Transcript 2.1 Plant Cells, Tissues, and Organs

2.1 Plant
Cells,
Tissues,
and Organs
Cell Specialization
cell specialization = when young cells develop from
similar cells into ones that have specific functions
 Occurs in multicellular organisms
 Cell differentiation is the development stage in a
young organism when specialized cells form
 Genes produce the proteins that cause cells to become
specialized into a bone cell, nerve cell, muscle cell, ...
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all cells contain the same DNA
certain genes “turn on” in one cell type, but not in others
New cells come from pre-existing cells by mitosis (&
cytokinesis), but they are not necessarily going to
perform the same function
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In all multicellular plants & animals:
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groups of specialized cells form tissues
groups of tissues form organs
groups of organs form systems
In plants, unspecialized cells are called meristematic
cells
In animals, unspecialized cells are called stem cells
Meristematic cells are sometimes called ‘permanent
embryos’, because unlike many animal stem cells, they
can produce cells that will become new tissues & organs
at any point in a plant’s life
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in humans, true undifferentiated stem cells exist only as a fetus
Plant: meristematic cells  tissues  organs  systems
Animal: stem cells  tissues  organs  systems
Plant
Animal
Meristematic Cells form Tissues
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Meristematic cells are constantly producing
new cells
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these new cells become specialized to form tissues
Types of plant tissues:
1. Dermal tissue
2. Ground tissue
3. Vascular tissue
Figure 2.3 on p.58
Types of Plant Tissues
1. Dermal tissue  outermost part
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Made of epidermal cells
These cells act as a barrier by:
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protecting the more delicate inner tissues
controlling the exchange of materials
2. Ground tissue  middle layer
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Made of photosynthesizing cells & support cells
3. Vascular tissue  innermost tissue
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Made of phloem cells & xylem cells
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phloem cells transport nutrients like water & sugar
throughout the plant
xylem cells provide physical support
Replacing Specialized Cells
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Cells, tissues, and organs of multicellular animals like humans
form as embryos inside the womb
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Unlike many animals, plants can replace cells, tissues and
organs throughout their lives
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while some of the cells can be replaced, organs have to last for life!
mammals cannot regenerate organs
they can produce new cells which will become organs like leaves,
roots, & stems
Bud = cluster of meristematic cells that often specialize to
form leaf or flower organs
Most of a plant’s growth takes place at the terminal bud
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see Figure 2.4 and 2.5 on p. 59
terminal bud produces the plant hormone auxin which ‘holds back’
growth in other places
Plant Organs
Tissues working together to form a leaf, one plant organ
How many plant organs
can you identify in the
diagram to the right?
1. Leaf
2. Stem
3. Roots
4. (Flower)
The Leaf
Its most important job is to provide a large surface area
for photosynthesis to take place
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Upper surface
Made of a layer of dermal tissue called the epidermis
Epidermal cells secrete a waxy cuticle (top layer) that:
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helps reduce water evaporation off of leaf
protects the leaf so it can perform its primary function,
photosynthesis
sunlight passes through epidermal cells to
photosynthesizing cells
Middle surface
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Made of mesophyll tissue (a special type of ground tissue),
which is composed of 2 types of cells:
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palisade cells, which are specialized to perform most of the
photosynthesis in the leaf
spongy parenchyma cells, which form a loose network of open
spaces (like a sponge), where gases for photosynthesis can be
contained (CO2, H2O, O2)
xylem and phloem cells are arranged in bundles that form
veins to help the palisade cells conduct photo. & cell resp.
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xylem delivers water for photosynthesis
phloem picks up sugars that have been produced in the palisade
cells to transport to the rest of the plant (in the form of sap)
Lower surface (underside)
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Also made of a layer of
dermal tissue, but the cells
are different
Guard cells allow gases to
move in & out of the leaf
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these cells have special pores
called stomata
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stomata are connected to the
open spaces in the mesophyll
transpiration happens through
these pores
CO2 enters, H2O and O2 leave
transpiration = evaporation of water from the leaves
Chloroplasts: the Leaf’s Key Organelle
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Located in palisade cells in the mesophyll
Where photosynthesis takes place
Palisade cells contain thylakoids
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thylakoids = sacs that contain chlorophyll to trap light
when thylakoids are stacked, they are called granum
(plural are grana)
A 3D look at a leaf organ
The Stem
Has 2 main jobs:
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Xylem & phloem tissues are found here
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Both are made of vertically stacked tubes
Most of a plant’s xylem tissue is found in the stem
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support
transportation of water, nutrients and sugars
when xylem cells die, they provide long, fibrous ‘pipes’
that allow water to be transported easily
Phloem’s cell walls are porous to allow materials to
move easily between phloem & neighbouring cells
Xylem & phloem are grouped together in vascular
bundles
See Figure 2.10 on p. 64
The Roots
Also has 2 main jobs:
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anchor the plant to the ground
allow for water and minerals to be taken out of the soil
In some plants, the roots act like a storage area for
See Figure 2.11 on p. 65
starch & minerals
Special cells in the roots called cortex cells serve to
store starch (long chains of glucose)
Endodermis tissue controls water and mineral flow to
vascular tissues
Pericycle tissue surrounds the xylem and phloem
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it allows roots to branch out
Cross section of an Actual Root
Also see Figure 2.12 on p. 65
Types of Roots
1. Taproot
 One main root that grows
larger and thicker than the rest
 Allows plant to reach far
underground for water
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ex: dandelion, thistle
2. Fibrous root
 Thin, hair-like roots
 Allows plant to absorb water
from near the surface of the
soil
 Help prevent erosion
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ex: grass, yarrow
Tissues in the 3 Main Organs