for cellular respiration
Cellular respiration must occur at a higher rate than during dormancy for growth to occur
Warmth - To facilitate movement in the water.
Too cold = slow movement = slow enzymes = slow growth
Too hot = atoms have too much kinetic energy, ie, they move too much, active site changes shape, enzyme denatures, less binding, less enzyme reactions = less growth = less likely to germinate
Germination = G in MRSGREN = Growth
Some seeds are picky germinators
Some seeds require more than some water for germination - desert plants need a large about to ensure it wasn't just a passing shower. Some seeds need fire to break their dormancy. some need heat to ensure they don.t germinate in winter. Some seeds will only germinate if the seed coat has been weakened by passing through have passed through the digestive track of an animal.
Germination Recap
3 Ways to get the first leaves out of the ground
Method 1 (dicot)
Bent or hook shaped Epicotyl
The Epicotyl grows and pulls the delicate plumule out of the ground
Method 2 (dicot)
Bent or hook shaped Hypocotyl
The Hypocotyl grows and pulls the delicate plumule out of the ground
Method 3 (monocot)
A sheath (like a closed tube) grows out of seed (this is called a coleoptile (think of corn))
First leaves grows up through the sheath
