Cell Structure and Function: Study Notes for Anatomy & Physiology

Cell Structure and Function

3.1 Cells are the Smallest Unit of Life

Cells are the fundamental building blocks of all living organisms. They perform all vital physiological functions and are the basis for structure and function in the body.

• Definition of a Cell: The smallest structural and functional unit of life, capable of independent existence.

• Principle of Complementarity: The biochemical activities of cells are determined by their subcellular structures.

• Generalized Cell Structure: Most cells have three main regions: the nucleus, cytoplasm, and plasma membrane.

• Extracellular Materials: Substances found outside cells, such as body fluids and extracellular matrix, which is especially abundant in connective tissues.

Part 1: Plasma Membrane

3.2 The Plasma Membrane is a Double Layer of Phospholipids with Embedded Proteins

The plasma membrane forms the boundary of the cell, controlling the movement of substances in and out, and facilitating communication with the environment.

• Structure: Composed of a bilayer of phospholipids, cholesterol, and proteins. The hydrophilic heads face outward, and hydrophobic tails face inward.

• Proteins: Integral and peripheral proteins serve as channels, carriers, enzymes, and receptors.

• Carbohydrates: Attach to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface, forming the glycocalyx for cell recognition.

• Junctions: Tight junctions, desmosomes, and gap junctions connect adjacent cells and facilitate communication.

• Selective Permeability: The membrane allows some substances to pass more easily than others, maintaining homeostasis.

3.3 Passive Membrane Transport: Substances Move Down Their Concentration Gradient

Passive transport does not require cellular energy (ATP) and relies on the kinetic energy of molecules.

• Diffusion: Movement of molecules from high to low concentration. Lipid-soluble substances diffuse directly through the lipid bilayer.

• Facilitated Diffusion: Uses membrane proteins (channels or carriers) to transport substances that cannot diffuse through the lipid bilayer.

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

• Tonicity: The ability of a solution to change the shape of cells by altering their internal water volume (isotonic, hypertonic, hypotonic solutions).

Key Equation:

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

3.4 Active Membrane Transport Directly or Indirectly Uses ATP

Active transport requires energy to move substances against their concentration gradients, often using ATP.

• Primary Active Transport: Direct use of ATP, e.g., the sodium-potassium pump ($\text{Na}^+/\text{K}^+$ ATPase).

• Secondary Active Transport: Uses energy stored in ionic gradients created by primary active transport.

• Vesicular Transport: Includes endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis.

Key Equation (Na+/K+ Pump):

$3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

3.5 Membrane Potential

Membrane potential is the voltage difference across the plasma membrane, essential for nerve and muscle function.

• Resting Membrane Potential: Typically -70 mV in neurons, due to differences in ion concentrations and membrane permeability.

• Key Ions: Sodium (Na+) and potassium (K+) are most important in generating membrane potential.

• Maintenance: The sodium-potassium pump and selective permeability maintain the resting potential.

Key Equation (Nernst Equation):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

3.6 Cell Adhesion Molecules and Membrane Receptors Allow the Cell to Interact with Its Environment

Cells communicate and adhere to each other and the extracellular matrix using specialized proteins.

• Cell Adhesion Molecules (CAMs): Mediate cell-cell and cell-matrix interactions, important for tissue structure and immune responses.

• Membrane Receptors: Proteins that bind signaling molecules (ligands) and initiate cellular responses, such as hormone action or neurotransmission.

Part 2: The Cytoplasm

3.7 Cytoplasmic Organelles Each Perform a Specialized Task

The cytoplasm contains organelles that carry out specific functions necessary for cell survival and activity.

• Cytosol: The fluid component of the cytoplasm, containing dissolved nutrients and ions.

• Mitochondria: The "powerhouse" of the cell, site of ATP production via aerobic respiration.

• Ribosomes: Sites of protein synthesis, found free in cytosol or attached to rough endoplasmic reticulum.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

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

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

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support and facilitate movement.

• Centrosome and Centrioles: Organize microtubules and are important in cell division.

3.8 Cilia and Microvilli are Two Main Types of Cellular Extensions

Cellular extensions increase surface area or aid in movement.

• Cilia: Hair-like projections that move substances across the cell surface (e.g., in respiratory tract).

• Microvilli: Finger-like extensions of the plasma membrane that increase surface area for absorption (e.g., in intestinal cells).

Part 3: Nucleus

3.9 The Nucleus Contains DNA, Nucleolus, and Chromatin

The nucleus is the control center of the cell, containing genetic material and directing cellular activities.

• Nuclear Envelope: Double membrane with nuclear pores for material exchange.

• Nucleolus: Site of ribosome subunit synthesis.

• Chromatin: DNA and associated proteins (histones); condenses to form chromosomes during cell division.

3.10 The Cell Cycle Consists of Interphase and a Mitotic Phase

The cell cycle describes the life of a cell from formation to division.

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for division) phases.

• Mitotic Phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

• DNA Replication: Ensures each daughter cell receives an identical set of chromosomes.

• Regulation: Controlled by cyclins and cyclin-dependent kinases (Cdks).

3.11 Messenger RNA Carries Instructions from DNA for Building Proteins

Protein synthesis involves transcription (DNA to mRNA) and translation (mRNA to protein).

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: Ribosomes read mRNA and assemble amino acids into a polypeptide chain.

• Genetic Code: Each triplet of mRNA bases (codon) specifies an amino acid.

Key Equation (Central Dogma):

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

3.12 Autophagy and Proteasomes Dispose of Unneeded Organelles and Proteins; Apoptosis Disposes of Unneeded Cells

Cells maintain homeostasis by removing damaged or unnecessary components.

• Autophagy: Damaged organelles are enclosed in vesicles and digested by lysosomes.

• Proteasomes: Degrade misfolded or damaged proteins tagged for destruction.

• Apoptosis: Programmed cell death, important for development and removal of harmful cells.

Developmental Aspects of Cells

• The first cell of an organism is the fertilized egg; cell specialization and differentiation occur during development.

• Cell numbers remain relatively constant in adulthood; cell division slows with age.

• Cell aging may result from genetic programming, immune system decline, or accumulated damage.