2.Plant Cell

The Plant Cell
Cells are the structural and
functional units of life
Cell theory:
1) All living organisms are composed of
one or more cells
2) The chemical reactions of a living
organism, including its energy-releasing
processes and its biosynthetic reactions,
take place within cells
3) Cells contain the hereditary
information of the organisms of which
they are part, and this information is
passed from parent cell to daughter cell
Cell Diversity
Cell similarity
Plasmalemma
Cytoplasm
DNA and genetic code
PROKARYOTES AND EUKARYOTES
• Prokaryotes: before a nucleus
• Prokaryotes are represented by the Archaea and the Bacteria
• They lack nuclei (no nuclear membrane)
• DNA a circular molecule
• Nucleoid
• No organelles (except acidocalcisomes)
• Eukaryotes with a true nucleus
• Nuclear envelope present (2 membranes)
• DNA is linear and bound to histones to form chromosomes
• Organelles present
• “Unit membrane” common
THE PLANT CELL: AN OVERVIEW
NUCLEUS
Two functions:
1. It controls of activities of the
cell by determining which protein
molecules are produced by the
cell and when they are produced
2. It stores the genetic
information, passing it on to the
daughter cells during cell division
Nuclear Genome: the total
genetic information stored in
the nucleus
Nuclear envelope: Pair of
membranes surrounding the
nucleus
Nuclear pores are complex
Outer nuclear membrane is
continuous with the
endoplasmic reticulum
Chromatin: thin threads of DNA
Nucleoplasma: nuclear matrix
Chromosomes: condensed
chromatin
Haploid (1n): number of
chromosomes in gametes
Diploid (2n): number of
chromosomes in somatic cells
Nucleoli: accumulations of RNA
and proteins inside the nucleus
RIBOSOMES : small particles of RNA and protein, protein synthesis
Polysomes: polyribosomes or aggregations of ribosomes
CHLOROPLASTS and other plastids
Plastids are typical structures in
plant cells
Several types of plastids:
a) Chloroplasts
b) Chromoplasts
c) Leucoplasts
Plastids are surrounded by a
double membrane
Thylakoids: internal membranes
Stroma: matrix
Chloroplasts are the site for photosynthesis, contain chlorophylls and
carotenoids pigments
Usually in green color, disk-shaped and numerous
Chloroplasts are positioned flat against the cell wall
Orientation depends on light intensity
Internal structure
Stroma is traversed by membranes or sacs, the thylakoids
Thylakoids can stack to form grana (granum) like coins!
These grana are interconnected by stroma thylakoids
Chloroplasts may contain starch grain and oil droplets
Chloroplasts are semiautonomous organelles
resembling bacteria:
• presence of a nucleoid
• DNA is circular
• absence of histones
• ribosomes are smaller
Most of chloroplasts proteins
are encoded by the nucleus
Thus, overall control resides in
the nucleus
Chloroplasts are the ultimate
source of all our food supplies
and our fuel Chromoplasts lack
chlorophyll but they are
pigmented
They contain carotenoid
pigments
Usually appear yellow,
orange or red
Leucoplasts lack pigments of
any kind and the elaborate
system of inner membranes
Amyloplasts is a type of
leucoplast containing starch
Proplastids are small,
colorless or pale green,
undifferentiated plastids
found in meristematic cells
They are the precursors of
the more highly
differentiated plastid
Etioplasts are plastids containing prolamellar bodies (semicrystalline
bodies made of tubular membranes) they are formed from proplastids
developed in the dark
MITOCHONDRIA
Like chloroplasts, mitochondria are
bound by two membranes
Cristae is a membranous system formed
by the inner membrane
They are the sites for respiration
They are semiautonomous organelles
Matrix: containing proteins, RNA,
DNA (nucleoid) and small ribosomes
(like chloroplasts)
Theory of Endosymbiosis
Mitochondria and chloroplasts
evolved from bacteria engulfed
by large heterotrophic cells
Can you cite some evidence for
the endosymbiosis theory?
