Cells: The Basic Unit of Life

Definition and Characteristics

• Cell: The fundamental unit of life in all living organisms.

• Cells can be specialized to perform many different functions (e.g., liver cells, muscle cells, red blood cells, bacterial cells).

• Cell function depends on which genes are active within the cell.

• Despite specialization, all cell types share many similarities.

• Specialization allows cells to perform unique roles (e.g., metabolizing drugs, carrying oxygen).

Examples of Cell Specialization

• Liver cells: Specialized to metabolize organic compounds, including drugs.

• Red blood cells (RBCs): Specialized to carry oxygen throughout the body.

Discovery and History of Cells

Robert Hooke and the Microscope

• Cells were first discovered in 1665 by Robert Hooke due to the development of the microscope.

Cell Theory (Developed in the 1800s)

• Cells are the basic structural and physiological units of all living organisms.

• Cells can exist as single entities (unicellular) or as building blocks in multicellular organisms.

• All cells arise from pre-existing cells.

• All cells share similar chemical composition.

• Most chemical reactions of life occur within cells.

• Cells contain genetic information that is replicated and passed to offspring.

Human Cells and Development

Cell Number and Developmental Stages

• Humans have approximately 37.2 trillion cells in adulthood.

• Development starts from a single cell called a zygote (fertilized egg).

• Developmental stages: Zygote → Morula (solid ball of cells) → Blastula (hollow ball of cells) → Gastrula (formation of germ layers).

Surface Area to Volume Ratio

• Cells must maintain a high surface area to volume ratio for efficient exchange of materials.

• As cell size increases, the amount of 'stuff' the cell can exchange decreases relative to its volume.

• If waste is not removed efficiently, the cell may die.

• Examples of cells with high surface area: eggs, skeletal muscle cells, neurons.

Cell Structure: Organelles and Their Functions

Overview of Animal Cell Structure

The animal cell contains various organelles, each with specialized functions essential for cell survival and activity.

Cell Membrane

• Composed of a phospholipid bilayer, cholesterol, and membrane proteins.

• Functions as a selectively permeable barrier, allowing certain substances to pass while blocking others.

• Contains embedded proteins for transport, signaling, and structural support.

Nucleus and Nucleolus

• Nucleus: Surrounded by a double-layered nuclear envelope; contains DNA.

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

Ribosomes

• Composed of ribosomal RNA (rRNA) and protein.

• Sites of protein synthesis; can be free in the cytoplasm or attached to the rough ER.

• Three binding sites: A (amino acid), P (peptide), and E (exit).

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; synthesizes and processes proteins for export or membrane insertion.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids, detoxifies drugs, and stores calcium.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for delivery to different destinations.

• Has a cis face (receiving side) and a trans face (shipping side).

Mitochondria

• Site of cellular respiration and ATP production in eukaryotic cells.

• Contains its own DNA and has double membranes (outer and inner).

Lysosomes and Peroxisomes

• Lysosomes: Contain enzymes to degrade and remove organic complexes or pathogens.

• Peroxisomes: Contain enzymes to degrade organic compounds using peroxide.

Cytoskeleton

• Provides structure, support, and enables movement within the cell.

• Includes microfilaments (actin), intermediate filaments, and microtubules.

• Forms cilia and flagella for movement; microfilaments aid in intracellular transport.

Centrioles

• Organize spindle fibers during cell division.

• Centrioles work in pairs and are located in the centrosome.

Movement Across Membranes

Selective Permeability

• Cell membranes are selectively permeable, allowing some substances to cross more easily than others.

Types of Membrane Transport

• Passive Movement: Does not require ATP; substances move from high to low concentration.

• Osmosis: Movement of water across a selectively permeable membrane from low solute concentration to high solute concentration.

• Tonicity: Describes the osmotic pressure of two solutions separated by a semi-permeable membrane.

Key Equations

• Surface Area of a Sphere:

• Volume of a Sphere:

• Surface Area to Volume Ratio:

Summary Table: Major Cell Organelles and Functions

Additional info: The notes above expand on the provided content with definitions, examples, and academic context to ensure a comprehensive understanding suitable for college-level Anatomy & Physiology students.