Carbon-Based Molecules in Biology

Carbon's unique bonding properties make it central to biological molecules. Its ability to form four covalent bonds and create stable chains enables the vast diversity of organic compounds essential for life.

Functional groups attach to carbon skeletons and determine molecular properties. Key groups include hydroxyl (-OH), amino (-NH₂), and carboxyl (-COOH), each contributing specific chemical characteristics to biological molecules.

Vocabulary: Dehydration synthesis joins monomers by removing water, while hydrolysis breaks bonds by adding water - these reactions are fundamental to building and breaking down biological molecules.

Macromolecules like proteins, carbohydrates, lipids, and nucleic acids rely on various bonding types. Proteins utilize peptide bonds (covalent) in their primary structure and hydrogen bonds in secondary structure. DNA combines covalent bonds in its backbone with hydrogen bonds between base pairs.

Understanding Cell Size and Membrane Transport in Biology

Cell size plays a crucial role in cellular function and survival. The relationship between surface area and volume (SA:V ratio) determines how efficiently a cell can exchange materials with its environment. For spherical cells, this ratio is calculated using SA = 4πr² and V = 4/3πr³, while cuboidal cells use SA = number of sides × length × width and V = height × width × length.

Definition: Surface area to volume ratio (SA:V) is the amount of cell membrane surface area available for exchange compared to the cell's internal volume.

Smaller cells have a higher SA:V ratio, making them more efficient at exchanging materials. This is critical because cells must maintain proper exchange of nutrients, waste products, and thermal energy across their membranes. As cells grow larger, their volume increases more rapidly than their surface area, potentially limiting their ability to sustain necessary life processes.

Membrane transport occurs through various mechanisms depending on the type of molecule being transported. Properties of covalent bonds and hydrogen bonds influence how molecules interact with the cell membrane. Nonpolar substances pass through easily, while polar, hydrophilic, and large molecules face more resistance. Transport can be either passive (requiring no energy) or active (requiring ATP).

Example: Water and small ions can move through channel proteins via facilitated diffusion, while larger molecules like glucose require specific carrier proteins.

Cellular Transport Mechanisms

Transport across membranes occurs through multiple pathways. Passive transport includes simple diffusion and facilitated diffusion, moving molecules down their concentration gradients. Variability and standard deviation in statistics examples can be used to analyze transport rates across different cell types.

Vocabulary: Active transport requires energy from ATP to move substances against their concentration gradients, often through specific protein pumps.

Exocytosis and endocytosis are vesicle-mediated transport processes. During exocytosis, vesicles fuse with the plasma membrane to release their contents. Endocytosis includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis for specific molecule uptake.

The sodium-potassium pump is a crucial example of active transport, maintaining essential ion gradients across cell membranes. This process requires ATP and helps establish the cell's membrane potential, vital for nerve cell function.

Plant Adaptations and Photorespiration Mechanisms

Photorespiration represents a significant metabolic challenge for plants, particularly during hot weather conditions. When plants close their stomata to conserve water, reduced oxygen levels trigger Rubisco (the primary carbon-fixing enzyme) to bind with oxygen instead of carbon dioxide. This process produces no sugar and wastes energy, potentially reducing plant productivity.

Highlight: Plants have evolved three distinct photosynthetic pathways (C3, C4, and CAM) to minimize the effects of photorespiration and optimize carbon fixation under different environmental conditions.

Plants have developed sophisticated adaptations to combat photorespiration's negative effects. C4 plants, such as corn and sugarcane, have evolved a spatial separation mechanism where initial carbon fixation occurs in mesophyll cells before being transferred to bundle sheath cells. This adaptation maintains high CO2 concentrations around Rubisco, effectively preventing photorespiration.

CAM (Crassulacean Acid Metabolism) plants like pineapples and cacti represent another evolutionary solution to photorespiration. These plants open their stomata at night to fix carbon dioxide, storing it as organic acids. During daylight hours, when stomata remain closed to conserve water, these stored compounds release CO2 for photosynthesis. This temporal separation of CO2 uptake and photosynthesis allows CAM plants to thrive in arid environments while minimizing water loss and photorespiration.

Vocabulary: Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the primary enzyme responsible for carbon fixation in photosynthesis, but it can also bind to oxygen, leading to photorespiration.