Membranes and cell signalling

Cell signalling- communication between cells.

The glycoproteins and glycolipids in the plasma membrane can act as receptors to receive signal molecules and bring about changes on the cytoplasmic side of the membrane.

The signal molecules are often hormones like testosterone, adrenaline or insulin that are released into the bloodstream via exocytosis. The clever thing is that these hormones only affect specific cells (Target Cells); they only bring about changes in those cells that have complementary receptors in their plasma membrane.

Overview of cell signalling:

• Communication between cells
• Signal molecule released from one cell by exocytosis
• Binds / attaches to receptor / glycoprotein
• On plasma membrane of second cell / target cell
• The receptor and signal molecule are specific
• Signal molecule and receptor have complementary shapes
• Bonding of signal molecule to receptor triggers response inside cell

Insulin: Insulin is a signal molecule allowing communication between the pancreas and the muscles tissue. When insulin binds to receptors in the plasma membrane of muscle cells it causes channel proteins to open allowing glucose to be taken into the cell from the bloodstream. The effect of insulin binding to muscle cells is a reduction in blood glucose concentration.

Pain Killers: Some pain killers work by blocking cell signalling between nerve cells, stopping the pain signal reaching the brain. The pain killer molecule binds to channel proteins on the nerve cell preventing them opening as usual. Without an influx of ions, no pain nerve signal is created and so no pain is felt.

 Virus Infection: This mechanism of signal molecules and receptors can be hijacked by viruses trying to enter cells. The virus particle is covered in proteins that are complementary to the cell’s receptors, when they bind it causes the cell to take the virus particle through the membrane into the cell; the first step in infection.        

The lipid bilayer is partially-permeable, allowing only certain molecules to diffuse across the membrane to enter or exit the cell.

Receptor acts an a ion channel- When a chemical signal attaches to the receptor, it makes the ion channel open, allowing ions into the cell.

Receptor activates a G-protein- When this happens, the G-protein then activates an enzyme which brings about a response.

Receptor acts a enzyme- The receptor is made up of 2 parts. The signal molecule attaches to both parts, connecting them together and forming an active enzyme, which bring about reactions in the cell.

• Form compartments / organelles within a cell
• Isolation of contents of organelle / reactions
• Site for attachment of enzymes
• Site for attachment of ribosomes
• Provide selective permeability
• Creation of concentration gradients
• Control what substances enter / leave organelles
• Isolates DNA
• Forms vesicles
• Intracellular transport
• Protects cells from hydrolytic enzymes in lysosomes
• Surrounds vacuole

Roles of cell surface membrane

• Separate cell from environment
• Control entry / exit of molecules
• Phospholipid bilayer allows the passage of non polar / lipid soluble molecules
• Aids facilitated diffusion via channel / carrier proteins
• Aids active transport via carrier proteins
• Endocytosis / Exocytosis
• Cell recognition
• Cell to cell attachment
• Receptors
• Microvilli increase surface area of cell

Phospholipid Bilayer- To act as a barrier to polar / charged particles. To allow the passage of non polar / lipid soluble particles by simple diffusion, e.g. O2. Select what enters or leaves cell

Cholesterol- Stabilise the membrane, Maintain fluidity, Reduces permeability to polar / charged particles

Channel Protiens- Allow the passage of polar / charged substances across the membrane by facilitated diffusion, e.g. Na+  ions

Carrier Protiens- Allow the passage of large substances across the membrane by facilitated diffusion, e.g. glucose. Allow the passage of any substance against the concentration gradient by active transport

Glycoprotein & Glycolipid- Acting as antigens,Recognition of cells as self / non-self, Cell signalling, Receptor / binding site for hormone / chemical signal / drugs, Trigger on transport proteins, Cell adhesion / to hold cells together in a tissue, Attach to water molecules to stabilise the membrane

Nucleus – Controls the activities of the cell.

Mitochondria – Aerobic respiration/ATP production/Release energy

Chloroplast - photosynthesis

RER – Protein synthesis and transport

SER – Lipid synthesis and transport

Golgi apparatus/body – chemically modifies and packages proteins for secretion

Lysosomes – contain hydrolytic enzymes to digest old/worn out organelles/cells

Biological Membranes are very thin (approximately 7 – 10 nm wide).  At this size it is very hard to see the exact structure, even with an electron microscope.  The structure of membranes does vary between different cells and organelles, however the Fluid Mosaic Model proposed by Singer and Nicolson in 1972 is generally accepted as describing the basic arrangement of biological membranes.

The Fluid Mosaic Model states that membranes are composed of a Phospholipid Bilayer with various protein molecules floating around within it.  The 'Fluid' part represents how some parts of the membrane can move around freely, i.e. they are not in a fixed position.  The 'mosaic' part illustrates the 'patchwork' of proteins that is found in the Phospholipid Bilayer and this makes the membrane look like a mosaic when viewed from above.

Phospholipids are the major component of cell membranes which enclose the cytoplasm and other contents of a cell.

Phospholipids form a lipid bilayer in which their hydrophilic ‘heads’ spontaneously arrange themselves to face the aqueous cytosol and the extracellular fluid, while their hydrophobic ‘tails’ face away from the cytosol and extracellular fluid.  

‘Bilayer’ means two layers. The phospholipids are not chemically bound to each other.  This is why the membrane is ‘fluid’.