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Flashcards in Cell Structure Deck (70):

What name is given to the basic functional and structural unit of all living organisms?

The basic structural and functional unit of all known life-forms is the cell.

For the AP Biology exam, you'll want to be familiar with animal and plant cells, as well as the general structure of bacterial cells.


The cell theory, originally composed in 1838, includes three primary tenets. Name them.

  1. The cell is the basic unit of life.
  2. All living things are composed of cells, whether one or many.
  3. All cells arise from other cells.


All cells can be categorized into which two broad groups?

Prokaryotic and eukaryotic cells

Prokaryotic cells are generally simpler and include bacterial species. Eukaryotes can range from single-celled organisms (like yeast) to complex animals (like humans).


What main features characterize eukaryotic cells?

Eukaryotic cells have membrane-bound organelles, including nuclei, and linear chromosomes. They are also larger than prokaryotic cells and differ in specific aspects like flagellum structure.

Eukaryotic cells can comprise either unicellular or multicellular organisms.


What main features characterize prokaryotic cells?

Prokaryotic cells lack membrane-bound organelles. They generally contain one circular chromosome within a nucleoid region, but can also possess circular plasmids outside the genome.

Prokaryotic cells always comprise unicellular organisms.


How do eukaryotic and prokaryotic cells differ with respect to organelles?

Unlike eukaryotes, prokaryotes lack a nucleus, as well as all membrane-bound organelles.

Note that membrane-bound organelles include mitochondria, lysosomes, the ER, and the Golgi apparatus, but not ribosomes. Prokaryotes do contain ribosomes, a fact that may appear on the AP Biology exam.


How do eukaryotic and prokaryotic organisms differ in their cellular organization?

Prokaryotes are always unicellular, while eukaryotes can be either unicellular or multicellular.

One common example of a unicellular eukaryote is yeast, a fungus. Most other single-celled eukaryotes are classified as protists.


Determine if an organism with the following traits is a prokaryote or a eukaryote:

  • composed of a single cell
  • linear DNA
  • has mitochondria

This organism is a eukaryote.

Only a eukaryote would possess mitochondria, since prokaryotes lack membrane-bound organelles. Eukaryotes also have linear, not circular, chromosomes. Note that both eukaryotes and prokaryotes can be unicellular.



The cytosol is the fluid contained within a cell.

In contrast, the cytoplasm includes both the intracellular fluid and all of the extranuclear organelles.



The cytoplasm includes both the intracellular fluid, or cytosol, and the organelles.

The only organelle that is not included in the cytoplasm is the nucleus.



An organelle is a separate, specialized structure within a cell.

Many organelles are enclosed by lipid bilayers, but some, including ribosomes, are not membrane-bound.


What is the cellular role of the plasma membrane?

The plasma membrane, also called the cell membrane, protects the interior of the cell from its environment. It also limits the movement of specific materials into and out of the cell.


Describe the composition of the plasma membrane.

The plasma membrane consists of a phospholipid bilayer, with polar heads on the exterior (pointing toward the extracellular fluid and cytoplasm) and nonpolar tails on the interior.

The membrane also contains cholesterol, associated large proteins, and sphingolipids, among other components.


Explain the fluid mosaic model.

The fluid mosaic model is used to describe the plasma membrane. It is composed of lipids with a "mosaic" of embedded proteins and other components, and its "fluidity" allows these macromolecule components to move laterally within the membrane.


In animal cells, which organelle serves as the location for DNA in the form of linear chromosomes?

The nucleus holds the cell's linear chromosomes. It is also the site of DNA replication and transcription.

While the mitochondria also include DNA, mitochondrial DNA is found in small circular chromosomes, not linear ones.


What structural features are present in the nucleus?

The nucleus is encased in a double membrane, known as the nuclear envelope. This membrane is marked by channels called nuclear pores. Inside, a fluid (the nucleoplasm) surrounds linear chromosomes.


In what part of the cell is the nucleolus located, and what function does it serve?

The nucleolus is located within the nucleus. It serves as the site of ribosomal RNA transcription and synthesis of ribosomal subunits.


Which organelle has two subunits and serves as the location for protein synthesis?

The ribosome

Ribosomes are small organelles found in both eukaryotic and prokaryotic cells. At these organelles, proteins are synthesized (translated). A typical ribosome includes a small and a large subunit, although the sizes of these subunits vary depending on the type of cell.


How do eukaryotic and prokaryotic organisms differ in the composition of their ribosomes?

