Cell Structure and Function

Basic Processes of Cells

Cells are the fundamental units of life, carrying out essential processes to maintain homeostasis and support the organism. These processes include metabolism, transport, communication, and reproduction.

• Cell Metabolism: The sum of all chemical reactions in a cell, including:

  • Anabolic reactions: Build complex molecules from simpler ones (e.g., protein synthesis).

  • Catabolic reactions: Break down complex molecules into simpler ones (e.g., cellular respiration).

  • Oxidation-Reduction reactions: Transfer electrons between molecules, crucial for energy production.

• Membrane Transport: Movement of substances into, out of, or within the cell.

• Communication: Cells interact with their environment and other cells via chemical and electrical signals.

• Cell Reproduction: Most cells divide to produce new cells, essential for growth and repair.

Overview of Cell Structure

Most animal cells share three main structural components, each with specialized functions.

• Plasma Membrane: The outer boundary that separates the cell from its environment.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing:

  • Cytosol: The fluid portion, also called intracellular fluid (ICF), rich in proteins and solutes.

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

  • Cytoskeleton: A network of protein filaments providing structural support, shape, and transport within the cell.

• Nucleus: The control center of the cell, surrounded by a double membrane (nuclear envelope), containing most of the cell's DNA and the site of RNA synthesis.

Functions of the Plasma Membrane:

• Physically isolates the cell from its surroundings.

• Provides structural support.

• Facilitates communication with other cells.

• Regulates transport of substances into and out of the cell.

• Identifies the cell to other cells (cell recognition).

• Defines fluid compartments:

  • Intracellular Space: Contains intracellular fluid (cytosol).

  • Extracellular Space: Contains extracellular fluid (ECF).

The Plasma Membrane

The Phospholipid Bilayer

The plasma membrane is primarily composed of a double layer of phospholipids, forming a barrier between the cell's interior and exterior environments.

• Phospholipids: Molecules with hydrophilic (water-attracting) phosphate heads and hydrophobic (water-repelling) fatty acid tails.

• In aqueous environments, phospholipids arrange themselves so that the hydrophilic heads face water and the hydrophobic tails face inward, away from water, forming a bilayer.

• This arrangement excludes water from the hydrophobic core, creating an effective barrier.

The Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which emphasizes its dynamic and heterogeneous nature.

• Membrane components (phospholipids, proteins, cholesterol, carbohydrates) move laterally within the bilayer, contributing to fluidity.

• The 'mosaic' aspect refers to the diverse array of proteins and other molecules embedded in or attached to the membrane.

Membrane Proteins

• Integral Proteins: Span the entire membrane; if they reach both sides, they are called transmembrane proteins.

• Peripheral Proteins: Attached to only one side of the membrane, often anchored by the cytoskeleton or floating within the bilayer.

Functions of Membrane Proteins:

• Channels: Allow specific substances to pass through the membrane.

• Carriers: Transport substances across the membrane, often changing shape in the process.

• Receptors: Bind signaling molecules (ligands) and trigger cellular responses.

• Enzymes: Catalyze chemical reactions at the membrane surface.

• Structural Support Proteins: Maintain cell shape and integrity.

• Linker Proteins: Connect adjacent cells within a tissue.

Membrane Transport Mechanisms

Passive Transport Processes

Passive transport moves substances across the membrane without energy input, relying on concentration gradients.

• Simple Diffusion: Movement of small, nonpolar molecules directly through the phospholipid bilayer from high to low concentration.

• Facilitated Diffusion: Movement of larger or polar molecules via protein channels or carriers.

• Osmosis: Diffusion of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.

  • Water moves through aquaporins (water channels) or between phospholipids due to its small size.

  • Osmosis can change the volume of fluid in cellular compartments.

Tonicity

Tonicity describes the ability of a solution to cause a cell to gain or lose water, based on solute concentration.

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink (crenate).

• Hypotonic: Lower solute concentration outside the cell; water enters the cell, causing it to swell and possibly burst (lyse).

Active Transport Processes

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

• Primary Active Transport: Direct use of ATP to transport molecules via carrier proteins called pumps.

  • Sodium-Potassium Pump (Na+/K+ ATPase): Moves 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP hydrolyzed.

  • This maintains essential ion gradients for processes like muscle contraction.

  • Equation:

• Secondary Active Transport: Uses the energy from primary active transport to move other substances against their gradients.

Vesicular (Bulk) Transport

Large particles or volumes of fluid are transported via vesicles, small membrane-bound sacs, in processes that require ATP.

• Endocytosis: Bringing substances into the cell.

  • Phagocytosis: "Cell eating"; ingestion of large particles (e.g., bacteria) by specialized cells called phagocytes.

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

  • Receptor-Mediated Endocytosis: Selective uptake of specific molecules via receptor binding.

• Exocytosis: Release of substances from the cell; also replenishes plasma membrane lost during endocytosis.

• Transcytosis: Substances are transported into, across, and then out of the cell.

Gradients Across the Membrane

• Chemical Gradient: Difference in concentration of a substance across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

• Electrochemical Gradient: Combined effect of chemical and electrical gradients, influencing the movement of ions.

Summary Table: Types of Membrane Transport

Example: The Na+/K+ pump is essential for maintaining the resting membrane potential in nerve and muscle cells, enabling electrical signaling and contraction.

Additional info: The concepts of tonicity and membrane transport are foundational for understanding fluid balance, nerve impulses, and muscle contraction in human physiology.