Human Cells: Structure and Function

• Cells are the basic living, structural, and functional units of the human body
• Cells perform many functions to maintain homeostasis at the system and whole-body levels

Human Cells

• Approximately 200 different types exist
• All originate from pre-existing cells
• Cell structure and function are closely linked
• Cells contain specialized structures for chemical reactions that are coordinated to maintain life at all levels of organization

Three Main Parts of a Human Cell

• Plasma Membrane: cell's outer boundary
• Cytoplasm: contains cellular contents between the plasma membrane and the nucleus
• Nucleus: the cell's control center containing most of the cell's DNA

Plasma Membrane

• It's a flexible, sturdy barrier surrounding the cytoplasm
• It's best described by the fluid mosaic model, depicting it as a dynamic sea of lipids (mainly phospholipids) with a mosaic of various proteins
• Some proteins float freely, while others are anchored
• Membrane lipids allow passage of lipid-soluble molecules but act as a barrier to charged or polar substances
• Membrane proteins facilitate movement of polar molecules and ions, act as receptors, or link the membrane to other proteins

Structure of the Plasma Membrane

Lipid Bilayer

• It's the basic framework with two back-to-back layers of lipid molecules
• Phospholipids comprise 75% of membrane lipids
  • They are amphipathic molecules with a polar, hydrophilic head (containing a phosphate group) and nonpolar, hydrophobic fatty acid tails
  • Heads face the watery cytosol and extracellular fluid, while tails face each other
• Cholesterol makes up 20% of membrane lipids
  • It is weakly amphipathic, and interspersed among other lipids
  • The polar hydroxyl group forms hydrogen bonds with polar heads of phospholipids and glycolipids
  • The nonpolar steroid rings and hydrocarbon tail fit among the fatty acid tails of phospholipids and glycolipids
• Glycolipids are about 5% of membrane lipids
  • They are lipids with attached carbohydrate groups
  • They are only located in the membrane layer facing the extracellular fluid, making the two sides of the bilayer asymmetric

Membrane Proteins

• They're classified as integral (firmly embedded) or peripheral (not firmly embedded)

Integral Proteins

• They extend into or through the lipid bilayer and are firmly embedded
• Most are transmembrane proteins, spanning the entire bilayer
• They're amphipathic, having hydrophilic regions protruding into the cytosol or extracellular fluid and hydrophobic regions extending among the fatty acid tails

Peripheral Proteins

• They attach to polar heads of membrane lipids or to integral proteins at the membrane surface

Glycoproteins

• Integral proteins with attached carbohydrate groups
• Carbohydrates are oligosaccharides (3 to 10 monosaccharides) protruding into the extracellular fluid

Glycocalyx

• Formed by the carbohydrate portions of glycolipids and glycoproteins
• Creates a sugary coat, which has a distinct pattern of carbohydrates that varies between cells, providing a "molecular signature" for cell recognition

Functions of Glycocalyx

• Cell recognition for distinguishing cells
• Cell-to-cell adherence
• Protection against digestion by extracellular enzymes
• Attraction of a film of fluid to prevent dehydration

Functions of Plasma Membrane Proteins

• Ion Channels: Integral proteins forming pores for specific ions (e.g., K+, Na+) to pass through, mostly selective for a single type of ion
• Carriers (Transporters): Integral proteins selectively moving polar substances or ions across the membrane by shape change
• Receptors: Integral proteins serving as cellular recognition sites; each recognizes and binds a specific ligand, such as insulin
• Enzymes: Integral or peripheral proteins catalyzing specific chemical reactions at the cell surface
• Linkers: Anchor proteins in plasma membranes of neighboring cells or to protein filaments inside and outside the cell
• Cell-identity markers: Membrane glycoproteins and glycolipids enabling a cell to recognize others of the same type

Selective Permeability of the Plasma Membrane

• The plasma membrane allows some substances to pass more easily than others
• Permeable: Nonpolar, hydrophobic molecules (e.g., oxygen, carbon dioxide, steroids) can pass through
• Impermeable: Ions and large, uncharged polar molecules (e.g., glucose) cannot pass through

Transmembrane Proteins

• Act as channels or transporters and increase membrane permeability to substances that cannot cross the lipid bilayer

Concentration Gradient

• A difference in the concentration of a chemical between one side of the plasma membrane and the other

Electrical Gradient

• A difference in electrical charge between the two sides of the plasma membrane
• The inside of the cell is typically more negatively charged than the outside

Electrochemical Gradient

• The combined influence of the concentration and electrical gradients on the movement of a particular ion

Cytoplasm

• Cellular contents between the plasma membrane and the nucleus

Components of the Cytoplasm

• Cytosol: The fluid portion, also called intracellular fluid, contains water, dissolved solutes (ions, glucose, amino acids, etc.), and suspended particles
• Organelles: "Little organs" within the cytosol, having characteristic shapes and specific functions (e.g., cytoskeleton, ribosomes, endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, and mitochondria)

Nucleus

• A large organelle containing most of the cell's DNA
• Chromosomes: Found within the nucleus, human cells have 46 chromosomes
  • A chromosome is a single molecule of DNA associated with several proteins
  • Each chromosome contains thousands of hereditary units called genes
• Genes: Control most aspects of cellular structure and function