Cellular Level of Organization

Question 1

Describe the cell theory and the process of cellular differentiation.

Answer 1

Cell Theory:

(1) Cells are the building blocks of all plants and animals. (2) All new cells come from the division of preexisting cells. (3) Cells are the smallest living units that carry out all vital physiological functions.

• Each cell maintains homeostasis at the cellular level.
• At higher levels of organization, many cells must coordinate to achieve homeostasis.
• All cells in the body are descendants of a single cell: the fertilized ovum (i.e., the zygote). This cell has the genetic potential to become any cell in the body.

Cellular Differentiation

Regional differences of the ovum cytoplasm at fertilization define how it will subdivide as the first cell divisions occur. During these first cell divisions, the new cells do not grow, but simply subdivide into smaller parcels.

The cytoplasmic difference affect the DNA of the new cells, turning specific genes on or off. The descendant (a.k.a. daughter) cells begin to develop specific structural and functional characteristics.

Question 2

Describe a body cell and its organelles.

Answer 2

Cells are the smallest living units of life. A plasma membrane separates the cytoplasm (a colloid) from the extracellular fluid. The cytoplasm contains cytosol (intracellular fluid and inclusions) and organelles (intracellular structures suspended within the cytosol with specific functions).

Nonmembranous [6] organelles include the cytoskeleton, microvilli, centrioles, cilia, flagella, and ribosomes.

Membranous [7] organelles include the nucleus, mitochondria, the endoplasmic reticulum, the golgi apparatus, lysosomes, and peroxisomes.

Question 3

Describe the structural and functional features of the plasma membrane.

Answer 3

• The plasma membrane is selectively permeable barrier that separates the inside of the cells from the surrounding extracellular fluid and controls the entry of ions and nutrients (like glucose), the elimination of wastes, and the release of secretions.
• The glycocalyx is a superficial layer of membrane carbohydrates that are components of complex molecules (like proteoglycans, glycoproteins, and glycolipids), and is important in cell recognition, binding to extracellular structures, and lubrication of the cell surface.
• Cholesterol, an amphipathic molecule, is an important component of the phospholipid bilayer. It stiffens the membrane at normal biological temperatures, which makes it less fluid and less permeable.
• Integral proteins, transmembrane proteins, and peripheral proteins have various functional classes.
• The general functions of the plasma membrane include physical isolation, regulation of exchange with the environment, and structural support.

Question 4

Differentiate among the structures and functions of the cytoskeleton. [8]

Answer 4

• Microvilli are finger-shaped extensions of the plasma membrane of some cells. A core of microfilaments stiffins each micovillus and anchors it to the cytoskeleton at the terminal web (a layer of microfilaments just inside the plasma membrane at the exposed surface of a cell, forming a lining). They greatly increase the surface area of the cell and enhance its ability to absorb materials from the extracellular fluid.
• The smallest of the cytoskeletal elements are microfilaments, composed of the protein actin. They provide strength, alter cell shape, bind the cytoskeleton to the plasma membrane, tie cells together, and are involved in muscle contraction.
• Intermediate filaments are the strongest most durable cytoskeletal component. They provide strength and move materials through the cytoplasm.
• Microtubules are the largest component of the cytoskeleton. They are hollow tubes built from the protein tubulin, and extend outward into the periphery of the cell from a region near the nucleus called the centrosome. They provide strength and move organelles.
• Thick filaments found in the cytoplasm of skeletal and cardiac muscle cells interacts with actin microfilaments to produce muscle contraction.
• Centrioles are comprised of nine groups of microtubule triplets that form a short cyllinder, and are found in the centrosome near the nucleus. They organize microtubules in the spindle to move chromosomes during cell division.
• Cilia are extensions of the cell membrane that as a group propel fluid or solids across a cell surface, or individually detect environmental stimuli.
• Flagella are extensions of the cell membrane comprised of nine groups of microtubule doublets and provides mobility to the cell. A flagellum propels sperm.

Question 5

Describe the ribosome and the smooth and rough endoplasmic reticula, and indicate their specific functions.

