Chapter 3: Cells – The Living Units (Anatomy & Physiology Study Notes)

Cells: The Living Units

Cell Theory

The cell is the fundamental structural and functional unit of all living organisms. All living things are composed of cells, which are the smallest units capable of life.

• Basic Building Units: Cells form the basis of all living organisms.

• Hierarchy of Organization: Cells → Tissues → Organs → Organ Systems → Organism.

• Unicellular vs. Multicellular: Some organisms consist of a single cell (unicellular), while others are made of many cells (multicellular).

Structure of a Generalized Cell

Major Parts of a Cell

Cells have several key components that perform specialized functions necessary for life.

• Plasma Membrane: The outer boundary of the cell, which protects and regulates the transport of nutrients and waste. It controls what enters and exits the cell.

• Cytoplasm: The material between the plasma membrane and the nucleus. It contains nutrients, metabolites, and organelles.

• Nucleus: The control center of the cell, containing genetic material (chromosomes). Human cells have 46 chromosomes, which carry genes.

Metabolism: The sum of all chemical reactions in the cell. It includes:

• Anabolism: Building up of molecules.

• Catabolism: Breaking down of molecules.

Organelles: Specialized structures within the cytoplasm that perform distinct functions.

Types of Cells: Evolutionary Perspective

Prokaryotes vs. Eukaryotes

Cells are classified based on their structural complexity and evolutionary development.

• Prokaryotes: Simple cells without a nucleus or membrane-bound organelles. Example: Bacteria.

• Eukaryotes: Advanced cells with a nucleus and membrane-bound organelles. All human cells are eukaryotic.

Comparison Table:

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Organelles	Absent	Present
Cell Type	Unicellular	Multicellular or unicellular

Types of Eukaryotic Cells

• Plant Cells: Rigid due to the presence of a cell wall. Contain chlorophyll for photosynthesis.

• Animal Cells: Soft, lack a cell wall. The outermost layer is the plasma membrane.

Cell Wall and Plasma Membrane

Cell Wall

The cell wall is a rigid structure found in plant cells and many bacteria, providing protection and structural support.

• Plant Cell Wall: Composed of cellulose, provides rigidity.

• Bacterial Cell Wall: Composed of peptidoglycan, provides protection.

Plasma Membrane

The plasma membrane is the outermost layer in animal cells and is covered by the cell wall in plant cells.

• Protection: Shields the cell from its environment.

• Boundary: Defines the limits of the cell.

• Transport: Regulates movement of substances in and out.

Functions of Membrane Proteins

Roles of Membrane Proteins

Proteins embedded in the plasma membrane perform various essential functions.

• Carriers: Transport molecules across the membrane.

• Channels: Allow specific ions or molecules to pass through.

• Enzymatic Activity: Catalyze chemical reactions at the membrane.

• Receptors: Receive and transmit signals (signal transduction).

Plasma Membrane Structure

Phospholipid Bilayer

The plasma membrane consists of a double layer of phospholipids with embedded proteins.

• Peripheral Proteins: Attached to the surface of the membrane.

• Integral Proteins: Embedded within the membrane.

Cell Connections: Membrane Junctions

Types of Membrane Junctions

Cells connect to form tissues through specialized junctions.

Junction Type	Function
Tight Junctions	Prevent molecules from passing between cells
Desmosomes	Anchor cells together, provide structural support
Gap Junctions	Allow communication and passage of small molecules

Membrane Transport

Types of Membrane Transport

Cells transport molecules and ions across the plasma membrane using various mechanisms.

• Passive Transport: Does not require energy (ATP).

• Active Transport: Requires energy (ATP).

• Bulk Transport: Movement of large molecules via endocytosis and exocytosis.

Passive Transport

• Simple Diffusion: Molecules move freely from high to low concentration.

• Facilitated Diffusion: Molecules move with the help of carrier proteins.

Diffusion Equation:

$J = -D \frac{dC}{dx}$

Where J is the flux, D is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Active Transport

• Requires ATP: Moves solutes against their concentration gradient.

• Uses Carrier Proteins: Specific proteins transport molecules.

Example: Sodium-potassium pump ($Na^+/K^+$ ATPase) moves sodium and potassium ions across the membrane.

$3Na^+_{out} + 2K^+_{in} + ATP \rightarrow 3Na^+_{in} + 2K^+_{out} + ADP + P_i$

Types of Active Transport

• Uniport: Transports one type of ion or molecule.

• Symport: Transports two ions/molecules in the same direction.

• Antiport: Transports two ions/molecules in opposite directions.

Osmosis

Osmosis and Tonicity

Osmosis is the movement of water across a selectively permeable membrane from an area of high water concentration to low water concentration.

• Isotonic Solution: Equal concentration of solutes inside and outside the cell; no net water movement.

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell, which may swell.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, which may shrink.

Bulk Transport: Endocytosis and Exocytosis

Exocytosis

Exocytosis is the process by which large molecules exit the cell via vesicles that fuse with the plasma membrane.

• Vesicle Formation: Molecules are packaged into vesicles.

• Fusion: Vesicle fuses with plasma membrane and releases contents outside.

Endocytosis

Endocytosis is the process by which large molecules enter the cell by engulfment.

• Engulfment: Plasma membrane extends to surround the molecule.

• Vesicle Formation: The engulfed material is enclosed in a vesicle inside the cell.

• Release: Vesicle contents are released into the cytoplasm.

Cytoplasm and Organelles

Cytoplasm/Cytosol

The cytoplasm is the material between the plasma membrane and the nucleus, containing nutrients, metabolites, and organelles.

• Storage: Stores nutrients and metabolites.

Organelles

Organelles are specialized structures within the cell that perform specific functions.

• Membranous Organelles: Surrounded by membranes (e.g., mitochondria, endoplasmic reticulum).

• Non-membranous Organelles: Not surrounded by membranes (e.g., ribosomes, cytoskeleton).

Key Organelles and Their Functions

• Ribosomes: Site of protein synthesis.

• Mitochondria: Site of ATP production via aerobic cellular respiration.

• Chloroplasts (in plants): Site of photosynthesis.

• Golgi Apparatus: Modifies, sorts, and packages proteins for transport.

• Lysosomes: Contain digestive enzymes to break down waste and foreign material.

• Peroxisomes: Break down hydrogen peroxide ($H_2O_2$) using catalase enzymes.

• Cytoskeleton: Provides structural support and facilitates cell movement.

• Centrioles: Involved in cell division.

Cilia and Flagella

Motile Structures

Cilia and flagella are extensions of the cell membrane that aid in movement.

• Cilia: Short, numerous projections that move substances across the cell surface.

• Flagella: Long, few in number, used for cell locomotion (e.g., sperm cells).

Structure: Both are composed of microtubules arranged in a characteristic pattern.

Additional info: Some explanations and terminology have been expanded for clarity and completeness, including standard definitions and examples relevant to Anatomy & Physiology students.