Introduction to Cells

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1 Introduction to Cells Key terms: Cell Microscopy Nucleus Organelle Micrograph Cytoplasm Define: Cell Microscopy Cell structure Today, all of biology is based on an understanding of what is going on within the cell. In order to understand how any cell function, a detailed knowledge of its underlying structure must be known. Starting as early as 1665 (Robert Hooke), scientists have been basic microscopes to view cells. One of the first, and distinctive, cell structures that scientists observed was the nucleus. Nucleus

2 The nucleus has three main parts: 1) Nucleus membrane 2) Nuclear pores 3) Nucleolus At the time, the nucleus was the only internal cell structure that could be seen. In more recent times, micrographs (photographs taken with a microscope) can now be taken using highly sophisticated microscopes, such as electron microscopes. However, based largely on these observations, scientists were still able to devise the Cell Theory. Cell Theory 1) 2) 3) Although early scientists could only see the nucleus, improvements in microscope technology allowed scientists to view a variety of different internal structures within the cell. These structures have a special name: Organelles Like the organs of your body, they perform specific tasks. Consider: How do organelles know exactly what task they should be performing? How do they actually go about performing their tasks?

3 Animal and Plant cell organelles Below, is a typical animal cell: The organelles are: 1. Nucleolus 2. Nucleus 3. Ribosome

4 4. Vesicle 5. Rough endoplasmic reticulum 6. Golgi apparatus (or "Golgi body") 7. Cytoskeleton 8. Smooth endoplasmic reticulum 9. Mitochondrion 10. Vacuole 11. Cytoplasm 12. Lysosome 13. Centriole

5 As you can see, the cell actually contains a variety of organelles, all of whom perform a specialized task. One organelle that s not included in this diagram is the cell membrane. Both animal and plant cells have one. Cell membrane A typical plant cell has a similar structure with a few exceptions: The additional organelles in a plant cell are: 1) Cell wall 2) Chloroplasts

6 Consider: By what process do plants convert sunlight into food energy they can use? Energy Like everything in universe, cells too need energy in order to perform their tasks. The process by which animals obtain energy is through a process called cellular respiration. Cellular respiration is the process of glucose reacting with oxygen to produce carbon dioxide, water, and energy. Task: Write down the skeletal equation, and balance it. In the skeletal equation, use the letter E to represent energy on the products side. You need not consider it when balancing (why?). Key terms: Chromosome DNA Gene DNA screening Transgenic organism Cloning Mutation Mutagen Genes and DNA As we ve already learned, the nucleus contains the master set of instructions that determine what each cell will become, what tasks it will perform, and how long it will live before being replaced. These instructions are carried by the Chromosomes. Chromosomes

7 Every plant and animal species has a specific set of chromosomes in the nucleus of each cell. In the cells of most plants and animal species, chromosomes come in pairs; one of each pair comes from each parent when an egg and sperm unite to produce a fertilized egg. The DNA code Chromosomes are made up of a material called deoxyribonucleic acid (DNA). DNA is a complex double helix shaped molecule made up of nitrogenous base pairs called adenine, thymine, cytosine, and guanine. In a DNA molecule, adenine always pairs with thymine, cytosine with guanine. DNA Each chromosome contains one molecule of DNA, which is subdivided into segments called genes. Genes Genes are located at specific locations on the DNA molecule. Most genes provide instructions for making proteins. This results in what kind of structure your cells are going to have and what thus what functions they can perform. The final results are well known feature such as hair colour, eye colour, height, and even whether or not you can digest milk or not. Consider: If DNA is capable of exerting control over our many of our features, does this mean that who we are is simple a result of our genes? Protein production As we have already stated, the job of our genes, and thus our DNA, is to manufacture proteins. Each protein is designed to do a specific job. Such build body parts, act as couriers, and carrying materials over long and short distances. Some even pick up or transfer signals from one body part to another, or catalyze various chemical reactions in our bodies. All of these various proteins get their instructions from DNA.

