Table of Contents
Overview: From Single Cells to Complex Bodies
Multicellular organisms are built in levels. Individual cells rarely work alone; they join, specialize, and cooperate. This creates a hierarchy:
- Cells → Tissues → Organs → Organ systems → Organism
This chapter focuses on how cells organize into tissues and organs, not on the detailed structure of cells (covered elsewhere).
Key ideas:
- Cells in multicellular organisms specialize (different shapes, functions, and gene activity).
- Specialized cells group into tissues.
- Different tissues combine to form organs with specific tasks.
- Organs are integrated into organ systems that keep the whole organism alive.
Why Multicellularity Matters
Becoming multicellular brought major advantages:
- Size: Larger organisms can escape some predators and exploit new habitats.
- Division of labor: Different cells perform different tasks more efficiently.
- Internal environment: Tissues and organs can stabilize conditions (e.g., temperature, pH) even when the outside environment changes.
- Longevity and repair: Some cells can be replaced (e.g., skin), while others are protected deep inside (e.g., neurons in the brain).
But multicellularity also creates challenges:
- Cells must stick together (adhesion).
- Cells must communicate (chemical and electrical signals).
- Cells must be supplied with nutrients, oxygen, and removed of waste.
- Cell numbers and growth must be regulated (to avoid uncontrolled growth like cancer).
Levels of Organization
Cells: The Starting Level
- Each cell is a unit of life but, in a multicellular organism, most cells cannot survive independently.
- Cell specialization (differentiation) creates many cell types (e.g., nerve, muscle, blood cells).
- Each cell type:
- Expresses a specific subset of genes.
- Has a characteristic structure related to its function.
- Occupies a defined place in tissues and organs.
Tissues: Groups of Similar Cells
A tissue is a group of similar cells (and the substances they produce) that work together to perform a particular function.
In animals, four basic tissue types are usually distinguished:
- Epithelial tissue
- Sheets of closely packed cells that cover surfaces and line cavities (skin surface, gut lining, blood vessel lining).
- Functions: protection, secretion, absorption, barrier formation.
- Connective tissue
- Cells embedded in an abundant extracellular matrix (fibers and ground substance).
- Examples: bone, cartilage, fat (adipose), blood, tendons, ligaments.
- Functions: support, connection, storage, transport.
- Muscle tissue
- Specialized for contraction.
- Three main types in vertebrates: skeletal muscle, cardiac muscle, smooth muscle.
- Functions: movement of body, pumping of blood, movement of organs (e.g., intestine).
- Nervous tissue
- Nerve cells (neurons) and supporting cells (glia).
- Functions: reception, processing, and transmission of information.
In plants, tissues are grouped differently, for example:
- Meristematic tissue: regions of cell division and growth (root and shoot tips).
- Dermal tissue: outer protective covering (epidermis).
- Ground tissue: filling and support, photosynthesis, storage (parenchyma, collenchyma, sclerenchyma).
- Vascular tissue: conduction of water and nutrients (xylem, phloem).
The key point: tissues are functional cell communities, not random cell clusters.
Organs: Combining Multiple Tissues
An organ is a structure made of at least two, usually several, types of tissues that together perform a specific, higher-level function.
Examples in animals:
- Heart
- Mainly muscle tissue (cardiac muscle) for pumping.
- Connective tissue for structure and support.
- Epithelial tissue lining the inside of chambers and blood vessels.
- Nervous tissue for rhythm and coordination.
- Stomach
- Epithelial tissue that secretes digestive enzymes and acid.
- Muscle tissue that churns food.
- Connective tissue that holds everything together and provides blood vessels.
- Nervous tissue that regulates activity.
Examples in plants:
- Leaf
- Dermal tissue (epidermis) with stomata for gas exchange.
- Ground tissue (mesophyll) with chloroplasts for photosynthesis.
- Vascular tissue (veins) transporting water, minerals, and sugars.
- Root
- Dermal tissue with root hairs for absorption.
- Ground tissue for storage and structure.
- Vascular tissue for transport to/from the shoot.
Organs show structural specialization (shape, layers, arrangements) that reflects their function.
Organ Systems and the Whole Organism
Organ systems are groups of organs that work together to perform major life functions.
Examples in animals:
- Digestive system: mouth, esophagus, stomach, intestines, liver, pancreas.
- Respiratory system: lungs and associated airways.
- Circulatory system: heart, blood vessels, blood.
- Nervous system: brain, spinal cord, nerves.
- Reproductive system: gonads and accessory organs.
In plants, we often refer to shoot system (stems, leaves, flowers) and root system as large-scale functional units.
