Chapter 3-The Cell



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Mrs. Verrastro

Chapter 3-The Cell


Lecture Notes
The cell is the body’s most basic unit of structure and function. The human body contains about 75 trillion cells. Cell size is measured in micrometersm), which are also called microns. 1 micrometer = 1/1000 millimeter (0.001 mm). A human egg cell is about 140 m and is just barely visible to the naked eye. A red blood cell is only about 7.5 m.
The cell: There is tremendous variation between cells. Most have three major parts:


Nucleus: this is the innermost part of the cell. It is enclosed by a thin membrane called the nuclear envelope, or nuclear membrane. The nucleus directs the cell’s activities, so it acts as the cell’s "brain."
Cytoplasm: this is the fluid surrounding the nucleus and filling the cell. It is enclosed in a membrane called the cell membrane (cytoplasmic membrane, plasma membrane). The cytoplasm contains organelles, which are structures that carry out metabolic functions.
Cell membrane: this is not just an outer covering. It is an active functioning part of the cell. This membrane is extremely thin, flexible, and relatively elastic and usually has many folds that increase its surface area. The cell membrane controls what enters and leaves the cell because it is selectively permeable. That means it allows only certain substances to pass through. If the membrane is permeable to a particular substance, that substance can move across the membrane.
Cell Membrane structure: The cell membrane is composed primarily of proteins and lipids, with some carbohydrate.

Phospholipid bilayer:

  • The cell membrane is organized into a double layer (bilayer) of phospholipid molecules.

  • The membrane has a fluid quality to it.

  • The phospholipid molecules are arranged so their water-soluble (hydrophilic) portions—the phosphate groups—are pointed outward, and the water-insoluble (hydrophobic) portions—the fatty acids—are pointing in toward the middle.

  • Because the middle part is "oily," water-insoluble substances, such as oxygen and carbon dioxide, are allowed through.

  • But water-soluble substances, such as amino acids, sugars, proteins, nucleic acids, and various ions, cannot pass directly through.

  • Also, the membrane has many cholesterol molecules in it that also hinder movement of water-soluble substances, as well as add some rigidity to the membrane structure.


Proteins: Proteins allow special membrane functions. Some are receptors, such as for hormones, which are typically long rod-like structures that span the full thickness of the membrane and may extend out from its surface. Some are globular and span the thickness of the membrane, forming small channels through it. These allow certain substances to pass through the membrane that otherwise could not. Still other globular proteins are located on the inside surface of the membrane and act as enzymes in metabolic reactions.

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Cytoplasm and organelles:


The cytoplasm is composed of a clear liquid in which a network of membranes and other organelles are suspended. It also contains protein rods and tubules that form the supportive cytoskeleton. The cytoplasm is where most metabolic reactions occur. This is where nutrients are received, processed, and used. Most reactions occur in association with the following organelles:


  • Endoplasmic reticulum (ER):

  • This is a complex organelle composed of many membranous sacs and channels interconnected and also connecting to the cell membrane, nuclear membrane, and other organelles.

  • The ER is widely distributed throughout the cytoplasm. It provides a tubular communication system to transport molecules to different areas within a cell.

  • The ER is also involved in protein and lipid synthesis, some of which is then used to make new membranes.

  • The outer surface of the ER often has numerous tiny spherical organelles attached to it.

  • These are called ribosomes. They give the ER a rough-textured appearance, so ER with ribosomes attached is referred to as rough endoplasmic reticulum, or rough ER, contrasted to smooth ER that doesn’t have them.

  • The ribosomes, and thus the rough ER, are involved in protein synthesis; smooth ER is involved primarily in lipid synthesis.




  • Ribosomes: not all ribosomes are attached to the ER. Many are freely scattered throughout the cytoplasm. As just mentioned, these are involved in protein synthesis. They are composed of protein and RNA (rRNA).


Golgi apparatus:

  • this is usually located near the nucleus.

  • The Golgi apparatus is composed of a stack of membranous sacks, called cisternae.

  • It is involved in refining, packaging, and distributing the proteins made by the ribosomes.

  • These (glyco) proteins then pass through the layers of the Golgi and are chemically altered as they progress.

  • When they reach the outer layer of the Golgi apparatus, the proteins are repackaged—parts of the Golgi membrane pinch off and form sacs (= transport vesicles) around the proteins.

  • The vesicles then may move to the cell membrane, fuse with it, and dump their contents outside the cell (cellular secretions).

  • The vesicle's membrane then becomes part of the cell membrane. Other vesicles may go to other organelles or remain loose in the cytoplasm.


