Quickly gain a reputation by helping other students with their questions. When students see your nickname, they'll immediately associate your answer with credibility and expertise. Also, earn credits for sharing your knowledge and redeem them for rewards.
Significant Step Towards Full 3-D INVISIBILITY CLOAK In Visible Light
Full 3-D invisibility cloak in visible light
Watching things disappear "is an amazing experience," admits Joachim Fischer of the Karlsruhe Institute of Technology in Germany. But making items vanish is not the reason he creates invisibility cloaks. Rather, the magic-like tricks are attractive demonst
That's what she said: Software that tells dirty jokes
Double entendres have been making us laugh since the days of Chaucer and Shakespeare, but up until now computers weren't in on the joke. Chloé Kiddon and
On Monday evening, TIME’s cover lit up the Nasdaq screen in Times Square in Manhattan. The cover features bin Laden’s face covered by a red “X.” This image has become iconic after its use with Hitler, Saddam Hussein and Abu Musab al-Zarqawi. The bin Laden cover was illustrated
Innate ‘Sense Of Justice’ Neural Circuitry Built Into The Brain
A new study from the Karolinska Institute and Stockholm School of Economics shows that the brain has built-in mechanisms that trigger an automatic reaction to someone who refuses to share. In the study publishing next week in the online open access journal PLoS Biology, the subjects' sense of justice was challenged in a two-player monetary fairness gam
Why is genetic drift a more powerful force in small populations?
The inheritance of genes in sexual reproduction has a major chance component. During meiosis homologous chromosomes separate and migrate to different poles. The haploid daughter cells each have one allele for each gene, but which allele they have is random. Essentially meiosis is like flipping thousands of
How is hunger related to the carrying capacity of the environment?
Every population occurs in an environment. Based on the resources in the environment each population has a carrying capacity. The carrying capacity is the number of individuals the environment will support. The amount of available food and other resources and the presence or absence of other species are t
What assumptions are built into estimations of life expectation?
This animation calculates a life expectancy based on information used by insurance companies. Data is collected on the number of people dying in each age class (year of age) for a particular year. This pattern is assumed to continue unchanged into the future. To calculate the average of life expectancy for
How is the type of risk to a fetus related to the developmental stage?
As animals develop they go from a small number of non-specialized cells to an larger number of more specialized cells. The increased size of the embryo means an increased nutritional demand. Damage to the embryo at a very early stage is likely to result in a non-viable embryo because the loss of even a
What is the difference between sperm and egg cells?
Gametes are haploid cells used in sexual reproduction. Plants and animals and many other organisms produce two types of gametes – eggs and sperm. Eggs are large gametes that are not capable of active movement and have substantial amounts of nutrients. Sperm are small gametes that are capable of movement in most organisms
In order to produce haploid gametes, individuals must have cells that undergo cell division that does not produce daughter cells that are genetically identical to the parent. Otherwise the gametes would be diploid and a zygote formed from diploid sperm and egg would be tetraploid. The two stage process of meiosis reduces the number of chro
The basic principles of genetic inheritance are segregation and independent assortment. Each individual has two alleles for a particular gene. Only one of these alleles is passed on to any particular offspring that individual might have. The alleles segregate. The principle of independent assortment states that the segregatio
What is the difference between positive and negative feedback?
Many molecular and physiological processes are controlled by feedback mechanisms. In a feedback loop the product of a process, such as the breakdown of proteins into amino acids, has an effect on the rate of the process. Negative feedback occurs when the rate of the process decreases as the concentratio
Each follicle is a single egg cell surrounded by several layers of follicle cells. An ovary consists of many follicles. The follicle cells protect and nourish the egg prior to its release into the oviducts during ovulation. The follicular tissue also release estrogens. The estrogens interact with other hormones produced in the brain t
What physical processes remove waste from the blood?
Waste material is removed from the blood in the nephrons of the kidney. In each nephron, the blood is forced through a ball of porous capillaries called the glomerulus. Pressure forces most of the fluid, solutes, and smaller suspended material out of the blood and into Bowman’s capsule. Not only is waste removed but als
What factors influence the concentration of urine?
rine contains water, nitrogenous wastes, and salts. The concentration of urine can vary considerably within our species and much more widely when we compare different species of animals. The variation in concentration depends on the structure of the kidney and on the environment the organism is experiencing. If protein o
What is the advantage of having more than one antibody attack the same structure?
