BIOL 2401Nursing Assignment help
Cell Structure and Function
Investigation Manual
Table of Contents
Overview………………………………………………………………………………………….. 3
Objectives………………………………………………………………………………………… 3
Time Requirements…………………………………………………………………………… 3
Background……………………………………………………………………………………… 3
Materials…………………………………………………………………………………………… 7
Safety……………………………………………………………………………………………….. 8
Activity 1: Simple Diffusion ………………………………………………………………… 9
Activity 2: Osmosis …………………………………………………………………………… 10
Disposal and Cleanup …………………………………………………………………….. 13
Overview
In this investigation, the student will explore the structure and function of the animal cell, particularly the selectively permeable plasma membrane. The student will model the processes of simple diffusion and osmosis and assess the tonicities of aqueous solutions.
Objectives
- Model the processes of simple diffusion and osmosis.
- Calculate the rate of diffusion and determine how it is affected by molecular weight.
- Assess the relative tonicities of aqueous solutions.
Time Requirements
Preparation …………………………………………………………………………………30 minutes
Activity 1 …………………………………………………………………………………….60 minutes Activity 2 …………………………………………………………………………………….75 minutes
Background
The cell is the fundamental structural and functional unit of all living things. Although the human body is composed of an amazing variety of different, specialized cell types, all of these cells have certain characteristics in common. Most importantly, all animal cells possess three main components: a nucleus, a cytoplasm, and a plasma membrane.
The Nucleus
The nucleus houses most of the genetic material of the cell. Most of the time, the genetic material exists in the form of a thread-like complex of DNA and proteins known as chromatin. When a cell goes through the process of division, the chromatin coils up tightly to form compact structures called chromosomes. Within the nucleus lies at least one nucleolus. This is where ribosomes—the machinery of protein synthesis—are assembled. The contents of the nucleus are separated from the rest of the cell by the nuclear envelope. This double membrane is penetrated by nuclear pores that permit materials to pass in and out of the nucleus.
The Cytoplasm
The cytoplasm occupies the area between the nucleus and the plasma membrane. It consists of the cytosol (which is mostly water with dissolved ions and proteins), the protein filaments of the cytoskeleton, and a variety of organelles, which are specialized structures devoted to specific cellular tasks (Table 1).
| Table 1. Cytoplasmic components of animal cells.
Structure Function |
| Ribosomes |
Protein synthesis |
| Rough endoplasmic reticulum (rough ER) |
Processing and transport of proteins |
| Smooth endoplasmic reticulum (smooth ER) |
Lipid and carbohydrate metabolism; detoxification |
| Golgi apparatus |
Processing and transport of proteins, especially secreted proteins |
| Lysosomes |
Intracellular digestion |
| Peroxisomes |
Catabolism of fatty acids |
| Mitochondria |
ATP production |
| Centrioles |
Organization and movement of chromosomes during cell division |
| Cilia |
Movement |
| Flagella |
Movement |
| Microfilaments |
Cytokinesis; changes in cell shape; cell motility |
| Intermediate filaments |
Strength and support for cells and tissues |
| Microtubules |
Motility (internal components of cilia and
flagella); intracellular transport;
chromosome movements during cell division |
The Plasma Membrane
The plasma membrane of the cell (also called the cell membrane or the cytoplasmic membrane) surrounds and defines each cell and separates its internal environment from the external environment. The plasma membrane is composed primarily of phospholipids. A phospholipid molecule consists of a glycerol skeleton with two fatty acids and a phosphate group attached. The fatty acids are nonpolar hydrocarbon chains, and thus, they are hydrophobic (i.e., repelled by water).
The negatively charged phosphate group forms the polar head of the molecule and is hydrophilic (i.e., attracted to water). Recall that the cytosol within the cell is mostly water, and most cells of the human body are bathed in extracellular fluid, which is also mostly water. These conditions cause the phospholipid molecules to cluster together so that their hydrophilic heads are oriented toward the water and their hydrophobic tails exclude water. The resulting structure is a phospholipid bilayer: two layers of molecules, with the hydrophilic heads directed to the inside and outside of the cell and the hydrophobic tails sandwiched in between.
