Plant Physiology

Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; ssaupe@csbsju.edu

I. A Water Balloon in a Box
This is a good model for a plant cell. Thus, there are two major components to a plant cell:

A. The Box
This part of the cell is analogous to the cell wall. Like a cardboard box, the cell wall is relatively rigid, it is non-living and highly structured. Its more obvious functions are to support and protect the cell. It is produced by the protoplast (see below).

If you stack up a bunch of boxes you generally can’t make a very large tower before it comes crashing down. Since a plant is essentially constructed of numerous small boxes, why don’t they fall apart, too? The answer is glue! Plants glue their cells together with pectic polysaccharides. These carbohydrates, which make up the outermost layer of the plant cell wall (called the middle lamella), bind adjacent cells together. Cooks use pectins extracted from the middle lamella to solidify jams and jellies.

Pores or air spaces (called intercellular spaces) exist between adjacent cells because of the difficulty of packing of cells with rigid walls. Try and squeeze a bunch of irregularly boxes into a room without leaving any space between them! The intercellular spaces are important for gas exchange and water transport, some movements (i.e., sensitive plants - water moves into/out of theses spaces; nyctinastic movements - sleep movements) and freezing protection (i.e., water moves out of cells into the spaces to minimize cellular damage on freezing. Trivia note: prized ginseng roots have a translucent appearance - apparently obtained by freezing).

Putting a cell in a box presents one major problem - how do cells talk to one another since they are now effectively isolated in their own small compartment? Plants solved this problem by putting windows in the box! Or in other words, there are lots of specialized pores through the wall called plasmodesmata that provide a cytoplasmic connection ("cytoplasmic bridges") between adjacent plant cells. Thus, the cytoplasm of a plant is essentially contiguous throughout the entire plant. Imagine hopping on board a tiny submarine like in the films Fantastic Voyage or Innerspace. You can essentially travel from cell-to-cell throughout the plant without ever leaving the cytoplasm nor crossing any cell membranes. Cool!

The plasmodesmata are 40-50 nm in diameter. The maximum sized object that can pass through as a molecular mass of 700-1000 daltons, which is equivalent to a molecule 1.5 - 2.0 nm. The plasmodesmata have a unique structure. The plasma membranes from adjacent cells are continuous through the pore. In addition, the ER (see below) is also continuous between adjacent cells. The narrow tubule of ER is called the desmotubule and in the center there may be dark staining rod (called then "central rod"). The cytoplasmic channel between desmotubule and membrane is called the "cytoplasmic sleeve." Protein fibers radiate from the desmotubule to the membrane like spokes on a bicycle. These fibers may act like a sieve or screen. Plasmodesmata are aggregated in pits and there are 1-15 per mm². They can make up as much as 1% of the cell surface. Plasmodesmata are formed during cell division (see cell notes) or secondarily after division. They may or may not be branched, too.

B. The water balloon - this is analogous to the protoplast. The protoplast is everything inside the cell wall. It is the "living" part of the cell and includes:

II. Separating the balloon from the box.
Just as you can remove a balloon from a box, so too can a plant physiologist liberate the protoplast from the cell wall. This is accomplished by slicing up a leaf (or other tissue) and then floating it in a solution containing wall-digesting enzymes like pectinase, cellulase, and hemicellulase suspended in an inert osmotic agent (0.7 M mannitol or sorbitol) in a buffered solution. The enzymes degrade the components of the cell wall releasing the protoplasts. This will be the focus of at least one or our lab sessions.

III. Organelles in Both Plant and Animal Cells
First, we will study the components of the protoplast, but will especially concentrate on those structures unique to plants. In the next lecture we’ll tackle the cell wall.

