4.1Cell Communication and Cell Cycle
Cell Communication — AP BIOLOGY Revision Notes & Topical Questions
5 min read 13 parts Lesson 1 of 6 4 quizzes
Lesson summary
4.1 Cell Communication — AP Biology notes: Describe how cells communicate by direct contact, local signalling and long-distance signalling using ligands.Coordination — cells in tissues, organs and systems must communicate to function effectively.Ligands — cells communicate using …
- Three scales
- Juxtacrine signalling
- Immune cells
- Gap junctions
- Plasmodesmata
- Cell–cell recognition
Section 1 of 13Why cells communicate
Coordination
- Coordination cells in tissues, organs and systems must communicate to function effectively.
Ligands
- Ligands cells communicate using chemical signalling molecules called ligands.
Receptors
- Receptors a ligand binds specifically to a receptor protein with a complementary shape, so only target cells with the right receptor respond.
Three scales
- Three scales communication can be by direct contact, over short distances or over long distances.
Section 2 of 13Direct contact (juxtacrine) signalling
Juxtacrine signalling
Juxtacrine signalling communication between cells that are in direct contact.
Immune cells
- T helper and T killer cells bind to complementary antigens displayed on antigen-presenting cells.
- Antigen-presenting cells can be phagocytes that engulfed pathogens, the pathogens themselves or infected body cells.
- Binding activates the immune cell and changes its activity, e.g. differentiation into memory cells.
Gap junctions
- Gap junctions animal cells pass signals through these channels between neighbouring cells.
Plasmodesmata
- Plasmodesmata cytoplasmic bridges in plant cells (e.g. in phloem) that let ligands and other materials move directly between neighbours.
Cell–cell recognition
- Cell–cell recognition immune cells recognise antigen-presenting cells by contact, and memory cells let a second infection be met more quickly
Gap junction example
- Gap junction example ions pass between cardiac cells through gap junctions
Section 3 of 13Local (paracrine) signalling
Local regulators
- Local regulators ligands that diffuse only through the spaces in the immediate area and affect cells close to their source.
Neurotransmitters
- Neurotransmitters released at synapses, they diffuse across the gap, bind receptors on the next neurone and continue the impulse.
Quorum sensing
- Quorum sensing bacteria release signalling molecules and use their concentration to judge population density and change their activities.
- Yeast mating factors (a and alpha) show signalling in single-celled eukaryotes.
Morphogens
- Morphogens signalling molecules that give cells information in early development.
- In the embryo the spinal cord releases Shh, whose concentration falls with distance and decides which neurone types form.
Plant immune responses
- Plant immune responses pathogen chemicals are detected by surface receptors, trigger a cascade in the cell,
- and signals are passed to neighbours to warn them.
Quorum sensing
- Quorum sensing bacteria release signals constantly and detect their level as a measure of population density,
- so a higher level means a larger local population
Section 4 of 13Long-distance (endocrine) signalling
1Hormones
Hormones ligands that travel a long way (in the blood or plant vascular tissue) and bind specific receptors on target cells.
2Insulin
- Insulin released by the pancreas when blood glucose rises, it travels in the blood to the liver and increases glucose uptake.
3Human growth hormone
- Human growth hormone released by the pituitary gland; regulates growth and metabolism.
4Thyroid hormones
- Thyroid hormones released by the thyroid; regulate metabolism, body temperature, bone health and digestion.
5Testosterone
- Testosterone released by testes or ovaries;
- regulates male secondary sexual characteristics and bone and muscle health.
6Estrogen
- Estrogen released by ovaries or testes;
- regulates female secondary sexual characteristics, the menstrual cycle and bone health.
7Sex hormones
- Sex hormones they regulate development of male and female secondary sexual characteristics
8Specificity
- Specificity a hormone carried throughout the blood affects only tissues whose cells have the complementary receptor
Section 5 of 13Stages of endocrine signalling
1
A stimulus causes an endocrine cell to secrete a hormone.
2
The hormone enters the blood and is carried around the body.
3
The hormone reaches cells throughout the body.
4
Only target cells with complementary receptors bind the hormone.
5
Binding triggers a response in the target cells.
