What you'll learn
Every living thing, from a bacterium to a blue whale, is built from cells. In this lesson you will learn:
- the cell theory and the difference between simple (prokaryotic) and complex (eukaryotic) cells;
- the main organelles and the job each one does, and how plant and animal cells differ;
- the levels of organization: cell → tissue → organ → organ system → organism;
- how specialized cells have structures that match their jobs;
- how cells divide by mitosis (growth and repair) and meiosis (making sperm and eggs).
How it shows up on the GED: Life Science is about 40% of the Science test, and cell questions are common. You will usually see a labeled cell diagram, a table of cell types, a graph of cells in each stage of division, or a short reading about an experiment, and you will need to identify a structure, compare two cells, interpret data, or choose the conclusion the evidence supports. Expect multiple choice, drop-down, drag-and-drop and hot-spot items.
Key terms
- cell — the smallest unit that can carry out all the processes of life (a single bacterium is one cell).
- cell theory — all living things are made of cells; the cell is the basic unit of life; all cells come from existing cells.
- prokaryote — a cell with no nucleus and no membrane-bound organelles (bacteria).
- eukaryote — a cell with a nucleus and membrane-bound organelles (plants, animals, fungi, protists).
- organelle — a "little organ" inside a cell with a specific job (mitochondrion, chloroplast).
- nucleus — holds the cell's DNA and controls its activities.
- cell membrane — thin, flexible boundary that controls what enters and leaves the cell (all cells have one).
- cell wall — stiff layer outside the membrane that supports and protects (plants, fungi, bacteria; not animals).
- mitochondrion (plural mitochondria) — releases energy from sugar as ATP (cellular respiration).
- chloroplast — captures light energy to make sugar (photosynthesis); found in plants and algae.
- vacuole — storage sac; plant cells have one large central vacuole that holds water.
- tissue — a group of similar cells doing the same job (muscle tissue).
- organ — several tissues working together (heart, leaf).
- organ system — organs that work together (digestive system).
- specialized cell — a cell whose shape and parts suit one job (a nerve cell is long to carry signals).
- mitosis — division that makes 2 identical cells; used for growth and repair.
- meiosis — division that makes 4 sex cells, each with half the chromosomes.
- chromosome — a tightly coiled strand of DNA carrying many genes.
- diploid (2n) / haploid (n) — the full chromosome number (human body cells: 46) / half that number (human sperm or egg: 23).
- gamete — a sex cell: sperm or egg.
1. The cell theory and two kinds of cells
By the 1800s, improved microscopes gave scientists enough evidence to state the cell theory:
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- All cells come from pre-existing cells.
The third part ended the old idea of spontaneous generation (that life could arise from mud or rotting meat). The cell theory is a true scientific theory: a broad explanation backed by a huge amount of evidence.
Two basic cell types
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No (DNA floats in the cytoplasm) | Yes |
| Membrane-bound organelles | No | Yes (mitochondria, etc.) |
| Examples | Bacteria | Plants, animals, fungi, protists |
Both types have a cell membrane, cytoplasm, ribosomes and DNA.
Example: a cell has a cell wall, ribosomes and DNA but no nucleus. It must be a prokaryote, such as a bacterium. A cell wall alone does not make it a plant cell.
2. Organelles: the cell's workers
Each organelle in a eukaryotic cell has a job.
| Organelle | Job |
|---|---|
| Nucleus | Stores DNA; directs the cell |
| Cell membrane | Controls what enters and leaves |
| Cytoplasm | Holds organelles; many reactions happen here |
| Mitochondria | Release energy from sugar (make ATP) |
| Ribosomes | Build proteins |
| Endoplasmic reticulum (ER) | Moves materials; rough ER helps make proteins |
| Golgi apparatus | Packages and ships proteins |
| Lysosomes | Break down wastes and worn-out parts |
| Vacuole | Stores water, food and wastes |
| Chloroplast (plants) | Makes sugar using light energy |
| Cell wall (plants) | Supports and protects |
Plant vs. animal cells
| Structure | Plant cell | Animal cell |
|---|---|---|
| Cell membrane, nucleus, mitochondria, ribosomes | Yes | Yes |
| Cell wall | Yes (made of cellulose) | No |
| Chloroplasts | Yes (in green parts) | No |
| Vacuole | One large central vacuole | Small or none |
| Shape | Boxy, fixed | Rounded, flexible |
Example: plants have both chloroplasts and mitochondria. Chloroplasts store light energy in sugar; mitochondria then release energy from that sugar. A common wrong answer claims plants have chloroplasts instead of mitochondria.
