topic 2: cell structure

Question 1

eukaryotic organisms and examples
What are the distinguishing features of eukaryotic cells?

Answer 1

-complex and larger structures e.g. animals, plants and fungi
● Cytoplasm containing membrane-bound organelles
● So DNA enclosed in a nucleus

Question 2

structure and function of nucleus

Answer 2

STRUCTURE
-NUCLEAR ENVELOPE(double membrane) contain NUCLEAR PORES which allows large molecules like RNA to move between the nucleus and cytoplasm
-CHROMOSOMES (which are made from protein histone bound linear DNA)
-has one or more NUCLEOLUS (dense region)
-chromatin = condensed and chromosome =highly condensed

FUNCTION
● Holds and stores genetic information which codes for polypeptides (proteins)
● Site of DNA replication
● Site of transcription (part of protein synthesis), producing mRNA
● Nucleolus makes ribosomes / rRNA

Question 3

structure of cell surface (plasma) membrane and function

Answer 3

STRUCTURE
-surrounds animal cells and is inside cell wall of bacteria and plant cells
-contains a phospholipid bilayer (made up of lipids and proteins) with molecules embedded (e.g. proteins, carbohydrates and cholesterol)

FUNCTION
● Selectively permeable → enables control of passage of substances in / out of cell
● Molecules / receptors / antigens on surface → allow cell recognition / signalling

Question 4

structure of RER (rough endoplasmic reticulum) and function

Answer 4

STRUCTURE
-system of membranes enclosing a fluid filled space
the surface is covered with ribosomes attached to membranes which makes it appear ROUGH

FUNCTION
● Ribosomes on surface synthesise proteins
● Proteins processed / folded / transported inside rER
● Proteins packaged into vesicles for transport eg. to Golgi apparatus

Question 5

structure of SER (smooth endoplasmic reticulum) and function

Answer 5

STRUCTURE
-system of membranes enclosing a fluid filled space without ribosomes attached to membranes

FUNCTION
● Synthesises and processes lipids
● Eg. cholesterol and steroid hormones

Question 6

structure and function of golgi apparatus

Answer 6

STRUCTURE
-a group of fluid filled, membrane-bound flattened sacs, with vesicles at the edges of the sacs
FUNCTION
● Modifies protein, eg. adds carbohydrates to produce glycoproteins
● Modifies lipids, eg. adds carbohydrates to make glycolipids
● Packages proteins / lipids into Golgi vesicles
● Produces lysosomes (a type of Golgi vesicle)

Question 7

structure and function of golgi vesicles

Answer 7

STRUCTURE
-small fluid-filled membrane bound sacs
-found at the edges of golgi apparatus

FUNCTION
● Transports proteins / lipids to their required destination
● Eg. moves to and fuses with cell-surface membrane

Question 8

structure and function of the lysosomes

Answer 8

STRUCTURE
-a round membrane-bound organelle, contains digestive lysosomal enzymes (hydrolytic enzymes)
-type of golgi vesicle

FUNCTION
● Release hydrolytic enzymes (lysozymes)
● To break down / hydrolyse pathogens or worn-out cell components

Question 9

structure and function of ribosomes

Answer 9

STRUCTURE
-small organelle
● Made of ribosomal RNA and protein (two subunits)
● Not a membrane-bound organelle

FUNCTION
-site of protein synthesis (Translation)

Question 10

structure and function of mitochondria

Answer 10

STRUCTURE
-oval shaped organelle surrounded by a double membrane - the inner membrane folded to form cristae and maximise surface area
-matrix (central part) is inside the inner membrane containing 70s ribosomes and circular DNA

FUNCTION
● Site of aerobic respiration
● To produce ATP for energy release
● Eg. for protein synthesis / vesicle movement / active transport

Question 11

structure and function of chloroplasts (5 structures)