The organism is the integrated whole: a living being in which all organ systems cooperate and are regulated so that internal conditions remain compatible with life.
How Cells Form Tissues: Adhesion and the Extracellular Environment
To build tissues and organs, cells must be held together and anchored.
Cell Adhesion
Cells adhere to each other using specialized proteins in their membranes (adhesion molecules).
In animals, important adhesive structures include:
- Tight junctions: seal spaces between cells, forming a barrier (e.g., gut epithelium).
- Adherens junctions and desmosomes: connect cell skeletons, providing mechanical strength (important in skin, heart).
- Gap junctions: small channels that allow direct exchange of small molecules and ions between neighboring cells (important in heart muscle coordination).
In plants:
- Cells are surrounded by cell walls and connected via plasmodesmata (fine channels through cell walls).
- Cell adhesion is largely due to components of the cell wall and middle lamella (rich in pectins).
Extracellular Matrix (ECM)
Beyond direct cell–cell contact, many tissues are reinforced by a matrix outside the cells.
- In animals, ECM is composed of:
- Fibrous proteins (e.g., collagen, elastin).
- Ground substance (proteoglycans, glycoproteins).
- Functions:
- Provides mechanical strength (bone, cartilage).
- Influences cell behavior (shape, division, movement, differentiation).
- Forms basement membranes under epithelia, anchoring them and regulating exchange.
In plants, the cell wall and associated materials (cellulose, hemicellulose, lignin) play comparable structural roles.
Coordination and Communication Between Cells
Organized tissues and organs require communication so that cells can coordinate their activities.
Local and Long-Distance Signals
Cells use multiple signaling modes:
- Direct contact:
- Through gap junctions (animals) or plasmodesmata (plants).
- Local chemical signaling:
- Signaling molecules diffuse short distances to nearby cells.
- Long-distance signaling:
- In animals: hormones via the blood, nerve impulses via neurons.
- In plants: hormones and signaling molecules transported in sap or through tissues.
These signals regulate:
- Cell division and growth.
- Cell differentiation (which specific type a cell becomes).
- Cell death (programmed cell death when no longer needed).
- Activity of organs (e.g., heart rate, opening and closing of stomata in leaves).
Cell Differentiation and Stable Tissues
To form stable tissues:
- Cells progressively specialize (different gene expression patterns).
- Once specialized, many cells maintain their identity for long periods.
- Some tissues have stem cells that can divide and replace older cells (e.g., skin, gut lining, plant meristems).
Coordinated differentiation ensures that each tissue and organ has the right types and numbers of cells in the right places.
Structural Principles in Tissues and Organs
Although there is great diversity, some general construction principles occur repeatedly.
Surface and Exchange
Many organs are specialized for exchange with the environment or between body compartments. They often show:
- Large surface area (folds, villi, alveoli).
- Thin barriers (thin epithelial layers).
- Rich vascularization (many blood vessels) or other transport tissues.
Examples:
- Intestinal villi increase area for nutrient absorption.
- Lung alveoli maximize gas exchange surface.
- Plant root hairs increase area for water and mineral uptake.
Support and Protection
Supporting tissues and organs:
- Use rigid or semi-rigid materials:
- Plants: lignified cell walls, woody tissue.
- Animals: cartilage, bone, exoskeletons.
- Often surround or overlay sensitive tissues (e.g., skull protecting the brain, bark protecting inner plant tissues).
Transport and Distribution
Transport structures are found in both animals and plants:
- Animals: blood vessels (arteries, veins, capillaries) form networks through organs.
- Plants: vascular bundles of xylem and phloem run through roots, stems, and leaves.
These ensure that all cells within tissues and organs receive supplies and can dispose of wastes, despite being far from the external environment.
Specialization vs. Independence
In unicellular organisms:
- One cell must do everything: nutrition, movement, reproduction, sensing.
In multicellular organisms:
- Individual cells lose independence but gain efficiency:
- Many cannot divide anymore (e.g., neurons).
- Most cannot survive on their own.
- Survival depends on cooperation and integration:
- If blood circulation stops, many tissues quickly die.
- If plant vascular transport is interrupted, organs above the blockage may wither.
This interdependence is the price and the power of complex multicellular life.
Summary
- Multicellular organisms show a hierarchy: cells → tissues → organs → organ systems → organism.
- Tissues are cooperative groups of similar cells (plus matrix) with specific functions.
- Organs combine several tissue types to perform more complex tasks.
- Adhesion, extracellular structures, and communication between cells enable stable, integrated structures.
- Structural principles (surface enlargement, support, transport) recur across very different organisms.
- The transition from independent cells to integrated organs underlies the complexity and adaptability of higher life forms.