Cell Structure, Membranes, and Transport

Cell Size and Surface Area:Volume Ratios

Cells are the basic units of life, and their size is limited by physical and functional constraints. The surface area-to-volume ratio is a key factor influencing cell size.

• Surface Area:Volume Ratio: As a cell grows, its volume increases faster than its surface area. This limits the rate at which materials can enter or leave the cell.

• Why Cells Are Small: Small cells have a higher surface area relative to their volume, allowing efficient exchange of nutrients, gases, and waste products with the environment.

• Example: Microvilli in intestinal cells increase surface area for absorption.

Cell Theory

The cell theory is a fundamental concept in biology, describing the properties of cells.

• Three Statements:

  1. All living things are composed of one or more cells.

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

  3. All cells arise from pre-existing cells.

• Major Contributors:

  • Matthias Schleiden: Proposed that all plants are made of cells.

  • Theodor Schwann: Proposed that all animals are made of cells.

  • Rudolf Virchow: Stated that all cells come from pre-existing cells.

Prokaryotic Cells: Bacteria vs. Archaea

Structure and Components

Prokaryotic cells are simpler than eukaryotic cells and lack membrane-bound organelles.

• Outside:

  • Cell Membrane: Phospholipid bilayer controlling entry and exit of substances.

  • Cell Wall: Provides structural support; composition differs between Bacteria (peptidoglycan) and Archaea (varied polysaccharides and proteins).

• Inside:

  • Ribosomes: Sites of protein synthesis (smaller than eukaryotic ribosomes).

  • Chromosome: Single, circular DNA molecule located in the nucleoid region.

  • Nucleoid: Region where the chromosome is found; not membrane-bound.

  • Cytoplasm: Gel-like substance containing enzymes and molecules for cellular processes.

• Appendages:

  • Fimbriae: Short, hair-like structures for attachment to surfaces.

  • Flagella: Long, whip-like structures for movement.

  • Pili: Involved in DNA transfer between cells (conjugation).

Eukaryotic Cells

Organelles and Functions

Eukaryotic cells contain membrane-bound organelles that compartmentalize cellular functions.

• Nucleus: Contains genetic material (DNA); site of transcription.

• Mitochondria: Site of cellular respiration and ATP production.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Synthesizes lipids and detoxifies toxins.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Chloroplasts: (Plants and algae) Site of photosynthesis.

• Vacuoles: Storage of substances; large central vacuole in plant cells maintains turgor pressure.

Cellular Connections and Systems

• Endomembrane System: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles; coordinates synthesis, modification, and transport of cellular products.

• Example: Proteins synthesized in the rough ER are transported to the Golgi for modification and then sent to their destination via vesicles.

Evolution of Eukaryotic Cells

• Endosymbiotic Hypothesis: Mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Autogenic Hypothesis: Eukaryotic organelles evolved from infoldings of the plasma membrane.

Membrane Structure and Function

Fluid-Mosaic Model

The plasma membrane is described by the fluid-mosaic model, which depicts a dynamic structure composed of lipids, proteins, and carbohydrates.

• Phospholipid Bilayer: Amphipathic molecules with hydrophilic heads and hydrophobic tails form a double layer, creating a semi-permeable barrier.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids); involved in cell recognition and signaling.

• Cholesterol: Maintains membrane fluidity and stability, especially in animal cells.

Membrane Transport

Types of Transport

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2).

  • Facilitated Diffusion: Movement of larger or polar molecules via transport proteins.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (ATP).

Tonicity and Water Potential

• Tonicity: Describes the relative concentration of solutes in solutions separated by a membrane.

  • Hypertonic: Higher solute concentration outside the cell; water leaves the cell.

  • Isotonic: Equal solute concentration; no net water movement.

  • Hypotonic: Lower solute concentration outside; water enters the cell.

• Water Potential (): Predicts the direction water will move; combines solute potential and pressure potential.

  • Equation 1:

  • Equation 2 (Solute Potential): Where:

    • i: Ionization constant

    • C: Molar concentration

    • R: Pressure constant

    • T: Temperature in Kelvin

Cellular Responses to Tonicity

• Crenation: Shrinking of animal cells in hypertonic solutions.

• Plasmolysis: Shrinking of plant cell cytoplasm away from the cell wall in hypertonic solutions.

• Turgor Pressure: Pressure exerted by the plasma membrane against the cell wall in plant cells; maintains rigidity.

• Flaccid: Plant cell in isotonic solution; no net water movement, cell is limp.

• Turgid: Plant cell in hypotonic solution; water enters, cell is firm.

Bulk Transport Mechanisms

• Phagocytosis: "Cell eating"; cell engulfs large particles or cells.

• Pinocytosis: "Cell drinking"; cell engulfs extracellular fluid and dissolved solutes.

• Endocytosis: General process of taking substances into the cell via vesicles.

• Exocytosis: Expulsion of materials from the cell via vesicles fusing with the plasma membrane.