Cells: The Living Units

Cell Theory

The cell theory is a fundamental concept in biology, stating that all living organisms are composed of cells, which are the basic structural and functional units of life. The activities of an organism depend on the combined and individual functions of its cells.

• Cell: The smallest unit of life, capable of performing all vital functions.

• Structure and Function: These are complementary; the shape and internal structures of a cell determine its biochemical activities.

• Biochemical Functions: Dictated by cell shape and specific subcellular structures.

Cell Diversity

Human bodies contain over 200 different types of cells, each specialized for particular functions.

• Cells differ in size, shape, and subcellular components.

• These differences result in functional diversity among cell types.

Generalized Cell Structure

Despite their diversity, all human cells share three basic structural components:

• Plasma membrane: Flexible outer boundary.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: DNA-containing control center.

The Plasma Membrane

Functions of the Plasma Membrane

The plasma membrane plays a dynamic role in cellular activity by regulating what enters and leaves the cell. It is also the site of many chemical reactions due to embedded enzyme systems.

• Also known as the cell membrane.

• Acts as a selective barrier and communication interface.

Structure: The Fluid Mosaic Model

The plasma membrane consists of a flexible bilayer of phospholipids with embedded proteins, forming a dynamic and semi-fluid structure.

• Phospholipid bilayer: Polar (hydrophilic) heads face outward toward water, nonpolar (hydrophobic) tails face inward.

• Membrane proteins: Specialized proteins float within the bilayer, creating constantly changing patterns.

• This arrangement is called the fluid mosaic model.

Membrane Proteins

Membrane proteins are essential for cell communication, transport, and structural support. They make up about half the mass of the plasma membrane.

• Integral proteins: Firmly inserted into the membrane, most span the membrane (transmembrane proteins).

• Have both hydrophobic and hydrophilic regions, allowing interaction with lipid tails and water.

• Functions include:

  • Transport proteins (channels and carriers)

  • Enzymes

  • Receptors for signaling molecules

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

• Functions include:

  • Enzymes

  • Motor proteins for cell division and muscle contraction

  • Cell-to-cell connections

  • Part of the glycocalyx, serving as identification markers for cell recognition

  • Receptors for recognizing molecules

Cell Junctions

Cells are bound to each other in tissues by specialized junctions, which maintain tissue integrity and allow communication.

• Tight junctions: Integral proteins fuse to form an impermeable barrier, preventing fluid and molecules from moving between cells. Found in epithelial cells lining the digestive tract.

• Desmosomes: Rivet-like junctions where linker proteins interlock, providing mechanical strength. Found in tissues under stress, such as skin and heart muscle.

• Gap junctions: Transmembrane proteins form tunnels for small molecules to pass between cells, facilitating the spread of ions and signaling molecules.

Membrane Transport

Selective Permeability

The plasma membrane is selectively permeable, allowing some molecules to pass while restricting others.

• Transport occurs via passive or active processes.

Passive Transport

Passive transport does not require energy and occurs down a concentration or pressure gradient.

• Diffusion: Movement of molecules from high to low concentration.

• Types of passive transport:

  • Simple diffusion: Hydrophobic substances diffuse directly through the phospholipid bilayer.

  • Facilitated diffusion: Hydrophilic molecules (e.g., glucose, amino acids, ions) are transported via:

    • Carrier-mediated: Specific polar molecules are transported by transmembrane integral proteins. Example: Glucose carriers only transport glucose.

    • Channel-mediated: Channels with aqueous cores formed by transmembrane proteins allow ions or water to pass. Channels may be always open or gated (controlled by signals).

  • Osmosis: Movement of water across a selectively permeable membrane. Water diffuses from areas of low solute (high water) concentration to high solute (low water) concentration.

  • Filtration: Transport across capillary walls due to pressure gradients.

Tonicity

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

• Isotonic: Same osmolarity as inside the cell; cell volume remains unchanged.

• Hypertonic: Higher solute, lower solvent concentration than the cell; water flows out, causing cell shrinkage.

• Hypotonic: Lower solute, higher solvent concentration than the cell; water flows in, causing cell swelling or lysis. Additional info: Hypotonic solutions can lead to cell bursting, called lysis.

Active Transport

Active transport requires ATP to move solutes against their concentration gradient. It is necessary when solutes are too large, not lipid soluble, or unable to move down the gradient.

• Primary active transport: Direct use of ATP to pump ions across the membrane. Example: Na+-K+ pump, calcium pump, hydrogen (proton) pump.

• Secondary active transport: Uses ion gradients created by primary active transport to move other substances. Example: Na+ can drag glucose or amino acids into the cell.

• Vesicular transport: Movement of large particles or fluids via vesicles.

  • Endocytosis: Cell engulfs material by surrounding it with membrane.

    • Phagocytosis: "Cell eating" – uptake of solids.

    • Pinocytosis: "Cell drinking" – uptake of liquids.

  • Exocytosis: Vesicle merges with plasma membrane to release substances out of the cell. Used for secretion of hormones, waste, mucus, etc.

Summary Table: Types of Membrane Transport

Key Equations

• Diffusion Rate (Fick's Law):

• Osmosis (Osmotic Pressure):

Where: = flux, = diffusion coefficient, = concentration gradient, = osmotic pressure, = van 't Hoff factor, = molarity, = gas constant, = temperature.