2.1.4 The ultrastructure of eukaryotic cells: membrane-bound organelles

Question 1

What are the characteristics of eukaryotic cells?

Answer 1

• a nucleus surrounded by a nuclear envelope and containing DNA organised and wound into linear chromosomes
• an area inside the nucleus called the nucleolus, containing RNA, where chromosomes unwind; the nucleolus is also involved in making ribosomes
• jelly-like cytoplasm in which the organelles are suspended
• a cytoskeleton – a network of protein filaments (actin or microtubules) within the cytoplasm that move organelles from place to place within the cell. They allow some cells to move e.g. amoeba and lymphocytes and allow contraction of muscle ells
• the plasma membrane (also called cell surface membrane or cytoplasmic membrane)
• membrane-bound organelles such as mitochondria, golgi apparatus and the endoplasmic reticulum
• small vesicles
• ribosomes, which are organelles without membranes where proteins are assembled

Question 2

What are examples of eukaryotic cells?

Answer 2

Animal, plant, fungal and protoctist cells.

Question 3

What are organelles?

Answer 3

Specialised structures in a cell that have a specific function.

Question 4

What does membrane-bound organelles mean?

Answer 4

Covered by a membrane and keeps each organelle separate from the rest of the cell. Prokaryotic cells do not have membrane-bound organelles.

Question 5

What is the structure of the nucleus?

Answer 5

• Surrounded by a double membrane, called the nuclear envelope. There are pores in the nuclear envelope.
• The nucleolus does not have a membrane around it. It contains RNA.
• Chromatin is the genetic material, consisting of DNA wound around histone proteins. When the cell is not dividing, chromatin is spread out or extended. When the cell is about to divide, chromatin condenses and coils tightly into chromosomes.

Question 6

What is the function of the nuclear envelope?

Answer 6

Separates the contents of the nucleus from the rest of the cell.

Question 7

What happens at some points where the inner and outer nuclear membranes fuse together?

Answer 7

Some dissolved substances and ribosomes can pass through.

Question 8

What do pores enable?

Answer 8

Enable large substances, such as messenger RNA to leave the nucleus. Substances, such as some steroid hormones, may enter the nucleus from the cytoplasm via these pores.

Question 9

Where are ribosomes made?

Answer 9

The nucleolus.

Question 10

What do chromosomes contain?

Answer 10

The organism’s genes.

Question 11

) Summarise the function of the nucleus:

Answer 11

• is the control centre of the cell
• stores the organism’s genome
• transmits genetic information
• provides the instructions for protein synthesis

Question 12

What is the structure of the rough endoplasmic reticulum (RER)

Answer 12

• This is a system of membranes containing fluid filed cavities, called cisternae, that are continuous with the nuclear membrane.
• It is coated with ribosomes

Question 13

What is the function of the RER?

Answer 13

• Acts as the intracellular transport system – the cisternae form channels for transporting substances from one are of a cell to another.
• It provides a large surface area for ribosomes, which assemble amino acids into proteins. These proteins then actively pass through the membrane into the cisternae and are transported into the Golgi apparatus for modification and packaging.

Question 14

What is the structure of the smooth endoplasmic reticulum (SER)?

Answer 14

• This is a system of membranes, containing fluid -filled cavities (cisternae) that are continuous with the nuclear membrane.
• There are no ribosomes on its surface.

Question 15

What is the function of the SER?

Answer 15

• SER contains enzymes that catalyse reactions involved with lipid metabolism, such as:
  • Synthesis of cholesterol
  • Synthesis of lipids/phospholipids needed by the cell
  • Synthesis of steroid hormones
  -It is involved with absorption, synthesis and transport of lipids from the gut.

Question 16

What is the structure of the Golgi apparatus?

Answer 16

This consists of a stack of membrane-bound flattened sacs. Secretory vesicles bring materials to and from the Golgi apparatus.

Question 17

What is the function of the Golgi apparatus?

Answer 17

• To modify proteins by:
  • Adding sugar molecules to make glycoproteins
  • Adding lipid molecules to make lipoproteins
  • Being folded into their 3D shape
  -The proteins are then packaged into vesicles that are pinched off and then:
  • Stored in the cell or
  • Moved to the plasma membrane, either to be incorporated into the plasma membrane of exported outside the cell

Question 18

What is the structure of the mitochondria?

Answer 18

• They may be spherical, rod-shaped or branched and are 2-5 μm long.
• They are surrounded by two membranes with a fluid-filled space between them. The inner membrane is highly folded into cristae.
• The inner part of the mitochondrion is a fluid-filled matrix.

Question 19

What is the function of the mitochondria?

Answer 19

• The site of ATP (energy currency) production during aerobic respiration.
• They are self-replicating, so more can be made if the cell’s energy needs increase.

Question 20

Where are mitochondria abundantly found?

Answer 20

In cells where a lot of metabolic activity takes place e.g. liver cells and at synapses between neurones where neurotransmitter is synthesised and released.

Question 21

What is the structure of the chloroplast?

Answer 21

• Large organelles, 4-10 μm long
• Found only in plant cells and in some protoctists
• They are surrounded by a double membrane or envelope
• The inner membrane is continuous with stacks of flattened membrane sacs called thylakoids, which contain chlorophyll
• Each stack or pile of thylakoids is called a granum (plural: grana)
• The fluid filled matrix is called stroma
• Chloroplasts contain loops of DNA and starch grains

Question 22

What is the function of chloroplasts?

Answer 22

• The site of photosynthesis
• The first stage of photosynthesis, when light energy is trapped by chlorophyll and used to make ATP, occurs in the grana. Water is also split to supply hydrogen ions.
• The second stage, when the hydrogen reduces carbon dioxide, using energy from ATP to make carbohydrates, occurs in the stroma.

Question 23

Where are chloroplasts abundantly found?

Answer 23

In leaf cells, particularly the palisade mesophyll layer.

Question 24

What is the structure of the vacuole?

Answer 24

Surrounded by a membrane called the tonoplast and contains fluid.

Question 25

What is the function of the vacuole?

Answer 25

• Only plant cells have a large permanent vacuole – it is filled with water and solutes and maintains cell stability because when full, it pushes against the cell wall, making the cell turgid (swollen due to high fluid content)
• If all the plant cells are turgid, then this helps to support the plant

Question 26

What is the structure of lysosomes ?

Answer 26

• Small bags, formed from the Golgi apparatus. Each is surrounded by a single membrane.
• They contain powerful hydrolytic (digestive) enzymes.

Question 27

Where are lysosomes abundantly found?

Answer 27

• Phagocytic cells such as neutrophils and macrophages (type of white blood cell) that can ingest and digest invading pathogens such as bacteria.

Question 28

What is the function of lysosomes?

Answer 28

• They keep the powerful hydrolytic enzymes separate from the rest of the cell.
• They can engulf old cell organelles and foreign matter, digest them and return the digested components to the cell for reuse.

Question 29

What is the structure of cilia and undulipodia?

Answer 29

• There are protrusions from the cell and are surrounded by the cell surface membrane.
• Each contains microtubules
• They are formed from centrioles

Question 30

What is the function of cilia and undulipodia?

Answer 30

• The epithelial cells lining your airways each have hundreds of cilia that move the band of mucus.
• Nearly all cell types in the body have one cilium that acts as an antenna – it contains receptors and allows the cell to detect signals about its immediate environment.

Question 31

What is an undulipodia (sometimes called flageullum)?

Answer 31

Longer cilium – they are only found in spermatozoon and enables it to move.