Fundamental Concepts in Biology: Measurements, Chemistry, Microscopy, and Cell Biology

Measurements and Experimental Design

Precision and Accuracy

Understanding the reliability and correctness of measurements is essential in biological experiments.

• Precision: How closely repeated measurements agree with each other.

• Accuracy: How closely measurements agree with the true or correct value.

Variables in Experiments

Variables are factors that can change in an experiment and are crucial for scientific inquiry.

• Dependent Variable: The factor being measured; predicted to be affected by manipulation of the independent variable.

• Independent Variable: The variable that is changed or manipulated.

• Controlled Experiment: An experiment in which only one variable is changed at a time, while others are kept constant.

• Hypothesis: A statement that clearly defines the specifics of the experimental variables being tested.

Measurements and Basic Properties

Accurate measurement is fundamental in biology for quantifying properties of substances and organisms.

• Meniscus: The curved surface of a liquid in a container; read the bottom at eye level for most liquids.

• Density: Mass per unit volume of a substance. Units: g/mL or g/cm3.

• Displacement: The amount of liquid moved when an object is placed into it; used to determine the object's volume.

• Volume: The amount of space an object or substance occupies.

Formula for Density:

$Density = \frac{Mass}{Volume}$

Example: If an object has a mass of 10 g and displaces 2 mL of water, its density is $\frac{10}{2} = 5$ g/mL.

Chemistry Basics

Atomic and Molecular Structure

Atoms and molecules are the building blocks of all biological matter.

• Atomic structure: Arrangement of protons and neutrons in the nucleus, electrons surrounding the nucleus.

• Molecular structure: Arrangement of atoms and chemical bonds within a molecule.

Chemical Bonds

Chemical bonds hold atoms together in molecules and are essential for biological structure and function.

• Covalent bond: Formed when atoms share electrons.

• Ionic bond: Formed by attraction between oppositely charged ions after electrons are transferred.

• Hydrogen bond: Weak attraction between a hydrogen atom bonded to oxygen, nitrogen, or fluorine and another electronegative atom.

Chemical Reactions and Properties

• Chemical reaction: Process in which reactants are converted into new substances called products.

• Octet rule: Atoms tend to gain, lose, or share electrons to have eight electrons in their outer shell.

• Dissociation: Process by which an ionic compound separates into ions when dissolved in water.

• Dissolve: Mixing a solute evenly into a solvent to form a solution.

• Electrolyte: Substance that dissociates into ions in water and conducts electricity.

• Cation: Positively charged ion.

• Anion: Negatively charged ion.

• Reactant: Starting substance in a chemical reaction.

• Product: Substance formed by a chemical reaction.

Example: Sodium chloride (NaCl) dissociates in water to form Na+ (cation) and Cl- (anion).

Solutions and Concentrations

Key Terms and Concepts

Solutions are homogeneous mixtures important in biological systems.

• Mole: Amount of substance containing $6.022 \times 10^{23}$ particles (Avogadro's number).

• Molarity (M): Concentration of a solution expressed as moles of solute per liter of solution.

• Molecular formula: Shows the actual number and types of atoms in a molecule.

• Molecular mass: Sum of atomic masses of all atoms in a molecule, usually in atomic mass units (amu).

• Solvent: Substance that dissolves a solute, usually present in greatest amount.

• Solute: Substance dissolved in a solvent.

• Solution: Homogeneous mixture of a solute dissolved in a solvent.

Formula for Molarity:

$Molarity = \frac{Moles\ of\ solute}{Liters\ of\ solution}$

Standard Curve and Absorbance

Standard curves are used to determine unknown concentrations using absorbance measurements.

• Standard curve: Graph used to determine concentration of an unknown sample by comparing to samples of known concentration.

• Absorbance: Measure of how much light a substance absorbs at a specific wavelength.

Example: A spectrophotometer measures absorbance; a standard curve relates absorbance to concentration.

Acids and Bases

pH and Buffers

Acids and bases are important for maintaining biological function and homeostasis.

• pH: Measure of how acidic or basic a solution is, based on its hydrogen ion (H+) concentration.

• Buffer: Solution that resists changes in pH when small amounts of acid or base are added.

• Acid: Substance that releases hydrogen ions (H+) in water.

• Base: Substance that accepts hydrogen ions (H+) or releases hydroxide ions (OH-) in water.

• Acidic solution: pH less than 7.

• Basic solution: pH greater than 7.

Formula for pH:

$pH = -\log_{10}[H^+]$

Example: A solution with [H+] = $1 \times 10^{-4}$ M has a pH of 4.

Compound Microscope Parts

Microscope Structure and Function

Microscopes are essential tools for observing cells and microorganisms.

• Compound microscope: Uses multiple lenses to magnify small objects.

• Base: Bottom support of the microscope.

• Arm: Supports the body and used for carrying.

• Oculars: Eyepieces, usually 10× magnification.

• Revolving nosepiece: Rotating part holding objective lenses.

• Objective lenses: Primary magnifying lenses (commonly 4×, 10×, 40×, 100×).

• Stage: Platform holding the microscope slide.

• Coarse adjustment knob: Large knob for rough focusing, usually on low power.

• Fine adjustment knob: Small knob for precise focusing, especially on high power.

• Condenser: Lens system focusing light onto the specimen.

• Iris diaphragm: Adjustable opening controlling light amount.

• Lamp: Built-in light source.

Example: To view bacteria, use the 100× objective lens with oil immersion for maximum resolution.

Cell Biology

Cellular Structures and Functions

Cells are the basic units of life, with specialized structures for movement and energy conversion.

• Cytoplasmic streaming: Movement of cytoplasm within a cell to help transport materials.

• Cilia: Short, hair-like projections moving the cell or substances across its surface.

• Pseudopodia: Temporary extensions of the cell membrane used for movement and engulfing food ("false feet").

Photosynthesis and Chloroplast Structure

Photosynthesis is the process by which plants, algae, and some bacteria convert sunlight into chemical energy.

• Photosynthesis: Conversion of carbon dioxide and water into glucose and oxygen using sunlight.

• Thylakoid: Flattened membrane sac inside chloroplasts where light-dependent reactions occur.

Photosynthesis Equation:

$6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$

Example: In green plants, thylakoids are stacked into grana, increasing surface area for light absorption.

Summary Table: Key Terms and Definitions

Term	Definition
Precision	Agreement among repeated measurements
Accuracy	Agreement with the true value
Meniscus	Curved surface of liquid; read at eye level
Density	Mass per unit volume
Covalent bond	Atoms share electrons
Ionic bond	Electrons transferred; ions attracted
Hydrogen bond	Weak attraction involving hydrogen
Mole	$6.022 \times 10^{23}$ particles
Molarity	Moles per liter of solution
pH	Measure of acidity/basicity
Buffer	Resists pH changes
Compound microscope	Multiple lenses for magnification
Cytoplasmic streaming	Movement of cytoplasm in cell
Photosynthesis	Conversion of light to chemical energy
Thylakoid	Site of light-dependent reactions