Cell Structure, Membranes, and Energy: A Study Guide

Prokaryotic and Eukaryotic Cells

Cell Types and Their Characteristics

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

• Prokaryotic Cells: Include Bacteria and Archaea. They lack a nucleus and membrane-bound organelles, and are typically unicellular.

• Bacteria: Possess circular DNA located in a region called the nucleoid, have 70S ribosomes, and divide by binary fission.

• Eukaryotic Cells: Found in animals, plants, fungi, and protists. They contain a nucleus and several membrane-bound organelles. DNA is linear and housed within the nucleus. Ribosomes are larger (80S), and division occurs via mitosis and cytokinesis. Only animal cells contain lysosomes.

• Common Features: Both cell types have a cell membrane and contain major biomolecules (carbohydrates, proteins, nucleic acids, lipids).

Ribosomes and Protein Synthesis

Structure and Function of Ribosomes

Ribosomes are non-membranous organelles responsible for protein synthesis in all living cells.

• Translation: The process by which ribosomes build proteins from mRNA templates.

• Free Ribosomes: Float in the cytoplasm and synthesize proteins for use within the cytosol.

• Attached Ribosomes: Bound to the rough endoplasmic reticulum (ER) and synthesize proteins for secretion or membrane insertion.

• Ribosomal RNA (rRNA): Assembled in the nucleolus of eukaryotic cells.

Endomembrane System

Components and Functions

The endomembrane system is a group of interconnected, membrane-bound organelles in eukaryotic cells that coordinate protein and lipid synthesis, modification, and transport.

• Vesicles: Small membrane-bound sacs that transport materials between organelles.

• Protein Secretion: Involves the nucleus, ER, and Golgi apparatus. Proteins are synthesized, modified, and exported from the cell.

Nucleus

• Nuclear Envelope: Double membrane surrounding the nucleus, containing nuclear pores for molecular exchange.

• Nucleolus: Site of ribosome assembly.

• Transcription: DNA is transcribed into RNA, which is then translated into protein.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; site of protein folding and modification (e.g., glycoprotein formation).

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, and stores calcium ions.

• ER Lumen: Internal space where protein folding and modification occur.

Golgi Apparatus

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

• Cis End: Receiving side for vesicles from the ER.

• Trans End: Shipping side where modified products are sent out.

Lysosomes and Peroxisomes

• Lysosomes: Acidic vesicles containing digestive enzymes; break down food, debris, and pathogens. Primarily in animal cells.

• Peroxisomes: Contain enzymes for breaking down toxic compounds and fatty acids; found in all eukaryotic cells.

Central Vacuole (Plants)

• Function: Stores water, degrades and recycles molecules, and maintains turgor pressure for plant rigidity.

• Turgor Pressure: High pressure keeps plants upright; low pressure leads to wilting.

Energy-Transforming Organelles

Mitochondria and Chloroplasts

• Mitochondria: Site of cellular respiration, converting food into ATP. Contains its own DNA and ribosomes. Structure includes outer membrane, folded inner membrane (cristae), intermembrane space, and matrix.

• Chloroplasts: Site of photosynthesis in plants and algae. Contains outer and inner membranes, thylakoids (pancake-shaped sacs), grana (stacks of thylakoids), and stroma (fluid containing enzymes, ribosomes, and DNA).

Endosymbiotic Theory

• Proposes that mitochondria and chloroplasts originated as free-living bacteria engulfed by ancestral eukaryotic cells.

• Evidence: Both have circular DNA, 70S ribosomes, replicate by binary fission, and possess double membranes.

Cytoskeleton and Cell Junctions

Cytoskeleton

• Microfilaments: Smallest, made of actin; involved in cell movement and muscle contraction.

• Intermediate Filaments: Provide structural stability; made of various proteins.

• Microtubules: Largest, made of tubulin; form tracks for vesicle transport, and are major components of cilia and flagella.

Cell Junctions

• Tight Junctions: Create leak-proof barriers between cells.

• Anchoring Junctions (Desmosomes): Connect cells via intermediate filaments; provide mechanical stability.

• Gap Junctions: Protein channels connecting cytoplasms of adjacent animal cells.