Antibodies recognize specific areas of antigens called epitopes. Several different antibodies can attack antigens in different areas. If an antigen evolves a different structure a specific antibody may no longer be able recognize and bind to it. The antigen is much less likely to evolve diff
Animation - Mechanical Events of the Cardiac Cycle
Why does the atrium not experience large changes in pressure?
The hearts of mammals and birds have four chambers consisting of two pairs – a left ventricle and atrium and a right ventricle and atrium. In both cases the atrium collects blood entering the heart and then transfers it to the ventricle. The force moving blood from atrium to ventricle is largely generated by th
How are all the components of the heart able to work together without direct control from the central nervous system?
For proper function of the heart it is necessary that all the muscle fibers in a region contract in unison. It is also necessary that all the different regions of the heart contract in a coordinated but NOT in unison. The non-synchronous coordination is ac
Animation - Baroreceptor Reflex Control of Blood Pressure
What causes abnormal blood pressure?
Blood pressure is determined by the force of the blood acting on the walls of the blood vessels. Two factors determine the size of this force. One is the volume of blood being pumped through the vessel. The other is the size of the vessels. Changes in blood pressure can be caused by either a change in the amount of blood being pumped o
Animation - Chemoreceptor Reflex Control of Blood Pressure
Why would the chemical composition of the blood affect heart rate?
Blood chemistry is determined, to a large extent, by the concentrations of gases in the blood. High carbon dioxide concentrations and low oxygen concentrations both indicate that gas exchange is occurring at a slower than ideal rate. The low rate of gas exchange could be due to rapid metabolism or to a low
Why is the processing of hemoglobin different from the processing of many other macromolecules?
Hemoglobin contains a heme group with contains iron. The heme group is used to bind oxygen. Iron is not used in most other types of macromolecules and is not common in biological systems. Conservation of iron is important as any iron lost will need to be replaced from the diet.
Animation - Alveolar Pressure Changes During Inspiration and Expiration
Why are lungs not good at breathing underwater?
Lungs and gills operate under the same basic principles. Both are highly branched structures with blood vessels inside a membrane through which gas exchange occurs. The differences between the two types of respiratory organs are related to physical differences between water and air. Water both supports gills and keeps the ga
Why are red blood cells needed to transport gases in the blood?
Oxygen and carbon dioxide will both go into solution in the plasma (the fluid part of the blood). Hemoglobin in red blood cells will bond to both oxygen and carbon dioxide. There are several reasons why red blood cells are necessary for respiration. First oxygen gas has a very low solubility in blood fluids a
All organisms require the intake of carbon, water, and nutrients. However, only animals have a system of organs whose primary function is to break down and absorb nutrients. Fungi absorb material from the environment as individual molecules. Plants obtain carbon through photosynthesis and other nutrients are absorbed from
[Animation] - Action Potentials and Muscle Contraction
How do nerve impulses cause muscle contraction?
Skeletal muscle is under voluntary control. Nerve impulses that originate in the central nervous system cause muscles to contract. Both neurons and muscle tissue conduct electrical current by moving ions across cellular membranes. A motor neuron ends in a synapse with a muscle fiber. The neuron releases acetylcholine
[Animation] - Breakdown of ATP and Cross-Bridge Movement During Muscle Contraction
What is the pathway of energy transfer during muscle movement?
The contraction of skeletal muscles is one of the most energetically expensive activities that the body does on a regular basis. Adenosine triphosphate (ATP) is split into adenosine diphosphate (ADP) and phosphate to supply the energy for muscle contraction. The free energy released by the ATP when the
What limits the extent to which muscles can contract?
Muscle contractions occur through the sliding of actin and myosin filaments past one another. The filaments themselves do not contract and the other elements of an individual sarcomere are not compressed. The sarcomere is able to contract without seriously bending or distorting itself. Contraction ends when the
Skeletal muscles, particularly those on the long bones are often organized into opposing groups of muscle fibers. For example the biceps and triceps muscles are on opposite sides of the humerus and cause opposite movements of the forearm. Opposing groups allow a wide range of movements to be powered by muscle contract
Bone strength is determined by the internal structure, shape, and size of the bone. Bone is dense relative to other tissues and therefore the size and strength of a bone is generally determined by both the need for strength and the cost of increasing the weight of the organism. Strength increases with bone density, however organisms a
Why is bone growth different from the growth of many other organs?
Although bone may appear to be rigid and lifeless it is actually living tissue that is capable of growth. Unlike soft tissues, bone cannot simply grow by adding additional cells and removing cells that are no longer necessary. The calcium laid down in bone gives the skeleton the strength and rigidity neede