Eukaryotic ribosomes are slightly larger, with a 40S and a 60S subunit combining to yield 80S. Prokaryotic ribosomes have a 30S and a 50S subunit, which combine to form 70S.

The abbreviation "S" refers to the rate at which a molecule settles in a centrifuge.


Describe the structural characteristics of the endoplasmic reticulum (ER).

The ER is a folded membrane divided into two regions: rough ER and smooth ER.

Rough ER contains ribosomes bound to its surface, while the smooth ER does not.


Within the cell, what is the role of the endoplasmic reticulum (ER)?

The ER is involved in a variety of processes, with smooth ER and rough ER performing different functions. The smooth ER is involved in lipid anabolism and detoxification, while the rough ER, with its many ribosomes, is the site of protein translation.

Both types of ER help synthesize macromolecules and shuttle them to the Golgi apparatus to be secreted from the cell.


What biological products are synthesized in the rough endoplasmic reticulum?

The rough ER synthesizes proteins.

These can include enzymes and peptide hormones, among other examples.


What biological products are synthesized in the smooth endoplasmic reticulum?

The smooth ER synthesizes lipids.

These include steroid hormones and phospholipids, among other examples.


What is the cellular role of the Golgi apparatus?

The Golgi apparatus modifies molecules that arrive from the ER. It has the ability to break off into vesicles and can thus facilitate the exocytosis of these modified products.


What main cellular function is performed by the mitochondria?

Mitochondria are involved in cellular metabolism, specifically the production of energy via aerobic respiration.

In the mitochondria, the Krebs cycle produces electron carriers, while the electron transport chain facilitates the formation of a proton gradient. This gradient is used to produce ATP.


What membranes and spaces are present in a mitochondrion?

Mitochondria contain both an outer and an inner membrane. The intermembrane space is located between the two, while the mitochondrial matrix is the innermost space, bounded by the inner membrane.

Both membranes are phospholipid bilayers.


Binary fission is most relevant to the production of which organelle?

Binary fission is the method of replication for mitochondria.

Just like prokaryotic asexual reproduction, which produces identical daughter cells, this method of division yields identical organelles.


Which organelle found in animal cells may have arisen as a result of mutualism?

Mitochondria may have evolved from a symbiotic relationship between small bacteria and larger cells. This is known as the endosymbiotic theory.

Like all instances of mutualism, this situation is thought to have provided benefits to both organisms. The smaller bacterium was given a livable environment while providing energy for the larger host.


What is the cellular role of lysosomes?

Lysosomes break down engulfed pathogens, nutrient molecules, and components of the cell itself that are no longer functional.

Like the stomach, a lysosome contains enzymes and an acidic interior.


What are peroxisomes, and what cellular function do they perform?

Peroxisomes are small membrane-bound organelles that contain enzymes. They function in fatty acid breakdown, detoxification, and facilitation of the pentose phosphate pathway.

Peroxisomes are named for hydrogen peroxide (H2O2), which can be both formed and broken down within the organelle. This is important because H2O2 is a poisonous radical initiator.



The cytoskeleton is a structural web of protein that can change shape to perform a variety of cellular processes.

The prefix "cyto-" means "cell," so the cytoskeleton is the "cell's skeleton."


What main functions are executed by the cytoskeleton?

The cytoskeleton mainly functions to provide shape to the cell, but is also involved in cell movement, endocytosis, and the formation of the spindle apparatus during cell division.

Cytoskeletal proteins have the unusual ability to self-assemble into larger units, and can thus change shape based on the cell's needs.


Name the three primary, protein-based components of the cytoskeleton.

The three primary components of the cytoskeleton are:

  1. microtubules
  2. intermediate filaments
  3. microfilaments


Describe the structural role of microtubules.

Microtubules are thick, hollow tubulin polymers. Specifically, units of alpha- and beta-tubulin dimerize, and many of these dimers come together to form a microtubule.

Microtubules form the spindle apparatus, an essential component of cell division. They also comprise cilia and flagella and perform various other functions.


Describe the structure and role of microfilaments.

Microfilaments are linear, thin filaments composed of many actin monomers. Microfilaments are thinner than either microtubules or intermediate filaments.

As actin polymers, microfilaments play a crucial role in muscle contraction and also facilitate cytokinesis.


Describe the structure and role of intermediate filaments.

Intermediate filaments are fibers composed of various cell-specific proteins; many are composed of keratin. Intermediate filaments are thicker than microfilaments but thinner than microtubules.

These fibers contribute to the rigidity of the cell and also form desmosomes, a type of cellular junction.


What term refers to the tubular structure used for locomotion in some bacteria?