Answer 11

STRUCTURE
-a small, flattened organelle in photosynthesizing cells
-ENVELOPE - surrounded by a double membrane
-contains THYKALOIDS which are flat disks with pigment/chlorophyll in
- GRANA is stacked thylakoids which are linked together by lamellae
-LAMELLAE are flattened membranes which link grana together
-STROMA is a fluid which contains thylakoid membranes, 70s ribosomes, circular DNA and starch granules

FUNCTION
● Absorbs light energy for photosynthesis
● To produce organic substances eg. carbohydrates / lipids

Question 12

structure and function of cell wall

Answer 12

STRUCTURE
-a rigid structure that surrounds cells
● Composed mainly of cellulose (a polysaccharide) in plants / algae
● Composed of chitin (a nitrogen-containing polysaccharide) in fungi

FUNCTION
● Provides mechanical strength to cell
● So prevents cell changing shape or bursting under pressure due to osmosis

Question 13

structure and function of vacuole

Answer 13

STRUCTURE
-a membrane-bound organelle found in the cytoplasm of plant cells
-contains cell sap (a weak solution of sugars and salts) which gives rigidity

FUNCTION
● Maintains turgor pressure in cell (stopping plant wilting)
● Contains cell sap → stores sugars, amino acids, pigments and any waste chemicals

Question 14

Describe how eukaryotic cells are organised in complex multicellular organisms

Answer 14

In complex multicellular organisms, eukaryotic cells become specialised for specific functions

Tissue: Group of specialised cells with a similar structure working together
to perform a specific function, often with the same origin
Organ: Aggregations of tissues performing specific functions
Organ system: Group of organs working together to perform specific functions

Question 15

prokaryotic cells, organelles and examples

Answer 15

prokaryotic cells are smaller and simpler e.g. bacteria and archaea

-plasmids
-a slime capsule surrounding the cell
-flagella
-cell surface membrane
-cell wall
-cytoplasm
-small ribosomes
-circular DNA

Question 16

What are the distinguishing features of prokaryotic cells?

Answer 16

● Cytoplasm lacking membrane-bound organelles
● So genetic material not enclosed in a nucleus

Question 17

Compare and contrast the structure of eukaryotic and prokaryotic cells

Answer 17

-eukaryotic cells Have membrane-bound organelles eg. mitochondria, endoplasmic reticulum whereas prokaryotic cells have No membrane-bound organelles eg. no mitochondria, endoplasmic reticulum

-eukaryotic cells Have a nucleus Containing DNA whereas prokaryotic cells have No nucleus, DNA is is free in cytoplasm

-eukaryotic cells DNA is long & linear
& associated with histone proteins where as prokaryotic cells DNA is short & circular
& not associated with proteins

-eukaryotic cells have Larger (80S) ribosomes (in cytoplasm) whereas prokaryotic cells have Smaller (70S) ribosomes

-eukaryotic cells have Cell wall only in plants, algae and fungi Containing cellulose or chitin
whereas Cell wall in all prokaryotic cells
Containing murein, a glycoprotein

-eukaryotic cells have Plasmids / capsule never present (sometimes flagella) whereas prokaryotic cells have Plasmids, flagella and a capsule sometimes present

-eukaryotic cells have Larger overall size whereas prokaryotic cells have Much smaller overall size

Question 18

circular/ chromosomal DNA

Answer 18

-free in cytoplasm
-not associated with proteins

Question 19

flagellum

Answer 19

-a long hair like structure that rotates, which is responsible for the movement of cells

Question 20

plasmids

Answer 20

-small circular loops of DNA, which passes on genetic information to other bacteria

Question 21

slime capsule

Answer 21

-the capsule is made up of secreted slime made of protein, which protects the cell from the attack by cells of the immune system (E.g. white blood cells/ phagocytosis)
-protects from pathogens and phagocytosis

