Genetics & Physiology: Plant Growth

Lesson Objective

In this lesson we will review plant growth, covering fertilization, cell division, secondary growth, and, finally, photosynthesis.

Previously we covered…

We looked at some of the important aspects of plant biology.

Beginning and Primary Growth

A plant begins life after double fertilization occurs within the flower’s ovary. As cells divide by mitosis, an embryo and endosperm are formed. Then the layers of tissue surrounding the embryo and endosperm toughen and become impermeable to water and oxygen. This causes the embryo to stop growing and start forming a mature seed.

At some point, the plant embryo will begin growing again. This is called germination. Before germination can occur at all, water and oxygen must penetrate the seed coat. This results in the breaking of the seed coat. Many plants require heat or cold to germinate; still others require exposure to light. The first visible evidence of this process is the emergence of the embryonic root. Then the shoot will elongate and emerge from the soil. The leaves will then push through, unfurl, and begin to photosynthesize. Eventually the food supply stored in the cotyledons is used up, causing them to shrivel up and fall off.

Plants grow by cell division that occurs in meristems located at the tips of the shoots and roots. This type of growth is called primary growth. It occurs in apical meristems, which are located at the tips of stems and roots, just behind the root cap. Primary growth also pushes roots through the soil. The root tip is covered by a root cap. Growth in length is concentrated near the root tip where meristems give rise to primary tissues:

• Protoderm gives rise to the epidermis.
• Procambium gives rise to the stele, which is where the xylem and phloem develop.
• Round meristems give rise to the ground tissue system; ground tissue fills the region between the stele and the epidermis; and ground tissues store food.

Question

The first sign of germination of a bean seed is the emergence of the embryo’s

1. hooked shoot
2. protected shoot
3. root
4. cotyledons

Reveal Answer

The correct answer is C. The embryonic root emerges from the seed first and is called the radical.

Secondary Growth

Most plants undergo secondary growth, in which the plant increases in width as well as length. The tissues that result from this growth are secondary tissues. The effects of secondary growth are most dramatic in woody plants. Two lateral meristems function in this process: vascular cambium and cork cambium. Cell division in the lateral meristems adds layers of new cells around the outside of the plant’s body.

The vascular cambium produces xylem (wood) and phloem. This layer is just beneath the bark. Xylem is produced in the interior of the stem and phloem is produced in the exterior. The new ring of xylem tissue formed every growing season and the older xylem become the wood of trees. Older xylem layers are dead, yet can still conduct water for several years. The phloem layer is never as thick as the xylem layer. Phloem tissue forms the inner half of tree bark.

The cork cambium lies within the bark of a woody stem. It produces the cork cells of the outer bark. Cork tissue functions as a barrier that helps protect the stem from physical damage and pathogens. The layers of cork tissue and the cork cambium make up the periderm, which is the protective coat of the secondary plant body that replaces the epidermis of the primary plant body.

The lateral meristems also develop and produce secondary growth in the roots. The vascular cambium forms within the stele and produces xylem and phloem. As the stele grows, the epidermis is split and shed. A cork cambium forms from the pericycle of the stele and produces periderm, which will become the secondary dermal tissue.

Question

Cell division that results in the increased width of a tree occurs in the

1. lateral meristems.
2. apical meristems.
3. primary tissues.
4. sieve tube vessels.

Reveal Answer

The correct answer is A. Cell division in the lateral meristems occurs in secondary growth which increases the length and width of a tree.

Growth Hormones

Hormones control the division, growth, maturation, and differentiation of plant cells. They are manufactured primarily in apical meristems, in young leaves, and in growing seeds and developing fruits. Plant hormones control a plant’s branching pattern, the rate at which its stems elongate, and its responses to environmental conditions.

Auxin in the stems causes the stem to grow toward light and away from the pull of gravity. High concentrations of auxin stimulate young stem cells to elongate. Auxin in the roots causes the roots to grow away from light and toward the pull of gravity. High concentrations of auxin inhibit root cell elongation. Auxin also can cause cell division in meristematic regions to stop or start. Finally, the high concentration of auxin produced by the apical meristem inhibits the growth of lateral buds near the tip of the shoot.

Cytokinins are manufactured by the growing roots. They stimulate cell division and cause dormant seeds to sprout. Several of the effects produced by cytokinins are the opposite of effects produced by auxins.

Gibberellin is manufactured by all higher plants. It stimulates growth in leaves and stems, but not in the roots. In stems, gibberellin stimulates cell elongation and cell division. It can also signal seeds to break from dormancy and begin germination.

Question

A chemical that directs the growth of a plant is called a (an)

1. apical meristem.
2. hormone.
3. androstrone.
4. citric acid.

Reveal Answer

The correct answer is B. All the other choices are not plant growth chemicals.