By themselves, the phospholipid molecules would form a relatively loose, fluid association, with a consistency similar to that of vegetable oil. However, phospholipid molecules are not the only type of molecule in the plasma membrane. In the membranes of animal cells, the phospholipids are stabilized by sterol molecules, such as cholesterol. Glycolipids, which have a carbohydrate group instead of a phosphate group, are also present in the outer portion of the bilayer.
Proteins constitute a major component of the plasma membrane and play important roles in cell signaling, adhesion, metabolism, and transport. Peripheral membrane proteins are weakly associated with the membrane, whereas integral membrane proteins are more firmly embedded. In fact, most integral membrane proteins are transmembrane proteins, meaning that they completely span the phospholipid bilayer and have exposed regions on both sides of the membrane. Many proteins are able to drift laterally within the phospholipid bilayer, which is why the plasma membrane is often described in terms of a fluid-mosaic model.
BIOL 2401Nursing Assignment help
Cell Transport and Cell Size
All living things take in nutrients and eliminate waste. These vital functions are facilitated at the cellular level by the selectively permeable (i.e., semipermeable) plasma membrane. The cell membrane permits the passage of molecules and ions of a certain size while restricting the passage of larger or differently charged molecules or ions. Some molecules, such as water, oxygen, and carbon dioxide, can move freely across the cell membrane’s lipid bilayer.
These molecules move into and out of the cell by diffusion, which can be defined as the net movement of molecules or ions down a concentration gradient, from a region of higher concentration to a region of lower concentration. Larger molecules, on the other hand, are excluded by the membrane and may enter or leave the cell only through processes mediated by dedicated transporter proteins located in the membrane.
In Activity 1, you will observe how diffusion occurs in the absence of a membrane. However, diffusion across a selectively permeable membrane, such as the plasma membrane, is subject to certain conditions. Four main factors determine the rate of diffusion of molecules or ions across a membrane:
- The steepness of the concentration gradient: The greater the difference between the concentrations on opposite sides of the membrane, the higher the rate of diffusion.
- Temperature: Molecules and ions have more kinetic energy at higher temperatures. When molecules and ions move more rapidly, diffusion proceeds more rapidly.
- The surface area of the membrane: The greater the surface area, the higher the rate of diffusion. A greater surface area allows more molecules or ions to cross the membrane at any point in time.
- The type of molecule or ion diffusing: Large molecules (those of higher molecular weight) tend to diffuse more slowly than smaller molecules (of lower molecular weight). If the large molecules are contained within a selectively permeable membrane, they may not be able to diffuse at all. Ions may move more readily along a charge gradient; for example, a cation (positively charged ion) may diffuse more quickly toward a region rich in anions (negatively charged ions) than toward a region with an overall positive charge.
Osmosis is the diffusion of water molecules across a selectively permeable membrane. The four factors listed above also apply to osmosis. The net movement of water molecules in osmosis is to the side of the selectively permeable membrane having the higher concentration of solute, and, therefore, the lower concentration of water. The cytoplasm is an aqueous solution, consisting of water with dissolved molecules and ions. If the cell is surrounded by solute-free, pure water, the concentration of water is actually lower inside the cell compared with the outside, and the net movement of water will be into the cell.
If the cell is in a solution with a high solute concentration, the concentration of water may be higher inside the cell compared with the outside, causing the net flow of water to be out of the cell.
The terms hypertonic, hypotonic, and isotonic are used to compare aqueous solutions of varying solute concentration in which the solute cannot cross the membrane. If the solutions have the same concentration of solute, they are called isotonic (iso-, “same”). When two solutions have different concentrations of a solute, the one with the higher solute concentration is called hypertonic (hyper-, “above”), and the one with the lower solute concentration is called hypotonic (hypo-, “below”). The hypertonic solution, which contains a higher solute concentration than the comparison solution, can also be thought of as having a lower concentration of water.
