Cells: The Living Units

Cell Structure and Function

The cell is the basic structural and functional unit of all living organisms. Each cell contains specialized organelles that perform distinct functions necessary for life.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Chromatin: Uncondensed DNA found in the nucleus; condenses to form chromosomes during cell division.

• Nucleolus: Site of ribosome production within the nucleus.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Mitochondria: Powerhouse of the cell; produces ATP through cellular respiration.

• Lysosomes: Contain digestive enzymes to break down waste and cellular debris.

• Peroxisomes: Detoxify harmful substances and break down fatty acids.

• Cytoskeleton: Provides structural support and facilitates cell movement.

• Plasma Membrane: Encloses the cell, regulates entry and exit of substances.

Plasma Membrane: Fluid Mosaic Model

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It regulates the movement of substances into and out of the cell and facilitates cell communication.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Integral Proteins: Span the membrane; involved in transport and cell signaling.

• Peripheral Proteins: Attached to the membrane surface; support and signaling.

• Glycoproteins & Glycolipids: Act as cell identifiers and facilitate cell-cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Membrane Transport Mechanisms

The plasma membrane is selectively permeable, allowing some molecules to pass freely while restricting others. Transport mechanisms are classified as passive or active.

• Passive Transport: No ATP required; includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires ATP; includes endocytosis, exocytosis, and membrane pumps.

• Primary Active Transport: Direct use of ATP to move ions (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy from ion gradients.

Example: The sodium-potassium pump maintains cellular ion balance by moving Na+ out and K+ into the cell against their concentration gradients.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, providing structural integrity and facilitating communication.

• Tight Junctions: Integral proteins join cells, forming an impermeable barrier.

• Desmosomes: Cadherin proteins join cells, providing mechanical strength.

• Gap Junctions: Connexon proteins form tunnels between cells, allowing direct communication.

Chemistry Comes Alive: Biomolecules

Basic Chemistry Concepts

Chemistry is fundamental to understanding biological processes. Matter is composed of atoms and elements, and energy is required for cellular activities.

• Matter: Anything that occupies space and has mass.

• Atoms: Basic units of matter; consist of protons, neutrons, and electrons.

• Elements: Pure substances made of one type of atom; major elements in the body include carbon, hydrogen, oxygen, and nitrogen.

• Energy: Capacity to do work; includes kinetic and potential energy.

Atomic Structure and Chemical Bonds

Atoms combine to form molecules and compounds through chemical bonds. The type of bond depends on electron sharing or transfer.

• Covalent Bonds: Electrons are shared between atoms; can be polar (unequal sharing) or nonpolar (equal sharing).

• Ionic Bonds: Electrons are transferred, resulting in charged ions.

• Example: Water (H2O) is a polar covalent compound; sodium chloride (NaCl) is an ionic compound.

Biochemistry: Inorganic and Organic Compounds

Biochemistry studies the chemical composition and reactions of living matter. Compounds are classified as inorganic or organic.

• Inorganic Compounds: Include water, salts, acids, and bases.

• Water: Most abundant compound; high heat capacity, solvent properties, and reactivity.

• Salts: Ionic compounds; dissociate into electrolytes.

• Acids/Bases: Affect pH; acids release H+, bases accept H+.

Organic Compounds: Carbohydrates, Lipids, Proteins

Organic compounds are essential biomolecules that provide energy, structure, and function in cells.

• Carbohydrates: Contain C, H, O; include monosaccharides, disaccharides, and polysaccharides. Function as energy sources and structural molecules.

• Lipids: Contain C, H, O (and sometimes P); include triglycerides, phospholipids, and steroids. Function in energy storage, membrane structure, and signaling.

• Proteins: Composed of amino acids linked by peptide bonds; serve structural, enzymatic, and transport roles.

Additional info:

• Biomolecules form the basis for cell structure and function, linking molecular biology to tissue and organ systems.

• Understanding cell structure and biomolecules is foundational for studying physiology, metabolism, and disease processes.