Topic 1 Cell Structure.

Question 1

Define magnification.

Answer 1

The number of times larger an image of an object is than the real size of the object.

Question 2

State the units for millimetres (mm), micrometres (µm), and nanometres (nm) relative to centimetres (cm).
State the formula for calculating magnification.

Answer 2

1 cm = 10 mm.
1 cm = 10000 µm.
1 cm = 10000000 nm (ten million).
Magnification = image size ÷ actual size.

Question 3

Define resolution.

Answer 3

The ability to distinguish between two objects that are very close together.

Question 4

Light Microscope vs. Electron Microscope:
- What is the main source of illumination for a light and electron microscope?
- Can light and electron microscopes be used to view living specimens?
- What type of images do light and electron microscopes produce?
- What is the typical resolution for light and electron microscopes?
- Which type of microscope has better resolution?

Answer 4

• What is the main source of illumination for a light and electron microscope?
  Light microscope = visible light whereas.
  Electron microscope = a beam of electrons.
• Can light and electron microscopes be used to view living specimens?
  Light microscopes = Yes, they can observe live cells.
  Electron microscopes = only observe non-living samples.
• What type of images do light and electron microscopes produce?
  Light microscopes = colour images.
  Electron microscopes = black and white images.
• What is the typical resolution for light and electron microscopes?
  Light microscope = About 200 nm.
  Electron microscope = Can achieve resolutions of 1 nm or better.
• Which type of microscope has better resolution?
  Electron microscopes because the wavelength of an electron is shorter than the wavelength of light.

Question 5

What is a cell, what is it surrounded by and what does it contain?

Answer 5

A cell is a basic unit of all living organisms; it is surrounded by a cell surface membrane and contains genetic material (DNA) and cytoplasm-containing organelles.

Question 6

What is an organelle? Give two examples.

Answer 6

An organelle is a functionally and structurally distinct part of a cell e.g. a ribosome and mitochondrion.

Question 7

What are eukaryotes?
Give examples of eukaryotes.
Name all of the organells and other cells structures found in eukaryotic cells.

Answer 7

An organism whose cells contain a nucleus and other membrane-bound organelles is a eukaryote.

Eukaryotes include animals, plants, fungi, and some other organisms.

Eukaryotes contain the following organelles and cell structures:
- Cell surface membrane.
- Nucleus, nuclear envelope and nucleolus.
- Rough endoplasmic reticulum.
- Smooth endoplasmic reticulum.
- Golgi apparatus.
- Mitochondria.
- Ribosomes.
- Lysosomes.
- Centrioles and microtubules.
- Cilia.
- Microvilli.
- Chloroplasts.
- Cell wall.
- Plasmodesmata.
- Large permanent vacuole and tonoplast of plant cells.

Question 8

What are prokaryotes?
- Give examples of prokaryotes.

• Outline key structural feautures of prokaryotic cells found in a typical bacterium.
• Outline the structural features and their functions that are sometimes found in prokaryotic cells found in a typical bacterium.

Answer 8

An organism whose cells do not contain a nucleus or any other membrane-bound organelles is a prokaryote.

Prokaryotes include bacteria.

Outline key structural feautures of prokaryotic cells found in a typical bacterium, including:
- Unicellular (organisms that consist of a single cell).
- Generally 1 - 5 µm diameter.
- Peptidoglycan cell walls.
- Circular DNA.
- 70S ribosomes.
- Absence of organelles surrounded by double membranes.

Outline the structural features that are sometimes found in prokaryotic cells found in a typical bacterium, including:
- Flagellum: Some bacteria are able to swim because they have one or more flagella.
- Infolding of the cell surface membrane: Forms a photosynthetic membrane or to carry out nitrogen fixation (converting nitrogen in the air to nitrogen containing compounds).
- Plasmid: Circular DNA (genetic material).
- Pili: Pili are fine protein rods used for attachment and interactions with other cells or surfaces.
- Capsule or slime layer: Made up of polysaccharides to protect the bacterium from drying out.

Question 9

Compare prokaryotic cells with eukaryotic cells, limited to:
- Diameter.
- DNA.
- Ribosomes.
- Cell wall.
- Cell division.
- Flagella.

