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Biology > Ch. 6 Mastering Biology > Flashcards

Ch. 6 Mastering Biology Flashcards

Glycolysis, acetyl CoA, citric acid cycle, and electron transport chain are the sequence of steps as what happens during cellular respiration?

Energy is extracted from glucose.

What is the correct general equation for cellular respiration?

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ATP.

What process takes place in the cytosol of a eukaryotic cell?

Glycolysis.

In what organelle do we find acetyl CoA formation, the citric acid cycle, and the electron transport chain?

Mitochondrion.

What does glycolysis do?

Splits glucose in half and produces 2 ATPs for each glucose.

What does the citric acid cycle do?

Produces some ATP and carbon dioxide in the mitochondrion.

What does the electron transport chain do?

Uses energy captured from electrons flowing to oxygen to produce most of the ATPs in cellular respiration.

How do cells capture the energy released by cellular respiration?

They produce ATP.

What are the by-products of cellular respiration?

Carbon dioxide and water.

What happens as electrons move through the mitochondrial space?

The pH of the intermembrane space decreases.

What does cellular respiration primarily do?

Makes ATP to power the cell’s activities.

What is the transfer of electrons from one molecule to another?

An oxidation-reduction reaction, or redox reaction.

In cellular respiration, what happens to glucose as it loses electrons (in hydrogen atoms)?

It becomes oxidized to carbon dioxide.

In cellular respiration, what happens to oxygen as it gains electrons (in hydrogen atoms) that came from glucose?

It becomes reduced to water.

In cellular respiration, organic molecules become oxidized as what happens?

NAD+ picks up electrons from H+ and becomes reduced to NADH.

Where does NADH deliver electrons?

To an electron transport chain.

What does an electron transport chain do?

Passes electrons through carrier molecules in a series of redox reactions to the final electron acceptor, oxygen.

What does the cell use to make ATP?

The energy released from the redox reactions in the electron transport chain.

What happens to a molecule that functions as the electron donor in a redox reaction?

It loses electrons and becomes oxidized.

In cellular respiration, glucose ___ electrons, whereas oxygen ___ electrons.

Loses … gains.

During cellular respiration, what happens to the energy in glucose?

It is carried by electrons.

What happens to NADH during cellular respiration?

It delivers its electron load to the first electron carrier molecule.

What is essentially a series of redox reactions that conclude cellular respiration?

The electron transport chain.

What happens during the electron transport chain (redox reactions)?

NADH is oxidized, which reduces an electron accept in the electron transport chain.

What is the final acceptor of cellular respiration?

Why can’t oxidative phosphorylation occur without glycolysis and the citric acid cycle?

The two stages supply the electrons needed for the electron transport chain.

What is oxidized and what is reduced in cellular respiration?

Glucose is oxidized and oxygen is reduced.

How many NADH are produced by glycolysis?

In glycolysis, how are ATP molecules produced?

Substrate-level phosphorylation.

What is not a product of glycolysis?

What are the products of glycolysis?

Water, ATP, pyruvate, and NADH + H+.

In glycolysis, what starts the process of glucose breakdown?

In glycolysis, there is a net gain of how many ATP?

How many NADH molecules are produced during glycolysis?

What is an end product of glycolysis?

Where do the reactions of the citric acid cycle occur in eukaryotic cells?

The mitochondrion.

What is the main process that produces glucose and oxygen gas?

Photosynthesis.

Which parts of cellular respiration require oxygen gas?

The citric acid cycle and the electron transport chain.

What are the most important outputs of glycolysis?

Two pyruvic acid and two NADH molecules.

Unlike the citric acid cycle and electron transport, where does glycolysis occur?

In the cytoplasm.

Because glycolysis is the multi-step breakdown of glucose, what play an important role in this process?

What forms at the end of glycolysis?

What is the process in which glucose is converted to pyruvate?

What is the process where CO2 and H2O convert into organic compounds using energy from light?

In the absence of oxygen, cells need a way to regenerate which compound?

Why is fat the most efficient molecule for long-term energy storage, even compared to carbohydrates?

Fats provide an abundant source of high-energy electrons with their numerous hydrogen atoms.

When a cell uses fatty acids for aerobic respiration, what does it do?

It first hydrolyzes fats to glycerol and fatty acids.

