Cells: Structure & Function – Chapter 3 Study Notes

Cells: Structure & Function

Introduction to Cell Theory

The cell is the fundamental structural and functional unit of life. All living organisms are composed of cells, which can be classified as prokaryotic or eukaryotic. Viruses, while not true cells, interact with cellular life. The activities of an organism depend on the collective activities of its cells, and the continuity of life has a cellular basis. The biochemical activities of cells are determined by their subcellular structures.

• Cell Theory: All living things are composed of cells; the cell is the basic unit of life; all cells arise from pre-existing cells.

• Major Components of Cells: Plasma membrane, cytoplasm, and nucleus.

Plasma Membrane

Structure and Function

The plasma membrane separates the intracellular fluid (ICF) from the extracellular fluid (ECF) and plays a dynamic role in cellular activity. It is selectively permeable, allowing the passage of materials by various mechanisms.

• Functions:

  • Maintains boundary between ICF and ECF

  • Regulates transport of substances

  • Facilitates communication and signaling

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins floating in or on a fluid lipid bilayer.

Membrane Proteins

• Channels/Transporters: Facilitate movement of substances across the membrane.

• Enzymatic Activity: Catalyze chemical reactions at the membrane surface.

• Receptors: Bind signaling molecules and initiate cellular responses.

• Intercellular Adhesion: Help cells stick together to form tissues.

• Cell-Cell Recognition: Allow cells to identify each other.

• Attachment to Cytoskeleton and Extracellular Matrix: Maintain cell shape and stabilize membrane location.

Cell Attachments

• Tight Junctions: Impermeable junctions that encircle the cell, preventing passage of substances between cells.

• Anchoring Junctions: Include desmosomes (provide mechanical stability) and hemidesmosomes (anchor cells to the basement membrane).

• Gap Junctions: Allow direct passage of chemical substances between adjacent cells through channels.

Transport Across the Plasma Membrane

Bulk (Vesicular) Transport

Bulk transport moves large particles and macromolecules across the plasma membrane via vesicles.

• Exocytosis: Moves substances from the cell interior to the extracellular space.

• Endocytosis: Brings substances into the cell. Includes:

  • Phagocytosis: Engulfment of solids by pseudopods.

  • Pinocytosis: Uptake of fluids and dissolved substances.

  • Receptor-Mediated Endocytosis: Specific uptake via receptor binding.

Passive Membrane Transport

• Simple Diffusion: Movement of small, nonpolar, and lipid-soluble substances directly through the lipid bilayer.

• Facilitated Diffusion: Transport of substances (e.g., glucose, amino acids, some electrolytes) via carrier or channel proteins down their concentration gradients.

• Osmosis: Diffusion of water across a semipermeable membrane from areas of higher to lower water concentration.

• Filtration: Movement of solute-containing fluid from higher to lower pressure areas.

Osmolarity: Total concentration of solute particles in a solution.

Tonicity: How a solution affects cell volume (isotonic, hypertonic, hypotonic).

• Water moves from areas of low solute concentration (high water) to high solute concentration (low water).

• Higher solute concentration = higher osmotic pressure.

Active Transport

• Requires ATP and carrier proteins to move substances against their concentration gradients.

• Symport System: Two substances moved in the same direction.

• Antiport System: Two substances moved in opposite directions.

• Primary Active Transport: Direct use of ATP (e.g., Na+-K+ ATPase pump).

• Secondary Active Transport: Indirect use of ATP via ion gradients established by primary active transport.

Na+-K+ ATPase Pump: Maintains electrochemical gradients by pumping 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

$3\,\mathrm{Na}^+_{\text{(in)}} + 2\,\mathrm{K}^+_{\text{(out)}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{\text{(out)}} + 2\,\mathrm{K}^+_{\text{(in)}} + \mathrm{ADP} + \mathrm{P}_i$

• Establishes electrochemical gradients essential for nerve and muscle function.

• Drives secondary active transport of other molecules (e.g., glucose).

Membrane Potential

• Membrane Potential: Voltage across the plasma membrane, typically -20 to -200 mV (inside negative).

• Results from Na+ and K+ gradients and differential membrane permeability.

• Maintained by active transport (Na+-K+ pump) and selective ion channels.

Cytoplasm and Organelles

Cytoplasm

The cytoplasm is the material between the plasma membrane and the nucleus. It consists of cytosol (fluid), cytoplasmic organelles (metabolic machinery), and inclusions (stored substances).

• Cytosol: Largely water with dissolved proteins, salts, sugars, and other solutes.

• Inclusions: Chemical substances such as glycogen granules and pigments.

Organelles

• Membranous Organelles: Mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes.

• Nonmembranous Organelles: Ribosomes, cytoskeleton, centrioles.

Mitochondria

• Powerhouse of the cell; site of ATP production via aerobic respiration.

• Contains its own DNA and double membrane structure.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Lysosomes

• Spherical membranous sacs containing digestive enzymes.

• Digest ingested bacteria, viruses, toxins, and degrade nonfunctional organelles.

• Break down bone to release Ca2+.

• Secretory lysosomes are found in immune cells and melanocytes.

Peroxisomes

• Membranous sacs containing oxidases and catalases.

• Detoxify harmful substances and neutralize free radicals (e.g., O2-).

Cytoskeleton

• Dynamic network of protein rods (microfilaments, intermediate filaments, microtubules).

• Maintains cell shape, enables movement, and organizes cell contents.

Centrioles

• Paired organelles involved in cell division and formation of cilia and flagella.

Nucleus

Structure and Function

The nucleus is the gene-containing control center of the cell. It consists of the nuclear envelope, nucleoli, chromatin, and compartments rich in regulatory proteins.

• Nuclear Envelope: Double membrane that encloses the nucleus.

• Nucleoli: Sites of ribosomal RNA synthesis and ribosome assembly.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Function: Contains the genetic library (DNA) with blueprints for nearly all cellular proteins; dictates the kinds and amounts of proteins to be synthesized.

From DNA to Protein

• Genetic information flows from DNA to RNA to protein (central dogma of molecular biology).

• Transcription: DNA is transcribed to messenger RNA (mRNA).

• Translation: mRNA is translated into a specific protein sequence at ribosomes.

Example: The gene for hemoglobin is transcribed in the nucleus and translated in the cytoplasm to produce hemoglobin protein in red blood cells.