Cell junctions and glands · Biology Class 11–12 | StemPy Lab
Course 32.1·Lesson 2 of 3
Cell junctions and glands
Cells of a tissue must hold together, seal gaps and talk to one another, and some of them must make products to send out. This lesson describes the three kinds of cell junction and the glands, from the goblet cell to the salivary gland, with the difference between a gland with a duct and one without.
about 25 min1 simulation1 practice question
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How cells hold together
Think of a wall of bricks. The bricks need mortar to stay together, and some walls also need a seal so that water does not leak through. Cells of a tissue have the same needs. They also need to talk to each other, which a brick wall does not.
Epithelial and other tissues are held by structures called cell junctions. There are three kinds.
Two neighbouring cells drawn side by side with a narrow gap between them. At the top the two membranes are fused in a tight junction, which stops substances from leaking between the cells. In the middle a dark plate on each membrane, joined by fibres, shows an adhering junction that cements the cells together. At the bottom channels that run straight through both membranes show a gap junction, which joins the cytoplasm of the two cells.
Junction
What it does
A way to remember it
Tight junction
stops substances from leaking across a tissue
a seal
Adhering junction
cements neighbouring cells together
mortar
Gap junction
joins the cytoplasm of adjoining cells so that ions, small molecules and sometimes big molecules pass quickly
a tunnel
A tight junction is useful in the lining of the intestine. The food stays in the gut and does not leak between the cells. A gap junction is useful in the heart, where a signal to contract must pass from cell to cell.
Other books give more names. Desmosomes are one kind of adhering junction. A gap junction is made of protein channels called connexons. NCERT keeps to the three names above.
Glands
A gland is made of cells that are specialised for secretion, which means making a product and sending it out. Most glands come from epithelium. The cells are columnar or cuboidal.
Glands are grouped in two ways.
By the number of cells. A unicellular gland is a single cell. The goblet cell of the alimentary canal is the standard example, and it makes mucus. A multicellular gland has many cells. The salivary gland is the standard example.
By the way the product leaves.Exocrine glands send their products through ducts or tubes. They secrete mucus, saliva, earwax, oil, milk and digestive enzymes. Endocrine glands have no duct. They secrete hormones straight into the fluid around the gland, and the blood carries the hormones away.
Some organs do both. The pancreas makes digestive enzymes that leave by a duct, so it is exocrine. It also makes the hormones insulin and glucagon that go into the blood, so it is endocrine as well.
A grid of six small drawings of epithelial tissue, each sitting on a thin line that is the basement membrane. Simple squamous: one layer of flat cells with flat nuclei, found in blood vessel walls and air sacs. Simple cuboidal: one layer of cube shaped cells with round central nuclei, found in gland ducts and kidney tubules. Simple columnar: one layer of tall cells with nuclei near the base and tiny projections on the free surface, found in the stomach and intestine lining. Ciliated columnar: tall cells with hair-like cilia on the free surface, found in the bronchioles and the fallopian tubes. Compound (stratified squamous): many layers with flat cells on top, found in skin and the mouth. Glandular: columnar cells with a goblet cell full of mucus, found in the alimentary canal.
A goblet cell fills with mucus until the top is swollen. Then it releases the mucus onto the surface and the cycle starts again. The mucus covers the lining and protects it.
Try it in Python
The program sorts five glands into exocrine and endocrine, by the route of the product.
Practise with new numbers
Common mistakes
Saying that all glands have ducts. Endocrine glands have none.
Saying that the goblet cell is multicellular. It is a single cell.
Mixing up the junctions. The tight junction seals. The adhering junction cements. The gap junction communicates.
Thinking that hormones travel along a duct. They enter the fluid around the gland and then the blood.
Forgetting that the pancreas is both. It has an exocrine part and an endocrine part.
Remember these
Three junctions: tight (no leakage), adhering (cementing), gap (communication).
Gland: cells that secrete. Goblet cell: unicellular. Salivary gland: multicellular.
Exocrine: product through a duct. Endocrine: no duct, hormones into the fluid around.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the three kinds of cell junction.
Which junction lets a signal pass from one heart cell to the next?
Give an example of a unicellular gland and a multicellular gland.
How are exocrine and endocrine glands different?
Why is the pancreas called both an exocrine and an endocrine gland?
References
OpenStax Biology 2e, chapter 33, The Animal Body: Basic Form and Function, section 33.2 Animal Primary Tissues.
NCERT (2023). Biology, Textbook for Class XI, chapter 7, Structural Organisation in Animals.
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