- Explain the dynamics of the transport of substances through the cell membrane including facilitated diffusion.
- Design and carry out an investigation to examine the movement of substances across a membrane.
The conditions inside a cell must remain nearly constant for it to continue performing its life functions. The steady state that results from maintaining these conditions is called homeostasis. The structure mainly responsible for this is the cell membrane.
The cell membrane is semi-permeable allowing some materials to pass in or out of the cell through the membrane. Water is the solvent inside and outside the cell. The external environment of a single-celled organism is mainly water with other organisms, decaying organic matter, dissolved gases, and other inorganic substances. In a multicellular organism, every cell is bathed in a thin layer of extracellular fluid, which consists of water and dissolved materials. Some are substances the cells need and others are waste products that the organism will eventually get rid of.
Diffusion is a passive method of transport of materials through the cell membrane. Molecules move from a region of high concentration to a region of low concentration (concentration gradient) and many molecules, especially small uncharged ones, can move easily through the cell membrane because of this process.
Molecules are in constant motion; in a liquid this means the molecules move around randomly and collide with one another and with the walls of their container causing them to rebound. This constant random movement is called Brownian motion and it drives the process of diffusion.
If molecules of another substance are added to water they will be bounced around by the water molecules and each other until the new substance is mixed throughout the water. The water is acting as a solvent and the other substance is a solute.
Over short distances diffusion works well to transport molecules across the cell membrane. For example oxygen and carbon dioxide cross the cell membrane by means of diffusion. As the cell uses up more oxygen the concentration inside the cell is less than outside so oxygen moves in. The cell also produces carbon dioxide, increasing the concentration inside the cell and carbon dioxide moves out.
Diffusion Limits Cell SizeEdit
Once molecules have diffused through the cell membrane their rate of diffusion slows down. If a cell has a large surface area relative to its volume it has more area available for materials to diffuse in and out.
Osmosis: The Diffusion of WaterEdit
Water inside and outside the cell also diffuses across the cell membrane so the concentration of water on both sides of the membrane is usually equal. The diffusion of solvent across a semi-permeable membrane separating two solutions is called osmosis.
The movement of water across the cell membrane depends on the relative concentration of water inside and outside the cell.
- Isotonic: The water concentration is equal on both sides of the cell membrane
- Hypertonic: The water concentration inside the cell is greater than outside and water diffuses out of the cell
- Hypotonic: The water concentration inside the cell is less than outside and water diffuses into the cell
The cell membrane cannot prevent the movement of water across it because it is permeable to water molecules. The only energy involved in this process is the Brownian motion of the water molecules.
The cell can only remain healthy, however, if the water concentrations inside and outside the cell remain balanced. Blood plasma and the extracellular fluid are usually isotonic. In hypotonic conditions the cell may burst; this is called lysis. In hypertonic conditions water diffuses out of the cell causing plasmolysis. A higher concentration of solutes in the extracellular fluid can cause plasmolysis.
Some substances cannot pass through the cell membrane without help, which makes the membrane a selectively permeable membrane. For example, molecules of glucose are too large to pass through the membrane and are insoluble in lipids. So specialized transport proteins embedded in the cell membrane will recognize a certain kind of dissolved molecule or ion based on its size, shape, and electrical charge and help move it across the membrane. In the case of glucose a carrier protein helps move glucose from a region of high concentration to one of low concentration. Carrier proteins will only accept a non-charged molecule of a certain shape and allows the molecule to move in and out of the cell. This is an example of facilitated diffusion another passive transport process.
Channel proteins are tunnel-shaped proteins that allow charged molecules to pass through the cell membrane. An ion must be small enough to fit through the channel protein and also have the opposite charge. No cellular energy is required to move the substance into or out of the cell.
In some cases a cell must move substances against their concentration gradient such as gathering nutrients inside the cell or completely removing toxic waste substances from the cell. Passive transport would not be effective for this task so the cell must use active transport which is a process that requires cellular energy to transport substances across the cell membrane. The greater the concentration gradient the cell is moving substances against the more energy it takes.
When a person is resting, most of their cells use up to 40% of their energy on active transport, but some specialized cells use more. Kidney cells that filter blood use up to 90% of their energy on active transport.
Some examples of active transport are:
- Kidney cells pump glucose and amino acids out of the urine and back into the blood
- Intestinal cells pump nutreints from the gut
- Root cells pump in nutrients from the soil
- Gill cells in fish pump out sodium ions (their extracellular fluid is less salty than sea water)
Active Transport PumpEdit
A transport pump is a membrane protein which actively pumps ions across the cell membrane against their concentration gradients. Cells have several different transporter pumps. The best understood example of an active transport pump is the sodium-potassium pump in animal cells. The cell membrane of every cell in the body uses these pumps.
When three (positive) sodium ions inside the cell and two (positive) potassium ions from the extracellular fluid bind to the transporter protein's complex, the transporter taps a form of cellular energy, ATP, allowing the protein to change its shape. In its new shape the three sodium ions move to the outside of the cell and the two potassium ions move inside. Then it releases all ions and returns to its original shape.
Tapping the Energy Stored by Active TransportEdit
The cell uses the artificial concentration gradient it has created for sodium ions to push molecules it needs, such as glucose and amino acids, into the cell. The cell cannot function if it only gets as many of these molecules as diffusion will allow into the cell so the cell must move extra glucose and amino acids in against the concentration gradient.
A type of carrier membrane protein helps the sodium ion and a molecule (such as glucose) enter the cell. When one sodium ion and one glucose molecule bind to this carrier protein, it changes shape allowing the sodium ion to move down its concentration gradient into the cell providing the energy to move the glucose molecule as well. Plant and bacterial cells use hydrogen instead of sodium ions to do this.
Another protein in the mebrane also taps the energy stored in the sodium-ion concentration gradient to push another positive ion out of the cell. A common use for this exchange of one ion for another is pumping unwanted hydrogen ions (H+) out of the cell against their concentration gradient. This keeps the interior of the cell from becoming too acidic.
The artificial concentration gradient that active transport creates for sodium and potassium ions result in a constant tendency for potassium ions to diffuse back into it. So the sodium-potassium pump must work constantly. In fact, even when resting it consumes nearly one third of the energy in the cells. This high-energy requirement is thought to be cause by the need for rapid, repeated changes of shape in the transporter protein complex.
Through its active transport system the cell stockpiles nutrients it needs for maintenance and growth and pumps out unwanted particles. In addition, it creates an electrical potential across the cell membrane that allows nerves and muscles to work. The higher concentration of positive ions outside the cell creates and electrical charge across the cell membrane.