A2.2 cell structure

Question 1

prokaryotic cells

Answer 1

• small
• unicellular
• simpler than eukaryotes
• no nucleus (instead nucleoid region with naked loop of DNA)

Question 2

classifying bacteria: gram positive vs. gram negative bacteria

Answer 2

bacteria classified by their ability to retain dye known as “crystal violet”

• gram positive bacteria e.g bacillus and staphylococcus: when cell wall is exposed to crystal violet, take on a violet/blue appearance
• gram negative: extra outer membrane so that when exposed to crystal violet, do not appear violet/blue under microscope as they do not retain “crystal violet”

Question 3

structure of prokaryotic cells

Answer 3

• nucleoid region
• pili
• flagella
• cytoplasm
• cell wall
• plasma membrane
• ribosomes (70s)

sometimes has a capsule

Question 4

nucleoid region in prokaryotes

Answer 4

structure: naked loop of DNA, contains single strand of long, continuous thread of DNA, non-compartmentalised

function: cell control and reproduction

Question 5

pili and flagella in prokaryotes

Answer 5

pili structure: hair-like growths on the outside of the cell wall

pili function: main role is conjugation, transfer of plasmid from one bacteria to another, attaches bacterium to other cells

flagella structure: long tail-like structure anchored to cell wall and plasma membrane

flagella function: allows for movement of bacterium

Question 6

cytoplasm in prokaryotes

Answer 6

structure: made of cytosol (water with dissolved substances e.g ions)

function: where all cellular processes take place because there is no compartmentalisation

Question 7

cell wall in prokaryotes

Answer 7

structure: made of peptidoglycan

function: protects and maintains shape of cell and prevents it from rupturing due to changes in water pressure, internal pressure being greater than external pressure

Question 8

plasma membrane in prokaryotes

Answer 8

structure: two layers of lipids creating bilayer, studded with proteins involved with cell recognition, communication and transport

function: surrounds and encloses cell contents, controls interactions between cell contents and exterior. plays a role in binary fission

Question 9

ribosomes in prokaryotes

Answer 9

structure: 70s ribosomes

function: site of protein synthesis (translation)

Question 10

capsule in prokaryotes

Answer 10

structure: some bacteria have a capsule. it is an extra layer of a type of polysaccharide outside the cell wall

function: makes it possible for bacteria to adhere to structures e.g teeth, skin, food. prevents cell from dehydration and attack of immune system cells e.g antibiotics

Question 11

plasmids in prokaryotes

Answer 11

structure: some bacteria contain plasmids. small, circular, DNA molecules. independent to the main bacterial chromosome and can replicate independently.

function: not required by cell under normal conditions, but can help cell adapt to unusual circumstances

Question 12

binary fission

Answer 12

type of asexual reproduction
- DNA is replicated semi-conservatively
- two DNA loops attach to opposite sides of membrane
- membrane elongates and pinches off (cytokinesis) forming two separate cells
- two genetically identical daughter cells are produced

Question 13

structure of eukaryotic cells

Answer 13

• plasma membrane
• cell wall (plants only)
• nucleus
• membrane riboses
• endoplasmic reticulum: smooth and rough
• golgi apparatus
• lysosomes
• centrosomes
• vacuole
• chloroplasts (plants only)
• cytoskeleton
• mitochondria

Question 14

cytoplasm in eukaryotes

Answer 14

compartmentalized, made up of cytosol and contains cytoskeleton which prokaryotes do not have

Question 15

cytoskeleton in eukaryotes

Answer 15

network of small fibres and roots:
- actin filaments
- intermediate filaments
- microtubules

made of protein and functions include:
- maintaining cell of shape
- aiding cellular movement
- anchoring some organelles

Question 15

endoplasmic reticulum in eukaryotes

Answer 15

extensive network of tubules that extends almost everywhere in the cell, main function is the transportation of materials throughout internal region of cell

smooth ER: no ribosomes, many enzymes on external surface: produces membrane phospholipids and cellular lipids, as well as sex hormones e.g testosterone

rough ER: ribosomes on external surface which are involved in protein synthesis so ER engages in protein development and transport

Question 16

membrane riboses in eukaryotes

Answer 16

80s ribosomes (larger than prokaryotic) conduct protein synthesis

Question 17

lysosomes

Answer 17

intracellular digestive centres that come from the golgi apparatus. they are vesicles bounded by a single membrane filled with hydrolytic enzymes that catalyse the breakdown of proteins, nucleic acids, lipids, and carbohydrates. they also breakdown the materials from phagocytosis (a form of endocytosis)

lysosomes have an acidic environment which is necessary for these hydrolytic reactions

Question 18

golgi apparatus

Answer 18

consists of flattened sacs called cisternae which are stacked. Involved in the transportation, modification and distribution of materials synthesised in the cell Two sides: “cis” side which is close to the rER and receives vesicles from the ER which then pass through the cisternae and come out of the “trans” side with modified materials, e.g lysosomes.

