# AP Biology/Cell Communication and Cell Cycle

Introduces how cells grow and reproduce, as well as how cells communicate.

## Objectives and Skills

Topics may include:
* The mechanisms of cell communication
* Signal transduction
* Cellular responses and feedback mechanisms
* The events in a cell cycle

## Cellular Communication

How do cells directly communicate with each other?

### Direct Contact

Molecules from the 1st cell, after being triggered to produce molecules, are sent to the proteins of the 2nd cell, to which it triggers a response. In plants, this is done through the plasmodesmata, while in animals, this is done through the gap junction.
Through surface proteins, they can bind to each other, altering the protein's shape--this can trigger a response in both cells.
Water filled channels allow intracellular mediators to diffuse between the cells. Big molecules, like proteins, require special assistance to get through.

#### Rember

* Intra = Inside
* Inter = Outside

### Paracrine Signaling

Short distance communication: a protein sends ligands through either diffusion or vesicles... and the ligands go to the cell with the right receptors.

### Endocrine Signaling

Long-distance communication: a hormone being sent into the bloodstream and reaches the receptor.

### Autocrine Signaling (Self)

A cell secretes a ligand which attaches to a receptor on the same cell, triggering a response.

## Unicellular Cells Communication

* Quorum Sensing (bacteria) - Bacteria monitor how much of themselves are in a given area. When the signaling reaches a threshold (enough bacteria in that place of the species), then the bacteria in that place will do their cellular response (attack the host/send fluorescent light). It is a form of paracrine signaling.

## Signal Transduction

See pic for diagram.

### Types

* Ligand-gated ion channels
* G-protein coupled receptors
* Tyrosine kinase receptors

### Ligand Receptors

* Polar cannot diffuse in; nonpolar can though
* Change in the receptor's shape causes an intracellular response.
* Steroid hormones = Lipids (nonpolar) - water-hating.
* Ligand-gated ion channels = Ions (polar) are prevaled from touching the hydrophobic tails.
* A G-protein attached to GTP = Active.
* A G-protein attached to GDP = Inactive.
* Gases that act as ligands can diffuse across the plasma membrane (due to its small size).
* Cell-receptors (protein receptors) bind to POLAR ligands.

### Signal Relay Pathways

* Bind to intracellular protein: The signaling process is complete.
* Bind to intercellular protein: The signaling process continues.
* Upstream: Earlier in the relay channel
* Downstream: Later in relay chain
* Second messenger: Small, non-protein molecules (intracellular) that pass a signal from the "first messenger" (intercellular).
* cAMP: Second-messenger molecule made from ATP (activates protein kinase A). Amplifies intracellular signals. [ATP --> cAMP with the addition of adenylyl cyclase].
* Same singling molecule may produce different results because the receptors are different kinds and therefore trigger dif. pathways.

## Homeostasis

    "Stability"

### Positive-feedback - An increase in product results in increasing the rate of the synthesis of that product. Amplifies initial stimuli and moves the system away from its starting point.

* Example: Childbirth - stretching of uterine walls cause contractions that further stretch the wall and continues until birth. Lactation - the child feeding stimulates milk production which causes more feeding until the baby stops feeding.
* Too much glucose!
* Receptors in the body senses a change in glucose levels.
* Beta cells of the pancreas release insulin, a hormone that stores glucose (protein). Insulin causes the liver to take up glucose and stores it as glycogen. Body cells also take up glucose.
* Blood glucose declines. The pancreas stops releasing insulin.
* Too little glucose!
* Alpha cells of the pancreas are alerted to release glucagon, a ligand, into the blood.
* Glucagon travels to liver receptors - causes the liver to break down glycogen --> glucose. Glucose is released to the blood.
* Blood glucose increases. The liver stops breaking down glycogen as glucagon (the release) has been stopped.

## Cell Cycle

Tell the difference between a cell in mitosis vs interphase (see nucleus vs. condensing/no nucleus).
* Mitosis
* Meiosis

### Bacteria: Binary Fusion instead of Mitosis

* 1 chromosome (nucleoid)
* Some changes occurred (evolutionary) between cell division and binary fission.

#### Protein Signals

* Cyclins
* Cdks

#### Chi-Square

* Used for count categorical data (used in genetics).
* Null hypothesis: No difference between expected numbers and observed numbers - no change [is the difference statistically significant or not?].
* Degrees of Freedom: [# of groups - 1].
* Significant: p=.05 (95% confident)
* "Fail to Reject" = "Accept" --> Calculated value > Critical value = "reject the null hypothesis".