A. Plasma or Cell membrane
Cell boundary; selectively permeable; bilayer of phospholipids with inserted protein. Phospholipids are unique molecules - they are amphipathic, meaning that they have both hydrophilic and hydrophobic regions. They have a glycerol backbone; one of the hydroxyls is bonded to a phosphate and another charged group, the other two hydroxyls are esterified to fatty acids. The fatty acids range in length from C14 - C24. One fatty acid is usually unsaturated and the other is saturated. The unsaturated fatty acid is kinked which helps to keep plant cell membranes fluid at cool temperatures. As a result plant phospholipids usually have a higher degree of unsaturation than animals. Hydrophobic interactions between the tail regions of the phospholipids hold the membrane together. Some proteins are found: (1) just on the outside or inside surfaces of the membrane (peripheral proteins - non-covalent interactions and anchored proteins - covalently bound to lipids, etc); or (2) embedded in the membrane (integral protein), many of which span the membrane (trans-membrane proteins). Hydrophilic regions of the integral proteins are oriented to the outside of the membrane whereas hydrophobic regions are embedded within the phospholipid bilayer. Lipid soluble materials can readily pass through but charged or ionized substances (hydrophilic) pass through very slowly, if at all. The function of the membrane is to: (1) regulate traffic; (2) separate the internal from external environment; (3) serve as a platform on which some reactions can occur; (4) participate in some reactions (i.e., the membrane components are important intermediates or enzymes); and (5) provide some structural integrity for the cell.

B. Nucleus
The cell "brain". Surrounded by a double membrane (two phospholipid bilayers) - the nuclear membrane. Has pores. The structure of the pores is complex comprised of more than 100 proteins. The pore opening is surrounding by a series of proteins and these are attached to a series radial spokes. Nucleoplasm - matrix within nucleus. DNA, which is found in the nucleus, may be condensed into chromosomes or not (chromatin). There may be one or more nucleolus (site of ribosome production). The nucleus is 5-20 mm in diameter. There is a layer of intermediate filaments (see below) just inside the nuclear envelope; called the nuclear lamina.

C. Cytoplasm/cytosol
The cytosol is the gel-like matrix within the cell in which the other structures are embedded. The cytoplasm refers to the cell contents inside the membrane.

D. Mitochondria
These organelles, like the nucleus and plastids, are double-membrane bound. They vary in shape from tubular (like sausages) to spherical. They reproduce by fission, have their own ribosomes and DNA (a circular loop like prokaryotic cells). The inner membrane has a larger surface area so it must be folded into finger-like projections (called cristae) to fit inside the outer membrane. Mitochondria are found in all eukaryotic cells. They are the sites of cellular respiration - process by which energy is released from fuels such as sugar. The mitochondria are the power plant of the cell. They are small (1-5 mm) and generally numerous (500-2000 per cell). A popular misconception is that "plants have chloroplasts, animals have mitochondria." Plant cells, at least green plant cells (i.e., leaf cells), have both. Root cells only have mitochondria. Mitochondrial DNA which comprises about 200 kbases, codes for some of the genes required for cellular respiration including the 70S ribosomes and components of the electron transport system. The inner membrane differs from the plasma membrane in that it has a higher protein content (70%) and unique phospholipids (i.e., cardiolipin).

E. Ribosome
Sites of protein synthesis (translation). Two subunits; one large and the other small. Made in the nucleus from rRNA and protein. Ribosomes are tiny (0.25 mm) and numerous (5 - 50 X 10¹⁰ per cell). Since ribosomes are not surrounded by a membrane, they are not considered to be "true" organelles. Some ribosomes are 'free' (produce proteins that remain in the cell) while others are attached to the ER (produce proteins for export). To export a protein, the mRNA and subunits of the ribosome bind together. A signal recognition particle (SRP) binds to specific amino acids in the newly forming protein. The SRP, which is bound to the protein/mRNA/ribosome, then binds to a receptor in the ER membrane. As the protein is made it is released into the lumen of the ER and the SRP sequence of the protein is snipped off.

F. Endoplasmic reticulum
A series of membranous tubes and sacs (cisternae) that run throughout the cell. Rough ER has ribosomes associated with it and is laminar while smooth ER lacks ribosomes and is tubular. Totally man. The ER has several functions including: (1) synthesis of lipids and membranes (smooth ER); (2) serving as a site for the synthesis of proteins by the ribosomes (rough ER); (3) transport (a type of cell 'highway' system); and (4) support.

