Case Study Conclusion: More Than Just Tired
Jasmin discovered that her extreme fatigue, muscle pain, vision problems, and vomiting were due to problems in her mitochondria. Mitochondria are small membrane-bound organelles found in eukaryotic cells that provide energy for the cells of the body. They do this by carrying out the final two steps of aerobic cellular respiration, the Krebs cycle, and electron transport, which is the major way that the human body breaks down the sugar glucose from food into a form of energy cells can use, namely the molecule ATP.
Because mitochondria provide energy for cells, you can probably easily see why Jasmin was experiencing extreme fatigue, particularly after running. Her damaged mitochondria could not keep up with her need for energy, particularly after intense exercise which requires a lot of additional energy. What is perhaps not so obvious are the reasons for her other symptoms, such as blurry vision, muscle spasms, and vomiting. But all the cells in the body require energy in order to function properly. Mitochondrial diseases can cause problems in mitochondria in any cell of the body, including muscle cells and cells of the nervous system, which includes the brain and nerves. The nervous system and muscles work together to control vision and digestive system functions, such as vomiting, so when they are not functioning properly, a variety of symptoms can emerge. This also explains why Jasmin’s niece, who also has mitochondrial disease, has symptoms related to brain function, such as seizures and learning disabilities. Our cells are microscopic and mitochondria are even tinier, but they are essential for the proper functioning of our bodies and when they are damaged, serious health effects can occur.
A one seemingly confusing aspect of mitochondrial diseases is that the type of symptoms, severity of symptoms, and age of onset can vary wildly between people — even within the same family! In Jasmin’s case, she did not notice symptoms until adulthood, while her niece had more severe symptoms starting at a much younger age. However, this makes sense when you know more about how mitochondrial diseases work.
Inherited mitochondrial diseases can be due to damage in either the DNA in the nucleus of cells or the DNA in the mitochondria themselves. Recall that mitochondria are thought to have evolved from prokaryotic organisms that were once free-living, but then infected or were engulfed by larger cells. One of the pieces of evidence that supports this endosymbiotic theory is that mitochondria have their own, separate DNA. When the mitochondrial DNA is damaged or mutated, it can result in some types of mitochondrial diseases. However, these mutations do not typically affect all of the mitochondria in a cell. During cell division, organelles such as mitochondria are replicated and passed down to the new daughter cells. If some of the mitochondria are damaged, and others are not, the daughter cells can have different amounts of damaged mitochondria. This helps explain the wide range of symptoms in people with mitochondrial diseases, even ones in the same family because different cells in their bodies are affected to different extents. Jasmin’s niece was affected strongly and her symptoms were noticed early, while Jasmin’s symptoms were milder and did not become apparent until adulthood.
There is still much more that needs to be discovered about the different types of mitochondrial diseases. But by learning about cells, their organelles, how they obtain energy, and how they divide, you should now have a better understanding of the biology behind these diseases. Apply your understanding of cells to your own life — can you think of other diseases that affect cellular structures or functions, maybe that even affect people you know? Since your entire body is made of cells, when they are damaged or not functioning properly it can cause a wide variety of health problems.
In this chapter, you learned many facts about cells. Specifically, you learned that:
- Cells are the basic units of structure and function of living things.
- The first cells, from cork, were observed by Hooke in the 1600s. Soon after, van Leeuwenhoek observed other living cells.
- In the early 1800s, Schwann and Schleiden theorized that cells are the basic building blocks of all living things. Around 1850, Virchow saw cells dividing and added that living cells arise only from other living cells. These ideas led to cell theory, which states that all organisms are made of cells, all life functions occur in cells, and all cells come from other cells.
- The invention of the electron microscope in the 1950s allowed scientists to see organelles and other structures inside cells for the first time.
- There is variation in cells, but all cells have a plasma membrane, cytoplasm, ribosomes, and DNA.
- The plasma membrane is composed mainly of a bilayer of phospholipid molecules and forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell. Some cells have extensions of their plasma membrane with other functions, such as flagella or cilia.
- The cytoplasm is a thick solution that fills a cell and is enclosed by the cell membrane. It helps give the cell shape, holds organelles, and provides a site for many of the biochemical reactions inside the cell. The liquid part of the cytoplasm is called cytosol.
- Ribosomes are small structures where proteins are made.
- Cells are usually very small so they have a large enough surface-area-to-volume ratio to maintain normal cell processes. Cells with different functions often have different shapes.
- Prokaryotic cells do not have a nucleus. Eukaryotic cells have a nucleus as well as other organelles. An organelle is a structure within the cytoplasm of a cell that is enclosed within a membrane and performs a specific job.
- The cytoskeleton is a highly organized framework of protein filaments and tubules that criss-cross the cytoplasm of a cell. It gives the cell structure and helps to hold cell structures such as organelles in place.
- The nucleus is the largest organelle in a eukaryotic cell and is considered to be the cell's control center. It contains DNA and controls gene expression, including which proteins the cell makes.
- The mitochondrion is an organelle that makes energy available to cells. According to the widely accepted endosymbiotic theory, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
- The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum (RER) is studded with ribosomes. Smooth endoplasmic reticulum (SER) has no ribosomes.
- The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
- Vesicles and vacuoles are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are vesicles that break down foreign matter, dead cells, or poisons.
