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CELL SPECIALIZATION
Is the process by which cells develop from similar cells into cells that
have specific functions within a multicellular organism.

This is how a seed changes into a seedling and finally into an adult tree. The seed is able
to specialize into all the different parts of the tree.
CELL DIFFERENTIATION
Is a stage of development of a living organism during which
specialized cells form.
 So, even though all the cells of a seed start out being the same, they will undergo
cell differentiation, which results in specialized cells for different functions.
 Cells are specialized according to the set of proteins they contain.
 Genes are responsible for producing proteins. Even though all the cells in an
organism contain all the same genes, not all genes are turned on in any given cell.
TISSUES
Are a cluster of similar cells that share the same specialized structure and function.
ORGANS
Are a combination of several types of tissue working together to perform a specific
function. A plant has 3 organs: roots, stems, and leaves.
MERISTEMATIC CELLS
Unspecialized plant cells that give rise to specific specialized cells.

Meristematic cells are responsible for a plant’s constant growth.

They are also called “permanent embryos” because of their lifelong ability to
produce the cells that can become new tissues and organs in their part of the plant.

They are constantly producing more cells, which then become specialized.

These cells combine to form the three types of tissues found in the body of a
plant: dermal tissue, ground tissue, and vascular tissue.

Dermal tissue forms the outermost covering of the plant’s organs. It is the
barrier between the plant and its external environment. It protects the delicate
inner tissues from damage, and controls the exchange of water and gases between
the plant and its environment.

Ground tissue has two main functions: to perform photosynthesis, as well as
provide support for the plant’s body.

Vascular tissue performs the critical job of transporting water, nutrients, and
sugars throughout the plant. It also helps to provide physical support for the
plant’s body.
Growing Up or Branching Out

The cells in the actively growing areas of the plant give off a chemical called auxin (a plant
hormone. These cells are usually located at a plant’s terminal bud (a bud is a swelling of the stem,
that contains meristem for new, not yet developed, tissues in organs such as leaves and flowers).

Auxin controls the cells below and behind them. For example, the cells at a plant’s terminal bud
produce auxin that inhibits, or holds back, the development of cells in the lateral buds. The
result is that the plant will tend to grow upward, rather than outward.

However, if you remove the tip of a plant (either cut it off, or it is eaten by an animal) this “hold
back” signal is removed. The plant will bush out at the lateral buds.
.
Tissues Working Together: Plant Organs
There are 3-4 organs that make up a plant. The leaves, stem, and root. The fourth organ is the
reproductive organ, which is usually the flower.
THE LEAF

A leaf’s most important job is to provide a large surface area where photosynthesis can take
place.

If photosynthesis produces more glucose (a simple sugar) than the leaf needs, the excess is
converted into starch and stored in the leaf.

The upper surface of a leaf is made of a sheet of DERMAL TISSUE called EPIDERMIS. The
cells of the epidermis secrete a waxy cuticle that helps reduce the amount of water that
evaporates from the leaf’s surface. The main function of the epidermis is protection – so the
epidermal cells do not perform photosynthesis. Sunlight passes through them to the
photosynthesizing cells.

Between the upper and lower surface of the leaf is the MESOPHYLL TISSUE (mesomeans middle). Mesophyll tissue consists of PALISADE TISSUE CELLS (perform most of the
photosynthesis in the leaf – they are arranged in lines that resemble long poles – the top of these
cells are arranged to meet the Sun’s rays head on, so that the rays pass through the length of
the cell, and encounter chloroplasts. Palisade tissue cells are packed with mitochondria, which
perform cellular respiration) and SPONGY PARENCHYMA CELLS (loosely packed to form a
network with open spaces – like a sponge. The spaces contain the gases needed or produced by
photosynthesis: water vapour, oxygen, and carbon dioxide).

The Centre of the leaf contains xylem and phloem tissue arranged into VASCULAR BUNDLES.
These vascular bundles form veins that dissect the inside of the leaf at regular intervals. The
ZYLEM delivers water, in the form of water vapour, to the photosynthesizing cells. Zylem is nonliving tissue; it is unidirectional (only brings water upward). The PHLOEM picks up the sugars
that have been produced and delivers them to cells throughout the rest of the plant. Phloem is
living tissue; it is bidirectional (it moves nutrients all over the plant).

The lower surface of the leaf contains GUARD CELLS (which allow the gases to move in and
out. They can change shape to control the opening and closing of pores in the leaf). These pores
are called STOMATA (singular are called stoma). The guard cells and stomata play a significant
role in transpiration. Carbon dioxide enters through these pores, and oxygen and water vapour
exit through them.

