2.4 Eukaryotic Cell Structure (2.1.1)

Question 1

What are the two fundamental types of cell?

Answer 1

• Prokaryotic
• Eukaryotic

Question 2

What are prokaryotes?

Answer 2

Prokaryotes are single-celled organisms with a simple structure of just a single undivided internal area called the cytoplasm (composed of cytosol, which is made up of water, salts and organic molecules). They lack a distinct nucleus and other organelles due to the absence of internal membranes.

Question 3

What do eukaryotic cells make up?

Answer 3

Eukaryotic cells make up multicellular organisms like animals, plants, and fungi.

Question 4

How do eukaryotic cells have a much more complicated internal structure?

Answer 4

They contain a membrane-bound nucleus (contains nucleoplasm) and cytoplasm, which contains many membrane-bound cellular components.

Question 5

What does metabolism involve?

Answer 5

Metabolism involves both the synthesis (building up) and the breaking down of molecules.

Question 6

Where do the reactions in eukaryotic cells take place?

Answer 6

In the cytoplasm

Question 7

In eukaryotic cells the cytoplasm is divided into many different membrane-bound compartments, known as ___________.

Answer 7

organelles

Question 8

All animal, plant, fungal and protoctist cells are eukaryotic. This means that they have: (8)

Answer 8

• a nucleus surrounded by a nuclear envelope 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; allow some cells (amoebae and lymphocytes) to move; and allow contraction of muscle cells.
• a plasma membrane (also called cell surface membrane or cytoplasmic membrane)
• membrane-bound organelles, other than the nucleus, such as mitochondria, the Golgi apparatus and endoplasmic reticulum.
• small vesicles
• ribosomes, which are organelles without membranes, where proteins are assembled.

Question 9

What are cells?

Answer 9

Cells are the fundamental units or building blocks of all living organisms. Within each cell there are various organelles, each having specific functions. This provides a division of labour, which means that every cell can carry out its many functions efficiently.

Question 10

Describe the structure of the nucleus, including the nuclear envelope and nucleolus (3)

Answer 10

• The nucleus is 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. These make up nearly all the organism’s genome.

Question 11

Describe the function of the nucleus, including the nuclear envelope and nucleolus (5)

Answer 11

• The nuclear envelope separates the contents of the nucleus from the rest of the cell to protect it from damage in the cytoplasm.
• In some regions the outer and inner nuclear membranes fuse together. At these points some dissolved substances and ribosomes can pass through.
• The pores enable larger substances, such as messenger RNA (mRNA) to leave the nucleus. Substances, such as some steroid hormones, may enter the nucleus, from the cytoplasm, via these pores.
• The nucleolus is where ribosomes are made. It is composed of proteins and RNA. RNA is used to produce ribosomal RNA (rRNA) which is then combined with proteins to form the ribosomes necessary for protein synthesis.
• Chromosomes contain the organism’s genes. DNA directs the synthesis of all proteins required by the cell.

Question 12

Describe the structure of mitochondria (singular: mitochondrion) (4)

Answer 12

• These may be spherical, rod-shaped or branched, and are 2-5 micrometres long.
• They are surrounded by two membranes with a fluid-filled space between them. The inner membrane is highly folded to form structures called cristae. The membrane forming the cristae contains the enzymes used in aerobic respiration.
• The inner part of the mitochondrion is a fluid-filled matrix.
• Mitochondria also contain a small amount of DNA, called mitochondrial (mt) DNA.

Question 13

Describe the function of mitochondria (singular: mitochondria) (4)

Answer 13

• They are the site of the final stages of cellular respiration, where the energy stored in the bonds of complex, organic molecules is made available for the cell to use by the production of the molecule ATP.
• Mitochondria are 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 to increase.
• Mitochondria can produce their own enzymes.

Question 14

What are vesicles?

Answer 14

Vesicles are membrane sacs that have storage and transport roles. They consist simply of a single membrane with fluid inside. Vesicles are used to transport materials inside a cell.

