Lectures 2 & 3

The Plant Cell

I. History

A. Original observations. Robert Hooke. First used in 1665 the word cell (like a monk’s cell) to describe structures seen while observing cork.

B. Cell theory. Matthias Schleiden 1837 - discovered all plants are composed of cells. In 1838 published this in Contributions to Phytogenesis. In 1839 Theodor Schwann published similar results in animals, then concluding that all living things are composed of cells). 1857, pathologist Rudolf Virchow proposed Omnis cellula e cellula—that every cell arises from another cell.

C. Organismal theory. Heinrich Anton de Bary (1879) “It is the plant that forms the cell, not the cells that form the plants”. Idea is that of cells are interconnected, they are not separated from one another, they form a continuous protoplasm. Later proponents are Kaplan & Hageman (1991).

II. The Plant Cell

Scales of size in biology
Generalized plant cell - diagramatic
Electron micrograph of a plant cell

A. Cell walls, which can be primary or secondary. Diagram. An entire lecture will be devoted to the cell wall later.

B. Protoplasm

1. Plasma membrane - Plasmalemma - cell membrane just internal to the wall. Composed of a double layer of phospholipids which can be seen with a transmission electron microscope (TEM). The plasma membranes of adjacent cells are connected by plasmodesmata. See webpage on composition and properties of cell membranes HERE. Various proteins are embedded in the plasma membrane - some on the outside, some inside, and some that traverse both layers. This view of the plasma membrane is called the Fluid Mosaic model (Wiki page).

Compartments inside and including the plasma membrane are:

a. Protoplasm - everything inside the cell wall, bounded by the plasma membrane.

b. Cytoplasm - protoplasm minus nucleus. Always in motion: cytoplasmic streaming (= cyclosis). Accomplished via actin and myosin. Here are some videos (from the Jaideep Mathur lab Univ. of Guelph, Ontario Canada) of the process:

cytoplasmic streaming in Arabidopsis trichomes
organellar movement in Arabidopsis.
movement of ER in Arabidopsis.

c. Cytosol - cell solution, contains sugars, proteins, amino acids, secondary metabolites. Some cell biologists use these terms interchangably.

d. Hyaloplasm - aqueous cytoplasmic matrix of the cell that organelles are in.

e. Organelles - nucleus, plastids, mitochondria, etc.

2. Nucleus - double membrane bound, forming the nuclear envelope. TEM. Diagram

a. Perinuclear space - space between membranes

b. Nuclear envelope contains pores, allows molecules (eg. mRNA, tRNA, ribosomal subunits) to go into the cytoplasm and allows molecules (eg. proteins, histones, enzymes) to go into the nucleus

c. Inside the nucleus

Nucleoplasm - matrix within the nucleus that contains:
Chromatin - DNA molecules complexed with proteins - histones. Composed of euchromatin - light regions in the nucleus, DNA active - transcribed, and heterochromatin - dark regions in the nucleus, DNA inactive, maybe just repetitive DNA, maybe structural
Nucleolus - dark region often in the center of the nucleus, rRNA synthesized and ribosomal subunits made

3. Plastids - an organelle that is double membrane bound that originates from a proplastid (Wiki page). Proplastids do not have a complex internal membrane system. Plastids are plastic! Many can convert and interconvert given different cellular and environmental conditions (Diagram). Plastids contain their own DNA and their own ribosomes. DNA is circular (like prokaryotes, doesn't have histones). Originated from endosymbiosis of an ancient cyanobacterium, later "degenerating" into an organelle.

a. Etioplast - occurs in low light - a plastid that starts to differentiate into a chloroplast.