Answer 5

• A functional ribosome consists of a small and a large subunit, each containing special proteins and ribosomal RNA. The 30S subunit contains 16S rRNA and 21 proteins; the 50S subunit contains 5S and 23S rRNA and 31 proteins.
• Ribosomes synthesize proteins.
• The smooth endoplasmic reticulum lacks ribosomes, and the cisternae are often tubular. Functions:
  
  • Synthesize phospholipids and cholesterol for maintenance and growth of membranous organelles and the plasma membrane.
  • In reproductive organs, synthesize steroid hormones.
  • In liver and fat cells, synthesize and store glycerides, especially triglycerides.
  • In skeletal muscle and liver cells, synthesize and store glycogen.
• The rough endoplasmic reticulum is where many newly synthesized proteins are chemically modified and packaged for export to the Golgi apparatus. Fixed ribosomes are bound to the RER. Proteins synthesized by these ribosomes enter the lumen of the RER’s cisternae.
• The ratio of SER to RER varies with cell type. Reproductive cells will have more SER than RER, for example, and pancreatic cells that produce digestive enzymes will contain more RER than SER.

Question 6

Describe the Golgi apparatus, and indicate its specific functions.

Answer 6

• The Golgi apparatus (1) renews or modified the plasma membrane; (2) modifies and packages secretions for release by exocytosis; and (3) package special enzymes within vesicles (lysosomes) for use in the cytosol.
• Typically consists of 5-6 flattened cisternae. A typical cell may contain more than one, located near the nucleus.
• Transport vesicles arrive at the cis face of the Golgi apparatus, and ultimately the product arrives at the trans face, which is usually oriented toward the free surface of the cell.

Question 7

Describe the structure of a mitochondrion, and explain the significance of mitochondria to cellular function.

Answer 7

• Double membrane; inner membrane contains numerous folds called cristae which increase the surface are in contact with the mitochondrial matrix, and so allow more attached protein complexes and enzymes.
• Contain their own DNA and ribosomes; however, their functions depend on imported proteins coded by nuclear DNA.

Question 8

Describe the role of the nucleus in maintaining homeostasis at the cellular level.

Answer 8

• The nucleus stores all the information needed to direct the synthesis of more than 100,000 different proteins. It determines the structure and function of the cell by controlling which proteins are synthesized, under what conditions, and in what amounts. Most cells contain a single nucleus. However, there are exceptions. For example, skeletal muscle cells have many nuclei, wheras mature red blood cells have none. A cell without a nucleus cannot repair itself, hence it will disintegrate within a few months.
• When the extracellular environment changes, the cell has mechanisms that can provide rapid short-term adjustments, and long-term adaptations.

Question 9

Describe the functions of the cell nucleus, and distinguish between chromatin and a chromosome.

Answer 9

• The nucleus is the control centre for chemical processes.
• The perinuclear space separates the two layers of the nuclear envelope.
• Nuclear pores permit communication between the nucleus and the cytosol.
• Nucleoli are transient nuclear organelles that synthesize ribosomal RNA and assemble ribosomal subunits. They are comprised of RNA, enzymes, and histone proteins.
• In the nucleus, the DNA strands are coiled. The coils wrap around histone molecules forming complexes known as nucleosomes.
• In cells not dividing, the nucleosomes are loosely coiled, forming chromatin.
• At the beginning of cell division, DNA coiling becomes tighter and more complex, forming distinct structures called chromosomes.
• In the nuclei of somatic cells, there are 23 pairs of chromosomes.

Question 10

Discuss the nature of genetic code, and summarize the process of protein synthesis.

Answer 10

• A gene is a functional unit of heredity.
• DNA triplets (codons) determine the sequence of mRNA codons (transcription).
• The mRNA codons determine the sequence of tRNAs.
• The sequence of tRNAs determines the sequence of amino acids in the polypeptide/protein. The ribosomal RNA of the ribosome strings amino acids together to form the protein.

Question 11

Summarize the process of transcription.

