The extracellular matrix and cell wall. Collagen, integrins, fibronectin, cellulose, and pectin.
We’ve spent a lot of time looking at what’s inside a cell. What, then, is on the outside? It depends a lot on what kind of cell you’re looking at.
Plants and fungi have a tough cell wall for protection and support, while animal cells can secrete materials into their surroundings to form a meshwork of macromolecules called the extracellular matrix. Here, we’ll look in more detail at these external structures and the roles they play in different cell types.
Extracellular matrix of animal cells
Most animal cells release materials into the extracellular space, creating a complex meshwork of proteins and carbohydrates called the extracellular matrix (ECM). A major component of the extracellular matrix is the protein collagen. Collagen proteins are modified with carbohydrates, and once they're released from the cell, they assemble into long fibers called collagen fibrils.
Collagen plays a key role in giving tissues strength and structural integrity. Human genetic disorders that affect collagen, such as Ehlers-Danlos syndrome, result in fragile tissues that stretch and tear too easily.
In the extracellular matrix, collagen fibers are interwoven with a class of carbohydrate-bearing proteoglycans, which may be attached to a long polysaccharide backbone as shown in the picture below. The extracellular matrix also contains many other types of proteins and carbohydrates.
Diagram showing the extracellular matrix and its connections to the cell. A network of collagen fibers and proteoglycans is found outside of the cell. Collagen connects to integrin proteins in the plasma membrane via fibronectin. On the inside of the cell, the integrins link up to the microfilaments of the cytoskeleton.
The extracellular matrix is directly connected to the cells it surrounds. Some of the key connectors are proteins called integrins, which are embedded in the plasma membrane. Proteins in the extracellular matrix, like the fibronectin molecules shown in green in the diagram above, can act as bridges between integrins and other extracellular matrix proteins such as collagen. On the inner side of the membrane, the integrins are linked to the cytoskeleton.
Integrins anchor the cell to the extracellular matrix. In addition, they help it sense its environment. They can detect both chemical and mechanical cues from the extracellular matrix and trigger signaling pathways in response.
Blood clotting provides another example of communication between cells and the extracellular matrix. When the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. When tissue factor binds to a molecule present in the extracellular matrix, it triggers a range of responses that reduce blood loss. For instance, it causes platelets to stick to the wall of the damaged blood vessel and stimulates them to produce clotting factors.
The cell wall
Though plants don't make collagen, they have their own type of supportive extracellular structure: the cell wall. The cell wall is a rigid covering that surrounds the cell, protecting it and giving it support and shape. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? A big part of that crunch is the rigidity of celery’s cell walls.
Fungi also have cell walls, as do some protists (a group of mostly unicellular eukaryotes) and most prokaryotes—though I don't recommend biting into any of those to see if they crunch!
Like the animal extracellular matrix, the plant cell wall is made up of molecules secreted by the cell. The major organic molecule of the plant cell wall is cellulose, a polysaccharide composed of glucose units. Cellulose assembles into fibers called microfibrils, as shown in the diagram below.
Image of the plant cell wall, showing the network of cellulose microfibrils and pectins (with pectins being particularly abundant in the middle lamella).
Most plant cell walls contain a variety of different polysaccharides and proteins. In addition to cellulose, other polysaccharides commonly found in the plant cell wall include hemicellulose and pectin, shown in the diagram above. The middle lamella, shown along the top of the diagram, is a sticky layer that helps hold the cell walls of adjacent plant cells together.
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- How is this related to cell signaling? Does the extracellular matrix act as receptors?(6 votes)
- The integrins, which connect the ECM to the cytoskeleton, act as receptors in the sense that they receive signals from the ECM and modulate the cell signaling pathways.(6 votes)
- Where are collagen synthesised and how does it reach its destination(3 votes)
- Proteins that get exported are (typically) synthesized by ribosomes bound to the ER (i.e. in the rough ER) and this is true of collagen.
The collagen precursor polypeptide then gets processed in the ER and Golgi before being exported via. exocytosis (vesicles bud off from the Golgi and then fuse with the plasma membrane).
Once outside the cell, collagen undergoes further modification by enzymes that are also secreted by cells.
To start learning more about the ER, Golgi apparatus, and exocytosis on Khan Academy:
(You can also find other material on this by searching for various keywords such as "rough ER", "Golgi", or "exocytosis".)
Wikipedia also has an article on collagen that describes these steps in more detail:
Does that help?(9 votes)
- What is a polysaccharide? Is this the sticky goo that bacteria produce and nestle in, when a biofilm forms on a surface? What makes a molecule sticky? Sorry - 3 questions.(3 votes)
- 1) Multi sugar/carbohydrate molecule. 2) Bacteria usually lives in colonies and sometimes due to overpopulation can kill itself with their own waste byproducts. 3) It usually depends on the type of molecules. Polarity matters as well. Polar wise molecules ( such as water) will attract same affinity molecules. Fats are non polar molecules and attract similar affinity molecules. Thisis also why soap has both polar and non polar sides so it will attach to both fats and water so it can remove fats when rinsed with water...(7 votes)
- The ECM and cell wall are used for support. Then what are cytoskeleton for?(4 votes)
- ECM is outsideof cell Cytoplasm-like material and cytoskeleton isinside of cells structural material(7 votes)
- why animals have extracellular matrix not a cell wall ?(2 votes)
- This allows us to bend and be flexible! With a cell wall our skin would be rigid and we might break if we bend our arm for example!(9 votes)
- Does proteins embedded in the cell membrane count as a part of the cell membrane or does it count as an individual apart from the cell membrane?(3 votes)
- does extracellular matrix come out in AP biology?(3 votes)
- Learning about the extracellular matrix won't hurt you because it exists.(1 vote)
- What are the chemical differences between the constituents of the ECM and the cell wall?(2 votes)
- What is the difference between Proteoglycan and Glycocalyx?
Many thanks!(1 vote)
- Glycocalyx is what surrounds a bacteria cell while Proteoglycan is what surround animal cell(3 votes)
- When fungi form symbiotic relationships with plant roots, can they connect directly with the plant's cells through their plasmadesmada, or is there a different part of the extracellular matrix that helps them to do this?
I ask because I've recently heard that fungi can receive nutrients they are unable to break down themselves from plants, and I'm wondering where the site of that transmission happens. Does it happen directly through the smooth ER?(1 vote)