Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are the basic units of life and can be classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a membrane-bound nucleus and organelles. Their genetic material is found in a nucleoid region.

• Eukaryotic cells possess a true nucleus enclosed by a nuclear membrane and contain various membrane-bound organelles.

• Fundamental difference: The presence of a nucleus and organelles in eukaryotes distinguishes them from prokaryotes.

• Examples: Bacteria are prokaryotes; plants and animals are eukaryotes.

Distinguishing Cell Types

Cells can be identified based on several features:

• Prokaryotes: No nucleus, circular DNA, cell wall (peptidoglycan in bacteria), smaller size.

• Eukaryotes: Nucleus, linear chromosomes, organelles (mitochondria, ER, Golgi), larger size.

• Plant cells: Cell wall (cellulose), chloroplasts, large central vacuole.

• Animal cells: No cell wall, centrioles, lysosomes.

Prokaryotic Chromosomes

Prokaryotic chromosomes are unique in their structure and organization:

• Typically a single, circular DNA molecule.

• Located in the nucleoid region.

• Lack histone proteins (in most bacteria).

Cellular Structures in Bacteria and Eukaryotes

Both cell types share some common structures:

• Plasma membrane

• Ribosomes

• Cytoplasm

Ribosomes: Structure and Function

Ribosomes are essential for protein synthesis:

• Composed of rRNA and proteins.

• Made of two subunits (large and small).

• Function: Translate mRNA into polypeptides.

Nucleus, Nucleolus, and Nucleoid

These terms refer to regions or structures associated with genetic material:

• Nucleus: Membrane-bound organelle in eukaryotes containing DNA.

• Nucleolus: Dense region within the nucleus where ribosomal RNA is synthesized.

• Nucleoid: Region in prokaryotes where DNA is located.

Endomembrane System

The endomembrane system coordinates the synthesis and transport of cellular materials:

• Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Organizes and transports proteins and lipids.

Nuclear Envelope and Nuclear Pore Complexes

The nuclear envelope surrounds the nucleus and contains nuclear pores:

• Nuclear pore complexes: Regulate transport of molecules between nucleus and cytoplasm.

• Ribosomal subunits require nuclear pores for export due to their size and specificity.

Cellular Organelles and Their Functions

Endoplasmic Reticulum (ER)

The ER is involved in protein and lipid synthesis:

• Rough ER: Studded with ribosomes; synthesizes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids:

• Receives products from ER.

• Prepares them for secretion or use within the cell.

Lysosomes and Peroxisomes

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

Cytoskeleton

The cytoskeleton provides structural support and facilitates movement:

• Includes microfilaments, intermediate filaments, and microtubules.

• Involved in cell division, shape, and transport.

Atoms, Bonds, and Molecules

Atomic Structure

Atoms consist of a nucleus (protons and neutrons) and electrons in orbitals:

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles in orbitals.

Example: An atom with 17 protons and 19 neutrons is chlorine (Cl).

Covalent and Hydrogen Bonds

Chemical bonds hold atoms together in molecules:

• Covalent bonds: Atoms share electrons; can be polar or non-polar.

• Non-polar covalent bonds: Electrons are shared equally (e.g., C-H bonds).

• Polar covalent bonds: Electrons are shared unequally (e.g., O-H bonds in water).

• Hydrogen bonds: Weak attractions between polar molecules, such as between water molecules.

Hydrophilic vs. Hydrophobic Molecules

Molecules interact with water based on their polarity:

• Hydrophilic: Water-loving; polar molecules that dissolve in water.

• Hydrophobic: Water-fearing; non-polar molecules that do not dissolve in water.

Properties of Water

Water is essential for life due to its unique properties:

• High heat capacity and heat of vaporization.

• Cohesion and adhesion.

• Universal solvent for polar substances.

• Evaporation of water cools the body (sweating).

Solutions: Solvent and Solute

A solution consists of a solvent and a solute:

• Solvent: The substance that dissolves the solute (e.g., water).

• Solute: The substance dissolved (e.g., sugar).

Cell Membranes and Transport

Phospholipid Bilayer Structure

The cell membrane is composed of a phospholipid bilayer:

• Hydrophilic heads: Face outward toward water.

• Hydrophobic tails: Face inward, away from water.

• Cholesterol and proteins are embedded for stability and function.

Membrane Proteins

Proteins in the membrane serve various functions:

• Integral proteins: Span the membrane; involved in transport.

• Peripheral proteins: Attached to the surface; involved in signaling and support.

• Functions include transport, enzymatic activity, cell recognition, and structural support.

Transport Across Membranes

Cells regulate the movement of substances across membranes:

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated diffusion: Uses transport proteins; does not require energy.

• Active transport: Requires energy (ATP) to move substances against their concentration gradient.

Selective Permeability

Cell membranes are selectively permeable, allowing some substances to pass while blocking others:

• Small, non-polar molecules pass easily.

• Large or charged molecules require transport proteins.

Osmosis and Tonicity

Osmosis affects cell volume and function:

• Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, causing shrinkage.

• Hypotonic solution: Lower solute concentration outside; water enters the cell, causing swelling.

• Isotonic solution: Equal solute concentration; no net water movement.

Endocytosis and Exocytosis

Cells transport large molecules via vesicles:

• Endocytosis: Cell engulfs material by forming a vesicle.

• Exocytosis: Vesicle fuses with membrane to release contents outside the cell.

HTML Table: Comparison of Cell Types

Key Equations

• Diffusion Rate:

• Osmosis:

• Atomic Number:

Additional info:

• Some context and definitions have been expanded for clarity and completeness.

• Table entries and equations inferred from standard biology curriculum.