Cell Biology 2415 Lab 5: The Plasma Membrane

Brush Border 3-D Viewer #12

1. Brush Border Microvilli 2. Proximal Tubule Lumen 3. Cell Membrane 4. Invaginations of Cell Membrane 5. Cytoplasm of Cell

Cell Membrane 3-D Viewer #8

1. Bulging nucleus under cells membrane 2. Surface of cell membrane facing lumen 3. Large vesicle 4. Small vesicle 5. Openings to vesicles on inside of cell membrane 6. Small interdigitations at cell-cell contact

Tight Junctions

Function in cell-to-cell communication. Prevents leakage across a layer of epithelial cells in animals. Adjacent cell membranes are fused forming a seal. (Ex. Epithelium in urinary bladder)

Molecular Size and Membrane Diffusion

Generally smaller molecules penetrate more rapidly than larger molecules of the same lipid solubility.

Would water molecules be moving across the membrane of a red blood cell when they are suspended in an isotonic solution?

If a cell is in a solution that has the same concentration outside the cell as it is inside the cell, it is said to be isotonic. This is the ideal situation for a cell, and the flow of water into the cell equals the flow of water out of the cell. However, cells often encounter environments that have different concentrations of solution, such as organisms found in fresh water (low concentration of solute) or ocean water (high concentration of solute).

The Fluid Mosaic Model of the Plasma Membrane

In 1972, Singer and Nicholson integrated that all of the available evidence regarding membrane structure proposed the *Fluid Mosaic Model*. This model is now widely accepted as the best available explanation of membrane structure. The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of individually inserted protein molecules drifting laterally in a fluid bilayer of phospholipids.

Glycolipid

Lipid molecule containing a bound carbohydrate group. Some are *glycerol based*, others are derivatives of *sphingosine* and are therefore called *glycosphingolipids*. Most common examples are *cerebroside* and *gangliosides*.

Adherens Junctions

Located just below the tight junctions in epithelial tissue where it functions to reinforce the tight junctions.

Microvilli 3-D Viewer #11

Long Microvilli (Stereocilia) 1. Long microvilli 2. Cell Membrane 3. Membrane base of microvilli 4. Luminal Openings of Microvilli 5. Cytoplasm of Cell

How can the activity or function of a cell or organelle can be reflected in the protein: lipid ratio of its membrane. Use the myelin sheath and the chloroplast as examples.

The fractured chloroplast membranes have an abundance of particles on them, which are known to contain complex enzyme systems. The myelin sheath is a relatively inert system and the exposed membrane surfaces are smooth with no suggestions of particulate substructure.

Visual Pigment

light-sensitive pigments that react to light and trigger electrical signals. Found in the rods and cones

Fenestration Vesicles 3-D Viewer #15

1. Endothelial Fenestrations 2. Endothelial Wall 3. Budding Vesicles

Photoreceptor (Stacked Membranes) 3-D Viewer #18

1. Photoreceptor outer membrane 2. Photoreceptor inner membrane 3. Connecting Cilium 4. Stacked Membrane 5. Plasma Membrane 6. Region of membrane infolding

Photoreceptor (Visual Pigment) 3-D Viewer #19

1. Photoreceptor outer segment 2. Stacked membrane 3. Plasma membrane 4. Membrane Disks

Fenestrations 3-D Viewer #14

1. Sinusoid 2. Erythrocyte 3. Endothelium 4. Endothelial Fenestrations 5. Microvilli 6. Space of Disse

Gap Junctions

A gap junction or nexus is a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between the cells.

Brush Border Function

A specialization of the free surface of a cell, consisting of minute cylindrical processes (microvilli) that greatly increase the surface area.

Plasma Membrane

All living cells are, by definition, surrounded by a plasma membrane. It has been long known that this important structure was composed of proteins and lipids but the exact arrangement of these molecules within the membrane was more difficult to determine.

Desmosomes

Anchoring junctions that prevents animal cells subjected to mechanical stress from being pulled apart; button like thickenings of adjacent plasma membranes connected by fine protein filaments.

Other Molecular Properties and Membrane Diffusion

For example, molecules such as *detergents* are able to disrupt the membrane and cause a massive and nonselective influx of extracellular molecules. This often results in the complete destruction of the cell.

Cell Junctions

Cell Junctions are specializations of the cell membrane surface. Include Gap Junctions, Tight Junctions, Desmosomes, Plasmodesmata, Adherens Junctions.

The Cell Membrane of Endothelia Cells 3-D Viewer #9

Outer Surface 1. Plasma Membrane 2. Luminal Vesicles 3. Vesicle Pinching off 4. Connection to endocytotic vesicle

Plasmodesmata

Channels between adjacent plant cells, which forms a circulatory & communication system connecting the cells in plant tissues.

In the plasmolyzed Elodea cell, what can be found in the space between the plasma membrane and the cell wall?

Chloroplasts all clump into the middle of the cell. This demonstrates the effect on a hypertonic solution on living cells.

Transport Across the Plasma Membrane

Electrolytes may be transported actively or passively through special protein channels. Nonelectrolytes, such as alcohols, generally penetrate the lipid bilayer by passive diffusion only. The rate of penetration by passive diffusion is determined by a number of factors: Lipid Solubility, Molecular Structure, Molecular Size, Other Molecular Properties.

