Cells - Cell Structure

Question 1

What is meant by an Eukaryotic cell

Answer 1

DNA is contained in a nucleus, contains membrane-bound specialised organelles.

Question 2

What is meant by a Prokaryotic cell

Answer 2

DNA is ‘free’ in cytoplasm, no organelles e.g. bacteria & archaea.

Question 3

State the relationship between a system and specialised cells

Answer 3

Specialised cells → tissues that perform specific function → organs made of several tissue types → organ systems

Question 4

Describe the structure and function of the cell-surface membrane.

Answer 4

‘Fluid mosaic’ phospholipid bilayer with extrinsic & intrinsic proteins embedded
Isolates cytoplasm from extracellular environment.
Selectively permeable to regulate transport of substances.
Involved in cell signalling / cell recognition.

Question 5

Explain the role of cholesterol, glycoproteins & glycolipids in the cell-surface membrane.

Answer 5

Cholesterol: steroid molecule connects phospholipids & reduces fluidity.
Glycoproteins: cell signalling, cell recognition (antigens) & binding cells together.
Glycolipids: cell signalling & cell recognition.

Question 6

Describe the structure of the nucleus.

Answer 6

Surrounded by nuclear envelope, a semi-permeable double membrane.
Nuclear pores allow substances to enter/exit.
Dense nucleolus made of RNA & proteins assembles ribosomes.

Question 7

Describe the function of the nucleus.

Answer 7

Contains DNA coiled around chromatin into chromosomes.
Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semiconservative replication.

Question 8

Describe the structure of a mitochondrion.

Answer 8

Surrounded by double membrane folded inner membrane forms cristae: site of electron transport chain
Fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins.

Question 9

Describe the structure of a chloroplast.

Answer 9

Vesicular plastid with double membrane.
Thylakoids: flattened discs stack to form grana; contain photosystems with chlorophyll
Intergranal lamellae: tubes attach thylakoids in adjacent grana.
Stroma: fluid-filled matrix.

Question 10

State the function of mitochondria and chloroplasts.

Answer 10

Mitochondria: site of aerobic respiration to produce ATP.
Chloroplasts: site of photosynthesis to convert solar energy to chemical energy.

Question 11

Describe the structure and function of The Golgi apparatus.

Answer 11

The Golgi apparatus is a series of flattened membranous sacs.
Transport vesicles form at the endoplasmic reticulum and fuse with the Golgi apparatus.
The transport vesicles empty proteins and lipids into the lumen of the Golgi apparatus.
As the proteins and lipids travel through the Golgi, they are sorted, packaged and tagged so that they can be sent to the right place for further modification

Question 12

Describe the structure and function of a lysosome.

Answer 12

Lysosomes are a special type of Golgi vesicle. They contain enzymes called lysozymes.
Lysozymes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids and old organelles.

Question 13

Describe the structure and function of a ribosome.

Answer 13

Formed of protein & rRNA
free in cytoplasm or attached to endoplasmic reticulum.
Site of protein synthesis via translation:
large subunit: joins amino acids
small subunit: contains mRNA binding site.

Question 14

Describe the structure and function of the endoplasmic reticulum

Answer 14

contains cisternae, network of tubules
flattened sacs extends from cell membrane through cytoplasm and connects to nuclear envelope:
Rough Endoplasmic Reticulum: many ribosomes attached for protein synthesis & transport.
Smooth Endoplasmic Reticulum: lipid synthesis.

Question 15

Describe the structure of the cell wall.

Answer 15

Bacteria: Made of the glycoprotein murein.
Plants: Made of cellulose microfibrils
plasmodesmata allow molecules to pass between cells,
middle lamella acts as boundary between adjacent cell walls.

Question 16

State the functions of the cell wall.

Answer 16

Mechanical strength and support.
Physical barrier against pathogens.
Part of apoplast pathway (plants) to enable easy diffusion of water.

Question 17

Describe the structure and function of the cell vacuole in plants.

Answer 17

Surrounded by single membrane:
tonoplast contains cell sap: mineral ions, water, enzymes, soluble pigments.
Controls turgor pressure.
Absorbs and hydrolyses potentially harmful substances to detoxify cytoplasm.

Question 18

Explain some common cell adaptations.

Answer 18

Folded membrane or microvilli increase surface area e.g. for diffusion.
Many mitochondria = large amounts of ATP for active transport.
Walls one cell thick to reduce distance of diffusion pathway.

Question 19

State the role of plasmids in prokaryotes.

Answer 19

Small circular loops of DNA that carries non-essential genes.
Can be exchanged between bacterial cells via conjugation.
Prokaryotes can have one or more plasmids

Question 20

State the role of flagellum in prokaryotes.

Answer 20

Long, rotating tail for motility
Some bacteria have multiple flagella

Question 21

State the role of the capsule in prokaryotes.

