Microfilaments - Stringy ProteinsYou will find microfilaments in most cells. They are the partner of microtubules. They are long, thin, and stringy proteins (mainly actin) compared to the rounder, tube-shaped microtubules. We'd like to say you can find them here or there, but they are everywhere in a cell. They work with microtubules to form the structure that allows a cell to hold its shape, move itself, and move its organelles.
Making the CytoskeletonAll of the microfilaments and microtubules combine to form the cytoskeleton of the cell. The cytoskeleton is different from cytoplasm (cytosol). The cytoskeleton provides structure. Cytoplasm is just a fluid. The cytoskeleton connects to every organelle and every part of the cell membrane. Think about an amoeba. All of the pieces work together so that the foot might reach out towards the food. Then lysosomes and peroxisomes are sent to begin digestion. The movement of the cell membrane, organelles, and cytoplasm is all related to the tubules and filaments.
You will also find many microfilaments in muscle tissue. They are called myofibrils when you find them in muscles. The two proteins myosin and actin work together to help the muscle cells relax and contract. The two proteins need each other and together they are called actomyosin. Combine those protein threads with some ions in the muscle cell and you get a huge contraction. The groups of actomyosin contracting are called sarcomeres. All of the muscle cells work together to make a muscle contract.
A Role in Cell MovementCells move in a variety of ways. We just talked about the contraction of a muscle cell. That is an extreme example. When you learn about single-celled organisms, you will understand that they need to move. They may need to glide from one area to another. The microfilaments are often found anchored to proteins in the cell membrane. Sometimes microfilaments are found floating free and connected to other filaments and tubules. Those binding proteins allow the microfilaments to push and pull on the cell membrane to help the cell move.
Modeling Contractile Forces (Univ. of Pennsylvania Video)
Encyclopædia Britannica (Cytoplasmic Streaming):