When a muscle cell is deprived of molecular oxgey, it will convert glucose to lactic acid to do what?

Recycle NADH through fermentation.

Why can yeasts produce ATP by either fermentation or oxidative phosphorylation?

They are facultative anaerobes.

What process produces the most ATP per molecule of glucose oxidized?

Aerobic respiration.

During fermentation, ___ that was produced during glycolysis is converted back to ___.

NADH … NAD+.

Each turn of the citric acid cycle generates one ATP and four additional energy rich molecules. What are they?

3 NADH and 1 FADH2.

What are the end products of the citric acid cycle?

ATP, FADH2, and CO2.

What is not an end product of the citric acid cycle?

At the end of the citric acid cycle, where is most of the energy remaining from the original glucose stored?

Where are most NADH molecules generated during cellular respiration?

The citric acid cycle.

In preparing pyruvate to enter the citric acid cycle, what happens?

A compound called coenzyme A binds to a two-carbon fragment.

In eukaryotes, what happens to most of the high-energy electrons released from glucose by cell respiration?

They reduce NAD+ to NADH, which then delivers them to the electron transport chain.

Why is the citric acid cycle called a cycle?

Acetyl CoA binds to oxaloacetate and this compound is restored at the end of the cycle.

What is the role of oxygen in cellular respiration?

It accepts high-energy electrons after they are stripped from glucose.

The breakdown of glucose is complete by the end of what process?

Most ATP molecules are produced during what process?

Electron transport.

Electron transport produces how many ATP molecules per NADH molecule?

Electron transport produces how many ATP molecules per FADH2 molecule?

The energy released from electron transport is used to do what?

Transport protons into the intermembrane space of the mitochondria, where they become concentrated.

What happens to protons after they become concentrated in the intermembrane space of the mitochondria?

They flow back out into the inner compartment (matrix) of the mitochondria.

What happens as protons exit into the matrix of the mitochondria?

They turn ATP synthase turbines and produce ATP.

For each glucose that enters glycolysis, how many NADH enter the electron transport chain?

In cellular respiration, most ATP molecules are produced by what process?

Oxidative phosphorylation.

What is the final electron acceptor of cellular respiration?

During electron transport, energy from ___ is used to pump hydrogen ions into the ___.

NADH and FADH2 … intermembrane space.

What is the proximate (immediate) source of energy for oxidative phosphorylation?

Kinetic energy that is released as hydrogen ions diffuse down their concentration gradient.

What is the energy production per glucose molecule through the citric acid cycle?

2 ATP, 6 NADH, 2 FADH2.

In oxidative phosphorylation, electrons are passed from one electron carrier to another. The energy released is used to do what?

Pump protons (H+) across the mitochondrial membrane.

The electron transport chain is a series of electron carrier molecules. Where can this be found in eukaryotes?

Mitochondria.

What is the ultimate fate of the electrons that are stripped from glucose during cellular respiration?

They are used to form water.

The majority of the energy the cell derives from glucose is found where?

In NADH and FADH2.

What happens to the energy that is released by electrons as they move through the electron transport chain?

It pumps H+ through a membrane.

In cellular respiration, what is the result of electrons moving through the electron transport chain (or its components)?

A proton gradient is formed.

A single glucose molecule produces about 38 molecules of ATP through the process of cellular respiration. However, this only represents approximately 38% of the chemical energy present in this molecule. What happens to the rest of the energy from glucose?

It is converted to heat.

What is the overall efficiency of respiration?

Approximately 40%.

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Chapter 06 - a tour of the cell.

Chapter 6 A Tour of the Cell Lecture Outline

Overview: The Importance of Cells

• Many organisms are single-celled.
• Even in multicellular organisms, the cell is the basic unit of structure and function.
• The cell is the simplest collection of matter that can live.
• All cells are related by their descent from earlier cells.