They have huge prolamellar bodies (PLB), in a crystalline arrangement (64 lammellae) - there is also some membrane, precursor to the chloroplast.
the PLB is quasi crystalline, made primarily of lipids that will go to make the membranes internally of the chloroplast
also contains the molecular precursor to chlorophyll (protochlorophyllide)
once exposed to light, they differentiate into chloroplasts. The chloroplast will revert back to the etioplast if kept in the dark.

b. Chloroplast - double membrane with invagination of the inner membrane. This micrograph shows chloroplasts in maize (a C4 plant), where the bundle sheath (BS) cell and mesophyll (M) cell chloroplasts are differentiated. Diagram.

internal membrane are called thylakoids
internal matrix - stroma, contains the DNA and ribosomes. Photosystems I and II occur in the grana thylakoids, also where chlorophyll molecules are located.
most important enzyme in the stroma is Rubisco (ribulose bis-phosphate carboxylase/oxygenase), the main carbon fixing enzyme in photosynthesis. Photorespiration is also carried out by Rubisco, but it is a wasteful reaction.
most chloroplasts have starch grains, the storage product of photosynthesis.

c. Amyloplast - starch storage, they begin as chloroplasts and can convert to one another. They also function in gravity perception. See Wikipedia page HERE.

d. Chromoplast - originate from chloroplasts. Contains pigments (plastoglobuli, composed of beta carotene), often reddish. May be a variety of shapes, as seen in Daucus carota (carrot). Often found in fruits, e.g. red fruits of Capsicum (bell pepper). Elongated chromoplasts with crystalline carotenoid pigments in petal cells of Strelitzia (bird-of-paradise) - viewed with polarizing microscope.

e. Elaioplast - stores oil (avocado, coconut, olives) plastoglobulin - when forming into oil

f. Leucoplast - clear plastid without a lot of internal membranes poorly developed membrane system.

4. Mitochondria (mitochondrion, singular) - double membrane bound, usually smaller than plastids, round to elongate to pleomorphic (irregular in shape) site of respiration. Diagram.

a. Cristae (crista, singular) - invagination of inner membrane - site of electron transport chain.

b. Stroma - has DNA and ribosomes, site of Krebs cycle, ATP (high energy molecule) generated to do work, breakdown of sugar, C₆H₁₂O₆

5. Microbodies = sphaerosomes - single membrane bound, small vacuoles that contain enzymes (2 kinds)

a. Glyoxysomes - associated with breakdown of lipids to be used as an energy source. Found in seeds and spores during germination.

b. Peroxisomes - Diagram - close to chloroplast, involved in recycling products of photorespiration. The enzyme catalase may form crystal-like structures. Videos showing peroxisomes form tubules ("peroxules") and are closely associated with ER.

6. Vacuole - single membrane bound (membrane called the tonoplast). In mature cells it occupies most of the volume. Diagram.

a. Main function is storage, including "waste" materials such as secondary metabolites: alkaloids - morphine, codeine, atropine, caffeine terpeneoids - essential oils, taxol (anti-cancer chemical), anthocyanins, phenolics (tannins), aspirin (salicylic acid). Role may be to deter herbivores. Ruptures when cell is damaged, releasing materials that are toxic even to the plant cell. May also provide some protection agains solar radiation.

b. Turgor pressure - maintaining turgidity (rigidity of the cell), prevents cell from becoming flacid (limp, wilting)

7. Ribosomes - not membrane bound, small, free or on rough endoplasmic reticulum.

a. the site of protein synthesis

b. made up of small and large subunits. Each subunit with an rRNA and several dozen ribosomal proteins.

c. polyribosome (polysome) - group of ribosomes reading the same mRNA molecule

8. Golgi bodies - sometimes called Dictyosomes (dissected bodies) in plants. TEM. Diagram.

a. "pita-like" stacks of membranes, the diameter is largest in the middle.

b. ER vesicles move towards and fuse with Golgi at the forming face (cis) .

c. Golgi vesicles move out of the Golgi at the maturing face (trans), then usually to the plasma membrane where they fuse

d. Golgi vesicles are packages that contain material released outside the cell by fusion of the vesicle with the plasma membrane for export (exocytosis) e.g. cell wall material and secretory materials (mucilage, nectar, salts)

9. Endoplasmic reticulum (ER). Diagram.

a. This is the membrane network within the cell that connects to other membranes throughout the cytoplasm. Its enclosed space = cisternae. Packages materials in vesicles that often fuse with Golgi body and contents are further modified.

