Eukaryotic Cell Flashcards
With the biological focus of the new MCAT, an understanding of eukaryotic cells is especially vital. Use these cards to familiarize yourself with microtubules and microfilaments, the phases of mitosis, and the various types of membrane transport. For enhanced practice, think about how these concepts relate to real-life topics like cancer and human aging.
The cell theory, originally composed in 1838, includes three primary tenets. Name them.
- The cell is the basic unit of life.
- All living things are composed of cells, whether one or many.
- All cells arise from other cells.
All cells can be categorized into which two broad groups?
- Prokaryotic cells
- 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?
- They 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?
- They 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.
Define:
cytosol
It is the fluid contained within a cell.
In contrast, the cytoplasm includes both the intracellular fluid and all of the extranuclear organelles.
Define:
cytoplasm
It includes both the intracellular fluid, or cytosol, and the organelles.
The only organelle that is not included in the cytoplasm is the nucleus.
Define:
organelle
It is a separate, specialized structure within a cell.
Many organelles are enclosed by lipid bilayers, but some, including ribosomes, are not membrane-bound.
Which organelles are membrane-bound?
- nucleus
- mitochondria
- Golgi apparatus
- endoplasmic reticulum
- peroxisomes
- lysosomes
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.
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.
Which genetic processes take place in the nucleus?
- replication
- transcription
In contrast, translation occurs at ribosomes outside the nucleus. These organelles can either be bound to the ER or located in the cytoplasm.
In what part of the cell is the nucleolus located, and what function does it serve?
within the nucleus
It serves as the site of ribosomal RNA transcription and synthesis of ribosomal subunits.
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.
What main cellular function is performed by the mitochondria?
They 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.
Binary fission is most relevant to the production of which organelle?
mitochondria
Just like prokaryotic asexual reproduction, which produces identical daughter cells, this method of division yields identical organelles.
What characteristic of mitochondrial inheritance deviates from Mendel’s laws?
Mitochondria are only passed down through the maternal line.
Though sperm contain paternal mitochondria, they are quickly destroyed by the egg after fertilization.
A woman has a muscle disease and passes it down to her two sons and three daughters. Her brother has the same disease, but none of his children do. Where is the gene for this condition likely located?
The gene is likely mitochondrial.
Unlike nuclear genes, mitochondrial genes are inherited only through the maternal line. A mother can pass the gene on to both sons and daughters.
Which organelle 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.
Which cell types are likely to contain an above-average number of mitochondria?
Muscle cells
Specifically, red or “slow-twitch” muscle fibers are high in mitochondria due to their increased need to perform aerobic respiration. This produces the ATP needed as part of the contraction cycle.
What is the cellular role of lysosomes?
They 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.
Describe the structural characteristics of the endoplasmic reticulum (ER).
It 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)?
- It 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?
proteins
These can include enzymes and peptide hormones, among other examples.
What biological products are synthesized in the smooth endoplasmic reticulum?
lipids
These include steroid hormones and phospholipids, among other examples.
What is the cellular role of the Golgi apparatus?
- It 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 are peroxisomes, and what cellular function do they perform?
They 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.
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.
A cell taken from a multicellular eukaryote lacks chloroplasts but does contain a cell wall. Which type of cell is this?
fungal
Plants, animals, and fungi are all multicellular, but plants do contain chloroplasts while animals lack cell walls. This sample, then, must come from a fungus.
Define:
cytoskeleton
It 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?
It mainly functions to provide shape to the cell, but also is 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.
- microtubules
- intermediate filaments
- microfilaments
Describe the structure and role of microfilaments.
They 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.
They 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.
Describe the structural role of microtubules.
They 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.
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 type of cytoskeletal fiber is associated with MTOCs?
Microtubules are anchored at MTOCs, or microtubule-organizing centers. The fibers attach to the MTOC at one end and radiate toward other parts of the cell.
Centrosomes are a specific type of MTOC involved in cell division. Spindle fibers attach to the centrosomes, each of which is composed of two tubulin-based centrioles.
