Unit 4: Cells and Cell Membranes
Cells are the smallest units of life. In this unit, we explore the characteristics, components, and functions of cells. Learning about the structures of cells allows us to see the similarities and differences among organisms. Bacteria, plant, animal, and fungus cells are similar in many ways and contain many of the same small structures known as organelles. However, some characteristics help distinguish whether a cell belongs to an animal, plant, fungus, or bacteria. For example, all plant cells contain cell walls, while animal cells lack this particular extracellular structure. The water within a cell that presses against the cell wall gives a plant its rigidity and your celery its crunch!
Completing this unit should take you approximately 10 hours.
Upon successful completion of this unit, you will be able to:
- describe and diagram the structure and function of a typical biological membrane;
- describe characteristics of a membrane, solutes, and solvents; predict where molecules will move and how the mass of a cell may change;
- describe characteristics of a cell, and classify the cell as a prokaryotic, animal, or plant;
- identify organelles that are found in typical prokaryotic, plant, and animal cells;
- indicate the functions of the various cellular organelles, such as the nucleus, cell membrane, cell wall, mitochondria, chloroplasts, ribosomes, Golgi body, central vacuole, rough endoplasmic reticulum, smooth endoplasmic reticulum, lysosome, and peroxisome;
- explain how large signal molecules get their signal into the cell; and
- describe the forms of transport across biological membranes.
4.1: Cellular Organization and Organelles
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.
We compartmentalize cells into several structures with specific functions in the cell. We call these structures organelles. Organelles are subunits in the anatomy of the cell. The advantage of compartmentalization inside the cell is that many different functions can be localized in specific places. This brings about a high level of organization and efficiency for the cell. In this section we discuss the structures and functions of the different parts of the cell.
Read this chapter to learn about important organelles. Most of these organelles are membrane-bounded and only appear in eukaryotic cells, which are structurally more complex than prokaryotic cells from which they evolved. Pay close attention to Figure 1 and to the differences in animal cells, plant cells, and bacterial cells.
Here is a summary of the organelles you should recognize:
- Cell wall – surrounds the plasma membrane of plant cells to provide strength and protection; found in most prokaryotic and some eukaryotic cells (but not animal cells)
- Central vacuole – regulates the cell's water concentration; membrane-bounded; found only in some eukaryotic cells, including plants and some protists
- Chloroplasts – organelle in plants where photosynthesis occurs; membrane-bounded; found only in photosynthetic eukaryotic cells (plants and algae)
- Golgi body – helps process and package proteins and lipid molecules; membrane-bounded; found only in eukaryotic cells
- Lysosomes – digest food and waste materials; membrane-bounded; found only in eukaryotic cells
- Mitochondrion – organelle called the "powerhouses" or "energy factories" where ATP is made; membrane-bounded; found only in most eukaryotic cells
- Nucleus – houses the cell's DNA and directs the synthesis of ribosomes and proteins; membrane-bounded; found only in eukaryotic cells
- Peroxisome – compartment for oxidation reactions, involved in lipid biosynthesis; membrane-bounded; found only in eukaryotic cells
- Plasma (cell) membrane – the cell membrane which protects the cell; found in prokaryotic and eukaryotic cells Ribosome – decodes the message and forms peptide bonds for protein synthesis; not membrane-bounded; found in prokaryotic and eukaryotic cells
- Rough endoplasmic reticulum – produces, folds and dispatches proteins; membrane-bounded; found only in eukaryotic cells
- Smooth endoplasmic reticulum – produces lipids, steroid hormones, and detoxifies harmful metabolic byproducts; membrane-bounded; found only in eukaryotic cells
After you read, respond to these review questions:- In your everyday life, you have probably noticed that certain instruments are ideal for certain situations. For example, you would use a spoon rather than a fork to eat soup because a spoon is shaped for scooping, while soup would slip between the tines of a fork. The use of ideal instruments also applies in science. In what situation(s) would the use of a light microscope be ideal, and why?
- In what situation(s) would the use of a scanning electron microscope be ideal, and why?
- In what situation(s) would a transmission electron microscope be ideal, and why?
- What are the advantages and disadvantages of each of these types of microscopes?
- Antibiotics are medicines that are used to fight bacterial infections. These medicines kill prokaryotic cells without harming human cells. What part or parts of the bacterial cell do you think antibiotics target? Why?
- Why are some microbes not harmful?
- You already know that ribosomes are abundant in red blood cells. In what other cells of the body would you find them in great abundance? Why?
- What are the structural and functional similarities and differences between mitochondria and chloroplasts?
- In the context of cell biology, what do we mean by form follows function? What are at least two examples of this concept?
- In your opinion, is the nuclear membrane part of the endomembrane system? Why or why not?
- What are the similarities and differences between the structures of centrioles and flagella?
- How do cilia and flagella differ?
- How does the structure of a plasmodesma differ from that of a gap junction?
- How does the extracellular matrix function?
Watch these short videos for another look at the different types of cells (prokaryotes and eukaryotes), intracellular and extracellular fluid, the nucleolus which builds ribosomes, and the other organelles, such as the mitochondria which create ATP from glucose, ribosomes, and chloroplasts, that inhabit them. Again, it is important to know the structures and functions of each organelle. The final video in the series discusses the cytoskeleton which is made out of protein filaments and helps with cell movement.
After you have watched the series of videos, you should be able to describe the structure and function of all organelles in the cell, explain how organelles control protein synthesis in the cytoplasm, discuss the difference in organelles between plant and animal cells, list the components of the endomembrane system, describe the energy conversions carried out by mitochondria and chloroplasts, and describe the functions of the cytoskeleton.
