Information

Why are the walls of the bronchioles folded?


The bronchiole shown in the section above has folded epithelium, why is this so?


Those foldings are called as mucosal folds, formed by the contraction of smooth muscles , are also present in trachea, and in many other organs such as gallbladder etc.

Physiologically: This folding causes an increase in area for better action of mucus which protects the body from entry of dirt, pathogens and loosing the moisture.

Anaomically: as @Bryan pointed out that the folding permits the contraction and relaxation of smooth muscles around the bronchioles.

Source 1

Source 2


Bronchiolitis obliterans is an inflammatory condition that affects the lung's tiniest airways, the bronchioles. In affected people, the bronchioles may become damaged and inflamed leading to extensive scarring that blocks the airways. Signs and symptoms of the condition include a dry cough shortness of breath and/or fatigue and wheezing in the absence of a cold or asthma. [1] [2] [3] Many different chemicals (such as nitrogen oxides, ammonia, welding fumes or food flavoring fumes) and respiratory infections can cause lung injury that leads to bronchiolitis obliterans. It can also be associated with rheumatoid arthritis and graft-versus-host disease following a lung or hematopoietic cell transplantation. [3] While there is no way to reverse the disease, treatments are available that may stabilize or slow the progression. [1] [3]

Another similarly named disease, bronchiolitis obliterans organizing pneumonia, is a completely different disease.


Lungs and Respiratory System

The lungs and respiratory system allow us to breathe. They bring oxygen into our bodies (called inspiration, or inhalation) and send carbon dioxide out (called expiration, or exhalation).

This exchange of oxygen and carbon dioxide is called respiration.

What Are the Parts of the Respiratory System?

The respiratory system includes the nose, mouth, throat, voice box, windpipe, and lungs.

Air enters the respiratory system through the nose or the mouth. If it goes in the nostrils (also called nares), the air is warmed and humidified. Tiny hairs called cilia (SIL-ee-uh) protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter the nose through the breathed air.

The two openings of the airway (the nasal cavity and the mouth) meet at the pharynx (FAR-inks), or throat, at the back of the nose and mouth. The pharynx is part of the digestive system as well as the respiratory system because it carries both food and air.

At the bottom of the pharynx, this pathway divides in two, one for food — the esophagus (ih-SAH-fuh-gus), which leads to the stomach — and the other for air. The epiglottis (eh-pih-GLAH-tus), a small flap of tissue, covers the air-only passage when we swallow, keeping food and liquid from going into the lungs.

The larynx, or voice box, is the top part of the air-only pipe. This short tube contains a pair of vocal cords, which vibrate to make sounds.

The trachea, or windpipe, is the continuation of the airway below the larynx. The walls of the trachea (TRAY-kee-uh) are strengthened by stiff rings of to keep it open. The trachea is also lined with cilia, which sweep fluids and foreign particles out of the airway so that they stay out of the lungs.

At its bottom end, the trachea divides into left and right air tubes called bronchi (BRAHN-kye), which connect to the lungs. Within the lungs, the bronchi branch into smaller bronchi and even smaller tubes called bronchioles (BRAHN-kee-olz). Bronchioles end in tiny air sacs called alveoli, where the exchange of oxygen and carbon dioxide actually takes place. Each person has hundreds of millions of alveoli in their lungs. This network of alveoli, bronchioles, and bronchi is known as the bronchial tree.

The lungs also contain elastic tissues that allow them to inflate and deflate without losing shape and are covered by a thin lining called the pleura (PLUR-uh).

The chest cavity, or thorax (THOR-aks), is the airtight box that houses the bronchial tree, lungs, heart, and other structures. The top and sides of the thorax are formed by the ribs and attached muscles, and the bottom is formed by a large muscle called the diaphragm (DYE-uh-fram). The chest walls form a protective cage around the lungs and other contents of the chest cavity.

How Do the Lungs and Respiratory System Work?

The cells in our bodies need oxygen to stay alive. Carbon dioxide is made in our bodies as cells do their jobs.

The lungs and respiratory system allow oxygen in the air to be taken into the body, while also letting the body get rid of carbon dioxide in the air breathed out.