Label the following structures on the diagram below: centrosome, centriole, spindle fiber, aster.
Describe the composition of the plasma membrane.
It 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, glycoproteins, and sphingolipids, among other components. Large proteins are associated with it, both integral (embedded within the membrane) and peripheral (bound to the outside of the membrane).
A certain plasma membrane protein is high in valine, isoleucine, and glycine, and contains very few aspartate or lysine residues. What type of protein is this likely to be?
membrane-embedded protein
Peripheral proteins are located on the exterior of the membrane and are likely to be hydrophilic / charged, while membrane-embedded proteins are positioned inside the bilayer and are generally hydrophobic / uncharged. Valine, isoleucine, and glycine are nonpolar (hydrophobic) amino acids.
What is a concentration gradient, and what is its biological significance?
It 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 facilitated diffusion, and is it an active or passive process?
It 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.
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.
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.
Define:
endocytosis
Itis 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.
Define:
exocytosis
It 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.
Define:
pinocytosis
It 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.
What features characterize tight junctions?
They 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 desmosomes?
They 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.
What features characterize gap junctions?
They 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.
Name the four phases of the cell cycle.
- G1
- S (synthesis)
- G2
- M (mitosis)
An additional phase is G0, in which the cell is not immediately preparing for cell division. G0 can be either temporary or permanent.
Which phases of the cell cycle are part of interphase?
- G1
- S
- G2
In these phases, the cell grows, replicates its DNA, and prepares for mitosis.
The M phase (mitosis itself) is not a part of interphase. G0 is generally thought to be distinct when it is permanent (as in neurons, which do not divide), but that is unlikely to be tested on the MCAT.
What does the cell assess at the G1 checkpoint?
The G1 checkpoint, also known as the G1/S checkpoint or the restriction checkpoint, assesses the DNA to ensure that it is in proper condition to be replicated.
Passage through this checkpoint is required to enter the S phase of the cell cycle.
What does the cell assess at the G2 checkpoint?
The G2 checkpoint, also known as the G2/M checkpoint, makes sure that sufficient cell growth has occurred and that the DNA was replicated properly.
Passage through this checkpoint is required to enter the M phase of the cell cycle.
Describe the role of the centrioles during cell division.
They are cylinder-shaped microtubular structures, generally found in pairs as part of the centrosome. Centrioles act to organize the spindle apparatus along which the chromosomes align.
Describe the role of the centromere during cell division.
At the centromere, sister chromatids attach in pairs. This region also contains the kinetochore, which binds to a spindle fiber and allows the chromatids to be pulled to opposite poles of the cell.
The centromere is the location on the chromosome that appears “pinched.”
Describe the role of the kinetochore during cell division.
It is a complex arrangement of various proteins that forms on the centromere. Here, spindle fibers bind, allowing sister chromatids to be separated to opposite poles of the cell.
Name the microtubule-based structure that separates chromosomes during cell division.
The spindle apparatus
Spindle fibers radiate from the centrosome, which contains centrioles, and bind at the kinetochore region of the centromere.
What cellular events characterize prophase?
- Loose chromatin condenses into visible chromosomes.
- The nuclear membrane disintegrates.
- The nucleolus disappears.
- The centrioles and spindle apparatus form.
What cellular events characterize anaphase?
- Sister chromatids are cleaved apart.
- Spindle fibers shorten, causing each sister chromatid to move to the opposite pole as its partner.
What cellular events characterize telophase?
- The cell elongates.
- Each daughter nucleus forms a new nuclear membrane and nucleolus.
- The chromosomes start to become less condensed.
Which two processes ensure that none of the four daughter cells produced during meiosis are genetically identical?
- Crossing over
- Independent assortment
Crossing over is a method of genetic recombination that occurs during prophase I of meiosis. In this process, homologous chromosomes exchange genetic material, increasing total variation.
The principle of independent assortment, or Mendel’s second law, states that genes for different traits separate into gametes independently of each other (as long as those traits are not linked).
