4.03 Membrane Proteins
1. Types of Membrane Proteins
- Integral (Intrinsic) Proteins:
- Embedded within the phospholipid bilayer, often spanning across the entire membrane (known as transmembrane proteins).
- Have both hydrophobic and hydrophilic regions, which allows them to interact with the bilayer’s hydrophobic tails and hydrophilic heads.
- Peripheral (Extrinsic) Proteins:
- Attached to the outer or inner surface of the membrane, not embedded within the bilayer.
- Often associated with integral proteins or the phospholipid heads through ionic bonds or hydrogen bonding.
2. Functions of Membrane Proteins
- Transport:
- Channel Proteins:
- Form hydrophilic pathways that allow specific ions and molecules to pass through, following concentration gradients (e.g., aquaporins for water).
- Carrier Proteins:
- Bind to specific substances and change shape to transport them across the membrane.
- Can facilitate active transport or facilitated diffusion.
- Channel Proteins:
- Enzymatic Activity:
- Some membrane proteins act as enzymes that catalyse reactions directly at the membrane surface, such as in cellular respiration or photosynthesis.
- Signal Transduction:
- Receptor Proteins:
- Bind specific signalling molecules (ligands) and initiate cellular responses.
- Common in cell communication, as seen with hormone or neurotransmitter receptors.
- Receptor Proteins:
- Cell Recognition:
- Glycoproteins:
- Proteins with attached carbohydrate chains that function in cell-cell recognition
- Allows cells to identify each other (e.g., immune response recognition).
- Glycoproteins:
- Cell Adhesion:
- Some proteins help bind cells together to form tissues and create stable structures by connecting with the cytoskeleton and extracellular matrix.
- Anchoring:
- Anchoring Proteins:
- Provide stability by linking the membrane to the cell’s cytoskeleton, or to extracellular structures.
- Anchoring Proteins:
3. Membrane Protein Structure
- Hydrophobic and Hydrophilic Regions:
- Membrane proteins have regions that interact with both the hydrophobic core of the bilayer and the aqueous environments on either side.
- Alpha-Helices and Beta-Barrels:
- Alpha-Helices: Common structural elements in transmembrane proteins, with hydrophobic side chains facing outward to interact with the lipid bilayer.
- Beta-Barrels: Form pore-like structures, often seen in porins which allow molecule passage in bacteria and mitochondria.
4. Role in Selective Permeability and Cellular Communication
- Membrane proteins contribute to the selective permeability of the cell membrane by controlling the passage of specific ions, nutrients, and waste products.
- Communication:
- Through receptor proteins, cells can receive and respond to extracellular signals, coordinating activities such as growth, metabolism, and immune responses.
5. Key Examples of Membrane Proteins
- Channel proteins that specifically facilitate the transport of water across the cell membrane.
- Sodium-Potassium Pump (Na⁺/K⁺ pump):
- A carrier protein that performs active transport to maintain the electrochemical gradient by pumping 3 Na⁺ ions out and 2 K⁺ ions into the cell, using ATP.
- G Protein-Coupled Receptors (GPCRs):
- A large family of receptors involved in signalling pathways, responding to a variety of external signals like hormones and neurotransmitters.
- Aquaporins:
- Channel proteins that specifically facilitate the transport of water across the cell membrane.
6. Fluidity and Movement:
- Proteins can move laterally within the membrane, resembling “icebergs” floating in the sea.
- Some proteins are fixed while others are mobile.
Proteins in Transport & Signalling
Membrane Proteins:
- Membrane proteins play crucial roles in cellular communication, recognition, and the transport of molecules across the cell membrane.
- Here’s a breakdown of the main types of membrane proteins and their functions:
1. Receptor Proteins
- Function: Receptor proteins bind with specific molecules such as hormones, neurotransmitters, and antibodies, allowing cells to recognize signals and respond to external cues.
- Roles:
- Cell Recognition: Involved in immune responses by recognizing antigens (e.g., antigen-antibody interactions).
- Signal Transduction: Receptor binding can initiate intracellular signaling pathways that alter cellular functions.
- Examples: Hormone receptors (e.g., insulin receptor), neurotransmitter receptors, and immune system receptors for antigen recognition.
2. Channel Proteins
- Function: Channel proteins form passages in the membrane, allowing specific ions or molecules to cross. This movement is typically passive, following the concentration gradient.
- Examples:
- Aquaporins: Specialized channels that permit rapid water transport across the membrane, essential for maintaining water balance in cells.
