Cell Structure, Function, and Metabolism: General Biology Study Notes

Cell Structure and Function

Cell Theory

The cell theory is a foundational concept in biology stating that all living organisms are composed of cells, and all cells arise from preexisting cells.

• Plasma membrane: All cells possess a plasma membrane that separates the cell from its environment.

• Cells are made of biomolecules:

  • Nucleotides

  • Amino acids

  • Carbohydrates

  • Lipids

• Cells as units: Cells are self-sustaining factories and represent the fundamental structural and functional unit of life.

Essential Functions of Cells

Cells must perform several key functions to sustain life:

• Metabolism: Digest molecules, build new molecules, and use energy (often facilitated by enzymes).

• Reproduction: Store and transmit genetic information; produce new cells.

• Gene expression: Respond to environmental changes using genetic information.

• Evolution: Genetic information in populations changes over time.

Types of Cells

Classification by Morphology

• Prokaryotes: Lack membrane-bound nucleus.

• Eukaryotes: Have membrane-bound nucleus.

Classification by Phylogeny

• Bacteria: Phospholipids consist of fatty acids bound to glycerol.

• Archaea: Phospholipids are branched isoprenoid chains bound to glycerol.

Prokaryotic Cell Structure

• Nucleoid region: Chromosomes (DNA) are organized here, often associated with proteins. Some DNA may be found in plasmids.

• Cytoplasm: All cell contents inside the plasma membrane.

• Ribosomes: Manufacture proteins; not membrane-bound.

• Cell wall: Provides shape and rigidity.

• Cytoskeleton: Protein filaments maintain cell shape and aid in cell division.

• Photosynthetic prokaryotes: Have internal membranes with pigments and enzymes.

• Specialized organelles: Some species have organelles for specific functions.

• External structures: Movement and attachment (e.g., flagella, fimbriae).

Eukaryotic Cell Structure

• May be unicellular or multicellular (depends on species).

• Organelles: Membrane-bound compartments that compartmentalize chemical reactions and increase efficiency.

• Nucleus: Highly organized, membrane-bound, protects genetic material, contains nucleolus for ribosomal RNA synthesis.

• Ribosomes: Sites of protein synthesis; not considered organelles.

• Endoplasmic reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for membranes or export.

  • Smooth ER: Synthesizes lipids and detoxifies molecules.

• Golgi apparatus: Processes, sorts, and ships proteins; packaging occurs in vesicles.

• Lysosomes: Hydrolyze macromolecules; contain enzymes functioning at acidic pH.

• Vacuoles: Found in plants, fungi, and some protists; roles include digestion, recycling, storage, and protection.

• Peroxisomes: Carry out reduction-oxidation reactions; contain catalase to detoxify H2O2.

• Mitochondria: Site of pyruvate processing and citric acid cycle; supply ATP; have double membrane; contain DNA.

• Chloroplasts: Site of photosynthesis in plants and algae; have double membrane; contain DNA.

• Cytoskeleton: Extensive system of protein fibers for shape and stability.

• Cell wall: Present in fungi, algae, and plants; provides structural support.

• Extracellular matrix (ECM): In animals, supports cells and helps with communication and linkage.

Cellular Organization and Communication

Unicellular vs. Multicellular

• Unicellular: Bacteria, archaea, some protists, some fungi; must respond dynamically to environment.

• Multicellular: Animals, plants, some protists (e.g., kelp), some fungi; cells communicate and cooperate.

Tissues and Extracellular Matrix

• Tissues: Groups of cells functioning as a unit; form organs in multicellular organisms.

• ECM: Protein fibers resist compression and tension; help cells recognize position and link together.

• Cell junctions: Openings between cells for communication (gap junctions in animals, plasmodesmata in plants).

• Quorum sensing: Unicellular organisms monitor population density and coordinate behavior.

Cell Signaling

• Cell signaling: Cells communicate via specific signaling molecules and receptors.

• Signals can be local or distant; cells must have the correct receptor to respond.

Enzymes and Metabolism

Role of Enzymes

Enzymes are proteins (or RNA) that catalyze specific chemical reactions by lowering activation energy and stabilizing transition states.

• Activation energy ($E_a$): Minimum energy required to start a reaction.

• Enzymes orient molecules and may form/break bonds to facilitate reactions.

• Enzymes are highly specific; defects can cause metabolic disorders (e.g., PKU).

Phenylalanine Metabolism and PKU

• Phenylalanine: Amino acid normally converted to tyrosine.

• PKU (Phenylketonuria): Defective enzyme prevents conversion, leading to buildup of phenylalanine and phenylpyruvic acid, which damages the brain.

• Dietary management can prevent damage.

Factors Affecting Reaction Rates

• Higher substrate concentration increases collision frequency.

• Higher temperature increases reaction rate.

Spontaneity and Coupling of Reactions

• Spontaneous reactions: Increase entropy, do not require energy input (exergonic).

• Non-spontaneous reactions: Require energy input, produce more ordered products (endergonic).

• Coupling: Transfer of electrons or phosphate groups links exergonic and endergonic reactions.

Redox Reactions

Reduction-oxidation (redox) reactions involve electron transfer:

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox reactions often transfer protons as well.

ATP and Energy Transfer

• ATP (Adenosine triphosphate): Main energy currency of the cell.

• Energy released during ATP hydrolysis is transferred to substrates by phosphorylation.

• Phosphorylation: Addition of a phosphate group; can energize molecules or change protein properties.

Equation for ATP hydrolysis:

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

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present (membrane-bound)
Organelles	Few, not membrane-bound	Many, membrane-bound
Cell wall	Present (peptidoglycan or other)	Present in plants, fungi, algae; absent in animals
Size	Generally smaller	Generally larger
DNA location	Nucleoid region	Nucleus

Additional info:

• Some context and terminology were expanded for clarity and completeness.

• Equations and table were added for academic context.