Concept 6.1 To study cells, biologists use microscopes and the tools of biochemistry

• The discovery and early study of cells progressed with the invention of microscopes in 1590 and their improvement in the 17th century.
• The lenses refract light such that the image is magnified into the eye or onto a video screen.
• Magnification is the ratio of an object’s image to its real size.
• It is the minimum distance two points can be separated and still be distinguished as two separate points.
• Resolution is limited by the shortest wavelength of the radiation used for imaging.
• The minimum resolution of a light microscope is about 200 nanometers (nm), the size of a small bacterium.
• At higher magnifications, the image blurs.
• Techniques developed in the 20th century have enhanced contrast and enabled particular cell components to be stained or labeled so they stand out.
• While a light microscope can resolve individual cells, it cannot resolve much of the internal anatomy, especially the organelles.
• Because resolution is inversely related to wavelength used, electron microscopes (whose electron beams have shorter wavelengths than visible light) have finer resolution.
• Theoretically, the resolution of a modern EM could reach 0.002 nanometer (nm), but the practical limit is closer to about 2 nm.
• A TEM aims an electron beam through a thin section of the specimen.
• The image is focused and magnified by electromagnets.
• To enhance contrast, the thin sections are stained with atoms of heavy metals.
• The sample surface is covered with a thin film of gold.
• The beam excites electrons on the surface of the sample.
• These secondary electrons are collected and focused on a screen.
• The result is an image of the topography of the specimen.
• The SEM has great depth of field, resulting in an image that seems three-dimensional.
• However, electron microscopes can only be used on dead cells.
• Light microscopes do not have as high a resolution, but they can be used to study live cells.
• Microscopes are major tools in cytology, the study of cell structures.
• Cytology combined with biochemistry, the study of molecules and chemical processes in metabolism, to produce modern cell biology.

Cell biologists can isolate organelles to study their functions.

• The goal of cell fractionation is to separate the major organelles of the cells so their individual functions can be studied.
• This process is driven by an ultracentrifuge, a machine that can spin at up to 130,000 revolutions per minute and apply forces of more than 1 million times gravity (1,000,000 g).
• Fractionation begins with homogenization, gently disrupting the cell.
• As the process is repeated at higher speeds and for longer durations, smaller and smaller organelles can be collected in subsequent pellets.
• Cell fractionation prepares isolates of specific cell components.
• For example, one cellular fraction was enriched in enzymes that function in cellular respiration.
• Electron microscopy revealed that this fraction is rich in mitochondria.
• This evidence helped cell biologists determine that mitochondria are the site of cellular respiration.
• Cytology and biochemistry complement each other in correlating cellular structure and function.

Concept 6.2 Eukaryotic cells have internal membranes that compartmentalize their functions

Prokaryotic and eukaryotic cells differ in size and complexity.

• All cells are surrounded by a plasma membrane.
• The semifluid substance within the membrane is the cytosol, containing the organelles.
• All cells contain chromosomes that have genes in the form of DNA.
• All cells also have ribosomes, tiny organelles that make proteins using the instructions contained in genes.
• A major difference between prokaryotic and eukaryotic cells is the location of chromosomes.
• In a eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus.
• In a prokaryotic cell, the DNA is concentrated in the nucleoid without a membrane separating it from the rest of the cell.
• In eukaryote cells, the chromosomes are contained within a membranous nuclear envelope.
• All the material within the plasma membrane of a prokaryotic cell is cytoplasm.
• These membrane-bound organelles are absent in prokaryotes.
• Eukaryotic cells are generally much bigger than prokaryotic cells.
• At the lower limit, the smallest bacteria, mycoplasmas, are between 0.1 to 1.0 micron.
• Most bacteria are 1–10 microns in diameter.
• Eukaryotic cells are typically 10–100 microns in diameter.
• Metabolic requirements also set an upper limit to the size of a single cell.
• Smaller objects have a greater ratio of surface area to volume.
• The plasma membrane functions as a selective barrier that allows the passage of oxygen, nutrients, and wastes for the whole volume of the cell.
• The volume of cytoplasm determines the need for this exchange.
• Rates of chemical exchange across the plasma membrane may be inadequate to maintain a cell with a very large cytoplasm.
• The need for a surface sufficiently large to accommodate the volume explains the microscopic size of most cells.
• Larger organisms do not generally have larger cells than smaller organisms—simply more cells.
• Cells that exchange a lot of material with their surroundings, such as intestinal cells, may have long, thin projections from the cell surface called microvilli. Microvilli increase surface area without significantly increasing cell volume.

Internal membranes compartmentalize the functions of a eukaryotic cell.