Section 6 of 13Comparing types of signalling
| Type | Distance | Signal | Example |
|---|---|---|---|
| Direct contact (juxtacrine) | Touching | Membrane proteins, gap junctions, plasmodesmata | Immune cell recognition |
| Paracrine | Short | Local regulators | Growth factors |
| Synaptic | Very short | Neurotransmitter | Neurone to neurone or muscle |
| Endocrine | Long | Hormones in blood | Insulin |
| Autocrine | Self | Local signal acting on the same cell | Some cancer cells |
Section 7 of 13Hormones and neurotransmitters
Peptide hormones
- Peptide hormones insulin, glucagon and ADH are water-soluble, cannot cross the bilayer, bind surface receptors and act through second messengers
Steroid hormones
- Steroid hormones testosterone and oestrogen are lipid-soluble, cross the bilayer and bind cytoplasmic or nuclear receptors, acting directly
Neurotransmitter
- Neurotransmitter it is released by exocytosis at a synapse, is not a hormone and is not carried in the blood
Hormone responses
- Hormone responses they are not instantaneous
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Message | Electrical impulse | Hormone |
| Transmission | Along neurones | In the bloodstream |
| Receiver | Effectors: muscles and glands | Target cells with complementary receptors |
| Speed | Very fast | Slower, not instantaneous |
| Duration | Short | Longer |
Section 8 of 13Transmission at a cholinergic synapse
1
Ca²⁺ enters through voltage-gated channels in the presynaptic membrane.
2
Acetylcholine is released by exocytosis into the cleft.
3
Acetylcholine binds receptors on the postsynaptic membrane.
4
Na⁺ channels open and Na⁺ enters.
5
The postsynaptic membrane is depolarised.
6
Acetylcholinesterase splits acetylcholine into acetate and choline, which stops the response.
Without the enzyme
- Without the enzyme the postsynaptic membrane would be permanently depolarised
Neuromuscular junction
- Neuromuscular junction the same mechanism, with the sarcolemma of a muscle fibre as the postsynaptic membrane
Section 9 of 13Receptors as transducers
Transducer
- Transducer a receptor that converts stimulus energy into an electrical impulse
Receptor potential
- Receptor potential the increased positive charge inside the receptor cell while positive ions enter through opened channels
Chemoreceptor
- Na⁺ enters through channels in the microvilli and depolarises the cell, voltage-gated Ca²⁺ channels open and neurotransmitter is released by exocytosis, starting an impulse in the next neurone
Threshold
- Threshold a stimulus must reach a threshold before a response follows
Venus flytrap
- Ca²⁺ channels open at a touched sensory hair, and the trap closes only if two hairs are touched, or one hair twice within about 30 seconds,
- so energy is not wasted on stimuli that are not prey
Section 10 of 13Plant chemical messengers
Auxin (IAA)
- Auxin (IAA) it binds a membrane receptor, proton pumps move H⁺ into the wall and acidify it, expansins loosen cellulose microfibril bonds,
- and water enters by osmosis so the cell elongates
Gibberellin
- Gibberellin it is made by the embryo and reaches the aleurone layer, where it switches on transcription of the amylase gene
1
The seed absorbs water.
2
The embryo makes gibberellin.
3
Gibberellin reaches the aleurone layer.
4
The amylase gene is transcribed.
5
Amylase hydrolyses starch to maltose.
6
The sugar reaches the embryo and is respired for growth.
Section 11 of 13More from the specification
1Extracellular signaling regulating nuclear genes
- Extracellular signaling regulating nuclear genes surface receptors relay extracellular chemical messages across membranes, altering transcription of developmental genes inside the nucleus.
2Adenylyl cyclase
- The enzyme that converts ATP to cyclic AMP.
3Transduction
- Transfer of DNA between bacteria by bacteriophages
4Feedback
- A change in a system that either amplifies or reduces the original change.
5Signal transduction
- Converting and relaying a signal inside the cell to a response.
6Negative feedback
- A mechanism in which a change from the set point triggers a response that returns the factor to the set point.
Section 12 of 13Key terms
| Term | Meaning |
|---|---|
| Ligand | a signalling molecule that binds a specific receptor |
| Receptor | a protein that binds a specific ligand |
| Juxtacrine | signalling by direct cell contact |
| Paracrine | local signalling to nearby cells |
| Endocrine | hormones carried in the blood to distant cells |
| Quorum sensing | bacterial signalling that reports population density |
| Hormone | Chemical carried in the blood from an endocrine gland to target cells. |
| Neurotransmitter | Chemical released by exocytosis at a synapse. |
| Morphogen | Signalling molecule that gives information to cells in early development. |
| Acetylcholinesterase | Enzyme that breaks down acetylcholine at the synapse. |
Section 13 of 13Exam technique
1
Saying all signals are hormones — only long-distance ligands carried in the blood are hormones; local regulators act on nearby cells.
2
Thinking any cell responds to any ligand — only cells with the complementary receptor respond.
3
Confusing plasmodesmata (plant cells) with gap junctions (animal cells).
4
Thinking neurotransmitters are hormones — they act at synapses and are not carried in blood.
5
Saying hormonal responses are instantaneous — they are slower and longer-lasting than nervous responses.
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