Question 19

mitochondria

Answer 19

have their own circular chromosomes of DNA and 70s ribosomes (like prokaryotes). Have double membrane: outer membrane is smooth while the inner membrane is folded into cristae with an area between known as the inner membrane space. the inner membrane contains a semi-fluid substance known as the matrix.

cristae give the mitochondria a large surface area where chemical reactions e.g the production of the usable cellular energy adenosine triphosphate (ATP) occur.

Question 20

nucleus

Answer 20

bordered by a double membrane known as the nuclear envelope to isolate the DNA from other potentially interfering chemical reactions. Not totally isolated, has nuclear pores for communication with cytoplasm. Inside the envelope is the DNA in the form of chromatin

Question 21

chloroplasts

Answer 21

only in algae and plant cells. double membrane, own circular DNA and 70s ribosomes (like prokaryotes and mitochondria). other internal structures include: stroma, fluid (like cytoplasm) with enzymes and chemicals necessary for photosynthesis, grana made up of stacked thylakoids which are flattened membrane stacks with components necessary for the absorption of light.

Question 22

centrosome

Answer 22

in animal cells they consist of a pair centrioles which are responsible for the assembly of microtubules (important for maintaining shape of cell, movement, and cell division). fungi and plants do not have centrioles, instead centrosome-like structures that produce microtubules.

centrosomes located close to nucleus and centrioles sometimes produce basal bodies located near pili and flagella which direct the assembly of microtubules.

Question 23

vacuoles

Answer 23

storage organelles formed from golgi apparatus. small in animal cells but large in plant cells. store food (to provide nutrition for cell), metabolic waste (to remove from cell), and water (to provide rigidity to plant cells).

Question 24

freeze fracture

Answer 24

rapid freezing of a biological specimen using liquified propane at -183 degrees and then fracturing the specimen using a steel blade. the fractured specimen is replicated using carbon or platinum vapour and the replica is observed under the microscope.

produces images of surfaces within cells

Question 25

cryogenic electron microscopy

Answer 25

to observe the function of proteins

solution of proteins poured on a grid and the molecules are flash frozen at -190 degrees. The frozen solution is then exposed to electrons to produce images of individual molecules.

Question 26

fluorescent stains

Answer 26

dyes that combine with specific cellular components in living samples that are then irradiated with UV light, the parts that accepted the dye will fluoresce which allows for more detailed visibility

Question 27

immunofluorescence

Answer 27

allows greater visibility of living tissue, involves antibodies tagged with fluorescent markers which then attach to specific antigens, marking them. allows viral proteins that have infected cells to be detected.

Question 28

light microscope

Answer 28

magnification: x500
resolution: 0.25nm
living specimen
color

Question 29

electron microscope

Answer 29

magnification: x500,000
resolution:0.25μm
dead specimen
no colour

Question 30

resolution

Answer 30

the minimum distance that an
instrument can perceive two points as being distinct

Question 31

unicellular organisms carrying out functions of life

Answer 31

• cell membrane controls movement of materials in and out of cell (homeostasis)
• vacuoles isolate and store waste (excretion)
• cells often have cilia and flagella that allow for movement in response to the environment (movement and response)
• vacuoles carry out digestion (nutrition)
• mitochondria produce energy
• ribosomes for growth and repair

Question 32

fungi

Answer 32

• cell wall chitin
• no chloroplasts
• large central vacuole similar to plants
• store carbs as glycogen
• no centrioles

Question 33

atypical eukaryotes

Answer 33

multinucleate:
aseptate hyphae: no divisions, many nuclei
skeletal muscle fibre: no divisions, many nuclei

no nuclei:
red blood cells: no nuclei to make more room for haemoglobin, biconcave shape
phloem sieve tube: no nuclei for room for movement, companion cell with nucleus

Question 34

origin of eukaryotic cells, theory of endosymbiosis

Answer 34

all eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually. 2 billion years ago this larger cell engulfed a smaller prokaryote that could produce energy which was engulfed and survived inside the vacuole, they developed a symbiotic relationship. smaller engulfed cell went through changes and became a mitochondrion. the smaller prokaryote provided the larger cell with ATP.

Question 35

evidence of endosymbiosis

Answer 35

mitochondria and chloroplasts have:
- double membrane
- their own naked loop of DNA (replicate independently)
- 70s ribosomes

Question 36

cell differentiation

Answer 36

the basis for differentiation is different patterns of gene expression often triggered by changes in the environment. genes code for specific proteins, so cell specialization allows cells to become adapted to perform specific functions which are controlled by specific proteins that depend on the gene expression of the organism

Question 37

evolution of multicellularity

Answer 37

multicellularity has evolved repeatedly, many fungi and eukaryotic algae are multicellular. all plants and animals are multicellular.

the advantages of multicellularity are that it allows a larger body size and cell specialization.

endosymbiosis was one factor in the development of the cell, but another important development was compartmentalization (membrane formation around the cell but also organelles). this resulted in specialized areas within the cell (advantage).