1. Light

Photons travel from the sun in waves that have specific lengths. Photosynthesis utilizes photons with wavelengths within the visible part of the wavelength spectrum, i.e., wavelengths that can be seen as colors. The pigments found within photosynthesizing cells of plants are capable of absorbing photons with the violet, blue and red parts of the spectrum, and reflecting back those in the green/yellow part of the spectrum. Thus, we "see" these reflected photons as the "green" of leaves and plants. Interestingly enough, less than 2% of all solar energy reaching the Earth is used in photosynthesis. The rest is reflected back as heat.

2. Chloroplasts

A chloroplast is a membrane-bounded organelle that contains the pigment, chlorophyll, and is found in most photosynthetic organisms. Chlorophyll is a green pigment that absorbs solar energy.

[See diagram --- p. 183 in textbook]

Chloroplasts have two recognizable parts. A double membrane surrounds a central area called the stroma. (Don’t confuse this with the stomata of leaves!) Inside the stroma, an enzyme-rich solution collects incoming CO₂, where it is chemically processed in a series of reactions (we’ll look at these in detail later). Within the stroma, there is another system of membrane structures called thykaloids, which are sometimes stacked lie dishes in grana(granum=singular). The spaces within each thykaloid are thought to be connected to spaces within all other thylakoids within a given area. This connected area is called a thykaloid space. Chlorophyll and other pigments (like carotene) are found within the thykaloid membranes. These pigments absorb the solar energy which in turn drives the reactions taking place in the stroma.

The Reactions of Photosynthesis

There are two separate sets of reactions involved in the process of photosynthesis:

• Light dependent reactions (the capture of solar energy)
  
• Light independent reactions (synthesis of carbohydrates)

Let’s look at each of these sets of reactions more closely.

Light Dependent Reactions

• Occur on thylakoid membrane where chlorophylls a and b and carotenoids are located

• These are the energy capturing reactions:

Light Independent Reactions

• Take place in the stroma section of chloroplasts
  
• Can take place either in the light or in the dark
  
• These reactions are called the synthesis reactions because they use the stored energy (in the form of ATP and NADPH) from the light-dependent reactions in the thylakoids to reduce (add electrons) CO₂ to a carbohydrate

This last step, reduction of CO₂ into a carbohydrate, is called the Calvin cycle, a process that has three stages:

PHOTOSYNTHESIS TAKING OTHER ROUTES

The form of photosynthesis that we’ve just looked at is used by plants referred to as C3 plants because the first carbohydrate molecule identified in the Calvin cycle contains three atoms of carbon. However, there are a couple of other plant groups that, because of living in harsher environments, produce their carbohydrates a little differently. These plants are called C4 and CAM plants.

Examples of C3 plants: wheat, rice oats
Examples of C4 plants: sugarcane, corn, Bermuda grass
Examples of CAM plants: cacti, orchids, bromeliads

In the C3 plant, the mesophyll cells of leaves contains the chloroplasts. These cells are arranged in parallel layers. In the leaves of C4 plants, the sheath cells of the vascular bundles, in addition to the mesophyll cells, contain chloroplasts. Also, the mesophyll cells in C4 plants are arranged in a circular fashion around the bundles’ sheath cells.

C3 plants fix CO₂ in the mesophyll cells during the light independent reactions. In C4 plants, an enzyme is added in the mesophyll cells to "fix" CO₂ to a C3 compound, thus forming a C4 or 4-carbon atom compound. This 4-carbon compound is then "pumped" into the vascular bundle sheath cells and then goes through the steps of the Calvin cycle.

C4 plants exist in hot, dry climates and even thought they receive less water than C3 plants, their photosynthesis rate is 2-3 times more efficient than C3 plants. Why is this? Because they avoid photorespiration, a process that can occur in plants when the oxygen competes with CO2 in the Calvin cycle. As a result, oxygen is absorbed into the plant and CO₂ is given off. This scenario is observed when the stomata in plant leaves close down in very dry and hot weather in order to conserve water. When the stomata are closed, the amount of CO₂ decreases in leaves while the amount of O₂, a by-product of photosynthesis, cannot be expelled so it accumulates inside the leaf.

This reaction does not occur in C4 plants even when the stomata are closed because CO₂ is delivered to the Calvin cycle in the bundle sheath cells. When weather is moderate and moist, C3 plants predominate; as weather becomes warmer and drier, C4 plants take over.

Example: Our class ecosystem. In the spring of this year, all plants were doing well… leafing out and growing. Summer came along with the drought. By the end of the drought, C4 plants were thriving: grasses, birch and paw-paw trees, sunflowers, dragonflowers, hydrangeas. C3 plants suffered; the hemlock and partridge berry bushes died, and the cranberry bush almost died.

