Biology Homework Chapter 5: Membrane Transport Mechanisms
Textbook assignment: Chapter 5:1-9 The Working Cell, sections 1-9.
- 5.1 Membranes function in a number of ways within the cell. We are going to spend a lot of time talking about how membranes control the flow of nutrients into and out of any cell, and how specialized tissues (like the lining of the lungs or the intestines) control the flow of nutrients into and out of organisms. But membranes also function within the cell by providing a surface for specific kinds of reactions, including those that result in the production of proteins and enzymes. The composition of membranes varies, depending on the organelle or cell type. The description here applies primarily to th plasma membrane found on eukaryotic cells; with some variation in detail, it also applies to the membranes of organelles like the nucleus, mitochondrion, and chloroplast, and to the inner cell container of bacteria. All such membranes have the same basic bilayer structure, so study the diagrams!
Notice that the composition of the membrane is fluid. Think of a crowd of people: they are bound to the same "layer" of space by the ground below and the fact that gravity holds them there, but on that surface, they can constantly move around and change positions. The lipids, proteins, and sugars of the membrane can move in the same way, "laterally" or sideways, throughout the membrane layer. This means they can move apart to let certain small molecules, such as water or oxygen gas, through.
- 5.2 Lipids form simple membranes or bilayers because their heads are attracted to water while their tails are not. Cell membranes are basically phospholipids with many added components that allow cells to filter and regulate the transport of specific nutritents and organelle components.
- 5.3 If the concentration of a particular substance outside the cell is greater than the concentration of that substance inside the cell, there is a tendency to push some of the substance through the cell wall into to cell. Passive transport or facilitated transport is simply the process of a membrane protein letting an otherwise restricted molecule into or out of the cell in the direction of the concentration gradient. The important point here is that the cell doesn't spend energy in this effort: it does no work.
- 5.4 If concentrations of a substance are different on two sides of the membrane, but the substance can't move through the membrane at all, water will move toward the side with the higher concentration, in the process called osmosis. Some cells, in particular plant cells, control their own water content collecting ions inside the cell (to pull water in) or pushing ions outside the cell (to force water out).
- 5.5 Controlling water levels is incredibly important; too little water and the cell cannot perform its metabolic functions; too much water and the cell actually pops open! We come back to the subject of osmoregulation when we discuss the functions of the excretory system later in the course.
- 5.6 If the substance is too big to fit through the possible openings between lipids in the membrane, or is ionized (has an electric charge) so that the lipids actually repel it, then it has to go through a protein gate. The gate doesn't have to do any work in this case, because the concentration "pushes" the molecule.
- 5.7 Aquaporins are special proteins that regulate the passage of water into cells.
- 5.8 In active transport, the protein burns cell energy to move a molecule against a concentration gradient. It gets the energy from a chemical called ATP, which we'll study more later. By controlling ion concentrations with active transport, cells perform certain vital functions in the body, such as the transmission of nerve signals.
- 5.9 Some things are just too big to push through even the largest proteins, so the cell actually surrounds the material with its own membrane, then brings it in or pushes it out by making a hole in the membrane structure. This method is very important for single-celled animals, which "eat" their prey by endocytosis, and for the immune system, which traps foreign bodies and absorbs them this way to prevent damage to the cell.
Read the following weblecture before chat: Transport across Membranes
Take notes on any questions you have, and be prepared to discuss the lecture in chat.
Perform the study activity below:
- Work through the BioCoach Activity Membrane Structure and Function.
- Optional Web site: Take a look at the Cellupedia site at Thinkquest. This site was designed, created, and executed by high school biology students! In particular, study the explanation of transport across cell membranes.
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- No quiz yet: the Chapter Quiz opens when we finish the chapter.
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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