BIO101: Introduction to Molecular and Cellular Biology

4a. Describe and diagram the structure and function of a typical biological membrane

• What is another name for the cell membrane?
• What types of molecules make up a cell membrane?
• How does the chemistry of the molecules in a membrane explain why a cell membrane forms?

The cell is the functional unit of life. Every organism features at least one cell; metabolism (the chemistry of life) occurs within cells. A membrane separates the cell from its surroundings.

Every cell features a cell membrane, which is also called the plasma membrane. The plasma membrane is a complex arrangement of several different types of molecules. The chief components are phospholipids. Each phospholipid molecule is an amphipathic molecule (polar at one end and non-polar at the other end). This explains why plasma (cell) membranes form.

In the presence of water, phospholipids self-assemble into a bilayer, with the non-polar tails in each monolayer pointing toward the non-polar tails of the other monolayer, and the polar heads of each monolayer pointing toward the watery solution on its side of the membrane (the water interior of the cell for one monolayer, and the water exterior of the cell for the other monolayer). In addition to the phospholipid bilayer, the plasma (cell) membrane features various other macromolecules, including proteins, sterols, and polysaccharides.

The plasma (cell) membrane is fundamental to life, so be sure to review its structure (and the structure of an individual phospholipid) in Parts of the Cell Membrane. Also, watch Review of the Cell Membrane and Structure of the Cell Membrane.

 

4b. Describe characteristics of a membrane, solutes, and solvents, as well as predict where molecules will move and how the mass of a cell may change

• What are the components of a solution?
• What is the difference between a solvent and a solute?
• What happens to cell volume when osmosis occurs?

A solution is a mixture that includes a solvent and a number of solutes. The solvent is the part of the solution that dissolves the solutes; the solutes are the parts the solvent has dissolved. In an aqueous solution, water is the solvent. A cell's plasma membrane forms a barrier between intracellular fluid and extracellular fluid (which are both aqueous solutions). The plasma membrane is selectively permeable, which means some particles easily pass through the membrane, while other particles cannot get through. Many solutes are effectively (although not perfectly) prevented from passing through the membrane, so we say the membrane is impermeable to these solutes. Water, on the other hand, can pass through to a certain degree.

Water passes through a plasma membrane using a mechanism called osmosis, a special type of diffusion process that is passive. The direction and rate of osmosis depend on the relative solute concentrations inside and outside the cell. Water always osmoses to the area that is less watery. This means water always moves away from the compartment that has a higher solute concentration. If the solute concentration of the extracellular fluid is higher than the solute concentration of the intracellular fluid, this means the extracellular fluid is less watery, so water will leave the cell by osmosis, and the cell volume will decrease.

If the reverse is true (the gradient is reversed), then water will enter the cell by osmosis, and cell volume will increase. In each case, notice that the water moves toward the less watery compartment. Organisms must regulate their osmotic conditions since changes in osmotic gradients can profoundly damage their cells.

Review solutions and osmosis in Passive Transport via Simple Diffusion, Passive Transport and Tonicity, Passive Transport via Osmosis, More on Osmosis, Facilitated Diffusion, Primary Active Transport, Secondary Active Transport, and Bulk Transport.

 

4c. Describe characteristics of a cell, and classify the cell as a prokaryotic, animal, or plant

• What distinguishes a eukaryotic cell from a prokaryotic cell?
• Are animal and plant cells eukaryotic or prokaryotic?

Although all cells share certain characteristics (for example, every cell has a plasma membrane), biologists recognize two fundamentally different categories of cells: prokaryotic cells and eukaryotic cells.

A prokaryotic cell does not feature membrane-bounded organelles; a eukaryotic cell does feature membrane-bounded organelles. A membrane-bounded organelle is an organelle (a tiny organ-like structure within a cell) that is enclosed by its own membrane, separate from the plasma membrane that encloses the entire cell.

Membrane-bounded organelles include diverse structures such as the nucleus, endoplasmic reticulum, lysosomes, mitochondria, chloroplasts, and others. Only eukaryotic cells feature these membrane-bounded organelles, though a eukaryotic cell might feature only some (but not all) of them.

