5.3: The Cytoplasmic Membrane
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- Ying Liu
- City College of San Francisco
State the chemical composition and major function of the cytoplasmic membrane in eukaryotic cells.
State the net flow of water when a cell is placed in an isotonic, hypertonic, or hypotonic environment and relate this to the solute concentration.
Define the following means of transport:
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passive diffusion
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osmosis
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active transport
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endocytosis
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phagocytosis
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pinocytosis
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exocytosis
- passive diffusion
- osmosis
- active transport
- endocytosis
- phagocytosis
- pinocytosis
- exocytosis
In addition, it contains glycolipids as well as complex lipids called sterols , such as the cholesterol molecules found in animal cell membranes, that are not found in prokaryotic membranes (except for some mycoplasmas). The sterols make the membrane less permeable to most biological molecules, help to stabilize the membrane, and probably add rigidity to the membranes aiding in the ability of eukaryotic cells lacking a cell wall to resist osmotic lysis. The proteins and glycoproteins in the cytoplasmic membrane are quite diverse and function as:
- channel proteins to form pores for the free transport of small molecules and ions across the membrane
- carrier proteins for facilitated diffusion and active transport of molecules and ions across the membrane
- cell recognition proteins that identifies a particular cell
- receptor proteins that bind specific molecules such as hormones and cytokines
- enzymatic proteins that catalyze specific chemical reactions.
All molecules and atoms possess kinetic energy (energy of motion). If the molecules or atoms are not evenly distributed on both sides of a membrane, the difference in their concentration forms a concentration gradient that represents a form of potential energy (stored energy). The net movement of these particles will therefore be down their concentration gradient - from the area of higher concentration to the area of lower concentration. Diffusion is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy.
A cell can find itself in one of three environments: isotonic, hypertonic, or hypotonic. (The prefixes iso-, hyper-, and hypo- refer to the solute concentration).
- In an isotonic environment (Figure \(\PageIndex{5}\)A) , both the water and solute concentration are the same inside and outside the cell and water goes into and out of the cell at an equal rate.
- If the environment is hypertonic (Figure \(\PageIndex{5}\)B) , the water concentration is greater inside the cell while the solute concentration is higher outside (the interior of the cell is hypotonic to the surrounding hypertonic environment). Water goes out of the cell.
- In an environment that is hypotonic (Figure \(\PageIndex{5}\)C) , the water concentration is greater outside the cell and the solute concentration is higher inside (the interior of the cell is hypertonic to the hypotonic surroundings). Water goes into the cell.
Transport of Substances Across the Membrane by Transport (Carrier) Proteins
For the majority of substances a cell needs for metabolism to cross the cytoplasmic membrane, specific transport proteins (carrier proteins) are required. Transport proteins allow cells to accumulate nutrients from even a scarce environment. Examples of transport proteins include channel proteins, uniporters, symporters, antiporters, and the ATP- powered pumps. These proteins transport specific molecules, related groups of molecules, or ions across membranes through either facilitated diffusion or active transport.
Facilitated diffusion is the transport of substances across a membrane by transport proteins, such as uniporters and channel proteins, along a concentration gradient from an area of higher concentration to lower concentration. Facilitated diffusion is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy.
1. Uniporter: Uniporters are transport proteins that transport a substance from one side of the membrane to the other (Figure \(\PageIndex{6}\)A1 and Figure \(\PageIndex{6}\)A2) . Amino acids, sugars, nucleosides, and other small molecules can be transported through eukaryotic membranes by different uniporters.
2. Channel proteins transport water or certain ions down either a concentration gradient, in the case of water, or an electric potential gradient in the case of certain ions, from an area of higher concentration to lower concentration (Figure \(\PageIndex{6}\)B) . While water molecules can directly cross the membrane by passive diffusion, as mentioned above, their transport can be enhanced by channel proteins called aquaporins.
Active transport is a process whereby the cell uses both transport proteins and metabolic energy to transport substances across the membrane against the concentration gradient. In this way, active transport allows cells to accumulate needed substances even when the concentration is lower outside. The energy is provided by either proton motive force, the hydrolysis of ATP, or by the electric potential (voltage) difference across the membrane.
Proton motive force is an energy gradient resulting from hydrogen ions (protons) moving across the membrane from greater to lesser hydrogen ion concentration. ATP is the form of energy cells most commonly use to do cellular work. Electric potential is the difference in voltage across the cytoplasmic membrane as a result of ion concentration gradients and the selective movement of ions across membranes by ion pumps or through ion channels.
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A Review of Proton Motive Force from Unit 6
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A Review of ATP from Unit 6
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Transport proteins involved in active transport include antiporters, symporters, the proteins of the ATP-powered pumps.
Antiporters are transport proteins that transport one substance across the membrane in one direction, while simultaneously transporting a second substance across the membrane in the opposite direction (Figure \(\PageIndex{6}\)C) . Antiporters use the potential energy of electrochemical gradients from Na + or H + to transport ions, glucose, and amino acids against their concentration gradient (Figure \(\PageIndex{6}\)E1) .
Symporters are transport proteins that simultaneously transport two substances across the membrane in the same direction (Figure \(\PageIndex{6}\)D) . Like antiporters, symporters use the potential energy of electrochemical gradients from Na + or H + to transport ions, glucose, and amino acids against their concentration gradient (Figure \(\PageIndex{6}\)E2) .
ATP- powered pumps couple the energy released from the hydrolysis of ATP with the transport of substances across the cytoplasmic membrane. ATP- powered pumps are used to transport ions such as Na + , Ca 2+ , K + , and H + across membranes against their concentration gradient.
An example of active transport via an ATP- powered pump is the sodium-potassium pump found in animal cells. Three sodium ions from inside the cell first bind to the transport protein (Figure \(\PageIndex{10}\)A) . Then a phosphate group is transferred from ATP to the transport protein causing it to change shape (Figure \(\PageIndex{10}\)B) and release the sodium ions outside the cell (Figure \(\PageIndex{10}\)C) . Two potassium ions from outside the cell then bind to the transport protein (Figure \(\PageIndex{10}\)D) and as the phosphate is removed, the protein assumes its original shape and releases the potassium ions inside the cell (Figure \(\PageIndex{10}\)E) .
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Flash animation showing the sodium-potassium pump.
Courtesy of Raymond Husthwaite |
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html5 version of animation showing the sodium-potassium pump.
Courtesy of Raymond Husthwaite |
Summary
The cytoplasmic membrane (also called the plasma or cell membrane) of eukaryotic cells is a fluid phospholipid bilayer embedded with proteins and glycoproteins. It contains glycolipids as well as complex lipids called sterols. The cytoplasmic membrane is a semipermeable membrane that determines what goes in and out of the cell. Substances may cross the cytoplasmic membrane of eukaryotic cells by simple diffusion, osmosis, passive transport, active transport, endocytosis and exocytosis.