• Plasmodesmata: Channels in plant cell walls connecting cytoplasms of adjacent cells.

Biological Membranes and Transport

Structure and Function

• Biological Membrane: Composed of a phospholipid bilayer with embedded proteins and cholesterol; forms the cell's boundary.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of membranes.

Membrane Proteins

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

• Peripheral Proteins: Attached to the membrane surface; involved in signaling and structural support.

• Functions: Recognition, anchorage, signal transduction, transport, linkage, and enzymatic activity.

Transport Mechanisms

• Passive Transport: Movement of molecules from high to low concentration without energy input (e.g., diffusion, osmosis).

• Active Transport: Movement of molecules from low to high concentration, requiring energy (ATP).

• Simple Diffusion: Small, uncharged, nonpolar molecules (O2, CO2, N2) cross membranes freely.

• Facilitated Diffusion: Large, charged, or polar molecules (sugars, ions, amino acids) require protein channels or carriers.

Osmosis and Tonicity

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

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

Environment	Effect on Animal Cells	Effect on Plant Cells
Hypotonic	Swells, may burst	Turgid (preferred)
Isotonic	Normal (preferred)	Flaccid
Hypertonic	Shrivels	Plasmolyzed (wilts)

Active Transport Examples

• Primary Active Transport: Directly uses ATP (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses the gradient established by primary transport (e.g., Na+-glucose symport).

• Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ in per ATP hydrolyzed; maintains electrochemical gradients.

Bulk Transport

• Endocytosis: Uptake of large molecules via vesicle formation.

• Phagocytosis: "Cell eating" of large particles (e.g., white blood cells engulf bacteria).

• Pinocytosis: "Cell drinking" of extracellular fluid.

• Receptor-Mediated Endocytosis: Specific uptake using receptor proteins.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell (e.g., hormones, neurotransmitters).

Energy, Thermodynamics, and Enzymes

Energy in Biological Systems

• Potential Energy: Stored energy (e.g., in chemical bonds of glucose).

• Kinetic Energy: Energy of motion.

• Chemical Energy: A form of potential energy stored in molecules.

Thermodynamic Laws

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

Types of Reactions

• Endergonic Reactions: Require energy input; products have higher energy than reactants (e.g., photosynthesis).

• Exergonic Reactions: Release energy; products have lower energy than reactants (e.g., cellular respiration).

ATP and Cellular Work

• ATP (Adenosine Triphosphate): Main energy currency of the cell; composed of adenine, ribose, and three phosphate groups.

• ATP Hydrolysis: Releases energy by converting ATP to ADP and a phosphate group.

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, energizing it.

Enzymes and Catalysis

• Enzymes: Biological catalysts that speed up chemical reactions without being consumed.

• Substrates: Reactants that bind to the enzyme's active site.

• Enzyme Activity: Influenced by temperature, pH, and substrate concentration. Extreme conditions can denature enzymes.

• Activation Energy: Minimum energy required to start a reaction. Enzymes lower activation energy, increasing reaction rate.

$\text{Enzyme} + \text{Substrate} \rightleftharpoons \text{Enzyme-Substrate Complex} \rightarrow \text{Enzyme} + \text{Product}$

• Enzyme Specificity: Each enzyme catalyzes a specific reaction due to its unique 3D structure.

• Feedback Inhibition: End product of a pathway inhibits an earlier enzyme, preventing overproduction and conserving resources.

Osmoregulation and Aquaporins

Water Balance in Cells

• Osmoregulation: Mechanisms that maintain water balance between a cell and its environment.

• Aquaporins: Protein channels that facilitate rapid water movement across membranes.

• Tonicity: Determines whether a cell gains or loses water in a given solution.

Additional Key Terms and Concepts

• Lysosomes in Neutrophils: Abundant for digesting pathogens and cellular debris.

• Red Blood Cells (RBCs): Lack lysosomes.

• Cisternae: Flattened sacs forming the Golgi apparatus.

Example: White blood cells (neutrophils) use lysosomes to digest engulfed bacteria via phagocytosis.

Additional info: The sodium-potassium pump is an example of an antiporter, moving ions in opposite directions across the membrane. Feedback inhibition is a common regulatory mechanism in metabolic pathways, ensuring efficient use of resources.