Answer 11

• Transcription is the synthesis of RNA from a DNA template.
• First, the hydrogen bonds between nitrogenous bases is disrupted and the the histone is removed to reveal the promoter (control segment), the first segment of a gene.
• RNA polymerase promotes hydrogen bonding between the nitrogenous bases of the DNA template and the complementary RNA nucleotides, which are strunge together via covalent bonds.
• At the stop signal, transcription ends and the complementary DNA strands reassociate as the enzyme and mRNA detach.
• Immature DNA contains introns and exons. RNA processing splices the introns out and exons are spliced together.

Question 12

Summarize the process of translation.

Answer 12

• Translation builds polypeptides as directed by an mRNA strand.
• Initiation begins when mRNA binds to a small ribosomal subunit. An initiator tRNA carrying methionine binds to the start codon on the mRNA strand.
  
  • The small and large ribosomal subunits join together and enclose the mRNA and tRNA. The large ribosomal subunit has three sites for tRNA binding, called the E (exit) site, P (polypeptide binding) site, and the A (arrival) site. The initiation complex is complete.
• In elongation, amino acids are added one by one to the growing polypeptide chain. When a complementary tRNA binds to the A site, ribosomal enzymes remove the amino acid from the tRNA at the P site and attach it to the amino acid delivered to the A site. The ribosome then moves one codon farther along the mRNA strand and the tRNA that moves from the P site to the E site is released into the cytoplasm.
• Termination occurs as a protein-releasing factor recognizes the stop codon. A ribosomal enzyme then breaks the bond between the polypeptide and the tRNA in the P site, releasing the polypeptide

Question 13

Contrast permeable, selectively permeable, and impermeable membranes.

Answer 13

• Freely permeable membranes allow any substance to pass without difficulty. No living cell has a freely permeable plasma membrane.
• Selectively permeable membranes permit the passage of some materials and prevent the passage of others. The distinction may be based on size, molecular shape, lipid solubility, electrical charge, or other factors. Plasma membranes allow lipids and water free passage. Cells differ in their permeabilities depending on what lipids and proteins are present and how they are arranged.
• Nothing can pass through impermeable membranes. Cells may be impermeable to specific substances, but no living cell has an impermeable plasma membrane.

Question 14

Explain the process of diffusion, and identify its signficance in the body. What factors influence diffusion rates?

Answer 14

• Diffusion is the net movement, a distribution, of a substance from an area of higher concentration to an area of lower concentration. After the concentration gradient is eliminated, the molecular motion continues, but net movement no longer occurs in any particular direction.
  
  • Diffusion of nutrients, waste products, and dissolved gases must keep pace with the demands of the active cells.
• Important factors that influence diffusion rates are as follows:
  
  • Distance; the shorter the distance, the more quickly concentration gradients are eliminated.
  • Molecule/ion size; the smaller, the faster the diffusion rate
  • Temperature; the higher the temperature, the faster the diffusion rate
  • Concentration gradient; the steeper the concentration gradient, the faster diffusion proceeds
  • Electrical forces; attraction or repulsion can accelerate or reduce the ion diffusion rate.

Question 15

Explain the process of osmosis, and identify its significance in the body.

Answer 15

The passive diffusion of water across a selectively permeable membrane down its concentration gradient is called osmosis.

The osmotic pressure of a solution is an indication of the force with which pure water moves into that solution as a result of its solute concentration.

Question 16

Describe carrier-mediated transport and its role in the absorption and removal of specific substances.

Answer 16

• In carrier-mediated transport, integral proteins facilitate membrane passage.
• Facilitated diffusion; no ATP is expended, passive transport through a carrier protein; diffusion cannot occur without the protein.
• Active transport; ATP provides the energy needed to move ions or molecules across the membrane; not dependent on a concentration gradient. (e.g., ion pumps)
• Secondary active transport; transport mechanism itself does not require energy from ATP, but the cell often needs to expend ATP at a later time to preserve homeostasis. Movement across the membrane follows an existing concentration gradient for one of two substances transported, typically sodium ions, so transport does not require energy.
  
  • The cell must then expend ATP to pump the arriving sodium ions out of the cell using the sodium-potassium exchange pump.

