The environment is dynamic – constantly subject to change. This is why the subject has become a field of study within academic settings. The complex interaction between organisms and their environment is known as ecology. When we say organism, we’re referring to living creatures, whether they are humans, beavers, or horses. Within the environment of the planet, one can find plants and animals occupying a distinct part of the environment. For example, certain communities of plants and animals will only occupy wet regions, while others other occupy dry regions. An ecosystem is, therefore, the name given to a community of animals and plants that occupy a specific region. This region is also known as its habitat.

An ecosystem has many different types of animals and plants – some of these will produce their own food, while others will need to eat food. The consumption of food is required to provide energy and nutrients to a given organism. An organism that produces its own food is known as a producer (for example: plants), while an organism that needs to eat things is known as a consumer (for example: animals). Some organisms, known as decomposers, break down waste plant and animal material. Bacteria, for instance, are examples of decomposers. Animals, or consumers, are divided according to what they need to consume for energy and nutrient purposes. The table below reflects these differences:

An ecosystem will typically have examples of producers, consumers, and decomposers. If there were no producers (plants), then herbivores could not survive. Given carnivores rely on herbivores as a source of food, they too would die out. So, the very existence of an ecosystem depends on the existence of a balance between producers, consumers, and decomposers. When the ecosystem becomes imbalanced, due to heavy pollution for example, it risks destroying the entire ecosystem. Pollution is just one example, of course, as we could also point to disease, which can wipe out particular species very quickly.

The vast array of living species makes it difficult to study ecology. This is why scientists invented the subject of taxonomy. Taxonomy is the science of classifying organisms into particular groups and families. This makes it considerably easier to document and identify species. In your ASVAB General Science exam, you will be expected to know the basics of taxonomy – possessing the knowledge to understand the classification and the various ranks within it. We will take a look at this system now – a system consisting of seven chief categories.

Note the difference in scale going from top to bottom. The broadest rank is the Kingdom (for example: animals), comprising many different phyla, classes, families, and species. The further we go down, the more specific we find the rank – to the point where we are only referring to one species at a time. Although this is a simple table, it hides the complexity of defining where a species should go. When a new species is discovered, scientists often have difficulty agreeing which order/family it should enter, though the same disagreement does not occur when it comes to defining its Kingdom or Species. Bear these factors in mind as you study this table.

Don’t worry – there is only one more sub-division to go! As part of your ASVAB General Science exam, you will be expected to know the 5 kinds of Kingdom. It’s not complex, but you will need to take the time out to learn them. Here is a list of all the facts you need to know:

This completes our examination of the environment. This comprehensive outline will prepare you for the ASVAB General Science questions in this area. This section is very much memorization oriented, meaning you need to take notes and continually revise over the next few days and weeks. Be sure to know the distinctions between the various definitions, as these often crop up as exam questions. In addition, be sure to know the classifications outlined in this section. We will now turn our attention to the next area of biology you need to know – the human body.

Your ASVAB General Science exam is replete with questions about the structure and function of the human body. Luckily, you are not expected to know this topic in detail but are, instead, expected to have a general understanding of how the body operates. This section provides you with that information, allowing you to digest (pardon the pun) and absorb the material at hand. At the same time, though, this section is remarkably simple to understand, the only issue you might have is memorizing the information. Thus, your study approach should be one of revision, revision and, you’ve guessed it, revision. A consistent study strategy such as this will reap dividends in the long run.

Just as the Kingdom is composed of many species, the human body is composed of many systems. There are 6 main systems you need to have knowledge of, and we will briefly go through them here to outline the main points you need to understand.

1. The Cardiovascular System

This system is characterized by the heart, blood, and blood vessels. The heart functions to pump blood around the body, courtesy of blood vessels called arteries and veins. Arteries are blood vessels that pump oxygenated blood (blood that contains oxygen) from the heart to the rest of the body. Oxygenated blood is necessary for every cell in the human body, as it allows cells to function properly. This blood travels around the body and deposits its oxygen to the cells that need it. When the blood loses its oxygen stores, it is now called deoxygenated blood. This deoxygenated blood needs more oxygen, so it must travel back to the heart in veins where the heart, in turn, sends it to the lungs. The deoxygenated blood travels across the lungs, picking up oxygen, and returning to the heart. The cycle now repeats, as the newly oxygenated blood travels back out the arteries to the rest of the body.

There are several key points you need to bear in mind:

2. The Immune System

The immune system is responsible for protecting us against infection. It achieves this thanks to the vast array of different cell types that serve distinct functions inside our bodies. White blood cells are responsible for conferring this protective function, as they consume bacteria in a digestive process known as phagocytosis. Most cell types, including white blood cells, are produced inside our bones. The core of our bones is composed of a substance called bone marrow, and it’s from here that many of our cells are produced. It would be wrong to think of bones as dead and useless for this reason. Other organs help our immune system to function. For example, the spleen is responsible for breaking down cells, extracting the useful substances and discarding the waste. These useful substances can be used to make more cells. The body needs to protect us against disease causing organisms – otherwise known as pathogens, and these bodily functions help to ward off these infections.

