Proteins and Their Structure

Proteins are the "doers" of the cell, handling everything from defense and transport to movement and structural support. Without proteins—especially enzymes that speed up chemical reactions—life simply couldn't exist. Each protein consists of one or more polypeptides, which are chains of amino acids linked by peptide bonds.

What makes proteins so versatile is their structure. The primary structure is just the unique sequence of amino acids in the chain. This sequence determines how the protein will fold into its secondary structure (coils and folds), and then into its tertiary structure (3D shape created by interactions between side chains). When multiple polypeptide chains interact, they create the quaternary structure.

The magic of proteins lies in their shape—their structure directly determines their function. This is why when proteins are damaged or "denatured," they stop working properly.

Quick Insight: Think of protein structure like origami—the specific sequence of folds (primary and secondary structure) creates a unique 3D shape (tertiary structure) that determines what the protein can do. If you unfold the paper, it loses its function!

Water and Macromolecules

Water is essential to life, with unique properties that make biochemistry possible. It's a polar molecule with covalent bonds within the molecule but hydrogen bonds between molecules. These hydrogen bonds give water its special properties: cohesion (water sticks to itself), adhesion (water sticks to other surfaces), and high specific heat.

The four main types of macromolecules in living things are carbohydrates, lipids, proteins, and nucleic acids. These large molecules form through dehydration synthesis, which removes water to join monomers together. Breaking them apart happens through hydrolysis by adding water back.

Carbohydrates (CHO) provide short-term energy and structure (like cellulose in plants). Lipids are mostly nonpolar and hydrophobic, offering long-term energy storage. They have a hydrophilic head and hydrophobic tail, with saturated fats having no double bonds (solid at room temperature) and unsaturated fats having at least one double bond (liquid at room temperature).

Remember This: Molecules that interact easily with water are hydrophilic $"water-loving"$ and usually have polar bonds. Those that don't mix with water are hydrophobic $"water-fearing"$ and have fewer polar bonds. This distinction is crucial for understanding cell membranes!

Proteins, Nucleic Acids, and Chemical Bonding

Proteins (CHON) are built from amino acids and have multiple levels of structure. The primary structure is the sequence of amino acids connected by peptide bonds. Secondary structures form alpha helices or beta sheets. Tertiary structure creates the functional 3D shape through R-group bonding. When multiple protein chains connect, they form quaternary structure.

Nucleic acids (CHONP) include DNA and RNA, with nucleotides as their building blocks. These store and transmit genetic information essential for life.

Chemical bonding is the foundation of all biomolecules. Covalent bonds involve sharing electrons—one pair for a single bond and two pairs for a double bond. Ionic bonds involve the transfer of electrons between atoms. Hydrogen bonds form between oxygen and hydrogen atoms of different molecules and are weaker but crucial for water properties.

Functional groups like carbonyl, carboxyl, hydroxyl, and amino groups give biomolecules their specific chemical properties. These small clusters of atoms determine how molecules interact with each other.

Pro Tip: The strength of bonds matters! Covalent bonds hold atoms together within molecules, while weaker hydrogen bonds allow for temporary interactions that can be easily broken and reformed—critical for processes like DNA replication.

Cells: The Fundamental Units of Life

Cells are the basic units of structure and function for all living organisms. There are two main types: prokaryotic cells (bacteria and archaea) that lack a nucleus and membrane-bound organelles, and eukaryotic cells (protists, fungi, animals, and plants) that have a nucleus and specialized organelles.

All cells share some basic features. The plasma membrane acts as a selective barrier, controlling what enters and exits the cell. Inside is the cytosol, a gel-like substance where many cellular reactions occur. All cells contain chromosomes that carry genes and ribosomes that manufacture proteins.

Eukaryotic cells are typically much larger than prokaryotic cells and contain a nucleus that houses DNA organized into chromosomes. When DNA combines with proteins, it forms chromatin. Inside the nucleus is the nucleolus, where ribosomal RNA is synthesized.

Size Matters: A cell's size is limited by its surface area-to-volume ratio. As a cell grows, its volume increases much faster than its surface area (volume increases by n³ while surface area increases by n²). This means larger cells have relatively less surface area for exchanging materials with their environment, which limits how big they can get.