3. Levels of organization and specialized cells
In a many-celled organism, cells are organized in levels. Each level is built from the one before it:
cell → tissue → organ → organ system → organism
| Level | Example (human) | Example (plant) |
|---|---|---|
| Cell | Muscle cell | Leaf cell |
| Tissue | Cardiac muscle tissue | Photosynthetic tissue |
| Organ | Heart | Leaf |
| Organ system | Circulatory system | Shoot system |
| Organism | A person | A tomato plant |
Humans have four main kinds of tissue: epithelial (covers and lines surfaces, like skin), connective (supports and binds, like bone and blood), muscle (moves) and nervous (carries signals). An organ such as the stomach contains all four.
Specialized cells: form fits function. Every body cell has the same DNA, but cells use different genes and develop different shapes and parts.
| Cell | Special feature | How it helps |
|---|---|---|
| Red blood cell | Disk shape, packed with hemoglobin, no nucleus in mammals | Carries more oxygen; squeezes through tiny vessels |
| Nerve cell (neuron) | Long, thin branches | Carries signals long distances |
| Muscle cell | Many mitochondria; proteins that contract | Supplies energy for movement |
| Sperm cell | Tail (flagellum); mitochondria in the midpiece | Swims to the egg |
| Root hair cell (plant) | Long, thin extension | Large surface area to absorb water |
4. How cells divide: the cell cycle, mitosis and meiosis
The cell cycle has two main parts:
- Interphase (most of the cycle): the cell grows, does its job and copies its DNA.
- Mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).
Mitosis has four stages, often remembered as PMAT:
- Prophase: chromosomes condense and become visible.
- Metaphase: chromosomes line up in the middle.
- Anaphase: copies are pulled apart, to opposite ends.
- Telophase: two new nuclei form; then the cell splits.
When a tissue has many cells spread randomly across these stages, the fraction of cells in a stage estimates the fraction of time the stage takes. Because most cells are in interphase, interphase is the longest part of the cycle.
Meiosis makes gametes. It has two divisions (meiosis I and II) after one round of DNA copying, so one diploid cell produces four haploid cells. In meiosis I, matching chromosomes can cross over (swap pieces), and pairs separate randomly. Both steps create genetic variety.
| Mitosis | Meiosis | |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Making sperm and eggs |
| Number of divisions | 1 | 2 |
| Cells produced | 2 | 4 |
| Chromosome number | Same as parent (2n) | Half of parent (n) |
| Genetically | Identical to parent | Different from parent and each other |
| Human example | Skin cell 46 → two cells of 46 | Cell of 46 → four gametes of 23 |
At fertilization, 23 + 23 = 46: meiosis keeps the chromosome number steady from one generation to the next. When the controls on the cell cycle fail, cells can divide out of control. That is what happens in cancer.
Estimating time spent in a stage
When cells are dividing independently, use:
\text{time in stage} = \frac{\text{cells in stage}}{\text{total cells counted}} \times \text{length of whole cycle}and
\text{percent in stage} = \frac{\text{cells in stage}}{\text{total cells}} \times 100Chromosome numbers: haploid n = \dfrac{2n}{2}. For humans, 2n = 46 and n = 23.
The Science test has no formula sheet; learn the part-over-whole idea. To change decimal hours to minutes, multiply by 60.
Worked example 1: Identify the cell type from a table
Stimulus: A scientist recorded which structures she observed in three unknown cells.
| Structure | Cell 1 | Cell 2 | Cell 3 |
|---|---|---|---|
| Cell membrane | Yes | Yes | Yes |
| Nucleus | Yes | No | Yes |
| Cell wall | Yes | Yes | No |
| Chloroplasts | Yes | No | No |
Question: Which cell is most likely a bacterium?
Think-aloud:
- A bacterium is a prokaryote, and the key feature of a prokaryote is no nucleus.
