Cell Structure, Membrane Transport, and Organelles: Study Notes for General Biology

Cell Form and Formation

Diffusion

Diffusion is a fundamental process by which molecules move from areas of higher concentration to areas of lower concentration, driven by their random motion. This process is essential for the movement of substances within and between cells.

• Definition: Net movement of molecules from an area of greater concentration to an area of lower concentration.

• Concentration Gradient: The difference in concentration between two regions; diffusion occurs "along" or "with" the gradient (from high to low).

• Energy Requirement: Diffusion does not require energy input.

Diffusion Through Membranes

Cell membranes regulate the passage of substances. The ability of a molecule to diffuse through a membrane depends on its size and polarity.

• Simple Diffusion: Small, nonpolar molecules (e.g., O2, CO2) can dissolve and pass easily through the lipid bilayer.

• Large or Polar Molecules: Large molecules, polar molecules, and ions cannot cross the membrane easily without assistance.

Osmosis

Osmosis is a specific type of diffusion involving water molecules. It is crucial for maintaining cell volume and internal conditions.

• Definition: Diffusion of water (or another solvent) from an area of high concentration to low concentration, often across a selectively permeable membrane.

• Properties: Water is small and has partial charges, allowing it to cross phospholipid bilayers more easily than ions.

Osmosis Scenarios

• Hypertonic Solution: Water moves to higher solute concentration inside the cell; the cell shrinks.

• Hypotonic Solution: Water moves to higher solute concentration outside the cell; the cell swells or bursts.

• Isotonic Solution: No net movement of water; cell volume remains stable.

Osmosis and Cell Walls

Osmosis affects most cells similarly, but cell walls provide additional protection against bursting in hypotonic environments.

• Plant cells, many bacteria, and fungi have cell walls that prevent bursting.

• Pure water (hypotonic) does not usually cause cells with cell walls to burst; it can be beneficial for turgor pressure.

Cell Size: Surface Area and Volume

Relationship Between Diffusion and Cell Size

The efficiency of diffusion is influenced by the surface area-to-volume ratio of a cell. This ratio limits the maximum size of cells and affects how efficiently they exchange materials with their environment.

• All nutrients and wastes must cross the cell membrane.

• Metabolic processes scale with cell volume.

• The larger the cell, the smaller its surface area relative to its volume.

• Surface area-to-volume ratio is a key concept in biology; smaller cells have a higher ratio, allowing more efficient exchange.

• Different shapes and membrane folds increase surface area.

• Tiny organisms can rely on diffusion for gas and waste exchange; larger organisms require specialized systems.

Examples: Intestines (villi increase surface area), mitochondria, and chloroplasts (internal membrane folds).

Membrane Transport Mechanisms

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules down their concentration gradient through membrane proteins.

• Definition: Movement of ions or molecules down their concentration gradient via a transport protein (channel or carrier) embedded in the membrane.

• Energy Requirement: No energy required.

• Examples: Glucose transporters, ion channels.

Active Transport

Active transport moves substances against their concentration gradient, requiring energy (usually from ATP).

• Definition: Movement of molecules or ions from low to high concentration, using energy.

• Examples: Sodium-potassium pump, proton pumps.

• Equation:

$\text{Na}^+/\text{K}^+\ \text{ATPase}:\ 3\ \text{Na}^+_{\text{in}} + 2\ \text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\ \text{Na}^+_{\text{out}} + 2\ \text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Membrane Proteins

Membrane proteins facilitate the movement of substances across the cell membrane and perform various other functions.

• Channel Proteins: Provide continuous passage for passive diffusion; allow multiple ions to pass through simultaneously.

• Carrier Proteins: Bind to specific molecules and change shape to transport them; generally slower than channels.

• Pumps: Use ATP to move substances against their electrochemical gradient (active transport).

• Cotransporters: Use the energy of one ion moving down its gradient to drive another substance against its gradient (secondary active transport).

Electrochemical Gradients

Electrochemical gradients combine differences in concentration and electrical charge across a membrane, storing potential energy used for various cellular processes.

• Gradients can be in concentration, charge, or both.

• Important in mitochondria, chloroplasts, and cellular respiration.

