The Plant Cell
&
The Cell Cycle
The Plant Cell
Plant cells are the basic units of plant
structure and function. Different plant
cells have different functions. Labeled
below are the organelles within a plant cell.
The Boundary Between Inside & Outside
Plasma Membrane – controls movement of material into and out of the cell.
Plasma membrane, a.k.a. cell membrane surrounds each cell. The
membrane is made of a phospholipid bilayer with imbedded proteins. The
membrane is selectively permeable (controls what goes through and some
items travel through easily while others travel slowly and others are totally
blocked).
Plasmodesmata - An open channel in the cell walls of plant cells allowing
for connections between the cytoplasm of adjacent cells. This is a
connection of tubes that allows two plant cells to be joined (Joins cells
together).
2 Plant Cells
Cytoplasm
Cell Wall
This continuous cytoplasm in
a set of cells connected by
plasmodesmata is called
symplast.
Cell Wall - Limits cell expansion; rigid structure outside the plasma
membrane. It is made of microfibrils (small fibers) formed from a
polysaccharide, cellulose.
Cellulose
Fibers of
a Plant
Cell Wall
Osmosis and Turgor Pressure
Osmosis - the flow of water from a relatively dilute solution to a
more concentrated solution across a selectively permeable
membrane.
Protoplast – the organized living unit of a single cell.
Turgor Pressure – the pressure within a cell resulting from the
osmotically generated imbibition (absorption of water) of
water into the protoplasm and vacuole. In plants, a large
turgor pressure can result and greatly support plants.
Turgid – means firm. Plant cells are swollen or turgid when they
absorb water.
Plasmolysis – the separation of the cytoplasm from the cell wall
caused by a removal of water from the protoplast.
The cell wall on the outside of the cell is rigid and the plasma
(cell) membrane can grow or shrink based on water pressure.
The plasma membrane grows in size with the influx of water but
cell wall limits the plasma membrane’s growth and eventually
balances the pressure of osmosis “stopping” the flow of
water into the cell.
When water is cut off, a leaf wilts. “This occurs because water is
lost from the cell and turgor pressure decreases.
Osmosis can work in and out of cells.
CELLS
SOLUTIONS
Around Cells
In this first row,
concentrations are
balanced between in and
out of the cell. Water is
not moving in or out.
Isotonic Solution – equal concentrations of solute in & out of cell.
In this row, concentration is
greater in the cell. Water will
move into the cell until
concentrations are balanced
or until cell wall limits the size
of the plasma membrane.
Hypotonic Solution – lower concentration of solute.
In this row, concentration is
lower in the cell. Water will
move out of the cell until
concentrations are balanced.
Protoplast will shrink away from the
cell wall and PLASMOLYSIS results.
Hypertonic Solution – greater concentration of solute.
A plasmolyzed cell has no turgor pressure. Thus, an
accumulation of salt in a soil can cause a plant to wilt.
Normal
Plasmolyzed
Elodea
Elodea
Cells
Cells
Osmosis and Turgor Pressure
Primary Cell Wall forms when a cell is still growing.
Secondary Cell Wall is between the primary cell wall and the
plasma membrane. This forms after the cell stops growing.
The secondary cell wall contains cellulose microfibrils and a
strong, water-impermeable substance call lignin. Lignin
makes the secondary cell wall especially rigid and much more
able to resist stretching or compression. Lignin in the
secondary wall provides strength for wood.
Other specialized cell walls have cutin covering them or suberin
embedded in them. Both are waxy compounds.
Cutin – cutin cell walls are formed on surface of leaves and other
organs exposed to air (protect against water loss and
disease.)
Suberin – a waxy tissue found in the cell walls of cork tissue.
The Organelles of Protein Synthesis &
Transport Much of the cell is made of protein.
Nucleus
-Stores and Expresses Genetic Information (DNA).
-Controls the cell’s activities by directing protein synthesis.
