Figure 3.36
How a G-protein linked receptor works with the help of a G-protein. In panel C, the second messenger cAMP can be seen moving away from the enzyme.
G protein-linked receptors are only found in higher eukaryotes, including yeast, plants, and animals. Your senses of sight and smell are dependent on G-protein linked receptors. The ligands that bind to these receptors include light-sensitive compounds, odors, hormones, and neurotransmitters. The ligands for G-protein linked receptors come in different sizes, from small molecules to large proteins. G protein-coupled receptors are involved in many diseases, but are also the target of around half of all modern medicinal drugs.
The process of how a G-protein linked receptor works is outlined in Figure above.
A. A ligand such as a hormone (small, purple molecule) binds to the G-linked receptor (red molecule). Before ligand binding, the inactive G-protein (yellow molecule) has GDP bound to it.
B. The receptor changes shape and activates the G-protein and a molecule of GTP replaces the GDP.
C. The G-protein moves across the membrane then binds to and activates the enzyme (green molecule). This then triggers the next step in the pathway to the cell's response. After activating the enzyme, the G-protein returns to its original position. The second messenger of this signal transduction is cAMP, as shown in C.
The sensing of the external and internal environments at the cellular level relies on signal transduction. Defects in signal transduction pathways can contribute or lead to many diseases, including cancer and heart disease. This highlights the importance of signal transductions to biology and medicine.
Signal Responses
In response to a signal, a cell may change activities in the cytoplasm or in the nucleus that include the switching on or off of genes. Changes in metabolism, continued growth, movement, or death are some of the cellular responses to signals that require signal transduction.
Gene activation leads to other effects, since the protein products of many of the responding genes include enzymes and factors that increase gene expression. Gene expression factors produced as a result of a cascade can turn on even more genes. Therefore one stimulus can trigger the expression of many genes, and this in turn can lead to the activation of many complex events. In a multicellular organism these events include the increased uptake of glucose from the blood stream (stimulated by insulin), and the movement of neutrophils to sites of infection (stimulated by bacterial products). The set of genes and the order in which they are activated in response to stimuli are often called a genetic program.
Lesson Summary
Molecules and ions cross the plasma membrane either by passive transport or active the transport.
Passive transport is the movement of molecules across the cell membrane without an input of energy from the cell.
Diffusion is the movement of molecules or ions from an area of high concentration to an area of lower concentration. The molecules keep moving down the concentration gradient until equilibrium is reached.
Osmosis is the diffusion of water molecules across a semipermeable membrane and down a concentration gradient. They can move into or out of a cell, depending on the concentration of the solute.
Active transport moves molecules across a cell membrane from an area of lower concentration to an area of higher concentration. Active transport requires the use of energy.
The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins that are found in the membrane.
The sodium-potassium pump is an example of a cell membrane pump. It moves three sodium ions out of the cell and two potassium ions into the cell. The sodium-potassium pump uses ATP.
Endocytosis and exocytosis are active transport mechanisms in which large molecules enter and leave the cell inside vesicles.
In endocytosis, a substance or particle from outside the cell is engulfed by the cell membrane. The membrane folds over the substance and it becomes completely enclosed by the membrane. There are two main kinds of endocytosis: pinocytosis and phagocytosis.
Communication between cells is important for coordinating cell function in an organism. Membrane proteins and vesicles are involved in cellular communication.
Review Questions
Identify the two ways that particles cross the plasma membrane.
How does osmosis differ from diffusion?
Outline how the sodium-potassium pump works.
Are vesicles involved in passive transport? Explain.
What is the difference between endocytosis and exocytosis?
Why is pinocytosis (cellular drinking) a form of endocytosis?
Identify which type of feedback mechanism is most common in homeostasis, and give an example of that type.
Imagine you have discovered a new cell that has not been seen before. How would you go about identifying it based on its structure alone?
Homeostasis can be thought of as a dynamic equilibrium rather than an unchanging state. Do you agree with this statement? Explain your answer.
This image shows plant cells. The central vacuole of each cell has shrunk and is smaller than normal. What is the likely solute concentration of the cells' environment which has caused this change?