8 DNA Screening Testing for the presence of genetic disorders is referred to as genetic screening, or DNA Screening. DNA Screening Some types of genetic disorders can be observed by looking at a person s chromosomes. Down syndrome is one such disorder: using a technique called amniocentesis, a needle is inserted into a pregnant woman s abdominal wall to withdraw a sample of fluid from the amniotic sac. Cells from the fetus are then isolated, and a micrograph of the chromosomes in these cells are taken. Although the micrograph, called a karyotype, does not show errors in individual genes, it can show if a person has too many or too few chromosomes, or if any are broken. In the case of Down syndrome, technicians look specifically at chromosome 21, and check if the person has 3 of these chromosomes instead of two. Question (page 19): how do scientists test for a) PKU b) Huntington Disease

9 Consider: should individuals at risk of having Huntington s disease be tested? Ethics and drug research Much of the research into treatments for various diseases is carried out by drug companies. Such research can be very costly, and take many years complete. As a result, many companies try and recover many of their losses by putting high prices on their products. Many critics argue that clinical trial results can be biased. Companies, in an effort to push their product and make their profits and recover from their losses will present the results of their trials as more positive than they really are. Other ethical issues, such as who owns a cure and what ethical obligations a company may have, often also arise. Consider: If a company discovered a cure for cancer or diabetes, but it is very expensive, does the company own the cure and therefore have the right to sell it? Or, does it have an ethical obligation to make life-saving cures available to everyone who needs them? Altering genes: transgenic organisms (GMO s) The genetic code is universal. All organisms use A, C, T and G to produce their proteins. That means that one organism s genetic code can be read by another organism. If a particular gene is transferred between organisms, one species could make the specific proteins that another organism usually makes. For many years now, scientists have been recombining the DNA of separate organisms in a process called genetic engineering. A species whose genes have been altered are genetically modified organisms, or transgenic organisms. Transgenic organism Examples:

10 Consider: is genetically modifying any species a good idea? Cloning Cloning Cloning is another potential solution to the problem of organ transplants. Although cloning has been used for centuries already (cloning plants), recently scientists have been able to clone both plants and mammals in the lab. The most famous example of an animal being cloned is Dolly, the sheep cloned in The process is illustrated below: Steps: 1) 2)

11 3) 4) 5) Mutations As we have already learned, the ordering of A, C, T, and G on the DNA molecule is what s responsible for the coding of specific proteins. Whenever there is a change in the ordering of these building blocks can lead to mutations. Mutation Such changes can affect how well the protein does its job. A common example of the effect that altering protein coding has on the function of the protein itself is sickle cell anemia. Although mutations can happen randomly and for no reason at all, often they are a result of mutagens. Mutagens Examples of mutagens include: - EM radiation such as x-rays and UV rays - Mercury and tar (cigarettes) - Inherited mutations Gene Therapy Consider: If science can correct mutations, should it?

12 Cells from cells Key terms: Cell division Mitosis Cytokinesis DNA replication Prophase Metaphase Anaphase Telophase Cell plate Cell reproduction Cellular reproduction is the process by which new cells are formed. An important difference between cellular reproduction of a multicellular organism and a single-celled organism is the number of parent involved. Single-celled organisms reproduce asexually; that is, they reproduce themselves identically. When a cell divides itself into 2 new cells, the newly reproduced cells are called daughter cells. In sexual reproduction, two parents mate and produce offspring that receive half the genes of both parents (one chromosome from each pair of chromosomes). However, although offspring share genetic material, they are not identical. Consider: During sexual reproduction, genetic material is shared between the parents. Now, if the chromosomes of the parents don t change, that implies the same genetic material is shared each time the parents reproduce. However, if this is the case, how is that, although similar, parents can produce genetically different offspring? Cell division Cell division