Question 15

Describe the structure of lysosomes (3)

Answer 15

• They are specialised forms of vesicles.
• They contain powerful hydrolytic (digestive) enzymes.
• These are small bags, formed by the Golgi apparatus. Each is surrounded by a single membrane.

Question 16

Describe the function of lysosomes (4)

Answer 16

• Lysosomes keep the powerful hydrolytic enzymes separate from the rest of the cell.
• They are responsible for breaking down waste material in cells, including old organelles, digesting them and returning the digested components to the cell for reuse.
• They play an important role in the immune system as they are responsible for breaking down pathogens ingested by phagocytic cells.
• They also play an important role in programmed cell death or apoptosis.

Question 17

Where is the cytoskeleton present?

Answer 17

Through the cytoplasm of all eukaryotic cells.

Question 18

What is the cytoskeleton?

Answer 18

It is a network of fibres necessary for the shape and stability of a cell. Organelles are held in place by the cytoskeleton and it controls cell movement and the movement of organelles within cells.

Question 19

What are the three components of the cytoskeleton?

Answer 19

• Microfilaments
• Microtubules
• Intermediate fibres

Question 20

What are microfilaments?

Answer 20

Contractile fibres formed from the protein actin. These are responsible for cell movement and also cell contraction during cytokinesis, the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells.

Question 21

Describe how microtubules are formed

Answer 21

Globular tubulin proteins polymerise to form microtubules that are used to form a scaffold-like structure that determines the shape of a cell.

Question 22

What do the immediate fibres in the cytoskeleton do? (2)

Answer 22

They:
- anchor the nucleus within the cytoplasm.
- extend between cells in some tissues, between special junctions, enabling cell-cell signalling and allowing cells to adhere to a basement membrane, therefore stabilising tissues.

Question 23

Describe the structure of the cytoskeleton (4)

Answer 23

A network of protein structures within the cytoplasm. It consists of:
- rod-like microfilaments made of subunits of the protein actin; they are polymers of actin and each microfilament is about 7nm in diameter.
- intermediate filaments about 10nm in diameter.
- straight, cylindrical microtubules, made of protein subunits called tubulin; about 18-30nm in diameter.
- the cytoskeletal motor proteins, myosins, kinesins and dyneins, are molecular motors. They are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy site.

Question 24

Describe the function of the cytoskeleton (7)

Answer 24

• The protein microfilaments within the cytoplasm give support and mechanical strength, keep the cell’s shape stable and allow cell movement.
• Microtubules also provide shape and support to cells, and help substances and organelles to move through the cytoplasm within a cell.
• Microtubules form the track along which motor proteins (dynein and kinesin) walk and drag organelles from one part of the cell to another.
• Microtubules form the spindle before a cell divides. These spindle threads enable chromosomes to be moved within the cell.
• Microtubules also make up the cilia, undulipodia and centrioles.
• Intermediate filaments are made of a variety of proteins. They anchor the nucleus within a cytoplasm.
• Intermediate filaments also extend between cells in some tissues, between special junctions, enabling cell-cell signalling and allowing cells to adhere to a basement membrane, therefore stabilising tissues.

Question 25

What does the movement of cells depend on?

Answer 25

The activity of the actin filaments in the cytoskeleton

Question 26

Describe cell movement (8)

Answer 26

The movement of cells like phagocytes depends on the activity of the actin filaments in the cytoskeleton.
- The filament lengths change with the addition and removal of monomer subunits.
- The rate at which these subunits are added is different at each end of a filament.
- The subunits are not symmetrical and can only be added if they are in the correct orientation.
- The subunits have to change shape before they are added to one end (the minus end) of the filament but not the other end (the plus end). This means that the subunits are added at a faster rate at the plus end.
- The filaments therefore increase in length at a faster rate in one particular direction.
- Whether subunits are added or removed, at either end, is determined by the concentration of subunits in the cytoplasm.
- Due to the different rates of addition at either end, at certain concentrations subunits will be added at one end and removed at the other. This is called treadmilling.
- The increasing length of the filaments at one edge of a cell, the leading edge, leads to cells such as phagocytes moving in a particular direction.

