Chapter 1 Plant Cells
Overview of Plant Structure
- Leaves are attached to the stem at the node. The region of the stem between two nodes is the
internodes. Gymnosperms (naked seeds) are less advanced, largest type is conifers (cone bearers)
and other trees like pine, spruce, etc. Angiosperms (vessel seed) are more advanced and are typically
Plant Cells are Surrounded by Rigid Cell Walls
- Primary cell walls are thin and characteristic of growing cells. Secondary cell walls are thicker
and stronger and are laid down after growth, composed of a tough material called lignin. Circular
gaps in the secondary cell walls form simple pits, which occur across simple pits from the adjacent
cell. Adjoining simple pits are pit pairs.
New Cells are Produced by Dividing Tissues Called Meristems
- In a young plant the most active meristems are the apical meristems located at the tip of the stem
and the root. At the nodes, axillary buds contain the apical meristems for branch shoots. Lateral
shoots arise from the pericycle. Next to the meristemic regions are zone of cell elongation which
cells increase dramatically in width and length.
- Primary growth gives rise to new organs and the basic plant form. Results in activity of apical
meristems and cell division is followed by elongation. After elongation, secondary growth occurs
which involves two lateral meristems: vascular cambium and cork cambium.
- The vascular cambium gives rise to secondary xylem (wood) and phloem. Cork cambium produces
periderm. Three major tissues are dermal tissue, ground tissue (parenchyma, collenchymas,
sclerenchyma) and vascular tissue.
The Plant Cell
Biological Membranes are Phospholipid Bilayers That Contain Proteins
- Plasma membrane (plasmalemma) allow cell to take up certain substances while excluding
others. According to the fluid mosaic model all biological membranes have same basic molecular
organization. Consist of bilayer of phospholipids or in the case of chloroplasts it is glycosylglycerides
in which proteins are embedded.
- Phospholipids: Two fatty acids covalently linked to glycerol, covalently linked to phosphate group.
Head is highly polar making phospholipid amphipathic. Plastids have lipid components consisting of
glycosylglycerides. In glycosylglyceride, polar head consists of galactose, digalactose or sulfated
galactose without a phosphate group. One of the fatty acids is usually saturated and the other is
- Unsaturated phospholipids have double bonds that prevent tight packing of phospholipids in the
bilayer and lead to fluidity of membrane. Fluidity is influenced by temp. At low temp, plant
phospholipids have high % of unsaturated fatty acids such as oleic acid and linoleic acid which
increase fluidity of membrane.
- Proteins: Integral proteins are embedded in lipid bilayer and usually span entire width of the
bilayer. Proteins that are ion channels are always integral membrane proteins. Peripheral proteins
are bound to membrane surface by noncovalent bonds such as ionic bonds or H bonds. Involved in
interactions between plasma membrane and components of cytoskeleton like MT and actin MF.
Anchored proteins bound to membrane surface via lipid molecules via covalent bonds. Includes
fatty acids (myristic acid), prenyl groups and glycosyl phosphatidylinositol (GPI) anchored proteins.
- Lipid rafts represent transient, rigid microdomains of tightly packed fatty acid chains enriched in
sphingolipids (lipids with amino alcohol sphingosine) and sterols. Proteins may be targeted to lipid
rafts by covalently attached lipids.
The Nucleus Contains Most of the Genetic Material of the Cell
- Nucleus is surrounded by double membrane called nuclear envelope. Space between two
membranes of nuclear envelope is called perinuclear space and two membranes of nuclear
envelope join at sites called nuclear pores. Nuclear pore contains nucleoporins arranged in
octagon to form a 120 nm nuclear pore complex. The channel is 40 nm wide. An NLS is required
for protein to enter nucleus.- Three proteins needed for nuclear import and export: Ran (GTP binding protein), and importin.
Review of chromosome compaction and rRNA from cell bio, already learned, not going to make notes
The Endoplasmic Reticulum is a Network of Internal Membranes
- The ER membranes are typical lipid bilayers with interspersed integral and peripheral proteins, and
these membranes often form flattened or tubular sacs known as cisternae. Rough ER is covered
with ribosomes and tends to be lamellar (flat sheet composed of two unit membranes) where as SER
- SER is site of lipid synthesis and membrane assembly. RER is site of synthesis of membrane proteins
and exctracellular proteins. Cortical ER refers to ER tubules that line the plasma membrane.
Perinuclear ER consists of ER tubules that are continuous with the nuclear envelope.
Secretion of proteins from cells begins with rough ER
- Proteins destined for secretion enter lumen of ER. Involves Golgi body and vesicles that fuse with
plasma membrane. A signal recognition particle (SRP) binds the signal peptide to the
ribosome, interrupting translation. SRP receptors associate with protein transporting channels
- Ribosome-SRP complex binds to SRP receptor on ER membrane and ribosome docks with
translocon. Translocon pore opens, SRP particle is released and elongating polypeptide enters
lumen of ER. Translation resumes. Signal sequence is cleaved off by signal peptidase. Carbohydrate
addition and chain folding allow the newly synthesized polypeptide to be shuttled to Golgi via
- An N linked glycan is usually attached to a specific asparagines residue of the secretory protein. N
linked glycan first assembled on lipid molecule dolichol diphosphate. Then transferred to nascent
polypeptide where certain sugars are removed by glucosidases and N linked glycoprotein is
transported to Golgi via small vesciels. Fuse with cisternae on cis face of Golgi.
Golgi Stacks Produce and Distribute Secretory Products
- Golgi appears as flattened membrane sacs called cisternae and network of tubules and vesicles
called trans Golgi network (TGN). Cistern