When you breathe in, the diaphragm moves downward toward the abdomen, and the rib muscles pull the ribs upward and outward. This makes the chest cavity bigger and pulls air through the nose or mouth into the lungs.

In exhalation, the diaphragm moves upward and the chest wall muscles relax, causing the chest cavity to get smaller and push air out of respiratory system through the nose or mouth.

Every few seconds, with each inhalation, air fills a large portion of the millions of alveoli. In a process called diffusion, oxygen moves from the alveoli to the blood through the capillaries (tiny blood vessels) lining the alveolar walls. Once in the bloodstream, oxygen gets picked up by the in red blood cells. This oxygen-rich blood then flows back to the heart, which pumps it through the arteries to oxygen-hungry tissues throughout the body.

In the tiny capillaries of the body tissues, oxygen is freed from the hemoglobin and moves into the cells. Carbon dioxide, made by the cells as they do their work, moves out of the cells into the capillaries, where most of it dissolves in the plasma of the blood. Blood rich in carbon dioxide then returns to the heart via the veins. From the heart, this blood is pumped to the lungs, where carbon dioxide passes into the alveoli to be exhaled.


300 Years Ago: No Smoking Please

Sir John Floyer (1649�), an English physician who had asthma, first described emphysema in the seventeenth century. Floyer was studying pulmonary (lung) disorders and described the characteristic prolonged expiration and progressive nature of emphysema.

Floyer warned his patients to avoid tobacco smoke, metallic fumes, and other potential irritants because he believed that they caused pulmonary disorders. He was right.

* elasticity is the ability to be stretched and to return to original shape.

Emphysema also may accompany diseases such as asthma and tuberculosis that can obstruct the airways in the lungs. A less serious form of emphysema sometimes develops in elderly people whose lungs have lost elasticity only as a part of the aging process. Another usually mild form, called compensatory emphysema, results when a lung overexpands to occupy the space of another lung that has collapsed or has been removed surgically.

A Close-Up Look at the Alveoli

Understanding the alveoli, and the airways in the lungs that lead to them, is key to understanding emphysema. These tiny sacs or pockets are grouped in grapelike clusters and are so small that each lung contains 300 to 400 million of them. Because there are so many alveoli, their total surface area is about 50 times greater than the entire surface area of the skin on the body. This huge surface area is important because it allows oxygen from the air we inhale to be transferred to the bloodstream, and it allows carbon dioxide in the bloodstream to be transferred out.

Inhaled air reaches the alveoli through bronchial tubes and repeatedly branching smaller bronchioles in the lungs that resemble an upside-down tree. The walls of the alveoli contain tiny blood vessels called capillaries, which lead to larger vessels that return blood to the heart to be pumped throughout the body. It is in the delicate capillaries of the alveoli that the transfer of oxygen and carbon dioxide takes place.

What Changes Occur in the Lungs from Emphysema?

In emphysema, tobacco smoke and other inhaled irritants damage the alveoli, causing them to lose elasticity. Moreover, smoking often causes chronic bronchitis, which tends to narrow and obstruct the bronchial airways with mucus, scarring, and muscle spasms in the walls of the bronchial tubes. As a result, air becomes trapped in the alveoli, stretching their walls and causing some to break down and form larger pockets by joining with other alveoli. As the lungs become less elastic, they tend to become distended, or overinflated.


Pathophysiology

Once innate respiratory defenses of the lung’s epithelial cell barrier and mucociliary transport system are infiltrated by foreign/invading antigens (noxious cigarette ingredients, for instance), the responding inflammatory immune cells (including polymorphonuclear cells, eosinophils, macrophages, CD4 positive and CD8 positive lymphocytes) transport the antigens to the bronchial associated lymphatic tissue layer (BALT). It is here where the majority of the release of neutrophilic chemotactic factors is thought to occur. Proteolytic enzymes like matrix-metalloproteinases (MMPs) are mainly released by macrophages, which lead to destruction of the lung’s epithelial barrier.

Macrophages are found to be 5- to 10-fold higher in the bronchoalveolar lavage fluid of emphysematous patients. [13] Also, along with macrophages, the release of proteases and free radical hydrogen peroxide from neutrophils adds to the epithelial ruination, specifically with emphasis on the basement membrane. This is why neutrophils are thought to be highly important in the pathogenesis of emphysema at the tissue level, a differentiator to the mainly eosinophilic inflammatory response in airways affected by asthma.