Answer 21

Prevents desiccation by having polyssacharide layer.
Acts as food reserve.
Provides mechanical protection against phagocytosis & external chemicals.
Sticks cells together.

Question 22

Compare eukaryotic and prokaryotic cells.

Answer 22

both have:
● Cell membrane.
● Cytoplasm.
● Ribosomes (don’t count as an organelle since not membrane-bound).

Question 23

Contrast eukaryotic and prokaryotic cells.

Answer 23

Question 24

Why are viruses referred to as ‘particles’ instead of cells?

Answer 24

Acellular & non-living: no cytoplasm, cannot self-reproduce, no metabolism.

Question 25

Describe the structure of a viral particle.

Answer 25

● Linear genetic material (DNA or RNA) & viral enzymes e.g. reverse transcriptase. ● Surrounded by capsid (protein coat made of capsomeres). ● No cytoplasm. ● not all viruses have an envelope

Question 26

State the role of the capsid on viral particles.

Answer 26

● Protect nucleic acid from degradation by restriction endonucleases. ● Surface sites enable viral particle to bind to host cells to enter and inject their genetic material.

Question 27

State the role of attachment proteins on viral particles.

Answer 27

Enable viral particle to bind to complementary rceptors on surface of host cell : entry via endosymbiosis.

Question 28

Describe how optical microscopes work.

Answer 28

1. Lenses focus rays of light and magnify the view of a thin slice of specimen. 2. Different structures absorb different amounts and wavelengths of light. 3. Reflected light is transmitted to the observer via the objective lens and eyepiece.

Question 29

Outline how a student could prepare a temporary mount of tissue for an optical microscope.

Answer 29

1. Obtain thin section of tissue e.g. using ultratome or by maceration. 2. Place plant tissue in a drop of water. 3. Stain tissue on a slide to make structures visible. 4. Add coverslip using mounted needle at 45° to avoid trapping air bubbles.

Question 30

Suggest the advantages and limitations of using an optical microscope.

Answer 30

+ colour image + can show living structures + affordable apparatus - 2D image - lower resolution than electron microscopes = cannot see ultrastructure.

Question 31

Describe how a transmission electron microscope (TEM) works.

Answer 31

1. Pass a high energy beam of electrons through thin slice of specimen. 2. More dense structures appear darker since they absorb more electrons. 3. Focus image onto fluorescent screen or photographic plate using magnetic lenses.

Question 32

Suggest the advantages and limitations of using a TEM.

Answer 32

+ electrons have a shorter wavelength than light microscope so high resolution, so the ultrastructure visible + high magnification (x 500000) - 2D image - requires a vacuum = cannot show living structures - extensive preparation may introduce artefacts - no colour image.

Question 33

Describe how a scanning electron microscope (SEM) works.

Answer 33

1. Focus a beam of electrons onto a specimen’s surface using electromagnetic lenses. 2. Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate.

Question 34

Suggest the advantages and limitations of using an SEM.

Answer 34

+ 3D image + electrons have a shorter wavelength than light so high resolution - requires a vacuum = cannot show living structures - no colour image - only shows outer surface. SEM images can be three-dimensional. Specimens do NOT have to be thin like when using a TEM.

Question 35

Define magnification and resolution.

Answer 35

Magnification: factor by which the image is larger than the actual specimen. Resolution: smallest separation distance at which 2 separate structures can be distinguished from one another.

Question 36

Explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure.

Answer 36

1. Place micrometer on stage to calibrate eyepiece graticule. 2. Line up scales on graticule and micrometer. Count how many graticule divisions are in 100μm on the micrometer. 3. Length of 1 eyepiece division = 100μm / number of divisions 4. Use calibrated values to calculate actual length of structures.

Question 37

State an equation to calculate the actual size of a structure from microscopy.

Answer 37

actual size = I / A M.

Question 38

Outline what happens during cell fractionation and ultracentrifugation.

Answer 38

1. Mince and homogenize tissue to break open cells & release organelles. 2. Filter homogenate to remove debris. 3. Perform differential centrifugation: a) Spin homogenate in centrifuge. b) The most dense organelles in the mixture form a pellet. c) Filter off the supernatant and spin again at a higher speed.

Question 39

State the order of sedimentation of organelles during differential centrifugation.

Answer 39

most dense → least dense nucleus → mitochondria → lysosomes → RER → plasma membrane → SER → ribosomes.

Question 40

Explain why fractionated cells are kept in a cold, buffered, isotonic solution.

Answer 40

cold: slow action of hydrolase enzymes. buffered: maintain constant pH. isotonic: prevent osmotic lysis/ shrinking of organelles.

Question 41

Describe the structure and function of the golgi vesicle

Answer 41

Golgi vesicles are membrane-bound, fluid-filled vesicles located in the cytoplasm. They are small, round and are seen in a high density near the edges of the sacs. Golgi vesicles store and transport modified proteins and lipids from the Golgi apparatus to target cells.