Grade 6 Science Practice Test: Cells, Body Systems and Weather

The cell theory says that all living things are made of one or more cells, that the cell is the basic unit of life, and that new cells come only from existing cells. That last idea rules out cells forming from nonliving matter, and bacteria and fungi are made of cells too.

Plant cells have a stiff cell wall outside the cell membrane and chloroplasts for photosynthesis; animal cells have neither. Both kinds of cells have a nucleus, a cell membrane, cytoplasm and mitochondria, which is why "nucleus and chloroplasts" is only half right.

The cell membrane surrounds the cell and lets some substances, such as water, oxygen and nutrients, pass through while keeping others out. The nucleus holds the cell's DNA and directs its activities, but it does not form the cell's border.

Sample 1 has a cell wall and chloroplasts, as a green leaf cell does. Sample 2 has a nucleus but no cell wall and no chloroplasts, which fits an animal cell. Sample 3 cannot be an animal cell because it has a cell wall; it could come from a plant root, where cells have no chloroplasts.

Similar cells working together form a tissue, different tissues form an organ such as the heart, organs work together in an organ system, and all the systems together make up the organism. The list that starts with "organism" has the right levels but runs from largest to smallest.

The respiratory system brings air into the lungs, where oxygen passes into the blood. The circulatory system (heart, blood and blood vessels) then carries that oxygen-rich blood to the muscles. The other systems have different main jobs, such as breaking down food or supporting the body.

Breathing rose from 14 to 32 breaths per minute as the activity got harder, because working muscle cells use more oxygen to release energy from food and give off more carbon dioxide. Five minutes after jogging the rate was almost back to normal, so it does not stay high all day.

Sense receptors in the nose detect the smoke and send signals along nerves to the brain. The brain processes the signals and can match them with memories stored from earlier experiences, which is how Mateo knows the smell means something is burning.

As the warm, moist air is forced upward, it cools and its water vapor condenses into clouds, often bringing showers or thunderstorms. Once the front has passed, the cold air mass brings lower temperatures. Long periods of light rain followed by warming are more typical of a warm front.

Falling air pressure often means a low-pressure system is arriving, where air rises, cools and forms clouds. The rising humidity and thickening clouds point the same way, so rain is the best forecast. It is rising pressure, not falling pressure, that usually brings clear, dry weather.

Energy from the Sun evaporates water and drives the movement of air that carries water vapor. Gravity pulls water back down as rain and snow and makes it flow downhill in rivers and soak into the ground.

Water needs more energy than land to change its temperature, so the ocean warms slowly in summer and cools slowly in winter. Air from the ocean keeps the coastal town cooler in summer and milder in winter: its range is 18°F, compared with 60°F inland. Both towns are the same distance from the Sun.