13 The cell membrane and diffusion In order for a cell to survive, it must be able allow certain substances to flow in and out of the cell. In order to do this, it must pass the cell membrane, which acts as a barrier to the movement of materials across it. The process by which materials pass through the cell membrane is called diffusion. Diffusion Diffusion is the movement of molecules from areas of high concentration to low concentration. The process by which water crosses from one side of the cell membrane to the other is called osmosis. Materials that the cell needs, such as oxygen, diffuse across the cell membrane from the outside of the cell where they are more concentrated to the inside where they are less concentrated. Question: In your own words, explain why particles will diffuse from an area of high concentration to an area of low concentration. In order for materials to be able to pass through the cell membrane, it must be permeable to some substances, and impermeable to others. Cells are thus referred to as selectively permeable because not all materials are allowed to enter or leave the cell. Growing Cells The surface area of a cell must be big enough in order to diffuse all needed materials it needs to survive. As cells use these materials, they produce more organelles and cytosol and get bigger, thus increasing their surface area. As the surface area of a cell increases, its volume also increases, and so does its need for supplies and the amount of waste it produces as well. As a result, there ends up being natural limits imposed on cell size.

14 Limiting cell size Every cell faces the problem of needing enough surface area to service its volume. As something gets larger, the ratio of its surface area to volume decreases. That is, there is less surface area for each unit of volume as the cell membrane expands. As such, a cell cannot get too big, otherwise it will not have enough surface area for all the nutrients it needs and waste it produces. Therefore, when a cell reaches a certain size, it must divide into smaller cells. Mitosis Mitosis and Cytokinesis Cytokinesis When cells divide, the nucleus, which contains the DNA, is so important that it has its own multi-stage division process called mitosis. The cytoplasm divides by a process called cytokinesis. Question: When dividing, the cell cannot just divide up its chromosomes equally between its two daughter cells. Why is this so? DNA replication DNA replication During mitosis, a parent cell makes a copy of every chromosome before it divides. As such, during replication, each chromosome is duplicated, although the two copies remain attached to each other. DNA replication is very precise. So much so that when copying errors occur, they are detected and fixed by special proofreading and repair proteins. At roughly the same time, an organelle called the centrosome also doubles, ensuring that the cell has two copies. Centrosomes help to organize the tubules that make up the cytoskeleton. They play an important role in mitosis.

15 Mitosis Prophase Metaphase Anaphase Telophase and cytokinesis

16 As we can see, after Telophase and cytokinesis, the cycle repeats itself. Because the process is continuous, we call it the cell cycle. The period between one cell division and another is called interphase. Interphase Cell cycle Gap 1 + S-phase: Gap 2 + Mitosis

17 Checkpoints Controlling the timing and rate of cell division in cells is vital to normal growth and development. Too few or too many cells in any one part of the organism can lead to serious problems. Although scientists are not sure about many of the details, it is known that there are 3 main points at which the cell checks its growth. Cell checkpoints At each point, specialized proteins act like stop signs if certain conditions aren t met. In general, the 3 main checkpoints are i. ii. iii. Many cells will leave after mitosis. This is because cells many aren t required to continue dividing. Cells that leave the cell cycle enter a non-dividing stage. Most cells in the human body muscle cells and nerve cells are in this stage. Cell death and cell suicide Some cells don t leave the cell cycle to become specialized cells. Instead, they leave to die. Sometimes this is because they are damaged beyond repair, perhaps by physical or chemical forces. On the other hand, some cells will commit cell suicide. In this case, cells break down in an organized way, and its contents are packaged and distributed so that they can be used by other cells. It has been discovered that this kind of cell death is actually pre-programmed into the cells by suicide genes. These are genes that code for proteins that kill cells in specific situations. In general, there are 2 distinct processes by which cells die: necrosis or apoptosis Necrosis

18 apoptosis Cancer Some cells start out normally, but are then transformed in such a way that they ignore the stop signs in the cell cycle. One possible result is the repetitious and excessive division, forming a clump of cells called tumours. Tumour Cells such as these can become cancerous. Cancer Normal stages of healthy development and cancerous development (for visual diagrams, refer to your textbook on page 43)

19 Losing control Task: Referring to your textbook on page 44, summarize the 3 paragraphs in point form.