Lecture Exam 1: Chapter 3 Cells

Cells: The Smallest Living Units

Cell Theory

The cell is the fundamental structural and functional unit of life. Understanding cell theory is essential for grasping how organisms function at the most basic level.

• Definition: A cell is the smallest unit of life capable of independent existence and performing all vital functions.

• Principles of Cell Theory:

  • All living organisms are composed of one or more cells.

  • The cell is the basic unit of structure and function in organisms.

  • Cells arise only from preexisting cells by division.

  • Structure and function are complementary: the biochemical activities of cells are dictated by their shapes and by the specific subcellular structures they contain.

• Example: Muscle cells are elongated to facilitate contraction, while nerve cells have long extensions for transmitting signals.

Basic Parts of a Human Cell

Common Structures and Functions

Despite differences in size and shape, most human cells share three basic components that are essential for their function.

• Plasma Membrane: A flexible boundary that separates the intracellular fluid (ICF) from the extracellular fluid (ECF). It controls what enters and leaves the cell and is also known as the "cell membrane."

• Cytoplasm: The intracellular fluid containing organelles, which are specialized structures that perform specific cellular functions.

• Nucleus: The control center of the cell, containing DNA and responsible for regulating cellular activities.

Extracellular Materials

Types and Functions

Substances found outside cells play important roles in supporting and connecting cells within tissues.

• Body Fluids: Includes interstitial fluid (surrounds cells), blood plasma (fluid of blood), and cerebrospinal fluid (surrounds nervous system organs).

• Cellular Secretions: Such as saliva and mucus, which aid in digestion and protection.

• Extracellular Matrix: A substance that acts as a glue to hold cells together, providing structural support.

Plasma Membrane Structure

Fluid Mosaic Model

The plasma membrane is a dynamic structure composed of lipids, proteins, and carbohydrates, forming a flexible barrier around the cell.

• Membrane Lipids:

  • Phospholipids: Consist of a polar (hydrophilic) phosphate head and nonpolar (hydrophobic) fatty acid tails. This arrangement forms a bilayer that is selectively permeable.

  • Glycolipids: Lipids with attached sugar groups, found on the membrane surface.

  • Cholesterol: Stabilizes the membrane and increases its fluidity.

• Membrane Proteins: Make up about half the mass of the plasma membrane and are responsible for most of its specialized functions.

  • Integral Proteins: Inserted into the membrane, often spanning it (transmembrane proteins). They have both hydrophobic and hydrophilic regions and function as transporters, enzymes, or receptors.

  • Peripheral Proteins: Loosely attached to integral proteins or membrane lipids. They function as enzymes, provide structural support, and assist in cell movement and division.

• Glycocalyx: A layer of carbohydrates attached to lipids and proteins on the cell surface, serving as biological markers for cell recognition.

Functions of Membrane Proteins

Key Roles

Membrane proteins perform a variety of essential tasks that support cell survival and communication.

• Transport: Move substances across the membrane via channels or carriers.

• Receptors for Signal Transduction: Bind chemical messengers and initiate cellular responses.

• Enzymatic Activity: Catalyze specific reactions at the membrane surface.

• Cell-Cell Recognition: Glycoproteins serve as identification tags recognized by other cells.

• Attachment to Cytoskeleton and Extracellular Matrix: Help maintain cell shape and stabilize the location of certain membrane proteins.

• Cell-to-Cell Joining: Membrane proteins of adjacent cells may hook together in various types of junctions.

Cell Junctions

Types and Functions

Cells are often bound together to form tissues. There are several types of cell junctions that facilitate this binding and communication.

• Tight Junctions: Impermeable junctions that prevent fluids and most molecules from moving between cells. Useful in tissues where a barrier is needed, such as the lining of the intestines.

• Desmosomes: Anchoring junctions that act like molecular "velcro," providing mechanical strength and allowing tissues to resist tearing under tension. Found in skin and heart muscle.

• Gap Junctions: Communicating junctions that allow ions and small molecules to pass between cells, facilitating electrical and metabolic coupling. Important in cardiac and smooth muscle cells.

Plasma Membrane Transport

Selective Permeability and Transport Mechanisms

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

• Passive Transport: Does not require energy. Includes:

  • Simple Diffusion: Movement of lipid-soluble and nonpolar substances directly through the phospholipid bilayer.

  • Facilitated Diffusion: Movement of polar or non-lipid soluble molecules via carrier or channel proteins.

  • Osmosis: Diffusion of water through the membrane, either directly or via aquaporins.

• Active Transport: Requires energy (ATP) to move substances against their concentration gradients. Additional info: Not covered in detail in these notes.

Factors Affecting Diffusion

• Concentration Gradient: Greater differences in concentration result in faster diffusion.

• Molecular Size: Smaller molecules diffuse more rapidly.

• Temperature: Higher temperatures increase kinetic energy and diffusion rate.

Osmosis and Osmolarity

Osmosis is the movement of water across a selectively permeable membrane. Osmolarity measures the concentration of solute particles in a solution.

• Water moves from areas of low solute (high water) concentration to areas of high solute (low water) concentration.

• Equilibrium: Achieved when osmolarity is equal on both sides of the membrane.

Pressure Effects in Osmosis

• Hydrostatic Pressure: Outward pressure exerted by increased cell volume.

• Osmotic Pressure: Inward pressure due to water being pulled into a cell with higher osmolarity.

• Balance: When hydrostatic and osmotic pressures are equal, there is no net movement of water.

Tonicity and Effects on Cells

Types of Solutions

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

Type of Solution	Osmolarity Compared to Cell	Effect on Cell
Isotonic	Same as inside cell	No net water movement; cell volume remains unchanged
Hypertonic	Higher than inside cell	Water flows out; cell shrinks (crenation)
Hypotonic	Lower than inside cell	Water flows in; cell swells and may burst (lysis)

Key Equations

• Osmosis: Water moves down its concentration gradient: $J = -D \frac{dC}{dx}$ Where: $J$ = flux, $D$ = diffusion coefficient, $\frac{dC}{dx}$ = concentration gradient

• Osmotic Pressure: $\Pi = iMRT$ Where: $\Pi$ = osmotic pressure, $i$ = van 't Hoff factor, $M$ = molarity, $R$ = gas constant, $T$ = temperature

Additional info: Active transport mechanisms, endocytosis, and exocytosis are also important for cellular transport but are not covered in these notes.