1.4 – Membranes
1.4.1 – Draw and label a diagram to show the structure of membranes
Phospholipid Bilayer – This is arranged with the hydrophilic phosphate heads facing outwards, and the hydrophobic fatty acid tails (consisting of hydrocarbon chains) facing into the middle of the bilayer. It is a barrier against all molecules except the smallest, CO₂ and
O₂. The phospholipids can change position on the horizontal plane, but not the vertical.
Integral Proteins – These usually span from one side of the phospholipid bilayer to the other. They are usually involved in transporting substances across the membrane.
Peripheral Proteins – These sit on the surfaces. They will slide around the membrane quickly and collide with each other, but will never flip from one side to the other. The ones on the inside of the membrane are often involved in maintaining the cell’s shape or motility. Thesemight also be enzymes, catalyzing reactions in the cytoplasm.
Glycoproteins – These are usually involved in cell recognition which is part of the immune system. They can also act as receptors in cell signaling such as with hormones.
Cholesterol – Binds together lipid in the plasma membrane reducing its fluidity as conferring structural stability.
This is called the fluid mosaic model because it is in a fluid state, and in electron micrographs of the membrane, the proteins form a mosaic pattern.
1.4.2 – Explain how the hydrophobic and hydrophilic properties of phospholipids help to
maintain the structure of cell membranes
The structure of the phospholipid bilayer is very stable, as the hydrophobic hydrocarbon tails are attracted to each other, and the hydrophilic phosphate heads are also attracted to each other. This attraction makes the barrier strong and stable.
The heads are suited to the high water content of the tissue fluid and cytoplasm on either side of the membrane. The tails repel water, creating a barrier between the internal and external water environments of the cell and a barrier to movement of charged molecules.
The charges on the phospholipids attract them to each other, making them fairly stable, though allowing for some movement. The presence of cholesterol molecules increases the stability of the phospholipid.
1.4.3 – List the functions of membrane proteins
Channel Protein – They span the membrane, allowing movement of large molecules across it. Within these are passive and active membrane pumps. They only allow specific ions through.
Receptor Protein – These detect hormones arriving at cells to signal changes in function.
They are also involved in other cell and substance recognition as in the immune system.
Enzymes – Integral proteins in the membrane may be enzymes (i.e. ATP Synthetase, Maltase)
Electron Carriers – These are a chain of peripheral and integral proteins that allow electrons to pass across the membrane. Active pumps use ATP to move specific substances across the membrane.
1.4.4 – Define diffusion and osmosis
Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration.
Diffusion through a cell membrane will occur if the membrane is fully permeable to the solute. In the case of the phospholipid
bilayer, it is permeable to non-polar substances, such as steroids and glycerol, as well as oxygen and carbon dioxide. They will diffuse
quickly via this route. It will also occur if the pores in the membrane are large enough for the solute to enter. Water diffusing through the plasma membrane passes via the protein-lined pores of the membrane, and tiny spaces between the phospholipid molecules.
This will occur more easily if the membrane contains phospholipids with unsaturated hydrocarbon tails, as they are spaced apart more widely. As a result, the membrane is especially leaky to water.
Osmosis is the passive movement of water molecules, across a partially permeable
membrane, from a region of lower solute concentration to a region of higher solute
The water moves through plasma membrane pores called aquaporin. When a solution is separated from water by a membrane permeable to water molecule, water molecules tend to diffuse, while dissolved molecules and their group of water molecules move less.
Osmosis can also be defined as the net movement of water molecules from a region of high concentration of water molecules to a region of lower concentration of lower concentration of water molecules, across a selectively permeable membrane.
1.4.5 – Explain the passive transport across membrane by simple diffusion and facilitated
Passive movement means that no energy (ATP) is used for the movement of molecules from one side of the membrane to the other.
Simple diffusion – The molecules are so small they can simply pass through the phospholipid molecules of the membrane, as it offers little resistance. Examples include O2 and CO2, as well as lipid molecules, even though they are large.
Facilitated diffusion – For larger molecules, there are channel proteins to take the through the membrane. These have complex shapes, which provide a channel through the protein, or the pore. It acts as a shield against the non-charged regions of the membrane for the molecule. These channels only allow a specific type of substance through, but there is no control over the direction of movement.
1.4.6 – Explain the role of protein pumps and ATP in active transport across membranes
In active transport, ATP is used to provide the energy necessary. It is hydrolysed into ADP. Protein pumps use this energy to pump molecules across the cell membrane, moving from a low concentration to a high concentration. They are called active because they need energy to function.
This energy causes the shape of the protein to change, allowing it to move the molecule across the membrane. Active transport is also highly selective, tending to absorb ions that reflect the cells needs. Sodium-Potassium pump creates an electro-chemical gradient across the membrane of all cells. The inside of the cell has a negative charge compared to the outside. In nerve cells, this pump is modified to create electrochemical phenomena.
1.4.7 – Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi, apparatus, and the plasma membrane
Cells manufacture molecules to be secreted outside the cell, which are sometimes a complex combination of proteins, carbohydrates and lipids. A gene is coded in the base protein, and the expression of it starts the process.
- A protein that has already been synthesises is present in the rER
- As the protein moves through the rER, it is modified
- At the end of the rER, a vesicle is formed containing the protein
- The vesicle then migrates to the Golgi Apparatus
- Vesicle and Golgi membranes fuse, and the protein enters the lumen of the Golgi. The Golgi further modifies the protein.
- Another vesicle is formed from the Golgi membrane and breaks away. It is transported to the plasma membrane, which fuse together, then secrete the protein. This is exocytosis.
1.4.8 – Describe how the fluidity of the membrane allows it to change shape, break and reform during endocytosis and exocytosis
Exocytosis – The vesicle membrane fuses with the plasma membrane, and its contents are secreted. The vesicle fuses with the plasma membrane, and its contents are expelled.
Endocytosis – A vesicle is formed when the plasma membrane infolds, then breaks off. Part of the membrane is pulled inwards, and a droplet of fluid is enclosed when it is pinched off.
They can then move the contents through the cytoplasm. The continuity of the plasma membrane is not disrupted.