- Scan the nucleus row: only Cell 2 says "No." Cell 2 is the bacterium.
- Don't be fooled by the cell wall. Cell 1 also has a wall, but its nucleus and chloroplasts make it a plant cell.
Answer: Cell 2.
Worked example 2: Reasoning from cell-cycle data
Stimulus: In a sample of 100 dividing skin cells from a mouse, a student counted 85 in interphase, 9 in prophase, 2 in metaphase, 2 in anaphase and 2 in telophase. The full cell cycle lasts 20 hours.
Question: About how long does prophase last?
Think-aloud:
- The fraction of cells in a stage estimates the fraction of time. Prophase: \dfrac{9}{100} = 0.09.
- Multiply by the whole cycle: 0.09 \times 20\text{ h} = 1.8\text{ h}.
- Convert if the choices are in minutes: 1.8 \times 60 = 108 minutes.
- Eliminate traps: "9 hours" treats the count as hours; "12 hours" uses only the 15 dividing cells (\tfrac{9}{15} \times 20). The whole is all 100 cells.
Answer: about 1.8 hours (108 minutes).
Worked example 3: Chromosome numbers in mitosis and meiosis
Stimulus: Dog body cells have 78 chromosomes.
Question: How many chromosomes are in (a) a dog skin cell made by mitosis and (b) a dog sperm cell?
Think-aloud:
- Mitosis copies the parent cell exactly, so (a) has 78.
- Sperm are gametes made by meiosis, which halves the number: 78 \div 2 = 39. So (b) has 39.
- Check with fertilization: sperm 39 + egg 39 = 78 in the puppy's first cell ✓
- A common trap is 19.5 (halving twice). Meiosis halves the number only once, because meiosis II only separates copied chromosomes.
Answer: (a) 78, (b) 39.
Worked example 4: Structure matches function
Stimulus: A sperm cell has a long tail (flagellum) that whips back and forth. Just behind its head is a "midpiece" packed with mitochondria wound around the base of the tail.
Question: What is the most likely reason so many mitochondria are packed into the midpiece?
A. To store the father's DNA B. To supply energy for the tail to move C. To make sugar from sunlight D. To protect the sperm from infection
Think-aloud:
- Mitochondria release energy (ATP). Ask: what in a sperm cell needs a lot of energy? The moving tail.
- Their location right at the base of the tail strongly supports this.
- Eliminate: DNA is stored in the nucleus (in the head), so A is wrong; making sugar from light is the chloroplast's job, so C is wrong; nothing in the stimulus mentions infection, so D is wrong.
Answer: B.
Common mistakes to avoid
- "Plants have chloroplasts instead of mitochondria." Wrong: plant cells have both. Chloroplasts make sugar; mitochondria release energy from it.
- "Any cell with a cell wall is a plant cell." Wrong: bacteria and fungi also have walls. Check for a nucleus (bacteria lack one) and chloroplasts (fungi lack them).
- Halving twice in meiosis. Wrong: human gametes have 23 chromosomes, not 11.5. One round of DNA copying plus two divisions gives a net result of half.
- Mixing up the purposes. Mitosis is for growth and repair, and its cells are identical. Meiosis makes gametes, and its cells are different. A skin wound heals by mitosis.
- Getting the order of levels wrong. Wrong order: organ → tissue. Right: cell → tissue → organ → organ system → organism. An organelle is inside a cell, below the cell level.
- Using the wrong total in cell-count problems. The fraction of time in a stage uses all cells counted, including interphase, not only the dividing cells.
GED strategy for cell questions
- Read the question before the stimulus. Know whether you need a structure, a comparison, a number or a conclusion.
- On diagrams, read every label and the key. Symbols like 2n and n, or shaded areas, often hold the answer.
- Compare, don't just list. To tell two cells apart, use features found in only one of them. Shared features are not evidence.
- Translate the job into the organelle: "energy from food" means mitochondria; "uses sunlight" means chloroplast; "controls what enters" means cell membrane; "contains DNA" means nucleus.
- For data, compute a quick fraction. 180 out of 200 is 90%. Write units at each step and remember 1 hour = 60 minutes.