After all, the T lymphocytes in the sputum of emphysematous smokers are mainly CD8 positive cells. [14] These cells release chemotactic factors to recruit more cells (pro-inflammatory cytokines that amplify the inflammation) and growth factors that promote structural change. The inflammation is further amplified by oxidative stress and protease production. Oxidants are produced from cigarette smoke and released from inflammatory cells. Proteases are produced by inflammatory, macrophage, and epithelial cells, which fuel bronchiolar edema from an elastin-destroying protease-antiprotease imbalance. This protease-menace is elastase, released by macrophages, and responsible for breakdown of the lung’s fragile elastic lamina (of which elastin is a structural protein component). [13] This is believed to be central in the development of emphysema. Peptides from elastin can be detected in increased quantities in patients with emphysema and AAT. [15]

The repair process of airway remodeling further exacerbates emphysema’s anatomical derangements with key characters such as vascular endothelial growth factor (VEGF), which is expressed in airway smooth muscle cells and is responsible for neovascularization and expression of increased and possibly abnormal patterns of fibroblastic development. It is these structural changes of mucus hyperplasia, bronchiolar edema, and smooth muscle hypertrophy and fibrosis in smokers’ airways that result in the small airways narrowing of less than two millimeters.

AATD lung disease is due to the relative deficiency in the blood and lungs of the alpha-1 antitrypsin (AAT) protein. Although evidence suggests a more complicated cascade of proteolytic and inflammatory factors as the cause of emphysema in AATD, unopposed neutrophil elastase activity within the pulmonary interstitium with resultant connective tissue destruction remains an important contributor to the pathogenesis of emphysema. [16]

Morphology

Pathologically defined as permanent enlargement of airspaces distal to the terminal bronchioles, emphysema creates an environment leading to a dramatic decline in the alveolar surface area available for gas exchange. Loss of individual alveoli with septal wall destruction leads to airflow limitation via two mechanisms. First, loss of alveolar wall results in a decrease in elastic recoil, which subsequently limits airflow. Second, loss of alveolar supporting structures is indirectly responsible for airway narrowing, again limiting airflow. [17]

Though the paradigm for classification continues to evolve, the described morphological pathology of region-specific emphysema remains in three types: [18]

Centriacinar emphysema is the most common type of pulmonary emphysema mainly localized to the proximal respiratory bronchioles with focal destruction and predominantly found in the upper lung zones. The surrounding lung parenchyma is usually normal with untouched distal alveolar ducts and sacs. Also known as centrilobular emphysema, this entity is associated with and closely-related to long-standing cigarette smoking and dust inhalation. [19, 20]

--> Emphysema. Centrilobular emphysema. Courtesy of Dr Frank Gaillard, Radiopaedia.org (http://radiopaedia.org/cases/emphysema-diagrams).

Panacinar emphysema destroys the entire alveolus uniformly and is predominant in the lower half of the lungs. Panacinar emphysema generally is observed in patients with homozygous (Pi ZZ) alpha1-antitrypsin (AAT) deficiency. In people who smoke, focal panacinar emphysema at the lung bases may accompany centriacinar emphysema. [19, 20]

--> Emphysema. Panlobular emphysema. Courtesy of Dr Frank Gaillard, Radiopaedia.org (http://radiopaedia.org/cases/emphysema-diagrams).

Paraseptal emphysema, also known as distal acinar emphysema, preferentially involves the distal airway structures, alveolar ducts, and alveolar sacs. The process is localized around the septae of the lungs or pleura. Although airflow is frequently preserved, the apical bullae may lead to spontaneous pneumothorax. Giant bullae occasionally cause severe compression of adjacent lung tissue. [19, 20]

--> Emphysema. Paraseptal emphysema. Courtesy of Dr Frank Gaillard, Radiopaedia.org (http://radiopaedia.org/cases/emphysema-diagrams).