Question 27

What are centrioles?

Answer 27

Centrioles are a component of the cytoskeleton present in most eukaryotic cells with the exception of flowering plants and most fungi. They are composed of microtubules. Two associated centrioles form the centrosome, which is involved in the assembly and organisation of the spindle fibres during cell division. In organisms with flagella and cilia, centrioles are thought to play a role in the positioning of these structures.

Question 28

Compare flagella and cilia

Answer 28

Both flagella (whip-like) and cilia (hair-like) are extensions that protrude from some cell types. Flagella are longer than cilia but cilia are usually present in much greater numbers.

Question 29

Describe the function of flagella

Answer 29

Flagella are used primarily to enable cells motility. In some cells they are used as a sensory organelle detecting chemical changes in the cell’s environment.

Question 30

What are cilia and undulipodia? (3)

Answer 30

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

Question 31

What are stationary cilia for?

Answer 31

Stationary cilia are present on the surface of many cells and have an important function in sensory organs such as the nose.

Question 32

What do mobile cilia do?

Answer 32

Mobile cilia beat in a rhythmic manner, creating a current, and cause fluids or objects adjacent to the cell to move. For example, they are present in the trachea to move mucus away from the lungs (helping to keep the air passages clean), and in fallopian tubes to move egg cells from the ovary to the uterus.

Question 33

What does each cilium contain?

Answer 33

Each cilium contains two central microtubules (black circles) surrounded by nine pairs of microtubules arranged like a “wheel”. This is known as the 9 + 2 arrangement. Pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion.

Question 34

Describe the function of cilia

Answer 34

• The epithelial cells lining your airways each have many hundreds of cilia that beat and 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.
• The only type of human cell to have an undulipodium (a longer cilium) is a spermatozoon. The undulipodium enables the spermatozoon to move.

Question 35

What is an undulipodium?

Answer 35

A longer cilium (flagella for eukaryotic cells)

Question 36

A key function of a cell is to synthesise proteins (including enzymes) for __________ use and for ___________ (transport out of the cell)

Answer 36

internal, secretion

Question 37

What are the three organelles that are involved in synthesising proteins?

Answer 37

• Ribosomes
• Endoplasmic reticulum
• Golgi apparatus

Question 38

What is the endoplasmic reticulum (RER)?

Answer 38

The endoplasmic reticulum (RER) is a network of membranes enclosing flattened sacs called cisternae. It is connected to the outer membrane of the nucleus. There are two types.

Question 39

Describe the structure of the rough endoplasmic reticulum (RER)

Answer 39

• This is a system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane.
• It is coated with ribosomes.

Question 40

Describe the function of the rough endoplasmic reticulum (RER) (2)

Answer 40

• RER is the intracellular transport system: the cisternae form channels for transporting substances from one area 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 to the Golgi apparatus for modification and packaging.

Question 41

Describe the structure of the smooth endoplasmic reticulum (SER)

Answer 41

• 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 42

Is the rough endoplasmic reticulum coated with ribosomes?

Answer 42

Yes

Question 43

Is the smooth endoplasmic reticulum coated with ribosomes?

Answer 43

No

Question 44

What is the function of the smooth endoplasmic reticulum (SER) (2)

Answer 44

• SER contains enzymes that catalyse reasons 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 45

Where can ribosomes be in the cell?

Answer 45

Ribosomes can be free-floating in the cytoplasm or attached to endoplasmic reticulum, forming rough endoplasmic reticulum.

Question 46

Are ribosomes surrounded by a membrane?

Answer 46

No

Question 47

What are ribosomes constructed of?

Answer 47

Ribosomal DNA and proteins with a small subunit and a large subunit.

Question 48

What are 2 organelles that contain ribosomes?

Answer 48

• Mitochondria
• Chloroplasts

Question 49

Describe the structure of the Golgi apparatus (2)

Answer 49

• A series of membrane bound, but flattened sacs called cisternae.
• The whole organelle is a mobile, flexible structure; vesicles are constantly added at one side and being lost from the other.

