# Epithelial Tissue and Glands Biology I · Cells and Tissues · https://tryals.app/learn/biology-i/epithelial-tissue-and-glands ## Every Surface Is Lined Epithelium covers every body surface, lines every cavity, and forms every gland. Its defining features follow from that job: cells packed tightly with almost no matrix between them, resting on a **basement membrane**, **avascular** so nutrients must diffuse from the tissue below, and **polarised** with a distinct apical and basal face. Epithelia are classified twice over, and the classification predicts function. By layers: **simple** is one cell thick and suits exchange; **stratified** is many cells thick and suits protection; **pseudostratified** is one layer that looks like several because the nuclei sit at different heights. By cell shape: **squamous** cells are flat, **cuboidal** are cube-shaped, **columnar** are tall. | Epithelium | Where | Why | |---|---|---| | Simple squamous | Alveoli, capillaries | Thin, so diffusion is fast | | Simple cuboidal | Kidney tubules, glands | Secretion and absorption | | Simple columnar | Gut lining | Absorption, with room for organelles | | Stratified squamous | Skin, oesophagus | Withstands abrasion | | Pseudostratified ciliated | Airways | Moves mucus by ciliary beating | The pattern is consistent: **thin where exchange matters, thick where protection matters**. An alveolus one cell thick allows oxygen across in fractions of a second; skin many cells thick sacrifices exchange entirely for durability. Apical surfaces are specialised. **Microvilli** multiply absorptive area, the intestinal brush border increases it around 20-fold, and combined with villi and folds the small intestine achieves roughly 200 m² from a tube a few centimetres wide. **Cilia** move material along a surface. **Tight junctions** seal neighbouring cells so nothing leaks between them. **Glands** are epithelium that has sunk inward and specialised in secretion. **Exocrine** glands keep a duct and secrete onto a surface, sweat, saliva, digestive enzymes. **Endocrine** glands lose their duct and secrete hormones into the blood, which is why they are richly vascularised while the epithelium they came from is not. **Skin** combines the two tissue types: a stratified squamous epidermis for protection over a connective-tissue dermis carrying blood vessels, nerves and glands. > **Common pitfall:** forgetting that epithelium is avascular. Every nutrient reaches it by diffusion from the connective tissue beneath, which puts a hard ceiling on how thick a living epithelium can be, and is why the outer layers of skin are dead. ## Practice questions 8 of this lesson's 11 practice questions, with answers. The full set is in the app. ### 1. Microvilli increase the absorptive surface of an intestinal cell about 20-fold. If the flat surface would be 50 µm², what is the effective area in µm²? **Answer:** 1000 (within ±10) **Why:** $50 \times 20 = 1000$ µm². Combined with villi and circular folds, this amplification is how a few metres of intestine achieve an absorptive area of roughly 200 m². Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/microvilli-increase-the-absorptive-surface-of-an-intestinal-cell ### 2. Which features are characteristic of epithelial tissue? A. Richly supplied with its own blood vessels B. Avascular, relying on diffusion for nutrients C. Cells packed tightly with very little matrix D. Resting on a basement membrane **Answer:** B. Avascular, relying on diffusion for nutrients; C. Cells packed tightly with very little matrix; D. Resting on a basement membrane **Why:** Tight packing, a basement membrane and avascularity are all defining. the first option contradicts the third, and that avascularity is what limits how thick a living epithelium can be. Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/which-features-are-characteristic-of-epithelial-tissue ### 3. Sort each epithelium by whether its structure favours exchange or protection. **Answer:** - Favours exchange: Simple squamous in the alveoli, Simple columnar in the gut - Favours protection: Stratified squamous in the skin, Stratified squamous in the oesophagus **Why:** The rule is consistent: thin where exchange matters, thick where protection does. The oesophagus is stratified for the same reason skin is, food scraping past would destroy a single layer. Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/sort-each-epithelium-by-whether-its-structure-favours-exchange-or ### 4. Why are endocrine glands richly supplied with blood vessels while the epithelium they developed from is not? A. Their dense secretory cells require rapid oxygen delivery to sustain synthesis B. Their chemical secretions are far too viscous to pass along an epithelial duct C. They secrete hormones directly into the bloodstream rather than onto a surface D. Capillary networks degrade and replace the basement membrane during development **Answer:** C. They secrete hormones directly into the bloodstream rather than onto a surface **Why:** An endocrine gland loses its duct, so the bloodstream becomes its delivery route and it must be richly vascularised. An exocrine gland keeps its duct and delivers onto a surface, so it does not need that supply. Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/why-are-endocrine-glands-richly-supplied-with-blood-vessels-while-the ### 5. The outer layers of the epidermis consist of dead cells. **Answer:** True **Why:** True, epidermal cells are pushed outward away from their nutrient supply, fill with keratin and die. The dead outer layer is not a defect but the functional barrier: tough, waterproof and continuously replaced from beneath. Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/the-outer-layers-of-the-epidermis-consist-of-dead-cells ### 6. Epithelial sheets are strictly avascular, yet they exhibit high metabolic activity across surfaces like the gut and kidney tubules. What structural constraint does this trade-off impose on epithelia specialised for exchange? A. They must remain limited to a single layer of living cells B. Their apical domains must possess cilia to transport nutrients C. Basement membranes must actively pump oxygen into the tissue D. Metabolising cells must rely on vesicular transport of blood **Answer:** A. They must remain limited to a single layer of living cells **Why:** Diffusion across unvascularised tissue drops sharply with distance; thus, active transport or rapid exchange requires a minimal diffusion pathway. Cilia move luminal fluids, not metabolic fuels, whilst the basement membrane acts as a passive anchor, not a vascular pump. Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/epithelial-sheets-are-strictly-avascular-yet-they-exhibit-high ### 7. Match each gland type to how it delivers its secretion. **Answer:** - Exocrine gland → Through a duct onto a surface - Endocrine gland → Into the bloodstream, with no duct - Sweat gland → Onto the skin surface through a duct - Thyroid gland → Hormones into the circulation **Why:** The duct is what distinguishes the two, and it determines everything else, including whether the gland needs a rich blood supply. Sweat and thyroid are the standard examples of each. Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/match-each-gland-type-to-how-it-delivers-its-secretion ### 8. Complete the classification of epithelia. **Answer:** An epithelium one cell thick is called **simple**, while one many cells thick is called **stratified**. Flat cells are described as **squamous**, and an epithelium that looks layered but whose cells all touch the basement membrane is called **pseudostratified**. **Why:** The double classification, layers and shape, predicts function directly. Simple squamous means thin and exchanging; stratified squamous means thick and protective. Pseudostratified is the trap, since it looks like one thing and is another. Page: https://tryals.app/practice/biology-i/epithelial-tissue-and-glands/complete-the-classification-of-epithelia