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7.1: The Cytoplasmic Membrane

Skills to Develop

  1. State the chemical composition and major function of the cytoplasmic membrane in eukaryotic cells.
  2. 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.
  3. Define the following means of transport:
    1. passive diffusion
    2. osmosis
    3. active transport
    4. endocytosis
    5. phagocytosis
    6. pinocytosis
    7. exocytosis

The cytoplasmic membrane (also called the plasma or cell membrane) in eukaryotic cells, as in prokaryotes, is a fluid phospholipid bilayer embedded with proteins and glycoproteins. The phospholipid bilayer is arranged so that the polar ends of the molecules (the phosphate and glycerol portion of the phospholipid that is soluble in water) form the outermost and innermost surface of the membrane while the non-polar ends (the fatty acid portions of the phospholipids that are insoluble in water) form the center of the membrane (Figure 7.1.1).

Figure 7.1.1: Diagram of a Cytoplasmic Membrane

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:

  1. channel proteins to form pores for the free transport of small molecules and ions across the membrane
  2. carrier proteins for facilitated diffusion and active transport of molecules and ions across the membrane
  3. cell recognition proteins that identifies a particular cell
  4. receptor proteins that bind specific molecules such as hormones and cytokines
  5. enzymatic proteins that catalyze specific chemical reactions.

As in prokaryotes, the cytoplasmic membrane is a semipermeable membrane that determines what goes in and out of the cell. In addition to . Substances may cross the cytoplasmic membrane of eukaryotic cells by simple diffusion, osmosis, passive transport, active transport, endocytosis and exocytosis. We will now review each of these methods of transport.

Passive Diffusion

Passive diffusion is the net movement of gases or small uncharged polar molecules across a phospholipid bilayer membrane from an area of higher concentration to an area of lower concentration (Figure 7.1.2). Examples of gases that cross membranes by passive diffusion include O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea.

   

Figure 7.1.2: Passive Diffusion, Step 1. Passive diffusion is the net movement of gases or small uncharge polar molecules across a phospholipid bilayer membrane from an area of higher concentration to an area of lower concentration . Examples of gases that cross membranes by passive diffusion include N2, O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea.

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.

Osmosis is the diffusion of water across a membrane from an area of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). Osmosis is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy. While water molecules are small enough to pass between the phospholipids in the cytoplasmic membrane, their transport can be enhanced by water transporting transport proteins known as aquaporins. The aquaporins form channels that span the cytoplasmic membrane and transport water in and out of the cytoplasm (channel proteins below).

To understand osmosis, one must understand what is meant by a solution. A solution consists of a solute dissolved in a solvent. In terms of osmosis, solute refers to all the molecules or ions dissolved in the water (the solvent). When a solute such as sugar dissolves in water, it forms weak hydrogen bonds with water molecules. While free, unbound water molecules are small enough to pass through membrane pores, water molecules bound to solute are not (Figure 7.1.3). Therefore, the higher the solute concentration, the lower the concentration of free water molecules capable of passing through the membrane.

   

Figure 7.1.3: Osmosis. Free Water Passing Through Membrane Pores. (left) When a solute such as sugar dissolves in water, it forms weak hydrogen bonds with water molecules. While free, unbound water molecules are small enough to pass through membrane pores, water molecules bound to solute are not. (right) When an ionic solute such as NaCl dissolves in water, the Na+ ion attracts the partial negative charge of the oxygen atom in the water molecule while the Cl- ion attracts the partial positive charge of the warter's hydrogen. While free, unbound water molecules are small enough to pass through membrane pores, water molecules bound to solute are not.

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 (Fig. 5A), 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 (Fig. 5B), 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 (Fig. 5C), 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 (Fig. 6A1 and Fig. 6A2). 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 (Fig. 6B). 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.

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 (Fig. 6C). Antiporters use the potential energy of electrochemical gradients from Na+ or H+ to transport ions, glucose, and amino acids against their concentration gradient (Fig. 6E1).

Symporters are transport proteins that simultaneously transport two substances across the membrane in the same direction (Fig. 6D). 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 (Fig. 6E2).

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+, Ca2+, 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 (Fig. 10A). Then a phosphate group is transferred from ATP to the transport protein causing it to change shape (Fig. 10B) and release the sodium ions outside the cell (Fig. 10C). Two potassium ions from outside the cell then bind to the transport protein (Fig. 10D) and as the phosphate is removed, the protein assumes its original shape and releases the potassium ions inside the cell (Fig. 10E).

Endocytosis

Endocytosis is a form of active transport in which a cell takes in solutes or particles by enclosing them in vesicles or vacuoles pinched off from its cytoplasmic membrane. There are three forms of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis is the ingestion of solid particles by endocytosis. The cytoplasmic membrane invaginates and pinches off placing the particle in a phagocytic vacuole or endosome (Fig. 11A and Fig. 11B). The phagocytic vacuole then fuses with lysosomes forming a phagolysosome and the material is degraded (Fig. 11C).

phagocyt.gif

Phagocytosis. Phagocytosis is theingestion of solid particles by endocytosis. The cytoplasmic membrane invaginates and pinches off placing the particle in a phagocytic vacuole. The phagocytic vacuole then fuses with lysosomes and the material is degraded.

Pinocytosis is the ingestion of dissolved materials by endocytosis. The cytoplasmic membrane invaginates and pinches off placing small droplets of fluid in a pinocytic vesicle. The liquid contents of the vesicle is then slowly transferred to the cytosol as seen in Fig. 12A, Fig. 12B, fFig. 12C and Fig. 12D.

pinocyt.gif

Pinocytosis. Pinocytosis is theingestion of dissolved materials by endocytosis. The cytoplasmic membrane invaginates and pinches off placing small droplets of fluid in a pinocytic vesicle. The liquid contents of the vesicle is then slowly transferred to the cytosol.

During receptor-mediated endocytosis, a specific molecule called a ligand binds to a receptor protein in the cytoplasmic membrane and subsequently enters the cytoplasm in coated vesicles. Receptor-mediated endocytosis is used by animal cells to take cholesterol up from the blood via low-density lipoprotein (LDL) particles. The LDL receptor proteins are concentrated in depressed regions of the membrane known as coated pits because they are coated with a layer of a protein called clathrin (Fig. 13A). After the LDL particle binds to the receptor protein, the coated pit invaginates forming a coated vesicle (Fig. 13B). The clathrin coating detaches and is recycled, leaving an uncoated vesicle called an endosome (Fig. 13C). The endosome then divides into two vesicles (Fig. 13D). One vesicle recycles the LDL receptor molecules back to the cytoplasmic membrane (Fig. 13E) while the other vesicle fuses with lysosomes so that the contents are digested and the cholesterol is released into the cytosol (Fig. 13F).

Exocytosis

During exocytosis, a cell releases waste products or specific secretion products by the fusion of a vesicle with the cytoplasmic membrane as seen in Fig. 14A, Fig. 14B, and Fig. 14C.

exocyt.gif

Figure 7.1.1: Exocytosis. During exocytosis, a cell releases waste products or specific secretion products by the fusion of a vesicle with the cytoplasmic membrane.

Concept map for Eukaryotic Cell Structure

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.

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)