The Endomembrane System

The endomembrane system is a complex network of organelles that work together to process and transport cellular materials. The endoplasmic reticulum (ER) accounts for more than half of all membranes in eukaryotic cells. Rough ER is studded with ribosomes and processes proteins, while smooth ER synthesizes lipids and regulates calcium.

The Golgi apparatus acts like a cellular post office, receiving products from the ER, modifying them, and packaging them into transport vesicles for delivery throughout the cell. It sorts and distributes proteins and lipids to their proper destinations, including the cell membrane or for secretion outside the cell.

Lysosomes are membrane-enclosed sacs containing digestive enzymes that break down macromolecules, cellular waste, and even old organelles through a process called autophagy. These enzymes work best in the acidic environment inside lysosomes, protecting the rest of the cell from their destructive activity.

In plant cells, plasmodesmata create channels through cell walls that allow water and small molecules to pass directly from one cell to another. This creates a continuous pathway for communication between neighboring cells.

Think About It: The endomembrane system works like an assembly line in a factory—materials move from one station to another, getting modified, packaged, and delivered to their final destinations. If one component fails, the entire production line can be affected!

Cell Membranes and Transport

Cell membranes are fluid mosaics of lipids and proteins that control what enters and exits the cell. The membrane consists of a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward. Embedded within this bilayer are various membrane proteins that serve as channels, receptors, and structural supports.

Cells carefully regulate transport across their membranes. Passive transport requires no energy and includes simple diffusion, where molecules move from areas of high concentration to low concentration. Transport proteins allow hydrophilic substances to cross the membrane, with aquaporins specifically facilitating water movement.

Water balance is critical for cell survival. When cells are placed in solutions with different concentrations, water moves by osmosis. In a hypotonic solution (less solute outside), water flows into the cell, causing it to swell. In a hypertonic solution (more solute outside), water leaves the cell, causing it to shrink. Cells function best in isotonic solutions, where there's no net movement of water.

Real-World Connection: Understanding osmosis explains why saltwater makes you thirstier—your cells become dehydrated as water moves out of them into the hypertonic environment created by salt intake. This same principle is used when preserving foods with salt or sugar!

Prokaryotic vs. Eukaryotic Cells and Osmosis

Prokaryotic cells (bacteria and archaea) are single-celled organisms without a nucleus—their DNA floats freely in the cytoplasm. Most have cell walls, but they lack membrane-bound organelles. Eukaryotic cells (found in protists, plants, animals, and fungi) contain a true nucleus that houses their DNA, along with various specialized organelles. Both cell types have ribosomes, DNA, cytoplasm, and cell membranes.

Osmosis is the movement of water through a selectively permeable membrane from areas of higher water concentration (lower solute) to areas of lower water concentration (higher solute). This is a form of passive transport requiring no energy input. Water potential (Ψ) determines the direction of water movement and includes both pressure potential and solute potential components.

The endosymbiotic theory explains how eukaryotic cells evolved—primitive prokaryotes were engulfed by larger cells and developed a mutually beneficial relationship, eventually becoming mitochondria and chloroplasts. This explains why these organelles have their own DNA and ribosomes.

Visual Aid: Think of osmosis like a crowded room. If one side of a room is packed with people (solutes) and the other side is nearly empty, people will naturally move from the less crowded area to the more crowded one. Water molecules do the opposite—they move away from the crowd toward areas with fewer solutes.

Cell Transport and Surface Area

Cells maintain homeostasis through various transport mechanisms across the cell membrane. Simple diffusion allows nonpolar molecules to pass directly through the phospholipid bilayer, moving from areas of high to low concentration. For polar molecules like water, facilitated diffusion uses transport proteins (including aquaporins) to cross the membrane—still requiring no energy.

Moving substances against their concentration gradient (from low to high concentration) requires energy from ATP in a process called active transport. Cells also use endocytosis to bring materials in by forming membrane-enclosed vesicles—either fluids (pinocytosis) or solid particles (phagocytosis). Exocytosis works in reverse to expel waste or secrete substances.

The relationship between surface area and volume is crucial for cellular function. As cells grow, their volume increases faster than their surface area. Since materials enter and exit through the cell surface, this ratio limits cell size—larger cells need proportionally more surface area to meet their metabolic needs.

Study Hack: When thinking about transport, remember "high to low, let it flow; low to high, ATP supply." Passive transport (diffusion and osmosis) always moves from high to low concentration and requires no energy, while active transport requires ATP to move substances against their concentration gradients.