Chapter Three (PART THREE)
Passive Mechanisms
- Filtration
- The process of forcing molecules through membranes due to the exertion of pressure.
- Blood pressure, due to the action of the heart, is a type of hydrostatic pressure used as the force for filtration.
- Filtration is used by the body to produce tissue fluid from blood plasma; water and small solutes are sent through the walls of capillaries to deliver O2 and nutrients to cells, while large particles (e.g., plasma proteins) remain in the capillaries.
- Particles are generally limited in movement by their size relative to the openings in the capillary wall.
- Diffusion
- Molecules move through the phospholipid bilayer from regions of higher concentration to regions of lower concentration.
- Examples include exchange of oxygen and carbon dioxide in the lungs.
- No cellular energy (ATP) is required.
- Facilitated diffusion
- Ions move through channels, or molecules move by carrier proteins, across the membrane from higher to lower concentration.
- Example: movement of glucose through a cell membrane.
- Osmosis
- Water molecules move through a selectively permeable membrane toward the solution with more impermeant solute (greater osmotic pressure).
- Example provided: distilled water entering a cell.
- Filtration (revisited)
- Emphasizes that smaller molecules are driven through porous membranes by hydrostatic pressure.
- Example: molecules leaving blood capillaries.
Active Mechanisms
- Active transport
- Substances moved from an area of lower concentration to an area of higher concentration.
- Requires cellular energy in the form of ATP and carrier molecules (pumps).
- The carrier proteins may also be called pumps, since they move substances against the concentration gradient (example: the ATPase pump).
- Up to about 40 rac{ ext{percent}}{} of a cell’s energy may be used to fuel this process.
- Substances moved by active transport include some sugars and some amino acids, as well as ions: ext{Na}^+, ext{K}^+, ext{Ca}^{2+}, ext{H}^+.
- Active Transport: Na⁺/K⁺ ATPase Pump
- A specific example of active transport that maintains essential ion gradients across the plasma membrane.
- Diagrammatically represented in the Na⁺/K⁺ ATPase pump (Fig. 3.18).
Endocytosis & Exocytosis
- Endocytosis and Exocytosis
- Large substances are moved into or out of a cell without crossing the cell membrane directly.
- Endocytosis: molecules too large to be transported by other means are brought into the cell inside a vesicle that forms from a section of the cell membrane.
- Exocytosis: movement of materials out of the cell in a vesicle that fuses with the cell membrane and releases contents to the outside.
Three Types of Endocytosis
- Pinocytosis (cell drinking)
- The cell engulfs liquids and dissolved molecules.
- A small indentation in the cell membrane surrounds the fluid, creating a vesicle.
- Phagocytosis (cell eating)
- The cell takes in solid particles (e.g., a white blood cell engulfing a bacterium).
- Once the vesicle enters the cell, it fuses with a lysosome and the contents are digested by lysosomal enzymes.
- Receptor-mediated endocytosis
- The cell takes in very specific molecules (ligands) that bind to specific receptors on the cell membrane to initiate vesicle formation.
Lysosomal Digestion of a Phagocytized Particle
- Following phagocytosis, phagosomes fuse with lysosomes where enzymatic digestion occurs.
- Molecules outside the cell (some ligands) bind to receptor proteins on the cell membrane.
- Receptor-ligand complexes form on the membrane.
- The receptor protein–ligand complex is internalized into a vesicle.
- Vesicle forms, enclosing and transporting receptor-ligand combinations for intracellular processing.
Movements Through Cell Membranes: Passive Mechanisms (Table 3.2)
- Diffusion
- Characteristics: Molecules move through the phospholipid bilayer from regions of higher concentration to regions of lower concentration.
- Energy source: Molecular motion (no cellular energy required).
- Examples: Movement of oxygen and carbon dioxide in the lungs.
- Facilitated diffusion
- Characteristics: Ions move through channels, or molecules move by carrier proteins, across the membrane from higher to lower concentration.
- Energy source: Molecular motion.
- Example: Movement of glucose through a cell membrane.
- Osmosis
- Characteristics: Water molecules move through a selectively permeable membrane toward the solution with more impermeant solute (greater osmotic pressure).
- Energy source: Molecular motion.
- Example: Distilled water entering a cell.
- Filtration
- Characteristics: Smaller molecules are forced through porous membranes from regions of higher pressure to regions of lower pressure.
- Energy source: Hydrostatic pressure.
- Examples: Molecules leaving blood capillaries.
Movements Through Cell Membranes: Active Mechanisms (Table 3.2)
- Active transport
- Characteristics: Carrier molecules transport molecules or ions through membranes from regions of lower concentration toward regions of higher concentration.
- Energy source: Cellular energy (ATP).