- Ion Channels: Regulate the flow of ions like sodium, potassium, chloride, and calcium, which helps maintain membrane potential and is crucial for nerve and muscle function.
- Examples: Sodium channels in neurons, potassium channels in muscle cells, aquaporins in kidney cells.
3. Transport Proteins
- Function: Transport proteins actively or passively move molecules and ions across the membrane, essential for nutrient uptake, waste removal, and ion balance.
- Types:
- Carrier Proteins:
- Mechanism: These proteins bind to specific molecules on one side of the membrane, undergo a conformational change, and release the molecule on the other side.
- Example: Glucose carriers that transport glucose into cells for energy.
- Active Transport Pumps:
- Mechanism: Use energy (usually ATP) to move ions or molecules against their concentration gradient, maintaining ionic and molecular balance within the cell.
- Example: The sodium-potassium pump, which moves sodium out of and potassium into the cell, crucial for nerve impulse transmission.
- Cotransporters (Symporters and Antiporters):
- Mechanism: These proteins use the gradient of one ion to move another molecule in the same direction (symport) or opposite direction (antiport).
- Example: Sodium-glucose cotransporter in kidney cells, which uses the sodium gradient to bring glucose into the cell.
- Carrier Proteins:
Summary Table of Membrane Proteins
| Type | Function | Examples |
|---|---|---|
| Receptor Proteins | Cell recognition, signal transduction | Hormone receptors, neurotransmitter receptors, immune system receptors |
| Channel Proteins | Permit specific ions or molecules to passively cross the membrane | Aquaporins, sodium channels, potassium channels |
| Carrier Proteins | Transport molecules by binding and changing shape | Glucose carriers |
| Active Transport Pumps | Use energy to move ions/molecules against gradient | Sodium-potassium pump |
| Cotransporters | Couple the movement of one molecule with another | Sodium-glucose cotransporter |
Practise Questions
Question 1
Differentiate between integral and peripheral membrane proteins in terms of structure and location. (5 marks)
Mark Scheme:
- Integral proteins are embedded within the phospholipid bilayer and often span across the entire membrane (transmembrane proteins). (1 mark)
- They have both hydrophobic and hydrophilic regions, interacting with the bilayer’s tails and heads. (1 mark)
- Peripheral proteins are attached to the surface of the membrane, either on the inner or outer side. (1 mark)
- They are not embedded but are associated with the membrane via ionic bonds or hydrogen bonding to phospholipid heads or integral proteins. (1 mark)
- Integral proteins play roles in transport and signalling, while peripheral proteins assist in cell recognition and adhesion. (1 mark)
Question 2
Explain the roles of channel and carrier proteins in transport across the cell membrane. (6 marks)
Mark Scheme:
- Channel proteins form hydrophilic pathways that allow specific ions and molecules to cross the membrane passively, following their concentration gradient. (1 mark)
- Example: Aquaporins permit rapid water movement across the membrane. (1 mark)
- Ion channels (e.g., sodium or potassium channels) regulate ion flow, critical for nerve and muscle function. (1 mark)
- Carrier proteins bind specific molecules and undergo a conformational change to transport them across the membrane. (1 mark)
- Carrier proteins can facilitate both active transport (requiring ATP) and facilitated diffusion (passive). (1 mark)
- Example: Glucose carriers transport glucose into cells for energy. (1 mark)
Question 3
Describe the function of receptor proteins in cellular communication. (5 marks)
Mark Scheme:
- Receptor proteins bind to specific molecules, such as hormones or neurotransmitters, to detect external signals. (1 mark)
- Signal transduction: Binding of a signalling molecule initiates an intracellular response, altering cell activity. (1 mark)
- Cell recognition: Receptor proteins identify and respond to antigens, playing a key role in immune responses. (1 mark)
- Example: Insulin receptors facilitate glucose uptake in response to insulin binding. (1 mark)
- Example: Neurotransmitter receptors enable nerve cells to transmit signals across synapses. (1 mark)
Question 4
How does the sodium-potassium pump function, and why is it important? (6 marks)
Mark Scheme:
- The sodium-potassium pump is an active transport pump that moves ions against their concentration gradient using ATP. (1 mark)
- It transports 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) into the cell per cycle. (1 mark)
- This process maintains the electrochemical gradient across the membrane. (1 mark)
- The pump is essential for maintaining resting membrane potential in nerve and muscle cells. (1 mark)
- It supports processes like nerve impulse transmission and muscle contraction. (1 mark)
- The energy expenditure by this pump represents a significant portion of a cell’s energy use. (1 mark)
Question 5