• A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell into compartments.
• These membranes also participate directly in metabolism, as many enzymes are built into membranes.
• The compartments created by membranes provide different local environments that facilitate specific metabolic functions, allowing several incompatible processes to go on simultaneously in a cell.
• The general structure of a biological membrane is a double layer of phospholipids.
• Other lipids and diverse proteins are embedded in the lipid bilayer or attached to its surface.
• Each type of membrane has a unique combination of lipids and proteins for its specific functions.
• For example, enzymes embedded in the membranes of mitochondria function in cellular respiration.

Concept 6.3 The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by the ribosomes

• Additional genes are located in mitochondria and chloroplasts.
• The nucleus averages about 5 microns in diameter.
• The two membranes of the nuclear envelope are separated by 20–40 nm.
• The envelope is perforated by pores that are about 100 nm in diameter.
• At the lip of each pore, the inner and outer membranes of the nuclear envelope are fused to form a continuous membrane.
• A protein structure called a pore complex lines each pore, regulating the passage of certain large macromolecules and particles.
• The nuclear side of the envelope is lined by the nuclear lamina, a network of protein filaments that maintains the shape of the nucleus.
• There is evidence that a framework of fibers called the nuclear matrix extends through the nuclear interior.
• Within the nucleus, the DNA and associated proteins are organized into discrete units called chromosomes, structures that carry the genetic information.
• Stained chromatin appears through light microscopes and electron microscopes as a diffuse mass.
• As the cell prepares to divide, the chromatin fibers coil up and condense, becoming thick enough to be recognized as the familiar chromosomes.
• A typical human cell has 46 chromosomes.
• A human sex cell (egg or sperm) has only 23 chromosomes.
• In the nucleolus, ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits.
• The subunits pass through the nuclear pores to the cytoplasm, where they combine to form ribosomes.
• The mRNA travels to the cytoplasm through the nuclear pores and combines with ribosomes to translate its genetic message into the primary structure of a specific polypeptide.

Ribosomes build a cell’s proteins.

• Cell types that synthesize large quantities of proteins (e.g., pancreas cells) have large numbers of ribosomes and prominent nucleoli.
• Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that function within the cytosol.
• These synthesize proteins that are either included in membranes or exported from the cell.
• Ribosomes can shift between roles depending on the polypeptides they are synthesizing.

Concept 6.4 The endomembrane system regulates protein traffic and performs metabolic functions in the cell

• Many of the internal membranes in a eukaryotic cell are part of the endomembrane system.
• In spite of these connections, these membranes are diverse in function and structure.
• The thickness, molecular composition and types of chemical reactions carried out by proteins in a given membrane may be modified several times during a membrane’s life.
• The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.

The endoplasmic reticulum manufactures membranes and performs many other biosynthetic functions.

• The endoplasmic reticulum (ER) accounts for half the membranes in a eukaryotic cell.
• The ER includes membranous tubules and internal, fluid-filled spaces called cisternae.
• The ER membrane is continuous with the nuclear envelope, and the cisternal space of the ER is continuous with the space between the two membranes of the nuclear envelope.
• Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside, including the outside of the nuclear envelope.
• Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.
• These include the sex hormones of vertebrates and adrenal steroids.
• Frequent use of these drugs leads to the proliferation of smooth ER in liver cells, increasing the rate of detoxification.
• This increases tolerance to the target and other drugs, so higher doses are required to achieve the same effect.
• Muscle cells have a specialized smooth ER that pumps calcium ions from the cytosol and stores them in its cisternal space.
• When a nerve impulse stimulates a muscle cell, calcium ions rush from the ER into the cytosol, triggering contraction.
• Enzymes then pump the calcium back, readying the cell for the next stimulation.
• As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the cisternal space through a pore formed by a protein complex in the ER membrane.
• As it enters the cisternal space, the new protein folds into its native conformation.
• Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is attached.
• Secretory proteins are packaged in transport vesicles that carry them to their next stage.
• Membrane-bound proteins are synthesized directly into the membrane.
• Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.
• As the ER membrane expands, membrane can be transferred as transport vesicles to other components of the endomembrane system.

The Golgi apparatus is the shipping and receiving center for cell products.

