Flashcards - Topic 2.1 Cell Structure - AQA Biology A-level

Question 1

Define eukaryotic cell.

Answer 1

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

Question 2

Define prokaryotic cell.

Answer 2

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

Question 3

What is 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 of the cell-surface membrane.

Answer 4

‘Fluid mosaic’ phospholipid bilayer with extrinsic & intrinsic proteins embedded.

Isolates cytoplasm from extracellular environment.

Question 5

What is the function of the cell-surface membrane?

Answer 5

Selectively permeable to regulate transport of substances.

Involved in cell signalling / cell recognition.

Question 6

Explain the role of cholesterol in the cell-surface membrane.

Answer 6

Cholesterol: steroid molecule connects phospholipids & reduces fluidity.

Question 7

Explain the role of glycoproteins in the cell-surface membrane.

Answer 7

Glycoproteins: cell signalling, cell recognition (antigens) & binding cells together.

Question 8

Explain the role of glycolipids in the cell-surface membrane.

Answer 8

Glycolipids: cell signalling & cell recognition.

Question 9

Describe the structure of the nucleus.

Answer 9

Surrounded by nuclear envelope, a semi-permeable double membrane.

Nuclear pores allow substances to enter/exit.

Question 10

What is the function of the nucleus?

Answer 10

Contains DNA coiled around chromatin into chromosomes.

Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semiconservative replication.

Question 11

Describe the structure of a mitochondrion.

Answer 11

Surrounded by double membrane folded inner membrane forms cristae: site of electron transport chain.

Fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins.

Question 12

Describe the structure of a chloroplast.

Answer 12

Vesicular plastid with double membrane.

Thylakoids: flattened discs stack to form grana; contain photosystems with chlorophyll.

Question 13

What is the function of mitochondria?

Answer 13

Site of aerobic respiration to produce ATP.

Question 14

What is the function of chloroplasts?

Answer 14

Site of photosynthesis to convert solar energy to chemical energy.

Question 15

Describe the structure of the Golgi apparatus.

Answer 15

Planar stack of membrane-bound, flattened sacs cis face aligns with rER.

Molecules are processed in cisternae vesicles bud off trans face via exocytosis.

Question 16

What is the function of the Golgi apparatus?

Answer 16

Modifies & packages proteins for export, synthesises glycoproteins.

Question 17

Describe the structure of a lysosome.

Answer 17

Sac surrounded by single membrane embedded H+ pump maintains acidic conditions contains digestive hydrolase enzymes glycoprotein coat protects cell interior.

Digests contents of phagosome, exocytosis of digestive enzymes.

Question 18

Describe the structure of a ribosome.

Answer 18

Formed of protein & rRNA free in cytoplasm or attached to ER.

Site of protein synthesis via translation: large subunit joins amino acids, small subunit contains mRNA binding site.

Question 19

Describe the structure of the endoplasmic reticulum (ER).

Answer 19

Cisternae: network of tubules & flattened sacs extends from cell membrane through cytoplasm & connects to nuclear envelope.

Rough ER: many ribosomes attached for protein synthesis & transport. Smooth ER: lipid synthesis.

Question 20

Describe the structure of the cell wall in bacteria.

Answer 20

Made of the polysaccharide murein.

Question 21

Describe the structure of the cell wall in plants.

Answer 21

Made of cellulose microfibrils.

Plasmodesmata allow molecules to pass between cells, middle lamella acts as boundary between adjacent cell walls.

Question 22

What are the functions of the cell wall?

Answer 22

Mechanical strength and support, physical barrier against pathogens, part of apoplast pathway (plants) to enable easy diffusion of water.

Question 23

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

Answer 23

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 24

Explain some common cell adaptations.

Answer 24

Folded membrane or microvilli increase surface area e.g. for diffusion.

Many mitochondria = large amounts of ATP for active transport.

Question 25

What is the role of plasmids in prokaryotes?

Answer 25

Small ring of DNA that carries non-essential genes. ## Footnote Can be exchanged between bacterial cells via conjugation.

Question 26

What is the role of flagella in prokaryotes?

Answer 26

Rotating tail propels (usually unicellular) organism.

