General Biology: Cell Structure, Membrane Function, and Enzyme Regulation Study Guide

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Cell Structure and Function

Cellular Organelles and Their Functions

Cells contain specialized structures called organelles, each with distinct roles in maintaining cellular function and organization.

Nucleus: Contains genetic material (DNA) and controls cellular activities.
Ribosomes: Sites of protein synthesis.
Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids such as oils, phospholipids, and steroids.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
Lysosomes: Contain digestive enzymes; act as cellular stomachs to break down waste and old organelles (autophagy).
Peroxisomes: Break down fatty acids and detoxify harmful substances.
Mitochondria: Powerhouse of the cell; site of cellular respiration. Cristae are folds of the inner membrane, increasing surface area for energy production.
Chloroplasts: Found in plant cells; site of photosynthesis. Thylakoids are membrane-bound compartments inside chloroplasts.
Centrosome: Organizes microtubules and is important for cell division.

Prokaryotic vs. Eukaryotic Cells:

Prokaryotic cells lack membrane-bound organelles (e.g., nucleus, mitochondria).
Eukaryotic cells have membrane-bound organelles.

Cell Membranes

The cell membrane is a semi-permeable barrier composed mainly of phospholipids and proteins, controlling the movement of substances in and out of the cell.

Phospholipid bilayer: Consists of hydrophilic (water-loving) phosphate heads and hydrophobic (water-fearing) fatty acid tails.
Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins and lipids moving laterally.
Integral proteins: Span the membrane from one side to the other.
Glycoproteins: Proteins attached to sugars, important for cell recognition.
Semi-permeable: Allows selective passage of molecules (e.g., sodium ions cannot pass without a protein channel).

Membrane Fluidity:

Influenced by temperature, cholesterol, degree of fatty acid saturation, and hydrogenation.
Hydrogenating fatty acid chains increases melting temperature and viscosity, but decreases fluidity.
Cholesterol can increase or decrease fluidity depending on temperature.

Transport Across Membranes

Cells use various mechanisms to move substances across membranes.

Passive Transport: Movement down a concentration gradient (e.g., diffusion, osmosis).
Active Transport: Requires energy (ATP) to move substances against a gradient.
Carrier proteins and ion pumps: Facilitate active transport and help establish membrane gradients.
Channel proteins: Allow specific ions or molecules to pass through the membrane.
Osmosis: Water moves toward higher solute concentration.
Hypertonic solution: Cell shrinks as water leaves.
Hypotonic solution: Cell swells and may burst as water enters.

Cellular Connections and Communication

Gap junctions (animals) and plasmodesmata (plants): Allow molecules to pass directly from one cell to another.
Cell wall: In plants, primarily made of cellulose.

Biological Molecules

Polymers and Monomers

Biological polymers are large molecules made by joining smaller units called monomers.

Proteins: Polymers of amino acids.
Carbohydrates: Polymers of monosaccharides (e.g., glucose, ribose).
Nucleic acids (DNA, RNA): Polymers of nucleotides.
Lipids: Not true polymers; include fats, phospholipids, steroids.

Polymerization Reactions:

Dehydration: Forms a bond by removing water.
Hydrolysis: Breaks a bond by adding water.

Carbohydrates

Monosaccharides: Simple sugars (e.g., glucose, ribose).
Disaccharides: Two monosaccharides joined together.
Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).
Aldose: Sugar with external carbonyl group.
Ketose: Sugar with internal carbonyl group.
Amylose: Unbranched starch found in plants.
Glycogen: Branched polysaccharide found in animals.

Lipids

Phospholipids: Major component of cell membranes; have a phosphate head and fatty acid tails.
Trans fats: Have incomplete hydrogenation and a double bond; associated with health risks.
Steroids: Lipids with a characteristic four-ring structure.

Proteins and Amino Acids

Amino acids: Building blocks of proteins; 20 standard types.
Charged amino acids: Typically 5; important for protein structure and function.
Hydrophilic amino acids: Interact with water; have polar or charged side chains.
Hydrophobic amino acids: Avoid water; have nonpolar side chains (e.g., tryptophan).
Special side chains: Serine (-CH2-OH), cysteine (-CH2-SH).
Protein structure: Primary (sequence), secondary (hydrogen bonds, e.g., alpha helix, beta sheet), tertiary (3D folding), quaternary (multiple polypeptides).

Genetics and Nucleic Acids

DNA and RNA Structure

DNA: Double-stranded helix; strands held together by hydrogen bonds between bases.
RNA: Single-stranded; backbone sugar (ribose) has an extra oxygen compared to DNA (deoxyribose).
Phosphate group: Provides negative charge to DNA and RNA.

Metabolism and Enzyme Function

Metabolic Pathways

Anabolic metabolism: Builds complex molecules from simpler ones; requires energy (e.g., synthesizing amino acids).
Catabolic metabolism: Breaks down molecules to release energy.

Thermodynamics in Biology

First Law of Thermodynamics: Energy cannot be created or destroyed.
Second Law of Thermodynamics: Entropy (disorder) of the universe increases.
Free energy (): The energy available to do work in a system.
Spontaneous reactions: Occur when free energy decreases ().
Equation:

= change in free energy
= change in enthalpy (heat)
= temperature (Kelvin)
= change in entropy

Enzymes and Regulation

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

Active site: Region on the enzyme where substrate binds and reaction occurs.
Competitive inhibitors: Bind to the active site, blocking substrate binding and decreasing enzyme activity.
Non-competitive inhibitors: Bind elsewhere, changing enzyme shape and function.
Allosteric regulation: Regulatory molecule binds at a site other than the active site, affecting enzyme activity.
Cooperativity: Binding of one substrate molecule affects binding of additional substrates; often seen in multi-subunit enzymes.
Enzyme activity: Influenced by pH, temperature, and regulatory molecules.

Examples and Applications

Autophagy: Lysosomes digest old organelles (e.g., mitochondria, peroxisomes).
Motor proteins: Myosin, kinesin, and dynein use ATP to move along cytoskeletal filaments.
Cell fractionation: Technique to separate cell components for study.

Tables

Comparison of Cell Structures

Structure	Function	Membrane Bound?
Nucleus	Contains DNA	Yes
Ribosome	Protein synthesis	No
Lysosome	Digestion	Yes
Peroxisome	Fatty acid breakdown	Yes
Mitochondria	Energy production	Yes (double membrane)
Chloroplast	Photosynthesis	Yes (double membrane)
Cell wall	Structural support	No

Types of Membrane Transport

Type	Energy Required?	Direction	Example
Passive Transport	No	Down gradient	Osmosis, diffusion
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Facilitated Diffusion	No	Down gradient	Glucose transporter

Protein Structure Levels

Level	Stabilizing Forces	Description
Primary	Peptide bonds	Amino acid sequence
Secondary	Hydrogen bonds	Alpha helix, beta sheet
Tertiary	Hydrogen, ionic, hydrophobic, disulfide bonds	3D folding
Quaternary	Same as tertiary	Multiple polypeptides

Additional info:

Some definitions and examples were expanded for clarity and completeness.
Equations and tables were inferred and formatted for academic context.