There are several key points you need to bear in mind:

3. Central Nervous System

Every time you move a finger, swallow food, or even think – you are using the central nervous system. This is composed of three distinct parts: the brain, the spinal cord, and the nerves that link up all these processes. Nerves are known anatomically as neurons, and our brain is composed of hundreds of millions of neurons. There are two main types of neuron: motor neuron and sensory neuron. The motor neuron, as the name implies, is required for movement. Sensory neurons, on the other hand, are required for sensation – such as touch, pain, and heat. If you were to touch the inside of a hot oven, this message would be sent by sensory neurons in your finger to the spinal cord and up to the brain. The brain will process this message and realize damage is being done, and so it sends a message back down the spinal cord to your hand via motor neurons – which move your hand away.

4. Digestive System

This system is responsible for digesting the food we eat every day. It begins at the mouth, and travels all the way through the esophagus, stomach, small intestine, and large intestine, until we finally remove the waste products. The purpose of digestion is to take all the useful nutrients out of food, all the while discarding the parts we do not need. The digestive process begins with our teeth and we have different types of teeth which have separate functions. Our canine teeth, as the name suggests, are used to tear food apart, while the molars (located toward the back) are required for grinding the food down. Substances throughout the digestive system, known as enzymes, help to break down food even further. When we finish chewing, the resulting mass of food is known as a bolus, and this bolus travels down the esophagus into the stomach.

The stomach is a highly acidic environment. Garden nails will, for example, break down in this environment over time. In science, when an environment has a high acidity, we say that it has a low pH. pH is a measure of the acidity of a substance or location. The lower the pH, the more potent the acidity, while the higher the pH, the less potent the acidity. The pH scale goes from 1 to 15, with the stomach ranging around 2 on this scale. The purpose of the stomach is to break down food even more, with the resulting slush known as chyme. This chyme pours into the small intestine. Almost all of the absorption of nutrients takes place in the small intestine, as the nutrients are absorbed through its walls. The pH of the small intestine is much higher than the stomach, rising to 6, meaning that it’s less acidic. Once the nutrients are absorbed, the remaining food passes further into the small intestine.

Toward the end of the small intestine, the remaining waste products pass into the large intestine. In the large intestine, some substances, such as water, are re-absorbed back into the body, and this means the mass of food becomes more solid. Other food, such as fiber, will not be absorbed by the body, and this forms the bulk of the feces. The feces pass slowly through the large intestine. The purpose of this slow transit is to absorb as much water and remaining nutrients as possible, resulting in a more solid waste product that’s easier to expel from the body. Eventually, thanks to nerve stimulation, the waste products are removed, and the cycle begins again.

There are numerous other organs of the digestive system you should also be aware of:

5. Respiratory System

The respiratory system is responsible for breathing. We breathe in air every day, and this air travels through our windpipe (trachea) into our lungs. The function of the lungs is to transport oxygen from the atmosphere into our blood, more specifically, red blood cells. The red blood cells are pumped around the body to deliver this oxygen to cells that need it. At the same time, though, carbon dioxide is exhaled from the lungs, as this gas can significantly alter the pH of our blood – which needs to be highly regulated. If our muscles, for example, did not receive enough oxygen, we feel the effects of this when we have a cramp – as a result of the build-up of lactic acid. Tiny structures in the lung, known as alveoli, are responsible for this exchange of gases, with the function of the alveoli becoming damaged through activities such as smoking or inhaling noxious substances.

6. Musculoskeletal System

The functions of the musculoskeletal system are as follows: to provide support and protection for the organs of our body, to produce cells inside the bone marrow of our bones, and to provide effective movement of our bodies. This is achieved through a complex meshwork of muscles and skeletons, all linked in sophisticated ways. Bones are, for example, attached to other bones by structures known as ligaments. Tendons, on the other hand, are structures that join muscle to bone. As part of your ASVAB General Science exam, you will be expected to know the anatomical name of common bones, and you will find these bones listed in the table below. It’s unlikely you will be asked a question about muscles but, if this does happen, it will ask about the whereabouts of common muscle groups – such as deltoids and biceps (arms), quadriceps and calves (legs), and trapezius muscle (shoulder).