Mitochondria: these are the "powerhouses" of the cells—they harness and provide cellular energy in the form of ATP.

  • Mitochondria are elongated, fluid-filled sacs that often move slowly through the cytoplasm.

  • Mitochondria can reproduce by division.

  • Mitochondria consist of a membrane with 2 layers. The inner layer is folded into partitions, called cristae, where enzymes are attached.

  • The mitochondria’s job is to release energy from glucose and other organic substances, and to convert that energy to a form the cells can use: adenosine triphosphate (ATP).




  • Lysosomes: these organelles are the cells’ "garbage-disposals." They come in various shapes. They are often tiny membranous sacs. They contain strong enzymes that break down substances such as proteins, carbohydrates, nucleic acids, and foreign matter. They also can destroy worn or damaged cell parts.




  • Peroxisomes: these are membranous sacs similar to lysosomes, located especially in the liver and kidney. Peroxisomes contain enzymes (peroxidases) that promote metabolic reactions that produce H2O2 (hydrogen peroxide) and enzymes that degrade it (peroxide is toxic). Peroxisomes play an important role in the breakdown of fatty acids and in the oxidation and detoxification of alcohol.




  • Centrosome: (central body). This is located in the cytoplasm near the Golgi apparatus and the nucleus. This is a nonmembranous structure. It has two hollow cylinders—centrioles—each containing tiny microtubules. These function in cell reproduction by directing the separation of the DNA strands. They also help form cilia and flagella.




  • Cilia and flagella: these are motile processes that extend out from some cells. Cilia are numerous hair-like structures that move in a wave-like manner. These are found, for example, in the respiratory tract. Flagella are usually single tail-like processes, such as the ones that give sperm their motility ("swimming" action).




  • Microfilaments and microtubules: these are thin thread-like structures. Microfilaments are tiny rods of protein organized in a meshwork or in bundles (such as in muscles). They are for cell movement. Microtubules are long slender tubes of globular proteins that form the internal skeleton of the cell. They provide the shape of the cell and its parts. Microtubules aid the movement of organelles through the cell.


Cell Nucleus: This is an organelle usually located near the center of the cell. It is relatively large and spherical. This organelle directs cellular activity. The nucleus is surrounded by a double-layered membrane = nuclear envelope/membrane. The layers have a narrow space between them but are joined at various spots by nuclear pores. These pores are relatively large openings that allow substances to be exchanged between the nucleus and the cytoplasm.

The nucleus is filled with fluid (nucleoplasm) in which are located the following:



  • nucleolus = "little nucleus"—a small dense body composed mostly of RNA and protein. It has no surrounding membrane; it’s associated with certain chromosomes. Function: production of ribosomes, which leave the nucleus via the pores.

  • chromatin = loosely coiled threadlike strands of DNA and protein.


Movements Through Cell Membranes: Oxygen and nutrients must enter the cell; carbon dioxide and other wastes must leave it. The basic processes by which substances move through cell membranes are simple diffusion, facilitated diffusion, osmosis, filtration, active transport, endocytosis, and exocytosis.
Simple Diffusion:

  • This is the process by which molecules or ions scatter from areas of high concentration to areas of lower concentration.

  • Anytime there are two adjacent areas with different concentrations, you have a concentration gradient.

  • Substances tend to move along concentration gradients from high to low concentration. They tend to disperse until the concentrations become equal, which is equilibrium.

  • The molecules continue moving even after equilibrium is reached, but the concentrations remain equal.

  • Consider what happens when you put sugar in a cup of coffee.

  • Simple diffusion is also how oxygen and carbon dioxide move into and out of cells.



  • The rate of diffusion is affected by the following:

  1. the amount of space available

  2. the concentrations involved

  3. the weight of the molecules

  4. the temperature


Facilitated Diffusion: this is especially important with glucose.

  • Most sugars can’t pass through the lipid portion of the membrane because they are too big; but glucose can pass through via facilitated diffusion.

  • A glucose molecule combines with a protein carrier molecule on the cell membrane’s surface.

  • This combination is lipid-soluble and can diffuse through the membrane. Once inside, the glucose is released and the carrier returns to the membrane surface to be used again.

  • Insulin promotes facilitated diffusion of glucose—it puts the circulating glucose in the cells to be stored.

  • As with simple diffusion, facilitated diffusion moves substances along the concentration gradient (high to low).

  • The rate is limited by the number of carrier molecules.




  • Osmosis: is a special type of diffusion that occurs when water molecules move along a concentration gradient through a selectively permeable membrane. So osmosis is merely diffusion of water through a selectively permeable membrane.


