Chapter 2: The Cell as the Basic Unit of Life

Question 1

State the Cell Theory.

Answer 1

1. All organisms are made up of 1 or more cells.
2. The cell is the basic unit of life; it is the smallest unit that shows all the characteristics of life.
3. All cells come from pre-existing cells.

Question 2

Definition of unicellular organisms and multicellular organisms.

Answer 2

Unicellular organisms:

• Organisms made up of only 1 cell (e.g. bacteria, Amoeba).

Multicellular organisms:

• Organisms made up of > 1 cells (e.g. plants, animals).

Question 3

What are some major events in cell biology and the development of microscopes?

Answer 3

1590: The first light microscope was invented (by Hans Janssen & his son).

1665: “Cells” (cell walls) were discovered (by Robert Hooke with his microscope).

1674: Living cells were observed (by Antoni van Leeuwenhoek with his microscope).

1831: The nucleus was discovered (by Robert Brown).

1839: The Cell Theory was proposed (by Theodor Schwann & Matthias Jakob Schleiden).

1886: The mitochondrion was discovered.

1933: The first electron microscope was made.

1950: The ribosome was discovered.

Question 4

What are the aspects of nature of science (NOS) are demonstrated in the discovery of cells and the development of the Cell Theory?

Answer 4

• Scientific knowledge is based on and derived from observations of the natural world.
• Science is affected by the technology and the types of equipment available at the time.
• Scientists build on the work of other scientists.

Question 5

Comparison of different types of microscopes.

Answer 5

(Refer to document)

Question 6

What are the differences between higher resolution and higher magnification?

Answer 6

Higher resolution:

• Clearer and more details of the image observed

Higher magnification:

• The specimen is enlarged to a larger extent

Question 7

What are the structures of a light microscope and their uses?

Answer 7

(Setup: refer to notes p.11)

1. Eyepiece: Magnifying lens → magnify specimens
2. Nosepiece: Holds different objectives (of different magnifications); Can be rotated
3. Objectives: Magnifying lens → magnify specimens
4. Stage: Where the microscope slide is clipped for observation; Have hole → light can pass through; Can be raised or lowered to focus
5. Condenser: Focuses light onto the specimen
6. Diaphragm: Control the amount of light shone onto the condenser
7. Coarse adjustment knob: Rotated to raise or lower the stage to obtain a rough focus; Causes a larger movement of the stage
8. Fine adjustment knob: Rotated to raise or lower the stage to obtain a sharp focus; Causes a smaller movement of the stage
9. Base: Foundation to support the whole microscope
10. Arm: For holding the microscope
11. Body tube: Connects the eyepiece and nosepiece
12. Mirror: Reflects light from an external light source onto the specimen
13. Light source: Provides light for viewing the specimen

Question 8

What are the steps for using a light microscope?

Answer 8

1. Insert a low-power eyepiece (e.g. 5X) into the body tube. Select a low-power objective (e.g. 4X) by rotating the nosepiece.
2. Turn on the light source, adjust the the diaphragm until the light is sufficient.
3. Clip the microscope slide with specimen onto the stage, with the specimen directly over the hole of the stage.
4. Steps for focusing on the specimen:
   (a) Look at the stage at eye level. Raise the stage (or lower the body tube) by turning the coarse adjustment knob until the objective almost touches the microscope slide.
   (b) Look through the eyepiece. Lower the stage (or raise the body tube) slowly by turning the coarse adjustment knob until the image of the specimen is in rough focus.
   (c) Turn the fine adjustment knob to get a sharp focus.
5. Search the field and move the region to be observed to the centre of the field of view.
6. Select a high-power objective (e.g. 40X) by rotating the nosepiece.
7. Turn the fine adjustment knob to get a sharp focus again.
8. Brighten the view by adjusting the diaphragm if needed. (Why? Higher power magnification → dimmer)

Question 9

Describe the image observed through a light microscope?

Answer 9

The image observed is magnified and inverted:

• Moving the slide right → Image moves left.
• Moving the slide away from the observer → Image moves towards the observer.

Question 10

Comparison of image observed through low-power magnification and high-power magnification.