Photosynthesis

Plants convert the energy of sunlight into the energy in the chemical bonds of carbohydrates (sugars and starches) in a process called photosynthesis. The overall process can be summarized by the following chemical equation:

CO2 + H2O + light                        C6H12O6 + O2

Photosynthesis occurs primarily within leaves. Cells in the leaves contain chloroplasts, which contain chlorophyll. Chlorophyll is the light absorbing substance needed for photosynthesis.

Photosynthesis requires:

• Sunlight
• Pigments
• Carbon dioxide
• Water
• Energy storing compounds

The stages of photosynthesis are:

• Energy is captured from sunlight.
• Light energy is converted to chemical energy.
• Chemical energy powers the synthesis of organic molecules, using carbon from carbon dioxide.

Question

Photosynthesis produces

1. carbon dioxide.
2. water.
3. alcohol.
4. glucose.

Reveal Answer

The correct answer is D, because the products of photosynthesis are glucose and oxygen.

Light Reactions

The process of photosynthesis is divided into two parts: light reactions and dark reactions. The light reactions require light. In these reactions, the energy of sunlight is captured and used to make energy storing compounds. The light reactions occur in the photosynthetic membranes of chloroplasts. The light reactions can be divided into four basic processes: light absorption, electron transport, oxygen production, and ATP formation. These processes are closely linked and dependent on each other.

• Light Absorption
  The photosynthetic membrane contains clusters of pigment, or photosystems. There are two photosystems in green plants. Each one contains hundreds of chlorophyll molecules, as well as other pigments. The other pigments absorb light in the regions of the spectrum that chlorophyll does not. This causes electrons in pigment molecules to be raised to an excited state. Only a special pair of chlorophyll molecules can process the light energy.
• Electron Transport
  The high-energy electrons produced in the previous process travel from a special pair of chlorophyll molecules to an adjacent molecule called an electron carrier. This begins electron transport. The high-energy electrons are transferred along a series of electron-carrier molecules in the photosynthetic membrane. At the end of the chain, an enzyme passes the high-energy electrons to an electron carrier called NADP+, which is converted into NADPH. Adding a pair of electrons to this electron carrier requires energy. The energy and electrons stored in the bonds of NADPH will be used later on in the overall process.
• Oxygen Production
  As the light continues to shine, the photosynthetic membrane contains a system that provides new electrons to chlorophyll to replace the ones that converted to NADPH. These electrons are taken from water. Four electrons are removed from two water molecules, leaving four hydrogen ions and two oxygen atoms. The two oxygen atoms combine to form oxygen gas that is eventually released into the air.
• ATP Formation
  The hydrogen ions left behind when water is “split” are released into the photosynthetic membrane. In addition, more hydrogen ions are pumped across the membrane as electrons are passed from chlorophyll to NADP+. Eventually, an enzyme attaches a phosphate molecule to ADP, forming ATP. This is the second way in which the energy of sunlight is trapped in chemical form. These energy-storing molecules will be converted to a more convenient form during the dark reactions.

Question

The oxygen that results from photosynthesis comes directly from the

1. splitting of carbon dioxide molecules by the Calvin cycle.
2. splitting of water molecules to provide electrons for photosystem II.
3. action of proton pumps in the electron transport chain.
4. absorption of photons by carotenoids in the photosystems.

Reveal Answer

B is correct. After the water molecules are split, the resulting 2 oxygen atoms combine to form oxygen gas.

Dark Reactions

The dark reactions generally take place in sunlight; however, light does not play a role in the dark reactions. In this part of photosynthesis, carbon dioxide is used to make a complex organic molecule. The dark reactions form a cycle or circular series of reactions. The dark reactions are also known as the Calvin cycle.

The Calvin cycle employs a complex battery of enzymes that are found in the stroma of a chloroplast. Photosynthesis itself usually occurs in the thylakoid membrane of a chloroplast. A total of six carbon dioxide molecules must enter the Calvin cycle to produce one six-carbon sugar molecule. The Calvin cycle takes place in five steps.

• The cycle begins when a carbon atom from a CO2 molecule is added to a five-carbon molecule (the starting material).
• The resulting six-carbon molecule is unstable and immediately splits, forming two three-carbon molecules. These first two steps tend to be relatively slow. In order to compensate, the chloroplast has an enzyme that catalyzes the reaction.
• Then energy in the form of a phosphate group from ATP is added to the three-carbon molecules, and they are reduced by the addition of hydrogen from NADPH.
• The reduced three-carbon molecules may combine to make glucose, a six-carbon sugar, or may be used to make other organic molecules.
• Most of the reduced three-carbon molecules are used to regenerate the five-carbon starting material, thus completing the cycle.

Question

Which of these are produced by the dark reactions?

1. Oxygen
2. Water
3. ATP
4. Glucose

Reveal Answer

D is the correct answer because the final end product of the dark reaction is glucose.

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