Diffusion, facilitated diffusion and osmosis
Many molecules pass across the plasma membrane. Water, oxygen, carbon dioxide, excretory products, nutrients and ions are continuously exchanged and many cells also secrete products such as hormones and enzymes through the membrane.
The simplest way in which a molecule could move into or out of a cell is by diff usion. No energy is required, and movement occurs by way of a simple concentration gradient. For example, as carbon dioxide concentration builds up in cells because of respiratory activity, it begins to diff use through the plasma membrane to an area where the concentration is lower. Diff usion occurs where the membrane is fully permeable to the substance or where protein channels in the membrane are large enough for it to pass through.
In cases where molecules are large, or where charged particles such as chloride ions (Cl–) must pass, simple diff usion is impossible. These substances are often transported across membranes by facilitated diff usion. Here an integral protein in the membrane forms a channel so that the substance particles can pass through them into or out of the cell (Figure 2.15). Some of these channels are permanently open whereas others can open and close to control the movement of the substance.
Furthermore, they are specifi c – that is, they only allow a particular substance to pass through. As in simple diff usion, no energy is used by the cell. In both cases, the transport relies on the kinetic energy of the particles moving down their concentration gradient.
Passive transport the movement of substances down a concentration gradient from an area of high concentration to an area of lower concentration without the need for energy to be used
Diff usion one example of passive transport; many molecules pass into and out of cells by diff usion e.g.
oxygen, carbon dioxide and glucose Osmosis another example of passive transport but the term is only used in the context of water molecules; osmosis is the movement of water molecules across a partially permeable membrane from a region of lower solute concentration, where there is a high concentration of water molecules, to a region of higher solute concentration, where the concentration of water molecules is lower
Active transport the movement of substances against the
concentration gradient, which always involves the expenditure of energy in the form of ATP
2 CELLS 31
chloride ion channel
phospholipid bilayer
hydrophilic parts of the channel protein Figure 2.15 Facilitated diffusion.
Figure 2.16 Osmosis.
a
A B A B
partially permeable b membrane
Before osmosis At equilibrium
solute molecule water molecule Two solutions are separated by a partially permeable
membrane. B has a higher solute concentration than A. The solute molecules are too large to pass through the pores in the membrane but the water molecules are small enough.
As the arrows in diagram a indicate, more water molecules moved from A to B than from B to A, so the net movement has been from A to B, raising the level to the solution in B and lowering it in A. The solute concentrations in A and B are now equal.
Two solutions are separated by a partially permeable membrane. B has a higher solute concentration than A. The soluble molecules are too large to pass through the pores in the membrane but the water molecules are small enough.
As the arrows in diagram a indicate, more water molecules moved from A to B than from B to A, so the net movement has been from A to B, raising the level of the solution in B and lowering it in A. The solute concentrations in A and B are now equal.
A special case of diff usion is osmosis (Figure 2.16). This is the passive movement of water across a partially permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
Active transport
Many of the substances a cell needs occur in low concentrations in the surroundings outside the plasma membrane. Plants must take in nitrate ions from very dilute solutions in the soil to build their proteins, and muscle cells actively take in calcium ions to enable them to contract. To move these substances into the cell against a concentration gradient, the cell must use metabolic energy released from the breakdown of ATP. This is called active transport (Figure 2.17). Specifi c proteins in the plasma membrane act as transporters or ‘carriers’ to move substances through.
Many of the carrier proteins are specifi c to particular molecules or ions so that these can be selected for transport into the cell.
Figure 2.17 Active transport of a single substance.
molecules or ions to be transported
carrier protein in lipid bilayer Inside the cell Outside the cell
ATP
carrier protein activated by reaction with ATP
change in shape and position of carrier protein
ADP and Pi released from carrier protein, which reverts to the receptive state
Figure 2.18 illustrates a very important example of active transport.
The sodium–potassium pump maintains the concentration of sodium and potassium ions in the cells and extracellular fl uid. Cells are able to exchange sodium ions for potassium ions against concentration gradients using energy provided by ATP. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell.