C4 plants have an advantage over C3 plants in hot, dry weather because photorespiration doesn’t occur.

Another alternative form of photosynthesis occurs in CAM (crassulacean-acid-metabolism) plants --- so named for plants in the Crasstulaceae family. These plants are the flowering succulent (water-containing) plants that we find in warm, arid ecosystems, like deserts. A prime example of these plants is the cactus.

In C4 plants, we saw CO₂ fixation occurring in the bundle sheath cells, partitioned off from the other cells. CAM occurs when CO₂ fixtation reactions are partitioned off part of the time. C4 plants store their energy at night and release CO₂ through their stomata during the day; however, CAM plants fix their CO₂ only at night, therefore keeping their stomata closed during the day and conserving H₂O. The stored CO₂ runs through the Calvin cycle during the day. Photosynthesis in CAM plants is minimal because of the limited amount of CO₂ fixed at night; however, the reaction does produce enough food to allow the plants to live in a harsh, dry environment.

REFERENCES

1. ----, The Way Nature Works, Macmillan. 1998.
2. Capon, Brian, Botany for Gardeners. Timber Press. 1990.
3. Mader, Sylvia S., Biology, 6th Edition McGraw-Hill. 1998.
4. Stern, Kingsley R., Introductory Plant Biology, 7th Edition. Wm. C. Brown Publishers. 1997.

Introduction

As we have seen in looking at the process of photosynthesis, for something that looks innocuous and simple, a plant can be quite complex and complicated. This statement certainly proves to be true when we look at how plants reproduce in nature. Plants follow a plan that provides for preservation and continuation of the species and allows for them to adapt to ever-changing environments. They accomplish this by alternating cycles of asexual and sexual reproduction. This process is called alternation of generations. Before we get into the details of this interesting story, we’ve got to backtrack and review a little bit about cell division.

Mitosis and Meiosis

As you recall from high school biology, cells in all living organisms multiply by simple binary fission , in which the nuclear and cytoplasmic material of a cells divide equally into two "daughter" cells, with the chromosomes duplicating a complete, exactly-the-same-as-the-parent-cell’s set of double chromosomes for each "daughter" cell. This process is called mitosis, and each of the daughter cells is exactly the same as the mother cell. Clean and simple. Heart muscle cells produce heart muscle cells; mesophyll cells produce mesophyll cells.

However, when it comes to producing a new individual organism, one in which it is prudent to provide for adaptation changes every once in a while to ensure survival in different or hostile environments, genetic structure should vary to provide for survival of the species. This action is accomplished by having two cells combine and produce a new arrangement of genetic material which is different from either of the parent cells. So, instead of a cell preparing to duplicate chromosomes in preparation for dividing, certain cells are formed into specific cells to provide for this formation of a new genetic pattern within a species. These cells are called gametes, and are further identified as male and female gametes. Each cell receives only one side of a set of chromosomes. The process of producing gametes or sex cells is called meiosis. When a male and a female gamete merge (fertilization), the resulting structure, the zygote, the two single strands of chromosomes match up and form the set of dual chromosome necessary for the growth of the new individual of the species. Cells with two sets of chromosomes are called diploid; the gametes, with only one set of chromosomes, are called haploid.

In animals, only the haploid cells are gametes. However, in plants, this is not the usual situation. In plants, the complete life cycle is an alternation of generations between a diploid or saprophyte (asexual) stage and a haploid or gametophyte (sexual) stage. Depending on what type of plant we’re looking at, one of these stages is much more visible than the other.

ALTERNATION OF GENERATIONS

Mosses and Ferns: Non-vascular plants

In mosses, the gametophyte or sexual form of the plant is predominant or more visible. The alternation of generations among this group progresses in this manner:

• There are separate male and female plants. The male plants produce antheridia and the females producearchegonia.
  
• Sperm from the antheridia swim in external water to reach the eggs of the archegonia.
  
• The egg is fertilized and develops into an embryo in the archegonium.
  A sporophyte, the vegetative plant stage of the life cycle, grows from the developing zygote – on top of the gametophyte.
  
• The sporophyte produces spores, which mature and are released when the weather is dry.
• Air disperses the spores.
  
• When spores land on suitable soil, they germinate into a new gametophyte.

In ferns, we see the sporophyte form of the plant become the predominant phase in its alternation of generations.

• Structures called sori are found on the underside of the frond leaflets of the large sporophyte.
  
• Spores are produced from sori by meiosis.
  
• Spores are released when the plant dries out and are dispersed by air.
  
• Spores that land in suitable environment germinate into a leaf-like structure called a prothallus.
  
