Organelles that Process Energy

A cell uses energy to synthesize cell-specific materials that it can use for activities such as growth, reproduction, and movement. Energy is transformed from one form to another in mitochondria (found in all eukaryotic cells) and in chloro-plasts (found in eukaryotic cells that harvest energy from sunlight). In contrast, energy transformations in prokaryotic cells are associated with enzymes attached to the inner surface of the plasma membrane or extensions of the plasma membrane that protrude into the cytoplasm.

Mitochondria are energy transformers

In eukaryotic cells, the breakdown of fuel molecules such as glucose begins in the cytosol. The molecules that result from this partial degradation enter the mitochondria (singular, mitochondrion), whose primary function is to convert the potential chemical energy of those fuel molecules into a form that the cell can use: the energy-rich molecule ATP (adeno-sine triphosphate). The production of ATP in the mitochondria using fuel molecules and molecular oxygen (O2) is called cellular respiration.

Typical mitochondria are small—somewhat less than 1.5 ^m in diameter and 2-8 ^m in length—about the size of many bacteria. The number of mitochondria per cell ranges from one contorted giant in some unicellular protists to a few hundred thousand in large egg cells. An average human liver cell contains more than a thousand mitochondria. Cells that require the most chemical energy tend to have the most mitochondria per unit of volume.

Mitochondria have two membranes. The outer membrane is smooth and protective, and it offers little resistance to the movement of substances into and out of the mitochondrion. Immediately inside the outer membrane is an inner membrane, which folds inward in many places, giving it a much greater surface area than that of the outer membrane (Figure 4.14). These folds tend to be quite regular, giving rise to shelflike structures called cristae.

The inner mitochondrial membrane contains many large protein molecules that participate in cellular respiration. The inner membrane exerts much more control over what enters and leaves the mitochondrion than does the outer membrane. The region enclosed by the inner membrane is referred to as the mitochondrial matrix. In addition to many proteins, the matrix contains some ribosomes and DNA that are used to make some of the proteins needed for cellular respiration.

Mitochondria Lack Cristae

Matrix

Cristae

Inner

Outer

Matrix

Cristae

Inner

Outer

What Cristae

4.14 A Mitochondrion Converts Energy from Fuel Molecules into

ATP The electron micrograph is a two-dimensional slice through a three-dimensional organelle. As the drawing emphasizes, the cristae are extensions of the inner mitochondrial membrane.

4.14 A Mitochondrion Converts Energy from Fuel Molecules into

ATP The electron micrograph is a two-dimensional slice through a three-dimensional organelle. As the drawing emphasizes, the cristae are extensions of the inner mitochondrial membrane.

In Chapter 7 we will see how the different parts of the mitochondrion work together in cellular respiration.

Plastids photosynthesize or store materials

One class of organelles—the plastids—is produced only in plants and certain protists. There are several types of plas-tids, with different functions.

chloroplasts. Chloroplasts contain the green pigment chlorophyll and are the sites of photosynthesis (Figure 4.15). In photosynthesis, light energy is converted into the chemical energy of bonds between atoms. The molecules formed in photosynthesis provide food for the photosyn-thetic organisms, as well as for other organisms that eat them. Directly or indirectly, photosynthesis is the energy source for most of the living world.

Chloroplasts are variable in size and shape (Figure 4.16). Like a mitochondrion, a chloroplast is surrounded by two membranes. In addition, there is a series of internal membranes whose structure and arrangement vary from one group of photosynthetic organisms to another. Here we concentrate on the chloroplasts of the flowering plants. Even these chloroplasts show some variation, but the pattern shown in Figure 4.15 is typical.

The internal membranes of chloroplasts look like stacks of flat, hollow pita bread. These stacks, called grana (singular, granum), consist of a series of flat, closely packed, circular compartments called thylakoids. In addition to phos-pholipids and proteins, the membranes of the thylakoids

Granum Plant

Inner membrane

Outer membrane

Stroma

Thylakoid

Granum (stack of thylakoids)

Double membrane

Inner membrane

Outer membrane contain chlorophyll and other pigments that harvest light for photosynthesis. The thylakoids of one granum may be connected to those of other grana, making the interior of the chloroplast a highly developed network of membranes, much like the ER.