Question 17

Describe vesicular transport as a mechanism for facilitating the absorption or removal of specific substances from cells.

Answer 17

• Vesicular transport, vesicles selectively carry materials into or out of the cell.
• Endocytosis is the import of extracellular substances through the formation of vesicles at the cell surface known as endosomes.
• Receptor-mediated endocytosis produces vesicles containing a specific target molecule.
• Pinocytosis is the formation of endosomes filled with extracellular fluid, ‘cell drinking’.
• Phagocytosis, ‘cell eating’ produces phagosomes containing solid objects that may be as large as the cell itself!
• Exocytosis is the functional opposite of endocytosis.

Question 18

Distinguish between interphase and cell division in the cell cycle.

Answer 18

• Interphase is the period in which the cell is performing normal functions and is not actively engaged in cell division.
• During interphase, the cell prepares for cell division.

Question 19

Describe interphase, and explain its significance.

Answer 19

• Interphase can be divided into G₁, S, and G₂ phases.
  
  • G₁: normal cell functions plus cell growth, organelle duplication, protein synthesis.
  • S: DNA replication, synthesis of histones
  • G₂: Protein synthesis
• Interphase is followed by mitosis (prophase, metaphase, anaphase, telophase)

Question 20

Describe the process of mitosis and its role in the cell life cycle.

Answer 20

• Mitosis is a series of events during which the duplicated chromosomes of a cell separate and migrate into two identical nuclei.
• Prophase begins when chromosomes coil into visible structures under a light microscope.
• Metaphase begins as the chromatids align along the metaphse plate.
• Anaphase begins when the centromere of each chromatid pair splits and the chromatids separate. They are pulled along the spindle fibres toward opposite poles.
• During telophase, each new cell prepares to return to interphase. The nuclear envelopes reform, the nuclei enlarge, and the chromosomes gradually uncoil to chromatin. This marks the end of mitosis.
• Cytokinesis begins with the formation of a cleavage furrow during anaphase and continues throughout telophase. The completion of cytokinesis marks the end of cell division.

Question 21

Discuss the relationship between cell division and cancer.

Answer 21

• Tumors (neoplasms) and cancer are characterized by abnormal cell growth and division.
• When the rates of cell division and growth exceed the rate of cell death, a tissue begins to enlarge. Cancer is an illness characterized by mutations - permanent changes in DNA nucleotide sequence and function - that disrupt normal control mechanisms that regulate the rates of cell division.
• In a benign tumour, the cells remain within the originating tissue. These are not life-threatening and are easily removed.
• Cells in a malignant tumour divide very rapidly, releasing chemicals that stimulate the growth of new blood vessels (angiogenesis) into the area. Increased nutrient availability accelerates tumour growth, and malignant cells can then migrate into surrounding tissues. This is called metastasis, and can produce secondary tumours in tissues remote from the primary tumour site.

Question 22

What is the structure and function of a peroxisome?

Answer 22

• STRUCTURE: Vesicles (membranous sacs) containing degradative enzymes.
• FUNCTION: Breakdown of organic compounds; neutralization of toxic compounds generated in the process.

Question 23

What is the structure and function of a lysosome?

Answer 23

• STRUCTURE: Vesicles containing digestive enzymes.
• FUNCTION: Breakdown of organic compounds and damaged organelles or pathogens.

Question 24

What is the structure and function of microvilli?

Answer 24

• STRUCTURE: Membrane extensions containing microfilaments.
• FUNCTION: Increase surface area to facilitate absorption of extracellular materials.

Question 25

What is the structure and function of Golgi apparatus?

Answer 25

• STRUCTURE: Stacks of flattened membranes (cisternae) containing chambers.
• FUNCTION: Stores, alters, and packages synthesized products.

Question 26

What is the structure and function of the nucleus?

Answer 26

• STRUCTURE: A fluid nucleoplasm containing enzymes, proteins, DNA, and nucleotides; surrounded by a double membrane, the nuclear envelope. The outer nuclear membrane is continuous with the endoplasmic reticulum, so the space between the inner and outer nuclear membranes is directly connected with the lumen of the endoplasmic reticulum.
• FUNCTION: Controls metabolism; stores and processes genetic information; controls protein synthesis.

