Everything you wanted to know about plant cells, but were afraid to ask

 Plant cells, like animals and other eukaryotes, have nuclei, mitochondria and cell membranes, along with a fundamentally similar living cytoplasm, but they have some unique features that adapt them to function quite differently:

1. A cell wall.  This is not the same thing as a cell membrane, but laid down outside of it by secretions of cellulose and other materials from the cytoplasm.  The cell wall is thus a non-living layer of material between cells. A cell wall is more-or-less rigid, yet generally porous.  Its primary function is to prevent the cell from bursting as water flows in through osmosis.  Animal cells don't have a wall, and if placed in distilled water will expand until they burst.  As I explained in a recent post, plants utilize that intrinsic turgor pressure for a variety purposes, such as expansion of tissues during growth, transport of food and other materials around the plant, holding soft foliage upright, and movements like the closing of venus fly traps.  

2. A large central vacuole.  This is essentially a big bag of water in the center of each cell, surrounded by its own membrane.  This is key to the phenomenal growth rates of young plant parts.  The living cytoplasm occupies a relatively thin zone around the periphery of the cell. Water absorbed by the cell can be stored in the vacuole without diluting the cytoplasm, allowing cells to expand many fold.  The central vacuole also serves as a depository for waste materials and defensive chemical substances.

3. Plastids.  The most recognizable plastid is the chloroplast, which gives the plant its signature property of photosynthesis.  Other forms of plastids are modified to store various materials, such as starch, oils, or pigments.

Parenchyma cells are complete living cells with thin  walls and
active cytoplasm.  This image is from a video posted on 
Youtube, in which you can see the chloroplasts moving 
around the cell.

There are many specialized cell types in plants, but we can begin with the generic living plant cell, such as might be depicted in an introductory biology text.  It is a living cell, capable of many metabolic functions, including photosynthesis, energy metabolism, food storage, and synthesis of structural molecules and secondary plant compounds.  It has functional mitochondria, active plastids, and a functioning nucleus. It has a relatively thin cell wall, and can even divide under certain circumstances, or be changed into another kind of cell.  This is called a parenchyma cell.

The parenchyma cells in a potato tuber are filled with
starch-storing leucoplasts (purple-stained bodies). 

Parenchyma cells fill the interior of all plant organs, filling in around the vascular tissues.  In the leaves and young stems, plastids in the parenchyma cells develop as chloroplasts, in the roots they develop primarily as starch-storing leucoplasts.  In flower petals, they become pigment-storing chromoplasts.

A tissue is a distinctive grouping of cells in a particular region and with a particular function.  Parenchyma tissue is a mass of parenchyma cells, which may seem obvious, but as you'll see later, some tissues are a mixture of different kinds of cells.  Parenchyma cells may also be embedded within the vascular tissues.

A mass of parenchyma tissue on land would flop on the ground and quickly dry out.  It would be okay underwater, and in fact most algae consist of nothing but parenchyma.  So land plants need specialized tissues for internal structural support (sclerenchyma and collenchyma), an external waterproofing layer (epidermis or cork), and internal water and food conducting tissues (vascular tissues: xylem and phloem).

In comparison with parenchyma, specialized cell types are characterized by something being missing, something being exaggerated, or both.  In other words, they are structurally modified for specialized functions.  In sclerenchyma tissues and the water-conducting xylem, cell walls are quite thick and very rigid, while the cytoplasm is essentially non-functional or completely gone. Once mature, these cells function only as empty cell walls.

Fibers, as in this palm fruit, are most often bundled into thick
strands.

These thickened walls, called secondary cell walls, consist of multiple layers of cellulose that are laid down before the cytoplasm dies.  They and may also be impregnated with a hard, resin-like substance called lignin, which gives them additional strength and durability.  Fibers are long slender sclerenchyma cells, usually bundled together to make rope- or cable-like strands of support.  They are mostly found around vascular bundles, but may form separate strands as well.


Fibers are particularly well-developed in monocots, which lack wood.  The trunk of a palm tree is made up of thousands of fibrous bundles running through a parenchyma matrix. Palm leaves are likewise reinforced with fibrous bundles that act as support cables.  The softer leaves of banana plants are also supported by bundles of fibers, and one species (Musa textilis) is the source of the commercial Manila hemp fiber.  Softer fibers in the flax plant (Linum usitatissimum), a eudicot, are used to make the fine fabric known as linen.

The epidermal cells of the developing bean have been
modified into closely packed elongate sclereids.  

The term sclereid is used for a great variety of thick-walled cells that are not long and narrow. They may be shaped like grains of sand, clubs, pillars, spikey stars, and other odd shapes. They are usually in patches or layers that create a barrier against herbivorous intruders. The seed coat that surrounds mature seeds is often made up of elongate, pillar-like sclereids.

