General Biology Study Guide: Cells, Membranes, and Molecular Structure

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

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

• Prokaryotic cells lack a membrane-bound nucleus and organelles. Their DNA is located in a region called the nucleoid.

• Eukaryotic cells have a true nucleus enclosed by a nuclear membrane and possess various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

• Key distinguishing features:

  • Prokaryotes: No nucleus, circular DNA, smaller size, no membrane-bound organelles.

  • Eukaryotes: Nucleus present, linear DNA, larger size, membrane-bound organelles.

• Examples: Bacteria and Archaea are prokaryotes; plants, animals, fungi, and protists are eukaryotes.

Cellular Components Shared by All Cells

• Plasma membrane: A phospholipid bilayer that encloses the cell, controlling the movement of substances in and out.

• Cytoplasm: The jelly-like substance within the cell where metabolic reactions occur.

• Ribosomes: Structures responsible for protein synthesis.

• Genetic material (DNA): Contains the instructions for cell function and replication.

Prokaryotic Chromosomes

Prokaryotic chromosomes are typically circular and located in the nucleoid region. They lack histone proteins and are not enclosed by a nuclear membrane.

Ribosomes: Structure and Function

• Structure: Composed of ribosomal RNA (rRNA) and proteins; consist of two subunits (large and small).

• Function: Translate messenger RNA (mRNA) into proteins.

• Location: Found in both prokaryotic and eukaryotic cells; in eukaryotes, they may be free in the cytoplasm or bound to the rough endoplasmic reticulum.

Nucleus and Nucleolus

• Nucleus: Contains the cell's genetic material and is the site of DNA replication and transcription.

• Nucleolus: Located within the nucleus; responsible for ribosomal RNA synthesis and ribosome assembly.

Nuclear Pore Complexes

• Function: Regulate the transport of molecules between the nucleus and cytoplasm.

• Structure: Large protein complexes embedded in the nuclear envelope.

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

Endoplasmic Reticulum (ER) and Golgi Apparatus

• Rough ER: Studded with ribosomes; synthesizes and processes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

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

Other Organelles

• Lysosomes: Contain digestive enzymes to break down waste.

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

• Mitochondria: Site of cellular respiration and energy (ATP) production.

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

Atoms, Bonds, and Molecules

Atomic Structure

Atoms are the basic units of matter, consisting of a nucleus (protons and neutrons) and electrons in orbitals.

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles in orbitals around the nucleus.

Example: An atom with 17 protons and 19 neutrons is chlorine (Cl). Its electron configuration determines its chemical properties and bonding behavior.

Covalent and Ionic Bonds

• Covalent bonds: Atoms share electron pairs. Can be polar (unequal sharing) or non-polar (equal sharing).

• Ionic bonds: Electrons are transferred from one atom to another, creating charged ions.

• Hydrogen bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen) and another electronegative atom.

Hydrophobic and Hydrophilic Molecules

• Hydrophobic: Molecules that repel water (e.g., non-polar molecules like oils).

• Hydrophilic: Molecules that interact well with water (e.g., polar molecules, ions).

Water: Properties and Importance

• Polarity: Water is a polar molecule, allowing it to form hydrogen bonds.

• Cohesion and adhesion: Water molecules stick to each other and to other surfaces.

• High specific heat: Water resists temperature changes, helping regulate cell and body temperature.

• Solvent properties: Water dissolves many substances, facilitating chemical reactions in cells.

Solutions

• Solution: A homogeneous mixture of two or more substances.

• Solvent: The substance that dissolves the solute (e.g., water in a sugar solution).

• Solute: The substance dissolved in the solvent (e.g., sugar in water).

Cell Membranes and Transport

Phospholipid Bilayer Structure

The cell membrane is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Hydrophilic heads: Face outward toward water.

• Hydrophobic tails: Face inward, away from water.

Membrane Proteins

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

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

• Functions: Transport, enzymatic activity, signal transduction, cell recognition, intercellular joining, attachment to cytoskeleton.

Transport Across Membranes

• Diffusion: Movement of molecules from high to low concentration.

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

• Facilitated diffusion: Movement of molecules via transport proteins; does not require energy.

• Active transport: Movement of molecules against a concentration gradient; requires energy (usually ATP).

Osmosis and Tonicity

• Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

• Hypotonic solution: Lower solute concentration outside the cell; water enters the cell, causing it to swell.

• Isotonic solution: Equal solute concentration; no net water movement.

Endocytosis and Exocytosis

• Endocytosis: Process by which cells take in substances by engulfing them in a vesicle.

• Exocytosis: Process by which cells expel substances using vesicles that fuse with the plasma membrane.

Energy and Membrane Transport

• ATP (adenosine triphosphate): The primary energy currency of the cell, used to power active transport and other cellular processes.

Sample Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
DNA Structure	Circular	Linear
Organelles	None (except ribosomes)	Membrane-bound organelles present
Cell Size	Small (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Equations and Concepts

• Osmosis: $\text{Osmosis} = \text{movement of water from low solute concentration to high solute concentration across a semi-permeable membrane}$

• Diffusion: $\text{Rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{distance}}$

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

Additional info:

• Some questions in the file prompt students to draw structures or label diagrams (e.g., water molecules, atomic structure). For study purposes, students should practice drawing these as part of their review.

• For more advanced study, students should explore the mechanisms of protein targeting, vesicle trafficking, and the role of cytoskeletal elements in cell structure and movement.