^Area of field of view, Details of specimen, Brightness of image

Answer 10

Area of field of view:

• Low-power magnification: Larger (More cells are observed)
• High-power magnification: Smaller (Fewer cells are observed)

Details of specimen:

• Low-power magnification: Less
• High-power magnification: More

Brightness of image:

• Low-power magnification: Brighter
• High-power magnification: Dimmer

Question 11

Definition of magnification.

Answer 11

Magnification represents how many times an image is larger than the object.

Question 12

What are the two equations for calculating the total magnification of a microscope?

^Reverse, Units

Answer 12

Total magnification = Power of eyepiece x Power of objective
Total magnification = Size of the image / Size of the object

Unit:

• 1cm = 10^-2m
• 1μm (micrometer/micron) = 10^-6m
• 1nm (nanometer) = 10^-9m

Question 13

How do we prepare temporary mounts of ox corneal cells?

^Animal cells

Answer 13

Ox corneal cells:

1. Touch the ox cornea gently with the middle of a clean slide.
2. Add a drop of methylene blue solution to the touched area to stain the cells.
3. Use a pair of forceps to place a cover slip over the cells.

Question 14

How do we prepare temporary mounts of onion epidermis, banana tissue?

^Plants cells that are mount with iodine solution

Answer 14

Diagram: Refer to book p.31

Onion epidermis

1. Peel off the inner epidermis of the fleshy layer of an onion.
2. Cut out a small piece of epidermis.
3. Mount it with a drop of iodine solution.

Banana tissue

1. Take a small amount of tissue from the soft white middle part of a banana.
2. Put the tissue into a drop of water on a slide.
3. Separate the cells with a toothpicks.
4. Mount the cells with a drop of iodine solution.

Question 15

How do we prepare temporary mounts of hydrilla leaf, onion root tip cells?

^Plants cells that are mount with water

Answer 15

Hydrilla leaf

1. Mount a hydrilla leaf with a drop of water.

Onion root tip cells

1. Put small pieces of onion root tips into freshly prepared macerating fluid (浸離液) for 2 to 3 days to soften the root tips.
2. Pour the content into a watch glass. Tear the tissue apart.
3. Filter off the macerating fluid. Wash the macerated onion root tip gently with water.
4. Mount the macerated onion root tip with a drop of water.

Question 16

How do we prepare temporary mount of pollen grains?

^Plants cells that does not require a mounting medium

Answer 16

Pollen grains

1. Touch a piece of sticky tape with the anther of a flower (e.g. a Gladiolus flower or a lily flower).
2. Stick the tape onto a slide.

Question 17

How do we place a cover slip on temporary mount of a specimen?

Answer 17

• Let the edge of a cover slip touch the mounting medium.
• Slowly lower the cover slip. Make sure no air bubbles are trapped (adding sufficient mounting medium can help minimize the chance of trapping air bubbles).
• Use tissue paper to soak up any excess mounting medium.

Question 18

What is the purpose of sectioning the specimen?

Answer 18

^ Cut the specimen into a thin (1-cell thick) section

• Allow light to pass through the specimen
• Obtain a thin section for clearer observation

Question 19

What is the purpose of staining the cells?

Answer 19

Make cell structures more distinct (especially for cells without pigments, e.g. ox cornea cells, onion epidermal cells)

Question 20

What is the purpose of adding a drop of water to the cells?

Answer 20

↓ light refraction

Question 21

What is the purpose of placing a cover slip over the specimen?

Answer 21

• Flattens the cells for observation
• Prevents the specimen from drying out
• Prevents the specimen from being contaminated by substances, e.g. dust

Question 22

List the structures in an animal cell (x8).

Answer 22

Labelled biological diagram: Refer to book p.21

Cell membrane, cytoplasm, nucleus, rough endoplasmic reticulum, smooth endoplasmic reticulum, ribosome, mitochondrion, vacuole

Question 23

List the structures in a plant cell (x10).

Answer 23

Labelled biological diagram: Refer to book p.21

Cell wall, cell membrane, cytoplasm, nucleus, rough endoplasmic reticulum, smooth endoplasmic reticulum, ribosome, mitochondrion, a large central vacuole, chloroplast

Question 24

What are the features and functions of cell wall?

