Cell Ultratructure

Question 1

What is a eukaryote?

Answer 1

Any organism consisting of one or more cells that contain DNA in a membrane-bound nucleus, separate from the cytoplasm.

Question 2

What are the three structural components of the cytoskeleton?

Answer 2

• Microfilaments

• Microtubules

• Intermediate fibres

Question 3

What is the function of microfilaments?

Answer 3

They are responsible for movement of the cell and cytoplasm during cytokinesis (division of the cell at the end of mitosis or meiosis). They are made from the protein ‘actin’.

Question 4

What is the function of microtubules?

Answer 4

They polymerise to form tubes that determine the shape of the cell. They also act as tracks for organelles moving around the cell. They’re formed by the globular protein tubulin.

Question 5

What is the function of intermediate fibres?

Answer 5

They give strength to cells and helps maintain integrity.

Question 6

Describe the structure of the nucleus.

Answer 6

• Double nuclear envelope - a double membrane which compartmentalises the nucleus and prevents damage, whilst protecting the DNA

• Nuclear pores - these allow molecules to enter (such as nucleotides for DNA replication) and leave the cell (such as mRNA)

• Nucleolus - the site of ribosome production, made of RNA and proteins

• Chromatin - this is the DNA (with associated histone proteins). Contains the genetic code which controls the activity of the cell

Question 7

What are the three functions of the nucleus?

Answer 7

• Controls all the activity of the cell

• It is where the genetic code (DNA) of the cell is stored, replicated, and copied into RNA (transcribed)

• The nucleus is attached to the rough endoplasmic reticulum (RER) so the mRNA can easily get to ribosomes

Question 8

Describe the structure of the rough endoplasmic reticulum (RER).

Answer 8

Stacks of membrane bound (fluid filled) sacs which form sheets called cisternae. It is attached to the nucleus and covered with ribosomes.

Question 9

What is the function of the rough endoplasmic reticulum (RER)?

Answer 9

It is the site of protein synthesis.

Question 10

Describe the structure of the smooth endoplasmic reticulum (SER).

Answer 10

Similar to the RER, it is a system of interconnected tubules, but it lacks ribosomes. It consists of stacks of membrane bound sacs forming sheets called cisternae.

Question 11

What is the function of the smooth endoplasmic reticulum?

Answer 11

It is responsible for carbohydrate and lipid synthesis and storage.

Question 12

Describe the structure of ribosomes.

Answer 12

They are a 2 subunit organelle made from RNA and protein. They are not membrane bound. They’re very small (about 22nm in diameter) and are found free floating in the cytoplasm or attached to the rough endoplasmic reticulum.

Question 13

What is the function of ribosomes?

Answer 13

They are where protein is made. They assemble amino acids into proteins in chains using mRNA.

Question 14

Describe the structure of mitochondria.

Answer 14

They are oval shaped and surrounded by a double membrane (inner and outer). The inner membrane forms a finger-like structure called ‘cristae’ which increases the surface area. The solution inside is called a matrix and contains enzymes for respiration. It has mitochondrial DNA, small amounts of DNA enabling the mitochondrion to reproduce and create enzymes.

Question 15

What is the function of mitochondria?

Answer 15

They are the site of aerobic respiration. As a result of this, they release ATP (energy carrier in cells).

Question 16

Describe the structure of the Golgi apparatus.

Answer 16

It is made of stacks of flattened, membrane bound sacs (cisternae). These are continuously formed from the ER at one end and budding off as Golgi vesicles at the other.

Question 17

What are the functions of the Golgi apparatus?

Answer 17

• Allows internal transport

• Receives proteins from the RER

• Modifies and processes molecules (such as new lipids and proteins) and packages them into vesicles, which may be secretory vesicles (leaving the cell) or lysosomes (staying in the cell)

• Makes lysosomes

• Lipid synthesis

Question 18

Describe the structure of lysosomes.

Answer 18

They are spherical sacs surrounded by a single membrane.

Question 19

What is the function of lysosomes?

Answer 19

They contain powerful hydrolytic digestive enzymes known as lysozymes. Their role is to break down worn out components of the cell or digest invading cells.

Question 20

Describe the structure of centrioles.

Answer 20

They are a component of the cytoskeleton, composed of many microtubules. They are small hollow cylinders that occur in pairs next to the nucleus in animal cells only. Each centriole contains a ring of 9 triplet microtubules.

Question 21

What is the function of centrioles?

Answer 21

They make a copy of themselves during cell division and then helps to form the spindle in cell division.

Question 22

Describe the structure of cilia.

Answer 22

• ‘Hair-like’ extensions that protrude from some animal cell types

• In cross section, they have an outer membrane (plasma membrane) and a ring of nine pairs of protein microtubules inside with two microtubules in the middle

• Known as 9 +2 arrangement

• Arrangement allows movement

Question 23

What is the function of cilia?

Answer 23

They have a sensory function (e.g. nose), and they beat creating a current to move fluid/mucous/objects. They are for locomotion.

Question 24

Describe the structure of flagella.

Answer 24

They are similar to cilia but longer. They protrude from the cell surface and are surrounded by the plasma membrane. They also have a 9 +2 arrangement.

Question 25

What is the function of flagella?

Answer 25

They move like whips which enables a cell’s mobility. The microtubules contract to make the flagellum move, propelling the cells forward (like sperm cells).

Question 26

Describe the structure of chloroplasts.

Answer 26

• Double membrane - encloses the stroma

• Stroma - contains starch grains, lipid stores, DNA, RNA, and ribosomes

• Thylakoids - series of membrane-bound flattened sacs found in the stroma. Thylakoids stacked together are called ‘grana’ and are linked together by lamellae. The grana contain chlorophyll.

Question 27

What is the function of chloroplasts?

Answer 27

They are the site of photosynthetic reactions.

Question 28

Describe the structure of the cell surface membrane (plasma membrane).

Answer 28

Found on the surface of animal cells and inside the cell wall of plant and prokaryotic cells. It has a phospholipid bilayer (two layers of phospholipids). It is made of proteins and lipids.

Question 29

What is the function of the cell surface membrane (plasma membrane)?

Answer 29

It regulates the movement of substances into and out of the cell. It also contains receptor molecules which allow it to respond to chemicals like hormones.

Question 30

Describe the structure of the cellulose cell wall.

Answer 30

It is made of B-cellulose microfibrils, a complex carbohydrate. It is fully permeable to substances. The thin layer called the middle lamella marks the boundary between adjacent cell walls and ‘cements’ adjacent cells together.

Question 31

What are the functions of the cellulose cell wall?

Answer 31

• Gives the plant mechanical strength

• Gives the plant cell support and its shape

• Contents of plant cell can ‘push’ against the cell wall (turgid cell), giving the cell good support

Question 32

Describe the structure of the large permanent vacuole.

Answer 32

It is bound by a single membrane (called a tonoplast). It contains a fluid called cell sap (a solution of mineral salts, sugars, amino acids, wastes etc.). It has a selectively permeable barrier.

Question 33

What are the functions of the large permanent vacuole?

Answer 33

• Stores cell sap

• Supports herbaceous plants by making cells turgid

• Helps maintain shape and gives support by maintaining turgor pressure

• Sugars and amino acids act as a temporary food store