GED Science Cells and the Organization of Life Summary

The cell theory has three main ideas: all living things are made of cells, the cell is the smallest unit of life, and new cells come only from existing cells.

Wrong choices

• Cells are larger than the organs they form.: Organs are made of many cells, so cells are much smaller than organs.
• Only animals are made of cells.: Plants, fungi, and bacteria are made of cells too.
• Cells can form from nonliving matter.: Cell theory says the opposite: new cells come only from existing cells.

The cell membrane is a thin layer around the cell. It lets some substances (such as water and oxygen) pass and keeps others out, so it controls what enters and leaves the cell.

Wrong choices

• Chloroplast: Chloroplasts carry out photosynthesis in plant cells.
• Nucleus: The nucleus holds DNA and controls cell activities, but it does not form the cell's outer boundary.
• Ribosome: Ribosomes make proteins.

Similar cells form a tissue (such as muscle tissue). Different tissues form an organ (such as the heart). Organs work together in an organ system (such as the circulatory system).

Wrong choices

• Organ → tissue → cell → organ system: Organs are made of tissues, so they cannot be smaller than tissues.
• Organ system → organ → tissue → cell: This goes from largest to smallest, the reverse of what was asked.
• Tissue → cell → organ → organ system: Tissues are made of cells, so cells come before tissues.

In mitosis, a cell copies its chromosomes and divides into two daughter cells. Each new cell has the same number and kind of chromosomes as the original. The body uses mitosis for growth and to replace or repair cells.

Wrong choices

• Cells that have no DNA: Each new cell gets a full copy of the DNA.
• Four cells with half the number of chromosomes: Four cells with half the chromosomes is the result of meiosis, which makes sex cells.
• One larger cell with twice the DNA: Mitosis divides one cell into two. It does not make one larger cell.

Meiosis makes sex cells (eggs and sperm) with half the usual chromosome number: 46 ÷ 2 = 23. When an egg (23) and a sperm (23) join, the fertilized egg has 46 again.

Wrong choices

• 12: 12 is not half of 46.
• 46: 46 is the number in body cells. Meiosis cuts the number in half.
• 92: 92 would be double. Meiosis halves the number.

The highest activity in the table is 40 units per minute, at 37°C. Activity rises up to 37°C and then falls at higher temperatures.

Wrong choices

• 20°C: 20°C gave only 12, much lower than the best value of 40.
• 50°C: At 50°C the activity had already dropped to 10.
• 60°C: 60°C gave the lowest activity, only 2.

Enzymes are proteins with a specific shape. Moderate warming speeds them up, but too much heat changes (denatures) their shape. Then they cannot fit with the substances they act on, so activity drops.

Wrong choices

• The temperature was too cold for the enzyme.: 50°C and 60°C are the hottest temperatures in the test, not the coldest.
• The enzyme needed sunlight and the room was dark.: Enzymes do not need sunlight to work.
• Enzymes always work faster when they are hotter.: The data show the opposite above 37°C: hotter gave slower activity.

Living things carry out life functions such as taking in nutrients (metabolism) and reproducing. Cells come from other cells. Taking in nutrients and dividing are strong evidence of life. Size, color, or dissolving are not.

Wrong choices

• It is very small and round.: Many nonliving things, such as grains of sand, are small and round.
• It changes color when placed in sunlight.: Some nonliving materials also change color in light.
• It dissolves when placed in salt water.: Many nonliving substances dissolve, so this does not show life.