GED Science: Cells and the Organization of Life Concepts
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Overview
This chapter covers cells, the basic units of life, and how they are organized into larger structures. It also explains how cells divide.
The cell theory says that all living things are made of cells, the cell is the smallest unit of life, and new cells come only from existing cells. Inside a cell, the cell membrane controls what enters and leaves, the nucleus holds the DNA, ribosomes make proteins, and mitochondria release energy. Plant cells also have a cell wall and chloroplasts. Enzymes are proteins that speed up chemical reactions; they work best at a certain temperature, and too much heat changes their shape so they stop working well.
In many-celled organisms, similar cells form tissues, tissues form organs, and organs work together in organ systems. Cells divide by mitosis to grow and repair: one cell makes two identical cells. Meiosis makes sex cells (eggs and sperm) with half the usual number of chromosomes, 23 in humans instead of 46.
Key concepts
Cell theory
All living things are made of cells; the cell is the smallest unit of life; cells come from existing cells.
Memory tipCells come only from cells.
Cell membrane
The outer boundary of the cell. It controls what moves in and out.
Memory tipThink of it as the cell's gatekeeper.
Levels of organization
Cell → tissue → organ → organ system → organism.
Memory tipEach level is made of the one before it.
Enzyme
A protein that speeds up a chemical reaction. It has a best (optimum) temperature; high heat changes its shape and slows it.
Memory tipActivity rises to a peak, then falls if it gets too hot.
Mitosis
Cell division that makes two identical cells with the full number of chromosomes. Used for growth and repair.
Memory tipMitosis = make more of the same.
Meiosis
Cell division that makes sex cells with half the number of chromosomes. Human body cells have 46; eggs and sperm have 23.
Memory tipMeiosis = half the chromosomes, for sex cells.
Often tested
- Signs of life include taking in nutrients, using energy, and reproducing.
- The best temperature for an enzyme is the highest point in the activity data.
- Mitosis: 2 identical cells. Meiosis: 4 sex cells with half the chromosomes.
Easy to confuse: Mitosis and meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth and repair | Making eggs and sperm |
| Cells made | 2 identical cells | 4 sex cells, each different |
| Chromosomes in humans | 46 in each new cell | 23 in each sex cell |
3 sample questions
Verified practice questions from this unit.
Question 1. Which statement is part of the cell theory?
- ① Cells are larger than the organs they form.
- ② All living things are made of one or more cells.
- ③ Only animals are made of cells.
- ④ Cells can form from nonliving matter.
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Answer: ② All living things are made of one or more cells.
Key point: Cell theory: all life is made of cells; cells come from cells
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.
Question 2. Which part of a cell controls what moves into and out of the cell?
- ① Cell membrane
- ② Chloroplast
- ③ Nucleus
- ④ Ribosome
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Answer: ① Cell membrane
Key point: The cell membrane controls what enters and leaves
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.
Question 3. Which list shows the levels of organization in the body from smallest to largest?
- ① Organ → tissue → cell → organ system
- ② Cell → tissue → organ → organ system
- ③ Organ system → organ → tissue → cell
- ④ Tissue → cell → organ → organ system
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Answer: ② Cell → tissue → organ → organ system
Key point: Cells → tissues → organs → organ systems
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.
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