b. Rough ER (RER) - studded with ribosomes. Proteins are synthesized and organized in cisternae then packaged into vesicles, pinched off at the end and these may fuse with something else within
the cell.

b. Smooth ER (SER) - No ribosomes. Often involved in lipid synthesis, provides membranes for cells major characteristic of plants is the formation of the cell membrane

10. Cytoskeleton - interconnected system of protein filaments or tubules

a. Microtubules. TEM.

involved in cell movement
composed of tubulin
they are the spindle fibers
components of flagella
anti-cancer drugs interfere with microtubules to stop reproduction of cells

b. Microfilaments

smaller than microtubules, found in bundles
composed of actin. Video showing F-actin (stained with Green Fluorescent Protein) and and red stained microtubules.
moves stuff around in the cell in a rapid motion e.g. cytoplasmic streaming in every cell
also divides organelles
Intermediate filaments

III. Ergastic Substances

Ergastic substances - non-living substances

A. Starch grains

1. carbohydrate composed of long chain molecules, amylose and amylopectin

2. first formed in chloroplasts, then broken down and moved as sugar to storage tissues where it is synthesized in amyloplasts

3. function as storage, gravity perception

4. shows purple/black with IKI stain. When viewed with polarized light, show a Maltese cross pattern.

5. shows layers around a point called the hilum, layers due to different density of molecules, center - more dense, outer layers - less dense and more hydrated

6. variations depends on position of the hilum (centric, eccentric or acentric) and on the number of hilums (single, compound, semi-compound) Solanum tuberosum (potato).

a. simple starch grains - centric, eccentric (acentric) from Phaseolus (bean)

b. compound starch grains. Ipomoea (sweet potato)

c. semi-compound starch grains

B. Lipids (oils and waxes)

a. glycerides of fatty acids

b. function - reserve material (oil), protective coating on epidermis (wax)

c. commonly present in seeds and fruits

d. produced by elaioplast or spherosome (lipid bodies) Persea americana (avocado)

e. stain reddish with Sudan III and IV

f. wax - suberin (of roots and in cork), cutin (of cuticles)

C. Proteins

a. found in the outermost endosperm layer (aleurone layer) in fruits and seeds

b. formed in vacuole of the storage cells, then water is lost by dehydration

c. function in storage. Cuboidal crystalloid in Solanum tuberosum.

D. Tannins

a. phenol derivatives

b. found in leaves, epidermis vascular tissues, unripe fruits, seed coats and in pathogenic growth (e.g. galls)

c. produced in vacuole or cytoplasm

d. forms granular mass as seen through EM and LM or bodies colored yellow, red, brown as seen in LM

e. functions against dehydration (forms a barrier against loss of water as it is a phenol and repels water), prevents rotting and damage by animals

E. Crystals

a. inorganic materials consisting mostly of calcium salts (less frequently silicon dioxide)

b. occurs in vacuole, cork, in cell wall or outside of cell membrane

c. the appearance location and type of crystals are often used in taxonomy and wood identification

d. functions in storage of waste deposition, deters herbivores

e. Ca oxalate - most common crystals appear in different forms

raphides - long needle-like, occurring in special cells in groups, not singly. Light microscope photo of raphides in Impatiens.
styloids (pseudoraphides) - long prismatic crystals with tapered ends; occur singly or in pairs, very large; common in monocots (cattail, Typha root, seen with polarizing microscope). Raphides involved in aerenchyma formation in Typha (paper HERE).

f. prismatic crystals in Tradescantia stem.

g. druses - compound crystals. In stem of Tilia (basswood or lime tree) or in Begonia. Large druses with and without polarized light in Begonia petioles. Druses in Myriophyllum aerenchyma.

F. Calcium carbonate

a. a cystolith forms inside a special cell called a lithocyst. In general, these cells are called idioblasts because they differ in size and content from surrounding cells

b. they occur in cells at or near the epidermal layer

c. cystolith (stone) eventually takes over and fills entire lithocyst cell. This cystolith is in the epidermis of Ficus elastica (rubber plant, Moraceae).

Last updated: 10-Oct-22 / dln