- Examples: Movement of various ions, sugars, and amino acids through membranes.
- Endocytosis
- Pinocytosis
- Energy source: Cellular energy.
- Example: Uptake of water and solutes by all body cells.
- Phagocytosis
- Energy source: Cellular energy.
- Example: White blood cell engulfing a bacterial cell.
- Receptor-mediated endocytosis
- Energy source: Cellular energy.
- Example: Cell removing cholesterol molecules from its surroundings.
- Exocytosis
- Energy source: Cellular energy.
- Example: Neurotransmitter release.
3.4: The Cell Cycle
- Cell Cycle
- The series of changes a cell undergoes from the time it is formed until it divides.
- Main phases: interphase, mitosis, and cytokinesis (division of the cytoplasm).
- Daughter cells may then undergo changes to become specialized.
- Checkpoints: transitions are controlled by interactions of special proteins.
- A restriction checkpoint determines the fate of the cell: continue the cycle and divide, enter a non-dividing specialized stage, or die.
- Telomeres on chromosome tips shorten with each cell division, contributing to a maximum number of divisions for most cells.
- Interphase
- Phase before division; cell grows and synthesizes new molecules, membranes, DNA, and organelles.
- High synthetic activity; not a period of rest.
- G1: cell growth; S phase: DNA replication; G2: additional growth.
- Mitotic phases
- Mitosis: nuclear division.
- Cytokinesis: cytoplasmic division.
- The cell cycle progresses from interphase to mitosis and cytokinesis, with a restriction checkpoint governing progression.
- Prophase, Metaphase, Anaphase, Telophase are the four stages of mitosis.
- Apoptosis can occur as part of cell fate decisions at checkpoints.
Interphase Details
- Interphase is the phase of growth and DNA synthesis prior to cell division.
- G1 and G2 are growth phases surrounding the S phase.
- S phase: DNA replication to prepare for division.
Cell Division: Meiosis vs. Mitosis
- Meiosis
- Only used for sperm and egg production.
- Ensures mature gametes have half the normal number of chromosomes.
- Mitosis + Cytokinesis
- More common; increases cell number for growth, development, and wound healing.
- Mitosis: division of the nucleus.
- Cytokinesis: division of the cytoplasm.
- Regulated to ensure each daughter cell receives an exact copy of the original DNA.
- Prophase
- DNA condenses into visible chromosomes (each consisting of two chromatids connected by a centromere).
- Centrioles migrate to opposite poles.
- Microtubules form spindle fibers.
- Nuclear membrane and nucleolus disassemble.
- Metaphase
- Spindle fibers attach to centromeres.
- Chromosomes align midway between centrioles.
- Anaphase
- Sister chromatids are pulled apart to become individual chromosomes.
- Chromosomes are pulled toward opposite poles by shortening spindle fibers.
- Telophase
- Chromosomes reach opposite poles and begin to de-condense.
- Nuclear envelope and nucleolus reassemble around each set of chromosomes.
- Spindle fibers disassemble.
Cytoplasmic Division (Cytokinesis)
- Begins during anaphase; membrane constricts to form a cleavage furrow.
- Furrow deepens through telophase, pinching the cell into two.
- Contractile ring of microfilaments assists membrane constriction.
- Result: two new cells with identical genetic information but possibly slightly different cytoplasmic content.
Meiosis vs. Mitosis (Comparison)
- A visual comparison exists in the text (Meiosis vs. Mitosis figure), illustrating differences in purpose, chromosome number, and outcomes.
Cell Differentiation
- Differentiation: process by which a cell develops/specializes into a specific type of cell with specialized functions.
- Humans have more than 290 types of differentiated cells.
- Differentiation allows cells to specialize by using different parts of the genome; genes are turned on or off in different cell types.
- Stem cells retain the ability to divide without specialization; their presence allows for continuous growth and renewal.
- Self-renewal: ability of a stem cell to divide and give rise to at least one other stem cell.
- Progenitor cells: daughters of stem cells that are partially specialized.
- Stem and progenitor cells enable production of differentiated cells across body tissues.
Stem and Progenitor Cells and Differentiation
- All cells in the human body derive from stem cells through mitosis and differentiation.
- There are over 290 types of human cell types.
Cell Death
- Not all cells divide or differentiate; some die.
- Apoptosis: a form of programmed cell death that is a normal part of development, not necessarily due to injury or disease.
- Role: removes overgrown tissues, damaged cells, and excess fetal cells.
- Steps of apoptosis:
- The cell becomes rounded and bulges (blebbing).
- Nuclear membrane breaks down.
- Chromatin condenses and enzymes cut up the chromosomes.
- The cell shatters into many membrane-bound pieces.
- Scavenger cells engulf and destroy the apoptotic fragments.