TRANSPIRATION:
the evaporation of water from leaves.

CHLOROPLASTS are where photosynthesis occurs.

Light energy from the Sun combines with carbon dioxide from the air and water from the
soil to produce glucose. Glucose is a carbohydrate used by both plant and animal cells as a
source of energy. Oxygen gas is a product of photosynthesis and is essential to cellular
respiration in both plants and animals.

Chloroplasts can change their shape and location in a cell to increase the amount of light
they capture. They contain little sacs called thylakoids, which contain light-trapping
CHLOROPHYLL molecules. This is the part of the chloroplasts where photosynthesis
occurs. The chlorophyll are arranged in a stack called a granum.
THE STEM

A plant’s stem has two main functions: physical support and transportation of water, nutrients,
and sugars.

Stems contain most of a plant’s xylem tissue. As xylem grows, it forms long, straw-like tubes, or
vessels. The cells then die, but their cell walls remain behind, forming “pipes” through which
water can flow. Water can move 10 billion times more easily through xylem than it would if it had
to travel through cells filled with cytoplasm!

Phloem (which works with xylem to form vascular bundles) is also made up of vertically stacked
tubes. Their cell walls are porous, which allows materials to be exchanged between the phloem
and the neighbouring cells.
THE ROOTS

Roots anchor a plant to the ground and allow it to take up water and minerals from the soil.

Roots also act as a plant’s storage area.

The ROOT HAIRS are the main site of water and mineral absorption.

CORTEX CELLS usually do not contain chlorophyll and can be used by the plant to store starch.
There are lots of spaces between these cells, through which water and minerals can flow to the
endodermis (which helps to control the transport of minerals between the cortex and the
vascular tissues).

Plants (like a dandelion) have a TAPROOT, which is the one main root that grows larger and
thicker than the rest. The taproot allows the plant to reach far underground for water. It
anchors the plant firmly in the ground.

Other plants (like grasses) have FIBROUS ROOTS, which have branches that are all about the
same size. They spread out horizontally near the surface of the soil. They provide the plant with
a large surface area over which water can be taken up from just under the surface of the soil.
Fibrous roots also stabilize the soil and help to prevent erosion and landslides.
Plant Organ Systems

The organs of a plant, work together in systems to maintain a constant flow of fluids, nutrients,
and hormones from one part of the plant to another.

SYSTEM:

In biology, a group of tissues and organs that perform specific functions.
ROOT SYSTEM:
An organ system in a plant, which takes in water and minerals
from the soil and transports these substances to the shoot
system.

A plant’s root system consists of all the roots that lie below the
surface of the ground.

It is constantly growing to keep pace with the plant’s increased
demand for materials as it grows.

SHOOT SYSTEM: An organ system in a plant, which supports the plant, performs
photosynthesis, and transports sap, water, nutrients, and
sugars.

A plant’s shoot system can include the flower, leaves, stem, and fruit
with seeds inside.

The root and shoot systems are connected by the flow of water, nutrients, and various hormones
through the vascular bundles that contain xylem and phloem. Maintaining this flow is essential
for the plant’s ability to survive.

Xylem tissues takes care of moving water and minerals from the roots to other plant
parts, including the leaves, where these materials are needed for photosynthesis.

Since xylem is dead cells, it does not use any of the plant’s energy stores. However,
phloem cells are alive, and do use these stores.

Phloem tissue moves the sugars produced by photosynthesis to other plant parts.

Phloem cells actively pump substances to where they are needed in the plant.
The Effect of Root Pressure on Water Movement

Once water from the roots reaches the xylem, how does it move upward against the force of
gravity? It is actually pushed from below, by root pressure, or pulled from above, as a result of
transpiration.

ROOT PRESSURE occurs at night (when transpiration is low and the soil is very moist). As root
cells bring minerals into the xylem, the mineral concentration in the xylem increases. This high
concentration of minerals increases the tendency of water to diffuse into the root xylem by
osmosis. As water flows in, root pressure builds in the xylem vessel. This pressure forces fluid
up the xylem.

TRANSPIRATION, along with some of water’s unique properties, moves water up the xylem.
Transpiration makes room for more water from the xylem to move into the leaves, pulling the
water column up.

COHESION (the ability of water molecules to cling to each other) holds the water column in the
xylem together. ADHESION (the tendency of water molecules to stick, or adhere, to certain
surfaces, such as the wall of a xylem vessel) helps water fight the force of gravity.

The rate of transpiration is controlled by the amount of water vapour in the leaves. When the
amount of water vapour is large, the guard cells open the stomata and water vapour moves out of
the leaves. If the amount of water vapour is small, the guard cells relax and the stomata close.