Cell Structure and Membrane Transport: Study Notes for General Biology

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are the basic units of life and can be classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a true nucleus and membrane-bound organelles. Their genetic material is located in a nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Plants, Animals, Fungi, and Protists.

• Key differences: Eukaryotes are generally larger, have complex internal structures, and can form multicellular organisms.

Surface Area-to-Volume Ratio

The surface area-to-volume ratio is a critical factor that influences cell size and function.

• As a cell grows, its volume increases faster than its surface area.

• A high surface area-to-volume ratio allows for efficient exchange of materials (nutrients, gases, wastes) with the environment.

• Cells remain small to maintain this ratio and ensure proper functioning.

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis.

• Composed of ribosomal RNA (rRNA) and proteins.

• Found free in the cytoplasm or attached to the rough endoplasmic reticulum (ER).

• Present in both prokaryotic and eukaryotic cells, but differ in size and structure.

• Function: Translate messenger RNA (mRNA) into polypeptide chains (proteins).

Membrane-Bound Organelles

Eukaryotic cells contain specialized structures called organelles, each with distinct functions.

• Nucleus: Stores genetic material (DNA) and controls cellular activities.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes and processes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Chloroplasts: (in plants and some protists) Site of photosynthesis.

• Vacuole: Storage organelle; large central vacuole in plants maintains turgor pressure.

Organelles in Plants vs. Animals

• Plant cells: Have cell walls, chloroplasts, and a large central vacuole.

• Animal cells: Lack cell walls and chloroplasts, have smaller vacuoles, and contain centrioles.

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, and facilitates movement.

• Microfilaments (actin filaments): Involved in cell movement and muscle contraction.

• Intermediate filaments: Provide mechanical support for the cell.

• Microtubules: Involved in cell division, intracellular transport, and the structure of cilia and flagella.

Functions of Organelles

• Nucleus: Genetic control center.

• ER: Protein and lipid synthesis.

• Golgi apparatus: Processing and packaging.

• Lysosomes: Digestion and waste removal.

• Mitochondria: Energy production.

• Chloroplasts: Photosynthesis (plants).

Pathways for Synthesis and Secretion of Proteins

Proteins destined for secretion or for certain organelles follow the endomembrane system pathway:

• Ribosomes (on rough ER) → ER lumen → Transport vesicles → Golgi apparatus → Secretory vesicles → Plasma membrane

Plasmodesmata, Gap Junctions, Tight Junctions, Desmosomes

• Plasmodesmata: Channels between plant cells for transport and communication.

• Gap junctions: Channels between animal cells for communication.

• Tight junctions: Seal cells together to prevent leakage of materials.

• Desmosomes: Anchor cells together, providing mechanical strength.

Cell Membrane Structure and Transport

Biological Membranes

Biological membranes are composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Structure: Fluid mosaic model describes the dynamic arrangement of lipids and proteins.

• Function: Selective barrier, communication, and compartmentalization.

Cholesterol in the Plasma Membrane

• Cholesterol stabilizes membrane fluidity, making the membrane less permeable to very small water-soluble molecules.

Membrane Proteins

• Peripheral proteins: Loosely attached to the membrane surface.

• Integral proteins: Embedded within the lipid bilayer; may span the membrane (transmembrane proteins).

Extracellular Matrix (ECM)

• Network of proteins and carbohydrates outside animal cells that provides structural support and mediates cell signaling.

Cytoskeleton

• Supports cell shape, anchors organelles, and enables movement.

Membrane Transport Mechanisms

Substances move across cell membranes by various mechanisms, depending on their size, polarity, and concentration gradients.

• Simple diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient without energy input.

• Facilitated diffusion: Movement of larger or polar molecules via transport proteins; no energy required.

• Passive transport: Includes simple and facilitated diffusion; does not require energy.

• Active transport: Movement of substances against their concentration gradient using energy (usually ATP).

Types of Membrane Proteins Involved in Transport

• Channel proteins: Form pores for specific molecules to pass through.

• Carrier proteins: Bind and transport substances across the membrane.

• Uniporter: Transports one type of molecule.

• Symporter: Transports two molecules in the same direction.

• Antiporter: Transports two molecules in opposite directions.

Osmosis

• Diffusion of water across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

Primary and Secondary Active Transport

• Primary active transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary active transport: Uses the energy from the movement of one molecule down its gradient to drive another molecule against its gradient.

Endocytosis and Exocytosis

• Endocytosis: Uptake of materials into the cell by engulfing them in vesicles.

• Exocytosis: Release of materials from the cell by fusion of vesicles with the plasma membrane.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

• Receptor-mediated endocytosis: Specific uptake of molecules after binding to cell surface receptors.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Osmosis	No	Down water gradient	Water movement
Primary Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Secondary Active Transport	Indirect (uses gradient)	Against gradient	Glucose-Na+ symporter
Endocytosis	Yes	Into cell	Phagocytosis of bacteria
Exocytosis	Yes	Out of cell	Secretion of hormones

Key Equations

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• Where J is the rate of diffusion, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Examples and Applications

• Example: The sodium-potassium pump maintains the electrochemical gradient in animal cells by pumping 3 Na+ ions out and 2 K+ ions in, using ATP.

• Application: Understanding membrane transport is essential for topics such as nerve impulse transmission, nutrient absorption, and drug delivery.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard biology curricula.