Question 27

What is the structure and function of the endoplasmic reticulum?

Answer 27

• STRUCTURE: Network of membranous sheets and channels extending throughout the cytoplasm.
• FUNCTION: Synthesis of secretory products; intracellular storage and transport; detoxification of drugs or toxins.
• Smooth ER, which has no attached ribosomes, synthesizes lipids and carbohydrates.
• Rough ER, which has ribosomes bound to the membranes, modifies and packages newly synthesized proteins.

Question 28

What is the structure and function of ribosomes?

Answer 28

• STRUCTURE: RNA and proteins; fixed ribosomes bound to rough ER; free ribosomes scattered in cytoplasm.
• FUNCTION: Protein synthesis.

Question 29

What is the structure and function of the cytoskeleton?

Answer 29

• STRUCTURE: Proteins organised in fine filaments (microfilaments) or slender tubes (microtubules); organizing center at the centrosome, a cytoplasmic region that contains a pair of centrioles.
• FUNCTION: Strengthens and supports cell; aids in movement of cellular structures and materials.

Question 30

What is the structure and function of mitochondria?

Answer 30

• STRUCTURE: Double membrane, with inner membrane folds enclosing important metabolic enzymes.
• FUNCTION: Produces 95% of the ATP (a high energy molecule) required by the cell.

Question 31

Describe the functional classes of membrane proteins [6].

Answer 31

• Anchoring proteins attach the plasma membrane to other structures and stabilize its position. Inside the cell, membrane proteins are bound to the cytoskeleton, a network of supporting filaments in the cytoplasm.
• Recognition proteins are detected by cells of the immune system.
• Enzymes may be integral or peripheral.
• Receptor proteins bind to specific extracellular molecules called ligands. Such receptor-ligand binding can affect a cell’s activities. A ligand can be anything from a small ion such as calcium, to a relatively large and complex horome such as insulin.
• Carrier proteins bind solutes and transport them across the plasma membrane.
• Channels are integral proteins containing a central pore that forms a passageway completely through the plasma membrane. The channel permits the passage of water and small solutes that cannot otherwise cross the lipid bilayer. (e.g., aquaporin)

Question 32

What are the three basic functions of lysosomes?

Answer 32

• Destruction of foreign material
• Removal of damaged organelles
• Autolysis; following injury to, or death of, the cell.

Question 33

What is membrane flow?

Answer 33

With the exception of mitochondria, all membranous organelles in the cell are either interconnected or in communication through the movement of vesicles. This is an example of the dynamic nature of cells, and gives cells a way to change the characteristics of their plasma membranes - the lipids, receptors, channels, anchoring proteins, and enzymes - as they grow, mature, or respond to a specific environmental stimulus.

Question 34

Briefly discuss glycolysis and aerobic metabolism.

Answer 34

Glycolysis occuring in the cytoplasm generates two molecules of pyruvate from a glucose molecule, which the mitochondria then absorb. After a CO₂ molecule is removed from each pyruvate molecule, the remainder enters the TCA, an enzymatic pathway that systematically breaks down the absorbed pyruvate remnant into carbon dioxide and hydrogen atoms. The CO₂ diffuses into the cytoplasm, and the protons are delivered to enzymes of the cristae, which catalyze ATP synthesis. Finally, oxygen combines with the protons to form water molecules.

Since mitochondrial activity requires oxygen, so we refer to it as cellular respiration. Glycolysis generates a very small amount of ATP compared to aerobic respiration performed by mitochondria.

Question 35

Describe the concept of tonicity.

Answer 35

• Descriptions of the effects of various osmotic solutions on cells = tonicity.
• Isotonic: a solution that does not cause an osmotic flow of water into or out of a cell.
• Hypertonic: a solution that causes osmotic water flow into the cell.
• Hypotonic: exposure to a hypotonic solution results in the osmotic movement of water out of the cell, and it shrivels.