For example, an animal cell (like one in a human body) features most of the membrane-bounded organelles, but it does not feature chloroplasts. A plant cell, on the other hand, typically includes the membrane-bounded organelles found in an animal cell, plus it also features chloroplasts. A bacterium, which is a prokaryotic cell, does not feature any of these membrane-bounded organelles. Ensure that you appreciate the differences between these major categories of cell types.

Review Prokaryotic Cells, Eukaryotic Cells, and Types of Cells.

 

4d. Identify organelles that are found in typical prokaryotic, plant, and animal cells 

• What are the names of the various organelles?
• Are all of the organelles membrane-bounded?
• What types of cells feature these various organelles?

You should recognize several organelles in this course:

• Ribosome - not membrane-bounded; found in prokaryotic and eukaryotic cells

• Plasma (cell) membrane - found in prokaryotic and eukaryotic cells

• Cell wall - found in most prokaryotic and some eukaryotic cells (though not animal cells)

• Nucleus - membrane-bounded; found only in eukaryotic cells

• Mitochondrion - membrane-bounded; found only in most eukaryotic cells

• Chloroplasts - membrane-bounded; found only in photosynthetic eukaryotic cells (plants and algae)

• Golgi body - membrane-bounded; found only in eukaryotic cells

• Central vacuole - membrane-bounded; found only in some eukaryotic cells, including plants and some protists

• Rough endoplasmic reticulum - membrane-bounded; found only in eukaryotic cells

• Smooth endoplasmic reticulum - membrane-bounded; found only in eukaryotic cells

• Lysosome - membrane-bounded; found only in eukaryotic cells

• Peroxisome - membrane-bounded; found only in eukaryotic cells

Notice that most of these organelles are membrane-bounded; therefore, they appear only in eukaryotic cells. These cells are structurally more complex than the prokaryotic cells they evolved from.

Review the structures of these important organelles in Cell Structure. Pay particular attention to Figure 1. Also, watch Parts of a Cell.

 

4e. Indicate the functions of the various cellular organelles, including the nucleus, cell membrane, cell wall, mitochondria, chloroplasts, ribosomes, Golgi body, central vacuole, rough endoplasmic reticulum, smooth endoplasmic reticulum, lysosome, and peroxisome

• What are the major functions of the various types of organelles?
• What advantage is gained by some organelles being membrane-bounded?

One difference between the various organelles is their shapes. However, our primary reason for classifying organelles differently is because they perform different functions, just as different organs in our body perform different functions.

• Ribosome - molecular machines that interpret codes in mRNA to build proteins

• Plasma (cell) membrane - defines the cell and forms the boundary between the contents of the cell and its surroundings

• Cell wall - thicker, more rigid than, and exterior to the plasma membrane; withstands pressure and prevents the cell from bursting

• Nucleus - enclosed by two membranes; houses the DNA

• Mitochondrion - enclosed by two membranes; site for cellular respiration

• Chloroplast - enclosed by two membranes; site for photosynthesis

• Golgi body - receives newly-formed proteins, modifies them, and packages them for transport to the plasma membrane or out of the cell

• Central vacuole - largely water-filled organelle that can also house pigments and wastes

• Rough endoplasmic reticulum - site for synthesis of proteins that the Golgi body will package

• Smooth endoplasmic reticulum - site for synthesis of lipids and storage of calcium ions

• Lysosome - digests materials by subjecting them to enzymes

• Peroxisome - safely breaks down harmful chemicals in the cell

The organelles that are membrane-bounded form sub-compartments, so they can perform their functions in isolation from the rest of the cellular contents. Before proceeding, be sure you know which functions each organelle performs. Review this material in Parts of a Cell.

 

4f. Explain how large signal molecules get their signal into the cell

• What are signal modules?
• What are receptors?

Signal molecules are examples of ligands, because they must bind to other molecules. We call the molecules that signal molecules bind to receptors. When a signal binds to a receptor, that binding causes changes in the cell. These changes are the responses to the signal. Some signals are small and non-polar, so they are easily able to pass through a cell's plasma membrane, and they bind to internal receptors. Most signals, however, are too large or too polar to pass through the plasma membrane, so they must bind to receptors on the exterior surface of the cell. Although these signal molecules do not actually enter the cell, they still cause changes inside the cell. This occurs using three primary mechanisms – the difference lies in what kind of receptor receives these signals.