Answer 24

• Thick, rigid outermost layer mainly made up of cellulose (polysaccharide)
  → Protects, supports and gives shape to the plant cell
• Fully permeable
  → Allows water and all dissolved substances to pass through

Question 25

What are the features and functions of cell membrane?

Answer 25

• Thin, flexible membrane mainly made up of phospholipids and proteins
  → Encloses the cell and separates the cell contents from the outside environment
• Differentially permeable
  → Controls the transport of substances into and out of the cell

Question 26

What are the features and functions of cytoplasm?

Answer 26

• Jelly-like fluid consisting of mainly water and proteins
  → Holds many organelles (e.g. nucleus, mitochondia)
  → Site for biochemical reactions (e.g. enzymatic)
  → Allows the movement of substances within the cells

Question 27

What are the features and functions of nucleus?

^Exceptions

Answer 27

• Double membrane (nuclear membrane) bounded
• Spherical structure
  → Holds DNA (genetic information) which regulates protein synthesis and thus controls cell activities
• Structure in nucleus: Nuclear membrane, DNA, nucleolus

^Absent in mature red blood cells; Muscle cells and some human liver cells have more than one nucleus

Question 28

What are the features and functions of nuclear membrane?

Answer 28

• Double membrane
  → Encloses the nucleus and separates the contents of the nucleus from the cytoplasm
• Have nuclear pores
  → Allow some molecules to be transported between the nucleus and the cytoplasm (e.g. RNAs + proteins are exported from the nucleus)

Question 29

(^^) What are the features and functions of nucleolus?

Answer 29

• Very dense structure in the nucleus
  → Involved in the making of ribosomes

Question 30

What are the features and functions of endoplasmic reticulum (ER)?

^Rough ER, Smooth ER

Answer 30

• Network of interconnected membrane-bounded sacs
• Continuous with the outer nuclear membrane and extends throughout the cytoplasm
• Large surface area
  → Facilitate protein and lipid synthesis and transport

Rough ER

• Have ribosomes on surface
  → Site for protein synthesis

Abundent in cells that produce a large amount of proteins, e.g.

• Saliva-secreting cells: produce enzymes in saliva
• Pancreatic cells: secrete enzymes for digestion, secrete insulin (胰島素) for regulating blood glucose level

Smooth ER

• Does not have ribosomes
  → Site for lipid synthesis

Abundent in cells that produce a large amount of lipids, e.g.

• Cells in testes: secrete male sex hormones

Question 31

What are the features and functions of ribosome?

Answer 31

• Small particle not surrounded by a membrane
• Attached on rough ER and lying free in the cytoplasm
  → Involved in protein synthesis

Question 32

What are the features and functions of mitochondrion?

Answer 32

• Double membrane bounded
  Outer membrane: smooth → Controls the entry and exit of substances;
  Inner membrane (Cristae): highly folded → ↑ Surface area to pack more enzymes involved in aerobic respiration
• Rod-shaped
• Contains matrix, a medium, where the main process of energy-releasing stage of cellular respiration occurs (i.e. powerhouse of cells)
  → Oxidation of glucose to release energy in the form of ATP

Abundent in cells with high energy requirements, e.g.
(… support high rate of respiration to release more energy to support the high demand for …)

• Muscle cells: … muscle contraction
• Liver cells: … various metabolic processes
• Sperms: … the motility of the sperm.
• Epithelial cells in the inner wall of the small intestine: … absorption of nutrients
• Root hair cells: … absorption of minerals

Question 33

What are the features and functions of vacuole in animal cells?

Answer 33

• Single membrane bounded
• Contains cell sap
  → For storage, holds dissolved substances (e.g. enzymes and food) and water

Question 34

What are the features and functions of large central vacuole in plant cells?

Answer 34

• Single membrane bounded
• Contains cell sap
  → For storage, holds dissolved substances (e.g. mineral salts, sugars, pigments, waste) and water
  → Provides support to plants, cell becomes turgid when fully filled with water as it exerts an outward pressure on the cell wall

^Some parts of plant may appear bluish/purplish ∵ pigments are present in the vacuole.

Question 35

What are the features and functions of chloroplast?