Cell Structure and Function

Cell Theory

Cell theory is a foundational concept in biology, describing the properties and functions of cells.

• All living things are made of one or more cells.

• All cells come from preexisting cells.

• Cells are the fundamental unit of life.

• Cells must carry out all functions of living things, including energy use, chemical composition, and DNA replication.

• Cells vary widely in size, shape, and function, even within multicellular organisms.

Cell Compartmentalization and Organelles

Eukaryotic cells contain membrane-bound organelles that compartmentalize functions, increasing efficiency and allowing for greater cell size and complexity.

• Organelles include the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and more.

• Cells without compartmentalization (e.g., prokaryotes) are generally smaller and less complex.

Eukaryotes vs. Prokaryotes

Eukaryotes

Eukaryotic cells are found in plants, animals, fungi, and many protists. They are characterized by extensive compartmentalization and the presence of a nucleus.

• Most multicellular organisms are eukaryotic.

• Contain many organelles, including the nucleus.

Prokaryotes

Prokaryotic cells (bacteria and archaea) lack a nucleus and most membrane-bound organelles. Their DNA is free-floating within the cell.

• Almost exclusively single-celled organisms.

• DNA is not in a nucleus; often circular.

• Important chemical differences from eukaryotes (e.g., antibiotic sensitivity).

• Archaea often live in extreme environments and have unique membrane chemistry.

Plasma Membrane

Structure and Function

The plasma membrane surrounds the cell, acting as a selectively permeable barrier that controls the movement of substances in and out of the cell.

• All living things have a plasma membrane.

• Composed of a phospholipid bilayer with embedded proteins and cholesterol.

Components of the Plasma Membrane

• Cholesterol: Regulates membrane fluidity.

• Proteins: Include transport proteins, receptors, enzymes, and cell recognition molecules (glycolipids and glycoproteins).

• Most organelles are also enclosed by membranes.

Genetic Material and Protein Synthesis

Ribosomes

Ribosomes are the sites of protein synthesis, translating messenger RNA into polypeptides.

• Can be free-floating in the cytoplasm or attached to the endoplasmic reticulum.

• Present in both prokaryotes and eukaryotes.

• Ribosomes differ in structure between prokaryotes and eukaryotes (important for antibiotic targeting).

• Prokaryotic DNA is circular; eukaryotic DNA is linear.

Endomembrane System

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; site of protein synthesis, especially for proteins destined for secretion or membranes.

• Smooth ER: Lacks ribosomes; site of lipid synthesis and detoxification of drugs and poisons.

Vesicles and Golgi Apparatus

• Vesicles: Small membrane-bound containers for storage and transport.

• Golgi Apparatus: Modifies, sorts, and ships proteins and lipids received from the ER to their final destinations.

Lysosomes and Peroxisomes

• Lysosomes: Contain digestive enzymes for breaking down waste, damaged organelles, and pathogens.

• Peroxisomes: Involved in redox reactions and breakdown of fatty acids and toxins.

Energy-Transforming Organelles

Mitochondria

Mitochondria are the powerhouses of the cell, converting energy from nutrients into ATP through cellular respiration.

• Contain their own DNA and ribosomes; replicate independently.

• Mitochondrial DNA is inherited maternally.

• Diseases can result from mutations in mitochondrial DNA.

Endosymbiosis Theory

This theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Mitochondria and chloroplasts are similar in size to bacteria, have circular DNA, and prokaryotic ribosomes.

• Replicate independently of the cell cycle.

Chloroplasts

Chloroplasts are found in plants and some protists; they are the site of photosynthesis, converting light energy into chemical energy (glucose).

• Contain their own DNA and ribosomes.

• Require mitochondria to convert glucose into ATP.

Cell Walls and Vacuoles

Cell Walls

Cell walls provide structural support and protection to plant cells, fungi, and many bacteria.

• Regulate cell volume and prevent bursting in hypotonic environments.

• Do not replace the plasma membrane; both can be present.

Vacuoles

Vacuoles are large membrane-bound organelles found in plants and fungi, serving various functions including storage and breakdown of macromolecules.

• Absent in animal cells.

• Store water, ions, and other molecules (including via osmosis).

• Sometimes break down macromolecules, similar to lysosomes.