• Ion-channel-linked receptors are transmembrane proteins that simultaneously serve as signal receptors and ion channels. When a signal molecule binds to this type of receptor, the ion channel either opens or closes its gate. This leads to changes in the flow of ions, which are charged particles. This redistribution of charge causes various responses.

• G-protein-linked receptors are transmembrane receptors that are associated with special proteins (G proteins) situated on the part of the protein that is in contact with the interior surface of the membrane. The binding of a signal molecule to the receptor activates (and frees) the G protein. This activation causes various responses.

• Enzyme-linked receptors are transmembrane proteins that simultaneously serve as signal receptors and enzymes. The binding of a signal molecule to the receptor activates the enzymatic portion of the receptor (which faces the interior of the cell). Once activated, the enzyme catalyzes various reactions, which causes various responses.

Be sure you understand the functional differences between these three classes of receptors; all three operate by binding to a signal molecule at the exterior surface. Since it helps to look at diagrams to make sense of the differences, review the text and figures in the section on types of receptors of Signaling Molecules and Cellular Receptors.

 

4g. Describe the forms of transport across biological membranes

• What are the primary categories of transmembrane transport?
• What is the fundamental difference between these primary categories?

Particles pass through biological membranes (including the plasma membrane) by various mechanisms, which we can lump into two primary categories. We can classify transmembrane transport (transport of a particle through a biological membrane) as active or passive. The distinction between the two is the requirement of an external source of energy.

• Active transport requires an additional (external) source of energy to drive it. ATP is often the energy source, but other energy sources can be used. Since additional energy is applied, active transport can move particles against their gradient (see definition in next paragraph), which causes gradients to become even steeper.

• Passive transport does not require additional (external) energy for the transport to occur. The energy that drives passive transport is in the form of a gradient. A gradient is a difference in magnitude. A gradient that drives passive transport can be a concentration gradient (when the concentration of the particle type is higher on one side of the membrane than the other), an electrical gradient (when the charge distribution is different on one side of the membrane than the other), or both. In all cases of passive transport, the transport occurs down the gradient, such as from the place of higher concentration to the place of lower concentration. Passive transport never occurs in the direction against the gradient.

There are important subcategories of passive transport:

• Simple diffusion is passive transport of solute particles down the gradient for that type of solute and directly through the phospholipid bilayer of the biological membrane. This can occur only for particles small enough or nonpolar enough to pass through the bilayer.

• Facilitated diffusion is also diffusion, but it requires the help (facilitation) of a transport protein to get the particle through the membrane. This occurs for particles that are too big or too polar to cross the phospholipid bilayer directly.

• Osmosis is passive transport of solvent particles (not solute particles) down the gradient for solvent particles and through a selectively permeable membrane. In biological systems, the solvent is always water, so biological osmosis is movement of water.

These transmembrane transport processes are fundamental to life because organisms must continuously exchange materials with their surroundings to stay alive. Review the categories and subcategories by watching Review of the Cell Membrane and reviewing subunit 4.4.

 

Unit 4 Vocabulary

You should be familiar with these terms as you prepare for the final exam.

• active transport
• amphipathic
• aqueous
• cell membrane
• cell wall
• cellular respiration
• central vacuole
• chloroplast
• enzyme-linked receptor
• eukaryotic
• extracellular fluid
• facilitated diffusion
• g-protein-linked receptor
• Golgi body
• intracellular fluid
• ion-channel-linked receptor
• ligand
• lysosome
• membrane-bounded
• mitochondrion
• non-polar
• nucleus
• organelle
• osmosis
• passive transport
• peroxisome
• phospholipid
• photosynthesis
• plasma membrane
• polar
• prokaryotic
• receptor
• ribosome
• rough endoplasmic reticulum
• selectively permeable
• signal
• simple diffusion
• smooth endoplasmic reticulum
• solute
• solution
• solvent