The tubular structure that facilitates bacterial motion is the flagellum.


How do eukaryotic and prokaryotic flagella differ in their protein composition?

Eukaryotic flagella are composed of tubulin while prokaryotic flagella are made up of flagellin.

Tubulin is the protein subunit of microtubules. Specifically, each eukaryotic flagellum consists of nine pairs of microtubules surrounding two individual ones.


What structural and functional differences exist between cilia and flagella?

Cilia are short, found in large numbers, and beat rhythmically in a back-and-forth pattern.

Flagella are longer, with only a few found per cell at the most, and beat more slowly but powerfully to propel the cell in a certain direction.

In eukaryotes, both cilia and flagella have a 9+2 method of organization. This consists of 9 microtubule pairs arranged in a circle surrounding 2 single microtubules.


Which organelles are membrane-bound?

The nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, peroxisomes, and lysosomes are membrane-bound. In plants, chloroplasts and vacuoles fall into this category as well.

Ribosomes, on the other hand, are not membrane-bound; while some are attached to the ER, they are not surrounded by membranes of their own.


Which organelles contain DNA?

The nucleus is the location of most genetic material, but the mitochondria also contain DNA.

In plants, chloroplasts have DNA as well.


Which two membrane-bound organelles are unique to plant cells?

Plant cells, unlike animal cells, contain chloroplasts and vacuoles.

Chloroplasts resemble mitochondria and perform the reactions of photosynthesis. Vacuoles are involved in storage.


Which eukaryotic cell types are encased in a cell wall?

Fungal and plant cells include cell walls, while animal cells do not. Bacterial cells, though not eukaryotic, also have cell walls.

Fungal walls are made of chitin, plant walls are made of cellulose, and bacterial cell walls are composed of peptidoglycan.


What traits distinguish molecules that can easily pass through the plasma membrane?

Molecules can easily travel through the membrane if they are small and nonpolar. To move passively, they also must be traveling down their concentration gradient.

Large molecules and ions must enter cells through special protein channels or via endocytosis.


In animal cells, where is most cholesterol found?

Cholesterol is found in the plasma membrane.

Specifically, cholesterol (shown here) is a lipid that confers rigidity to the membrane. Cholesterol can either be acquired through the diet or synthesized by cells.


What is the difference between peripheral and integral membrane proteins?

Peripheral proteins are loosely attached to either the inner or outer face of the plasma membrane. These proteins can be easily removed from the lipid bilayer.

Integral proteins are firmly attached to the lipid bilayer and are very difficult to remove. These proteins extend at least partially into the hydrophobic interior region of the bilayer.

A transmembrane protein is a specific type of integral protein that spans, or extends through, the entire lipid bilayer.


What are four functions that can be served by plasma membrane proteins?

Plasma membrane proteins can:

  • function in cell-to-cell adhesion
  • function in cell-to-cell communication and recognition
  • serve as receptors for hormones and other ligands
  • assist in transport across the membrane



In a solution, the solute is the substance whose phase is lost after solvation (or the substance of which less is present). For example, dissolving a small amount of solid NaCl into liquid water produces a liquid solution. In this case, NaCl is the solute.

On the AP Biology exam, you will often see references to solutes dissolved in plasma or in other bodily fluids.


What is a concentration gradient, and what is its biological significance?

A concentration gradient is a difference in the amount of solute molecules per unit volume between one region and another.

Molecules are prone to moving down their concentration gradient. For example, if the outside of a cell contains a much higher concentration of glucose than the inside, glucose will tend to move into the cell (if possible). If the membrane is impermeable to glucose, water will tend to move out instead.


What is the difference between passive and active transport?

Passive transport involves the movement of a substance down its concentration gradient without the use of energy.

Active transport requires energy, usually in the form of ATP, and moves a substance against its gradient.


What traits distinguish molecules that require membrane channels to enter a cell?

Substances that require membrane channels include large molecules, polar molecules, and ions.

When these materials move down their concentration gradient, the process is known as facilitated diffusion. Small, nonpolar molecules do not require membrane channels and can travel via simple diffusion.


What is facilitated diffusion, and is it an active or passive process?

Facilitated diffusion is the movement of a substance down its concentration gradient with the help of transmembrane protein channels.

Since facilitated diffusion does not require energy, it is a form of passive transport.


What is the meaning of the term “isotonic?”

An isotonic solution is one with the same solute concentration as a solution to which it is compared.

For example, fluids that are administered in an IV should be isotonic with human cells. In other words, they should have the same osmolality, or solute concentration.


What is the difference between a hypertonic and a hypotonic solution?