G. Peroxisomes
Membrane sac containing enzymes for metabolizing waste products from photosynthesis, fats and amino acids. Hydrogen peroxide is a product of metabolism in peroxisomes. Catalase, which breaks down the peroxide is also present and serves as a marker enzyme for these organelles.

H. Glyoxisomes
Membrane sac containing enzymes for fat metabolism. Especially common in seeds. Also contain catalase.

I. Golgi apparatus
Pancake- or pita bread-like stack of membranes. Particularly important in cells that produce materials for export (secretion). They have a polarity (cis - imports vesicles from ER; trans - exports vesicles). The Golgi is the site of processing and packaging cellular components. Vesicles containing proteins, lipids and other materials, fuse with the Golgi (cis side), release contents, which then get processed, sorted, packaged and re-released from the other side (trans face). The Golgi also is active in synthesizing many cell components, especially carbohydrates and is involved in tagging proteins with carbohydrates and other side chains for sorting them to their final destination. There are two models for the movement of materials thru the Golgi: (1) Vesicle Migration Model - in this case a vesicle fuses with the cis side, then ultimately a new vesicle pinch off this stack and fuses with teh next one, and so on, until the vesicle reaches the trans side; and (2) Escalator Model - a vesicle fuses with the cis side and never leaves this stack. Rather, the stack on the trans side releases vesicles and then disintegrates while a new stack forms on the cis side. The original vesicle is now in the "second" stack, and so on until it reaches the trans side. Vesicles are tagged with various proteins to direct them to the appropriate locations.

J. Microtubules
Hollow tubes made of a mix of alpha and beta tubulin, which are globular proteins. There are essentially 13 columns of proteins. The tubes are about 25 mm in diameter. Microtubules are involved in the cell cytoskeleton (for support), cell movements (cilia, flagella) and cell division (spindle). Assembly of microtubules is prevented by colchicine, an inhibitor derived from Crocus bulbs. Low calcium concentration favors the formation of microtubules

K. Microfilaments
Protein strands. Solid. Made from G-actin. Involved with the cell cytoskeleton. Main function is support. They are about 7 nm in diameter.

L. Intermediate filaments
These are similar to microfilaments. They are also made of protein in the keratin family; about 10 nm diameter.

M. Cilia/flagella
For cellular movements. Cilia = many, short; flagella = few, long. Have a 9+2 arrangement of microtubules. Prongs on the tubules are ATPases (dynein) to hydrolyze ATP to provide energy for movement. These are not particularly common in plants.

N. The Cytomembrane system
The membranous organelles (ER, vesicles, golgi, cell membrane) comprise a group of organelles that cooperate and function together. For example, imagine the synthesis of cellulose in the cell wall of a plant. Cellulose synthesis requires the enzyme cellulose synthase. Ribosomes (rough ER) → makes enzyme → passes through RER to smooth ER → packaged into a vesicle → pinches off → to golgi (cis face) → processed → repackaged into vesicle → pinches off (trans face) → cell membrane → fuses → releases contents → cellulose synthase makes cellulose.

IV. Organelles Unique to Plants - Plastids
Plastids are double membrane-bound organelles in plants. They contain their own DNA (in nucleoid region) and ribosomes. They are semi-autonomous and reproduce by fission similar to the division process in prokaryotes. If plastids only arise from other plastids and can’t be built "from scratch", then where do they come from? The egg. Plastids are inherited cytoplasmically, primarily through the female - however, there are examples of paternal inheritance of plastids. The plastid DNA carries several genes including the large subunit of rubisco and those for resistance to some herbicides. The chemistry of the membranes differs from the plasma membrane - plastid membranes are comprised of glycosylglycerides rather than phospholipids (the phosphate in the polar head group in glycosylglycerides is replaced with galactose or a related sugar).

Plastids can dedifferentiate and convert from one form into another. For example, think about the ripening processing in tomato. Initially, green tomatoes have oodles of chloroplasts which then begin to accumulate lycopene (red) and become chromoplasts. Usually you find only chromoplasts or chloroplasts in a cell, but not both.

