AP Biology
Roots
Plant Unit Guided Notes
Anatomy of Plants
• The anatomy of a plant in its most simplistic form can be divided into the roots and the shoots
– are portions of the plant that are below the ground
– are the portion of the plants that are above the ground
• The roots work their way through the terrain, working as an anchor to keep the plants in place.
• In addition, the roots work as gatherers, absorbing the water and nutrients vital to a plants survival
Tissue System
• There are three plant tissue systems to know:
–
–
–
Ground Tissue
• The ground tissue, that makes up most of the the dermal and vascular systems and is subdivided into three cell types:
–
–
–
Collenchyma Cells
• Live cells that provide flexible and mechanical support
– Often found in
, is found between
Parenchyma Cells
• The most
• When found in leaves are called
of the three types, with many functions
– Allow CO2 and O2 to diffuse through intercellular spaces
• (owing to the presence of large vacuoles, these cells play a role in storage and secretion for plants
Sclerenchyma Cells
• Protect
Vascular Tissue
and support the plant
Xylem
• This structure has multiple functions
– It is a structure that the plant
– Functions as a passageway for the transport of water and minerals from the soil
• Xylem can be divided into two categories:
– and
• They both function in the passage of water, but vessel elements move water more efficiently because of structural differences
Phloem
• This structure also functions as a “ ” for plants
– Assisting in the movement of sugars from one place to another
• Unlike the xylem, the functionally mature cells of a phloem, sieve-tub elements, are alive and well
Dermal Tissue
• Dermal tissue provides the
• It is the skin, or

– This coating attempts to keep the bad guys (infectious agents) out, and the good guys (water and nutrients) in
• Within the epidermis are called
• What do you think they do?
– They control the opening and closing of gaps called stomata that are vital to the process of photosynthesis
Roots
• How do plants get their nutrients?
• Through the hard work of roots, whose tips absorb nourishment for the plant (
) via root hairs
• Most of the water and minerals are absorbed by plants at the extending from their surface
• These hairs create a larger surface area for
• A root is not just a root, for not all root structures are the same
• There are two types of angiosperm plants-
• Dicots are known for having a
• Monocots are associated with
, which have root hairs
Taproot System
• The taproot system (e.g. carrot) branches in a way similar to the human lungs;
– The roots start as on thick root on entrance to the ground
– then divide into smaller and smaller branches called underneath the surface which serve to hold the plant in place
Fibrous Roots
• These roots provide plants with a very strong anchor in the ground without going very deep into the soil
Root System Summary
•
•
Root Structure
• Lets take a look at the structure of a root moving from outside to inside
• The root is lined by the
– Cells that give rise to root hairs (that plants must thank for their ability to absorb water and nutrients)
• Moving further in, we come to the ,
– the majority of the root that functions as a starch storage receptacle
– Why would plants need starch? What type of organic compound does it fall under?
• The inner most layer of the cortex is composed of a cylinder of cells known as the
– These cells are important to the plant because walls between these cells create an obstacle known as the, , which blocks water from passing
• This is one of the mechanisms by which plants control the flow of water
• Moving in through the endodermis, we come to the , which is composed of a collection of cells known as the
– The lateral roots of the plant are made from the pericycle and hold the vascular tissue of the root

Root Growth
• Plants grow as long as they are alive as a result of the presence of
• Early on in the life of a plant, after a seed matures, it sits and waits until the time is right for germination
• At the point of , water is absorbed by the embryo, which begins to grow again
• When large enough, it busts through the seed coat, beginning the journey into plant-hood
• At the start of the journey, the growth is concentrated in the actively dividing cells of the
• Growth in this region leads to an increase in the length of a plant:
• Later on growth occurs in cells known as the , which extend all the way through the plant
• This growth that leads to an increase in the width of a plant and is known as
Regions of Growth
• : protective structure that keeps roots from being damaged during push through soil
•
•
: section of root where cells are actively dividing
: next section up along the root, where cells absorb H2O and increase in length to make the plant longer
• : section of root past the zone of elongation where the cells differentiate to their finalized form (phloem, xylem, parenchyma, epidermal, etc).
Shoots and Hormones
Structure of a Leaf
• Leaves are protected by the waxy cuticle of the , which functions to decrease the
• Inside the epidermis lies the ground tissue of the leaf, the mesophyll, which is involved in the everso-important process of photosynthesis.
• There are two important layers to the mesophyll
– The - where most of the photosynthesis of the leaf occurs, where there are MANY
– The spongy mesophyll- a bit farther in where cells provide to the cells performing photosynthesis
• Stomata are important structures to successful photosynthesis
– Controlled by
• In
that line the walls of the plants, a second type of cell called a surrounds the vascular tissue to make the use of CO2 more efficient and allow the stomata to remain closed during the hot day time hours, These cells prevent excessive
Structure of Stems
• Again, lets travel from the outside in and discuss the basic structure
• The epidermis of the stem provides protection and is covered by , a waxy protective coat
• The cortex of a stem contains parenchyma, collenchyma, and schlerenchyma cells.
• You’ll notice that there is no in the stem

