Cellular Structure and Membrane Transport: Biochemistry and Physiology of the Cell

Organic and Inorganic Biochemistry and the Cell

Introduction to Cellular Biochemistry

Cells are the fundamental units of life, composed of organic and inorganic molecules that participate in essential biochemical processes. Understanding the structure and function of cellular components is crucial for studying anatomy and physiology.

Active Membrane Transport

Overview of Active Transport Mechanisms

• Active transport and vesicular transport are two major processes that move substances across the plasma membrane using cellular energy (ATP).

• Active transport is required when solutes are too large for channels, are not lipid soluble, or are moving against their concentration gradient.

Active Transport

Carrier Proteins and Transport Types

• Active transport requires carrier proteins (solute pumps) that bind specifically and reversibly to the substances being moved.

• Some carriers transport more than one substance:

  • Antiporters: transport one substance into the cell while transporting a different substance out.

  • Symporters: transport two different substances in the same direction.

• Moves solutes against their concentration gradient (from low to high concentration), requiring energy (ATP).

• Primary active transport: energy comes directly from ATP hydrolysis.

• Secondary active transport: energy is obtained indirectly from ionic gradients created by primary active transport.

Primary Active Transport

Sodium-Potassium Pump (Na+-K+ ATPase)

• The sodium-potassium pump is the most studied pump, present in all plasma membranes, especially in excitable cells (nerves and muscles).

• Pumps 3 Na+ out of the cell and 2 K+ into the cell for each ATP molecule hydrolyzed.

• Functions as an antiporter, maintaining electrochemical gradients essential for muscle and nerve function.

Equation:

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

Secondary Active Transport

Cotransport and Ion Gradients

• Depends on ion gradients created by primary active transport (e.g., Na+-K+ pump).

• Energy stored in gradients is used to drive transport of other solutes (e.g., glucose, amino acids).

• Example: Na+-glucose symporter uses Na+ gradient to bring glucose into the cell.

Vesicular Transport

Transport of Large Particles and Macromolecules

• Involves movement of large particles, macromolecules, and fluids in membranous sacs called vesicles.

• Requires cellular energy (usually ATP).

• Types of vesicular transport:

  • Endocytosis: transport into cell (phagocytosis, pinocytosis, receptor-mediated endocytosis).

  • Exocytosis: transport out of cell.

  • Transcytosis: transport into, across, and out of cell.

  • Vesicular trafficking: transport within the cell.

Endocytosis

• Formation of protein-coated vesicles, often involving receptors for selectivity.

• Substances must bind to unique receptors to be internalized.

• Pathogens may hijack receptors for entry.

• Once inside, vesicles may fuse with lysosomes or undergo transcytosis.

Phagocytosis

• "Cell eating"; membrane projections (pseudopods) engulf solid particles, forming a phagosome.

• Used by macrophages and certain white blood cells.

Pinocytosis

• "Cell drinking"; plasma membrane infolds to bring extracellular fluid and dissolved solutes inside the cell.

• Fuses with endosome.

• Main mechanism for nutrient absorption in the small intestine.

Receptor-mediated Endocytosis

• Highly selective; involves concentration of specific molecules via receptors embedded in clathrin-coated pits or caveolae.

• Examples: uptake of enzymes, LDL, iron, insulin, and some viruses and toxins.

Exocytosis

• Process by which material is ejected from the cell, usually in secretory vesicles.

• Triggered by cell-surface signals or changes in membrane voltage.

• Docking proteins (v-SNARE and t-SNARE) facilitate fusion and release.

• Substances exocytosed include hormones, neurotransmitters, mucus, and cellular wastes.

Membrane Potential: Resting Membrane Potential (RMP)

Electrical Properties of the Plasma Membrane

• RMP is the electrical potential energy produced by separation of oppositely charged particles across the plasma membrane in resting cells.

• Difference in electrical charge between two points is called voltage.

• Cells with a charge are polarized.

• Voltage occurs only at the membrane surface; the rest of the cell and extracellular fluid are neutral.

• Typical RMP values: -50 to -100 mV (negative inside relative to outside).

K+ as Key Player in RMP

• K+ diffuses out of the cell through leakage channels, making the cytoplasmic side more negative.

• Electrochemical gradient of K+ sets the RMP (most cells have RMP around -90 mV).

• Na+-K+ pump maintains RMP by pumping 3 Na+ out and 2 K+ in.

Other Players in RMP

• Na+ also affects RMP; entry can bring RMP up to -70 mV.

• Membrane is more permeable to K+ than Na+.

• Cl- does not influence RMP due to balanced concentration and electrical gradients.

Cell-Environment Interactions

Glycocalyx and Cellular Communication

• Cells interact with their environment via direct contact or extracellular chemicals.

• Interactions involve the glycocalyx, a carbohydrate-rich area on the cell surface.

• Key components: Cell adhesion molecules (CAMs) and plasma membrane receptors.

Role of Cell Adhesion Molecules (CAMs)

• Anchor cells to extracellular matrix or to each other.

