A Level Biology : Module 1 - Cell Ultrastructure

Question 1

Cell Surface Membrane

Answer 1

• Surrounds ALL cells
• Controls exchange of materials between internal environment and external environment
• Formed from a phospholipid bilayer spanning around 10nm

Question 2

Cell Wall

Answer 2

• Freely permeable to most substances
• Plant cells, NOT ANIMAL CELLS
• Offer structural support
• In plant cells, it is made of polysaccharide cellulose
• In most bacterial cells, it is made of peptidoglycan

Question 3

Nucleus

Answer 3

• In all eukaryotic cells apart from RBCs
• Separated from the cytoplasm via a double membrane (nuclear envelope)
• Nuclear envelope has many pores, allowing mRNA and ribosomes to travel out of the nucleus, as well as enzymes and signalling molecules to travel in
• Contains chromatin (making up chromosomes)
• Chromosomes are tightened around proteins called histones

Question 3

Mitochondria

Answer 3

• Site of aerobic respiration in all EUKARYOTIC cells
• Surrounded by a double membrane with the inner membrane folded inwards to form cristae
• Within the cristae is the matrix, containing enzymes needed for aerobic respiration, producing ATP
• Smaller pieces of DNA and ribosomes are found in the matrix for replication

Question 4

Chloroplasts

Answer 4

• Plant cells
• Larger than mitochondria, also surrounded by a double membrane
• Thylakoids are membrane bound compartments containing chlorophyll
• They stack to form structures called grana
• Grana join together by lamellae (thin flat thylakoid membranes)
• Site of photosynthesis
• Light dependent stage takes place within thylakoids
• Light independent stage takes place in stroma
• Small pieces of DNA and ribosomes are used to synthesise proteins needed in replication and photosynthesis

Question 5

Ribosomes

Answer 5

• Synthesised in the nucleolus
• Composed of almost equal amounts of protein and RNA
• Found in ALL cells
• Found in cytoplasm of all cells, or as part of the ENDOPLASMIC RETICULUM in eukaryotic cells
• Ribosomes are complexes of ribosomal RNA and proteins
• 80S ribosomes are found in eukaryotic cells
• 70S ribosomes are found in prokaryotes, mitochondria and chloroplasts
• Site of translation and proteinsynthesis

Question 6

Rough Endoplasmic reticulum

Answer 6

• Found in plant and animal cells
• Surface covered in ribosomes
• Formed from continuous colds of membrane continuous with the nuclear envelope
• This processes proteins made by ribosomes

Question 7

Smooth Endoplasmic Reticulum

Answer 7

• Found in animal and plant cells
• Does not have ribosomes on its surface
• Function is distinct to the RER
• Involves production, processing and storage of steroids, lipids and carbohydrates

Question 8

Golgi Apparatus

Answer 8

• Found in plant and animal cells
• Flattened sacs of membrane similar to ER
• Modifies proteins and lipids before packaging them into Golgi vesicles
• Vesicles then transport proteins and lipids to required destination
• ## Proteins go through Golgi apparatus are exported, put into lysosomes or delivered to membrane bound organelles

Question 9

Large Permanent Vacuole

Answer 9

• Sac in plant cells surrounded by the tonoplast (selectively permeable)
• Vacuoles in animal cells are not permanent and small

Question 10

Vesicles

Answer 10

• Found in plant and animal cells
• Membrane bound sacs for transport and storage

Question 11

Lysosomes

Answer 11

• Special forms of vesicles
• They contain hydrolytic enzymes (which are enzymes that break down biological molecules)
• Break down waste materials, such as worn out organelles
• Used extensively by cells of the immune system and in apoptosis

Question 12

Centrioles

Answer 12

• Hollow fibres made of microtubules
• Two centrioles formed at right angles to each other form a centrosome, which organises spindle fibres during division
• Not found in flowering plants or fungi

Question 13

Microtubules

Answer 13

• Found in all eukaryotic cells
• Makes up cytoskeleton
• Made of α and β tubulin which combine to form DIMERS
• Dimers join to form protofilaments
• 13 protofilaments in a cylinder make a microtubule
• It provides support and movement of the cell

Question 14

Microvilli

Answer 14

• Found in specialised animal cells
• Cell membrane projections
• Increase surface area of cell surface membrane in order to increase the rate of exchange of substances

Question 15

Cilia

Answer 15

• Hair like projections which are made of microtubules
• Allows movement of substances over the cell surface

Question 16

Flagella

Answer 16

• Found in specialised cells
• Similar in structure to the CILIA, but is made of longer microtubules
• Contracts to provide cell movement, for example in sperm cells

Question 17

What is the Cytoskeleton?

Answer 17

• Extensive network of protein fibres
• Made if microfilaments and microtubules
• There are intermediate filaments also found within cytoskeleton

Question 18

Function of the cytoskeleton?

Answer 18

• Strengthening and support
• Intracellular movement
• Cellular movement

Question 19

What are microfilaments?

Answer 19

• Solid strands mostly made of actin
• Fibres can cause cell movement and movement of organelles

Question 20

What are microtubules?

Answer 20

• Hollow (tubular) strands made of tubulin.
• Organelles and other cell contents are moved along these fibres using ATP to drive movement

Question 21

How does the cytoskeleton strengthen and support the cell?

Answer 21

• With mechanical strength (like scaffolding), maintaining shape of cell
• Supports organelles and keeps them in position

Question 22

How does the cytoskeleton provide intracellular movement?

Answer 22

• Aids transport by forming ‘tracks’ along which organelles can move
• Examples include movement of vesicles and chromosomes to opposite ends of a cell during cell division

Question 23

How does the cytoskeleton provide cellular movement?

Answer 23

• Via cilia and flagella
• They contain microtubules responsible for moving them

Question 24

How do prokaryotic cells differ from eukaryotic cells?

Answer 24

• Eukaryotic cells have membrane bound organelles
• Eukaryotic cells have nuclei (and not a single circular dna molecule)
• Prokaryotic cells have cell walls containing murein.
• Ribosomes are 70S (smaller) as opposed to 80S

Additional :
- Flagellum
- Plasmids (small loops of DNA)
- Capsules (protects bacteria from drying out)

• Prokaryotes divide by binary fission, Eukaryotes by mitosis or meiosis, involves spindle
• In prokaryotes, cell walls are made of peptidoglycan, Eukaryotes it is polysaccharide cellulose
• Prokaryotes are 0.5-5um, eukaryotes are up to 100um