Answer 9

• Prokaryotes are typically 1-5 µm in diameter wherease eukaryotes are up to 40 µm in diameter.
• Prokaryotes have circular DNA which is free in the cytoplasm and is not surrounded by a double membrane whereas eukaryotes have non-circular DNA which is contained in the nucleus where the nucleus is surrouned by a double membrane (nuclear envelope).
• Prokaryotes have 70S ribosomes whereas eukaryotes have 80S ribosomes.
• Prokaryotes have a peptidoglycan cell wall whereas eukarytoes have a cellulose cell wall (plants) or chitin cell wall (fungi).
• Prokaryotic cells divide by binary fission and does not involve a spindle whereas eukaryotic cells divide by mitosis or meiosis and involves a spindle.
• Prokaryotic flagella are simple and lack microtubules wherease eukaryotic flagella and cilia are complex with a ‘9 + 2’ arrangement of microtubules.

Question 10

What are the features that animal and plant cells have in common?

Answer 10

• Cell surface membrane.
• Nucleus.
• Cytoplasm.
• Mitochondria (singular: mitochondrion).
• Golgi apparatus.

Question 11

Cell surface membrane sometimes referred to as the plasma membrane:

• What cells are surrounded by a very thin cell surface membrane?
• What is the function of the cell surface membrane?

Answer 11

• All cells, including those of both eukaryotes and prokaryotes, are surrounded by a very thin cell surface membrane.
• It is partially permeable and controls the exchange of materials between the cell and its environment.
• Contains proteins and important molecules for cell signalling and recognition.

Question 12

Nucleus:
- What is a nucleus (plural: nuclei)?

• What cells contain a nucleus?
• Describe the structure of the nucleus.

Answer 12

• A large organelle found in eukaryotic cells.
  The nucleus contains the cell’s DNA and therefore controls the activities of the cell. It is surrounded by two membranes which together form the nuclear envelope.
• All eukaryotic cells contain a nucleus.
• The nucleus is surrounded by the nuclear envelope, a double membrane. The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum. The nuclear envelope has many small pores called nuclear pores which enable movement of molecules into and out of the nucleus.
• The nucleus contains chromosomes and chromosomes contain DNA (deoxyribonucleic acid), the genetic material. Chromosomes are made of chromatin which is a combination of DNA and proteins known as histones.
• The nucleolus is a region within the nucleus that synthesises ribosomes using the information in its own DNA.

Question 13

What is the role of nucleus?

Answer 13

• To store the genetic material (DNA) of the cell.
• To synthesise ribosomes.
• To regulate the production of messenger RNA for protein synthesis.

Question 14

Rough endoplasmic reticulum:
- Describe the structure of the rough endoplasmic reticulum.
- What is the function of the rough endoplasmic reticulum?

Answer 14

• The rough endoplasmic reticulum (RER) is continuous with the nuclear envelope and is a network of flattened sacs called cisternae. Its surface is studded with ribosomes which are the sites of proytein synthesis.
• The primary function of the RER is protein synthesis, folding and modification of proteins and transport of proteins to the Golgi apparatus for further modification.

Question 15

Smooth endoplasmic reticulum:
- Describe the structure of the smooth endoplasmic reticulum.
- What is the function of the smooth endoplasmic reticulum?

Answer 15

• The smooth endoplasmic reticulum (SER) is a network of tubular membranes without ribosomes, giving it a smooth appearance.
• The SER synthesises and transports lipids and steriods such as cholesterol and the reproductive hormones oestrogen and testosterone. SER is also a major storage site for calcium ions. In the liver, SER is involved in drug metabolism.

Question 16

Golgi apparatus:
- Describe the structure of the Golgi apparatus.
- What is the function of the Golgi apparatus?

Answer 16

• The Golgi apparatus is a series of flattened, membrane-bound sacs called cisternae.
• The primary role of the Golgi apparatus is to modify, and package proteins and lipids received from the rough and smooth ER for exocytosis or distributiom within the cell. Golgi vesicles are used to make lysosomes.

Question 17

Lysosomes:
- Describe the structure of the lysosomes.
- What is the function of the lysosomes?

Answer 17

• Lysosomes are simple spherical sacs, surrounded by a single membrane.
• Lysosomes contain hydrolytic enzymes that digest macromolecules such as ingested pathogens in phagocytes, cells after cell death and old or worn out organelles within cells.

Question 18

Mitochondria:
- Describe the structure of the mitochondria.
- What is the function of the mitochondria?

Answer 18

• Mitochondria are double-membrane-bound organelles with an outer membrane (mitochondrial envelope) and a highly folded inner membrane, forming structures called cristae. The space inside the inner membrane is known as the matrix.
• The primary function of mitochrondria is to produce adenosine triphosphate (ATP) through cellular aerobic respiration. The ATP is used for energy-requiring processes.

Question 19

Ribosomes:
- Describe the structure of the ribosomes.
- What are the two types of ribosomes?
- What is the function of the ribosomes?

