Must Know High School Biology - Kellie Ploeger Cox 2019

PART FIVE Forms of Life
Plant Structure and Transport

If you recall from way back in Part One, one characteristic of all life was the cell as its basic building block. A living thing can be made of a single cell (such as bacteria and many protists), or it could be multicellular. Multicellular organisms include fungi, some forms of protist, plants, and animals. Each of these multicellular life-forms are fascinating and worth studying in great detail. In this book, however, we will focus on two major players: the plants and the animals. I mean no offense to fungi and bacteria—bacteria are actually my favorite biological topic—but plants and animals provide the perfect opportunity to learn how a complex organism relies on smaller systems within it to function properly. As cells divide in the process of growth, they begin to differentiate (Chapter 11), meaning certain genes turn on and the cell adopts a specific shape and function. These specialized cells form tissues, and multiple tissues come together to create the organs that perform specific tasks for the organism. This process of creating small systems in the larger organism is key to the success of both animals and plants.

MUST KNOW

Plants have specialized tissues capable of performing different functions.

Transport in plants is driven by water potential.

Plants are easy to define: they are autotrophic (photosynthetic), multicellular organisms made from a specific type of cell that contains chloroplasts and has a cell wall made of cellulose. A plant’s body is composed of organs such as leaves (for photosynthesis), a stem (for structure), and roots (for water absorption). It becomes even more fascinating when you look a bit closer at these organs. Like, how exactly is a leaf’s form fit for its function of photosynthesis? How does a huge tree move water into its roots and pull it all the way up its body?

Just like in animals, plants’ organs are composed of a number of different tissues: dermal, ground, and vascular. This is, in fact, our must know when learning about plant structure: plants have different tissues, just like animals do. The dermal tissue is what it sounds like: the epidermis (or skin) of the plant. The dermal tissue often secretes a waxy coating to help reduce water loss. The ground tissue comprises the major “filling” of the plant and often specialize in photosynthesis and storage. The vascular tissue is composed of phloem (the transport system for sugar produced through photosynthesis) and xylem (the transport system for water). No matter what plant organ you are studying—stems, roots, or leaves—they all contain these three tissue types.

We will be focusing on flowering plants, the most evolutionarily advanced type of plant. Please understand, however, there are other categories of plants that do not necessarily possess all the traits that we are talking about. Moss, for example, doesn’t have vascular tissues and relies on flagellated sperm to swim over to the egg (weird, right?). Ferns have vascular tissues (which enables them to grow much larger than the more primitive moss), but lack seeds to protect their embryos. Conifers (such as pine trees) have vascular tissues and seeds, but the seeds are said to be “naked” because they’re not encased and protected in flowers!

Not meaning to be animal-centric, but once again I want to point out that, like animals, plants’ organization follows the same hierarchy of organs → tissues → cells. Plants’ organs are made of specific types of tissues, and those tissues are made up of specific types of cells! Parenchyma cells are considered the stereotypical plant cells—they are the models for the plant cells you see in your biology book. Parenchyma plant cells have the usual cell wall composed of cellulose, are chock full of chloroplasts, and house a large central vacuole. If a cell is going to do a lot of photosynthesis, it is most likely a parenchyma cell; this cell type tends to do all the work (storage, metabolism, etc.).

IRL

It’s possible to grow an entire plant out of a single parenchyma cell! These cells retain the ability to differentiate into other types of plant cells (with the correct chemical coaxing). If you have ever propagated a plant by cutting off a stem and stimulated root growth—often simply by placing the cutting in water—the parenchyma cells found in the stem are responsible for differentiating and forming the new roots.

The cool thing about these different cell types, however, is their structure starts to specialize a bit to help in its function, and a major function besides photosynthesis is support. Recall that plants rely on their cell walls to provide structure, so the next two cell types are modified to provide a bit more support. Can you probably guess what part of the plant cell is modified? Yup, the cell wall. For example, collenchyma cells are found in young parts of a plant and provide structure without restraining growth. Their cell walls are thicker than in parenchyma cells, yet are still flexible.

