SAT Biology E/M Subject Test

Part II: Subject Review

Chapter 12 Plants

All the information about plant structure and plant function could fill an entire textbook. Fortunately, this chapter will help you focus in on the material you really need to know for the SAT Biology E/M Subject Test. We will concentrate on leaf structure, photosynthesis, transport, flower structure, and plant reproduction.

LEAF STRUCTURE

Generally speaking, photosynthesis involves the use of solar energy (sunlight) to produce food (carbohydrates). It makes sense then that photosynthesis would occur where sunlight is most likely to strike the plant: the leaf. The leaves of a plant provide the plant with a large surface area for sunlight to hit. Here’s a cross section of a plant leaf:

The outer cell layer of a leaf, both top and bottom, is called epidermis (just as it is in skin). The epidermis has a layer of wax on it called the cuticle. The job of the cuticle is (1) to protect the leaf from attack by things like fungi and (2) to keep water from escaping the leaf. The cuticle is always found on the top surface of a leaf and is often found on the bottom surface as well.

The palisade layer, which lies just under the surface, is where most photosynthesis takes place. The spongy cells beneath the palisade layer also carry out photosynthesis, but this layer is more important for gas exchange. That’s why there are so many air pockets in the spongy layer.


Photosynthesis requires the intake of carbon dioxide and the release of oxygen. The exchange of these gases, as well as water, occurs through openings on the bottom surface of the leaf, called stomates. Stomates are opened and closed by special cells called guard cells.


Photosynthesis

Before you start this section on photosynthesis, you should go back and review the information we gave you on cellular respiration in Chapter 5. Many of the ideas and concepts for the two processes are essentially the same, and if you understand cellular respiration, you will find the concepts of photosynthesis easier. Let’s start by talking about where the specific reactions of photosynthesis take place. The organelle in which these reactions occur is the chloroplast.

The chloroplast is a double-membraned organelle, just like the mitochondria. Here’s a look:

The inner fluid of the chloroplast is the stroma. Inside the stroma are flattened membrane structures called thylakoids. The inner space of the thylakoid is simply called the thylakoid space, and thylakoids themselves are found in stacks called grana. The membranes of the thylakoids are filled with green pigments called chlorophyll. This is what gives plants their green color.

Basic Reactions of Photosynthesis

In cellular respiration, we basically took some glucose, added some oxygen, and produced energy (ATP). Carbon dioxide and water were released as waste products. Well, photosynthesis is essentially the reverse of cellular respiration. Plants take carbon dioxide, water, and energy and use these substances to produce glucose, and oxygen is released as a waste product.


6 CO2 + 6 H2O + energy → C6H12O6 + 6 O2


Even though the overall reaction for photosynthesis looks like the reverse of cellular respiration, the reactions that make up the process are not just those of respiration in reverse. In other words, we can’t run the electron transport chain, the Krebs cycle, the pyruvate dehydrogenase complex (PDC), and glycolysis backward to make glucose. The reactions of photosynthesis are a little different. They’re split into two main groups: the light-dependent reactions and the light-independent reactions.

The Light-Dependent Reactions

These reactions get their name from the fact that they require light—solar energy. (Sometimes they’re just called the light reactions, but light-dependent is more accurate.) The whole point to the light-dependent reactions is to convert solar energy into a usable form of energy—namely ATP and a reduced electron carrier. (Remember from Chapter 5 that reduced electron carriers, which are electron carriers that are actually carrying electrons, are sources of energy.) The light-dependent reactions occur in the membranes of the thylakoids. Remember we said there was a lot of green pigment called chlorophyll there? The chlorophyll absorbs sunlight, and this causes some of its electrons to be excited.

Excited?!

When we say the electrons become “excited,” we just mean that they now have more energy than they did before the chlorophyll absorbed sunlight. These energetic electrons are now able to pass down an electron transport chain, just as the electrons did in cellular respiration. At the end of the chain, the electrons are handed off to NADP+, forming a molecule of NADPH. NADP+ is just an electron carrier, and NADPH is its reduced form. You should think of NADPH as usable energy.

The other thing that happens when chlorophyll absorbs sunlight is that a molecule of water is split into hydrogen ions and oxygen. The oxygen is released as a waste product. The hydrogen ions are used to make ATP through an ATP synthase, just as they did in cellular respiration. The ATP synthase is located in the membranes of the thylakoids, and as H+ ions exit the thylakoids, ATP is made. The ATP is, of course, usable energy.