The shell of a walnut is mostly sclereids, densely packed and glued together like bricks to create a protective wall that can be breached only by animals with special tools, such as rodents with their chisel-like teeth.  The gritty patches of sclerieds just below the surface of a pear protect it from insects when it is small, but spread apart as it matures, making it palatable to the larger animals that disperse its seeds.  The outer husk of a coconut is made of fibers intermixed with light, airy tissues that allow the whole fruit to float on water.

The long "strings" that line the angles of a celery stalk are
made of collenchyma cells.  The thickened primary walls run
together to form a light colored matrix around the darker
protoplasts.  

Like the walnut shell. the heavy-duty inner shell of a coconut consists primarily of sclereids.   In nature, this shell can only be broken by a specialized crab (Birgus latro) that has enormous, powerful pincers.  Such crabs, the largest land-dwelling arthropods, live on islands in the Indian and Pacific Oceans, where it is believed that the coconut evolved.

Collenchyma is a specialized soft supportive tissue, where the rigidity comes from thick, water-filled cell walls. These are primary cell walls, consisting of a loose matrix of cellulose, unlike the dense secondary walls of sclerenchyma.  Collenchyma is rapidly produced and common in young, growing herbaceous stems and leaves.

In this succulent stem of Hoya, sclereids (S) form a continuous "brick wall" that protects the inner tissues.  Bundles
of fibers (F) serve as reinforcing cables inside the sclereid wall, and parenchyma (P) tissues fill the interior spaces.
The epidermis to the right has a thin, orange-stained cuticle. Vascular bundles are out of view to the left. 

A xylem vessel consists of a stack of thickened, cylindrical cell
walls (vessel elements) abandoned by the protoplasts that formed
 them.  Tracheids are single cells. 
Modified from Slideshare.net.

The tracheids and vessels that conduct water through the plant  also have thickened walls, but much larger hollow centers in their mature, functional state.  Fibers may be evolutionarily related to these conducting cells, and in wood (secondary xylem) they intergrade with one another.  Xylem tissue is a mix of tracheids, vessels, fibers, and parenchyma.

In this cross-section of wood from Celtis occidentalis (hackberry), wide vessels (V) are surrounded by narrow fibers (F). Rays of parenchyma (P) run through the wood at right angles to the vessels and fibers. Vessels are narrower and fibers have thicker walls in the late Summer and Fall (lower part of the picture), helping to mark the annual growth ring.  Wood is laid down by the cylindrical layer of dividing cells known as the vascular cambium, which slows down its activity during the late summer.

The cells that make up a sieve tube (sieve tube members) 
are partially broken down.  The nucleus, central vacuole, 
and mitochondria are gone, and the modified cytoplasm
serves as the medium for carrying dissolved sugar through
the tube. The end walls are also called sieve plates, for 
they contain large pores to accomodate the flowing
fluid. The companion cells are fully functioning 
parenchyma cells. Modified from Slideshare.net

Phloem tissue consists of the food-conducting sieve tubes, companion cells, parenchyma and fibers.  The sieve tubes consist of a series of cells that contain a cytoplasm-like fluid, but lack nuclei and other organelles. Sugar and other organic substances are loaded into the fluid and flow from one part of the plant under the force of turgor pressure.  Large pores at the ends of the sieve tube elements allow the fluids to pass through freely.  Companion cells are specialized, metabolically active parenchyma cells that run parallel to the sieve tubes and perform some metabolic functions for them. They also help load the sugar into the phloem.

A vascular bundle, as seen here in a Ranunculus stem, is a
complex assemblage of xylem, phloem, narrow
parenchyma cells, and fibers, embedded in the mass
of larger parenchyma cells.

In this succulent stem, the epidermis is reinforced by a layer of sclereids,
 and covered by a thick cuticle. No stomata are visible in this picture. 

Stiff bristles emerge from the epidermal cells of maize leaves.
The outgrowths of epidermal cells are generally known as
trichomes.

Epidermal cells are living cells on the surface of plant organs and so are modifed for protection against water loss and invading organisms.  They don't usually contain chloroplasts, but are active in other ways, and under some circumstances can divide.  The epidermis usually contains specialized cells, including embedded sclereids, glandular cells and the important guard cells that open and close pores (stomata) in the epidermis, allowing for gas exchange.  Epidermal cells also secrete on their outer surface a cuticle, made of a hard polymer called cutin.  The cuticle prevents water loss from the plant, except through the stomata.  Some plants even secrete a layer of was over the cuticle for additional waterproofing.  The thick layer of wax on the leaves of the Carnauba wax palm (Copernicia prunifera) is used commercially to make fine, hard car waxes.  Epidermal cells may also sport a wide variety of hairs, bristles and scales, collectively known as trichomes.

Cork tissue may be laid down on older stems and roots through cell divisions in the epidermis, the parenchyma below the epidermis, or from older phloem tissues.  It consists of empty, thin-walled, waterproof cells.