In comparison to a reference solution, a hypertonic solution has a greater solute concentration, while a hypotonic solution has a smaller solute concentration.

For example, say that a certain compartment contains 4 grams of NaCl per liter of solution. A solution of 2 g NaCl per L solution would be hypotonic, while a solution of 8 g NaCl per L solution would be hypertonic.


What term describes the passive movement of water or another solvent down its concentration gradient?

This process is called osmosis. Water will always move from areas of low solute to areas with a higher solute concentration.

Osmosis is generally tested in cases where two compartments are separated by a semipermeable membrane. The membrane allows water, but not solute, to pass through; osmosis is thus required to promote similar solute concentrations on both sides.


If a normal human cell is placed in a container of pure water, what will result?

The cell will swell, possibly to the point of lysing (rupturing).

Since pure water contains no solute, it is hypotonic in comparison to the cell. Water will travel down its concentration gradient from the exterior of the cell to the interior.


The hammerhead shark lives in a tropical ocean environment. Are its cells likely to be hypertonic or hypotonic compared to human cells?

Shark cells are likely hypertonic to human cells.

To avoid excessive water loss or gain, shark cells must be specialized to be isotonic, or at least close, to their surroundings. Since the ocean is composed of salt water, shark cells likely contain more solute than the cells of land-dwelling species.


What will happen to a human cell when it is placed in a hypertonic solution?

Water will exit the cell, causing it to shrivel.

A hypertonic solution is one that contains a comparatively high amount of solute (salt). Due to osmosis, water will flow from a low-solute to a high-solute environment in an attempt to equalize the solute concentrations.



Endocytosis is a form of transport in which a material is engulfed by the cell membrane, then enters the cell in a vesicle. Endocytosis requires the expenditure of energy.

Phagocytosis and pinocytosis are both subtypes of endocytosis.



Exocytosis is a form of transport in which a material is packaged into a vesicle which then fuses with the cell membrane. This process, which requires energy, allows the material to be exported from the cell.

Waste products and secreted hormones often leave a cell via exocytosis.



Pinocytosis is a type of endocytosis. In this process, the cell membrane engulfs the extracellular fluid, as well as the small particles it contains, in a vesicle.

Other types of endocytosis include phagocytosis and receptor-mediated endocytosis.


bulk flow

Bulk flow is the movement of molecules (typically fluid molecules) in one direction in response to pressure.

A classic example is the movement of blood through blood vessels. Blood, like typical fluids, tends to travel from areas of high pressure to areas of low pressure.


What term describes diffusion between two regions in which fluids are flowing in opposite directions?

Countercurrent exchange

This form of transport allows for a fairly consistent difference in concentration gradient between the two regions, maximizing diffusion. A classic example of this mechanism occurs in the loop of Henle, a region of the nephron (the structural unit of the kidney).


What term is given to the separation of molecules based on their differing abilities to diffuse through a semipermeable membrane?


In a dialysis procedure, large particles are unable to diffuse through the dialysis membrane, so small particles are left to diffuse in an attempt to reduce the overall concentration gradient between the compartments. This technique is often used to remove toxins from the blood in cases of kidney failure.


Name the three distinct junctions that separate the cells of vertebrates.

The three main types of cell junction to know for the AP Biology exam are:

  1. tight junctions
  2. desmosomes
  3. gap junctions


What features characterize tight junctions?

Tight junctions are composed of multiple proteins, including claudins, and form an impermeable seal between adjacent cells. This prevents fluid and solutes from going "around" the cell to enter a cavity.

Tight junctions are found between cells in the same epithelial layer.


What features characterize gap junctions?

Gap junctions are small channels formed from connexin proteins. They allow small solutes and fluid to pass from one cell to another.

While gap junctions are found between most cells, they are generally associated with cells that must communicate or function together, like neurons.


What features characterize desmosomes?

Desmosomes are composed of intermediate filaments and are found at localized regions throughout a cell's membrane. They generally attach one epithelial or cell layer to another.

Desmosomes are found in epithelial and muscle tissue.


Which type of junction is especially important in the heart?

Gap junctions

The electrical impulse that triggers contraction must travel throughout the entire heart muscle, beginning at the SA node. Gap junctions facilitate the movement of ions, allowing the cells to act together as one unit.


What factor plays the biggest role in limiting the size of a cell?

The cell must have a sufficiently large surface area-to-volume ratio. As the cell becomes larger, volume increases faster than surface area, causing this ratio to decrease.

A large surface area gives the cell a greater ability to obtain nutrients and eliminate waste through its cell membrane.