Unit 5 Plant and animal cells
Key unit competence: Differentiate between animal and plant cells.
Key ideas
- The cell is the smallest unit of life and the basic unit of structure and function of all living things. Unicellular organisms (bacteria, Amoeba, yeast) are one cell; multicellular organisms (plants, animals) are made of many cells.
- Prokaryotic cells (bacteria) have no true nucleus: their DNA is not surrounded by a membrane. Eukaryotic cells (plants, animals, fungi, protoctists) have a true nucleus and membrane-bound organelles.
- Every plant and animal cell has a cell membrane (controls what enters and leaves; it is partially (selectively) permeable), cytoplasm (jelly, mostly water, where most chemical reactions take place) and a nucleus (contains DNA; controls the cell's activities and passes on hereditary information).
- Mitochondria are found in both plant and animal cells. They carry out aerobic respiration, releasing energy from glucose for growth, movement and cell division. Very active cells (muscle, sperm) have many mitochondria.
- Only plant cells have a cellulose cell wall (support, keeps a fixed shape; it is fully permeable), chloroplasts containing chlorophyll (photosynthesis) and a large permanent vacuole filled with cell sap (stores water, sugars and salts, keeps the cell firm). Animal cells have only small, temporary vacuoles.
- Shape: plant cells are usually regular (box-shaped) because of the cell wall; animal cells are irregular or round and smaller. Animal cells store carbohydrate as glycogen; plant cells store starch.
- Other organelles (seen with an electron microscope): ribosomes (make proteins), endoplasmic reticulum (transport inside the cell), Golgi apparatus (packages and sends out proteins).
- Practical: onion epidermis cells (stained with iodine) show a cell wall, nucleus, cytoplasm and vacuole but no chloroplasts (onion bulbs are not green). Human cheek cells (scraped with a clean spatula, stained with methylene blue) are irregular, with a nucleus, cytoplasm and membrane, and no cell wall.
Common misconceptions
- "All plant cells have chloroplasts." No: only cells in green parts have chloroplasts; root cells and onion bulb cells have none.
- "Mitochondria make energy." No: energy cannot be made; mitochondria release energy from food by respiration.
- "The cell wall controls what enters the cell." No: the cell wall is fully permeable; the cell membrane controls entry and exit.
- "Animal cells have no vacuoles." No: they have small temporary vacuoles, but not a large permanent one.
Worked examples
Example 1. An unknown cell has a regular shape, a cell wall and a large vacuole, but no chloroplasts. Is it a plant or an animal cell? Where might it come from?
Solution
It is a plant cell (cell wall and large vacuole are only in plants). With no chloroplasts, it comes from a part not exposed to light, such as a root or an onion bulb.
Example 2. Explain why a muscle cell has many more mitochondria than a skin cell.
Solution
Muscle cells contract and need a lot of energy. Mitochondria release energy by aerobic respiration, so more mitochondria supply more energy for movement.
Practice questions
State three structures found in both plant and animal cells, and give the function of each.
Show answer
Cell membrane: controls movement of substances in and out. Cytoplasm: site of chemical reactions. Nucleus: controls cell activities and contains DNA. (Mitochondria: release energy by respiration.)
Common mistake: Listing the cell wall as common to both.
Give four differences between a plant cell and an animal cell.
Show answer
Plant: cell wall present, chloroplasts present, large permanent vacuole, regular shape. Animal: no cell wall, no chloroplasts, small temporary vacuoles, irregular shape.
Common mistake: Saying animal cells have no nucleus.
Explain why the nucleus is called the control centre of the cell.
Show answer
It contains DNA, the genetic instructions that control which proteins the cell makes and therefore its activities, growth and division.
Common mistake: Saying the nucleus makes energy.
Describe how to prepare a slide of human cheek cells.
Show answer
Gently scrape the inside of the cheek with a clean spatula, smear it on a clean slide, add a drop of methylene blue, lower a cover slip at an angle, blot and observe; then disinfect the spatula and slides.
Common mistake: Using a sharp object that hurts the cheek.
State the functions of: (a) chloroplast; (b) vacuole; (c) cell wall.
Show answer
(a) Photosynthesis (traps light to make food). (b) Stores cell sap: water, sugars and salts; keeps the cell firm. (c) Supports and protects the cell, gives it a fixed shape.
Common mistake: Saying the vacuole controls the cell.
Explain why muscle cells contain many mitochondria.
Show answer
Muscles contract and use a lot of energy; mitochondria release energy from glucose by aerobic respiration.
Common mistake: Saying mitochondria create energy.
What is the difference between a prokaryotic and a eukaryotic cell? Give an example of each.
Show answer
A prokaryotic cell has no nucleus surrounded by a membrane (bacteria). A eukaryotic cell has a true nucleus and membrane-bound organelles (plant, animal, fungal cells).
Common mistake: Saying prokaryotes have no DNA.
Name the stain used for onion epidermis and for cheek cells, and explain why stains are used.
Show answer
Iodine for onion cells, methylene blue for cheek cells. Stains colour parts such as the nucleus so they can be seen clearly under the microscope.
Common mistake: Thinking stains make cells bigger.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Onion epidermis cells have no chloroplasts because: a. the bulb grows underground without light 2. The organelle where aerobic respiration releases energy is the: b. Chloroplast 3. Which structure is found only in plant cells? c. mitochondrion 4. The green pigment in chloroplasts is: d. chlorophyll Show answer
1 → a (the bulb grows underground without light) · 2 → c (mitochondrion) · 3 → b (Chloroplast) · 4 → d (chlorophyll)
- Only cells in green, lit parts have chloroplasts.
- Mitochondria are called the powerhouse of the cell.
- Chloroplasts carry out photosynthesis.
- Chlorophyll traps light energy.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
vacuole · cell · glycogen · cytoplasm- The jelly-like part of the cell where most reactions take place is the .
- Animal cells store carbohydrate as .
- The basic unit of structure and function of all living things is the .
Show answer
1: cytoplasm · 2: glycogen · 3: cell
- Cytoplasm is mainly water with dissolved substances.
- Plants store starch; animals store glycogen.
- All organisms are made of one or more cells.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The part of the cell that controls its activities is the: a. nucleus 2. The structure that gives support and protection to a plant cell is the: b. lets some substances through and stops others 3. The cell membrane is described as partially permeable because it: c. Ribosome 4. Which structure makes proteins in the cell? d. cell wall Show answer
1 → a (nucleus) · 2 → d (cell wall) · 3 → b (lets some substances through and stops others) · 4 → c (Ribosome)
- The nucleus contains DNA, the instructions for the cell.
- The cell wall is strong and rigid.
- It controls movement into and out of the cell.
- Ribosomes join amino acids into proteins.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
plant cells · unicellular · Golgi apparatus · tissues- Organisms made of only one cell are called .
- A large permanent vacuole containing cell sap is found in .
- The organelle that packages proteins for export is the .
Show answer
1: unicellular · 2: plant cells · 3: Golgi apparatus
- Bacteria, Amoeba and yeast are unicellular.
- Animal cells have only small temporary vacuoles.
- It modifies and packages proteins.