• Prothallus produces the male antheridia (near the tip) and female archegonia (near the notch).
  
• External moisture is necessary for the sperm to swim from the antheridia to the eggs within the archegonia.
  
• When fertilization occurs, a zygote is formed. Zygote grows within archegonium until it bursts out as the first leaf.
  
• Roots develop and then a rhizome bears the beginnings of the characteristic fronds of the sporophyte.

As we get more complex in the plant kingdom, the sporophyte generation remains the prominent generation. In the vascular plants of the gymnosperms (conifers) and angiosperms (flowering plants), spores no longer give rise to sporophytes, but rather seeds do. In fact, the true meaning of the above terms mean "naked" seed" (gymnosperm) and "covered seed" (angiosperm). A seed contains a sporophyte embryo. This embryo is surrounded by a protective layer of stored food. In gymnosperms, this seed is not covered; in angiosperms, the seeds are enclosed in a fruit.

Let’s look at how the alternation of generations takes place in gymnosperms and angiosperms.

Gymnosperms

1. We start with the dominant sporophyte, the pine tree you see in the woods. It forms asexual sporangia in the cones.

2. Each tree has two types of cones:

• Small pollen cones, found near the tips of the lower branches. Each scale of these cones has 2 or moremicrosporangia on the underside. Each of these microsporangia undergoes meiosis and forms 4 haploid microspores. Then each of these develops into a microgametophyte or pollen grain. These grains have "wings" and they are carried by wind to the seed cone.
  
• The seed cones are larger than the pollen cones and are located near the branch tips. Each scale has 2 ovules, a structure that will later contain the seed. The ovules are covered with a thick coating with an opening in one end. Each ovule contains a megasprorangium which, like the microsporangium, undergoes meiosis and produces four haploid megaspores. Only one of these spores will develop into a megagametophyte, the structure containing the egg. This structure is positioned next to the opening of the megasporangium.

3. Pollen grains are delivered by wind to the seed cones. Once enclosed by the seed cone, the pollen grain develops a long tube called the pollen tube. This tube grows slowly towards the megasporangium with its egg. When the pollen tube arrives, it releases 2 sperm. One sperm fertilizes the egg and the other disintegrates. It takes a YEAR for the pollen tube to make this trip, before fertilization can occur.

4. After fertilization, action speeds up. The ovule matures into a seed, composed of the embryo, reserve food, and the seed coat.

5. After another season, in the fall, the seed cone is woody and hard and opens to release winged seeds.

6. The seeds germinate and a sporophyte embryo grows into a tree.

Angiosperms

The seeds of angiosperms are covered by fruit, and fruit are what flowers are all about!

Let’s first review the parts of a flower.

A flower is actually an arrangement of highly specialized leaves arranged in concentric rings and attached to a stem tip called the receptacle.

1. The outermost leaves, the sepals, (or calyx) are around the base of the flower and are usually green. They enclose the flower before it opens.
2. The next layer is comprised of petals which are frequently colorful. This is the corolla (The color attracts insect pollinators.)
3. The third layer is a ring of long thin structures called stamens. The stamens are further broken down into the filament, or stem-like part, and the anther at the tip of the stamen. (Anthers are sporophylls, where pollen is produced.)
4. The stamens surround the innermost part of a flower, the carpel or pistil. This part is also a sporophyll --- it contains the ovule which in turn houses megagametophytes. The pistil contains 3 parts:

• The lower most part is the ovule, which becomes a seed.
  
• The ovule is contained in the ovary, which becomes the fruit.
  
• The top part is called the style, which is capped by the stigma, the receiving point for pollen.

Angiosperms are interesting in that they undergo a double-fertilization process. The end-product of one part of the process produces the embryo sporophyte, while the other fertilization process produces endosperm, a rich food supply for the developing embryo.

There are actually four processes in the life cycle of angiosperms:

1. Development of the megagametophyte
2. Development of the microgametophyte
3. Double fertilization
4. Formation of the seed

Here’s how all of it comes together.

1. Development of the Megagametophyte

• The ovary at the base of the pistil contains one or more ovules.
  
• Within each ovule, a megasporocyte undergoes meiosis and produces 4 haploid megaspores.
  
• Only one megaspore survives; the other 3 disintegrate.
  
• The remaining haploid megaspore divides by mitosis and forms 8 haploid nuclei surrounded by cytoplasm. This is the megagametophyte.
  
• The cytoplasm then differentiates into 2 cells. One cell is the egg; the other, containing 2 nuclei, is the endosperm (its two nuclei are now called polar nuclei. ) The other nuclei are incorporated into the cytoplasm of the cells and eventually disintegrate.