- Watch absolute words (always, never, only, all). Biology has exceptions (mammal red blood cells have no nucleus), so absolute choices are often wrong.
- Reject choices that add information the passage never gave, even if they sound scientific.
Check yourself
Two Cells Under the Microscope
A student looked at two kinds of cells under a light microscope. One sample was scraped gently from the inside of her cheek. The other was a thin leaf from a pond plant. She drew one cell from each sample, labeled the structures she could see, and named the drawings Cell X and Cell Y. Later she realized she had forgotten to write down which drawing came from which sample.
Her teacher reminded her that all plant and animal cells share some structures, such as a cell membrane, cytoplasm, a nucleus and mitochondria. Other structures are found only in certain kinds of cells, so they can be used as clues.
Which statement correctly identifies Cell Y and gives evidence from the diagram that supports it?
Hint
Look for structures that appear in only one of the two drawings. Structures found in both cells cannot tell them apart.
Step-by-step solution
Where the answer comes from: the passage says all plant and animal cells share "a cell membrane, cytoplasm, a nucleus and mitochondria," so those structures cannot tell the cells apart. The diagram shows that only Cell Y has a cell wall (the thick outer border) and chloroplasts (the green ovals). Cell X has neither.
Step by step:
- List what both cells have: cell membrane, cytoplasm, nucleus, mitochondria. These are useless as clues.
- List what only Cell Y has: cell wall, chloroplasts, one large central vacuole.
- Cell walls and chloroplasts are found in plant cells, not animal cells. So Cell Y is the leaf cell, and Cell X is the cheek cell.
Answer: Cell Y came from the leaf, because it has a cell wall and chloroplasts.
Tip: when a question asks for evidence, the evidence must actually distinguish the choices. A true fact that applies to both cells (both have a nucleus) is not evidence.
Check yourself
Two Cells Under the Microscope
A student looked at two kinds of cells under a light microscope. One sample was scraped gently from the inside of her cheek. The other was a thin leaf from a pond plant. She drew one cell from each sample, labeled the structures she could see, and named the drawings Cell X and Cell Y. Later she realized she had forgotten to write down which drawing came from which sample.
Her teacher reminded her that all plant and animal cells share some structures, such as a cell membrane, cytoplasm, a nucleus and mitochondria. Other structures are found only in certain kinds of cells, so they can be used as clues.
Click on a structure in Cell Y where the energy of sunlight is captured and used to make sugar.
Click on the figure to mark your answer (or focus it and use the arrow keys).
Hint
The process that uses light energy to make sugar is photosynthesis. Which organelle carries it out, and which cell in the diagram has it?
Step-by-step solution
Where the answer comes from: the diagram labels two green ovals in Cell Y as chloroplasts. Chloroplasts are the organelles where photosynthesis happens: they capture light energy and use it to build sugar from carbon dioxide and water.
Step by step:
- Translate the question: "captures sunlight and makes sugar" = photosynthesis.
- Recall the organelle: photosynthesis takes place in chloroplasts.
- Find it in Cell Y: the green ovals, one below the nucleus at the lower left and one at the bottom center. Either one is correct.
- Avoid the traps: the mitochondrion releases energy from sugar (cellular respiration), the opposite job; the nucleus holds DNA; the vacuole stores water and dissolved materials.
Answer: click either chloroplast in Cell Y.
Tip: pair the two energy organelles: chloroplasts store light energy in sugar; mitochondria release that energy from sugar as ATP.
Summary
- Cell theory: all living things are made of cells; the cell is the basic unit of life; all cells come from existing cells.
- Prokaryotes (bacteria) have no nucleus; eukaryotes do. Plant cells add a cell wall, chloroplasts and a large vacuole, but they also have mitochondria.
- Match jobs to organelles: nucleus = DNA/control, mitochondria = energy, chloroplasts = photosynthesis, ribosomes = proteins, membrane = gatekeeper.
- Levels: cell → tissue → organ → organ system → organism. Specialized cells have shapes and parts that fit their jobs.
- Mitosis: 1 division, 2 identical 2n cells, for growth and repair. Meiosis: 2 divisions, 4 different n gametes. Fertilization restores 2n.
- On data items, the fraction of cells in a stage estimates the fraction of time in that stage. Use all cells as the whole.