Cell Structure and Organization

Prokaryotes vs. Eukaryotes

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: Lack a membrane-bound nucleus and organelles. Examples include Bacteria and Archaea.

• Eukaryotes: Possess a membrane-bound nucleus and various organelles. Examples include Plants, Animals, Fungi, and Protists.

Endosymbiotic Theory: Proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Animal vs. Plant Cells

Animal and plant cells share many organelles but also have distinct differences.

• Plant Cells: Have a cell wall, chloroplasts, and large central vacuole.

• Animal Cells: Lack cell walls and chloroplasts, but contain centrioles and lysosomes.

Surface Area to Volume Ratio and the Cell Size

The surface area to volume ratio limits cell size because as a cell grows, its volume increases faster than its surface area, reducing efficiency in nutrient uptake and waste removal.

• Smaller cells have a higher surface area to volume ratio, facilitating efficient exchange with the environment.

Cell Organelles: Structure and Function

Organelles are specialized structures within eukaryotic cells that perform distinct functions.

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

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids.

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

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis in plant cells.

• Lysosomes: Contain digestive enzymes to break down waste.

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

Endomembrane System: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane. These structures work together to modify, package, and transport lipids and proteins.

Mitochondria vs. Chloroplasts

Both are energy-converting organelles with their own DNA, supporting the endosymbiotic theory.

• Mitochondria: Found in all eukaryotic cells; site of aerobic respiration.

• Chloroplasts: Found only in plant and algal cells; site of photosynthesis.

Extracellular Matrix (ECM)

The ECM is a network of proteins and carbohydrates outside animal cells that provides structural support, mediates cell signaling, and anchors cells.

Cilia and Flagella

Cilia and flagella are hair-like structures that aid in cell movement.

• Cilia: Short, numerous, move substances across cell surfaces.

• Flagella: Longer, usually one or two per cell, propel cells (e.g., sperm).

Fluid Mosaic Model of the Cell Membrane

The cell membrane is described by the fluid mosaic model, which depicts a flexible bilayer of phospholipids with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Proteins: Integral (span the membrane) and peripheral (attached to surface).

• Cholesterol: Stabilizes membrane fluidity.

Membrane Structure and Function

Hydrophilic vs. Hydrophobic Regions

The cell membrane has hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, influencing the arrangement of proteins and other molecules.

• Hydrophilic: Interact with aqueous environments inside and outside the cell.

• Hydrophobic: Form the interior of the membrane, creating a barrier to most water-soluble substances.

Membrane Proteins

Proteins embedded in the membrane perform various functions:

• Integral Proteins: Span the membrane; involved in transport and signaling.

• Peripheral Proteins: Attached to the membrane surface; involved in signaling and maintaining cell shape.

Transport Across Membranes

Cells regulate the movement of substances across their membranes through various mechanisms.

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

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

Types of Passive Transport

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or polar molecules via transport proteins (channels or carriers).

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

Types of Active Transport

• Pumps: Proteins that move substances against their gradient (e.g., sodium-potassium pump).

• Endocytosis: Uptake of large particles by engulfing them in vesicles.

• Exocytosis: Release of substances from the cell via vesicles.

Selective Permeability

The cell membrane allows some substances to pass more easily than others, maintaining homeostasis.

• Small, nonpolar molecules pass easily.

• Ions and large polar molecules require transport proteins.

Concentration Gradient

A concentration gradient exists when the concentration of a substance differs across a space or membrane, driving passive transport.

Water Potential

Water potential determines the direction of water movement. It is calculated as:

• = Water potential

• = Solute potential

• = Pressure potential

Water moves from areas of higher to lower water potential.

Transport Proteins

Transport proteins facilitate the movement of substances across the membrane.

• Channel Proteins: Form pores for specific molecules or ions.

• Carrier Proteins: Bind and transport substances by changing shape.

Examples of Membrane Transport

• Osmosis: Water uptake by plant roots.

• Facilitated Diffusion: Glucose transport into cells via GLUT proteins.

• Active Transport: Sodium-potassium pump in animal cells.

Potassium Pump

The sodium-potassium pump is an example of active transport, moving Na+ out and K+ into animal cells, maintaining electrochemical gradients.

Proton Pump

Proton pumps actively transport H+ ions across membranes, creating a proton gradient used for ATP synthesis in mitochondria and chloroplasts.

Summary Table: Types of Membrane Transport

Additional info: Some details, such as the specific structure of the phospholipid bilayer and the function of the proton pump, were expanded for academic completeness.