Step 1 potassium ions
Step 1 sodium ions Step 3
potassium ions
Step 3 sodium ions Step 2
sodium ions
Step 2 potassium ions inside
the cell
outside The breakdown of ATP provides energy for the proteins to change shape and to allow the ions to move.
ATP ADP + Pi + energy
Figure 2.18 An example of active transport – the sodium–potassium pump. Start at step 1 for each ion in turn and work round clockwise.
There are two types of endocytosis.
If the substances being taken in are particles, such as bacteria, the process is called phagocytosis. If the substances are in solution, such as the end products of digestion, then it is called pinocytosis.
Exocytosis and endocytosis
Cells often have to transport large chemical molecules or material in bulk across the plasma membrane. Neither diff usion nor active transport will work here. Instead, cells can release or take in such materials in vesicles, as shown in Figure 2.19. Uptake is called endocytosis and export is exocytosis. Both require energy from ATP.
During endocytosis, part of the plasma membrane is pulled inward and surrounds the liquid or solid that is to be moved from the extracellular space into the cell. The material becomes enclosed in a vesicle, which pinches off from the plasma membrane and is drawn into the cell. This is how white blood cells take in bacteria (Figure 2.19).
2 CELLS 33 Materials for export, such as digestive enzymes, are made in the rER and
then transported to the Golgi apparatus to be processed. From here they are enclosed within a membrane-bound package known as a vesicle, and moved to the plasma membrane along microtubules. The vesicles fuse with the plasma membrane and in doing so release their contents to the outside.
The fl exibility and fl uidity of the plasma membrane allow this to happen.
11 Outline the difference between simple diffusion and facilitated diffusion.
12 Suggest why the term ‘fl uid mosaic’ is used to describe membrane structure.
13 Suggest why the fatty acid ‘tails’ of the phospholipid molecules always align themselves in the middle of the membrane.
14 Outline the difference between integral membrane proteins and peripheral membrane proteins.
15 List the six ways that substances move from one side of a membrane to the other.
16 State which of these transport mechanisms require energy from ATP.
17 List the functions of proteins that are found in a membrane.
phagocytic vacuole
lysosomes, containing digestive enzymes, fuse with phagocytic vacuole
bacterium being digested
bacterium engulfed bacterium plasma membrane of white blood cell (phagocyte)
product released
secretory product e.g. enzyme
Golgi apparatus secretory vesicle
undigested remains of bacterium can be removed by exocytosis
Phagocytosis of a bacterium by a white blood cell – an example of endocytosis. Exocytosis in a secretory cell. If the product is a protein, the Golgi apparatus is often involved in chemically modifying the protein before it is secreted, as in the secretion of digestive enzymes by the pancreas.
phagocytic vacuole
lysosomes, containing digestive enzymes, fuse with phagocytic vacuole
bacterium being digested
bacterium engulfed bacterium plasma membrane of white blood cell (phagocyte)
product released
secretory product e.g. enzyme
Golgi apparatus secretory vesicle
undigested remains of bacterium can be removed by exocytosis
Phagocytosis of a bacterium by a white blood cell – an example of endocytosis. Exocytosis in a secretory cell. If the product is a protein, the Golgi apparatus is often involved in chemically modifying the protein before it is secreted, as in the secretion of digestive enzymes by the pancreas.
Phagocytosis of a bacterium by a white blood cell – an example of endocytosis.
Figure 2.19 Examples of endocytosis and exocytosis.
Exocytosis in a secretory cell. If the product is a protein, the Golgi apparatus is often involved in chemically modifying the protein before it is secreted, as in the secretion of digestive enzymes by the pancreas.
2.5 Cell division
Assessment statements
•
Outline the stages in the cell cycle, including interphase (G1, S and G2), mitosis and cytokinesis.•
State that tumours (cancers) are the result of uncontrolled cell division and that these can occur in any organ or tissue.•
State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and chloroplasts (in some plant cells).•
Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase).•
Explain how mitosis produces two genetically identical nuclei.•
State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.New cells are needed to replace cells which have died or to allow an organism to grow. Cells divide by a process known as mitosis, which is one phase of a series of events known as the cell cycle.