• This is because this portion of the plant is not involved in the absorption of water
• As a result, the next structure moving inward is the vascular cylinder and once again the xylem and phloem
• A term to know is the
• It extends along the entire length of the plant and gives rise to
• Over time, the stem of a plant will increase in because of the secondary xylem produced each year
• Another term to know is the
• Produces
• This covering replaces the
-
when it dries up and falls off the stem during secondary growth, forming a protective barrier against infection and physical damage
• The growth of plants is not in seasonal environments
• There are periods of dormancy in between phases of growth
• Rings of a stem- These rings produced each year are a window into the past, and give insight into the amount of rain a tree has encountered in a given year. The wider the ring,
Plant Hormones
• Hormones perform the same general function for plants that they do for humans
–
• There are 5 main plant hormones you will need to know; abscisic acid, auxin, cytokinins, ethylene, gibberellins
• Abscisic Acid
– The “
– It makes sure that seeds do not
” hormone
, inhibits cell growth, and stimulates the to make sure that the plant maintains
• Auxin enough water
– A hormone that performs several functions
• Leads to the
• Plays a role in phototrophism and gravitropism
• Cytokinins
– Hormones that
– Seem to slow down the
– Supermarkets use synthetic cytokinins to keep their veggies fresh
• Ethylene
– This hormone
– Causes flowers and leaves to drop from trees (associated with aging)
• Gibberellins

– A hormone group that assists in
– When you think gibberellins, think “grow”
– It induces the growth of
Plant Tropism
• A tropism is growth that occurs in response to an environmental stimulus such as sunlight or gravity
The three tropisms you should familiarize yourself with are gravitropism, phototropism, and thimotropism
• Gravitropism
– This is a plants growth response to
– Two of the hormones that play a role in this movement are and
– A plant placed on its side will show gravitropism growth in which the cells on the upward facing side will not grow as much as those on the downward side
– It is believed that the relative concentrations of these hormones in the various areas of the plant are responsible for this imbalanced growth of the plant
• Phototropism
– This is a plants growth response to
– is the hormone in charge here, working in the zone of elongation
– When a plant receives light on all sides, auxin is distributed equally around the zone of elongation and growth is even
– When one half of the plant is in the sun, and the other is in the shade, auxin (almost as if it feels bad for the shady portion) focuses on the darker side
– This leads to unequal growth of the stem with the side receiving less light growing faster- causing the movement of the plant toward the light source
• Thigmotropism
– This is a plants growth response to
– One example involves vines, which wind around objects with which they make contact as they grow
Photoperiodism
• Like all of us, plants have a biological clock that maintains a
– A physiological cycle that occurs in time increments that are roughly equivalent to the length of a day
• The month of June has the longest days- the most sunlight. The month of December have the shortest days- least sunlight
• How is it that plants, which are so dependent on light, able to survive through these varying conditions?
• This is thanks to photoperidism
–

• One commonly discussed example of photoperiodism involves flowering plants (angiosperms)
– A hormone known as is thought to assist in the blooming of flowers
– An important pigment to the process of flowering is which is involved in the production of florigen
• Because plants differ in the conditions required for flowering to occur, different amounts of
, florigen are needed to initiate this process from plant to plant
• One interesting application of photoperiodism involves the distinction bewteen short day plants and long day plants which flower only if certain requirements are met:
Plant Type Example Flowering
Requirements
Flowers During
Short Day plants
Long Day Plants
Go with the Flow
•
Osmosis drives the of water and minerals from the soil by the
•
Water then moves deeper into the root until it reaches the
•
Once there, because of the , it can only travel through the selective endodermal cells that choose which nutrients and minerals they let through to the vascular cylinder beyond

•
The casparian strip essentially lets only those with a backstage pass through
• has a backstage pass and can go into the vascular cylinder
•
•
Once the water gets to the
does not and gets deeeeenied
, it has reached the H2O super highway and is ready to go all over the plant
•
There are few driving forces responsible for the movement of a plants water supply
•
The three main forces responsible are
•
Of these three
•
Osmosis
pulls the most weight
•
The driving force that moves from the into
•
How in the world does the plant keep the concentration gradient such that it promotes the movement of water in the appropriate direction? There are two contributing factors
•
The water is constantly moving from the root tips creating the space for more water to enter and
•
Osmosis is defined as the of water down its concentration gradient across
•
It flows from a region with a high water concentration to a region with a low water concentration
•
There is a higher mineral concentration inside the , which drives water into the xylem contained in this cylinder by a force known as
•
Capillary action
•
The force of between water and a passageway that pulls water up along the sides
•
Along with osmosis, this mechanism is a minor contributor to the
due to the counteracting force of gravity
•
Cohesion-Tension Theory and Transpiration-
•
This is the major mover of water in the xylem
•
creates a negative pressure in the leaves and xylem tissue due to evaporative loss of water
•
Water molecules display molecular attraction ( ) for other water molecules, in effect creating a single united water molecule that runs the length of the plant

•
When water evaporates off the surface of the leaf, the water is pulled through the xylem toward leaves- transpiration is the force pulling water through the plant.
T he Changing of the Guard; Regulating Stomata Activity
•
The are structures vital to daily workings of a plant
•
When closed, photosynthesis is because
• are inaccessible
When open, cells have access to water and carbon dioxide
•
But with every reward, there is always a risk
•
When the stomata are open, the plant could
•
This process of opening and closing the stomata must therefore be very carefully controlled
•
are the ones for the job
•
They surround and tightly regulate the actions of the stomata
•
When water flows into neighboring guard cells (leading to an increase in ), a structural change occurs that causes the opening of the stomata
•
When water flows out of the guard cells (a decrease in turgor pressure) the stomata will
•
It is by this mechanism that guard cells control the opening and closing of the stomata
“Move, Over Sugar”: Carbohydrate Transport Through Phloem
•
The transport of carbohydrates through the phloem is called
•
After their production, carbohydrates, the all important product of photosynthesis, are dumped into the phloem (the sugar super highway) near the site of their creation, to be distributed throughout the plant
•
The movement of the sugar into the phloem creates a driving force because it establishes a
•
This gradient leads to the of water into the , causing an increase in the pressure of these cells
•
This pressure drives the movement of sugars and water through the phloem
•
As the sugars arrive at various destination sites, the is consumed by plant cells, causing a reversal in the driving force for water that pushes water
•
As water exits the phloem, the increased pressure disappears and all is good once again
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