The fluid in which the grana are suspended is the stroma. Like the mitochondrial matrix, the chloroplast stroma contains ribosomes and DNA, which are used to synthesize some, but not all, of the proteins that make up the chloroplast.

Animal cells do not produce chloroplasts, but some do contain functional chloroplasts. These are either taken up as free chloroplasts derived from the partial digestion of green plants or contained within unicellular algae that live within the animal's tissues. The green color of some corals and sea anemones results from the chloroplasts in algae that live within those animals (Figure 4.16c). The animals derive some of their nutrition from the photosynthesis that their chloroplast-containing "guests" carry out. Such an intimate relationship between two different organisms is called symbiosis.

other types of plastids. Other types of plastids also store pigments or polysaccharides:

► Chromoplasts contain red, orange, and/or yellow pigments and give color to plant organs such as flowers (Figure 4.17a). The chromoplasts have no known chemical func

4.15 The Chloroplast:The Organelle That Feeds the World The electron micrograph shows a chloroplast from a leaf of corn. Chloroplasts are large compared with mitochondria and contain an extensive network of photosynthetic thylakoid membranes.

Stroma

Thylakoid

Granum (stack of thylakoids)

Double membrane

Figure Chloroplast Structure

ATP is used in converting CO2 to glucose in the stroma, the area outside the thylakoid membranes.

Thylakoid membranes are sites where light energy is harvested by the green pigment chlorophyll and converted into ATP.

ATP is used in converting CO2 to glucose in the stroma, the area outside the thylakoid membranes.

Chloroplasts

Leaf cell

Chloroplasts

Leaf cell

Spiral Chloroplasts

The chloroplasts in these single-celled green algae have assembled into spirals.

4.16 Being Green (a) In green plants,chloroplasts are concentrated in the leaf cells. (b) Green algae are photosynthetic and filled with chloroplasts. (c) No animal species produces its own chloroplasts, but in this symbiotic arrangement, unicellular green algae nourish a sea anemone.

The chloroplasts in these single-celled green algae have assembled into spirals.

4.16 Being Green (a) In green plants,chloroplasts are concentrated in the leaf cells. (b) Green algae are photosynthetic and filled with chloroplasts. (c) No animal species produces its own chloroplasts, but in this symbiotic arrangement, unicellular green algae nourish a sea anemone.

tion in the cell, but the colors they give to some petals and fruits probably encourage animals to visit flowers and thus aid in pollination, or to eat fruits and thus aid in seed dispersal. (On the other hand, carrot roots gain no apparent advantage from being orange.)

► Leucoplasts are storage depots for starch and fats (Figure 4.17b).

Endosymbiosis may explain the origin of mitochondria and chloroplasts

Although chloroplasts and mitochondria are about the size of prokaryotic cells and have the genetic material and protein synthesis machinery needed to make some of their own components, they are not independent of control by the nucleus. The vast majority of their proteins are encoded by nuclear DNA, made in the cytoplasm, and imported into the organelle. Observations of these organelles have led to the proposal that they originated by endosymbiosis—that is, that they were once independent prokaryotic organisms.

Chromoplasts

4.17 Chromoplasts and Leucoplasts (a) Colorful pigments stored in the chromoplasts of flowers like this bego nia may help attract pollinating insects. (b) Leucoplasts in the cells of a potato are filled with white starch grains.

4.17 Chromoplasts and Leucoplasts (a) Colorful pigments stored in the chromoplasts of flowers like this bego nia may help attract pollinating insects. (b) Leucoplasts in the cells of a potato are filled with white starch grains.

Leucoplast

^^ Leucoplast

Starch grains

^^ Leucoplast

Starch grains

Membrane of larger cell

Double membranes may have originated when one cell engulfed another.

Membrane of larger cell

Double membranes may have originated when one cell engulfed another.