Gross Anatomy

The trachea is the segment that connects the upper airways to the bronchi. It has 16-22 cartilaginous rings in the anterior and lateral walls (cartilaginous portion) and a thin band of smooth muscle in the posterior wall (membranous portion). This configuration supports the tracheal anatomy during inspiration and expiration. [3]

The trachea extends distally 10-12 cm and divides into the right and left mainstem bronchi (primary) at the level of the T5 vertebra (see image below).

The right mainstem bronchus originates higher than the left mainstem bronchus it is also shorter, wider, and more vertical. It measures about 2.5 cm and appears as a direct continuation of the trachea. The left mainstem bronchus is about 5 cm in length.

Mainstem bronchi divide into the lobar bronchi (secondary) and subsequently into the segmental (tertiary) bronchi (see the image below). Arteries, veins, and lymphatics also enter the lungs at the hilum along with the bronchi. A bronchopulmonary segment is a portion of lung that is supplied by a segmental bronchus and its adjacent blood vessels.

The right mainstem bronchus divides into the right upper lobe bronchus and the bronchus intermedius (BI). The former then divides into 3 segments: apical (RB1), posterior (RB2), and anterior (RB3). The bronchus intermedius divides into the right middle lobe and right lower lobe bronchi. The right middle lobe bronchus has two segments: lateral (RB4) and middle (RB5). The right lower lobe bronchus has 5 segments: superior (RB6), medial basal (RB7), anterior basal (RB8), lateral basal (RB9), and posterior basal (RB10).

The left mainstem bronchus divides into the left upper lobe and lower lobe bronchi. The left upper lobe bronchus subsequently divides into the left upper division bronchus and the lingular division. The former gives rise to 3 segments: apical (LB1), posterior (LB2), and anterior (LB3). The apical and posterior segments are usually fused in a single apicoposterior (LB1/2) segment. The lingular bronchus has 2 segments: superior lingular (LB4) and inferior lingular (LB5). The left lower lobe bronchus branches into the superior (LB6), anteromedial basal (LB7/8), lateral basal (LB9), and posterior basal (LB10) segments.

Bronchi undergo multiple divisions (on average 23) along the bronchial tree. The initial 16-17 generations of bronchi make up the conducting zone of the airways and do not participate in gas exchange. The surface of the airways that does not contribute to gas exchange is referred to as “dead space.”

As bronchi divide into smaller airways, the respiratory epithelium undergoes histological changes and gives rise to terminal bronchioles. The 17 th to 19 th generations of bronchioles constitute the transitional zone. These bronchioles enter pyramid-shaped pulmonary lobules separated from one another by a thin septum, with the apex directed toward the hilum, comprising 5-7 terminal bronchioles. The last 2-3 generations of bronchioles have some alveoli in their walls and make up the respiratory zone.

The area of the lung that is distal to a terminal bronchiole is termed the acinus. [2, 4] The final division is called the respiratory bronchiole, which further branches into multiple alveolar ducts. Alveoli, the functional units of the respiratory system, start appearing at the level of the respiratory bronchioles.


Overview of the Respiratory System

To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation (whereby food molecules are combined with oxygen). Oxidation involves carbon and hydrogen being combined with oxygen to form carbon dioxide and water. The consumption of oxygen and the production of carbon dioxide are thus indispensable to life. It follows that the human body must have an organ system designed to eliminate carbon dioxide from the circulating blood and absorb oxygen from the atmosphere at a rate rapid enough for the body’s needs, even during peak exercise. The respiratory system enables oxygen to enter the body and carbon dioxide to leave the body.

The respiratory system starts at the nose and mouth and continues through the airways and the lungs. Air enters the respiratory system through the nose and mouth and passes down the throat (pharynx) and through the voice box, or larynx. The entrance to the larynx is covered by a small flap of tissue (epiglottis) that automatically closes during swallowing, thus preventing food or drink from entering the airways.

The windpipe (trachea) is the largest airway. The trachea branches into two smaller airways: the left and right bronchi, which lead to the two lungs.

Each lung is divided into sections (lobes): three in the right lung and two in the left lung. The left lung is a little smaller than the right lung because it shares space in the left side of the chest with the heart.