Question 22

cell wall

Answer 22

-contains murein, a glycoprotein

Question 23

Explain why viruses are described as acellular and non-living

Answer 23

● Acellular - not made of cells, no cell membrane / cytoplasm / organelles
● Non-living - have no metabolism, cannot independently move / respire / replicate / excrete

Question 24

Describe the general structure of a virus particle
-nucleic acid/ genetic material
-attachment protein
-capsid

Answer 24

1. Nucleic acids surrounded by a capsid
   (protein coat)
2. Attachment proteins allow attachment
   to specific host cells
3. No cytoplasm, ribosomes, cell wall,
   cell-surface membrane etc.
4. Some also surrounded by a lipid
   envelope eg. HIV

Question 25

Describe the difference between magnification and resolution

Answer 25

● Magnification = number of times greater image is than size of the real (actual) object ○ Magnification = size of image / size of real object ● Resolution = minimum distance apart 2 objects can be to be distinguished as separate objects

Question 26

OPTICAL MICROSCOPE

Answer 26

-Light focused using glass lenses -Light passes through specimen, different structures absorb different amounts & wavelengths -Generates a 2D image of a cross-section -Low resolution due to long wavelength of light -Can’t see internal structure of organelles or ribosomes -Specimen = thin -Low magnification (x 1500) -Can view living organisms -Simple preparation -Can show colour

Question 27

TRANSMISSION ELECTRON MICROSCOPE (TEM)

Answer 27

-Electrons focused using electromagnets -Electrons pass through specimen, denser parts absorb more and appear darker -Generates a 2D image of a cross-section -Very high resolution due to short wavelength of electrons -Can see internal structures of organelles and ribosomes -Specimen = very thin -High magnification (x 1,000,000) -Can only view dead / dehydrated specimens as uses a vacuum -Complex preparation so artefacts often present -Does not show colour

Question 28

SCANNING ELECTRON MICROSCOPE (SEM)

Answer 28

-Electrons focused using electromagnets -Electrons deflected / bounce off specimen surface -Generates a 3D image of surface -High resolution due to short wavelength of electrons -Can’t see internal structures -Specimen does not need to be thin -High magnification (x 1,000,000) -Can only view dead / dehydrated specimens as uses a vacuum -Complex preparation so artefacts often present -Does not show colour

Question 29

Suggest how the scientific community distinguished between artefacts (eg. dust, air bubbles occurring during preparation) and cell organelles

Answer 29

● Scientists prepared specimens in different ways ● If an object was seen with one technique but not another, it was more likely to be an artefact than an organelle

Question 30

Describe how the size of an object viewed with an optical microscope can be measured

Answer 30

1. Line up (scale of) eyepiece graticule with (scale of) stage micrometre 2. Calibrate eyepiece graticule - use stage micrometre to calculate size of divisions on eyepiece graticule 3. Take micrometre away and use graticule to measure how many divisions make up the object 4. Calculate size of object by multiplying number of divisions by size of division 5. Recalibrate eyepiece graticule at different magnifications

Question 31

cell fractionation

Answer 31

used to separate and isolate the different organelles, so they can be studied easily under an electron microscope

Question 32

homogenisation

Answer 32

● Disrupts the cell membrane, breaking open cells to release contents / organelles into solution

Question 33

3 characteristics the solution must have

Answer 33

-ice cold: to reduce enzyme activity so organelles are not broken down and damaged -isotonic: so water doesnt move in or out of organelles by osmosis, so they dont burst -buffered: to keep pH constant, so enzymes dont denature

Question 34

2. filtration

Answer 34

● Remove large, unwanted debris through a gauze eg. whole cells, connective tissue

Question 35

3. ultracentrifugation - separates organelles in order of density / mass

Answer 35

● Centrifuge homogenate in a tube at a low speed ● Remove pellet of heaviest organelle and respin supernatant at a higher speed ● Repeat at increasing speeds until separated out, each time the pellet is made of lighter organelles (nuclei → chloroplasts /mitochondria → lysosomes → ER → ribosomes)