Question 27

What is the role of the capsule in prokaryotes?

Answer 27

Polysaccharide layer: prevents desiccation, acts as food reserve, provides mechanical protection against phagocytosis & external chemicals, sticks cells together.

Question 28

Compare eukaryotic and prokaryotic cells.

Answer 28

Both have: cell membrane, cytoplasm, ribosomes (don't count as an organelle since not membrane-bound).

Question 29

Contrast eukaryotic and prokaryotic cells.

Answer 29

Prokaryotic: small cells & always unicellular, no membrane-bound organelles & no nucleus, circular DNA not associated with proteins, small ribosomes (70S), binary fission - always asexual reproduction, cellulose cell wall (plants)/ chitin (fungi), capsule, sometimes plasmids & cytoskeleton. ## Footnote Eukaryotic: larger cells & often multicellular, always have organelles & nucleus, linear chromosomes associated with histones, larger ribosomes (80S), mitosis & meiosis - sexual and/or asexual, murein cell walls, no capsule, no plasmids, always cytoskeleton.

Question 30

Why are viruses referred to as 'particles' instead of cells?

Answer 30

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

Question 31

Describe the structure of a viral particle.

Answer 31

Linear genetic material (DNA or RNA) & viral enzymes e.g. reverse transcriptase. ## Footnote Surrounded by capsid (protein coat made of capsomeres), no cytoplasm.

Question 32

Describe the structure of an enveloped virus.

Answer 32

Simple virus surrounded by matrix protein. ## Footnote Matrix protein surrounded by envelope derived from cell membrane of host cell, attachment proteins on surface.

Question 33

What is the role of the capsid on viral particles?

Answer 33

Protect nucleic acid from degradation by restriction endonucleases. ## Footnote Surface sites enable viral particle to bind to & enter host cells or inject their genetic material.

Question 34

What is the role of attachment proteins on viral particles?

Answer 34

Enable viral particle to bind to complementary sites on host cell: entry via endosymbiosis.

Question 35

Describe how optical microscopes work.

Answer 35

Lenses focus rays of light and magnify the view of a thin slice of specimen. ## Footnote Different structures absorb different amounts and wavelengths of light.

Question 36

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

Answer 36

Obtain thin section of tissue e.g. using ultratome or by maceration. ## Footnote Place plant tissue in a drop of water, stain tissue on a slide to make structures visible, add coverslip using mounted needle at 45 to avoid trapping air bubbles.

Question 37

What are the advantages and limitations of using an optical microscope?

Answer 37

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

Question 38

Describe how a transmission electron microscope (TEM) works.

Answer 38

Pass a high energy beam of electrons through thin slice of specimen. ## Footnote More dense structures appear darker since they absorb more electrons.

Question 39

What are the advantages and limitations of using a TEM?

Answer 39

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

Question 40

Describe how a scanning electron microscope (SEM) works.

Answer 40

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

Question 41

What are the advantages and limitations of using an SEM?

Answer 41

+ 3D image, + electrons have shorter wavelength than light = high resolution. ## Footnote - requires a vacuum = cannot show living structures, - no colour image.

Question 42

Define magnification.

Answer 42

Factor by which the image is larger than the actual specimen.

Question 43

Define resolution.

Answer 43

Smallest separation distance at which 2 separate structures can be distinguished from one another.

Question 44

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

Answer 44

Place micrometer on stage to calibrate eyepiece graticule. ## Footnote Line up scales on graticule and micrometer. Count how many graticule divisions are in 100 m on the micrometer.

Question 45

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

Answer 45

actual size = image size / magnification.

Question 46

Outline what happens during cell fractionation and ultracentrifugation.

Answer 46

Mince and homogenize tissue to break open cells & release organelles. ## Footnote Filter homogenate to remove debris.

Question 47

State the order of sedimentation of organelles during differential centrifugation.

Answer 47

Most dense ? least dense: nucleus ? mitochondria ? lysosomes ? RER ? plasma membrane ? SER ? ribosomes.

Question 48

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

Answer 48

Cold: slow action of hydrolase enzymes. ## Footnote Buffered: maintain constant pH, isotonic: prevent osmotic lysis/ shrinking of organelles.