Unit 4 Study Guide: Cells and Cell Membranes

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

The functional unit of life is the cell. Every organism features at least one cell, and metabolism (the chemistry of life) occurs within cells. A cell is separated from its surroundings by a 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?

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 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, a plasma membrane features various other macromolecules, including proteins, sterols, and polysaccharides. The plasma membrane is fundamental to life, so be sure to review its structure (and the structure of an individual phospholipid) by reading the beginning of this chapter.


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 consisting of a solvent and some number of solutes. The solvent is the part of the solution that dissolves the solutes, and the solutes are the parts that are dissolved by the solvent. An aqueous solution is one in which water is the solvent. A cell's plasma membrane is a barrier between intracellular fluid and extracellular fluid, both of which are aqueous solutions. The plasma membrane is selectively permeable, because certain particles pass through the membrane easily and other particles are blocked from passing through. Many solutes are effectively (though not perfectly) prevented from passing through the membrane, so we say that the membrane is impermeable to those solutes. Water, on the other hand, can pass through to a certain degree.

The mechanism by which water passes through a plasma membrane is called osmosis, which is a special case of diffusion (and it is therefore a passive process). The direction and rate of osmosis depends on the relative solute concentrations inside and outside the cell. Water always osmoses to where it is less watery (which means it always moves to 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, that 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 water is moving toward the less watery compartment. Changes in osmotic gradients can have profoundly damaging effects on a cell, which is why organisms must regulate the osmotic conditions.

Watch this video to review your understanding of solutions and osmosis.


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

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

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

A prokaryotic cell is one that does not feature membrane-bounded organelles, whereas a eukaryotic cell is one that does feature membrane-bounded organelles. A membrane-bounded organelle is an organelle (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 such diverse structures 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.

Reading the chapter sections on prokaryotic cells and eukaryotic cells will help you review.


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 features these various organelles?

There are several organelles that you should recognize 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, and they therefore appear only in eukaryotic cells, which are structurally more complex than prokaryotic cells from which they evolved. Review the structures of these important organelles by reading this section. Pay particular attention to Figure 1.


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, the primary reason for classifying them as different types of organelles is that they perform different sets of functions, just as different organs in the body perform different sets of functions.

  • Ribosome - molecular machines that interpret codes in mRNA’s 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 bursting of the cell
  • Nucleus - enclosed by two membranes; houses the DNA
  • Mitochondrion - enclosed by two membranes; site of cellular respiration
  • Chloroplast - enclosed by two membranes; site of 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 of synthesis of proteins that will be packaged by Golgi body
  • Smooth endoplasmic reticulum - site of 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

Those organelles that are membrane-bounded form sub-compartments, so their functions can be performed in isolation from the rest of the cellular contents. Before proceeding, be sure to refresh your memory of which functions are performed by which organelles. This video will help.


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

Signal molecules are examples of ligands, because they must bind to other molecules. Those molecules to which signals bind are called receptors. When a signal binds to a receptor, that binding causes changes in the cell. Those 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 therefore 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. Though these signals don’t actually enter the cell, they still cause changes inside the cell. There are three primary mechanisms by which this takes place, and the difference lies in what kind of receptor receives these signals.

  • Ion-channel-linked receptors are transmembrane proteins that simultaneous serve as signal receptors and ion channels. When a signal molecule binds to such a 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 to the receptor activates (and frees) the G protein, and that activation leads to various responses.
  • Enzyme-linked receptors are transmembrane proteins that simultaneous serve as signal receptors and enzymes. The binding of a signal to the receptor activates the enzymatic portion of the receptor (which faces the interior of the cell), and once activated, the enzyme catalyzes various reactions, leading to the various responses.

Be sure you understand the functional differences between these three classes of receptors, even though all three operate by binding to a signal molecule at the exterior surface. It helps to look at diagrams to make sense of the differences, so review the text and figures under the Types of Receptors section of this chapter.


4g. describe the forms of transport across biological membranes

Particles pass through biological membranes (including the plasma membrane) by various mechanisms, all of which can be lumped into two primary categories.

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

Transmembrane transport (that is, transport of a particle through a biological membrane) can be classified as either active or passive. The distinction between the two is the requirement for an external source of energy.

  • Active transport is so called because it requires an additional (external) source of energy to drive it. Often that source of energy is ATP, but other sources can be used, as well. Since additional energy is applied, active transport can move particles against their gradient, which causes gradients to become even steeper.
  • Passive transport is called passive because it 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 (if the concentration of the particle type is higher on one side of the membrane than the other), an electrical gradient (if 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 (for example, 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 non-polar 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 process are fundamental to life, because organisms must continuously exchange materials with their surroundings to stay alive. Review the categories and subcategories by watching this video and reading these subsections of Chapter 4.


Unit 4 Vocabulary

This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.

Try to think of the reason why each term is included.

  • 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
Last modified: Wednesday, July 17, 2019, 5:48 PM