Inside the Lungs and Airways

The bronchi themselves branch many times into smaller airways, ending in the narrowest airways (bronchioles), which are as small as one half of a millimeter (or 2/100 of an inch) across. The airways resemble an upside-down tree, which is why this part of the respiratory system is often called the bronchial tree. Large airways are held open by semiflexible, fibrous connective tissue called cartilage. Smaller airways are supported by the lung tissue that surrounds and is attached to them. The walls of the smaller airways have a thin, circular layer of smooth muscle. The airway muscle can relax or contract, thus changing airway size.

Thousands of small air sacs (alveoli) are at the end of each bronchiole. Together, the millions of alveoli of the lungs form a surface of more than 100 square meters (1111 square feet). Within the alveolar walls is a dense network of tiny blood vessels called capillaries. The extremely thin barrier between air and capillaries allows oxygen to move from the alveoli into the blood and allows carbon dioxide to move from the blood in the capillaries into the air in the alveoli.

When you breathe, air travels through your nose, down the trachea, and into smaller and smaller airways called bronchi. These bronchi branch into smaller passages called bronchioles and finally into small, thin, fragile sacs called alveoli. During inspiration, the alveoli in the lungs are filled with air. It is here that oxygen is exchanged for carbon dioxide. Blood cells absorb oxygen from the capillaries in the alveoli as carbon dioxide, a waste product, is released back into the lungs from the veins. During expiration, the carbon dioxide is expelled from the body. Oxygen-rich blood then travels to the heart so it can be pumped back to the body where it is needed.

The pleura is a slippery membrane that covers the lungs as well as the inside of the chest wall. It allows the lungs to move smoothly during breathing and as the person moves. Normally, the two layers of the pleura have only a small amount of lubricating fluid between them. The two layers glide smoothly over each other as the lungs change size and shape.


Mistakes in Scientific Studies Surge

It was the kind of study that made doctors around the world sit up and take notice: Two popular high-blood-pressure drugs were found to be much better in combination than either alone.

"There was a 'wow' reaction," recalls Franz Messerli, a New York doctor who, like many others, changed his prescription habits after the 2003 report.

Unfortunately, it wasn't true. Six and a half years later, the prestigious medical journal the Lancet retracted the paper, citing "serious concerns" about the findings.

The damage was done. Doctors by then had given the drug combination to well over 100,000 patients. Instead of protecting them from kidney problems, as the study said the drug combo could do, it left them more vulnerable to potentially life-threatening side effects, later studies showed. Today, "tens of thousands" of patients are still on the dual therapy, according to research firm SDI.

When a study is retracted, "it can be hard to make its effects go away," says Sheldon Tobe, a kidney-disease specialist at the University of Toronto.

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Can the intrauterine contraceptive device be used as emergency contraception?

The IUCD is a very effective form of emergency contraception which is effective for up to five days after you have had unprotected sex (intercourse). It prevents more than 9 out of 10 pregnancies which would otherwise have occurred. It can be left in place to provide ongoing contraception.

The IUCD is the most effective method of emergency contraception - but it is not available everywhere, not all doctors are able to fit it and there may not be emergency appointments available at just the right time. Therefore, if you need emergency contraception it is important to try to make arrangements for this as early as possible.

If you don't want to continue to use the IUCD as contraception it can be removed after you have had your next period.


Why are the walls of the bronchioles folded? - Biology

Which organelle often takes up much of the volume of a plant cell?

The liver is involved in detoxification of many poisons and drugs. Which of the following structures is primarily involved in this process and, therefore, abundant in liver cells?

A) rough endoplasmic reticulum

B) smooth endoplasmic reticulum

B) smooth endoplasmic reticulum

Which of the following frequently imposes a limit on cell size?

A) the absence of a nucleus

B) the number of mitochondria in the cytoplasm

C) ratios of surface area to volume

D) the volume of the endomembrane system

C) ratios of surface area to volume

What kinds of molecules pass through a cell membrane most easily?

Which of the following would be most appropriate method to observe the three-dimensional structure and organization of microvilli on an intestinal cell?

A) a hand lens (magnifying glass)

B) standard light microscopy

C) scanning electron microscopy

D) transmission electron microscopy

C) scanning electron microscopy

Which organelle is the primary site of ATP synthesis in eukaryotic cells?

