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.
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.
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.
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.
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.
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.
Unit 4 Assessment