Cellular Movement and Locomotion

Muscles facilitate movement in vertebrates and invertebrates.
Cells, materials, and transport fluids (e.g., red blood cells, white blood cells) move within the body.
Oxygen is inhaled, while carbon dioxide is exhaled.
The digestive system utilizes smooth muscles to move food through the small intestine.
Movement occurs at various scales, including subcellular motion (e.g., chromosome movement).
Vesicles are transported by motor proteins.
Individual cells like bacteria and gametes can move.
Body parts move, such as the heart and fingers.

Movement typically involves one part pushing against another.
Stationary structures, like cytoskeleton filaments, act as tracks for motor proteins that drag vesicles.
Muscles contract against a hard skeleton to facilitate movement.
Hydroskeletons use squishing and squeezing motions to enable movement.
Organisms can push against their environment for propulsion, like flagellated bacteria.
Some organisms remain stationary and filter food from the moving environment.

Flagella are long (approximately $20$ microns compared to cell size of approximately $2$ microns).
They can be located at the posterior end, both ends, or all sides of the cell.
Flagella move clockwise or counterclockwise.
Flagella enable taxis, which is movement towards or away from stimuli like light or chemicals.
Bacteria can move at speeds of up to $60$ body lengths per second, which is faster than a cheetah's $25$ body lengths per second.

The bacterial flagellum is a true biological wheel.
It consists of a long tail made of flagellin proteins.
The tail is attached to the cell body via a hook.
The structure rotates due to a motor embedded in the membrane.
A proton gradient drives the rotation via motor proteins (similar to ATP synthase).
- It takes about $1000$ protons for one rotation.
- Bacteria can move at $1000$ rotations per second. Therefore, a million protons pass through the motor protein per second.
Bacterial flagella differ significantly from eukaryotic flagella.

The flagellum's complexity is sometimes used as an argument against evolution.
Critics argue that the flagellum is irreducibly complex; it can't properly function unless all parts are present.
However, components of the flagellum exist independently with other functions.
The type three secretase system, used to inject toxins, is related to one key element of the flagellum.

Bacteria can orient flagella in different ways.
Some bacteria can only rotate flagella clockwise, requiring them to stop and reorient to change direction.
Other bacteria can rotate flagella both clockwise and counterclockwise for forward and backward movement.
Vibrio cholerae, which causes cholera, uses a single flagellum.
E. coli uses multiple flagella that twist together to push against the environment.
When changing direction, the flagella unwind and splay out, causing the bacterium to stop and reorient.
Bacteria move in a random walk pattern, sampling the environment.
They exhibit directed movement towards attractants like glucose, oxygen, or light.

Microtubules are hollow filaments made of alpha and beta tubulin subunits.
They are dynamic, growing and shrinking to restructure themselves.
Microtubules maintain cell shape, enable cilia and flagella movement, move chromosomes during cell division, and transport organelles (e.g., mitochondria and chloroplasts).

Actin filaments are smaller twisted chains of actin subunits.
They also grow and shrink dynamically.
Actin filaments contribute to the cytoskeleton and help cells change shape (e.g., pseudopodia in amoebas).
They are important for muscle contraction, organelle movement, and cell division.

Bacteria possess a cytoskeleton.
MREB protein filaments maintain the rod-like shape of some bacteria.
FETZ filaments help cells divide during binary fission.
MREB is homologous to actin, while FETZ is homologous to beta tubulin, indicating shared evolutionary origins.
Alpha helices are present in both prokaryotic and eukaryotic versions.
Actin and MREB filaments provide cell structure.
FETZ, a tubulin homolog, splits cells during division in bacteria.
In eukaryotes, tubulin drags chromosomes during cell division.
Both filaments can move things inside the cell and help cells divide.
Chromosomes are taken to daughter cells by the tubulin.

Filaments grow by adding monomers or dimers at the plus end and dissolve at the minus end.
Actin subunits bound to ATP dimerize, hydrolyzing ATP to ADP.
Actin monomers bounce around until nucleation occurs, leading to growth.
Filaments can grow at the plus end while disassembling at the minus end, a process called treadmilling.

Motor proteins move along filaments.
They have head groups that walk along the filament and tails that connect to cargo.
Movement of the head group is driven by ATP hydrolysis.
Kinesin moves on microtubules towards the plus end, delivering vesicles.
Tubulin interacts with kinesin (moves to the plus end) or dynein (moves to the minus end).
Myosin interacts with actin, but cannot walk backwards.
Filaments can move relative to each other using motor proteins and ATP.

Cilia and flagella are structures composed of filaments and motor proteins.
Cilia line the lungs and help clear debris.
Flagella, like those on sperm, enable movement.
Both structures have a central pair of tubules surrounded by nine pairs of outer tubules.
Motor proteins cause tubules to walk up neighboring tubules, causing bending.
Eukaryotic flagella move back and forth, unlike the rotating bacterial flagellum.
Cilia move with a power stroke and a recovery stroke, similar to the backstroke swimming technique.

Sarcomeres contain actin filaments and myosin motor proteins.
Myosin head groups walk along actin filaments, pulling them inward and contracting the muscle.
Actin and myosin interactions also extend pseudopodia in amoebas.
Myosin pushes on filaments, which are attached to the cell membrane, and cause the cell to extend, allowing the amoeba to move.
Actin and myosin facilitate movement inside plant cells, like the movement of chloroplasts.

Neurons transport proteins and neurotransmitters along microtubules using motor proteins.
Vesicles are carried by motor proteins along microtubule highways.
Concussions can damage these microtubule highways, leading to a buildup of materials and cell death.

Motor proteins exist in both prokaryotes and eukaryotes, functioning similarly.
Bacteria lack motor proteins, relying on structural filaments.
Prokaryotic and eukaryotic flagella evolved separately, using different structures.
The structure and protein composition of flagella differ between prokaryotes and eukaryotes.