In contrast, the hypotonic solution has a lower concentration of solute, but a higher concentration of water. Because the solute cannot cross the membrane, osmosis occurs between solutions of different tonicities. The water will move from the solution in which it is more concentrated to the solution in which it is less concentrated. In other words, water will move from the hypotonic solution to the hypertonic solution. In the figures that follow, the larger black circles represent the solute molecules, and the smaller open circles represent the water molecules. The vertical center line represents a selectively permeable membrane.
Figure 1. The concentration of solute molecules is higher on the left side of the membrane, so the left side is hypertonic relative to the right side. The right side is hypotonic relative to the left side.
Figure 2. The left and right sides are at equilibrium and are isotonic relative to each other. The concentration of molecules on both sides of the membrane is equal.
Osmotic pressure is the measure of a solution’s tendency to gain water when separated from pure water by a selectively permeable membrane. A solution’s osmotic pressure is proportional to its solute concentration; the greater the solute concentration, the greater the osmotic pressure and, therefore, the greater the tendency for the solution to gain water. In isotonic solutions, water diffuses across the membrane from one solution to another at an equal rate in both directions. There is no net osmotic movement of water and no net osmotic pressure.
Water enters our cells passively through osmosis. For instance, most water absorption in the digestive tract occurs in the large intestine, and there are no channels in the plasma membranes of intestinal cells that actively transport water. While water transport relies on osmosis, there are membrane channels that actively transport sodium and other ions into the cytoplasm, using ATP for energy. In order to manipulate the characteristics of osmosis, the concentration of solutes can be increased in the cells of the intestinal lining such that the cytoplasm becomes hypertonic relative to the lumen of the large intestine. Then, water flows into the cells by osmosis.
The kidneys regulate the water balance in our bodies. Like the large intestine, the movement of water by osmosis is regulated by the active transport of salts. In addition, some cells of the kidneys have selective channels called aquaporins, which allow water to move across the membrane very quickly in response to osmotic pressure.
In Activity 2, you will use dialysis tubing to simulate the plasma membrane of a cell. The flat, transparent dialysis tubing has microscopic pores that permit the passage of water, but not larger solutes such as sugars.
Materials
Included in the materials kit:
Sucrose, 100-g packet 1
250-mL beaker 3
Dialysis tubing, 8” piece 3
100-mL graduated cylinder 1
10-mL graduated cylinder 1
Weigh boats 3
Grease pencil 1
Plastic cup, 10 oz 4
Pipette 3
Teaspoon 1
Agarose 30 mL
Petri dish 1
Potassium permanganate 1 g
Methylene blue 1 g
Micro spoons 2
Ruler 1
Needed, but not supplied:
Tap water
Blank white paper
Pen or pencil
Timing device
Paper towels
Digital camera or mobile device capable of taking digital photos Pot holder or oven mitt (recommended but not required)
Safety
Read all of the instructions for this laboratory activity before beginning. Follow the instructions closely and observe established laboratory safety practices, including the use of appropriate personal protective equipment (PPE).
Wear safety glasses, gloves, and a lab apron while performing this laboratory investigation. Work in close proximity to a sink or other source of running water. A kitchen sink sprayer or a shower may serve as an emergency eye wash station if needed.
Potassium permanganate is an oxidizing agent. Methylene blue is an irritant of the skin, eyes, and respiratory passages; exposure may result in drowsiness or dizziness and may impair fertility or, if pregnant, cause harm to an unborn child. If either substance is inhaled, seek fresh air immediately and seek medical attention. In case of contact with the eyes, rinse immediately with plenty of water and seek medical attention. In case of contact with skin, wash immediately with soap and rinse with plenty of water. If skin irritation results, seek medical advice or attention. If swallowed, call a poison center and/or seek medical attention immediately.
Do not eat, drink, or chew gum while performing this activity. Wash your hands with soap and water before and after performing the activity. Clean up the work area with soap and water after completing the investigation. Keep pets and children away from lab materials and equipment.
Concentration is defined as the amount of a substance per unit volume, such as the mass of sucrose (table sugar) in a milliliter (mL) of water.
