IB Biology HLChemical SignallingPaper 1 & 2~11 min read
Receiving the Signal
There are only two ways for a message to get into a cell. Either the messenger comes inside, or it stays outside and knocks. Which one happens is decided entirely by whether the molecule can survive the oily middle of the membrane.
📚 What you need to know
Transmembrane receptors sit in the cell membrane with an external binding site and an internal region extending into the cytoplasm.
They have hydrophilic regions at either end, in contact with the aqueous solution inside and outside, and a hydrophobic region in contact with the phospholipid tails.
Intracellular receptors are used by non-polar, hydrophobic ligands such as steroid hormones, which diffuse through the bilayer.
These ligands bind to receptors in the cytoplasm or on the DNA in the nucleus.
Binding to either receptor type starts a cascade of events called the signal transduction pathway.
Transmembrane binding causes a change in shape of the internal region; intracellular binding forms a ligand–receptor complex.
Transmembrane pathways involve phosphorylation events and a second messenger.
Cellular responses include changes to gene expression, metabolic activity, enzyme activity, membrane proteins, cytoplasm rearrangement and cell death.
Transmembrane receptor proteins
Receptors located in the cell membrane have an external binding site and an internal region that extends into the cytoplasm. They are called transmembrane proteins because they stretch right across the width of the membrane.
That position creates a problem the protein has to solve. The two ends sit in water; the middle sits in fat. A single protein has to be comfortable in both. It manages this with different amino acid regions:
Hydrophilic amino acid regions at either end, in contact with the aqueous solution inside and outside the cell.
A hydrophobic amino acid region within the membrane, in contact with the hydrophobic tails of the phospholipids.
Some ligands bind to these receptors instead of entering the cell cytoplasm at all.
The internal region is the important bit. When the ligand binds outside, this region changes shape, and that shape change is the start of the signal inside the cell.
Intracellular receptors
Non-polar, hydrophobic ligands, such as steroid hormones, can diffuse through the phospholipid bilayer. They do not need a doorway.
These ligands bind to receptors in the cytoplasm, or on the DNA in the nucleus.
A steroid hormone such as oestradiol binds to the receptor molecule and activates it, so that protein synthesis is initiated.
Why steroids get in and proteins do not. The centre of the membrane is made of hydrophobic fatty acid tails. A hydrophobic steroid dissolves into that layer happily and slides out the other side. A hydrophilic protein hormone is repelled by it and is stuck outside — which is exactly why transmembrane receptors exist.
Signal transduction pathways
When a ligand binds to either kind of receptor, a cascade of events follows that leads to a change in cell activity. That sequence of events is the signal transduction pathway.
Different ligands and different receptors trigger different pathways, but all of them follow the same three-part shape.
Read both columns downwards. The only real difference is where the binding happens and how many steps it takes to reach the response.
Binding with the receptor
Receptor type
What happens on binding
Transmembrane receptor
The ligand binds to the extracellular region, causing a change in shape of the internal region of the protein
Intracellular receptor
The ligand binds to the receptor inside the cell, forming a ligand–receptor complex
Signal transduction
Receptor type
How the signal is passed on
Transmembrane receptor
Initiates a multistep pathway involving phosphorylation events and a second messenger
Intracellular receptor
The activated ligand–receptor complex follows its own signal transduction pathway
A second messenger is just a small molecule inside the cell that carries on the message the ligand started outside. The ligand is the first messenger; it never enters. Getting that distinction clear now will make the epinephrine and cAMP page much easier.
Cellular responses
A signal transduction pathway can end in any of these:
Regulation of gene expression, through control of transcription or translation
Change in metabolic activity
Regulation of enzyme activity
Cell death
Rearrangement of the cytoplasm of the cell
Regulation of proteins, for example channels in the plasma membrane
🔑
Three words, in order
Bind → transduce → respond. If you can name what does the binding, what carries the message, and what changes at the end, you can answer almost any question in this topic — including ones about pathways you have never seen.
Worked examples
WE 1
Describe the structure of a transmembrane receptor
Describe how the structure of a transmembrane receptor protein suits its position in the cell membrane. (3 marks)
Point 1: the two ends
It has hydrophilic amino acid regions at either end, which are in contact with the aqueous solution inside and outside the cell.
Point 2: the middle
It has a hydrophobic amino acid region within the membrane, in contact with the hydrophobic tails of the phospholipids.
Point 3: the function
The external binding site holds the ligand, while the internal region extends into the cytoplasm and passes the signal on.
Water-loving ends, fat-loving middle, binding site outside, message insidesay “amino acid regions” — the marks are about the protein’s own structure
WE 2
Compare the two receptor types
Compare and contrast transmembrane and intracellular receptors. (4 marks)
Point 1: location
Transmembrane receptors span the cell membrane; intracellular receptors are in the cytoplasm or nucleus.
Point 2: type of ligand
Transmembrane receptors are used by hydrophilic ligands that cannot cross the bilayer; intracellular receptors are used by hydrophobic ligands such as steroid hormones that can.
Point 3: what binding does
Transmembrane binding causes a shape change in the internal region; intracellular binding forms a ligand–receptor complex.
Point 4: what is shared
Both bind ligands with a complementary shape and both begin a signal transduction pathway ending in a cellular response.
Different doors, different keys, same destination“compare and contrast” needs similarities as well as differences — give at least one of each
WE 3
Explain the role of transduction
Explain why a signal transduction pathway is needed when a hydrophilic ligand binds to a cell. (3 marks)
Point 1: the barrier
A hydrophilic ligand cannot cross the phospholipid bilayer, so it stays outside the cell.
Point 2: the conversion
Binding changes the shape of the receptor’s internal region, converting the external signal into an internal one — this is transduction.
Point 3: relaying it
The message is then carried onwards by phosphorylation events and second messengers until a response, such as a change in enzyme activity or gene expression, is produced.
The message gets in even though the messenger does notthat last sentence is worth memorising — it is the whole point of transduction
💡 Exam tips
Use the exact terms external binding site and internal region.
Describe the receptor’s regions as hydrophilic and hydrophobic amino acid regions.
Say a transmembrane ligand causes a change in shape, and an intracellular ligand forms a ligand–receptor complex.
Mention phosphorylation and second messenger for transmembrane pathways.
Learn at least four cellular responses from the list.
If a question gives you an unfamiliar pathway, still structure your answer as bind, transduce, respond.
⚠ Common mistakes
Saying the ligand enters the cell through a transmembrane receptor. It binds on the outside; the receptor is not a tunnel.
Confusing first and second messengers. The ligand is the first messenger and stays outside.
Saying steroid hormones need a transport protein. They diffuse straight through.
Describing transduction as “the signal moves”. It is converted into a different signal.
Listing only gene expression as a response. Enzyme activity, metabolic activity and cell death all count.
Saying all pathways are identical. Different ligands and receptors trigger different pathways with the same overall shape.
Up next: Transmembrane Receptors. Time to open up the surface receptors properly — ligand-gated ion channels at a synapse, G-protein-coupled receptors, and the tyrosine kinase receptor that lets insulin do its job.
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