Answer 19

• Ribosomes are small, spherical organelles composed of ribosomal RNA (rRNA) and ribosomal proteins.
• 70S ribosomes are smaller and lighter and found in prokaryotic cells in the chloroplasts and mitochondria and 80S ribosomes are larger and heavier and found in eukaryotic cells in the cytoplasm and on the RER.
• Ribosomes are the sites of protein synthesis in the cell. They translate messenger RNA (mRNA) sequences into polypeptide chains.

Question 20

Centrioles and Microtubules:
- Describe the structure of the centrioles and microtubules.
- What is the function of the centrioles and microtubules?

Answer 20

• Centrioles are cylindrical organelles made up of nine triplets of microtubules arranged in a ring. Microtubules are hollow, cylindrical structures found in the cytoplasm (about 25nm in diameter) composed of a-tubulin and b-tubulin protein subunits.
• Centrioles and microtubules play a key role in cell division by organizing the spindle fibers that separate chromosomes during mitosis. They also help in the formation of cilia and flagella, which are important for cell movement and fluid movement across cell surfaces.

Question 21

Cilia:
- Describe the structure of the cilia.
- What is the function of the cilia?

Answer 21

• Cilia are short, hair-like projections that extend from the surface of some eukaryotic cells. Each cilium is composed of microtubules arranged in a characteristic “9+2” pattern.
• The cilia on respiratory epithelial cells waft mucus and pathogens or foreign material upwards and out of the lungs.

Question 22

Microvilli:
- Describe the structure of the microvilli.
- What is the function of the microvilli?

Answer 22

• Microvilli are tiny, finger-like projections that extend from the surface of epithelial cells.
• Microvilli increase the surface area of a cell to increase the efficiency of exchange. They play a crucial role in nutrient absorption, helping to transport molecules across the cell membrane efficiently.

Question 23

Chloroplasts:
- Describe the structure of the chloroplasts.
- What is the function of the chloroplasts?

Answer 23

• Chloroplasts are double-membrane-bound organelles found in inside plant cells. They contain a system of flattened sacs called thylakoids, which are stacked into structures known as granum. The fluid-filled space surrounding the thylakoids is called the stroma. They also contain 70S ribosomes and small circular DNA.
• Chloroplasts capture sunlight using chlorophyll, the green pigment, and convert carbon dioxide and water into glucose and oxygen. The glucose produced can be stored or used immediately for energy.

Question 24

Cell wall:
- Describe the structure of the cell wall.
- What is the function of the cell wall?

Answer 24

• The cell wall is a rigid, protective layer that surrounds the cell membrane in plant cells, fungi, and bacteria. In plants, it is primarily composed of cellulose, while fungal cell walls contain chitin, and bacterial cell walls are made of peptidoglycan.
• It provides mechanical strength to the plant. It protects the cell from osmotic lysis as it regulates the movement of water.

Question 25

Plasmodesmata:
- Describe the structure of the plasmodesmata.
- What is the function of the plasmodesmata?

Answer 25

• Plasmodesmata are microscopic channels that traverse the cell walls of plant cells, connecting the cytoplasm of adjacent cells.
• Plasmodesmata allow the undisrupted flow of material via the symplastic route, and the efficient exchange of substances between cells without having to cross the cell wall and membrane.

Question 26

Large permanent vacuole:
- Describe the structure of the large permanent vacuole.
- What is the function of the large permanent vacuole?

Answer 26

• The large permanent vacuole is a membrane-bound organelle found primarily in plant cells. It is filled with a solution of water, ions, sugars, and other substances.
• The vacuole stores nutrients, waste products, and other substances, helping to regulate the cell’s internal environment.
  By filling with water, the vacuole helps maintain turgor pressure, which supports the plant’s structure and keeps it rigid.

Question 27

Tonoplast:
- Describe the structure of the tonoplast.
- What is the function of the tonoplast?

Answer 27

• The tonoplast is the selectively permeable membrane that surrounds the large permanent vacuole.
• The tonoplast is composed of a lipid bilayer with various proteins embedded in it, facilitating the transport of substances in and out of the vacuole.

Question 28

Viruses:
- Describe the structure of the viruses.
- What is the function of the viruses?

Answer 28

• Viruses are microscopic infectious agents that consist of genetic material (either DNA or RNA) enclosed in a protein coat called a capsid and some viruses have a membrane-like outer layer made of phospholipids called the envelope.
• All viruses are parasitic because they can only reproduce by infecting and taking over living cells. The virus DNA or RNA takes over the protein synthesising machinery of the host cell, which then helps to make new virus particles.