Cells: The Living Units

Cell Theory and Cell Diversity

The cell is the fundamental structural and functional unit of all living organisms. The activities of an organism depend on the individual and collective functions of its cells. Cell structure and function are closely related, and the continuity of life is maintained through cellular reproduction.

• Cell Theory: All living things are composed of cells; cells are the basic units of structure and function; all cells arise from preexisting cells.

• Cell Diversity: The human body contains over 250 different types of cells, varying in size, shape, and specialized function.

Examples: Red blood cells transport oxygen, neurons transmit information, and muscle cells enable movement.

Generalized Cell Structure

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

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

• Cytoplasm: The intracellular fluid containing organelles.

• Nucleus: The control center containing genetic material (DNA).

Extracellular Materials

Substances found outside cells are termed extracellular materials and include:

• Extracellular fluids: Interstitial fluid (bathes cells), blood plasma, and cerebrospinal fluid.

• Cellular secretions: Such as saliva and mucus.

• Extracellular matrix: A network of proteins and polysaccharides that provides structural support and acts as a "glue" to hold cells together.

Structure of the Plasma Membrane

Overview and Fluid Mosaic Model

The plasma membrane acts as a dynamic barrier, separating the intracellular fluid (ICF) from the extracellular fluid (ECF). It controls the entry and exit of substances, maintaining cellular homeostasis. The membrane is described by the fluid mosaic model, which depicts a flexible bilayer of lipids with embedded proteins and carbohydrates.

Membrane Lipids

The lipid bilayer is primarily composed of:

• Phospholipids (75%): Each molecule has a polar, hydrophilic head and two nonpolar, hydrophobic tails. This arrangement forms a bilayer with heads facing outward toward water and tails facing inward, away from water.

• Glycolipids (5%): Lipids with attached sugar groups, found on the outer membrane surface.

• Cholesterol (20%): Stabilizes the membrane and increases its fluidity.

Membrane Proteins

Membrane proteins account for about half the mass of the plasma membrane and are responsible for most of its specialized functions. There are two main types:

• Integral proteins: Firmly embedded in the membrane, often spanning its entire width (transmembrane). They function as transporters, receptors, or enzymes.

• Peripheral proteins: Loosely attached to integral proteins or membrane lipids. They function as enzymes, motor proteins, or provide structural support.

Functions of Membrane Proteins

• Transport: Move substances across the membrane, sometimes using ATP for active transport.

• Receptors for signal transduction: Bind chemical messengers and initiate cellular responses.

• Enzymatic activity: Catalyze metabolic reactions.

• Cell-cell recognition: Serve as identification tags for cell recognition by other cells.

• Attachment to cytoskeleton and extracellular matrix (ECM): Maintain cell shape and stabilize membrane location.

• Cell-to-cell joining: Form intercellular junctions for communication and adhesion.

Glycocalyx

The glycocalyx is a carbohydrate-rich area on the cell surface, formed by glycoproteins and glycolipids. It serves as a biological marker for cell recognition and helps the immune system distinguish self from non-self.

Cell Junctions

Most cells are bound together to form tissues and organs. The main types of cell junctions are:

• Tight junctions: Form impermeable barriers that prevent molecules from passing between cells.

• Desmosomes: Provide anchoring strength, allowing cells to resist mechanical stress.

• Gap junctions: Allow communication by permitting the passage of ions and small molecules between cells.

Passive Membrane Transport

Overview of Passive Transport

Passive transport does not require cellular energy (ATP). It relies on the natural movement of molecules from areas of high concentration to low concentration (down their concentration gradient). The main types are:

• Simple diffusion

• Facilitated diffusion

• Osmosis

Simple Diffusion

Nonpolar, lipid-soluble substances (e.g., oxygen, carbon dioxide, steroid hormones) diffuse directly through the phospholipid bilayer. Small polar molecules like water can also pass in small amounts.

Facilitated Diffusion

Certain hydrophilic molecules (e.g., glucose, amino acids, ions) require assistance to cross the membrane. This occurs via:

• Carrier-mediated facilitated diffusion: Specific molecules bind to protein carriers, which change shape to transport them across the membrane.

• Channel-mediated facilitated diffusion: Substances move through water-filled protein channels, which may be always open (leakage channels) or gated (open in response to signals).

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from areas of low solute concentration (high water concentration) to areas of high solute concentration (low water concentration), either directly through the lipid bilayer or via aquaporins (water channels).

Osmolarity and Tonicity

Osmolarity measures the total concentration of solute particles in a solution. Water moves to balance osmolarity across membranes. Tonicity describes how a solution affects cell volume:

• Isotonic: Same osmolarity as the cell; no net water movement.

• Hypertonic: Higher osmolarity than the cell; water leaves the cell, causing it to shrink (crenation).

• Hypotonic: Lower osmolarity than the cell; water enters the cell, causing it to swell and possibly burst (lysis).

Key Equations

• Osmolarity calculation:

Example: 1 M NaCl dissociates into Na+ and Cl−, so it is a 2 Osm solution.

Summary Table: Types of Passive Transport

Additional info: Plant cells have cell walls that limit osmotic swelling, but animal cells do not, making them susceptible to bursting in hypotonic solutions.