• Many transport vesicles from the ER travel to the Golgi apparatus for modification of their contents.
• The Golgi is a center of manufacturing, warehousing, sorting, and shipping.
• The Golgi apparatus is especially extensive in cells specialized for secretion.
• The membrane of each cisterna separates its internal space from the cytosol.
• One side of the Golgi, the cis side, is located near the ER. The cis face receives material by fusing with transport vesicles from the ER.
• The other side, the trans side, buds off vesicles that travel to other sites.
• During their transit from the cis to the trans side, products from the ER are usually modified.
• The Golgi can also manufacture its own macromolecules, including pectin and other noncellulose polysaccharides.
• According to the cisternal maturation model, the cisternae of the Golgi progress from the cis to the trans face, carrying and modifying their protein cargo as they move.
• Molecular identification tags are added to products to aid in sorting.
• Products are tagged with identifiers such as phosphate groups. These act like ZIP codes on mailing labels to identify the product’s final destination.

Lysosomes are digestive compartments.

• A lysosome is a membrane-bound sac of hydrolytic enzymes that an animal cell uses to digest macromolecules.
• Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids.
• Proteins in the lysosomal membrane pump hydrogen ions from the cytosol into the lumen of the lysosomes.
• Rupture of one or a few lysosomes has little impact on a cell because the lysosomal enzymes are not very active at the neutral pH of the cytosol.
• However, massive rupture of many lysosomes can destroy a cell by autodigestion.
• Lysosomal enzymes and membrane are synthesized by rough ER and then transferred to the Golgi apparatus for further modification.
• Proteins on the inner surface of the lysosomal membrane are spared by digestion by their three-dimensional conformations, which protect vulnerable bonds from hydrolysis.
• Lysosomes carry out intracellular digestion in a variety of circumstances.
• The food vacuole formed by phagocytosis fuses with a lysosome, whose enzymes digest the food.
• As the polymers are digested, monomers pass to the cytosol to become nutrients for the cell.
• This recycling, or autophagy, renews the cell.
• During autophagy, a damaged organelle or region of cytosol becomes surrounded by membrane.
• A lysosome fuses with the resulting vesicle, digesting the macromolecules and returning the organic monomers to the cytosol for reuse.
• This process plays an important role in development.
• The hands of human embryos are webbed until lysosomes digest the cells in the tissue between the fingers.
• This important process is called programmed cell death, or apoptosis.

Vacuoles have diverse functions in cell maintenance.

• Food vacuoles are formed by phagocytosis and fuse with lysosomes.
• Contractile vacuoles, found in freshwater protists, pump excess water out of the cell to maintain the appropriate concentration of salts.
• The membrane surrounding the central vacuole, the tonoplast, is selective in its transport of solutes into the central vacuole.
• The functions of the central vacuole include stockpiling proteins or inorganic ions, disposing of metabolic byproducts, holding pigments, and storing defensive compounds that defend the plant against herbivores.
• Because of the large vacuole, the cytosol occupies only a thin layer between the plasma membrane and the tonoplast. The presence of a large vacuole increases surface area to volume ratio for the cell.

Concept 6.5 Mitochondria and chloroplasts change energy from one form to another

• Mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work.
• Mitochondria are the sites of cellular respiration, generating ATP from the catabolism of sugars, fats, and other fuels in the presence of oxygen.
• They convert solar energy to chemical energy and synthesize new organic compounds such as sugars from CO2 and H2O.
• In contrast to organelles of the endomembrane system, each mitochondrion or chloroplast has two membranes separating the innermost space from the cytosol.
• Their membrane proteins are not made by the ER, but rather by free ribosomes in the cytosol and by ribosomes within the organelles themselves.
• Both organelles have small quantities of DNA that direct the synthesis of the polypeptides produced by these internal ribosomes.
• Mitochondria and chloroplasts grow and reproduce as semiautonomous organelles.
• There may be one very large mitochondrion or hundreds to thousands of individual mitochondria.
• The number of mitochondria is correlated with aerobic metabolic activity.
• A typical mitochondrion is 1–10 microns long.
• Mitochondria are quite dynamic: moving, changing shape, and dividing.
• The inner membrane divides the mitochondrion into two internal compartments.
• The first is the intermembrane space, a narrow region between the inner and outer membranes.
• The inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA, ribosomes, and enzymes.
• Some of the metabolic steps of cellular respiration are catalyzed by enzymes in the matrix.
• The cristae present a large surface area for the enzymes that synthesize ATP.
• Amyloplasts are colorless plastids that store starch in roots and tubers.
• Chromoplasts store pigments for fruits and flowers.
• Chloroplasts contain the green pigment chlorophyll as well as enzymes and other molecules that function in the photosynthetic production of sugar.
• Chloroplasts measure about 2 microns × 5 microns and are found in leaves and other green organs of plants and algae.
• The contents of the chloroplast are separated from the cytosol by an envelope consisting of two membranes separated by a narrow intermembrane space.
• The stroma contains DNA, ribosomes, and enzymes.
• The thylakoids are flattened sacs that play a critical role in converting light to chemical energy. In some regions, thylakoids are stacked like poker chips into grana.
• The membranes of the chloroplast divide the chloroplast into three compartments: the intermembrane space, the stroma, and the thylakoid space.
• Their shape is plastic, and they can reproduce themselves by pinching in two.
• Mitochondria and chloroplasts are mobile and move around the cell along tracks of the cytoskeleton.