- Centrioles are organelles located near the nucleus that help organize the chromosomes before cell division so each daughter cell receives the correct number of chromosomes.
- There are two basic ways that substances can cross the cell’s plasma membrane: passive transport, which requires no energy; and active transport, which requires energy.
- No energy is needed for passive transport because it occurs when substances move naturally from an area of higher concentration to an area of lower concentration. Types of passive transport in cells include:
- Simple diffusion, which is the movement of a substance due to differences in concentration without any help from other molecules. This is how very small, hydrophobic molecules, such as oxygen and carbon dioxide, enter and leave the cell.
- Osmosis, which is the diffusion of water molecules across the membrane.
- Facilitated diffusion, which is the movement of a substance across a membrane due to differences in concentration but only with the help of transport proteins in the membrane, such as channel proteins or carrier proteins. This is how large or hydrophilic molecules and charged ions enter and leave the cell.
- Active transport requires energy to move substances across the plasma membrane, often because the substances are moving from an area of lower concentration to an area of higher concentration or because of their large size. Two examples of active transport are the sodium-potassium pump and vesicle transport.
- The sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, both against a concentration gradient, in order to maintain the proper concentrations of both ions inside and outside the cell and to thereby control membrane potential.
- Vesicle transport uses vesicles to move large molecules into or out of cells.
- Energy is the ability to do work and is needed by every living cell to carry out life processes.
- The form of energy that living things need is chemical energy, and it comes from food. Food consists of organic molecules that store energy in their chemical bonds.
- Organisms mainly use glucose and ATP for energy. Glucose is the compact, stable form of energy that is carried in the blood and taken up by cells. ATP contains less energy and is used to power cellular processes.
- Cellular respiration is the aerobic process by which living cells break down glucose molecules, release energy, and form molecules of ATP. Overall, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and water.
- The first stage of cellular respiration, called glycolysis, takes place in the cytoplasm. In this step, enzymes split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.
- The second stage of cellular respiration, called the Krebs cycle, takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their chemical bonds being stored in a total of 16 energy-carrying molecules (including 4 from glycolysis).
- The third stage of cellular respiration, Oxidative Phosphorylation, takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions across the membrane, creating an electrochemical gradient that drives the synthesis of many more molecules of ATP.
- In all three stages of aerobic cellular respiration combined, as many as 36 molecules of ATP are produced from just one molecule of glucose.
- Some organisms can produce ATP from glucose by anaerobic respiration, which does not require oxygen. Many human cells perform fermentation that also does not require oxygen. It is performed to recycle NADH back into NAD+. There are two types: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis.
- Alcoholic fermentation is carried out by single-celled organisms including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
- Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
- Anaerobic respiration produces far less ATP than does aerobic cellular respiration, but it has the advantage of being much faster.
Chapter Summary Review
1. For the following questions, choose whether the description applies to eukaryotic cells, prokaryotic cells, or both.
a. Has a nuclear membrane
b. Has a plasma membrane made of a phospholipid bilayer
c. Can be in a multicellular organism
d. Has ribosomes
e. Has an endoplasmic reticulum
f. Its DNA replicates before cell division
g. Has a single circular chromosome
h. Has cytoplasm that splits into two daughter cells during cell division
i. Has a cell cycle that includes interphase and mitosis
j. The type of cell that most likely evolved to become mitochondria
2. Name one example of a prokaryotic organism and one example of a eukaryotic organism.
3. Neurons are cells in the nervous system that transmit messages. They use energy to maintain the balance of sodium and potassium ions inside and outside of them, which is critical for their ability to send messages.
a. What kind of transport is this maintenance of sodium and potassium ion concentrations – active or passive? Explain your reasoning.
b. What creates the barrier between the inside and the outside of these cells?
c. What molecule uses energy to maintain the balance of sodium and potassium ions inside and outside of neurons? Describe two reasons why such a molecule is required.
d. What form of energy is used in this process?
e. Briefly explain how the energy in the food you eat gets there and provides energy for your neurons in the form necessary to power this process.
4. Explain why the inside of the plasma membrane, the side that faces the cytoplasm of the cell, must be hydrophilic.
5. True or False. Anaerobic and aerobic cellular respiration both produce ATP.
6. True or False. The cell membrane can also be called the plasma membrane.
7. True or False. Each phospholipid molecule in the cell membrane has two heads and a tail.
8. True or False. For cells, a smaller size is generally more efficient.
9. True or False. DNA is located in the nucleus of prokaryotic cells.
10. True or False. Cilia and flagella stick out of the cell membrane but are not made of cell membrane themselves.
11. Which statement about the cell membrane is false?
A. It encloses the cytoplasm
B. It protects and supports the cell
C. It keeps all external substances out of the cell
D. None of the above
12. During diffusion, substances move from an area of ____________ concentration to an area of ___________ concentration.
A. higher, lower
B. lower, higher
C. higher, equal
D. lower, equal
13. Which process produces glucose?
A. Anaerobic respiration
B. Aerobic cellular respiration
14. a. Which type of respiration involves electron transport?
b. Where does this electron transport occur within the cell?
c. Energy from electron transport is used to pump hydrogen ions across a membrane. Is this active or passive transport of hydrogen ions? Explain your answer.
d. After the process described in part c, hydrogen ions then flow from a region of higher concentration to a region of lower concentration. Is this active or passive transport of hydrogen ions? Explain your answer.