CELLS

Introduction

Cells are the fundamental structural and functional units of all living organisms. In the human body, there are over 200 different cell types, each with specialized structures and functions. Despite their diversity, all cells share certain common features that enable them to sustain life.

CELL STRUCTURE

Overview of Cell Types

• Fibroblasts: Cells that produce fibrous connective tissue.

• Erythrocytes: Also known as red blood cells, responsible for oxygen transport.

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

• Epithelial cells: Form protective layers on body surfaces and line cavities.

• Myocytes / Muscle fibers: Skeletal muscle cells responsible for voluntary movement.

• Myocytes / Muscle fibers (Smooth muscle): Involuntary muscle cells found in organs.

• Lipocyte / Lipocyte: Fat storage cells.

• Neuron: Nerve cells specialized for communication.

• Spermatozoon: Male reproductive cell.

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

Additional info: Human cells vary greatly in size and shape, reflecting their specialized functions.

EXTRACELLULAR MATERIALS

Components Outside the Cell

• Extracellular fluids: Include interstitial (tissue) fluid, blood plasma, cerebrospinal fluid, and lymph. These fluids provide nutrients and remove waste from cells.

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

• Extracellular matrix: A network of proteins and polysaccharides that provides structural support and helps bind cells together, especially abundant in connective tissue.

PLASMA MEMBRANE

Structure and Function

The plasma membrane defines the boundary of the cell and acts as a selectively permeable barrier between the intracellular and extracellular environments. According to the fluid mosaic model, the membrane is composed of a dynamic mixture of phospholipids, cholesterol, and proteins.

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

• Integral proteins: Span the membrane and function as channels, carriers, or receptors.

• Peripheral proteins: Attached to the membrane surface, often involved in signaling or maintaining cell shape.

• Glycocalyx: A carbohydrate-rich area on the cell surface, important for cell recognition and interaction.

• Cholesterol: Stabilizes the membrane and maintains its fluidity.

Membrane Junctions

• Tight junctions: Seal adjacent cells to prevent passage of molecules between them (e.g., in the digestive tract).

• Desmosomes: Anchoring junctions that provide mechanical strength by connecting the cytoskeletons of adjacent cells.

• Gap junctions: Allow direct communication between cells through connexon channels, important in cardiac and smooth muscle tissue.

TRANSPORT ACROSS THE PLASMA MEMBRANE

Selective Permeability

The plasma membrane allows certain substances to enter or exit the cell while restricting others, maintaining homeostasis.

Passive Transport

• Simple diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer down their concentration gradient.

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

• Osmosis: Diffusion of water across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

Osmolarity: The total concentration of solute particles in a solution.

Active Transport

• Primary active transport: Uses ATP to move substances against their concentration gradient (e.g., sodium-potassium pump).

• Vesicular transport: Movement of large particles or macromolecules via vesicles. Includes endocytosis (into the cell) and exocytosis (out of the cell).

Types of Endocytosis

• Phagocytosis: "Cell eating"; engulfment of large particles by the cell.

• Pinocytosis: "Cell drinking"; uptake of extracellular fluid and dissolved solutes.

• Receptor-mediated endocytosis: Selective uptake of specific molecules via receptor binding.

Exocytosis

• Release of substances (e.g., neurotransmitters, hormones) from the cell by fusion of a vesicle with the plasma membrane.

CHARACTERISTICS OF CYTOPLASM

Components of the Cytoplasm

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

• Inclusions: Chemical substances such as glycogen, lipids, or pigments, which may or may not be present depending on cell type.

• Organelles: Specialized structures that perform specific cellular functions (see below).

CYTOPLASMIC ORGANELLES

Mitochondria

• Powerhouse of the cell; site of ATP production via aerobic respiration.

• Double-membraned with inner folds called cristae.

• Contain their own DNA and ribosomes.

Ribosomes

• Sites of protein synthesis.

• Free ribosomes produce proteins for use within the cell; membrane-bound ribosomes (on rough ER) produce proteins for export or membrane insertion.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes and processes proteins.

• Smooth ER: Involved in lipid metabolism, detoxification, and calcium storage.

Golgi Apparatus

• Stack of flattened membranes; modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Lysosomes

• Membranous sacs containing digestive enzymes; break down waste materials and cellular debris.

Peroxisomes

• Contain enzymes that detoxify harmful substances and neutralize free radicals.

Cytoskeleton

• Microfilaments: Thin strands of actin; involved in cell movement and shape changes.

• Intermediate filaments: Provide mechanical support and resist stress.

• Microtubules: Hollow tubes that maintain cell shape and serve as tracks for organelle movement.

Centrosome and Centrioles

• Microtubule organizing center; contains a pair of centrioles important for cell division.

Cellular Extensions

• Cilia: Motile projections that move substances across cell surfaces.

• Flagella: Longer projections that propel the cell (e.g., sperm cell).

• Microvilli: Finger-like extensions that increase surface area for absorption.

NUCLEUS

Structure and Function

• Control center of the cell; contains genetic material (DNA).

• Most cells are uninucleate; some are multinucleate (e.g., skeletal muscle), while others are anucleate (e.g., mature erythrocytes).

• Key structures: nuclear envelope, nucleoli, chromatin.

Nuclear Envelope

• Double membrane with nuclear pores for material exchange.

• Outer membrane continuous with rough ER.

• Inner membrane supported by nuclear lamina (intermediate filaments).

Nucleoli

• Sites of ribosomal RNA synthesis and ribosome assembly.

Chromatin and Chromosomes

• DNA is packaged with histone proteins as chromatin.

• During cell division, chromatin condenses into chromosomes.

• Humans have 23 pairs (46 total) of chromosomes.

OVERVIEW OF THE CELL CYCLE

Phases of the Cell Cycle

• Interphase: Period of cell growth and normal function; includes G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• Cell Division (Mitotic Phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis (PMAT)

1. Prophase: Chromatin condenses into chromosomes; nuclear envelope breaks down; spindle forms.

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

3. Anaphase: Sister chromatids separate and move to opposite poles.

4. Telophase: Chromosomes decondense; nuclear envelope reforms; nucleoli reappear.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Additional info: The cell cycle is tightly regulated by checkpoints to ensure proper division and function. Errors in cell cycle regulation can lead to diseases such as cancer.