Cell Structure and Function: Study Notes for Anatomy & Physiology

CELLS

Introduction to Cells

Cells are the fundamental microscopic living units that serve as the basic structural and functional units of all living organisms. The human body contains trillions of cells, with over 200 different cell types, each specialized for particular functions. While all human cells share some common structures and functions, they vary greatly in size and shape, reflecting their diverse roles.

• Definition: A cell is the smallest unit of life capable of performing all vital physiological functions.

• Examples of Cell Types:

  • Fibroblasts: Cells that produce fibrous connective tissue.

  • Erythrocytes: Red blood cells responsible for oxygen transport.

  • Leukocytes (Macrophages): White blood cells involved in immune defense.

  • Epithelial cells: Cells forming protective layers on body surfaces.

  • Myocytes / Muscle fibers: Skeletal muscle cells specialized for contraction.

  • Myocytes / Muscle cells: Smooth muscle cells found in organs.

  • Adipocyte / Lipocyte: Fat cells storing energy.

  • Neuron: Nerve cells transmitting electrical signals.

  • Spermatozoon: Sperm cell involved in reproduction.

  • Escherichia coli: Example of a prokaryotic cell (bacterium).

• Key Point: Cell diversity in structure and function underlies the complexity of multicellular organisms.

Additional info: The study of cells is called cytology.

CELL STRUCTURE

Generalized Cell Structure

All eukaryotic cells share three main structural components:

• Plasma Membrane: Defines the cell boundary and acts as a semi-permeable barrier, controlling the movement of substances in and out of the cell.

• Cytoplasm: The region between the plasma membrane and nucleus, packed with organelles and cytosol (fluid component).

• Nucleus: The control center containing genetic material (DNA) and directing cellular activities.

Additional info: Organelles within the cytoplasm perform specialized functions essential for cell survival.

EXTRACELLULAR MATERIALS

Types of Extracellular Materials

Extracellular materials are substances located outside the cells, playing crucial roles in tissue structure and function.

• Extracellular Fluids:

  • Interstitial (tissue) fluid: Surrounds cells in tissues.

  • Blood plasma: Fluid component of blood.

  • Other fluids: Cerebrospinal fluid, lymph, etc.

• Cellular Secretions: Substances released by cells, such as mucus, saliva, and acids.

• Extracellular Matrix: A network of proteins and polysaccharides that acts as a 'glue' to bind cells together, especially abundant in connective tissue.

Additional info: The extracellular matrix provides structural support and regulates cell behavior.

PLASMA MEMBRANE

Structure and Function

The plasma membrane is a dynamic boundary separating the intracellular and extracellular environments. According to the fluid mosaic model, it consists of a mosaic of phospholipids, cholesterol, and proteins that move fluidly within the membrane.

• Phospholipid Bilayer: Main structural component, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Membrane Proteins:

  • Integral proteins: Span the membrane, forming channels or acting as receptors.

  • Peripheral proteins: Attached to the membrane surface, often involved in signaling or structural support.

  • Glycoproteins and Glycolipids: Carbohydrate-attached proteins/lipids forming the glycocalyx, important for cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Functions:

• Selective permeability

• Cell communication and recognition

• Physical barrier

TRANSPORT ACROSS THE PLASMA MEMBRANE

Transport Mechanisms

The plasma membrane is selectively permeable, allowing the cell to maintain homeostasis by regulating the entry and exit of substances. Transport mechanisms are classified as passive or active.

Passive Transport

• Simple Diffusion: Unassisted movement of small, non-polar, or lipid-soluble molecules (e.g., O2, CO2) down their concentration gradient.

• Facilitated Diffusion: Movement of water-soluble or larger molecules (e.g., glucose, Na+) via carrier proteins or channel proteins, still down the concentration gradient.

• Osmosis: Diffusion of water across a selectively permeable membrane from an area of higher to lower water concentration.

Active Transport

• Primary Active Transport: Uses ATP to move substances (e.g., Na+, K+, Ca2+) against their concentration gradients via pumps (e.g., sodium-potassium pump).

• Vesicular Transport: Movement of large particles or macromolecules via vesicles (endocytosis and exocytosis).

Key Equations:

• Osmolarity: $\text{Osmolarity} = \sum \text{concentration of all solute particles}$

Additional info: Passive transport does not require energy, while active transport requires ATP.

CHARACTERISTICS OF CYTOPLASM

Cytoplasm Components

The cytoplasm is the site of most cellular activities and consists of three major elements:

• Cytosol: Semi-transparent fluid containing water, proteins, salts, sugars, and ions.

• Inclusions: Chemical substances that may or may not be present, depending on cell type (e.g., glycogen, lipids, pigment granules).

• Organelles: Specialized structures performing specific functions (e.g., mitochondria, ribosomes, endoplasmic reticulum).

NUCLEUS

Structure and Function

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

• Nuclear Envelope: Double membrane barrier with nuclear pores for transport.

• Nucleoli: Sites of ribosome assembly.

• Chromatin: DNA wrapped around histone proteins; condenses to form chromosomes during cell division.

Key Point: Most cells have one nucleus, but some are multinucleate (e.g., skeletal muscle cells), while others are anucleate (e.g., erythrocytes).

OVERVIEW OF THE CELL CYCLE

Phases of the Cell Cycle

The cell cycle is a series of changes a cell undergoes from formation to division. It consists of two main phases: interphase and cell division.

• Interphase: Cell grows, carries out physiological functions, and replicates DNA.

• Cell Division (Mitosis & Cytokinesis): Cell divides into two daughter cells.

Interphase Subphases

• G1 (Gap 1): Cell growth and protein synthesis.

• S (Synthesis): DNA replication.

• G2 (Gap 2): Preparation for cell division.

Mitosis Phases (PMAT)

• Prophase: Chromatin condenses, nucleoli disappear, centrosomes migrate.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Chromatids separate and move to opposite poles.

• Telophase: Chromatids decondense, nuclear envelope reforms.

• Cytokinesis: Division of cytoplasm, forming two daughter cells.

Additional info: The goal of mitosis is growth and repair; meiosis (covered in another course) produces gametes.