What is the role of glycoproteins in cell recognition? (4 marks)
Mark Scheme:
- Glycoproteins are membrane proteins with attached carbohydrate chains. (1 mark)
- They function in cell-cell recognition, enabling cells to identify each other. (1 mark)
- Example: Glycoproteins on immune cells recognize antigens, distinguishing self from non-self. (1 mark)
- They also play a role in tissue formation and the immune response. (1 mark)
Question 6
Describe the structural features of transmembrane proteins and their significance. (6 marks)
Mark Scheme:
- Transmembrane proteins span the entire phospholipid bilayer. (1 mark)
- They have hydrophobic regions that interact with the lipid tails and hydrophilic regions exposed to aqueous environments. (1 mark)
- Alpha-helices are common structural elements that anchor proteins within the membrane. (1 mark)
- Some proteins form beta-barrels, creating pore-like structures for molecule passage. (1 mark)
- These structural features enable transmembrane proteins to perform roles such as transport, signalling, and enzymatic activity. (1 mark)
- Example: Ion channels use their structure to allow specific ions to pass across the membrane. (1 mark)
Question 7
Explain how the fluidity of the cell membrane affects the movement and function of membrane proteins. (5 marks)
Mark Scheme:
- The fluid nature of the membrane allows proteins to move laterally within the phospholipid bilayer. (1 mark)
- This movement supports functions like cell signalling and transport, ensuring proteins can interact with other molecules. (1 mark)
- Proteins can cluster in specific regions to perform specialized tasks. (1 mark)
- Some proteins are anchored to the cytoskeleton, limiting their movement and stabilizing the membrane structure. (1 mark)
- The dynamic nature of the membrane is essential for processes like endocytosis, exocytosis, and receptor activation. (1 mark)
Question 8
What is the role of aquaporins, and why are they essential for cellular function? (5 marks)
Mark Scheme:
- Aquaporins are channel proteins that facilitate the rapid transport of water across the cell membrane. (1 mark)
- They are essential for maintaining water balance in cells, particularly in osmotically active tissues like kidneys. (1 mark)
- By allowing water to pass through, aquaporins help regulate cell volume and prevent damage due to osmotic pressure. (1 mark)
- They ensure efficient water movement during processes like reabsorption in the kidneys. (1 mark)
- Without aquaporins, water transport would be too slow to meet cellular needs. (1 mark)
Question 9
Compare the roles of channel proteins and carrier proteins in membrane transport. (6 marks)
Mark Scheme:
- Channel proteins form hydrophilic pathways, allowing specific ions or molecules to move passively along their concentration gradient. (1 mark)
- Example: Ion channels regulate the flow of sodium, potassium, and calcium. (1 mark)
- Carrier proteins bind specific molecules, change shape, and transport them across the membrane. (1 mark)
- Carrier proteins can facilitate active transport (e.g., sodium-potassium pump) or facilitated diffusion. (1 mark)
- Channel proteins enable rapid transport, while carrier proteins are slower but highly specific. (1 mark)
- Both are critical for maintaining homeostasis by regulating the exchange of molecules and ions. (1 mark)
Question 10
Explain how membrane proteins contribute to the selective permeability of the cell membrane. (5 marks)
Mark Scheme:
- Membrane proteins regulate the movement of specific molecules that cannot pass freely through the phospholipid bilayer. (1 mark)
- Channel proteins allow small ions and polar molecules to move along their concentration gradient. (1 mark)
- Carrier proteins transport larger molecules (e.g., glucose) and ions, either passively or via active transport. (1 mark)
- Receptor proteins detect signalling molecules, triggering intracellular responses that regulate permeability. (1 mark)
- These proteins ensure that essential nutrients enter, waste products exit, and the cell maintains its internal environment. (1 mark)
Exam Style Questions
Quiz 1
1. Which of the following best describes integral proteins?
2. What is the main role of channel proteins in the cell membrane?
3. Which statement best explains the function of receptor proteins in the cell membrane?
4. What is the primary function of glycoproteins in the cell membrane?
5. Which of the following statements about the sodium-potassium pump is correct?
Correct Answers: 0%
Quiz 2
1. What characteristic allows integral proteins to interact with both the hydrophobic core of the membrane and the aqueous environment?
2. Which membrane protein structure is commonly associated with forming pore-like channels in bacterial and mitochondrial membranes?
3. How do carrier proteins differ from channel proteins in terms of transport?
4. Which of the following is a key function of anchoring proteins in the cell membrane?
5. Which of the following best describes how G Protein-Coupled Receptors (GPCRs) function in cellular signaling?
Correct Answers: 0%