2.1 - Cell Structure and Function

Cells are the basic units of all living things.

Preparing and Examining Cell Slides

Plant Cells

1. Remove the translucent epidermis from the inner surface of an onion scale leaf.
2. Place the epidermis in a drop of distilled water on a glass slide.
3. Lower a cover slip at an angle of 45° using a mounting needle. Ensure that the epidermis is not folded and that no air bubbles are trapped.
4. Use the irrigation technique to stain the specimen: place a drop of iodine solution at one side of the cover slip and draw it across with filter paper at the opposite side.
5. Absorb excess iodine solution and distilled water with filter paper.
6. Examine the slide first with a low-power objective lens and then with a high-power objective lens.

Animal Cells

1. Gently scrape the inside of the cheek with a blunt toothpick.
2. Transfer the scraping into a drop of distilled water on a clean glass slide and cover it with a cover slip.
3. Stain the cheek cells with methylene blue solution using the irrigation technique.
4. Examine the cells through a light microscope.

Components of Animal and Plant Cells and Their Functions

Plasma Membrane

• Characteristics:
  • Outer membrane that surrounds the entire content of a cell.
  • Made of proteins and phospholipids.
  • Thin, elastic and partially permeable.
• Functions:
  • Separates the cell content from the external environment.
  • Controls the movement of substances into and out of the cell.
  • Allows the exchange of nutrients, respiratory gases and waste materials between the cell and its surroundings.

Cell Wall

• Characteristics:
  • Strong and rigid outer layer made of cellulose.
  • Fully permeable.
• Functions:
  • Maintains the shape of plant cells.
  • Provides mechanical support to plant cells.

Cytoplasm

• Characteristics:
  • Jelly-like substance that contains the cell components.
  • Contains organic compounds such as proteins, lipids and carbohydrates, as well as inorganic compounds such as potassium ions.
• Functions:
  • Acts as a medium for biochemical reactions in cells.

Nucleus

• Characteristics:
  • The largest component in the cell.
  • Spherical or compressed and enclosed by a nuclear membrane with many pores.
  • Contains chromosomes, a nucleolus and nucleoplasm.
• Functions:
  • Controls all cell activities.
  • Chromosomes contain deoxyribonucleic acid (DNA), which determines cell characteristics and metabolic function.

Mitochondrion (Plural: Mitochondria)

• Characteristics:
  • Rod-shaped or spherical.
  • Has a smooth outer membrane and a folded inner membrane.
  • Contains enzymes involved in cellular respiration.
• Functions:
  • Generates energy through glucose oxidation during cellular respiration.
  • Releases energy in the form of adenosine triphosphate (ATP) molecules for use by cells.

Ribosome

• Characteristics:
  • Small, compact and spherical granules made of protein and ribonucleic acid (RNA).
  • Found freely in the cytoplasm or on the surface of the rough endoplasmic reticulum.
• Function: Site of protein synthesis.

Endoplasmic Reticulum

• Characteristics:
  • A system of interconnected folded, flattened sacs.
  • Its membrane is continuous with the nuclear membrane.
  • Rough endoplasmic reticulum has ribosomes attached to its surface; smooth endoplasmic reticulum does not.
• Functions:
  • Acts as a transport system within the cell.
  • Provides a wide surface for enzyme attachment and biochemical reactions.
  • Rough endoplasmic reticulum transports proteins synthesised by ribosomes.
  • Smooth endoplasmic reticulum synthesises and transports glycerol and lipids, and detoxifies drugs and metabolic by-products.

Golgi Apparatus

• Characteristics:
  • A stack of parallel flattened sacs, each coated by a single membrane.
  • New membrane is added at one end, while vesicles bud off from the other end.
• Function: Processes, modifies, packs and transports chemicals such as proteins, carbohydrates and glycoproteins.

Lysosome

• Characteristics:
  • Small spherical sac enclosed by a single membrane.
  • Contains hydrolytic enzymes.
• Functions:
  • Hydrolyses complex organic molecules such as proteins, nucleic acids and lipids.
  • Breaks down bacteria and components of damaged cells.

Centriole

• Characteristics:
  • Small cylindrical components that exist in pairs in animal cells.
  • Made of a complex arrangement of microtubules.
  • Absent from plant cells.
• Function: Forms spindle fibres during cell division in animal cells.

Chloroplast

• Characteristics:
  • Oval-shaped and consists of two layers of membrane.
  • Contains chlorophyll pigments in the grana that give plants their green colour.
• Function: Chlorophyll absorbs sunlight and converts it to chemical energy during photosynthesis.

Vacuole

• Characteristics:
  • A sac filled with cell sap.
  • Surrounded by a semi-permeable membrane called the tonoplast.
  • Cell sap contains water, organic acids, sugars, amino acids, enzymes, mineral salts, oxygen, carbon dioxide and metabolic by-products.
  • Plant cells have large vacuoles; animal cells have small vacuoles.
• Functions:
  • Water is absorbed into a plant-cell vacuole and the cell becomes turgid.
  • In unicellular organisms, the contractile vacuole is involved in osmoregulation and excretion.

Comparison between Animal Cells and Plant Cells

Similarities

• Both have a nucleus, cytoplasm, plasma membrane, Golgi apparatus, mitochondria, ribosomes and endoplasmic reticulum.

Differences

Animal Cells	Plant Cells
Do not have a fixed shape.	Have a fixed shape.
Do not have a cell wall.	Have a cell wall.
Do not have chloroplasts.	Have chloroplasts.
Have no vacuoles or have small vacuoles.	Have large vacuoles.
Store carbohydrates as glycogen.	Store carbohydrates as starch.
Have centrioles.	Do not have centrioles.