A cell with a predominance of smooth endoplasmic reticulum is likely specialized to ________.

A) store large quantities of water

B) import and export large quantities of protein

C) actively secrete large quantities of protein

D) synthesize large quantities of lipids

D) synthesize large quantities of lipids

According to the fluid mosaic model of cell membranes, phospholipids ________.

A) can move laterally along the plane of the membrane

B) frequently flip-flop from one side of the membrane to the other

C) occur in an uninterrupted bilayer, with membrane proteins restricted to the surface of the membrane

D) have hydrophilic tails in the interior of the membrane

A) can move laterally along the plane of the membrane

Which of the following statements describes a characteristic feature of a carrier protein in a plasma membrane?

A) It exhibits specificity for a particular type of molecule.

B) It requires the expenditure of cellular energy to function.

C) It works against diffusion.

D) It has no hydrophobic regions.

A) It exhibits specificity for a particular type of molecule.

When a plant cell, such as one from a tulip leaf, is submerged in a hypertonic solution, what is likely to occur?

B) Plasmolysis will shrink the interior of the cell.

C) The cell will become flaccid.

D) The cell will become turgid.

B) Plasmolysis will shrink the interior of the cell.

Thylakoids, DNA, and ribosomes are all components found in ________.

A cell with a predominance of rough endoplasmic reticulum is most likely ________.

A) producing large quantities of proteins for secretion

B) producing large quantities of proteins in the cytosol

C) producing large quantities of carbohydrates to assemble an extensive cell wall matrix

D) producing large quantities of carbohydrates for storage in the vacuole

A) producing large quantities of proteins for secretion

An animal cell lacking carbohydrates on the external surface of its plasma membrane would likely be impaired in which function?

A) transporting ions against an electrochemical gradient

C) attaching the plasma membrane to the cytoskeleton

D) establishing a diffusion barrier to charged molecules

Which of the following statements about diffusion is true?

A) It is very rapid over long distances.

B) It requires an expenditure of energy by the cell.

C) It is an active process in which molecules move from a region of lower concentration to a region of higher concentration.

D) It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration.

D) It is a passive process in which molecules move from a region of higher concentration to a region of lower concentration.

A newly discovered unicellular organism isolated from acidic mine drainage is found to contain a cell wall, a plasma membrane, two flagella, and peroxisomes. Based just on this information, the organism is most likely ________.

Motor proteins provide for molecular motion in cells by interacting with what types of cellular structures?

A) membrane proteins of the inner nuclear envelope

B) free ribosomes and ribosomes attached to the ER

C) components of the cytoskeleton

D) cellulose fibers in the cell wall

C) components of the cytoskeleton

Tay-Sachs disease is a human genetic abnormality that results in cells accumulating and becoming clogged with very large, complex, undigested lipids. Which cellular organelle is most likely defective in this condition?

C) the smooth endoplasmic reticulum

D) the rough endoplasmic reticulum

Which of the following statements is a reasonable explanation for why unsaturated fatty acids help keep a membrane more fluid at lower temperatures?

A) The double bonds form kinks in the fatty acid tails, preventing adjacent lipids from packing tightly.

B) Unsaturated fatty acids have a higher cholesterol content, which prevents adjacent lipids from packing tightly.

C) Unsaturated fatty acids are more nonpolar than saturated fatty acids.

D) The double bonds block interaction among the hydrophilic head groups of the lipids.

A) The double bonds form kinks in the fatty acid tails, preventing adjacent lipids from packing tightly.

What will happen to a red blood cell (RBC), which has an internal ion content of about 0.9%, if it is placed into a beaker of pure water?

A) The cell would shrink because the water in the beaker is hypotonic relative to the cytoplasm of the RBC.

B) The cell would shrink because the water in the beaker is hypertonic relative to the cytoplasm of the RBC.

C) The cell would swell because the water in the beaker is hypotonic relative to the cytoplasm of the RBC.

D) The cell will remain the same size because the solution outside the cell is isotonic.

C) The cell would swell because the water in the beaker is hypotonic relative to the cytoplasm of the RBC.

The evolution of eukaryotic cells most likely involved ________.

