General Biology: Cell Structure, Macromolecules, and Membrane Function Study Guide

Macromolecules in Biology

Classes of Macromolecules

Biological macromolecules are large, complex molecules essential for life. They are primarily polymers, made by joining smaller subunits (monomers).

• Carbohydrates: Sugars and their polymers; provide energy and structural support.

• Proteins: Polymers of amino acids; perform a wide range of functions including catalysis, structure, and signaling.

• Nucleic acids: DNA and RNA; store and transmit genetic information.

• Lipids: Not true polymers; include fats, phospholipids, and steroids; important for energy storage and membrane structure.

Carbohydrates: Aldoses and Ketoses

Types of Monosaccharides

Monosaccharides are classified based on the location of their carbonyl group:

• Aldoses: Carbonyl group (aldehyde) is at the end of the carbon chain (outside).

• Ketoses: Carbonyl group (ketone) is within the carbon chain (inside).

Amino Acids and Proteins

Asymmetrical (Chiral) Carbon

An asymmetrical carbon (also called a chiral or alpha carbon) is a carbon atom attached to four different groups. In amino acids, the alpha carbon is central to the molecule's structure.

Structure of an Amino Acid

Amino acids are the building blocks of proteins. Each amino acid has:

• An amino group (-NH2)

• A carboxyl group (-COOH)

• A hydrogen atom

• An R group (side chain, variable among amino acids)

Peptide Bond Formation

Peptide bonds are formed by dehydration (condensation) reactions, which link the carboxyl group of one amino acid to the amino group of another, releasing water.

R Groups and Amino Acid Properties

The R group (side chain) determines the chemical properties and reactivity of each amino acid. R groups can be:

• Nonpolar (hydrophobic)

• Polar (hydrophilic, uncharged)

• Charged (acidic or basic)

Nucleic Acids: DNA Structure

Base Pairing and Hydrogen Bonds

DNA is composed of two strands forming a double helix, held together by hydrogen bonds between complementary bases:

• Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds

• Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds

Cell Structure and Function

Basic Features Shared by All Cells

All cells, whether prokaryotic or eukaryotic, share several fundamental features:

1. Surrounded by a selectively permeable plasma membrane

2. Cellular components are suspended in semifluid, jellylike substance called cytosol

3. Have chromosomes which contain DNA

4. Have ribosomes which make proteins from DNA

5. Cell interior is the cytoplasm (excludes nucleus in eukaryotes)

Prokaryotic vs. Eukaryotic Cells

Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells have both. Eukaryotes include plants, animals, fungi, and protists.

Organelles and Their Functions

Each organelle within a eukaryotic cell has a specific function. Students should be able to identify and describe the function of each organelle (e.g., nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.).

Endosymbiosis Theory

The endosymbiosis theory explains the origin of mitochondria and chloroplasts in eukaryotic cells. It proposes that these organelles originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence: Mitochondria and chloroplasts have their own circular DNA, ribosomes, and double membranes. Their DNA closely matches that of certain extant prokaryotes.

Compartments of Mitochondria and Chloroplasts

• Mitochondria: Known as the "powerhouse" of the cell; site of cellular respiration.

• Chloroplasts: Found in plants and algae; site of photosynthesis.

Cytoskeleton and Motor Proteins

Functions of the Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, and enables cell movement. Motor proteins interact with the cytoskeleton to move the entire cell or parts of the cell.

Extracellular Matrix (ECM)

Composition and Components

The extracellular matrix (ECM) is a complex network outside animal cells, providing structural and biochemical support.

• Collagen: Forms strong fibers outside of cells.

• Proteoglycans: Network formed by a small protein core with many noncovalently attached polysaccharide chains.

• Fibronectin: Attaches the cell membrane to the ECM.

• Integrins: Plasma membrane proteins that link the ECM (outside the cell) to the cytoskeleton (inside the cell).

Functions of the Extracellular Matrix

• Acts as a physical barrier or movement track for cell migration.

Plasma Membrane Structure and Function

Composition of the Plasma Membrane

The plasma membrane is primarily composed of phospholipids and associated proteins. Phospholipids are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Functions of membrane proteins include:

  • Transport

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and ECM

Membrane Transport: Diffusion, Osmosis, and Tonicity

Key Concepts

• Diffusion: Movement of a substance from a region where it is more concentrated to a region where it is less concentrated.

• Osmosis: Specific type of diffusion where water moves across the membrane (from low solute to high solute concentration).

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

Summary Table: Types of Membrane Transport

Type	Description	Example
Simple Diffusion	Movement of small, nonpolar molecules directly through the membrane	O2 and CO2 exchange in lungs
Facilitated Diffusion	Movement of molecules via membrane proteins (channels/carriers)	Glucose transport into cells
Osmosis	Diffusion of water through a selectively permeable membrane	Water uptake by plant roots
Active Transport	Movement of substances against their concentration gradient using energy (ATP)	Na+/K+ pump in animal cells

Key Equations

• Osmotic Pressure:

$\Pi = iMRT$

• Where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature in Kelvin.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard General Biology curriculum.