Light-Dependent Reactions

Here’s a quick summary of what we’ve learned so far:

•   The light-dependent reactions convert solar energy into usable energy—NADPH and ATP.

•   These reactions occur along the thylakoid membranes.

•   The substance that absorbs sunlight is the green pigment, chlorophyll.

•   This is the part of photosynthesis in which oxygen is released.


The Light-Independent Reactions (AKA the Calvin Cycle)

The light-independent reactions are sometimes called the dark reactions, but this is a bad name, because it makes it sound like the reactions must occur in the dark. They don’t have to occur in the dark; they just don’t need light in order to occur.

Calvin Cycle

It is important to realize 
that the Calvin cycle only 
depends on light indirectly. 
The Calvin cycle is 
dependent on the input 
supplied by ATP
and NADPH. These are
the products of 
light-dependent reactions.


The light-independent reactions take place in the stroma of the chloroplast and are also known as the Calvin cycle.


If this reminds you of the Krebs cycle, it should. The basic concept is the same: We start with a certain molecule that’s regenerated each time the cycle takes a turn. In this case, the molecule is ribulose bisphosphate. Ribulose bisphosphate is made of five carbon atoms. A molecule of carbon dioxide is added to this, to make a six-carbon molecule that, in a series of reactions, is broken in half to form two three-carbon molecules called glyceraldehyde-3-phosphate (G3P). In each turn of the cycle, one of the G3P molecules formed in this pathway can be used to make glucose and other carbohydrates, and the other G3P molecules are used to regenerate the original ribulose bisphosphate.

The light-independent reactions require energy, and they use the ATP and NADPH produced by the light-dependent reactions. This process (the Calvin cycle), which is used by plants to produce carbohydrates, is also known as carbon fixation.


Light-Independent Reactions

•   The light-independent reactions are sometimes called the “dark reactions,” even though they do not have to occur in the dark.

•   They are also known as the Calvin cycle, which is also known as carbon fixation.

•   These reactions take place in the stroma of the chloroplast.

•   They rely on the energy (ATP and NADPH) from the light-dependent reactions in order to run.

•   Their product is a three-carbon carbohydrate called glyceraldehyde- 3-phosphate (G3P), which can be used to form glucose and other carbohydrate.


One Last Thing

Organisms that can make (and store) glucose in this way (plants) are essentially making (and storing) their own food. Troph is a suffix meaning “related to nutrition,” so plants are referred to as autotrophs (auto means “self”); they are “self-feeders.”

Organisms that cannot make their own food are called heterotrophs. Heterotrophs rely on the consumption of plants and other animals for nutrition. Humans and other animals are all heterotrophs.

Quick Quiz #1

Fill in the blanks and check the appropriate boxes:

  1. The inner fluid of the chloroplast is called [  grana  stroma ].

  2. The layer of wax on the leaf surface is called the _________________________.

  3. Most photosynthesis occurs in the [  palisade layer  spongy layer].

  4. The Calvin cycle is a series of [  light-dependent  light-independent ] reactions.

  5. Oxygen is released during the [  light-dependent  light-independent ] reactions.

  6. The light-dependent reactions convert solar energy to usable energy, namely ATP and _________________________.

  7. The ______________________________ are the membranes inside the chloroplast where the light-dependent reactions take place.

  8. Plants are [  autotrophs  heterotrophs ] and [  can  cannot ] make their own food.

  9. Stomates are opened and closed by cells called ___________________________________.

10. Forming carbohydrate from carbon dioxide is also known as _________________________.

Correct answers can be found in Chapter 15.

Transport Within the Plant

Plants, like animals, have a need to transport materials. Water and minerals must be transported through the roots to the rest of the plant. Nutrients from photosynthesis must be transported from the palisade layers of the leaves to the rest of the plant. Water and nutrients are carried through vessels called veins, but they are carried by specific tissues within those veins, called xylem and phloem.

Let’s Start at the Bottom

Roots anchor a plant to the ground and root hairs that grow at the tip of the roots increase the surface area for absorption of water and minerals. The water and minerals are transported through the xylem upward to the rest of the plant. Xylem tissue is made of several types of cells. The specific cells that transport water are called tracheids and vessel elements.

Which One?

An easy way to remember 
which type of tissue 
carries food and which
carries water is to 
remember that the ph 
sound is like the f sound, 
so phloem carries food.
Xylem, then, must 
carry water.