PEROXISOMES
They are bound by a single
membrane
They contain a crystalline body of
protein
They are self-replicating organelles
There is no DNA or ribosomes
Involved in photorespiration
Glyoxysomes, a type of peroxisomes,
convert fat to sucrose during
germination of seeds
VACUOLES
They are characteristics of a plant cell
They are bounded by a single
membrane (tonoplast)
Cell sap: liquid inside the vacuole,
usually water and other diluted
substances, crystals (Ca oxalate) are
also common in the form of druses or
raphides
They may contain pigments (blue and
red anthocyanins)
They can act as lysosomes or digesting
vacuoles
ENDOPLASMIC RETICULUM (ER)
ER is a complex membranous system
found in the cytosol
Appear as two parallel membranes
with a space or lumen inside
Rough ER, with flattened sacs or
cisternae (cisterna) and many
polysomes; common in cells storing
proteins
Smooth ER, lacks ribosomes and
appear tubular; common in cells
secreting lipids
ER forms an extensive network of
cisternae and tubules just inside the
plasma membrane in the periphery
(cortical) cytoplasm and forming a
Cortical ER
Cortical ER have many important
functions in the cell
En general, the ER functions as a
communications system and as a
system for channeling materials
(proteins and lipids) around the cell
GOLGI APPARATUS
A system of golgi bodies or dictyosomes
Golgi bodies are involved in secretion
Each golgi body consists of stacks of
flattened, disk-shaped sacs, or cisternae, often
branched into complex series of tubules at
their margins
One face CIS is forming while the other face
TRANS is maturing
The endomembrane system consists of an interconnected network
of the ER, the nuclear envelope, Golgi bodies, secretory vesicles,
the plasma membrane, and vacuolar membranes; secretion of
substances from cells in vesicles is called exocytosis
CYTOSKELETON
Network of protein filaments throughout the cytosol
Involved in cell division, growth, differentiation, organelle
movement, etc
Two types of filaments:
a) Microtubules
b) Microfilaments
MICROTUBULES
Made of protein tubulin
24 nm in diameter
Arranged in a helix to form 13 rows or
protofilaments
Dynamic inestability: shifting
configurations
Microtubule organizing centers: specific
sites for assembling
Functions:
Orderly cell wall growth
Directing Golgi vesicles
Chromosome movement
Cell plate formation Components of flagella and cilia
MICROFILAMENTS OR ACTIN FILAMENTS
Polar filaments
Made of protein Actin
5-7 nm in diameter
Sometimes occurring in bundles
Functions:
Cell wall deposition
Nucleus movement
Organelle movement
ER organization
Cytoplasmic streaming
Cytoplasmic streaming in giant algal cells
FLAGELLA AND CILIA
Hair-like structures of eukaryotic cells
Covered by the cell membrane
0.2 micrometer in diameter
Longer and few: flagella
Short and numerous: cilia
Locomoter structures
Usually found in reproductive cells
(zoospores, sperm)
Basic unit 9+2
Nine external pairs of microtubules
Two central microtubules
CELL WALL
A typical structure of plant
cells
Constrains expansion of
protoplast
Prevents rupture of plasma
membrane
Determines cell size and shape
Contains enzymes for the
absorption and secretion of
substances
Defense against bacterial and
fungal pathogens
Cellulose is the main cell wall
component
Cellulose is a polymer of glucose
Cellulose polymers are bundled
into microfibrils
Cell Wall
a) Cellulose fibers b) Matrix of molecules made up of Hemicelluloses and Pectins
Glycoproteins such as Extensins
Enzymes
Lignin for rigidity
Cutin
Suberin (cork)
Waxes
Primary Wall: formed first, before and during the growth of the plant cell
Middle lamella: region of union between two adjacent primary walls made up
pectins
Secondary Wall: additional wall layers formed inside the 1ary wall
Primary walls are not of uniform thickness
Thin areas are called Primary Pit-Fields
Plasmodesmata or cytoplasmic threads connect living protoplasts among adjacent
cells
Secondary walls are formed AFTER primary walls have stopped
increasing in size
2ary walls are rigid Function: strength and water conduction
Cells with secondary walls usually die to function properly
Cellulose is abundant, no pectins, matrix of hemicellulose
Three distinct layers (S1, S2, S3) are seen in 2ary walls
Secondary walls are not deposited over primary pit-fields
These interruptions are called Pits
A pit usually is found opposite to another pit from the adjacent cell
Pit membrane: area between two pits formed the middle lamella and two primary
walls
Pit-pair: the two opposite pits plus de membrane, two types:
• Simple: no overarching (stone cells in pears)
• Bordered, with the 2ary wall arched over the pit cavity
Expansins are enzymes that loose the cell wall structure allowing the
expansion of the cell New cellulose microfibrils are placed over the previous layer
(cellulose synthase)
Cells expanding uniformly have microfibrils laid down at random
Cells expanding by elongation: the microfibrils are deposited in a
plane at right angles (perpendicular) to the axis of elongation
Synthesis of cellulose microfibrils
Plasmodesmata connect the protoplasts of adjacent cells
They occur throughout the cell or aggregated in primary pit-fields
or in the membranes between pit-pairs
The channels are lined by the plasma membrane and traversed by
a modified ER known as desmotubule
Plasmodesmata
provide a pathway for
the transport of
substances between
cells
CELL CYCLE SUMMARY
Two unique events in plants at Interphase
a) Phragmosome: cytoplasm layer formed across the
cell where the nucleus becomes located and divide
b) Preprophase band (ring-like band of microtubules
below the plasma membrane
MITOSIS AND CYTOKINESIS
Mitosis: Nuclear division
Cytokinesis: Cell division
MITOSIS
Can you identify the
mitotic phases in this
section of onion root?
CYTOKINESIS
Usually, but not always, follows mitosis
Cell division occur by formation of a Phragmoplast and a Cell Plate
Phragmoplast: Spindle-shaped system of microtubules at telophase,
perpendicular to the plan of the cell division
Cell plate: initial cross wall that starts in the middle of the cell and
grows outward
SUMMARY
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The cell is the fundamental unit of life
Cells are of two fundamentally different types: prokaryotic and eukaryotic
Plant cells typically consists of a cell wall and a protoplast
The nucleus, bounded by a nuclear envelope, contains nucleoplasm, chromatin and nucleoli
Ribosomes are the sites of protein synthesis
There are three main types of plastids: chloroplasts, chromoplasts, and leucoplasts
Mitochondria are the sites of respiration
Plastids and mitochondria share certain features con prokaryotic cells
Peroxisomes are bounded by a single membrane
Vacuoles perform a variety of functions
The ER is an extensive 3D system of membranes with a variety of roles
The Golgi apparatus is a highly polarized membrane system involved in secretion
The cytoskeleton is composed of microtubules and microfilaments
The cell wall is a major distinguishing feature of the plant cell
Cells reproduce by cell division
Dividing eukaryotic cells pass through a regular sequence of events: the cell cycle
During prophase, the duplicated chromosomes shorten and thicken
Metaphase, anaphase, and telophase followed by cytokinesis result in two daughter cells