Movements Through Cell Membranes
Passive Mechanisms: Diffusion, Facilitated Diffusion, Osmosis, Filtration
Active Mechanisms: Active Transport, Endocytosis, Exocytosis
Cells of our bodies vary greatly in structure which reflects that their functions also vary. However all cells have the same basic organization. Therefore in this chapter we will look at the parts of a generalized cell called the Composite Cell. Most cells in the body have a nucleus, cytoplasm and a cell membrane. The other parts of the Composite Cell may or may not be included in all cell types in the body.
Click here to see the various shapes of different cell types in the human body
Click here to see the composite cell and its parts labeled
The cell membrane is also called the plasma membrane. It is the outer limit of a cell.
The cell membrane is very thin and is described as selectively permeable. In other words, it allows some substances to pass through it while preventing other substances from passing through.
The cell membrane is composed of two layers of phospholipids, different types of proteins, cholesterol, and a few carbohydrates.
Recall from Chapter 2 that a phospholipid molecule has a phosphate group and 2 chains of fatty acids attached to a glycerol molecule. The phosphate groups are hydrophilic, meaning they are attracted to water, while the fatty acids are hydrophobic meaning they repel water. The two layers of phospholipids in the cell membrane are arranged so that the phosphate groups touch the inside and the outside of the cell (where there is water) and the fatty acids touch each other so they do not have to touch water. The phospholipid bilayer is about the consistency of oil. In general, it will allow oxygen and carbon dioxide to pass through it. It also allows lipids and alcohol to pass through it. Proteins, carbohydrates, various ions and water cannot pass through the phospholipid bilayer!
Cholesterol molecules are dispersed into the phospholipid bilayer. Cholesterol molecules act to stabilize the membrane.
In general 2 types of proteins are part of the cell membrane, globular proteins and fibrous proteins. Globular proteins are very large while fibrous proteins tend to be small and "stringy". Proteins have many functions in the cell membrane. Some proteins act as channels and allow substances to pass through the cell membrane. Others act as receptors and receive chemical messages for the cell.
A small amount of carbohydrates usually cover the outside of the cell membrane. When carbohydrate associate with proteins of the membrane, they act to identify the cell as being a specific type of cell. For example, a liver cell will have different types of carbohydrates on its surface than the carbohydrates of a blood cell.
Click here to see the structure of the cell membrane
The cytoplasm of a cell is the "inside" of the cell. It is mostly water, proteins, ions and nutrients. Cytoplasm may also contain many of the following organelles:
Rough Endoplasmic Reticulum (RER) - RER is a series of channels that has ribosomes attached to it. These channels usually transport substances throughout the cell and participate in protein synthesis and some lipid synthesis.
Smooth Endoplasmic Reticulum (SER) - SER is a series of channels that does not have ribosomes attached to it. It also transports substances throughout the cell and also participates in lipid synthesis.
Ribosomes - Ribosomes are granular-like structures made of RNA and proteins. They may be attached to RER or they many float in the cytoplasm. They participate in protein synthesis.
Golgi Apparatus - The Golgi apparatus is a series of flattened sacs stacked on top of each other. The Golgi apparatus modifies proteins and packages them into small sac called vesicles.
Mitochondria - Mitochondria are elongated rod-like organelles. They are composed of an inner membrane and an outer membrane. The inner membrane is highly folded into structures called cristae. These cristae contain enzymes to make ATP (energy) for the cell; therefore we say mitochondria are the "powerhouse" organelles that generate energy.
Lysosomes - Lysosomes are small membranous sacs filled with digestive enzymes. These digestive enzymes are used by the cell to destroy unwanted materials such as bacteria, toxins, or viruses.
Peroxisomes - Peroxisomes are small membranous sacs filled with enzymes that break down hydrogen peroxide into water and oxygen. It is important for a cell to break down hydrogen peroxide as it is toxic to cells. Peroxisomes can also contain enzymes that are important for breakdown of alcohol and lipids.
Microfilaments and Microtubules - Microfilaments and microtubules are thread-like strands that form an "internal skeleton" for a cell. This internal skeleton helps to maintain the shape of a cell. The microfilaments are used in the movement of a cell. For example a muscle cell contracts because its microfilaments are shortening. Microtubules are thicker than microfilaments and are used to make cilia and flagella.