Stomata are important elements of the epidermis.  They consist
of a pore flanked by a pair of guard cells that can change shape
to open or close the pore. This allows for gas exchange needed
for photosynthesis, or to seal the plant against water loss during
dry or inactive periods.  From micro-scopic.tumbler

The trichomes produced by the epidermal cells of a carnivorous sundew (Drosera)
contain glands that exude a sticky, digestive juice.

Within these basic cell types there is amazing variation in shape and distribution. Families, genera, and even species in some cases, can be recognized by the pattern of their internal tissues. The study of this variation is plant anatomy, which in turn is one of the most important tools in forensics, as well as in archeology and paleobotany.  In coming posts, I will explore some of this variation.

How does water get to the top of a redwood tree?

The upward movement of water more than 100 meters in a redwood or eucalyptus (there is  a traditional dispute between Americans and Australians over who has the tallest trees!) seems to be a gravity-defying task of epic proportions.  Gravity is certainly a factor, and ultimately limits how tall a tree can get, but there are other forces at work that can meet that challenge.  The amazing thing about plants is that the process is largely passive, in the sense that plants expend practically no energy to accomplish it.  There are no muscles and no heart in a tree to pump water upward.  What there is, is basically a gigantic paper towel.

You may recall the ads for the “Bounty Quicker-Picker-Upper”  a decade or two ago.  In the ads, these paper towels quickly absorbed any spilled liquid.  You can take a paper towel (even a cheap slower-picker-upper), roll it into a tube, and stick the end in water. You’ll quickly see the water begin creeping up the paper towel.  We see the same gravity-defying process in plants.  What is a paper towel made of?  Wood!  And the magic ingredient in both a paper towel and a living plant is cellulose. 

Every plant cell is wrapped in a layer of cellulose – the cell wall.   In wood, the cells of the xylem die after laying down strong cellulose walls, leaving the latter as narrow, empty conducting tubes.  These tubes line up so as create masses of interconnected passageways through which water can move freely.  So a tree trunk is essentially a massive, non-living, interwoven paper towel.   

But what is the interaction between cellulose and water?  What is the force that can overcome gravity?  The short answer is magnetism.

Magnets can defy gravity by lifting nails and other iron objects.  No, neither cellulose nor water is made of iron, but the forces involved are similar.  A molecule of water is electrically charged.  Imagine each molecule as a Mickey Mouse head.  The face is an atom of oxygen, and the ears a pair of hydrogen atoms.   The two types of atoms are bound together by sharing electrons, but the oxygen atom holds onto the electrons much more tightly than the hydrogen atoms do.  the result is that the electrons hang out around the oxygen atom most of the time, giving its side of the molecule a net negative charge.  The two hydrogen atoms are left with a net positive charge as they are visited less often by the electrons.   Thus water molecules are thus like tiny magnets and tend to stick together.  This actually is what makes water liquid, rather than a gas, at room temperature, and which accounts for a lot of other properties that we don’t have time to review here.

It so happens that the complex surface of cellulose fibers also have positive and negative charges, and so attracts water molecules.  In a paper towel, water molecules are pulled into every available niche in the cellulose matrix.  Those at the top are pulled into higher niches, which pulls more water molecules from the bottom.  Narrow spaces within the matrix also fill with water molecules, which attract and pull each other in.  This happens until the towel is saturated and there is no room for any more water. 

The water molecule (A) consists of a negatively charged oxygen atom and two positively charged hydrogen atoms.  This causes them to stick together in chains (B) and to the walls of cellulose fibers (C).

If we return to our rolled up paper towel with the end sitting in the water, we can watch something else happening over time.  The paper towel only gets saturated at the base, and even remains fairly dry at the top.  But given enough time, the container of water it is standing in will completely dry up.  Of course what is happening is that the water that has moved up the column continually evaporates when exposed to the air, leaving spaces for more water molecules to move up.  The result is a steady stream of water moving upward, drawn by both evaporation and the electromagnetic attraction of water molecules to the cellulose and to each other. 

This is the process of transpiration, which is powerful enough to lift water and dissolved minerals to the top of a tall tree.   It continues as long as water is evaporating through the leaves.  In extremely humid weather, or if the leaf pores (stomata) shut down for the night, the stream is suspended in place until evaporation resumes again later. 

I should note that a tall tree, if dried out, cannot start this process from scratch.  Gravity will stop the electromagnetic movement of water long before it can reach the evaporation zone of the leaves.  The transpiration stream develops in a germinating tree seedling and is maintained and strengthened as the tree grows, but if that stream should be broken (interrupted by extensive air bubbles) in an extreme drought, it cannot be repaired and the tree will die.   Only small plants like mosses can completely dehydrate and recover when wet conditions return.