Food from photosynthesis (carried out in the palisade layers of the leaves) is transported through the plant in tissue called phloem. Phloem contains two types of cells: sieve cells and companion cells. Sieve cells are the ones that actually carry out the transport, and companion cells help the sieve cells with their metabolic functions.

Flowers and Reproduction

Flowering plants are called angiosperms, and for the test you should know something about how they reproduce. Here’s a typical flower and its parts:

The stamen.  The stamen is the plant’s male component. It consists of the anther and the filament. The filament supports the anther, which makes pollen. Pollen is made from little cells called microspores, and mature pollen grains contain a cell that can divide to form two sperm cells.

The pistil.  The pistil is the plant’s female component. It consists of the stigmastyleovule, and ovary. Inside the ovary is the ovule, which forms cells called megaspores. Megaspores can divide to form eggs and polar bodies.


Plant Reproduction

Pollen falls on stigma

Pollen germinates

Pollen tube grow

Two sperm enter the ovule

Egg is fertilized and becomes embryo

Polar bodies are fertilized and become endosperm

The entire ovule becomes a seed and the ovary becomes a fruit


Here’s how a flowering plant reproduces:

1.   Some pollen grains fall onto the stigma, which is sticky. There are many ways this can happen. They can be blown there, fall there, be moved there by insects, etc. Once on the stigma, the pollen grains germinate.

2.   During germination, a tube called the pollen tube grows down through the style to connect to the ovary.

3.   The two sperm (from the pollen) travel down the pollen tube to enter the ovary and the ovule, where they undergo a double fertilization; one sperm fertilizes the egg, and the other sperm combines with the polar bodies.

4.   The fertilized egg becomes the plant embryo and the polar bodies become endosperm. Endosperm is a food-storing tissue that surrounds the plant embryo.

5.   The entire ovule, which contains the embryo and the endosperm, develops into a seed, and the ovary develops into a fruit. The fruit protects the seed and helps it disperse by wind or animals.

6.   The seed is released (the fruit drops, the plant is eaten, etc.), and, when it finds a suitable environment, it develops into a new plant.

Quick Quiz #2

Fill in the blanks and check the appropriate boxes:

  1. The female part of a flower is the [  stamen  pistil ].

  2. The tissue within plant veins that carries water from the roots up to the rest of the plant is the [  phloem  xylem ].

  3. The specific cells that transport water are the ____________________ and the ___________________.

  4. The ovary develops into a [  fruit  seed ].

  5. The polar bodies, when fertilized, become a nutrient-rich tissue called __________________.

  6. _________________________ is the type of tissue that carries food from photosynthesis in the leaves to the rest of the plant.

  7. The specific cells that transport food are the [  companion cells  sieve cells ].

  8. The anther and the filament are [  male  female ] parts of the flower.

  9. In a flowering plant, pollen is produced by and located on the [  stigma  anther ].

10. The [  seed  fruit ] develops from the ovule.

11. A pollen tube grows down through the [  stigma  style ].

Correct answers can be found in Chapter 15.

Key Words

photosynthesis

epidermis

cuticle

palisade layer

spongy cells

stomates

guard cells

chloroplast

stroma

thylakoids

thylakoid space

grana

chlorophyll

light-dependent reactions

light-independent reactions

light reactions

dark reactions

Calvin cycle

ribulose bisphosphate

gylceraldehyde-3-phosphate (G3P)

carbon fixation

autotrophs

heterotrophs

xylem

phloem

tracheids

vessel elements

sieve cells

companion cells

stamen

anther

filament

pollen

microspores

pistil

stigma

style

ovule

ovary

megaspores

Summary

•   Photosynthesis is a process in which plants use energy from the sun to produce food.

•   Photosynthesis occurs in the part of the plant that sunlight is most likely to hit—the leaves.

•   Photosynthetic reactions take place in the chloroplasts of a plant cell.

•   Plants take in carbon dioxide, water, and energy and use them to produce glucose and release oxygen.

•   The summary equation for photosynthesis:
6 CO2 + 6 H2O + energy → C6H12O6 + 6 O2

•   There are two types of reactions in the process of photosynthesis: light-dependent reactions and light-independent reactions.

•   Minerals, water, and nutrients are transported within a plant by xylem and phloem.

•   A flowering plant reproduces in the following way: Pollen falls on the stigma, the pollen then germinates, pollen tubes grow, two sperm enter the ovule, the egg is fertilized and becomes an embryo, the polar bodies are fertilized and become endosperm, and eventually the entire ovule becomes a seed and the ovary becomes a fruit.