The Cell: Structure, Function, and Membrane Transport

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The Cell

Cell Diversity

Human cells exhibit remarkable diversity in size, shape, and function, with over 200 different types identified. This diversity allows for specialization and efficient functioning of tissues and organs.

Erythrocytes: Cells that transport gases (e.g., oxygen and carbon dioxide).
Fibroblasts: Cells that produce fibers and ground substance for connective tissue.
Epithelial cells: Cells that form linings and coverings of organs.
Skeletal and Smooth Muscle Cells: Cells responsible for movement of body parts and organs.
Nerve Cells: Cells that gather information and control body functions.
Macrophages: Cells that fight disease by engulfing pathogens.
Fat Cells: Cells that store nutrients.
Sperm: Cells of reproduction.

Basic Processes of Cells:

Cell metabolism: Sum of all chemical reactions in the cell.
Transport: Movement of substances produced or ingested by the cell.
Communication: Chemical and electrical signaling within and between cells.
Cell reproduction: Division for growth, development, and replacement of cells.

Generalized animal cell with labeled organelles

Cell Structure

Plasma Membrane

The plasma membrane surrounds each cell, isolating internal structures from the external environment. It provides structural support, communication, and cell identification. The membrane contains intracellular fluid (ICF) and separates it from extracellular fluid (ECF).

Plasma membrane structure with proteins and carbohydrates

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, consisting of a phospholipid bilayer with embedded proteins, cholesterol, glycolipids, and glycoproteins.

Integral proteins: Span the entire membrane (transmembrane proteins).
Peripheral proteins: Located on one side of the membrane.

Detailed structure of the plasma membrane showing proteins and carbohydrates Schematic structure of a phospholipid molecule

Functions of Membrane Proteins

Carrier proteins: Bind and transport substances across the membrane.
Channels: Allow specific substances to cross the membrane.
Receptors: Bind ligands and trigger cellular responses.
Enzymes: Catalyze chemical reactions.
Structural support: Maintain cell shape and integrity.
Linker proteins: Anchor cells and facilitate communication.

Carrier protein transporting a substance Protein channel allowing passage of molecules Ligand-gated protein channel Enzyme catalyzing a reaction at the membrane Structural support proteins in the membrane Linker proteins connecting adjacent cells

Other Membrane Components

Cholesterol: Stabilizes membrane fluidity during temperature changes.
Glycolipids and glycoproteins: Involved in cell recognition and immune response.

Membrane components: cholesterol, glycolipids, glycoproteins

Transport Across the Plasma Membrane

Selective Permeability

The plasma membrane is selectively permeable, allowing certain molecules to cross while restricting others. Transport occurs via passive or active mechanisms.

Passive Transport

Diffusion: Movement of solute from high to low concentration until equilibrium is reached.
Simple diffusion: Direct movement through the lipid bilayer (e.g., O2, CO2).
Facilitated diffusion: Movement of charged or polar solutes via membrane proteins (channels or carriers).
Osmosis: Diffusion of water across a selectively permeable membrane.

Diffusion of dye in water Diffusion across a membrane Simple diffusion through the plasma membrane Osmosis across a selectively permeable membrane Osmosis through aquaporins in the plasma membrane Facilitated diffusion: channels and carriers

Tonicity and Red Blood Cells

The effect of solution tonicity on cells:

Isotonic: No net water movement; cell shape maintained.
Hypertonic: Water leaves cell; cell shrinks (crenation).
Hypotonic: Water enters cell; cell swells and may burst (lysis).

Effect of isotonic, hypertonic, and hypotonic solutions on red blood cells

Active Transport

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

Primary active transport: Direct use of ATP (e.g., Na+/K+ pump).
Secondary active transport: Uses energy from the movement of another substance down its gradient.

Sodium-potassium pump mechanism Secondary active transport: symport of sodium and glucose

Membrane Potential

Cells maintain a membrane potential, an electrical gradient across the plasma membrane, essential for processes such as nerve impulse transmission.

Separation of charges across the plasma membrane

Vesicular Transport

Endocytosis: Uptake of materials via vesicles (includes phagocytosis, pinocytosis, and receptor-mediated endocytosis).
Exocytosis: Release of substances from the cell via vesicles.
Transcytosis: Transport of substances across the cell via vesicles.

Phagocytosis process Pinocytosis process Receptor-mediated endocytosis Exocytosis process Electron micrograph of exocytosis

Summary Tables: Plasma Membrane Transport

Type of Transport	Definition	Examples
Simple Diffusion	Movement of solute with its concentration gradient through the plasma membrane unaided by a transport protein.	Oxygen, Carbon dioxide
Facilitated Diffusion	Movement of solute with its concentration gradient using a membrane protein (channel or carrier).	Sodium ions, Glucose
Osmosis	Movement of solvent (water) across a membrane from lower to higher solute concentration.	Water absorption in kidneys
Primary Active Transport	Movement of solute against its gradient using ATP.	Na+/K+ ATPase pump
Secondary Active Transport	Uses energy from another solute's gradient to move substances.	Symport of sodium and glucose
Phagocytosis	"Cell eating"; ingestion of large particles.	Uptake of bacteria by macrophages
Pinocytosis	"Cell drinking"; uptake of extracellular fluid.	Nutrient transport
Receptor-Mediated Endocytosis	Uptake of specific molecules via receptors.	Cholesterol, iron
Exocytosis	Release of substances from cell via vesicles.	Secretion of hormones, neurotransmitters

Table of passive membrane transport Table of active membrane transport Table of vesicular transport

Cytoplasmic Organelles

Membranous and Non-membranous Organelles

Membranous: Mitochondria, Endoplasmic reticulum, Golgi apparatus, Peroxisomes, Lysosomes
Non-membranous: Ribosomes, Cytoskeleton, Centrioles

Generalized cell with labeled organelles

Mitochondria

Known as the "power plant" of the cell, mitochondria produce the majority of ATP. They have a double membrane, their own DNA, and ribosomes.

Structure of mitochondrion

Peroxisomes

Organelles that use oxygen to oxidize organic molecules, producing hydrogen peroxide (H2O2). They detoxify substances and break down fatty acids.

Peroxisome structure and function

Ribosomes

Sites of protein synthesis, composed of large and small subunits made of rRNA and proteins. Free ribosomes synthesize proteins for the cell; bound ribosomes produce proteins for export.

Schematic structure of the ribosome

Endoplasmic Reticulum (ER)

Rough ER: Studded with ribosomes; synthesizes and packages proteins for export.
Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Rough and smooth endoplasmic reticulum

Golgi Apparatus

Modifies, sorts, and packages proteins and lipids for export or delivery to other organelles.

Golgi apparatus structure and function

Additional info: Lysosomes, cytoskeleton, and nucleus are also covered in the full notes but not shown in the selected images above.