IRL

You are familiar with collenchyma cells if you have broken a piece of celery in half and noticed those long strings. Those are cylinders of collenchyma cells. It makes sense, because it’s the stalk of a young celery plant that needs to keep elongating and growing. The collenchyma cells allow this growth while providing structure.

One step along the specialized-for-structure chain involves the sclerenchyma cells. These guys are like tiny little bricks: very strong and inflexible. Once sclerenchyma cells have finished growing, they produce a second cell wall that is chock full of a polymer called lignin, which provides even more rigidity. These cells cannot elongate and thus are found in areas of the plant that are no longer growing in length.

IRL

A specific type of sclerenchyma cell called a sclereid is responsible for the slightly gritty texture when you eat a pear!

The vascular tissue of the plant is also composed of specific cell types. The water-conducting xylem is made of tracheids and vessel elements, both of which are tube-shaped hollow cells that are quite dead at functional maturity. The sugar-conducting phloem is composed of sieve-tube elements, that unlike the cells of the xylem, are still alive and functioning (though the contents of the cell are reduced to help provide more space for transport). These cells must retain a bit of function because movement of phloem relies on solute gradients and active transport, and a dead cell is unable to actively do anything.

Now that we have covered the building blocks for a plant (cells and tissues), we can zoom out a bit and focus on the plant’s organs and their functions.

Leaves

A leaf’s form is adapted for its primary function: photosynthesis. The driving force behind the creation of glucose is the sun, so leaves tend to have a high surface area to increase the light absorption. There are exceptions to this rule, and they are quite interesting. The following table shows some leaves that have been modified to the point of no longer looking like leaves!

The dermal layer of the leaves has a waxy coating to help reduce water loss by evaporation. The underside of the leaf, however, has special holes called stomata (singular, stoma) that are very important for photosynthesis. If you think back to the process of photosynthesis, you can probably guess what those holes are for (hint: they provide an entryway for an atmospheric gas to enter the leaf airspaces). Yes, stomata are for carbon dioxide to move into the plant leaves! The carbon dioxide gas enters these pores and then moves throughout the leaf until it gets to the cells that are specialized to make glucose. The holes are flanked by special cells called guard cells that are in charge of either opening or closing the stomata. The organization of the leaf tissue is logical, if you think about the function:

Cross section of a leaf

Source: https://commons.wikimedia.org/wiki/File:Leaf_anatomy.jpg

The top layer of the leaf is filled with column-shaped cells packed as tightly together as possible (the palisade cell layer). This ensures that there are a ton of chloroplasts hanging out on the sunny (upper) side of the leaf, grabbing as many photons of light as possible. These cells use the carbon dioxide that comes in through the stomata, and the water that is brought up from the roots, in order to create glucose. The gaseous CO₂ can get to that tightly packed upper layer because the lower layer is composed of loosely packed cells (the spongy cell layer) that has a bunch of airspaces. Once the glucose is created, it needs to get all throughout the plant’s body. The glucose superhighway—the phloem—is conveniently branched and dispersed all throughout the leaf. Alongside the sugar highway is the water conduit (the xylem), and both of these vascular tissues are bundled together to create the veins you see when you hold a leaf up to the light.

The leaf is clearly an important structure for photosynthesis, but it also plays an important role in transport of water up the plant. The movement of a column of water molecules up the xylem is no easy feat. The main driving force is a “pull” that originates from the leaves, specifically, all those open stomata that allow CO₂ to enter. The truth is, when those tiny doorways are open, it also allows water to leave through evaporation. Sure, this isn’t necessarily a good thing; too much water loss will cause a plant to wilt and eventually die. But the evaporation of water from the top of a plant (referred to as transpiration) is the driving force behind how a plant pulls water up from the soil through the roots. Imagine the column of water in the plant's xylem as a chain of water molecules, each holding on to the other through hydrogen bonds (cohesion). The chain of water molecules is stabilized by the adhesion to the inside of the xylem conduit. If something introduces a bubble into the water column, this disrupts the flow of the xylem because the continuous cohesive chain of water molecules is broken. This phenomenon is called cavitation.

Don’t forget that cohesion and adhesion are both possible because of water’s ability to form hydrogen bonds:

When a water molecule bonds with itself (such as in the figure to the right), it is called cohesion. If a water molecule hydrogen bonds with something else, it is called adhesion.

