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Anatomy and Physiology Unit 2: Cells and tissues

Similar cells form tissues. There are only four tissue types, and every organ is built from them.

Covers: Cell organelles, membrane transport, four tissue types: epithelial, connective, muscle, nervous

Study guide

Cells are the body's basic units. The plasma membrane controls what enters and leaves. Inside, the nucleus holds DNA, ribosomes make proteins, the rough and smooth ER make proteins and lipids, the Golgi apparatus packages them, mitochondria produce ATP, and lysosomes digest waste. The cytoskeleton gives shape and allows movement, and cilia and flagella move fluids or cells.

Substances cross the membrane by passive transport, which needs no energy: diffusion from high to low concentration, osmosis of water, and facilitated diffusion through proteins. Active transport uses ATP to move substances against a gradient, like the sodium-potassium pump that keeps nerve and muscle cells ready to fire. Large particles enter by endocytosis and leave by exocytosis. Tonicity matters clinically: red blood cells swell and burst in hypotonic fluids and shrink in hypertonic ones, which is why IV fluids are isotonic.

Tissues are groups of similar cells doing a common job, and there are four types. Epithelial tissue covers surfaces and lines cavities and glands; it's classified by cell shape (squamous, cuboidal, columnar) and layers (simple or stratified), and it protects, absorbs and secretes. Connective tissue supports and connects; it includes loose and dense connective tissue, cartilage, bone, adipose tissue and blood, which is a fluid connective tissue. Muscle tissue contracts: skeletal muscle is voluntary and striated, cardiac muscle is involuntary and striated with intercalated discs, and smooth muscle is involuntary and lines organs and vessels. Nervous tissue, made of neurons and supporting glial cells, conducts electrical signals.

Body cells divide by mitosis after interphase, during which the cell grows and copies its DNA in the S phase. Mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis produce two identical cells for growth and repair. Some cells, such as most neurons, rarely divide after maturity. Connective tissue is made of cells in an extracellular matrix of ground substance and fibers. Collagen fibers give strength, elastic fibers let tissue stretch and recoil, and reticular fibers form delicate supporting networks in the spleen, lymph nodes and bone marrow. Dense regular connective tissue forms tendons and ligaments.

Simple squamous epithelium lines the alveoli and capillaries, where substances need to diffuse quickly. Stratified squamous epithelium protects surfaces that face wear, like the skin and esophagus. Simple columnar epithelium lines the stomach and intestines for absorption and secretion, and cuboidal epithelium lines kidney tubules and glands. Exocrine glands release their products through ducts onto a surface, as sweat and salivary glands do. Endocrine glands are ductless and release hormones into the blood. Body membranes include mucous membranes (lining openings to the outside), serous membranes (lining closed cavities), the cutaneous membrane (skin) and synovial membranes (lining joints).

Key ideas

• Epithelial tissue covers surfaces and lines cavities.
• Connective tissue supports and binds: bone, blood, cartilage, fat.
• Muscle tissue contracts: skeletal, cardiac and smooth.
• Nervous tissue sends electrical signals.
• Organs combine several tissue types.

Formulas and rules

Common mistakes

• Forgetting blood is a connective tissue.
• Calling cardiac muscle voluntary.

Words to know

Four tissue types

Epithelial, connective, muscle, nervous.

Epithelial tissue

Covers surfaces and lines cavities.

Blood

A fluid connective tissue.

Smooth muscle

Involuntary muscle in the walls of organs.

Nervous tissue

Tissue that conducts electrical signals.

Muscle tissue types

Skeletal (voluntary), cardiac (heart), smooth (organs).

Plasma membrane

Controls what enters and leaves the cell.

Mitochondria

Produce ATP.

Cytoskeleton

Gives the cell shape and allows movement.

Osmosis

Diffusion of water across a membrane.

Sodium-potassium pump

Active transport that keeps nerve and muscle cells ready to fire.

Isotonic solution

Same solute concentration as cells; used for IV fluids.

Squamous / cuboidal / columnar

Flat / cube-shaped / tall epithelial cells.

Stratified epithelium

Several cell layers; protects against abrasion.

Connective tissue

Supports and connects: bone, cartilage, fat, blood.

Cardiac muscle

Involuntary, striated, with intercalated discs.

Skeletal muscle

Voluntary and striated.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.