Cell Structure and Function: Anatomy & Physiology Study Notes

Cell Structure and Function

Cells: The Smallest Unit of Life

Cells are the fundamental building blocks of all living organisms. Their structure and function are essential for understanding anatomy and physiology.

• Cell Theory:

  • All living organisms are composed of one or more cells.

  • The cell is the basic unit of life in all living things.

  • All cells arise from pre-existing cells.

• Cell Diversity:

  • Over 250 different cell types exist in the human body.

  • Cells vary in shape, size, and function (e.g., red blood cells, nerve cells, epithelial cells).

Generalized Cell Structure

Most human cells share three main structural regions:

• Plasma Membrane: The outer boundary, selectively permeable, regulates movement of substances.

• Cytoplasm: Intracellular fluid containing organelles and the cytoskeleton.

• Nucleus: Control center containing genetic material (DNA).

Plasma Membrane

Structure

The plasma membrane is a dynamic structure composed of a double layer of phospholipids with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid Bilayer: Provides the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

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

• Cholesterol: Stabilizes membrane fluidity.

• Carbohydrates: Attach to proteins and lipids, forming the glycocalyx for cell recognition and protection.

Functions

• Physical Barrier: Separates intracellular and extracellular environments.

• Selective Permeability: Controls entry and exit of substances.

• Cell Communication: Membrane proteins and carbohydrates facilitate signaling and recognition.

• Cell Adhesion: Proteins and junctions maintain tissue integrity.

Intercellular Junctions

Junctions connect cells and regulate communication and transport.

• Tight Junctions: Prevent leakage between cells.

• Desmosomes: Provide mechanical stability.

• Gap Junctions: Allow direct communication via channels.

Membrane Transport

Substances move across the plasma membrane by passive or active processes.

• Passive Transport: No energy required.

  • Simple Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Movement via transport proteins.

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

• Active Transport: Requires energy (ATP).

  • Primary Active Transport: Direct use of ATP to transport molecules against their concentration gradient.

  • Secondary Active Transport: Uses energy from electrochemical gradients.

  • Vesicular Transport: Movement of large substances via vesicles (endocytosis, exocytosis, pinocytosis, phagocytosis).

Membrane Potential

The voltage difference across the cell membrane is established by selective diffusion of ions, crucial for nerve and muscle function.

• Resting Membrane Potential: Maintained by ion gradients and active transport (e.g., sodium-potassium pump).

Membrane Proteins and Receptors

• Cell Adhesion Molecules (CAMs): Facilitate cell interactions and signaling.

• Membrane Receptors: Involved in signal transduction and cellular responses (e.g., hormone binding).

Cytoplasm

Cytosol and Inclusions

The cytosol is the fluid component containing dissolved substances; inclusions are stored nutrients or pigments.

Cytoplasmic Organelles

Organelles perform specialized functions within the cell.

• Mitochondria: Powerhouse of the cell, site of ATP production.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes, synthesizes proteins.

  • Smooth ER: Synthesizes lipids, detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes for breakdown of substances.

• Peroxisomes: Detoxification and breakdown of fatty acids.

• Centrosome: Microtubule organizing center, important for cell division.

• Cytoskeleton: Provides structural support and intracellular transport.

Cellular Extensions

• Cilia: Move substances across cell surfaces.

• Microvilli: Increase surface area for absorption.

• Flagella: Enable cell movement (e.g., sperm cells).

Nucleus

Structure

The nucleus is surrounded by a double membrane (nuclear envelope) and contains chromatin and the nucleolus.

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

• Nucleolus: Site of ribosome production.

Functions

• Genetic Control: Houses DNA, controls cellular activities.

• Cell Growth and Division: Regulates the cell cycle.

• Protein Synthesis: Directs transcription and translation.

Cell Cycle

Phases

The cell cycle consists of interphase (cell growth and DNA replication) and mitotic phase (cell division).

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

• Mitotic Phase: Mitosis (division of nucleus) and cytokinesis (division of cytoplasm).

Checkpoints

• G1 Checkpoint: Ensures cell is ready for DNA synthesis.

• G2/M Checkpoint: Ensures DNA is correctly replicated before mitosis.

Protein Synthesis

Transcription and Translation

Protein synthesis involves two main steps:

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a protein at the ribosome.

Genetic Code

• Codons: mRNA triplets that specify amino acids.

• Anticodons: tRNA triplets that pair with codons.

• Triplets: DNA sequences corresponding to codons.

RNA Types

• mRNA (Messenger RNA): Carries genetic instructions from DNA.

• rRNA (Ribosomal RNA): Combines with proteins to form ribosomes.

• tRNA (Transfer RNA): Brings amino acids to the ribosome.

Cell Death and Recycling

Autophagy

Autophagy is the process by which cells dispose of damaged organelles and proteins, maintaining cellular health.

Apoptosis

Apoptosis is programmed cell death, essential for development and disease prevention.

Ubiquitin-Proteasome System

Damaged or unnecessary proteins are tagged with ubiquitin and degraded by proteasomes.

Extracellular Materials

Types

• Interstitial Fluid: Bathes all cells, contains nutrients, wastes, and regulatory substances.

• Blood Plasma: Fluid component of blood.

• Cerebrospinal Fluid: Surrounds brain and spinal cord.

• Extracellular Matrix: Jellylike substance composed of proteins and polysaccharides, acts as "cell glue".

Summary Table: Cell Organelles and Functions

Organelle	Main Function
Mitochondria	ATP production (energy supply)
Ribosomes	Protein synthesis
Rough ER	Protein synthesis and modification
Smooth ER	Lipid synthesis and detoxification
Golgi Apparatus	Processing and packaging of proteins
Lysosomes	Digestion of cellular debris
Peroxisomes	Detoxification
Centrosome	Microtubule organization, cell division
Nucleus	Genetic control, cell regulation

Key Equations and Concepts

• Diffusion Rate: $J = -D \frac{dC}{dx}$ (Fick's Law, where J is flux, D is diffusion coefficient, dC/dx is concentration gradient)

• Osmosis: $\pi = iMRT$ (Osmotic pressure, where $\pi$ is pressure, i is ionization constant, M is molarity, R is gas constant, T is temperature)

• Sodium-Potassium Pump: $3\,\text{Na}^+_{\text{out}} : 2\,\text{K}^+_{\text{in}}$ per ATP hydrolyzed

Additional info:

• Cell signaling pathways often involve second messengers such as cyclic AMP (cAMP).

• Cell cycle checkpoints are crucial for preventing mutations and ensuring proper cell division.

• Autophagy and apoptosis are essential for tissue homeostasis and development.