Centrosome - A centrosome is made of two hollow cylinders called centrioles. The two centrioles sit at right angles to each other near the nucleus of a cell. The centrioles pull chromosomes apart during mitosis.
Cilia - Cilia are "hair-like" structures that extend from the surface of a cell's plasma membrane. Cilia are used to move substances away from the surface cells.
Flagella - Flagella are also "hair-like" extensions from the surface of a cell. However flagella are much longer that cilia. Also, the only human body cell to have flagella are the sperm cells of the male. A normal sperm cell will only have one flagellum. It is used to propel the cell or allow it to "swim".
Vesicles - Vesicles are also called vacuoles. They are small membranous sacs that store a variety of substances. Vesicles can store, water, ions, proteins, sugars, etc. Vesicles typically are made from the RER or the Golgi apparatus.
The nucleus of a cell is typically spherical in structure and placed near the center of a cell. It is enclosed by a nuclear membrane (nuclear envelope) that is similar in structure to the plasma membrane. However the nuclear membrane contains nuclear pores so that larger substances can move in and out of the nucleus.
Within the nucleus, you find a nucleolus. The nucleolus is made of RNA and proteins and functions to make new ribosomes.
The nucleus also contains chromatin which is made of DNA and proteins. Chromatin is the material that stores genetic information for the cell. Genetic material contains the information needed for protein synthesis in the cell. When chromatin condenses, it forms bar-like structures called chromosomes.
Click here for a summary of cell organelles
The cell membrane controls what moves into and out of the cell. Some substances move across the cell membrane without the use of energy. These movements are called passive mechanisms. Sometimes the cell has to use energy to move a substance across its membrane. In this case we say the substances move through active mechanisms.
Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. In other words, it is the spreading out of a substance. Think about when you spray air freshener in the corner of a room. Eventually the air freshener will spread out evenly across the entire room. This is diffusion. Substances that easily diffuse across the cell membrane include gases such as oxygen and carbon dioxide.
Facilitated diffusion is simply diffusion of a substance with the help of a carrier molecule. For example, if glucose is concentrated on the outside of a cell, it will want to diffuse into the cell. However glucose cannot pass through the membrane unless it can find a carrier molecule (a protein) to allow it to pass into the cell. Therefore, we say glucose moves across the cell membrane through facilitated diffusion.
Click here to see facilitated diffusion
Osmosis refers to the diffusion of water across a semi-permeable membrane, such as a cell membrane. You want to remember that water will always try to diffuse or move toward the higher concentration of solutes (solids in solution).
Let's say you put a red blood cell into a solution that contains 10% salt. Salt is the solute in the solution. The solution inside the cell (cytoplasm) only contains 1% solute. Therefore, water from the inside of the cell will move to the outside of the cell because that is where the greater concentration of solutes are. The 10% salt solution is said to be a hypertonic solution meaning it is the solution with the greatest amount of solutes. Cells shrink when placed in hypertonic solutions because water always leaves the cell.
Let's say you put a red blood cell into a pure water solution (100% water). The solution inside the cell contains a 1% solute concentration. Therefore, water outside the cell will move into the cell because that is where the highest concentration of solutes are. We say the 100% water solution is a hypotonic solution meaning it has the lowest concentration of solutes. Cells swell and sometimes burst when place in hypotonic solutions since water always enters them.
If you want the cell size to remain the same, you need to keep cells in isotonic solutions. An isotonic solution has the same concentration of solutes and the solution inside the cell. So, if a red blood cell contains a 1% solute concentration, you could place it in a 1% glucose solution and water would neither leave or enter the cell; therefore the cell size would remain the same.
In filtration, substances are forced by some type of pressure, such as gravity or blood pressure, across a membrane that acts like a filter. Filtration separates substances in solutions. For example, you could separate sand from water by pouring the sand/water mixture through a filter. In the body, capillaries in the kidneys act as filters to separate components in blood.
In active transport, substances move across the cell membrane with the help of carrier molecules from an area of low concentration to an area of high concentration. In other words, substances are gathered together; this is the opposite of diffusion. Active transport is similar to facilitated diffusion in that both require carrier molecules. However, facilitated diffusion does not require energy. In real life, most cells tend to have a higher concentration of sodium ions outside their membranes because as sodium ions diffuse into the cell, the cell moves the sodium ions to the outside through active transport. The net result is that sodium ions are gathered outside the cell. Other substances that are moved across the cell membrane through active transport include sugars, amino acids, potassium, calcium, and hydrogen ions.