Figure 3.37
Further Reading / Supplemental Links
Biology © 2002 6th Edn. Campbell and Reese. Published by Benjamin Cummings.
http://fig.cox.miami.edu/~cmallery/150/unity/cell.text.htm
http://en.wikipedia.org
Vocabulary
active transport
The energy-requiring process of pumping molecules and ions across membranes against a concentration gradient.
carrier protein
A transport protein that is specific for an ion, molecule, or group of substances; carries the ion or molecule across the membrane by changing shape after the binding of the ion or molecule.
channel protein
A transport protein that acts like a pore in the membrane that lets water molecules or small ions through quickly.
contractile vacuole
A type of vacuole that removes excess water from a cell.
diffusion
The movement of molecules from an area of high concentration of the molecules to an area with a lower concentration.
endocytosis
The process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane.
exocytosis
The process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell.
facilitated diffusion
The diffusion of solutes through transport proteins in the plasma membrane.
gated channel protein
A transport protein that opens a "gate," allowing a molecule to flow through the membrane.
ion channel
A protein that transports ions across the membrane by facilitated diffusion.
ligand
A small molecule that binds to a larger molecule.
osmosis
The diffusion of water molecules across a selectively permeable membrane from an area of higher concentration to an area of lower concentration.
passive transport
A way that small molecules or ions move across the cell membrane without input of energy by the cell.
second messenger
A small molecule that starts a change inside a cell in response to the binding of a specific signal to a receptor protein.
selectively permeable
The characteristic of the cell membrane that allows certain molecules to pass through the membrane, but not others.
signal-transduction pathway
The signaling mechanism by which a cell changes a signal on it surface into a specific response inside the cell; most often involves an ordered sequence of chemical reactions inside the cell which is carried out by enzymes and other molecules.
sodium-potassium pump
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A carrier protein that moves sodium and potassium ions against large concentration gradients, moves two potassium ions into the cell where potassium levels are high, and pumps three sodium ions out of the cell and into the extracellular fluid.
transport protein
A protein that completely spans the membrane, and allows certain molecules or ions to diffuse across the membrane; channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion.
Points to Consider
Next we turn our attention to photosynthesis.
What is photosynthesis?
Where to plants get the "food" they need?
Where does most of the energy come from?
Chapter 4: Photosynthesis
Lesson 4.1: Energy for Life: An Overview of Photosynthesis
Lesson Objectives
Identify the kind of energy which powers life.
Contrast the behavior of energy to that of materials in living systems.
Analyze the way in which autotrophs obtain energy and evaluate the importance of autotrophs to energy for all life.
Explain the relationship between autotrophs and heterotrophs.
Discuss the importance of glucose to all life on earth.
Compare the energy-carrying role of ATP to that of glucose.
Explain the roles of chlorophyll and NADPH as sources of energy for life.
Summarize the process of photosynthesis and write out the overall chemical equation for photosynthesis.
Identify reactants, necessary conditions, and products in the chemical equation for photosynthesis.
Describe the roles of chlorophyll and chloroplasts in photosynthesis.
Identify the groups of organisms which are capable of photosynthesis.
Discuss the many reasons photosynthesis is important to humans.
Introduction
All living things require an ongoing source of energy to do the work of life. You often see energy in action on a large scale: a whale breaches, apple blossoms swell and burst, a firefly glows, or an inky cap mushrooms overnight. However, energy works constantly to maintain life on a very small scale as well. Inside each cell of every organism, energy assembles chains of information and constructs cellular architecture. It moves tiny charged particles and giant protein molecules. Moreover, it builds and powers cell systems for awareness, response, and reproduction. All life’s work requires energy.
Physics tells us that organized systems, such as living organisms, tend to disorder without a constant input of energy. You have direct, everyday experience with this law of nature: after a week of living in your room, you must spend energy in order to return it to its previous, ordered state. Tides and rain erode your sandcastles, so you must work to rebuild them. And your body, after a long hike or big game, must have more fuel to keep going. Living things show amazing complexity and intricate beauty, but if their source of energy fails, they suffer injury, illness, and eventually death.
Physics also tells us that, although energy can be captured or transformed, it inevitably degrades, becoming heat, a less useful form of energy. This is why organisms require a constant input of energy; the work they must do uses up the energy they take in. Energy, unlike materials, cannot be recycled. The story of life is a story of energy flow – its capture, transformation, use for work, and loss as heat.
Energy, the ability to do work, can take many forms: heat, nuclear, electrical, magnetic, light, and chemical energy. Life runs on chemical energy - the energy stored in covalent bonds between atoms in a molecule. Where do organisms get their chemical energy? That depends…
How Do Organisms Get Energy? Autotrophs vs. Heterotrophs
Living organisms obtain chemical energy in one of two ways.
