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

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Chapter 3: Cells – The Living Units

3.1 Cells are the Smallest Unit of Life

Cells are the fundamental structural and functional units of all living organisms. Every process essential to life occurs within cells, making them the basic building blocks of biology.

Definition: A cell is the smallest unit capable of independent life and self-replication.
Cell Theory: All living things are composed of cells; cells arise only from pre-existing cells.
Example: Human body contains trillions of cells, each specialized for different functions.

3.2 The Plasma Membrane

The plasma membrane surrounds each cell, providing a selective barrier between the internal and external environments. Its structure is crucial for maintaining cellular integrity and communication.

Structure: Composed of a phospholipid bilayer with embedded proteins.
Function: Regulates entry and exit of substances, supports cell signaling.
Fluid Mosaic Model: Describes the dynamic arrangement of lipids and proteins.
Example: Transport proteins facilitate movement of ions like Na+ and K+ across the membrane.

3.3 Intercellular Junctions

Cells connect to each other via specialized structures called intercellular junctions, which enable communication and adhesion.

Tight Junctions: Prevent leakage of extracellular fluid.
Desmosomes: Anchor cells together, providing mechanical strength.
Gap Junctions: Allow direct passage of ions and small molecules between cells.
Example: Cardiac muscle cells use gap junctions for synchronized contraction.

3.4 Passive Membrane Transport

Passive transport mechanisms move substances across the membrane without energy input from the cell.

Diffusion: Movement of molecules from high to low concentration.
Osmosis: Diffusion of water across a selectively permeable membrane.
Example: Oxygen diffuses into cells, while carbon dioxide diffuses out.
Equation: (Fick's Law of Diffusion)

3.5 Active Membrane Transport

Active transport requires cellular energy (usually ATP) to move substances against their concentration gradients.

Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).
Secondary Active Transport: Uses energy from ion gradients established by primary transport.
Example: Na+/K+ ATPase maintains electrochemical gradients.
Equation: (Hydrolysis of ATP)

3.6 Selective Diffusion and Membrane Potential

Selective diffusion of ions across the plasma membrane creates an electrical potential difference, known as the membrane potential.

Resting Membrane Potential: Typically negative inside the cell due to selective permeability to K+.
Key Ions: Na+, K+, Cl-
Equation: (Nernst Equation)
Example: Neurons use changes in membrane potential to transmit signals.

3.7 Cell Adhesion Molecules and Membrane Receptors

Cells interact with their environment through specialized proteins on their surface.

Cell Adhesion Molecules (CAMs): Mediate binding between cells and the extracellular matrix.
Membrane Receptors: Bind signaling molecules (ligands) to initiate cellular responses.
Example: Integrins help cells attach to the extracellular matrix; insulin receptors mediate glucose uptake.

3.8 Cytoplasmic Organelles

Organelles are specialized structures within the cytoplasm that perform distinct cellular functions.

Mitochondria: Produce ATP via cellular respiration.
Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).
Golgi Apparatus: Modifies, sorts, and packages proteins.
Lysosomes: Digest cellular waste.
Example: Pancreatic cells have abundant rough ER for enzyme production.

3.9 Cellular Extensions: Cilia and Microvilli

Cells may possess surface extensions that aid in movement or increase surface area.

Cilia: Motile projections that move substances across cell surfaces.
Microvilli: Non-motile projections that increase surface area for absorption.
Example: Ciliated cells in the respiratory tract move mucus; microvilli in the intestine enhance nutrient absorption.

3.10 The Nucleus

The nucleus is the control center of the cell, containing genetic material and structures for gene expression.

Nuclear Envelope: Double membrane that encloses the nucleus.
Nucleoli: Sites of ribosome synthesis.
Chromatin: DNA-protein complex that condenses to form chromosomes during cell division.
Example: Liver cells have prominent nucleoli due to high protein synthesis.

3.11 The Cell Cycle

The cell cycle describes the sequence of events leading to cell division and replication.

Interphase: Period of growth and DNA replication (G1, S, G2 phases).
Mitotic Phase: Division of the nucleus (mitosis) and cytoplasm (cytokinesis).
Example: Skin cells undergo frequent cell cycles for tissue renewal.
Additional info: Cell cycle checkpoints ensure proper division and prevent errors.

3.12 Messenger RNA (mRNA) and Protein Synthesis

Messenger RNA carries genetic instructions from DNA to ribosomes, where proteins are synthesized.

Transcription: DNA is copied into mRNA in the nucleus.
Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.
Equation:
Example: Hemoglobin synthesis in red blood cells.

3.13 Autophagy, Proteasomes, and Apoptosis

Cells maintain homeostasis by removing damaged components and, when necessary, undergoing programmed cell death.

Autophagy: Process by which cells digest and recycle their own organelles and proteins.
Proteasomes: Protein complexes that degrade unneeded or damaged proteins.
Apoptosis: Programmed cell death, essential for development and disease prevention.
Example: Apoptosis removes cells with DNA damage to prevent cancer.