02 Cells, Tissues, and Membranes
Trace how cells organize into tissues, membranes, and organs, and learn how their structures support the body's functions.
From cells to organ systems
The body is organized in levels: cells form tissues, tissues combine in organs, and organ systems coordinate the work of multiple organs. A is the smallest living unit, while a is an organized group of cells and surrounding material performing related functions. An organ combines two or more types.
Structure and function are linked at every level. The arrangement and types of cells in a help determine what an organ can do, and an organ's specialized tissues work together rather than functioning as isolated collections of cells.
structures and their roles
A typical human has a surrounding and a nucleus. The is a flexible phospholipid bilayer containing proteins that regulate what enters and leaves the and help cells communicate. consists of cytosol, the 's fluid, and its organelles. The nucleus contains most of the 's DNA, which provides instructions for activities.
Organelles perform specialized tasks that support cellular activity:
Mitochondria release usable energy from nutrients.
Ribosomes build proteins.
Endoplasmic reticulum (ER) helps make and process proteins in rough ER and lipids in smooth ER.
Golgi apparatus modifies, sorts, and packages cellular products for transport.
Lysosomes digest materials and cellular waste.
Cytoskeleton is a network of protein fibers that helps maintain shape, move materials within the , and support movement or division.
These structures can work in sequence. A that secretes a protein can use DNA instructions in the nucleus, ribosomes and rough ER to make the protein, and the Golgi apparatus to package it for release.
The four basic types
The body has four broad types, each with characteristic structures suited to its main roles.
consists of closely packed cells forming sheets. It covers surfaces, lines cavities and passageways, and makes glands. Its functions include protection, absorption, filtration, and secretion. Examples include the skin surface, intestinal lining, and glands.
has cells spread within an , material outside cells that may contain fibers and ground substance. It supports, binds, protects, stores energy, and transports materials. Examples include bone, cartilage, tendons, adipose , and blood.
consists of contractile cells that shorten in response to stimulation, producing movement or force. Skeletal muscle moves the skeleton, cardiac muscle pumps blood, and smooth muscle moves substances through hollow organs. Examples include muscles attached to bones, the heart wall, and the intestinal wall.
contains neurons that transmit electrical signals and supporting cells that help protect and maintain them. It detects changes and coordinates responses. Examples include the brain, spinal cord, and nerves.
Membranes that cover and line
A is a sheet that covers a body surface, lines a cavity, or surrounds an organ. Epithelial membranes combine epithelium with underlying ; membranes consist of .
Cutaneous membrane (skin): The protective outer covering that helps resist injury, pathogens, and water loss.
Mucous membranes (mucosae): Line passageways that open to the exterior, including the digestive and respiratory tracts. Their epithelial cells produce mucus, which helps protect and lubricate the lining.
Serous membranes (serosae): Line body cavities that do not open to the exterior and cover organs within them. Serous fluid reduces friction as organs move. Examples include the pleura around the lungs and the pericardium around the heart.
Synovial membranes: membranes lining freely movable joint cavities. They contribute to synovial fluid, which lubricates joint surfaces.
Tissues working together in organs
An organ's function depends on several types working together. In the small intestine, absorbs digested nutrients and secretes substances; supports the lining and contains blood vessels; smooth muscle mixes and moves intestinal contents; and helps regulate movement and secretion. Together, these activities support digestion and nutrient absorption.
The skin combines epithelial cells that form the exposed barrier with underlying that provides strength and support. Together, these tissues protect the body and help maintain a stable internal environment. These examples show how specialized tissues depend on one another within organs.