Autotrophs, shown in Figure below, store chemical energy in carbohydrate food molecules they build themselves. Food is chemical energy stored in organic molecules. Food provides both the energy to do work and the carbon to build bodies. Because most autotrophs transform sunlight to make food, we call the process they use photosynthesis. Only three groups of organisms - plants, algae, and some bacteria - are capable of this life-giving energy transformation. Autotrophs make food for their own use, but they make enough to support other life as well. Almost all other organisms depend absolutely on these three groups for the food they produce. The producers, as autotrophs are also known, begin food chains which feed all life. Food chains will be discussed in the Principles of Ecology chapter.
Heterotrophs cannot make their own food, so they must eat or absorb it. For this reason, heterotrophs are also known as consumers. Consumers include all animals and fungi and many protists and bacteria. They may consume autotrophs, or other heterotrophs or organic molecules from other organisms. Heterotrophs show great diversity and may appear far more fascinating than producers. But heterotrophs are limited by our utter dependence on those autotrophs which originally made our food. If plants, algae, and autotrophic bacteria vanished from earth, animals, fungi, and other heterotrophs would soon disappear as well. All life requires a constant input of energy. Only autotrophs can transform that ultimate, solar source into the chemical energy in food which powers life, as shown in Figure below.
Figure 4.1
Photosynthetic autotrophs, which make food for more than 99% of the organisms on earth, include only three groups of organisms: plants such as the redwood tree (a), algae such as kelp (b), and certain bacteria like this (c).
Photosynthesis provides over 99 percent of the energy supply for life on earth. A much smaller group of autotrophs - mostly bacteria in dark or low-oxygen environments - produce food using the chemical energy stored in inorganic molecules such as hydrogen sulfide, ammonia, or methane. While photosynthesis transforms light energy to chemical energy, this alternate method of making food transfers chemical energy from inorganic to organic molecules. It is therefore called chemosynthesis, and is characteristic of the tubeworms shown in Figure below. Some of the most recently discovered chemosynthetic bacteria inhabit deep ocean hot water vents or “black smokers.” There, they use the energy in gases from the Earth’s interior to produce food for a variety of unique heterotrophs: giant tube worms, blind shrimp, giant white crabs, and armored snails. Some scientists think that chemosynthesis may support life below the surface of Mars, Jupiter's moon, Europa, and other planets as well. Ecosystems based on chemosynthesis may seem rare and exotic, but they too illustrate the absolute dependence of heterotrophs on autotrophs for food.
Figure 4.2
Food chains carry energy from producers (autotrophs) to consumers (heterotrophs). 99 percent of energy for life comes from the sun via photosynthesis. Note that only nutrients recycle. Energy must continue to flow into the system.
Figure 4.3
Tubeworms deep in the Gulf of Mexico get their energy from chemosynthetic bacteria living within their tissues. No digestive systems needed! Photo: Charles Fisher
Food and Other Energy-Carrying Molecules
You know that the fish you had for lunch contained protein molecules. But do you know that the atoms in that protein could easily have formed the color in a dragonfly’s eye, the heart of a water flea, and the whiplike tail of a Euglena before they hit your plate as sleek fish muscle? As you learned above, food consists of organic (carbon-containing) molecules which store energy in the chemical bonds between their atoms. Organisms use the atoms of food molecules to build larger organic molecules including proteins, DNA, and fats and use the energy in food to power life processes. By breaking the bonds in food molecules, cells release energy to build new compounds. Although some energy dissipates as heat at each energy transfer, much of it is stored in the newly made molecules. Chemical bonds in organic molecules are a reservoir of the energy used to make them. Fueled by the energy from food molecules, cells can combine and recombine the elements of life to form thousands of different molecules. Both the energy (despite some loss) and the materials (d
espite being reorganized) pass from producer to consumer – perhaps from algal tails, to water flea hearts, to dragonfly eye colors, to fish muscle, to you!
The process of photosynthesis, which usually begins the flow of energy through life, uses many different kinds of energy-carrying molecules to transform sunlight energy into chemical energy and build food.
Some carrier molecules hold energy briefly, quickly shifting it like a hot potato to other molecules. This strategy allows energy to be released in small, controlled amounts. An example is chlorophyll, the green pigment present in most plants which helps convert solar energy to chemical energy. When a chlorophyll molecule absorbs light energy, electrons are excited and “jump” to a higher energy level. The excited electrons then bounce to a series of carrier molecules, losing a little energy at each step. Most of the “lost” energy powers some small cellular task, such as moving ions across a membrane or building up another molecule. Another short-term energy carrier important to photosynthesis, NADPH, holds chemical energy a bit longer but soon “spends” it to help to build sugar.
Two of the most important energy-carrying molecules are glucose and ATP, adenosine triphosphate. These are nearly universal fuels throughout the living world and both are also key players in photosynthesis, as shown below.
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