A) endosymbiosis of an oxygen-using bacterium in a larger bacterial host cell-the endosymbiont evolved into chloroplasts

B) endosymbiosis of a photosynthetic archaeal cell in a larger bacterial host cell to escape toxic oxygen—the anaerobic archaea evolved into chloroplasts

C) endosymbiosis of an oxygen-using bacterium in a larger bacterial host cell-the endosymbiont evolved into mitochondria

D) evolution of an endomembrane system and subsequent evolution of mitochondria from a portion of the smooth endoplasmic reticulum

C) endosymbiosis of an oxygen-using bacterium in a larger bacterial host cell-the endosymbiont evolved into mitochondria

Plant cell walls are composed of cellulose, while fungal cell walls are composed of chitin. A group of scientists hypothesize that this difference means the cell wall has largely different functions in plant cells and fungal cells. Alternatively, another group of scientists hypothesize that despite their biochemical differences, plant and fungal cell walls serve similar functions.

Which of the following observations would best support the alternative hypothesis described above?

A) Plant cell walls are found just outside the plasma membrane, while fungal cell walls are found just beneath the plasma membrane.

B) In both plant cells and fungal cells, the cell wall surrounds the outside of the cell membrane.

C) Some plant cells have secondary cell walls that confer additional rigidity, while fungal cells do not.

D) Photosynthesis occurs in plant cells, but it does not occur in fungal cells.

B) In both plant cells and fungal cells, the cell wall surrounds the outside of the cell membrane.

Cholesterol is an important component of animal cell membranes. Cholesterol molecules are often delivered to body cells by the blood, which transports the molecules in the form of cholesterol-protein complexes. The complexes must be moved into the body cells before the cholesterol molecules can be incorporated into the phospholipid bilayers of cell membranes.

Based on the information presented, which of the following is the most likely explanation for a buildup of cholesterol molecules in the blood of an animal?

A) The animal’s body cells are defective in exocytosis.

B) The animal’s body cells are defective in endocytosis.

C) The animal’s body cells are defective in cholesterol synthesis.

D) The animal’s body cells are defective in phospholipid synthesis.

B) The animal’s body cells are defective in endocytosis.

Organelles such as mitochondria and the endoplasmic reticulum have membranes that compartmentalize reactions and other metabolic processes. To function properly, the organelles must move substances across their membranes.

Which of the following statements describes a feature shared by mitochondria and the endoplasmic reticulum that increases the efficiency of their basic functions?

A) They have rigid, nonfluid membranes.

B) They have highly folded membranes.

C) They have membranes composed of many carbohydrates.

D) They have double membranes, with one membrane enclosed within the other.

B) They have highly folded membranes.

Which of the following statements is true regarding the movement of substances across cell membranes?

A) Ions are unable to move through the phospholipid bilayer because the nonpolar tail regions of the phospholipids are hydrophobic.

B) Ions are able to move through the phospholipid bilayer because the polar head regions of the phospholipids are charged.

C) Water is able to move through the phospholipid bilayer because the nonpolar tail regions of the phospholipids are charged.

D) Water is unable to move through the phospholipid bilayer because the polar head regions of the phospholipids are charged.

A) Ions are unable to move through the phospholipid bilayer because the nonpolar tail regions of the phospholipids are hydrophobic.

Aquaporins are channel proteins that facilitate the transport of water across the cell membrane. One group of researchers hypothesizes that without functional aquaporins, no water will be able to enter the cell. A different group proposes an alternative hypothesis, stating that even with nonfunctional aquaporins, a small amount of water will still cross the cell membrane. An experiment is set up in which plant cells with mutated (nonfunctional) aquaporins and plant cells with normally functioning aquaporins are both placed in distilled water.

Which of the following data would support the alternative hypothesis?

A) Cells with functional aquaporins exhibit low turgor pressure and are hypertonic.

B) Cells with functional aquaporins exhibit high turgor pressure and are hypotonic.

C) Cells with mutated aquaporins exhibit an absence of turgor pressure and are completely plasmolyzed.

D) Cells with mutated aquaporins exhibit moderate turgor pressure and are hypertonic.

D) Cells with mutated aquaporins exhibit moderate turgor pressure and are hypertonic.


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