Cells: The Living Units – Structure, Function, and Membrane Transport

Cells: The Living Units

Cell Theory and Cell Diversity

Cells are the fundamental structural and functional units of all living organisms. The human body contains trillions of cells, each specialized for distinct functions. The cell theory states:

• The cell is the smallest unit of life

• All organisms are composed of one or more cells

• Cells arise only from pre-existing cells

Human cells exhibit remarkable diversity in size, shape, and internal components, leading to over 250 different cell types with specialized functions.

Generalized Cell Structure

Despite their diversity, all human cells share three basic structural components:

• Plasma membrane: Flexible outer boundary that separates the cell from its environment

• Cytoplasm: Intracellular fluid containing organelles

• Nucleus: Control center containing DNA

Plasma Membrane Structure and Function

Fluid Mosaic Model

The plasma membrane is a dynamic, selectively permeable barrier composed of a phospholipid bilayer with embedded proteins and surface carbohydrates. This arrangement is described by the fluid mosaic model.

• Phospholipids: Form the bilayer; hydrophilic heads face outward, hydrophobic tails face inward

• Cholesterol: Interspersed within the bilayer, increasing membrane stability

• Proteins: Integral and peripheral proteins perform various functions

• Carbohydrates: Form the glycocalyx, important for cell recognition

Membrane Proteins and Their Functions

Membrane proteins are essential for communication, transport, and structural support. They are classified as:

• Integral proteins: Span the membrane; function as channels, carriers, receptors, and enzymes

• Peripheral proteins: Loosely attached; function as enzymes, motor proteins, and in cell-to-cell connections

Key functions include:

• Transport: Channels and carriers move substances across the membrane

• Receptors: Receive and transmit signals

• Enzymatic activity: Catalyze reactions

• Cell-cell recognition: Glycoproteins serve as identification tags

• Cell-to-cell joining: Form junctions for tissue integrity

• Attachment: Anchor cytoskeleton and extracellular matrix

Glycocalyx and Cell Recognition

The glycocalyx is a carbohydrate-rich area on the cell surface, composed of glycolipids and glycoproteins. It functions as a biological marker for cell recognition and immune response.

Intercellular Junctions

Cells are connected by specialized junctions:

• Tight junctions: Create impermeable barriers

• Desmosomes: Provide mechanical strength

• Gap junctions: Allow communication via small molecules

Membrane Transport

Passive Transport

Passive transport moves substances across the membrane without energy input, relying on concentration gradients. Types include:

• Simple diffusion: Direct movement of lipid-soluble substances

• Facilitated diffusion: Movement via carrier or channel proteins

• Osmosis: Diffusion of water through aquaporins or the bilayer

Key factors affecting diffusion:

• Concentration gradient

• Molecular size

• Temperature

Selective permeability ensures only certain substances can cross the membrane.

Osmosis and Tonicity

Osmosis is the movement of water from areas of low solute concentration to high solute concentration. Tonicity describes the effect of solutions on cell volume:

• Isotonic: No net change

• Hypertonic: Cell shrinks (crenation)

• Hypotonic: Cell swells (lysis)

Active Transport

Active transport requires ATP to move substances against their concentration gradient. Types include:

• Primary active transport: Direct use of ATP (e.g., sodium-potassium pump)

• Secondary active transport: Indirect use of ATP via ion gradients

Vesicular Transport

Vesicular transport moves large substances via vesicles:

• Endocytosis: Intake of substances (phagocytosis, pinocytosis, receptor-mediated)

• Exocytosis: Release of substances

Cytoplasm and Organelles

Cytoplasmic Components

The cytoplasm contains:

• Cytosol: Gel-like fluid

• Inclusions: Stored nutrients and pigments

• Organelles: Specialized structures for cellular functions

Major Organelles

• Mitochondria: ATP production via aerobic respiration

• Ribosomes: Protein synthesis

• Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies

• Golgi apparatus: Modifies, sorts, and packages proteins

• Lysosomes: Digestive enzymes for breakdown

• Peroxisomes: Detoxification and fatty acid metabolism

Cytoskeleton

The cytoskeleton provides structural support and facilitates movement:

• Microfilaments: Actin filaments for cell shape and movement

• Intermediate filaments: Mechanical strength

• Microtubules: Tubulin-based tracks for organelle movement

Cellular Extensions

• Cilia: Move substances across cell surfaces

• Flagella: Propel cells (e.g., sperm)

• Microvilli: Increase surface area for absorption

Nucleus and Genetic Control

Nuclear Structure

The nucleus contains:

• Nuclear envelope: Double membrane with pores

• Nucleoli: Ribosome assembly

• Chromatin: DNA and associated proteins

Cell Cycle and Division

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

• Prophase

• Metaphase

• Anaphase

• Telophase

• Cytokinesis: Division of cytoplasm

Protein Synthesis

Transcription and Translation

Protein synthesis involves two main steps:

• Transcription: DNA code is transcribed to mRNA in the nucleus

• Translation: mRNA is decoded by ribosomes to assemble polypeptides

Key molecules:

• mRNA: Carries genetic code

• rRNA: Structural component of ribosomes

• tRNA: Transfers amino acids to ribosome

Genetic Code

The genetic code is based on triplets of nucleotides (codons) that specify amino acids. There are 64 codons for 20 amino acids, including start and stop signals.

Cellular Homeostasis and Aging

Autophagy, Proteasomes, and Apoptosis

Cells maintain homeostasis by disposing of damaged organelles and proteins:

• Autophagy: Degradation of obsolete organelles

• Proteasomes: Breakdown of misfolded proteins

• Apoptosis: Programmed cell death

Cell Differentiation and Aging

Cells differentiate during development, acquiring specialized functions. Aging theories include wear and tear, mitochondrial dysfunction, immune decline, and genetic programming (telomere shortening).

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required	Direction	Examples
Simple Diffusion	No	High to Low	O2, CO2, fatty acids
Facilitated Diffusion	No	High to Low	Glucose, amino acids, ions
Osmosis	No	High to Low (water)	Water
Primary Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Secondary Active Transport	Indirect (ATP)	Low to High	Glucose, amino acids
Vesicular Transport	Yes (ATP)	Varies	Endocytosis, exocytosis

Additional info: This study guide expands on brief points from lecture slides, providing definitions, examples, and academic context for each topic. Images included are directly relevant to the explanation of cell structure, membrane composition, and transport mechanisms.