Cell Structure and Function

• All organisms are composed of cells, which are the basic structural and functional units of life.
• There are over 200 different cell types in the human body.
• Most cells range in size from 10 to 100 micrometers.
• Cells facilitate communication, gas transport, secretion/reabsorption, nutrient absorption, cushioning and protection, movement, and heat production.

Common Features and General Functions

• The nucleus is the largest cell structure, enclosed by a plasma membrane, and contains DNA within the nucleoplasm.
• Cytoplasm includes all cellular contents between the plasma membrane and the nucleus, where most cellular activities occur.
• Cytosol is a viscous, water-rich fluid within the cytoplasm that contains dissolved macromolecules and ions.
• Organelles are complex structures within cells with unique shapes and functions, categorized as either membrane-bound or non-membrane-bound.
• Inclusions are temporarily stored materials like melanin, glycogen, and triglycerides.

Plasma Membrane

• Functions as a physical barrier, is selectively permeable, establishes an electrochemical gradient, and facilitates communication via receptors.
• Acts as the limiting barrier, separating the cell's internal environment from the external environment.
• It consists of lipids and proteins; only small, nonpolar substances can pass through unaided.
• Lipids in the plasma membrane include phospholipids (75%), cholesterol (20%), and glycolipids (5%).

Phospholipids

• Phospholipids, comprising 75% of the membrane lipids, feature a polar, hydrophilic head and two nonpolar, hydrophobic tails.
• Hydrophobic interactions between phospholipids create two parallel sheets arranged tail to tail, which form the phospholipid bilayer and the basic structure of the plasma membrane.

Cholesterol

• Cholesterol accounts for 20% of membrane lipids, strengthens the membrane, and stabilizes it against temperature extremes.

Glycolipids

• Glycolipids make up 5% of the membrane lipids and are phospholipids with attached carbohydrate groups, located only on the outer phospholipid region.
• Glycolipids contribute to the glycocalyx and function in cell recognition and intercellular adhesion.

Membrane Proteins

• Membrane proteins float and move laterally within the fluid bilayer, performing most of the membrane's functions.
• The two structural types are integral and peripheral.
• Integral proteins are embedded in the phospholipid bilayer; many are glycoproteins and components of the glycocalyx.
• Peripheral proteins are not embedded in the lipid bilayer but are loosely bound to the interior or exterior surfaces.

Functions of Membrane Proteins

• Transport proteins regulate the movement of substances across the membrane.
• Cell surface receptors bind ligands, such as neurotransmitters, to initiate cellular responses.
• Identity markers communicate a cell's belonging to the body, distinguishing healthy cells from those to be destroyed.
• Enzymes catalyze chemical reactions on either the internal or external cell surface.
• Anchoring sites secure the cytoskeleton to the plasma membrane.
• Cell-adhesion proteins facilitate cell-to-cell attachments.

Explore cell structure and function. Learn about the nucleus, cytoplasm, cytosol, organelles, and inclusions. Discover how cells facilitate communication, gas transport, and nutrient absorption.