Ch 3 Part 2 Notes (Cells: The Living Units)

Vesicular Transport and Endocytosis/Exocytosis

Vesicular transport: transport of large particles, macromolecules, and fluids across the plasma membrane in membranous sacs called vesicles; requires cellular energy (usually ATP).
$\text{Energy: } ATP$
Vesicular transport types:
- Endocytosis: transport into the cell; main route into the cell for macromolecules and fluids; types include phagocytosis and pinocytosis.
- Exocytosis: transport out of the cell; Endo = into, Exo = out; cyt- meaning cell.
Endocytosis vs Exocytosis (summary):
- Endocytosis brings material into the cell via vesicles formed from the plasma membrane.
- Exocytosis releases substances from the cell by vesicles fusing with the plasma membrane.
Phagocytosis (Endocytosis type):
- Meaning: "cell eating".
- Process: cell engulfs external particle (e.g., bacteria, cell debris) and encloses it in a vesicle.
- Function: disposal of foreign substances or dying cells to prevent inflammation from debris.
- Examples: immune system cells (macrophages, certain white blood cells) ingest bacteria.
- Often depicted in teaching figures and videos.
Pinocytosis (Endocytosis type):
- Meaning: "cell drinking".
- Step 1: cell gulps extracellular fluid into a vesicle.
- Step 2: vesicle fuses with endosome for sampling contents.
- Function: routine sampling of extracellular fluid (ECF) by most cells.
- Examples: hypothalamus samples blood to maintain homeostasis; parathyroid checks blood calcium levels.
Exocytosis:
- Vesicular transport to eject substances from the cell into the extracellular fluid.
- Step 1: Substance enclosed in membranous vesicle.
- Step 2: Vesicle fuses with plasma membrane.
- Step 3: Substance released to ECF.
- Function: secretion of neurotransmitters, hormones, mucus, wastes, etc.
Figure 3.10 (Endocytosis and Exocytosis) summary of steps:
1) Coated pit ingests substance; protein coat forms.
2) Protein-coated vesicle detaches.
3) Coat proteins recycled to plasma membrane.
4) Uncoated endocytic vesicle fuses with an endosome.
5) Transport vesicle with membrane components moves to plasma membrane for recycling.
6) Fused vesicle may (a) fuse with lysosome for digestion or (b) deliver contents to the opposite side of the cell (transcytosis).

Resting Membrane Potential (RMP)

Definition: potential energy produced by separation of oppositely charged particles across the plasma membrane in excitable cells (nerves and muscles).
Location: RMP occurs at the membrane surface; the rest of the cell and extracellular fluid are effectively neutral.
Typical value for muscle/nerve cells: $V_{mem} \approx -70\text{ mV to } -90\text{ mV}$ (inside becomes more negative).
Quick Na+-K+ Pump review:
- Na+-K+ Pump creates both chemical and electrical gradients.
- Chemical gradient: more Na+ outside, more K+ inside the cell.
- Electrical gradient: membrane interior becomes more negative due to ion movement.
- Ion movement specifics:
- 3 Na+ are pumped out of the cell per cycle.
- 2 K+ are pumped into the cell per cycle.
- This contributes to the overall electrochemical gradient across the membrane.
Resting Membrane Potential maintenance mechanisms:
- Potassium (K+) leakage channels: the membrane is highly permeable to K+; K+ leaks down its chemical gradient out of the cell.
- Large intracellular proteins remain inside (too large to pass through channels), contributing to negative interior.
- Na+–K+ pump continuously ejects 3 Na+ for every 2 K+ brought in, maintaining the gradient.
- Result: cytoplasmic side becomes more negative relative to the extracellular fluid.
Why Na+ ions don’t play a major role in RMP:
- Fewer Na+ leakage channels; Na+ movement across the membrane is limited, so Na+ contributes less to RMP compared to K+.
- Facilitated diffusion is limited by channel availability.
Key takeaways about RMP:
- Both K+ leakage and the Na+-K+ pump are critical for maintaining RMP.
- RMP provides the potential for excitable cells to respond to stimuli (e.g., muscle contraction, nerve impulse).
- Without a negative RMP (~ -70 to -90 mV), cells cannot be excited.

Cytoplasm and Its Three Elements

Inside the cell: cytoplasm = cellular material between plasma membrane and nucleus; location of most cellular activities.
Three elements of cytoplasm:
- Cytosol
- Organelles
- Inclusions
Cytosol (the fluid):
- Fluid in which other structures are suspended.
- Largely composed of water with solutes such as proteins, sugars, and salts.
Organelles (metabolic machinery):
- Each organelle carries out specific cellular functions.
Inclusions (variable by cell type):
- Chemical substances present depending on the cell type.
- Examples: glycogen in liver and muscle cells.