V. Organelles Unique to Plants - Vacuoles
This is the large, central cavity containing fluid, called cell sap, found in plant cells. The vacuole is surrounded by a membrane (tonoplast). Back to the water balloon in the box model - imagine the vacuole to be analogous to another water balloon inside our protoplast balloon. This water balloon is a separate entity that can be physically removed from the cell. The vacuole is penetrated by strands of cytoplasm - transvacuolar strands.

The tonoplast and plasma membrane have different properties such as thickness (tonoplast thicker).

Virtually every plant cell has a large, well-developed vacuole that makes up to 90% or more of the cell volume. Wow! Meristematic and embryonic cells are exceptions. Young tissues have many small vacuoles. As the cell grows the vacuoles expand and eventually coalesce. These small vacuoles appear to be derived from the Golgi.

The central vacuole contains water, ions, organic acids, sugars, enzymes, and a variety of secondary metabolites. Among the hydrolytic enzymes are proteases (digest protein), ribonucleases (digest RNA) and glycosidases (break links between monosaccharides). These enzymes are typically not used for recycling cellular components but rather leak out on cell senescence. There are smaller lytic vacuoles, which contain digestive enzymes, that are used for this purpose. Another type of vacuole, protein bodies, are vacuoles that store proteins.

Why do plants cells have a large central vacuole? Important roles of the vacuoles are:

A. Energetically efficient means to increase surface to volume ratio in the dendritic growth form
Since 90% of the cell volume is vacuole, therefore 90% of the cell is water, which is relatively cheap in metabolic terms. Thus, it allows plants to get big with a minimal energy investment. Plants are particularly 'smart,' since, the cell wall, which comprises much of the remaining 10% or so of the cell is a polymer of glucose. Cellulose is a great bargain! It is stronger and cheaper than comparable polymers. Let’s compare:

polymer	compound/production value (weight product/weight glucose to make)	tensile strength (newton m² x 10⁹)
cellulose	0.83	30
collagen	0.40	2
silk	0.40	10

C. Waste disposal
The vacuole can be considered the cell cesspool. It contains many secondary metabolites including a variety of hydrolytic enzymes like the "marker enzyme" alpha mannosidase. In this regard, the vacuole is analogous to the lysosome.

Let’s consider differences between plants and animals in terms of wastes. Plants have little waste. Their nutrients are in dilute form, they use them efficiently and hence, there is little left over. Plants remind me of my Dad who was a Marine during WWII. He said that the Marine philosophy at dinner was to "eat all you want, but all that you take." Plants do just that - most everything they "ingest" they use. The minimal wastes plants produce can be stored in the cells in the vacuoles (or disposed of in other ways - released as a gas into the air, leached out of roots or leaves).

In contrast, animals rely on nutrients in a concentrated form. They are rarely selective about what they eat; much un-usable material is ingested along with the "good stuff". Thus, they have a large quantity of waste and needed to evolve specialized digestive/excretory systems to get rid of it. Further, animals don’t have the luxury of having bulky vacuoles and internally storing wastes because these adaptations would limit motility.

D. pH regulation
The vacuole is a pool to dump excess protons. There is an active proton pump in the tonoplast. The cell sap has a pH of 2-5.7, whereas the cytosol is ca. 7.0.

E. Storage of essential ions
Ions are pumped into the vacuole for water balance. Potassium and calcium. in particular, are stored in the vacuole.

F. Cell enlargement
Cell growth requires some force to allow for the cell to increase in size. Water pressure provides the force and it moves into the vacuole. For example, root hair enlargement is due entirely to vacuolar enlargement.

G. Facilitates diffusion
The cytosol essentially forms a thin coating around the large vacuole which in effect, increases the surface-to-volume ratio of the cytoplasm. It provides easier access and shorter diffusion distances between any part of the cytoplasm and the external environment of the cell. This can be particularly important for chloroplasts.

H. Structural support
The vacuole helps to maintain turgor pressure in plant cells due to the opposing forces of tensile strength of the wall vs. compression strength of water.