Peroxisomes generate and degrade H2O2 in performing various metabolic functions.

• An intermediate product of this process is hydrogen peroxide (H2O2), a poison.
• The peroxisome contains an enzyme that converts H2O2 to water.
• Some peroxisomes break fatty acids down to smaller molecules that are transported to mitochondria as fuel for cellular respiration.
• Peroxisomes in the liver detoxify alcohol and other harmful compounds.
• Specialized peroxisomes, glyoxysomes, convert the fatty acids in seeds to sugars, which the seedling can use as a source of energy and carbon until it is capable of photosynthesis.
• Peroxisomes are bound by a single membrane.
• They form not from the endomembrane system, but by incorporation of proteins and lipids from the cytosol.
• They split in two when they reach a certain size.

Concept 6.6 The cytoskeleton is a network of fibers that organizes structures and activities in the cell

• The cytoskeleton is a network of fibers extending throughout the cytoplasm.

The cytoskeleton provides support, motility, and regulation.

• The cytoskeleton provides mechanical support and maintains cell shape.
• The cytoskeleton provides anchorage for many organelles and cytosolic enzymes.
• The cytoskeleton is dynamic and can be dismantled in one part and reassembled in another to change the shape of the cell.
• The cytoskeleton also plays a major role in cell motility, including changes in cell location and limited movements of parts of the cell.
• Cytoskeleton elements and motor proteins work together with plasma membrane molecules to move the whole cell along fibers outside the cell.
• Motor proteins bring about movements of cilia and flagella by gripping cytoskeletal components such as microtubules and moving them past each other.
• The same mechanism causes muscle cells to contract.
• Inside the cell, vesicles can travel along “monorails” provided by the cytoskeleton.
• The cytoskeleton manipulates the plasma membrane to form food vacuoles during phagocytosis.
• Cytoplasmic streaming in plant cells is caused by the cytoskeleton.
• Recently, evidence suggests that the cytoskeleton may play a role in the regulation of biochemical activities in the cell.
• There are three main types of fibers making up the cytoskeleton: microtubules, microfilaments, and intermediate filaments.
• Microtubule fibers are constructed of the globular protein tubulin.
• Each tubulin molecule is a dimer consisting of two subunits.
• A microtubule changes in length by adding or removing tubulin dimers.
• Microtubules shape and support the cell and serve as tracks to guide motor proteins carrying organelles to their destination.
• Microtubules are also responsible for the separation of chromosomes during cell division.
• These microtubules resist compression to the cell.
• Before a cell divides, the centrioles replicate.
• Many unicellular eukaryotic organisms are propelled through water by cilia and flagella.
• For example, cilia lining the windpipe sweep mucus carrying trapped debris out of the lungs.
• They are about 0.25 microns in diameter and 2–20 microns long.
• Flagella are the same width as cilia, but 10–200 microns long.
• A flagellum has an undulatory movement that generates force in the same direction as the flagellum’s axis.
• Cilia move more like oars with alternating power and recovery strokes that generate force perpendicular to the cilium’s axis.
• Both have a core of microtubules sheathed by the plasma membrane.
• Nine doublets of microtubules are arranged in a ring around a pair at the center. This “9 + 2” pattern is found in nearly all eukaryotic cilia and flagella.
• Flexible “wheels” of proteins connect outer doublets to each other and to the two central microtubules.
• The outer doublets are also connected by motor proteins.
• The cilium or flagellum is anchored in the cell by a basal body, whose structure is identical to a centriole.
• Addition and removal of a phosphate group causes conformation changes in dynein.
• Dynein arms alternately grab, move, and release the outer microtubules.
• Protein cross-links limit sliding. As a result, the forces exerted by the dynein arms cause the doublets to curve, bending the cilium or flagellum.
• Each microfilament is built as a twisted double chain of actin subunits.
• Microfilaments can form structural networks due to their ability to branch.
• The structural role of microfilaments in the cytoskeleton is to bear tension, resisting pulling forces within the cell.
• They form a three-dimensional network just inside the plasma membrane to help support the cell’s shape, giving the cell cortex the semisolid consistency of a gel.
• In muscle cells, thousands of actin filaments are arranged parallel to one another.
• Thicker filaments composed of myosin interdigitate with the thinner actin fibers.
• Myosin molecules act as motor proteins, walking along the actin filaments to shorten the cell.
• A contracting belt of microfilaments divides the cytoplasm of animal cells during cell division.
• Microfilaments assemble into networks that convert sol to gel.
• According to a widely accepted model, filaments near the cell’s trailing edge interact with myosin, causing contraction.
• The contraction forces the interior fluid into the pseudopodium, where the actin network has been weakened.
• The pseudopodium extends until the actin reassembles into a network.
• This creates a circular flow of cytoplasm in the cell, speeding the distribution of materials within the cell.
• Intermediate filaments range in diameter from 8–12 nanometers, larger than microfilaments but smaller than microtubules.
• Intermediate filaments are specialized for bearing tension.
• Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes.
• They reinforce cell shape and fix organelle location.