Endosymbiotic Theory Steps

Membrane of smaller cell

Double membrane

4.18 The Endosymbiosis Theory Chloroplasts and mitochondria may be descended from a small prokaryote that was engulfed by another, larger prokaryote.

Membrane of smaller cell

Double membrane

4.18 The Endosymbiosis Theory Chloroplasts and mitochondria may be descended from a small prokaryote that was engulfed by another, larger prokaryote.

About 2 billion years ago, only prokaryotes inhabited Earth. Some of them absorbed their food directly from the environment. Others were photosynthetic. Still others fed on smaller prokaryotes by engulfing them (Figure 4.18).

Suppose that a small, photosynthetic prokaryote was ingested by a larger one, but was not digested. Instead, it somehow survived, trapped within a vesicle in the cytoplasm of the larger cell. The smaller, ingested prokaryote divided at about the same rate as the larger one, so successive generations of the larger cell also contained the offspring of the smaller one. This phenomenon, called endosymbiosis (endo-, "within"; symbiosis, "living together"), exists today, as in the case of the algae that live within sea anemones (see Figure 4.16c).

According to this scenario, endosymbiosis provided benefits for both partners: The larger cell obtained the photo-synthetic products from the smaller cell, and the smaller cell was protected by the larger one. Over evolutionary time, the smaller cell gradually lost much of its DNA to the nucleus of the larger cell, resulting in the modern chloroplast.

Much circumstantial evidence favors the endosymbiosis theory:

► On an evolutionary time scale of millions of years, there is evidence for DNA moving between organelles in the cell.

► There are many biochemical similarities between chloro-plasts and modern photosynthetic bacteria.

► DNA sequencing shows strong similarities between modern chloroplast DNA and that of a photosynthetic prokaryote.

► The double membrane that encloses mitochondria and chloroplasts could have arisen through endosymbiosis. The outer membrane may have come from the engulfing cell's plasma membrane and the inner membrane from the engulfed cell's plasma membrane.

Similar evidence and arguments also support the proposition that mitochondria are the descendants of respiring prokary-

otes engulfed by larger prokaryotes. The benefit of this en-dosymbiotic relationship might have been the capacity of the engulfed prokaryote to detoxify molecular oxygen (O2), which was increasing in Earth's atmosphere as a result of photosynthesis.

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  • basso
    Are plasma membranes organelles in plants?
    8 years ago
  • daniel
    Why is the endosymbiotic theory a theory circumstantial evidence?
    8 years ago
  • HABTE
    Which organelle is contained in the plasma B cell?
    8 years ago
  • Odo Lightfoot
    What organelle converts one form of energy into another?
    8 years ago
  • jago
    How to make flowers made of white tissue explain with steps?
    8 years ago
  • OLIVIER
    What is the difference of thylakoid to granum?
    7 years ago
  • tesfay
    What is the organelle that processes energy?
    7 years ago
  • sebastian
    Which organelle or structure is generated by the plasma membrane and contains molecules?
    7 years ago
  • rudigar
    What color is the outside of a chloroplast?
    7 years ago
  • casey
    Which organelles process energy?
    7 years ago
  • yvonne
    Which organelle is NOT bound by a membrane ribosomes or chloroplasts?
    7 years ago
  • fnan
    Which organelle converts protein to energy?
    7 years ago
  • GLEN
    Is the relationship between the mitochondria in the plasma membrane?
    7 years ago
  • zewdi
    Which organelle contains produces plasma membranes for cells?
    6 years ago
  • martha yonas
    What organelle does the cellular process that uses oxygen to make energy?
    6 years ago
  • bladud
    What two organelles process engery?
    6 years ago
  • Fre-weini Aziz
    What two organelles process engergy?
    4 years ago
  • liviana
    What organelles are energy transformers?
    3 years ago
  • Noah
    What organelles are energy trasformers?
    3 years ago
  • Harold Hayes
    What distinct organelles perform energy transformation?
    1 year ago
  • kevin
    Which organells are used for the harvesting of energy from the sun?
    4 months ago

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