The Plant Cell and Cell Wall
Biology 241, General Botany
4 February 2015
Distinguishing features of plant cells:
• Cell wall composed of
cellulose, hemicellulose,
pectins, lignins in many
cases, various proteins.
• Large central vacuole filled
with water (+)
• Plasmodesmata
“Plants build cells, cells do not build ,”
-J. von Sachs
• Cell Division by
phragmoplast/cell plate
• Plastids
(integrated endosymbiont)
Evert & Eichorn Fig 3.7
Nucleus
• Controls cellular activity by determining
which proteins are produced, and when
(in response to signals from…)
• Stores genetic information (DNA) and
passes it on to descendant nuclei via
Mitosis & Meiosis.
– Nuclear Genome = total genetic
information stored in the nucleus.
– Organized into linear chromosomes
(occur as diffuse chromatin, which
condenses during cell division)
– # of chromosomes varies from species
to species; polyploidy (genome
multiplication) is common in plants.
– E.g. in Linum (flax),
n of different species = 8, 16, 24, 32
(diploid, tetraploid, hexaploid)
Mitochondria and Chloroplasts have
separate genomes, remnants
of their ancestral bacterial genome.
Inherited differently.
Nucleolus (nucleoli): usually
visible in non-dividing
nucleus.
- Sites of ribosomal subunit
RNA transcription
Organization of Genetic Information
Plastids: Chloroplasts
(amyloplasts, chromoplasts)
Chloroplast
Photosynthesis
Amyloplasts
Starch synthesis, storage,
and digestion
Chromoplasts
Thylakoids = continuous membrane system (grana = stack)
Thylakoid lamellae = strands between
Lumen = thylakoid interior
Stroma = fluid surrounding thylakoids
Chlorophyll removed, other
pigments remain  color change
Chloroplast Structure
Nuclear and Plastid DNA
Chloroplast Movement
Movement mediated by filaments of
Actin and Myosin.
- Myosin (attached to organelle)
has ATP-ase “head” that uses
ATP energy to “walk” along actin
Endosymbiotic origins
of Mitochondria and
Chloroplasts
Vacuole
• 1 large central vacuole or
several smaller vacuoles with
different functions.
• Young undifferentiated cells
have numerous small
vacuoles that later grow and
fuse
• 90% of cell volume may be
occupied by vacuole(s)
– Remaining cytoplasm is a
thin layer pressed against
the cell wall. Any
advantages?
– Water and other vacuolar
contents are “cheap,”
whereas cytoplasmic
contents are not.
Storage of primary metabolites.
E.g. in bean seeds: sugars, organic acids, and
proteins are stored in the vacuoles of cells of the
cotyledons.
Storage of toxic secondary metabolites. (e.g.
nicotine, tannins), isolating them from the rest of
the cytoplasm (where they might interfere with
cellular processes.)
Those secondary metabolites may be toxic or
repellent pathogens and predators. the vacuole
has a role in defense.
Pigment storage. Anthocyanins (water-soluble
pigments) are deposited in vacuole. Give flowers
and other plant parts their red and blue colors:
grapes, radishes, roses, Delphinium, etc.
Degredation/recycling of macromolecules and
organelles (mitochondria, chloroplasts).
Cytoskeleton
Microtubules: α and β tubulin dimers; self-assemble @ Nucleating sites, specific
areas near nucleus and in cortical region of cytoplasm
Functions: guide cellulose deposition in cell wall, direct golgi vessicles to cell
wall, chromosome movement and cell plate formation, flagella & cilia
Actin filament = double
helix of actin molecules
- act as binding site and
pathway for myosin
“walking”
- Form arrays and
networks at particular
places in cell.
Functions: pollen tube
growth, vesicular secretion,
cytoplasmic streaming,
orientation of chloroplasts.
Cell Wall Deposition: movement of cellulose synthase
rosettes is guided by microtubules in the cytoplasm.
Cortical Microtubules
Cell shape: determined by the pattern of cellulose
synthase rosette movement, which is guided by the
underlying microtubule network…
Cellulose microfibrils constrain cell expansion.
Isodiametric
cell
Microfibril
Deposited
in random
orientations:
Microfibrils
deposited
Parallel to
eachother :
elongated
cell
Important in cell differentiation and morphogenesis process.
Cell Shapes: form follows function
astrosclereids
Leaf Cross-section from Linanthus
Unicellular trichomes of Arabidopsis
Stem longitudinal section
http://ceplas.eu/public/images/Huelskamp_research_big.png
Root Hairs: specialized epidermal cells
Inheritance
Environment
Gene Expression
Development &
Survival
Central Dogma of Molecular Biology
Genome
Genotype
(DNA)
Modern
Evolutionary
Synthesis
Organism
Proteome
Metabolites
Phenotype
s
Darwinian Natural Selection
Survival &
Reproduction
Population Genetics
Mutation
climate change, habitat loss,
predation, disease, competition
A Model Organism for Molecular Biology
Arabidopsis thaliana
- n=5
- 6 week generation time
- ~10,000 seeds/plant
- Very small plants
- Grow well under artificial light
- Self-fertilize
- Easily infected by
Agrobacterium tumefasciens
Arabidopsis root cells: microtubules visualized by immunoflorescence labeling.
Wild Type
Fra2 Mutant
The Plant Cell Wall
Functions of cell walls:
•Provide tensile strength and limited plasticity which are important for keeping cells
from rupturing from turgor pressure (balance of turgor pressure pushing out, cell wall
pusgshing ba
•Turgor pressure provides support for non-woody tissues.
• Allow turgor-driven cell expansion during cell differentiation
•Thick walled cells provide mechanical support
Tubes for long-distance transport
•Cutinized walls prevent water loss
Provide mechanical protection from insects & pathogens
•Physiological & biochemical activities in the wall contribute to cell-cell
communication