Question 50

Describe the structure of ribosomes (4)

Answer 50

• Small spherical organelles, about 20nm in diameter.
• Made of ribosomal RNA.
• Made in the nucleolus, as two separate subunits, which pass through the nuclear envelope into the cell cytoplasm and then combine.
• Some remain free in the cytoplasm and some attach to the endoplasmic reticulum.

Question 51

Describe the function of ribosomes (2)

Answer 51

• Ribosomes bound to the exterior of RER are mainly for synthesising proteins that will be exported outside the cell.
• Ribosomes that are free in the cytoplasm, either singly or in clusters, are primarily the site of assembly of proteins that will be used inside the cell.

Question 52

Describe the function of the Golgi apparatus (3)

Answer 52

• All proteins produced in rough ER go through Golgi apparatus. The cis-Golgi sorts and returns wrongly secreted proteins. The cisternae modify the protein and add labels to them. The trans-Golgi recognises the labels and sends them to their final destination.
• Proteins are modified for example 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, or exported outside the cell.

Question 53

State the steps in which proteins are made and secreted (8)

Answer 53

• The gene that has the coded instructions for the protein, housed on chromatin in the nucleus, is transcribed into a length into RNA, called messenger RNA (mRNA).
• Many copies of this mRNA are made and they pass out of the pores in the nuclear envelope to the ribosomes.
• At the ribosomes, the instructions are translated and insulin molecules are assembled.
• The insulin molecules pass into the cisternae of the rough endoplasmic reticulum (RER) and along these hollow sacs.
• Transport vesicles with insulin inside are pinched off from the RER and pass, via microtubules and motor proteins, to the Golgi apparatus.
• The vesicles fuse with the cis face of the Golgi apparatus, where the insulin protein molecules may be structurally modified for release.
• Inside vesicles pinched off from the trans face of the Golgi apparatus, these molecules pass to the plasma membrane.
• Secretory vesicles carry proteins that are to be released from the cell. The vesicles move towards and fuse with the cell surface membrane, releasing their contents by exocytosis. Some vesicles form lysosomes - these contain enzymes for use in the cell.

Question 54

Are transport and secretory vesicles the same?

Answer 54

No

Question 55

What are transport vesicles? (2)

Answer 55

• They are small spheres of membranes that carry cargo, such as proteins or carbohydrates, between organelles or within the same organelle.
• They move molecules around the inside of a cell.

Question 56

What are secretory vesicles? (2)

Answer 56

• They are membrane-bound sacs that transport secretory proteins, such as hormones or neurotransmitters. Secretory vesicles are also known as dense core vesicles (DCV)s.
• They move molecules outside of the cell, and are located near the plasmalemma and the trans face of the Golgi Complex.

Question 57

Ribosomes can bind to ____ and ____.

Answer 57

mRNA and tRNA

Question 58

What types of cells are lysosomes abundant in?

Answer 58

They are abundant in phagocytic cells such as neutrophils and macrophages (types of white blood cell) that can ingest and digest invading pathogens such as bacteria.

Question 59

In what types of cells are lysosomes abundant in?

Answer 59

They are abundant in phagocytic cells such as neutrophils and macrophages (types of white blood cell) that can ingest and digest invading pathogens such as bacteria.

Question 60

What type of cells are mitochondria abundant in?

Answer 60

They are abundant in cells where a lot of metabolic activity takes place, for example in liver cells and at synapses between neurones where neurotransmitter is synthesised and released.

Question 61

Describe the function of microtubules (4)

Answer 61

• Microtubules provide shape and support to cells, and help substances and organelles to move through the cytoplasm within a cell.
• Microtubules form the track along which motor proteins (dynein and kinesin) walk and drag organelles from one part of the cell to another.
• Microtubules form the spindle before a cell divides. These spindle threads enable chromosomes to be moved within the cell.
• Microtubules also make up the cilia, undulipodia and centrioles.