Ch. 3: The Cellular Level of Organization – Study Guide

Body Fluid Compartments

Extracellular and Intracellular Fluids

The body’s fluids are divided into compartments that are separated by cell membranes. Understanding these compartments is essential for studying cellular function and homeostasis.

• Extracellular Fluid (ECF): Fluid outside the cell, further divided into:

  • Blood plasma (Intravascular): The liquid component of blood.

  • Interstitial fluid: Fluid between cells, providing nutrients and removing waste.

• Intracellular Fluid (ICF): Fluid inside the cell.

  • Cells receive nourishment from the ECF.

  • Cells release waste products into the ECF.

  • Cells communicate by releasing chemical regulators (e.g., hormones) into the ECF.

Cell (Plasma) Membrane

Functions of the Plasma Membrane

The plasma membrane is a dynamic structure that separates the ICF from the ECF and mediates interactions between these compartments.

• Physical barrier: Separates ICF from ECF.

• Regulation of exchange: Controls entry and exit of substances.

• Communication: Facilitates signaling between the cell and its environment (e.g., secretion of substances).

• Structural support: Maintains cell shape and anchors cytoskeletal elements.

Composition of the Plasma Membrane

• Lipids:

  • Phospholipids: Form the basic bilayer structure; have a hydrophilic (polar) head and hydrophobic (nonpolar) tail.

  • Cholesterol: Embedded within the bilayer; stiffens the membrane, decreases water solubility, and is a type of steroid.

• Carbohydrates:

  • Act as identity molecules, collectively called the glycocalyx (short chains of linked monosaccharides).

  • Include glycolipids and glycoproteins.

• Proteins: Determine membrane functions.

  • Integral proteins: Embedded in the bilayer; involved in transport, signal transduction, and cell joining.

  • Peripheral proteins: Loosely attached to integral proteins; involved in support and signaling.

Membrane Proteins: Functions

• Transport: Move substances across the membrane.

• Receptors: Bind signaling molecules and initiate cellular responses.

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Cell-to-cell recognition: Allow cells to identify each other.

• Attachment: Anchor the cytoskeleton and extracellular matrix.

• Cell junctions: Connect adjacent cells.

Cell Junctions

• Tight Junctions: Integral proteins fuse adjacent cells, preventing passage of molecules between them. Found in epithelial tissues (e.g., digestive tract).

• Desmosomes: Anchoring junctions that bind cells together like Velcro; provide mechanical strength (e.g., skin, heart muscle).

• Gap Junctions: Allow direct communication between cells via channels; permit passage of ions and small molecules (e.g., heart, smooth muscle).

Transport Across the Plasma Membrane

Passive Transport

Passive transport does not require energy and moves substances down their concentration gradients.

• Simple Diffusion: Substances move directly through the lipid bilayer (e.g., O2, CO2, steroid hormones, fatty acids). Limited by lipid solubility and size.

• Facilitated Diffusion:

  • Carrier-mediated: Transport of polar molecules (e.g., sugars, amino acids) via protein carriers; subject to saturation.

  • Channel-mediated: Ions or water move through protein channels; can be leaky (always open) or gated (open/close in response to signals).

• Osmosis: Diffusion of water through a selectively permeable membrane, often via aquaporins. Occurs when water concentration differs across the membrane.

Osmolarity and Tonicity

• Osmolarity: Total concentration of solute particles in a solution.

• Tonicity: The ability of a solution to change the shape or tone of cells by altering their internal water volume.

Solution Type	Osmolarity (relative to cell)	Effect on Cell
Isotonic	Same	No net water movement; cell volume unchanged
Hypertonic	Higher	Water leaves cell; cell shrinks (crenation)
Hypotonic	Lower	Water enters cell; cell swells and may burst (lysis)

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

• Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

• Secondary Active Transport: Indirect use of ATP; relies on gradients established by primary active transport.

• Vesicular Transport: Movement of large particles or fluids via vesicles (e.g., endocytosis, exocytosis).

• Requires carrier proteins, which may function as antiporters (opposite directions) or symporters (same direction).

Membrane Potential

The membrane potential is the electrical potential difference across the plasma membrane, essential for nerve and muscle function.

• Resting Membrane Potential (RMP): Typically ranges from -50 to -90 mV; inside of the cell is negative relative to the outside.

• Established mainly by K+ diffusion; Na+ also contributes.

• Maintained by active transport (e.g., sodium-potassium pump).

Equation for membrane potential (Nernst equation):

$E = \frac{RT}{zF} \ln \left( \frac{[\text{ion outside}]}{[\text{ion inside}]} \right)$

Additional info: R = gas constant, T = temperature, z = charge of ion, F = Faraday's constant.

Protein Synthesis

Overview

Protein synthesis is the process by which cells build proteins based on genetic instructions.

• Functions of proteins: Structural support, enzymes, signaling, transport, etc.

• DNA/genes/chromosomes: DNA contains genes, which are segments coding for proteins; chromosomes are DNA-protein complexes.

• Parts of a gene: Regulatory (controls expression) and coding (specifies amino acid sequence).

Key Steps

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA is used to assemble amino acids into a protein.

Central Dogma of Molecular Biology:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Mutations

Types of Mutations

Mutations are changes in the DNA sequence that can affect protein function.

• Substitution: One base is replaced by another.

  • Silent: No change in amino acid sequence.

  • Missense: Changes one amino acid in the protein.

  • Nonsense: Creates a premature stop codon.

• Frameshift: Insertion or deletion of bases alters the reading frame.

  • Insertion: Addition of one or more bases.

  • Deletion: Removal of one or more bases.

Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.