This completes this section on the human body. This is, of course, quite a lot of study material. Still, there are often a disproportionate number of questions devoted to this section, so it’s certainly worth your while to learn everything documented here. Your best approach would be one that sticks to one section at a time, that way you will not be confused about the different functions and names. With this in mind, we will advance to the next section of our biology study guide, this time focusing on the cell – another section that often appears on the ASVAB General Science exam.

We are all composed of cells – all 37.5 trillion of them. There are hundreds of different cell types, and this can often be a source of confusion for many candidates. Throughout this discussion on biology, we have already noted several examples: such as white blood cells, red blood cells, platelets, protozoa, and many more. What we need to do now, though, is understand the fundamental structure and function of the typical cell, outlining how and why they work. Just like the human body has many organs, cells also have organs – though these organs are instead known as organelles. We will look at two types of common cell here: the plant cell and the animal cell. Note the different organelles in each cell, as this could be a possible ASVAB General Science exam question.

Both the plant cell and the animal cell have three structures in common. Both of them, for example, contain a nucleus. The nucleus is like the control center of the cell, giving out instructions due to the presence of DNA. Bacteria, on the other hand, do not contain a nucleus. The gooey substance that fills animal and plant cells is known as cytoplasm. Cytoplasm is mostly composed of water, but also contains some essential chemicals necessary for the efficient operation of the cell. The third structure, the cell membrane, is the name of the structure that holds the cell together. So, animal and plant cells are common in many respects – they all share a nucleus, cytoplasm and a cell membrane.

It’s worth recognizing, though, that plant and animal cells do have notable differences. We will examine these differences below:

There are several other structures found in animal and plant cells – ribosome’s and mitochondria. Ribosome’s are responsible in the cell for producing proteins. The DNA of the cell, found in the nucleus, sends a message to the ribosome’s to produce a particular protein. This protein then travels to its site of action to illicit its effect. Mitochondria, on the other hand, are the equivalent of a power plant – in the sense that they’re responsible for the production of energy. Interestingly, mitochondria also contain their own version of DNA. As stated previously, both ribosome’s and mitochondria are present in both animal and plant cells.

This completes our discussion of the cell. One of the major complications with this topic, for many students, is the amount of organelles you are required to remember. Yes, it is difficult, but try not to lose sight of their importance. It’s quite common to see, on the ASVAB General Science exam, questions asking about the functions of various organelles. You will also need to understand the differences between plant and animal cells, and what organelles are present in both. If you know these basic points, you will undoubtedly score well on this section of your exam. The final section of this guide looks at genetics, as a couple of your questions may be directed to this subject on the day of your exam.

Genetics is the study of genes and heredity in living organisms. Parents pass their genes into their offspring, with each gene coding for a particular trait. These traits include things such as eye color, hair color, size, and personality. Understanding the role genetics plays in our lives has helped to make huge advances within medicine, as we are now capable of isolating genes responsible for particular diseases. This helps us to develop a cure. In most cells of our body, we will find our genetic material. This material is stored as DNA, with the DNA organized into particular structures known as chromosomes. Some cells, though, such as the red blood cell, lack a nucleus and, as a result, do not contain these chromosomes.

In your ordinary healthy adult, you will find 23 pairs of chromosomes. These chromosomal structures contain all the genes we just described, allowing each cell to know how to behave and what way they should be. Note that we have 23 pairs of chromosomes – meaning we have 46 chromosomes in all. In each pair, one of the chromosomes came from the mother, while the other chromosome comes from the father. The cells we have in our body, as well as every other cell, need to multiply and divide at some stage. As a result of this division, the chromosomes are separated and divided, helping to form two new cells. The vast majority of cells, with the exception of bacteria, reproduce in this way in a process known as mitosis.

Not all cells divide by mitosis, there are two notable exceptions – bacteria and sex cells. Bacteria divide through a process known as binary fission (or ‘budding’), whereas sex cells divide through a process known as meiosis. Unlike mitosis which divides into two equal cells, meiosis divides one cell into four cells – each of which only contains half the number of chromosomes than in mitosis. This is a crucial distinction you need to think about. The reason sex cells only have half the number of chromosomes, is that they must match up with the other half during reproduction. When they join, you will find 23 pairs, or 46 chromosomes – the amount needed for human cells to operate and work effectively.

There is one special pair of chromosomes, though, known as the sex chromosomes. This pair determines the gender of the individual. One is known as the X chromosome while the other is known as the Y chromosome. If the pair of chromosomes is in the configuration of XX, the child will be female, but if the configuration is XY, the child will be male. Given the father is the individual with the XY pair of chromosomes, it is he who determines the sex of the child. This is just one aspect of the story. Some genes are expressed more than others in the child. So, if the father has brown eyes but the mother has blue eyes, the brown eyed gene is known as the dominant gene while the blue eyed gene is known as the recessive gene. As a result, the dominant gene will be expressed and the child will have brown eyes.