Cells: The Smallest Living Units

Cell Theory

The cell theory is a fundamental concept in biology, stating that the cell is the structural and functional unit of life. The health and function of an organism depend on the activities of its individual and collective cells. The structure of a cell is closely related to its function, and all cells arise from preexisting cells, ensuring the continuity of life.

• Structural and functional unit: All living things are composed of cells.

• Complementarity: The biochemical activities of cells are determined by their shapes and subcellular structures.

• Cellular continuity: New cells are produced only from existing cells.

Cell Diversity

Human bodies contain over 250 different types of cells, each specialized in size, shape, and subcellular components to perform unique functions.

• Examples: Muscle cells, nerve cells, fat cells, and blood cells all have distinct structures and roles.

Structure of the Generalized Cell

Despite their diversity, all human cells share three basic 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 DNA.

Extracellular Materials

Types of Extracellular Materials

Substances found outside cells are essential for cellular function and include:

• Extracellular fluids: Such as interstitial fluid (surrounds cells), blood plasma, and cerebrospinal fluid (surrounds nervous system organs).

• Cellular secretions: Examples include saliva and mucus.

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

The Plasma Membrane

Structure and Function

The plasma membrane, also known as the cell membrane, acts as an active barrier separating the intracellular fluid (ICF) from the extracellular fluid (ECF). It controls what enters and leaves the cell, playing a dynamic role in cellular activity.

• Fluid mosaic model: The membrane is composed of a flexible lipid bilayer with proteins floating within it, creating a constantly changing pattern.

• Glycocalyx: Surface sugars that form a protective and recognition layer.

• Cell junctions: Structures that help hold cells together.

Membrane Lipids

The lipid bilayer is primarily made up of phospholipids, glycolipids, and cholesterol:

• Phospholipids (75%): Have hydrophilic (water-loving) heads and hydrophobic (water-hating) tails, forming a bilayer.

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

• Cholesterol (20%): Stabilizes the membrane.

Membrane Proteins

Membrane proteins make up about half the mass of the plasma membrane and are responsible for most of its specialized functions. They can be:

• Integral proteins: Firmly inserted into the membrane, often spanning it (transmembrane). Functions include transport, acting as enzymes, and serving as receptors.

• Peripheral proteins: Loosely attached to the membrane, functioning as enzymes, motor proteins, or in cell-to-cell connections.

Functions of Membrane Proteins

• Transport: Proteins may form channels or act as pumps to move substances across the membrane, sometimes using ATP.

• Receptors for signal transduction: Proteins act as binding sites for chemical messengers, triggering cellular responses.

• Enzymatic activity: Some membrane proteins are enzymes that catalyze reactions at the membrane surface.

• Cell-cell recognition: Glycoproteins serve as identification tags for cell recognition.

• Attachment to cytoskeleton and extracellular matrix (ECM): Helps maintain cell shape and stabilize membrane proteins.

• Cell-to-cell joining: Membrane proteins may form junctions between adjacent cells.

Cell Junctions

Types of Cell Junctions

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

• Tight junctions: Encircle the cell and prevent the movement of molecules between cells.

• Desmosomes: Rivet-like junctions anchored to the cytoskeleton, providing strength.

• Gap junctions: Transmembrane proteins form tunnels for the movement of molecules and ions between cells.

Membrane Transport

Selective Permeability

The plasma membrane is selectively permeable, allowing only certain molecules to cross. Substances move across the membrane by passive or active transport.

Passive Membrane Transport

Passive transport requires no energy input and involves diffusion, the movement of molecules from high to low concentration (down a concentration gradient).

• Simple diffusion: Nonpolar, lipid-soluble substances (e.g., oxygen, carbon dioxide) diffuse directly through the lipid bilayer.

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

• Osmosis: The movement of water across a selectively permeable membrane, either through the lipid bilayer or via aquaporins.

Osmolarity measures the concentration of solute particles in a solution. Water moves from areas of low solute (high water) concentration to high solute (low water) concentration.

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

• Hypertonic solution: Higher osmolarity than inside the cell; water leaves the cell, causing shrinkage.

• Hypotonic solution: Lower osmolarity than inside the cell; water enters the cell, causing swelling.

Active Membrane Transport

Active transport requires ATP to move solutes against their concentration gradient (from low to high concentration) using carrier proteins (solute pumps).

• Primary active transport: Direct use of ATP (e.g., Na+-K+ pump).

• Secondary active transport: Indirect use of ATP, relying on ionic gradients created by primary active transport.

Vesicular transport moves large particles and fluids via vesicles and includes:

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

• Exocytosis: Transport out of the cell.

• Transcytosis: Transport into, across, and out of the cell.

• Vesicular trafficking: Movement within the cell.

Membrane Potential

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential energy produced by the separation of oppositely charged particles across the plasma membrane. It is typically –50 to –100 mV, with the inside of the cell being more negative relative to the outside. The electrochemical gradient of potassium (K+) is key in generating the RMP.

Cell-Environment Interactions

Cell Adhesion Molecules (CAMs) and Receptors

Cells interact with their environment through direct contact or by responding to extracellular chemicals. CAMs are glycoproteins that anchor cells to the extracellular matrix or to each other and attract white blood cells to sites of injury or infection. Plasma membrane receptors serve as binding sites for chemical signals (ligands), such as neurotransmitters and hormones, triggering changes in cellular activity.