4.2: The Cell or Plasma Membrane
The cell membrane, also called the plasma membrane, protects the contents within the cell and monitors all of the components that enter and exit it. The cell or plasma membrane also plays a role in communication and attachment in tissues. Its structure is made of phospholipids and proteins. These biological molecules play an important role in the different actions of the cell’s membrane. In this section, we discuss the structure and function of the membrane and its impact on the life of the cell.
Watch this short video for a visual overview of the molecular structure and function of the cell or plasma membrane. Note that the membrane is composed mainly of phospholipids and proteins. It is selectively permeable, allowing certain organic molecules to pass into and out of the interior.
The chief components of the cell or plasma membrane 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.
As you read this chapter on the cell membrane, pay attention to how certain molecules "transport" across the cell membrane. We will examine these mechanisms in more detail later in the unit.
After you read, you should be able to describe how phospholipids and most other membrane constituents are amphipathic molecules, list the major functions of membrane proteins, and define the three types of transport: selective permeability, active transport, and passive transport.
4.3: Membranes, Lipids, and the Fluid Mosaic Model
We measure the components of the membrane that are fluid by the types and amounts of lipids found in the membrane. We describe the protein components of the membrane as a mosaic with different functions. In this section, we discuss the structures and functions of the components of the plasma membrane.
Watch this lecture that reviews cell membranes and details their central role in cell function and survival. Learn the components on the plasma membrane and reflect on their macromolecule characteristics.
After you have watched the videos in this section, you should be able to explain how molecules cross cell membranes, discuss the channel proteins, carrier proteins, pumps, and aquaporins involved in transport, distinguish between osmosis, facilitated diffusion, and active transport, and describe how large molecules are transported across a cell membrane.
Watch these videos, which describe the properties of lipids that form a major component of the plasma membrane. As you review these characteristics, keep the material you learned in the previous resources in mind. Pay attention to the difference between unsaturated and saturated fatty acids of phospholipids. You should also be able to explain how cholesterol and steroids resist changes in membrane fluidity.
This video gives an additional explanation of the chemical and visual structure of lipids.
Read this text, which explains how the Fluid Mosaic model describes the structure of the plasma membrane as a mosaic of components – including phospholipids, cholesterol, proteins, and carbohydrates – which gives the membrane a fluid character. These macromolecules have special characteristics that relate to the functionality of the plasma membrane.
After reading, you should be able to define the fluid mosaic model, explain why membranes with different functions have different types of membrane proteins, describe the fluidity of the components of a cell membrane, and distinguish between peripheral and integral membrane proteins and their major functions.
4.4: Diffusion, Osmosis, and Active Transport
The plasma membrane is a selectively permeable barrier. We can classify transmembrane transport (transport of a particle through a biological membrane) as active or passive. The distinction refers to the requirement of an external source of energy.]
- Active transport, as its name implies, requires an additional (external) source of energy to drive it. Often that source of energy is ATP, but other sources can also be used. The additional energy allows active transport to move particles against their gradient.
- Passive transport does not require additional (external) energy for the transport to occur. A gradient that drives passive transport can be a concentration gradient (when the concentration of a certain particle 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, the transport occurs "down" the gradient – from the place of higher to lower concentration). Passive transport never occurs in the direction against the gradient.
Three subcategories of passive transport include:- Simple diffusion is the passive transport of solute particles down the gradient through the phospholipid bilayer of the biological membrane. This only occurs for particles small or non-polar enough to pass through the bilayer.
- Facilitated diffusion requires the help or 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.
- Osmosis is the passive transport of solvent particles (not solute particles) down the gradient through a selectively permeable membrane. Since the solvent is always water in biological systems, biological osmosis refers to the movement of water.
These transmembrane transport processes are fundamental to life because organisms must continuously exchange materials with their surroundings to stay alive.Watch this video to see how hydrophobic molecules move from a high to a lower concentration across the plasma membrane via simple diffusion. Notice that proteins are not required for this process.
This video discusses simple diffusion and introduces the concept of osmosis, the movement of water across the plasma membrane to a place where there is a higher concentration of solutes. Make sure you can define hypertonic, hypotonic, and isotonic solutions.
After you have watched this video, you should be able to define diffusion and explain why it is a passive process that does not require energy, define terms hypertonic, hypotonic, and isotonic solutions, and define osmosis and predict the direction of water movement based on differences in solute concentrations in the cell or solutions.
This video provides some animations of osmosis and diffusion. Notice that molecules on both sides of the membrane will continue to move until they reach equilibrium.
Watch this video to review what we have learned about passive transport and osmosis.
Facilitated diffusion is the movement of hydrophilic molecules from a high concentration to a lower concentration across the plasma membrane. Due to the partially hydrophobic nature of the membrane, facilitated diffusion requires a protein. As you watch, pay attention to how transport proteins facilitate diffusion. Make sure you can distinguish between osmosis, simple diffusion, and facilitated diffusion.
4.5: Transporting Signal Molecules across the Plasma Membrane
Signal molecules are examples of ligands (a molecule that binds to another, usually larger, molecule) because they bind to other molecules to pass through the plasma membrane. The molecules the signals bind to are called receptors. When a signal binds to a receptor, the binding causes changes in the cell. These changes are the responses to the signal. Some signal molecules are small and non-polar, so they are easily able to pass through a cell's plasma membrane, and therefore, 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 signals do not actually enter the cell, they still cause changes inside the cell.
There are three primary mechanisms for this occurrence – 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 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 simultaneously 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 various responses.
As you read this text, be sure you understand the functional differences between the three classes of receptors (ion channel linked, g-protein-linked, and enzyme-linked), 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.
Unit 4 Assessment
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Take this assessment to see how well you understood this unit.
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