AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/4/4-1)
Unit 4 · Topic 4.1
4.1 Cell Communication
Cells coordinate with each other by sending and receiving signals. They can signal by direct contact, by releasing local signals that reach nearby cells, or by sending hormones that travel long distances. Matching a signal to its range and its target cells is the foundation for everything else in this unit.
Key terms
- chemical signal
- direct contact
- local regulator
- neurotransmitter
- hormone
- target cell
Why cells communicate
In a multicellular organism, trillions of cells have to work together: heal a cut together, release sugar when you need energy, or fight off an infection. Even single-celled organisms like bacteria and yeast communicate, to find mates or to act together as a group.
Most signals are chemicals. A signal works only on target cells, which are cells that have the right receptor protein to recognize it. A cell without the receptor ignores the signal, even if it's bathed in it.
Signaling by direct contact
Some cells communicate by touching. Molecules on the surface of one cell bind directly to receptors on the surface of another. Neighboring cells can also be connected by channels that let small signal molecules pass straight from one cytoplasm to the next: gap junctions in animal cells and plasmodesmata in plant cells.
Your immune system relies on contact signaling. An antigen-presenting cell, such as a macrophage that has engulfed a pathogen, displays pieces of that pathogen on its surface. A helper T cell has to physically bind to those pieces to become activated. Killer (cytotoxic) T cells likewise bind directly to infected cells to recognize and destroy them.
Local signaling: short distances
A local regulator is a signal that a cell releases into the space around it, where it affects target cells nearby.
- Neurotransmitters: a nerve cell releases them across the tiny gap (synapse) to the next cell, which may be another nerve cell or a muscle cell. The signal travels a very short distance and acts in milliseconds.
- Quorum sensing in bacteria: each bacterium releases a small signal molecule. When enough bacteria are packed together, the signal gets concentrated enough to switch on certain genes in all of them at once, such as genes for glowing (bioluminescence) or forming a biofilm.
- Morphogens in embryos: a signal spreads out from one group of cells, forming a concentration gradient. Cells respond differently depending on how much they receive, which helps tell them what to become.
- Plant immune responses: a plant cell under attack releases signals that put nearby cells on alert.
Long-distance signaling: hormones
Hormones are signals that travel far through the body to reach target cells of a different type. In animals, endocrine cells release hormones into the blood, and the blood carries them everywhere, but only cells with the matching receptor respond. Insulin is made in the pancreas and acts on liver, muscle and fat cells. Other examples include human growth hormone, thyroid hormones, testosterone and estrogen. Plants have hormones too, which move through their vascular tissue or through the air.
Hormone signals are slower than nerve signals but can last longer and reach many tissues at once.
| Type | Range | Example |
|---|---|---|
| Direct contact | Touching cells only | Helper T cell binding an antigen-presenting cell |
| Local regulator | Nearby cells | Neurotransmitters; quorum-sensing signals |
| Hormone | Long distance, via the blood or vascular tissue | Insulin; testosterone |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying signals
Classify each signal as direct contact, local or long-distance, and justify: (a) A nerve cell releases acetylcholine onto a muscle cell next to it. (b) The thyroid gland in the neck releases thyroid hormone, which raises the metabolic rate of cells throughout the body. (c) A helper T cell binds to a protein fragment on the surface of a macrophage.
Show the solutionHide the solution
- Step 1: (a) The signal is released into the small gap between two adjacent cells and acts on the cell right next to it. That's local signaling by a neurotransmitter.
- Step 2: (b) The signal is released into the blood and acts on cells far from the gland. That's long-distance signaling by a hormone.
- Step 3: (c) The two cells physically bind; nothing is released and carried away. That's direct contact.
Answer: (a) Local (neurotransmitter); (b) long-distance (hormone); (c) direct contact.
- Example 2
Explaining a quorum-sensing result
A species of marine bacterium glows only when its population is dense. Researchers grow a sparse culture that doesn't glow, then add liquid filtered from a dense culture (the liquid contains no cells). The sparse culture begins to glow. Explain this result.
Show the solutionHide the solution
- Step 1: Identify the mechanism: quorum sensing, in which bacteria release a signal molecule into their surroundings.
- Step 2: In a sparse culture, there are too few cells to make the signal concentrated enough to switch on the glowing genes.
- Step 3: The filtered liquid from the dense culture contains a high concentration of signal molecules, even with no cells.
- Step 4: Adding it raises the signal concentration around the sparse bacteria above the threshold, so their receptors are activated and the glowing genes switch on.
Answer: The filtered liquid contains the quorum-sensing signal at high concentration. It reaches the threshold that triggers gene expression for glowing, even though the sparse bacteria couldn't make that much signal themselves.
Common mistakes
- Saying every cell responds to a hormone in the blood. Only target cells with the matching receptor respond.
- Calling neurotransmitters hormones. Neurotransmitters act locally across a synapse; hormones travel long distances.
- Forgetting that bacteria communicate too. Quorum sensing is chemical signaling between single-celled organisms.
On the exam
- Expect scenarios where you identify the type of signaling (contact, local or long-distance) and justify it with the distance the signal travels.
- Questions often ask why a signal affects one cell type and not another. The answer is the presence of a specific receptor.
Connected topics
Videos
Check yourself
4 questions on 4.1 Cell Communication. Pick an answer to see if you got it, and why.
A gland at the base of the brain releases a signal molecule that travels through the blood to cells in the kidney, where it changes how much water the kidney reabsorbs. This is an example of which kind of signaling?
Neighboring plant cells are connected by channels called plasmodesmata that pass through their cell walls. A small signal molecule made in one cell can reach the next cell without crossing a plasma membrane. This is best described as
A helper T cell becomes activated only when a receptor on its surface binds a molecule displayed on the surface of another immune cell that it is touching. If the two cells are kept apart by a thin membrane that lets dissolved molecules through, the T cell is not activated. Which kind of signaling does this best show?
In a developing limb, a small group of cells on one edge releases a signal molecule that diffuses through the tissue. Cells close to the source develop into one type of digit, and cells farther away develop into other types. When a bead soaked in the signal is placed on the opposite edge of the limb, extra digits form in a mirror-image pattern. Which conclusion is best supported?
0 of 4 answered