Organelles

Mitochondria
- Function: provide most of the cell’s ATP supply via aerobic cellular respiration (oxidative phosphorylation).
- Structure: double-membrane bound, “ATP power plant”; numerous in cells with high energy demand (e.g., muscle).
- Cellular respiration overview: breakdown of glucose by oxidation to replenish ATP stores:
 $\text{C}6\text{H}{12}\text{O}6 + 6\,\text{O}2 \rightarrow 6\,\text{CO}2 + 6\,\text{H}2\text{O} + \text{Energy}$
- Why many mitochondria in muscle?: muscle contraction requires substantial energy; more mitochondria meet this demand.
- Mitochondrial diseases: lack of energy production due to mitochondrial dysfunction can affect neurons, muscles, heart, kidneys, etc.
Ribosomes
- Function: site of protein synthesis.
- Locations: two places where ribosomes are found:
- Floating freely in the cytosol, synthesizing cytosolic proteins.
- Attached to the rough endoplasmic reticulum (RER), synthesizing proteins destined for secretion or membranes.
- Appearance: tiny red dots on diagrams.
Endoplasmic Reticulum (ER)
- Structure: continuous with the outer nuclear membrane; two types:
- Rough Endoplasmic Reticulum (RER)
- Smooth Endoplasmic Reticulum (SER) (surface studded with ribosomes on the RER).
Smooth Endoplasmic Reticulum (SER)
- Structure: continuous with rough ER; no ribosomes on the surface; membranous sacs and tubules.
- Function:
- Lipid synthesis (phospholipids and cholesterol).
- Detoxification of drugs (e.g., liver).
- Breakdown of stored glycogen to free glucose (e.g., liver).
- Ca2+ storage (especially in skeletal and cardiac muscle; known as sarcoplasmic reticulum).
Rough Endoplasmic Reticulum (RER)
- Structure: surface studded with ribosomes; continuous with the nuclear envelope; extensive system of parallel sacs.
- Functions:
- Synthesis of proteins.
- Folding of proteins.
- Proteins are enclosed in vesicles and sent to the Golgi apparatus for further processing (protein factory).
Golgi Apparatus
- Structure: stack of flattened membranous sacs and associated vesicles.
- Function: modify, concentrate, and package proteins and lipids made in the ER for export or for membranes.
- Phrase: often described as a "warehouse + UPS" for cellular products.
Peroxisomes
- Structure: spherical membranous sacs containing a variety of detoxifying enzymes.
- Function:
- Detoxifying enzymes to neutralize toxins and free radicals.
- Abundant in kidney and liver cells active in detoxification.
Lysosomes
- Structure: spherical membranous organelles containing acidic enzymes.
- Function:
- Digestion of pathogens (e.g., viruses, bacteria) within cells.
- Autophagy: digestion of stressed or dead cells.
Cytoskeleton (general reference)
- Protein-based internal framework that supports cell shape, organizes intracellular components, and facilitates movement and intracellular transport.

Interrelationships of the Protein/Synthesis Pathway

Ribosomes (on RER or free in cytosol) synthesize polypeptides.
Proteins synthesized on RER are packaged into vesicles and transported to the Golgi apparatus.
Golgi modifies, sorts, and packages proteins for secretion, delivery to lysosomes, or incorporation into the plasma membrane.
Secretory vesicles from the Golgi fuse with the plasma membrane to release products via exocytosis.

Lysosomes vs Peroxisomes (Functions and Roles)

Lysosomes:
- Contain acidic hydrolases; digest cellular debris and pathogens; key for autophagy.
Peroxisomes:
- Contain enzymes for detoxification; break down very long chain fatty acids; produce hydrogen peroxide and convert to water and oxygen via catalase in some cases.

Clinical Note

Tay-Sachs disease:
- Lysosomal storage disorder where a lysosomal enzyme needed to break down glycolipids in brain cells is lacking.
- Glycolipids accumulate, impairing nervous system function.
- Predominantly seen in infants of Central European Jewish descent; causes seizures, mental retardation, blindness, and death before age 5.
- Note: This is provided as a clinical aside and not necessarily on exams in all editions.

Quick Review Questions (Ch 3, Part 2)

Resting membrane potential: at rest, is the inside of the cell membrane more positive or more negative?
What is the name for cell eating? cell drinking?
Where are substances transported during exocytosis?
What are the three parts of the cytoplasm?
Which organelle produces ATP through cellular respiration?
Which cell part attaches to vesicles to eat viruses or bacteria? (phagocytosis hosts or the phagosome formation step in endocytosis)
Which organelle is the site of protein synthesis?
Which organelle detoxes?
Which organelle modifies and repackages proteins and lipids?
What two factors establish and maintain the resting membrane potential?
Why is the resting membrane potential important?
What types of cells have a resting membrane potential of -70 mV to -90 mV?
Name two types of endocytosis.
Which one is defined as cell eating? cell drinking?
Which process would be used to release a hormone, and does it require ATP?
Which process would be used to see how many calcium ions are in the blood passing by the cell?
What are the three elements of cytoplasm?
What are common solutes in cytosol?
List the steps of protein synthesis to make a protein for the cell membrane.
Where does the folding (origami) of proteins occur?