Hydrogen bonding between water molecules (cohesion)

IRL

You are supposed to cut the stems of flowers under water to prevent the introduction of an air bubble into the xylem (cavitation)!

As transpiration occurs, it draws the column of water from the roots all the way up the plant body. Imagine this happening in a giant tree! There’s also a bit of help from the roots (a slight “push” of the water column), but it’s not enough to have an effect on larger plants. We’ll talk about this a bit more when we get to roots.

Stems

In a typical plant, the leaves are attached to the stem. The stem helps the leaves reach out and grab as much sunlight as possible, and the stem will also grow tall to outcompete neighboring plants for sun. The stem will also push the reproductive bits as high as possible to help disperse pollen and facilitate fertilization. Stems provide the highway through which sugar (in the phloem) and water (in the xylem) will flow. Some plants have modified stems, the most surprising of which is the glorious potato. Even though potatoes grow underground, each tuber is actually a stem modified to store large amounts of starch.

Roots

The roots of a plant extend into the soil and absorb water and nutrients. Some species of plants have one strong major root (the taproot), whereas others have a bunch of smaller, shallower fibrous roots. If you have ever tried to pull up a dandelion from the soil, you have battled the strong grip of a taproot. On a related note, if you have ever ripped up a piece of sod (such as a clump of grass from your lawn), the roots on the underside are perfect examples of fibrous roots. Some plants use their roots to store food and water, such as beets and carrots.

IRL

If you have ever pulled a carrot from the ground, the part that gets all the glory is the carroty part (the orange root) that you eat. Sure, it’s delicious and good for you, but what was the purpose for the plant? What would a carrot plant do with all that stored starch if we hadn’t sauntered along and so rudely ripped it from the soil and ate it? If a carrot plant is left to its own devices, it would eventually use all that stored glucose (starch is the polymer a plant creates to store tons of glucose) and convert it into energy (ATP) to use toward flowering! A carrot flower is actually quite pretty.

The flow of the water in the xylem originates in the roots and moves upward into the rest of the plant’s body. As we mentioned, the movement of the water is mostly due to transpiration (evaporative water loss) from the top of the plant, which pulls the cohesive chain of water molecules upward. The root, however, can provide a tiny bit of “push” due to the difference in water potential in the root tissue and surrounding soil. Recall from Chapter 7 that water will diffuse from a region of high water potential to low water potential, and the more solute that is dissolved in water, the lower the water potential. Therefore, the roots of a plant need to have a lower water potential than the surrounding soil in order to facilitate osmosis from the soil into the roots. The cells of the roots work to make this happen by actively transporting solute (ions and salts) from the soil into the root cells. This lowers that water potential in the plant’s tissues, and water will diffuse in after it! This increased volume of water creates a slightly higher pressure in the roots compared to the rest of the plant, but the impact the pressure has on water movement is minimal. Yet if you have ever noticed droplets of water emerging from the edges and tips of the leaves, you have witnessed the effect of root pressure on xylem flow (called guttation). The bulk of the xylem movement up a plant is due mostly to the transpiration from the top.

REVIEW QUESTIONS

1. List the three plant tissue types and briefly explain their function.

2. ________________ cells are the “typical” plant cell, whose main roles are photosynthesis and storage. ________________ cells provide structure without restraining growth. ________________ cells are specialized for structure and are no longer able to elongate.

3. Explain why a leaf’s structure is perfectly adapted for photosynthesis.

4. How does water potential play a role in movement of water up a plant?

5. Compare and contrast phloem and xylem.

6. For each of the following descriptions, indicate whether it applies to parenchyma (P), collenchyma (C), or sclerenchyma (S) cells:

a. Has two cell walls

b. Found in strings of celery because they provide structure without restraining growth

c. Has lignin in its cell walls

d. Primary function is photosynthesis

e. The reason a pear has a gritty feel

7. The process of ________________ is the major force that pulls water from the roots up a plant’s body. As water ________________ from the top of the plant, it pulls a chain of water molecules up the xylem. Each water molecule creates a chain through the process of ________________, and the entire chain sticks to the sides of the hollow tracheids and vessel elements through the process of ________________.