Answer 35

• Double membrane bounded
• Filled with a jelly-like fluid called stroma, where all thylakoids are suspended
• Contains a network of thykoids (fluid-filled, disc-like sacs bounded by membrane); p.s. Stacks of thykloids called grana (singular: granum)
  → Provide larger surface area to pack more chlorophyll to absorb more light for photosynthesis
• Contains photosynthetic pigments, e.g. chlorophyll, on the thylakoid membrane
  → To absorb light energy and converts it into chemical energy in photosynthesis
• Contains enzymes for photosynthesis
  → Catalyse photosynthetic reactions.
• Starch grains are often presented inside ∵ Some of the glucose produced during photosynthesis is converted into starch and temporarily stored in chloroplast

^Present in green plant cells only (e.g. palisade mesophyll cells, guard cells in leaves)

Question 36

(^^) What are the functions of Golgi apparatus?

Answer 36

Modify and package proteins / lipids for internal use / transport out of the cell.

Question 37

What are the same sub-cellular structures shared among an animal cell and a plant cells?

Answer 37

Cell membrane, cytoplasm, nucleus, endoplasmic reticulum, mitochondia

Question 38

What are the differences between an animal cell and a plant cell?

Answer 38

Structure
Cell wall:

• Animal cells: ✘
• Plant cells: ✔

Vacuole:

• Animal cells: Small or absent
• Plant cells: Often a large central vacuole

Chloroplasts:

• Animal cells: ✘
• Plant cells: ✔ (in green plant cells only)

Size

• Animal cells: Smaller
• Plant cells: Larger

Shape

• Animal cells: Irregular (usually)
• Plant cells: Regular (∵cell wall)

Location of the nucleus

• Animal cells: Center of the cell (often)
• Plant cells: Side of the cell

Question 39

What are the features of an eukaryotic cells?

Answer 39

Eukaryotic cell contains a true nucleus. It has a nucleus surrounded by a nuclear membrane, with genetic material (DNA) enclosed in the nucleus.

It also contains various other membrane-bound organelles, include the endoplasmic reticulum (both the smooth and the rough ER), the Golgi apparatus, vacuoles, mitochondria, lysosomes (found in animal cells), and chloroplasts (found in plant cells).

Some ribosomes in eukaryotic cell are attached to ER, while some are lying free in the cytoplasm.

Cell membrane is present in all eukaryotic cells. Cell wall is present in plant cells but absent in animal cells. Only cell wall in plant cells contains cellulose.

In terms of size, eukaryotic cells are usually larger than prokaryotic cell.

Question 40

What are eukaryotes?

Answer 40

Eukaryotes are organisms consisting of eukaryotic cells. Most eukaryotes are multicellular (e.g. animals and plants), but some are unicellular.

Question 41

What are the features of an prokaryotic cell?

Answer 41

Prokaryotic cell does not have a true nucleus. DNA is found in the nucleoid region, lying free in the cytoplasm.

They do not contain membrane-bound organelles. Ribosomes in prokaryotic cells are lying free in the cytoplasm.

Cell membrane is present in all prokaryotic cells. Cell wall is present in most prokaryotic cells, it does not contain cellulose.

Some prokaryotic cells have flagella for cell movement, while some have capsule surround the cell wall for protection.

In terms of size, prokaryotic cells are usually smaller than eukaryotic cell.

Question 42

What are prokaryotes?

Answer 42

Prokaryotes are organisms consisting of prokaryotic cells. All prokaryotes are unicellular (e.g. bacteria).

^They are believed to be the earliest types of organisms that appeared on Earth.

Question 43

What are the similarities among eukaryotic cells and prokaryotic cells?

Answer 43

Both are bounded by a cell membrane.
Their genetic material is DNA.

Question 44

What are the levels of body organization in muticellular organisms?

Answer 44

Cell → Tissue → Organ → System → Organism

Question 45

What are the levels of body organization in unicellular organisms?

Answer 45

Molecule → Organelle→ Cell

Question 46

Cells are small. Why doesn’t a cell keep increasing in size?

Answer 46

When a cell ↑ in size, its surface area ↑ at a ↓er rate than that of the cell’s volume, thus the surface to volume ratio of the cell ↓.

When its surface area is not large enough to meet the demand of its volume, i.e. substances cannot enter the cell quickly enough to meet its metabollic demand, the cell stop growing.