Molecular Structure and Membrane Diffusion

For example, hydroxyl groups form hydrogen bonds with the water molecules surrounding the cell, therefore, a compound with one or more hydroxyl groups would tend to remain in the aqueous phase outside of the cell.

Examining Membrane Structure According to the Fluid Mosaic Model

Include lipid bilayer, peripheral proteins, integral proteins (both transmembrane and monotopic), lipid-anchored membrane proteins, carbohydrates, and cholesterol. Label cytoplasmic side and extracellular matrix side of the membrane.

Microvilli Function

Increases surface area of cell for increased absorption of secretion.

Interdigitations

Interlocking folds that appear where cells meet.

Lipid Solubility and Membrane Diffusion

Molecules with high lipid solubility have a faster rate of penetration. Lipid solubility is frequently measured as the *ether: water partition coefficient*. This value is calculated as: water partition coefficient = (solubility of diethyl ether)/(solubility in water).

Fenestrations

Pores in the glomerular endothelial cells that allow filtration of blood solutes, but not proteins and formed elements

Glycoprotein

Protein with one or more carbohydrate groups linked covalently to amino acid side chains (peripheral proteins) on the outside. Acts as a recognition site for hormones and neurotransmitters. Also allows cells to attach to one another to form tissues and recognize other cells.

Hemolysis RBCs

Red blood cells in a hypotonic solution will undergo hemolysis and this can be seen macroscopically when the blood solution becomes transparent red.

The Effect of Osmosis on Red Blood Cells (Isotonic)

Red blood cells in an isotonic solution have a disk-like appearance with a dimple. A salt or *saline* solution that is isotonic to red blood cells is known as *physiological saline* and has a concentration of 0.85% NaCl.

Plasma Membrane Functions

The plasma membrane functions as a dynamic barrier that separates the orderly internal structure and activity of the cell from the relative disorder of its external environment. In addition, it selectively regulates the movement of water and other substances in and out of the cell. This is done by allowing *passive diffusion* of substances along the concentration gradient or by actively moving substances against their concentration gradient in a process called *active transport*. Active transport requires the expenditure of ATP energy by the cell.

Deplasmolysis

The reverse of plasmolysis; a plasmolysed cell is moved to a hypotonic solution, water re-enters the cell, the central vacuole swells and the cell returns to a turgid state.

Explain why plasmolysis occurred. Include your description an explanation of exactly what occurred inside the cell.

There are high concentrations of glucose outside the cell which cause water to leave and glucose to enter as the water will move to a lower concentration causing the cell to shrink as the concentration of water outside the cell is lower. Glucose moves in as there is lower concentration inside the cell.

Photoreceptor Functions

These two types of receptor cells subserve different functions because of their differences in light sensitivity, density within the retina and types of neuronal connections they make with other retinal neurons. In general, we refer to rods as mediating dim light, low acuity, non-color vision (scotopic) and cones mediating bright light, high acuity, and color vision (photopic). Note, in the figure on the next page, that the fovea region of the retina only contains cones. The bipolar and ganglion cells in this area are also shifted to the side to that the light rays have almost a direct path to the photoreceptors. As a result, this is the area of greatest visual acuity

Hemolysis

Unlike plant cells, animal cells do not have a cell wall to counteract an incoming flow of water or molecules. So while plant cells placed in a hypotonic solution will become swollen, turgid and healthy, animal cells placed in a hypotonic solution will *lyse* (burst) as water moves into the cell by osmosis. *Hemolysis* specifically refers to the bursting of red blood cells as result of an osmotic imbalance. Homeless can also result from the passive diffusion of alcohol solutions and organic molecules into the cell. This diffusion results in the cell becoming hypertonic to the environment and creates the osmotic imbalance necessary for hemolysis. Molecules that diffuse faster will result in hemolysis more quickly. During hemolysis, a solution of red blood cells will change from translucent red (allowing light though) to transparent red (easy to see through). In this way, hemolysis can be observed macroscopically.

Plant Cells in Isotonic Solution

When a plant cell is placed in an isotonic solution, there is no net inward moment of water and the cells tend to become *flaccid* (limp).

Plant Cells in Hypertonic Solution

When a plant is placed in a hypertonic solution, water will move out of the cell by the process of osmosis. The net moment of water causes the cell to shrink and, as a result, the plasma membrane pulls away from the cell wall. This phenomenon is known as *plasmolysis*. Often under a microscope, a cell membrane is visible but plasmolysis is observed because chloroplasts and other organelles appear to move to the centre of the cell.

Plant Cells in Hypotonic Solution

When a plant is placed in a hypotonic solution, the cell wall maintains the cells water balance. The elastic cell wall will expand only up until a point and then it will begin to oppose further water uptake. At this point, the central vacuole is fully swollen and the cell membrane is pressing up against the cell wall; the cell is *turgid* (firm).

Crenation RBCs

When red blood cells are placed in a hypertonic solution they undergo crenation. Crenated red blood cells take on a scalloped or spiked appearance when water leaves the cell and the cells to shrivel.

Cell Membrane Outer Surface of Endothelia Cells 3-D Viewer #10

Where Cells Touch 1. Interdigitations