BSC2010 Exam #2 Study Guide: Chapters 6–8 (Cell Structure, Membrane Function, and Metabolism)

Cell Structure and Function

Basic Features of All Cells

• All cells share certain features: a plasma membrane, cytosol, chromosomes (DNA), and ribosomes.

• Prokaryotic cells (e.g., bacteria and archaea) lack a nucleus and membrane-bound organelles; their DNA is concentrated in a region called the nucleoid.

• Eukaryotic cells (e.g., plants, animals, fungi, protists) have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles.

• Cell size is limited by the surface area-to-volume ratio; as a cell grows, its volume increases faster than its surface area, limiting efficient exchange of materials.

Plant vs. Animal Cells

• Plant cells have a cell wall, chloroplasts, and a large central vacuole; animal cells lack these structures but have lysosomes and centrosomes.

• Both types contain mitochondria, endoplasmic reticulum, Golgi apparatus, and a nucleus.

Cell Organelles: Location and Function

• Nucleus: Contains genetic material; site of DNA replication and transcription.

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs.

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

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion (mainly in animal cells).

• Vacuoles: Storage and structural support (large central vacuole in plants).

• Mitochondria: Sites of cellular respiration; generate ATP.

• Chloroplasts: Sites of photosynthesis (plants and algae).

Organelle Function and Environmental Changes

• Changes in pH, temperature, or chemical agents can denature proteins or disrupt membranes, impairing organelle function.

• Structural changes (e.g., mutations in membrane proteins) can alter organelle efficiency or cause disease.

Definition of Organelle; Free vs. Bound Ribosomes

• Organelle: A specialized subunit within a cell with a specific function, usually membrane-bound in eukaryotes.

• Free ribosomes: Float in cytosol; synthesize proteins for use within the cell.

• Bound ribosomes: Attached to rough ER; synthesize proteins for secretion or for membranes.

Golgi Apparatus and Endoplasmic Reticulum (ER)

• Golgi apparatus: Consists of flattened sacs (cisternae); has a cis face (receiving) and trans face (shipping).

• ER: Network of membranes; rough ER (with ribosomes) and smooth ER (without ribosomes) have distinct functions.

• Cells with prominent ER or Golgi are often specialized for secretion (e.g., pancreatic cells).

Cytoskeleton: Components and Functions

• The cytoskeleton provides structural support, cell shape, and facilitates movement.

• Three main components:

  • Microtubules: Hollow rods; maintain cell shape, guide organelle movement, form cilia and flagella.

  • Microfilaments (actin filaments): Thin rods; support cell shape, involved in muscle contraction and cell division.

  • Intermediate filaments: Fibrous proteins; provide mechanical support, anchor organelles.

Cytoskeletal Component	Structure	Main Functions
Microtubules	Hollow tubes of tubulin	Cell shape, organelle movement, cilia/flagella
Microfilaments	Two intertwined actin strands	Cell shape, muscle contraction, cell division
Intermediate Filaments	Fibrous proteins coiled into cables	Cell shape, anchor organelles

Cilia and Flagella

• Both are structures for cell movement, composed of microtubules in a "9+2" arrangement.

• Cilia: Short, numerous, move fluid over cell surface.

• Flagella: Longer, usually one or a few per cell, propel cell through fluid.

Cell Wall Structure and Layers

• Cell wall: Rigid structure outside the plasma membrane in plants, fungi, and some protists.

• In plants, the primary cell wall is secreted first, followed by the secondary cell wall (if present).

Cell Junctions

• Allow communication and adhesion between cells.

• Plant cells: Plasmodesmata (channels through cell walls).

• Animal cells: Tight junctions (prevent leakage), desmosomes (anchor cells), gap junctions (communication).

Extracellular Matrix (ECM)

• Network of proteins and carbohydrates outside animal cells; provides structural support, regulates cell behavior.

Membrane Structure and Function

Selective Permeability

• Cell membranes allow some substances to cross more easily than others, maintaining internal conditions.

Fluid Mosaic Model of Membranes

• Membranes are a mosaic of lipids, proteins, and carbohydrates; components move laterally within the layer.

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward; move laterally and rarely flip-flop.

• Proteins: Embedded (integral) or attached (peripheral); serve as transporters, receptors, enzymes, etc.

• Membrane fluidity: Maintained by unsaturated fatty acids (increase fluidity) and cholesterol (buffers fluidity).

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

Transport Across Membranes

• Small, nonpolar molecules (e.g., O2, CO2) cross easily; large or charged molecules require transport proteins.

• Passive transport: Diffusion of substances down their concentration gradient; includes simple diffusion and facilitated diffusion (via proteins).

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

• Active transport: Movement against a gradient, requiring energy (usually ATP).

• Cotransport: Coupling the movement of one substance with another (e.g., sucrose-H+ cotransport in plants).

• Electrochemical gradient: Combination of concentration gradient and membrane potential (voltage across membrane).

• Membrane potential: Voltage difference across a membrane, important for nerve and muscle function.

Tonicity and Cell Behavior

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

• Animal cells: Prefer isotonic solutions; in hypotonic solutions, they may burst (lyse); in hypertonic, they shrink (crenate).

• Plant cells: Prefer hypotonic solutions (turgid); in isotonic, become flaccid; in hypertonic, undergo plasmolysis.

Bulk Transport Processes

• Endocytosis: Uptake of materials via vesicles; includes phagocytosis ("cell eating"), pinocytosis ("cell drinking"), and receptor-mediated endocytosis (specific uptake).

• Exocytosis: Secretion of materials via vesicle fusion with the plasma membrane.

• Pinocytosis vs. Receptor-mediated endocytosis: Both involve uptake of extracellular fluid, but receptor-mediated is specific for certain molecules via receptors.

Introduction to Metabolism

Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed (conservation of energy).

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

Forms of Energy

• Kinetic energy: Energy of motion (e.g., heat, movement of molecules).

• Potential energy: Stored energy (e.g., chemical bonds, concentration gradients).

Spontaneous vs. Nonspontaneous Processes

• Spontaneous: Occur without energy input; increase entropy.

• Nonspontaneous: Require energy input; decrease entropy or increase order.

Entropy and Free Energy

• Entropy (S): Measure of disorder or randomness.

• Free energy (G): Portion of a system's energy available to do work.

• Change in free energy equation: $\Delta G = \Delta H - T\Delta S$ where $\Delta G$ is change in free energy, $\Delta H$ is change in enthalpy (total energy), $T$ is temperature in Kelvin, and $\Delta S$ is change in entropy.

ATP: Structure and Role

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

• Hydrolysis of ATP releases energy for cellular work.

• ATP is regenerated from ADP and inorganic phosphate.

Energy Coupling

• Cells couple exergonic (energy-releasing) and endergonic (energy-consuming) reactions using ATP hydrolysis.

• Energy coupling allows cells to drive unfavorable reactions by pairing them with favorable ones.

Enzymes and Activation Energy

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy.

• Activation energy (Ea): Initial energy input required to start a reaction.

• Enzymes are specific for their substrates and are not consumed in the reaction.

Induced Fit Model

• Enzyme changes shape slightly to fit the substrate more closely, enhancing catalysis.

Factors Affecting Enzyme Function

• Temperature, pH, substrate concentration, and presence of inhibitors or activators can affect enzyme activity.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins) required for enzyme function.

Enzyme Inhibition and Metabolic Pathways

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

• Enzymes regulate metabolic pathways via feedback inhibition and other mechanisms.