Basic terminology for osmosis:

solvent = a substance, usually fluid, in which something is dissolved. In the body, the fluid is water.
solute = the substance that is dissolved in the fluid.
solution = a mixture composed of a solute dissolved in a solvent.

With osmosis, the selectively permeable membrane allows water to pass freely, but the solute cannot. If the solute is initially in higher concentration on side A than on side B, side A will hold fewer water molecules than side B (because the excess solute on side A takes up space that could be occupied by water molecules). In other words, the water concentration is lower on side A than on side B:

membrane
|


A

B

O O O O O O O O O O O O O O O O O O O O O OOOOOOOOO
O O O O O O O O O O OOOOO
O O O O O O O O O O O O O O O O O O O O O OOOOOOOOO

O O O

O O O

O O O

The glucose can’t diffuse because of the membrane, but the water can move rather freely.
Water will move by osmosis from side ______ (where its concentration is higher) to side ______.
The end result is:

  • The water concentration moves toward equilibrium.

  • The solute concentration moves toward equilibrium (Water’s movement out of ___ increases the concentration there, and its movement into ____ dilutes the concentration there).

  • More solution on side ____ than on side ____ (side ___ takes on water; side ____ loses it).

osmotic pressure = a potential pressure in a solution that is due to its nondiffusible solutes.

> solute concentration = > osmotic pressure

Water moves toward areas with high osmotic pressure (to dilute the solute concentration and thus decrease the osmotic pressure). Think about how salt and water interact: the more salt you eat, the more water your body retains.



When comparing two solutions, for example inside and outside a cell:

  • if the solute concentration is the same inside and outside, the solutions are isotonic.

  • if the solute concentration outside is greater than inside, the solution outside is hypertonic.

  • if the solute concentration outside is less than inside, the solution outside is hypotonic.

Water tends to move from ________________________areas to___________________________areas.

  • Filtration: This is the movement process by which molecules are forced through membranes. Both force and a membrane are required. In the body, this happens when blood pressure forces substances through the vessel walls. The blood pressure results primarily from the water in the blood so it is called hydrostatic pressure. When the fluid pressure in the vessel is greater than the fluid pressure outside the vessel, substances are forced out of the blood vessel, which is a selectively permeable membrane.

  • Active Transport: All the methods of movement we’ve covered thus far are passive—substances move from areas of high concentration to low concentration. Sometimes substances need to be moved against the concentration gradient. To do this, energy must be used, thus the process is called active transport. Sodium and potassium are kept in high concentrations on opposite sides of the cell membrane via this mechanism. The process is much like facilitated diffusion—it uses specific carrier molecules in the cell membrane. When the substance to be moved binds to the carrier, energy is released. This energy changes the shape of the carrier so it can move the substance through the membrane to be released inside the cell. Active transport is used to move some sugars, amino acids, and ions.

  • Endocytosis: is a way by which to transport molecules or particles that are too large to move by diffusion or active transport. They are moved within a vesicle that forms from a section of the cell membrane. There are 3 types of endocytosis.

Pinocytosis = "cell drinking". This is the process by which cells take in tiny droplets of liquid. A small part of the cell membrane indents and the indentation contains some of the surrounding fluid. The open end of the pocket seals and forms a small vesicle (sac) that detaches from the cell membrane and moves into the cell. Its membrane opens, dumping the contents inside the cell. By this means, cells can take in water and substances it contains, such as proteins and other larger molecules.

Phagocytosis = "cell eating". This is very much like pinocytosis, but the vesicle forms around a solid. A solid touches the cell membrane, which responds by projecting outward and surrounding the solid. The vesicle forms and moves inwards, often combining with a lysosome that dumps its enzymes into the vesicle to digest the solid material. Contents of the vesicle can dump into the cytoplasm to be used for metabolic reactions, or be expelled from the cell (exocytosis). Some cells, especially some white blood cells, are phagocytes that conduct phagocytosis as a major function. This provides a major defense against invasion by infectious microorganisms.

Receptor-mediated endocytosis—moves only substances for which the cell membrane has specific receptors. The substances bond to the membrane receptors, and the binding triggers the cell membrane to indent and form a vesicle around the substance and pull it inside, much like pinocytosis (except the receptor protein is required). This mechanism allows cells to respond to specific substances, such as cholesterol and hormones, when they are present in very low concentrations.

  • Exocytosis: This is essentially the reverse of endocytosis. A vesicle, packaged inside the cell, makes its way to the cell membrane and fuses with the membrane. The vesicle then pushes into the membrane, with the leading edge joining the cell membrane and splitting, thus releasing the contents of the vesicle outside of the cell.



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