Concept 6.7 Extracellular components and connections between cells help coordinate cellular activities

Plant cells are encased by cell walls.

• The cell wall, found in prokaryotes, fungi, and some protists, has multiple functions.
• In plants, the cell wall protects the cell, maintains its shape, and prevents excessive uptake of water.
• It also supports the plant against the force of gravity.
• The thickness and chemical composition of cell walls differs from species to species and among cell types within a plant.
• The basic design consists of microfibrils of cellulose embedded in a matrix of proteins and other polysaccharides. This is the basic design of steel-reinforced concrete or fiberglass.
• A mature cell wall consists of a primary cell wall, a middle lamella with sticky polysaccharides that holds cells together, and layers of secondary cell wall.
• Plant cell walls are perforated by channels between adjacent cells called plasmodesmata.

The extracellular matrix (ECM) of animal cells functions in support, adhesion, movement, and regulation.

• Though lacking cell walls, animal cells do have an elaborate extracellular matrix (ECM).
• The primary constituents of the extracellular matrix are glycoproteins, especially collagen fibers, embedded in a network of glycoprotein proteoglycans.
• The interconnections from the ECM to the cytoskeleton via the fibronectin-integrin link permit the integration of changes inside and outside the cell.
• Embryonic cells migrate along specific pathways by matching the orientation of their microfilaments to the “grain” of fibers in the extracellular matrix.
• This may coordinate the behavior of all the cells within a tissue.

Intercellular junctions help integrate cells into higher levels of structure and function.

• Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact.
• Water and small solutes can pass freely from cell to cell.
• In certain circumstances, proteins and RNA can be exchanged.
• Animals have 3 main types of intercellular links: tight junctions, desmosomes, and gap junctions.
• This prevents leakage of extracellular fluid.
• Intermediate filaments of keratin reinforce desmosomes.
• Special membrane proteins surround these pores.
• Ions, sugars, amino acids, and other small molecules can pass.
• In embryos, gap junctions facilitate chemical communication during development.
• A cell is a living unit greater than the sum of its parts.
• For example, macrophages use actin filaments to move and extend pseudopodia to capture their bacterial prey.
• Food vacuoles are digested by lysosomes, a product of the endomembrane system of ER and Golgi.
• The enzymes of the lysosomes and proteins of the cytoskeleton are synthesized on the ribosomes.
• The information for the proteins comes from genetic messages sent by DNA in the nucleus.
• All of these processes require energy in the form of ATP, most of which is supplied by the mitochondria.

Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 6-1

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