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Endocytosis is an active mechanism that moves substances into the cell through the use of vesicles. In this process think of a cell reaching out with its cell membrane to grab a substance. Once it contacts a substance, it will form a vesicle around it with its cell membrane. The substance- containing vesicle is then brought into the cell. Endocytosis of mostly water and small ions is termed pinocytosis. Endocytosis of relatively large, solid particles is called phagocytosis. Bacteria often enter a body cell through phagocytosis. Receptor-mediated endocytosis involves receptors on the surface of the cell membrane. In this type of endocytosis, a substance binds to the receptor and that triggers the cell to bring in the substance through endocytosis. Cholesterol molecules enter cells through receptor-mediated endocytosis.
Click here to see pinocytosis and phagocytosis
Exocytosis is the opposite of endocytosis. In exocytosis, a substance-containing vesicle inside the cell fuses with the cell membrane. Once they fuse together the substance inside the vesicle is released to the outside of the cell. Neurons release neurotransmitters this way.
A cell cycle is the series of changes that a cell goes through from the time it is formed to the time it divides or dies. The stages of the cell cycle are describe below.
Click here to see the entire cell cycle
During interphase a cell carries out its normal daily functions. For example, if a liver cell is in this phase, it is making liver enzymes and proteins. If a bone cell is in interphase, it is maintaining bone tissue. Also during interphase a cell will duplicate its DNA and cytoplasmic organelles such as mitochondria, ribosomes, centrioles, etc. The cell duplicates such structures in order to prepare for cell division (mitosis and cytokinesis). The original cell is termed the mother cell. When the mother cell divides, it forms two daughter cells. Each daughter cell will have the exact same copy of DNA and organelles as the original mother cell. Interphase is also a time for growth of the cell.
Click here to see a cell in interphase
Mitosis is the part of cell division in which the nucleus of the cell divides. There are four phases of mitosis:
Prophase - This is the first phase of mitosis. During this phase the chromatin in the nucleus condenses to form chromosomes. The nuclear membrane dissolves as well as the nucleolus. The centriole pairs migrate to opposite sides of the nucleus and form a spindle apparatus out of microtubules. These microtubules attach to the chromosomes and will eventually pull chromosome halves (chromatids) apart.
Metaphase - This is the second phase of mitosis. During this phase the chromosomes line up half-way between the centrioles.
Anaphase - This is the third phase of mitosis. During this phase, the microtubules of the spindle apparatus start to pull the chromosome halves apart.
Telophase - This is the fourth and final phase of mitosis. During this phase, the chromosomes are completely pulled apart. Now the chromosomes can spread back out to form chromatin. Nuclear membranes are formed around the each chromosome set. New nucleoli also appear in the newly formed nuclei.
Click here to see the events of mitosis
Click here for a table summarizing events of each phase of mitosis
Click here to see a cell in prophase
Click here to see a cell in late prophase
Click here to see a cell in metaphase
Click here to see a cell in anaphase
Click here to see a cell in telophase
Cytoplasmic division is also termed cytokinesis. This is the other part of cell division. Cytokinesis occurs about the same time that anaphase and telophase are happening during mitosis. During this process the cell membrane constricts to divide the cytoplasm of the cell. The result is that the organelles of the mother cell get distributed almost evenly into the two new daughter cells.
Most of the time when a cell divides, the daughter cells will have the same structure and function as the mother cell they came from. However, sometimes the daughter cells take on different structures and functions from that of the mother cell. We call this process cell differentiation. For example, when a sperm cell and an egg unite they form one cell called a zygote. The zygote will begin dividing and eventually some of the cells will have to differentiate into blood cells, liver cells, bone cells, nerve cells, and all the other cell types of the body.
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If a cell does not want to go through cell division, it can go through cell death. Cell death is also called aptosis. For example, it is normal for skin cells to undergo aptosis when they are sunburned. The peeling of skin is evidence of aptosis of these cells.
Click here to see the events of the entire cell cycle