• Assist in movement of cells past one another.

• Attract white blood cells to injured or infected areas.

• Stimulate synthesis or degradation of adhesive membrane junctions (e.g., tight junctions).

• Transmit intracellular signals for cell migration, proliferation, and specialization.

Roles of Plasma Membrane Receptors

• Contact signaling: cells recognize each other by unique surface membrane receptors; important in development and immunity.

• Chemical signaling: interaction between receptors and ligands (chemical messengers) causes changes in cellular activities.

• Activated G protein-linked receptors cause cellular changes by activating G proteins, which affect ion channels, enzymes, or release second messengers (e.g., cyclic AMP, calcium).

Cytoplasm

Components of the Cytoplasm

• All material between the plasma membrane and nucleus.

• Composed of:

  • Cytosol: gel-like solution of water and soluble molecules (proteins, salts, sugars).

  • Inclusions: insoluble molecules (glycogen granules, pigments, lipid droplets, vacuoles, crystals).

  • Organelles: specialized metabolic machinery, either membranous or nonmembranous.

Cytoplasmic Organelles

Classification of Organelles

• Membranous organelles: mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes, lysosomes.

• Nonmembranous organelles: ribosomes, cytoskeleton, centrioles.

Mitochondria

• "Powerhouse" of the cell; produces ATP via aerobic cellular respiration.

• Enclosed by double membranes; inner membrane has folds called cristae for increased surface area.

• Contains own DNA, RNA, and ribosomes; capable of fission.

Ribosomes

• Sites of protein synthesis; made of protein and ribosomal RNA (rRNA).

• Two forms:

  • Free ribosomes: synthesize soluble proteins for cytosol and organelles.

  • Membrane-bound ribosomes: attached to ER, synthesize proteins for export or membranes.

Endoplasmic Reticulum (ER)

• Series of interconnected cisterns; two types:

  • Rough ER (RER): studded with ribosomes; site of protein and phospholipid synthesis.

  • Smooth ER (SER): no ribosomes; involved in lipid synthesis, detoxification, glycogen conversion, and calcium storage.

Golgi Apparatus

• Stacked, flattened membranous sacs; modifies, concentrates, packages, and secretes proteins and lipids.

• Receives proteins from RER (cis face) and releases them from trans face.

Peroxisomes

• Membranous sacs with enzymes that neutralize toxins (e.g., free radicals).

• Contain oxidase (converts toxins to hydrogen peroxide) and catalase (converts hydrogen peroxide to water).

• Also involved in breakdown and synthesis of fatty acids.

Lysosomes

• Spherical membranous bags with digestive enzymes (acid hydrolases).

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

• Release Ca2+ from bone and break down glycogen.

• Intracellular release causes autolysis (cell self-digestion).

Cytoskeleton

Structure and Function

• Network of rods throughout cytosol; provides structural support and enables movement.

• Three types:

  • Microfilaments (actin filaments): thinnest, support cell surface, involved in cell motility.

  • Intermediate filaments: tough, rope-like fibers, resist pulling forces, found in desmosomes.

  • Microtubules: largest, hollow tubes of tubulin, determine cell shape and organelle distribution.

Motor Proteins

• Complexes that function in motility; move organelles and substances around the cell using microtubules and microfilaments as tracks.

• Powered by ATP.

Centrosome and Centrioles

• Microtubule organizing center near the nucleus; consists of a granular matrix and paired centrioles.

• Centrioles form the basis of cilia and flagella.

Cellular Extensions

• Cilia: whip-like extensions that move substances across cell surfaces (e.g., mucus in respiratory tract).

• Flagella: longer extensions that propel the whole cell (e.g., sperm).

• Both are made of microtubules.

• Microvilli: fingerlike projections that increase surface area for absorption; core of actin filaments.

Structure	Main Function	Core Component
Cilia	Move substances across cell surface	Microtubules
Flagella	Propel entire cell	Microtubules
Microvilli	Increase surface area for absorption	Actin filaments

Nucleus

Structure and Function

• Largest organelle; contains genetic library for synthesis of cellular proteins.

• Most cells are uninucleate; some are multinucleate (skeletal muscle, bone, liver), and some are anucleate (red blood cells).

• Three main structures:

  • Nuclear envelope: double membrane with nuclear pores for transport.

  • Nucleolus(i): dark-staining bodies involved in rRNA synthesis and ribosome assembly.

  • Chromatin: DNA wrapped around histone proteins, forming nucleosomes; condenses into chromosomes during cell division.

Discussion Points

• Receptor-mediated endocytosis is more selective than phagocytosis or pinocytosis because it requires specific binding to cell surface receptors, allowing concentration of particular molecules.

• Lysosomes vs. Peroxisomes: Lysosomes digest cellular debris and pathogens; peroxisomes neutralize toxins and break down fatty acids.

• Cilia move substances, flagella propel cells, microvilli increase surface area for absorption.

Additional info: These notes cover key aspects of cell structure and function, membrane transport, and cellular interactions, relevant to Anatomy & Physiology chapters on cells and tissues.