Cell Structure and Membrane Transport: Anatomy & Physiology Study Notes

Cell Structure

Overview of Animal Cells

Animal cells are the basic structural and functional units of the body. They exhibit diversity in shape and function, such as red blood cells (transport oxygen), nerve cells (transmit signals), epithelial cells (form protective layers), and skeletal muscle cells (enable movement).

• Three main compartments:

  • Plasma membrane: Outer boundary, regulates entry/exit of substances.

  • Cytoplasm: Contains cytosol, organelles, and cytoskeleton.

  • Nucleus: Contains genetic material (DNA), controls cell activities.

Plasma Membrane

Structure and Function

The plasma membrane is a dynamic boundary that separates the cell's internal environment from the external environment. It provides structural support, enables communication, and allows cell identification.

• Separates: Extracellular fluid (ECF) from intracellular fluid (cytosol).

• Functions:

  • Structural support

  • Communication with other cells

  • Cell identification

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer. Phospholipids are amphipathic molecules with hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Bilayer arrangement: Hydrophobic tails face inward, hydrophilic heads face outward toward water.

• Fluid Mosaic Model: Membrane contains proteins, lipids, and carbohydrates scattered throughout, allowing fluidity and movement.

Membrane Proteins

Proteins embedded in the plasma membrane perform various essential functions.

• Types:

  • Integral proteins: Span the entire membrane.

  • Peripheral proteins: Located on one side of the membrane.

• Functions:

  • Transport: Channels and carriers move substances across the membrane.

  • Receptors: Bind chemical messengers (ligands) to trigger cellular responses.

  • Enzymes: Catalyze chemical reactions.

  • Structural support: Bind cytoskeleton and extracellular proteins for cell shape.

  • Linking cells: Attach adjacent cells for strength and communication.

Other Membrane Components

• Cholesterol: Stabilizes membrane fluidity.

• Glycolipids and glycoproteins: Located on the outer surface, function in cell recognition.

Cytoplasm

Components of Cytoplasm

The cytoplasm is the region inside the plasma membrane, excluding the nucleus. It consists of:

• Cytosol: Intracellular fluid, mostly water with dissolved proteins and solutes; site of many chemical reactions.

• Organelles: Specialized structures (e.g., mitochondria, endoplasmic reticulum) that perform specific cellular functions.

• Cytoskeleton: Network of protein filaments that support cell shape, organize organelles, and enable movement.

Nucleus

Structure and Function

The nucleus is typically a single, spherical structure surrounded by a phospholipid bilayer called the nuclear envelope. It contains DNA and produces most of the cell's RNA.

• Functions:

  • Stores genetic information

  • Controls cell activities

  • Directs protein synthesis

Transport Across the Plasma Membrane

Selective Permeability

The plasma membrane is selectively permeable, allowing certain substances to pass while restricting others. Transport can be passive (no energy required) or active (requires energy, usually ATP).

Passive Transport

• Diffusion: Movement of molecules from higher to lower concentration, driven by kinetic energy.

  • Simple diffusion: Non-polar solutes pass directly through the bilayer (e.g., O2, CO2).

  • Facilitated diffusion: Polar or charged solutes cross via membrane proteins (channels or carriers).

• Osmosis: Movement of water across the membrane, mainly through aquaporins, from lower to higher solute concentration.

• Filtration: Movement of substances due to pressure differences (not detailed in provided notes).

Osmosis and Tonicity

Tonicity describes the effect of extracellular solution on cell volume:

• Isotonic: Solute concentration equal inside and outside; no net water movement.

• Hypertonic: Higher solute concentration outside; water leaves cell, causing shrinkage (crenation).

• Hypotonic: Lower solute concentration outside; water enters cell, causing swelling and possible lysis.

Active Transport

• Primary Active Transport: Uses ATP directly to pump substances against their concentration gradient.

  • Types of pumps:

    • Uniport: Moves one substance in one direction.

    • Symport: Moves two or more substances in the same direction.

    • Antiport: Moves two or more substances in opposite directions.

  • Sodium-Potassium Pump (Na+/K+ ATPase): Maintains steep gradients by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed. $3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

• Secondary Active Transport: Uses energy from primary transport to move other substances (e.g., glucose transport coupled with Na+ gradient).

Vesicular Transport

• Endocytosis: Movement of substances into the cell via vesicles.

  • Phagocytosis: "Cell eating"; ingestion of large particles.

  • Pinocytosis: "Cell drinking"; ingestion of fluid and dissolved substances.

  • Receptor-mediated endocytosis: Specific molecules are concentrated and internalized via receptor binding.

• Exocytosis: Release of substances from the cell via vesicles fusing with the plasma membrane.

Resting Membrane Potential

Charge Distribution Across Membrane

The unequal distribution of ions (mainly Na+ and K+) across the plasma membrane creates a transmembrane potential. The inside of the cell is typically negative relative to the outside.

• Maintained by: Active transport (Na+/K+ pump) and selective permeability of the membrane.

• Importance: Essential for nerve impulse transmission and muscle contraction.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, ions
Osmosis	No	Water: Low to High solute	Water via aquaporins
Primary Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Low to High (coupled)	Glucose/Na+ symport
Endocytosis	Yes	Into cell	Phagocytosis, pinocytosis
Exocytosis	Yes	Out of cell	Neurotransmitter release

Additional info: Filtration is another passive process, important in capillary exchange, but was not detailed in the provided notes.