Links: Cell Theory Ultrastructure Membrane Structure & Transport
Textbook Readings: Chapter 1, section 1.1-1.3
2. Calculating size
3. Missions 1 & 2
Let's go here to talk about magnification and calculating size.
Mission 1: It's Not About the Dance, It's About SCIENCE!!!
Mission Objectives: You should be able to...
1. Describe the three components of cell theory.
2. Explain the significance of the creation of the microscope
3. List the seven functions all living organisms carry out.
This is a short TedEd video talking about microscopes, cells and accidental science. What three components make up the foundation of cell theory? How did we get from the rudimentary two-lens scope of the 1500s to now? Do you think science occurs in a vacuum? Why do you think that some scientists couldn't get along? What does this tell you about the nature of scientific discovery.
From the IB Guides:
Many reactions occur within the cell. Substances need to be taken into the cell to fuel these reactions and the waste products of the reactions need to be removed. When the cell increases in size, so does its chemical activity. This means that more substances need to be taken in and more need to be removed.
The surface area of the cell is vital for this. Surface area affects the rate at which particles can enter and exit the cell (the amount of substances that it takes up from the environment and excretes into the environment), whereas the volume affects the rate at which materials are made or used within the cell, hence the chemical activity per unit of time.
As the volume of the cell increases so does the surface area however not to the same extent. When the cell gets bigger its surface area to volume ratio gets smaller.
You can read more about it here. But, simply put, the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, the ratio decreases, and therefore not enough material (nutrients in, waste out) will be able to cross the membrane fast enough to accommodate the increased cellular volume. Also the cell will not be able to release heat and thus may overheat. What do you think happens when that point is reached?
Mission 2: Itty Bitty Things.
Mission Objectives. You should be able to...
2. Describe the parts of a prokaryotic and eukaryotic cell.
3. Explain the functions of each and every organelle in an animal and plant cell.
4. Evaluate how form should follow function when it comes to cells and their structures.
5. Understand the relative sizes of cells and their organelles.
Cells are like small cities. Each organelle carries out a specific function to maintain the life of the cell. Who does what and why? If you had to create an analogy of the cell using a city, how would you do it and why?
Mission Objectives. You should be able to...
1. Sketch and annotate a model of the plasma membrane.
2. Describe the structure of the plasma membrane.
3. Explain the purpose of cholesterol and proteins in the membrane.
4. Compare and contrast the Davson-Danielli model with the Fluid Mosaic Model.
What you should focus on is the role of proteins, cholesterol and the orientation of the membrane. You should also be able to complete a basic sketch of the plasma membrane and describe it using the following terms: phospholipid bilayer, amphipathic, phosphate head, fatty acid tail, hydrophilic, hydrophobic, polar, nonpolar, cholesterol function and protein functions. Additional information about membrane structure is of course found here.
As you know, nature of science is one of the tenets of the IB curriculum. A component is understanding the dynamic nature of scientific knowledge and how what we know changes based on observable evidence.
Davson-Danielli's Lipid Bilayer. Basically, in 1935, these scientists developed a model of the plasma membrane and called it the lipid bilayer. Lipids, of course, are fats (and oils) and bilayer means "two layers." Their model suggested that the membrane was constructed like an ice cream sandwich, with the lipid bilayer acting as the cream and being surrounded by globular proteins (aka the cookies).
All cellular membranes, whether plasmic or organelle, have the same general structure. It is a phospholipid bilayer. A phospholipid has a phosphate head and a fatty acid tail. The phosphate head is polar and hydrophilic, meaning it interfaces with water extremely well. The fatty acid tail is nonpolar and hydrophobic, meaning it does not interface with water. Similar in idea to the lipid bilayer, the Fluid Mosaic forms a sandwich with the phosphate heads on the outside and the fatty acid tails tucked in. However, there are cholesterol and protein molecules embedded throughout the phospholipid bilayer, giving it a mosaic appearance (see p. 28 for a nice image).
Mission Objectives. You should be able to...
1. Explain how substances move through the plasma membrane.
2. Describe the processes involved in active and passive support.
3. Compare and contrast endocytosis and exocytosis.
4. Describe the function of the Na/K pump.
Let's talk about substance transportation into and out of the cell. The function of the plasma membrane is to maintain homeostasis by allowing substances and nutrients into and out of the cell. Basically, it acts like a bouncer at Club Cell. What processes are at work? There are two main types of transport: passive and active.
Terms to know: diffusion, facilitated diffusion, osmosis, Na/K pump, endocytosis, and exocytosis. What is the relationship of equilibrium to passive transport but not active transport?
Big Question: How does the plasma membrane allow nutrients and water into and out of cell? Is this an easy process or no? Is there a difference between how substances enter and how they exit? Are the substances that enter the same as the ones which exit?
Diffusion: This is the net movement of particles from an area of high concentration to an area of low concentration. Think of how scents and fragrances make their way to your nose. If someone sprays perfume, the scent is strongest near the bottle, but in time, the scent particle will make their way to your nose. At some point, there will be an even distribution of scent particles in the air.
Facilitated diffusion: When substances move through the plasma membrane via a water-filled transport protein (called a channel protein). Channel proteins open and close to allow necessary substances to diffuse through the membrane.
Osmosis: This is a form of specialized diffusion of water across a selectively permeable membrane. Usually the water is mixed with a solute (sometimes sugar or salt). The water can pass through the membrane but the solute cannot. The water will diffuse through the membrane towards the side with the greater concentration of solute particles until dynamic equilibrium is reached (when the concentration of the solution is the same on both sides of the membrane).
Here's a nice passive transport animation with a few practice questions. Here is another one on membrane transport.
Energy (ATP) is required to move substances into and out of the cell. The proteins embedded in the plasma membrane are positioned so that part lies inside the bilayer and parts outside the bilayer. They must work against a concentration gradient. This means instead of substances (usually ions) moving from areas of high concentration to areas of low concentration, the proteins must work in reverse: moving ions from low concentration to high concentration.
The Sodium/Potassium pump
Animal cells have a higher concentration of potassium ions outside the cell whereas sodium ions are more concentrated inside the cell. The cell maintains these conditions by pumping K+ ions into the cell and pumping Na+ ions out of it. A membrane protein is required for this to occur. Here is a lovely animation showing how it works.
Steps of the Na+/K+ pump:
1. A protein binds to three intracellular Na ions.
2. When the Na ions bind, ATP (adenosine triphosphate) loses a phosphate ion and turns into ADP (adenosine diphosphate).
3. This loss of a phosphate causes the protein to change shape and push the Na ions out of the cell.
4. Random K ions hanging outside of the cell bind to the protein which releases the phosphate group that was captured in step 2.
5. The loss of the phosphate group restores the protein's original shape and allows the release of the K ions into the cell.
Endocytosis is the active transport process by which a cell encloses a substance in a portion of the plasma membrane. This process is dependent of the fluidity and the orientation of the plasma membrane to enclose particles or macromolecules to form a vesicle that then enters the cytoplasm of the cell. The membrane then reattaches itself with the vesicle enclosed.
Exocytosis is the opposite process. Materials are secreted or expelled from the cell at the plasma membrane. Protein exocytosis begins in the ribosomes of rough ER and progresses through a series of steps (p. 37-38) until the substance is secreted to the extracellular environment. Both processes require ATP and are used for the transport of large particles. Can you explain the differences between pinocytosis and phagocytosis?
Homework: Corresponding workbook pages.