-Inside the Nucleus, DNA and proteins make up chromosomes.
-Nucleus is surrounded by the nuclear membrane (nuclear envelope).
-Nucleolus is within the nucleus, where ribosomal RNA (rRNA) is
made and assembled with proteins imported from the cytoplasm.
Resulting is the ribosomal subunits which will form
functional
ribosomes when they exit to the cytoplasm of the
cell.
Ribosomes
A ribosome is a cell structure consisting of RNA and protein organized into 2
subunits and functioning as the site of protein synthesis in the cytoplasm.
Ribosomes occur on the Endoplasmic Reticulum and Cytoplasm.
RNA along with ribosomes carry the genetic information of its DNA template
out of the nucleus and into the cytoplasm for protein synthesis.
Endoplasmic Reticulum
Endoplasmic Reticulum is a membranous system of channels and flattened
sacs that traverse the cytoplasm. There are 2 varieties.
Rough Endoplasmic Reticulum – This is rough because it has bumps.
These bumps are ribosomes. This is the site of protein
synthesis.
Smooth Endoplasmic Reticulum – Does not have ribosomes so it is
smooth. The Smooth E.R is the site of lipid synthesis
and it stores calcium.
Nucleus
Golgi Apparatus
The golgi apparatus is a stack of membranous sacs surrounded by vesicles.
It stores and ships products from the endoplasmic reticulum. It guides the
movement of protein to certain compartments.
The Organelles of Energy Metabolism
Plastid – A cellular organelle in which carbohydrate metabolism is located.
Plastids are double-membraned. Plastids are complex organelles found in
every living plant cell. Types of plastids: leukoplasts, amyloplasts,
chromoplasts and chloroplasts.
Chloroplast and Mitochondria are energy converting organelles in a plant.
Chloroplast – the site of photosynthesis.
Chloroplasts are found in the
green parts of plants. They contain the pigment chlorophyll.
Mitochondria
Mitochondria is the
site of cellular
respiration (also
named cell respiration,
oxidative respiration,
and aerobic
respiration).
Mitochondria make
useful forms of
chemical energy. ATP
generation occurs
here. ATP powers
many of the important
chemical reactions in
the cell.
Other Organelles
Vacuole – The vacuole is often the largest organelle in a plant cell. It is mainly
for storage. It is a large compartment. It may bring water into the cell,
store nutrients and/or chemicals meant for pigmentation or chemical
defense.
Another
cellular
structure is
the
cytoskeleton,
which holds
organelles in
place and
directs their
movement.
The Cell Cycle (Cell Division)
Cells divide in regions called meristems.
Meristems – sites in the plant body where cells divide and where
differentiation into specialized cells and tissues are initiated. Meristems
are found in the tips of roots and shoots and in some other regions of the
plant (This will be discussed in later chapters).
The Cell Cycle,
consisting of the G1,
S, G2 and M phases.
Approximately 50%
of new daughter
cells leave the cycle
at either G1 or G2
and begin to
differentiate; the
remaining cells will
cycle again in the
meristems.
Cell Cycle
G1 –
S –
G2 –
M –
Pre-DNA synthetic Phase
Synthesis (Duplication) of DNA
Premitosis Phase
Mitosis
Interphase
During mitosis, nuclear division (Karyokinesis) and
cytoplasmic division (cytokinesis) occurs.
During mitosis, the mother nucleus (2n chromosomes) divides
and resulting 2 daughter nuclei have identical number of
chromosomes (2n chromosomes each). The main point of
mitosis is to separate the DNA and result in 2 nuclei with the
same DNA.
Mitosis
Note the stages and the order of the stages of mitosis:
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Chromosomes are
condensing. Nuclear
membrane is breaking
down.
Chromosomes
are lined up at
equatorial plane
Chromosomes
separate and
move to
opposite poles
Chromosomes
begin to uncoil.
Nuclear
membrane
reforms.
BIO 141 Botany with Laboratory
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