2. Development of the Microgametophyte

• Pollen sacs on the anthers contain many microsporocytes.
  
• Each microsporocyte undergoes meiosis and produces 4 haploid microspores.
  
• The microspores separate, each becoming a microgametophyte or pollen grain.
  
• Each microgametophyte contains 2 nuclei. One is the generative cell and the other is the tube cell.
  
• Pollen is spread from the anthers of one plant by wind, insects, birds or other animals to the stigma of the same kind of plant.
  
• The pollen grain "germinates" and produces a long pollen tube which progresses down the style until it reaches an ovule within an ovary.
  
• The generative cell inside the pollen grain divides and produces two sperm.

3. Double Fertilization (Unique to angiosperms!)

• When the pollen tube reaches the ovule, the two sperm are released.
  
• One sperm migrates to the egg within the ovule and fertilizes it. This forms a zygote.
  
• The second sperm migrates to the 2 polar nuclei, producing a triploid endosperm nucleus (3 strings of chromosomes). This 3n nucleus then divides and forms the endosperm, food for the developing plant.

4. Seed Formation

• Ovules now develop into seeds, which contain the embryo and its food, covered by a protective seed coat.
  
• The wall of the ovary and frequently other surrounding parts continue growing into a thick fleshy covering for the seed(s) called a fruit.

Asexual Reproduction of Plants

There are several ways that plants reproduce asexually. Such reproduction is referred to as propagation or vegetative reproduction. Examples of these modes are:

1. Bulbs – these dormant plants can produce daughter bulbs or corms, budding into new plants. EX - Daffodils; onions; "eyes" on potatoes.

2. Runners or stolons – Roots grow out of stems which growing across the ground surface. These stems give rise to new plants. EX - Strawberries; forsythia

3. Stem and leaf cuttings – A branch or leaf is inserted into a growth medium and new roots are produced from the stem or pedicle. EX - African violets; geraniums

BIRTH --- NOW GROWTH…

Before we leave our discussions of plants, let’s look a bit at how they grow. Plants go through a number of steps in their growth cycle:

seed ® root system started ® sprout or shoot ® formation of stem(s) and leaves ® flowers ® fruit

Some plants progress through these stages quickly (one season) and then die, such as tomatoes or corn. These plants are said to have determinate growth, i.e., there is a known beginning and end to their life span. Determinate growth can be defined further as:

• annual – the plant completes a life cycle in one year or growth season
  
• biennial – the plant completes a life cycle in two years or growth seasons
  
• perennial – the plant lives for many years ("indefinitely") after flowering

Plant Hormones

Just what sparks these growth cycles in plants? Implanted in the genetic code on the genes of each species are the "recipes" for certain hormones (organic substances produced in minute amounts in one part of an organism that is transported to other areas of the organism where it controls or affects growth and development). These hormones contribute to the many growth characteristics in plants.

Recalling from our discussion of plant structure, plant hormones are produced in the apical meristemic areas of developing roots and stems. The hormones are transported throughout the plant via the phloem contained with the vascular bundles.

The most important plant hormones and their functions are:

1. auxins – Regulate plant growth through cell elongation. Also affects several other growth characteristics, including flower initiation; how a flower bends towards light; rooting; aging

2. gibberellins – Promote cell elongation. Can activate dormant seeds and buds. When applied artificially, can increase the size of plants and fruit.

3. cytokinins – promote cell division

4. abscisic acid – the plant "stress hormone". Brings about closure of stoma when a plant is under the "stress" of too much heat and threat of loss of moisture. Also initiates and maintains seed and bud dormancy.

5. ethylene – Involved in fruit ripening and abscission (the dropping off of leaves and fruit). When applied externally as a gas, will ripen fruit within an area. (Put a cut apple in a paper bag with unripened fruit and they will ripen quickly.) Also – produced as a by-product of gasoline combustion. When there are high levels in the area, can stunt plant growth, wither existing plants.

Responses of Plants to Stimuli

Plants are "easy" – they can be led almost anywhere, given the proper conditions. Such directional stimuli are called tropisms and there are several associated with plants.

1. phototropism – a movement in response to a light stimulus

2. gravitropism – a movement in response to gravity

3. thigmotropism – a movement in response to touch (ex – twining of morning glory tendrils)

REFERENCES

1. ----, The Way Nature Works, Macmillan. 1998.
2. Capon, Brian, Botany for Gardeners. Timber Press. 1990.
3. Mader, Sylvia S., Biology, 6th Edition .McGraw-Hill. 1998.
4. Stern, Kingsley R., Introductory Plant Biology, 7th Edition. Wm. C. Brown Publishers. 1997.

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