Cracking the AP Biology Exam

3

Cells

PLANT CELLS VERSUS ANIMAL CELLS

Plant cells contain most of the same organelles and structures seen in animal cells, with several key exceptions. Plant cells, unlike animal cells, have a protective outer covering called the cell wall (made of cellulose). A cell wall is a rigid layer just outside of the plasma membrane that provides support for the cell. It is found in plants, protists, fungi, and bacteria. (In fungi, the cell wall is usually made of chitin, a modified polysaccharide. Chitin is also a principle component of an arthropod’s exoskeleton.) In addition, plant cells possess chloroplasts (organelles involved in photosynthesis). Chloroplasts contain chlorophyll, the light‑capturing pigment that gives plants their characteristic green color. Another difference between plant and animal cells is that most of the cytoplasm within a plant cell is usually taken up by a large vacuole that crowds the other organelles. In mature plants, this vacuole contains the cell sap. Plant cells also differ from animal cells in that plant cells do not contain centrioles.

To help you remember the differences among prokaryotes, plant cells, and animal cells, we’ve put together this simple table. Make sure you learn it! ETS is bound to ask you which cells contain which structures:

Why do we need to know about the structure of cells? Because biological structure is often closely related to function. (Watch out for this connection: It’s a favorite theme for the AP Biology Exam.) And, more important, because ETS likes to test you on it!

TRANSPORT: THE TRAFFIC ACROSS MEMBRANES

We’ve talked about the structure of cell membranes, now let’s discuss how molecules and fluids pass through the plasma membrane. What are some of the patterns of membrane transport? The ability of molecules to move across the cell membrane depends on two things: (1) the semipermeability of the plasma membrane and (2) the size and charge of particles that want to get through.

First let’s consider how cell membranes work. For a cell to maintain its internal environment, it has to be selective in the materials it allows to cross its membrane. Since the plasma membrane is composed primarily of phospholipids, lipid-soluble substances cross the membrane without any resistance. Why? Because “like dissolves like.” Generally speaking, the lipid membrane has an open-door policy for substances that are made up of lipids. These substances can cross the plasma membrane without any problem. However, if a substance is not lipid-soluble, the bilipid layer won’t let it in.

One exception to the rule is water:

Although water molecules are polar (and therefore not lipid-soluble) they can rapidly cross a lipid bilayer (at a rate of about 3 billion water molecules a second, in fact) through aquaporins, which are integral membrane proteins that regulate the flow of water.

Diffusion

We’ve just seen that lipid-soluble substances can traverse the plasma membrane without much difficulty. But what determines the direction of traffic across the membrane? Some substances move across a membrane by simple diffusion. That is, if there’s a high concentration of a substance outside the cell and a low concentration inside the cell, the substance will move into the cell. In other words, the substance moves down a concentration gradient:

It’s like riding a bicycle downhill. The bike “goes with the natural flow.” Another name for this type of transport is passive transport. Here’s one more thing you must remember:

Simple diffusion does not require energy.

A special kind of diffusion that involves the movement of water is called osmosis.

Facilitated Transport

How do lipid-insoluble substances, which are dissolved in the fluid on either side of the cell membrane, get in and out of the cell? These dissolved substances, or solutes, rely on the proteins embedded in the plasma membrane. Special proteins—called channel proteins—can help lipid-insoluble substances get in or out:

These proteins pick up the substance from one side of the membrane and carry it across to the other. This type of transport is known as facilitated transport, or facilitated diffusion. Facilitated diffusion is just like simple diffusion in one respect: The flow of the substance is down the concentration gradient. Therefore, it doesn’t require any energy.

Active Transport

Suppose a substance wants to move in the opposite direction—from a region of lower concentration to a region of higher concentration. A transport protein can help usher the substance across the plasma membrane, but it’s going to need energy to accomplish this. This time it’s like riding a bicycle uphill. Compared with riding downhill, riding uphill takes a lot more work. Movement against the natural flow is called active transport.

But where does the protein get this energy? Some proteins in the plasma membrane are powered by ATP. The best example of active transport is a special protein called the sodium-potassium pump. It ushers out sodium ions (Na+) and brings in potassium ions (K+) across the cell membrane. These pumps depend on ATP to get ions across that would otherwise remain in regions of higher concentration. Where do we usually find these proteins? In vertebrates, they’re found in neurons and skeletal muscle fibers.

We’ve now seen that small substances can cross the cell membrane by:

  • Simple diffusion
  • Facilitated transport
  • Active transport

Endocytosis

When the particles that want to enter a cell are just too large, the cell uses a portion of the cell membrane to engulf the substance. The cell membrane forms a pocket, pinches in, and eventually forms either a vacuole or a vesicle. This is called endocytosis.

Three types of endocytosis exist: pinocytosis, phagocytosis, and receptor-mediated endocytosis. In pinocytosis, the cell ingests liquids (“cell-drinking”). In phagocytosis, the cell takes in solids (“cell-eating”). A special type of endocytosis, receptor-mediated endocytosis, involves cell surface receptors that are covered in clathrin-coated pits. (Clathrin is a kind of protein.) When a particle, or ligand, binds to one of these receptors, the ligand is brought into the cell by the invagination or “folding in” of the cell membrane. A vesicle then forms around the incoming ligand and carries it into the cell’s interior.

Bulk Flow

Other substances move by bulk flow. Bulk flow is the one-way movement of fluids brought about by pressure. For instance, the movement of blood through a blood vessel or movement of fluids in xylem and phloem of plants are examples of bulk flow.

Dialysis

Dialysis is the diffusion of solutes across a selectively permeable membrane. For example, a cellophane bag is often used as an artificial membrane to separate small molecules from large molecules.

Exocytosis

Sometimes large particles are transported out of the cell. In exocytosis, a cell ejects waste products or specific secretion products such as hormones by the fusion of a vesicle with the plasma membrane.