Cells

Question 1

describe the structure and function of a nucleus

Answer 1

-a large organelle surrounded by a nuclear envelope which contains many nuclear pores which allow passage of substances between the nucleus and cytoplasm.
-the nucleus contains chromosomes which are made from protein bound linear DNA
-the nucleus controls the cell’s activities by controlling the transcription of DNA.
- the dense nucleolus made of RNA and proteins works to assemble ribosomes.

Question 2

describe the structure and function of cell wall

Answer 2

• a rigid cellulose structure that surrounds the plants cells
• in fungi it is composed of chitin
• it provides structural support to the plant cells so to keep it rigid

Question 3

describe the structure and function of the vacuole

Answer 3

• they are fluid filled sacs enclosed in a membrane called tonoplast
• filled with cell sap which is a solution of salts and sugars
  -they help to maintain the pressure inside the cell and keep the cell rigid to prevent the cell from wilting
  -it isolated unwanted chemicals in the cell

Question 4

describe the structure and function of the mitochondria

Answer 4

• a mitochondrion is surrounded by a double membrane and the inner membrane is is folded to form cristae
• the matrix contains a mixture of enzymes and proteins with the enzymes present required in the synthesis of ATP molecules during aerobic respiration.

Question 5

describe the structure and function of chloroplasts

Answer 5

-small,flattened structure found within plant and algae cells
-contains a double membrane
-stroma is an aqueous liquid found in these membranes containing enzymes
-contains structures called thylakoids which are stacked up to form granum
-contains circular DNA and ribosomes
-chloroplasts conduct photosynthesis as the chlorophyll pigment captures the energy from sunlight and converts it to chemical energy

Question 6

describe the structure and function of golgi apparatus

Answer 6

-packages and modify proteins
-golgi vesicles are small fluid filled sacs surrounded by a membrane
-they store and transport the proteins and lipids to the cell surface membrane using exocytosis

Question 7

describe the structure and function of lysosomes

Answer 7

-lysosomes are membrane bound vesicles found in the cytoplasm that contain a hydrolytic enzyme called lysozymes
-they are responsible for digesting invading cells and breaking down worn out components

Question 8

describe the function and structure of a ribosome

Answer 8

-small organelles which is formed of protein and rRNA
-they are the site of protein synthesis as they carry out condensation reaction required to join amino acids into polypeptides

Question 9

describe the structure and function of the cell surface membrane

Answer 9

-isolates the cytoplasm from the extracellular environment
- it has a selectively permeable barrier that regulates the movement of substances into and out of the cell
- they have receptor molecules which allow it to play a role in cell signalling and recognition by allowing it to respond to chemicals like hormones

Question 10

describe the structure and function of the rough endoplasmic reticulum.

Answer 10

-a system of membranes enclosing a fluid filled space. the surface is covered with ribosomes
-folds and processes proteins that have been made at the ribosomes.

Question 11

describe the structure and function of the smooth endoplasmic reticulum

Answer 11

-a network of fluid filled membranes
-produced and processes lipids

Question 12

describe the role of plasmids, capsule and the flaggelum in a prokaryotic cell

Answer 12

• plasmids are small loops of DNA that carries non essential genes such as genes for antibiotic resistance. these genes can be exchanged between prokaryotes
• capsule is a mucilaginous layer which prevents desiccation, acts a food reserve, and provides mechanical protection against phagocytosis and external chemicals
• flagellum is a rotating tail that propels the organism

Question 13

outline the process of cell fractionation and ultracentrifugation

Answer 13

• the cells are blended in an homogeniser in order to break up the plasma membrane and release organelles into solution
  -filter homogenate to remove large debris
• the homogenate, is placed in a cold, buffered, isotonic solution.
  -the tube of homogenate is then placed in a centrifuge and spun at slow speed
• the heaviest organelles form a pellet and the fluid at the top, called the supernatant, is filtered off and transferred to another tube
  -the supernatant is spun on a slightly higher speed, this time the pellet formed will contain the next heaviest organelle

Question 14

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

Answer 14

• cold to reduce the activity of the enzymes that break down organelles
• buffered solution added to maintain a constant pH so proteins do not denature
• isotonic to prevent damage to the organelles through osmosis

Question 15

describe an optical light microscope

Answer 15

-lenses focus rays of light and magnify the view of a thin slice of specimen
-they have a lower resolution than electron microscope as they have a maximum resolution of 0.2 micrometres
-they have a maximum magnification of about x1500
-they show a colour image
-they can be used to observe living structure
-it is affordable apparatus

Question 16

describe a transmission electron microscope

Answer 16

-they use electromagnets to focus a beam of electrons which is then transmitted through the specimen
-denser parts of the specimen will absorb more electrons which makes them look darker on the image
-they have a higher resolution so the ultra structure is visible
-they have a higher magnification of x500000
-only shows a 2D image
-requires a vacuum so cannot be used to observe living structure
-extensive preparation may introduce artefacts
-no colour image

Question 17

describe a scanning electron microscope

Answer 17

-they scan a beam of electrons across the specimen surface using electromagnetic lenses. reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate
-they produce a 3D image
-give lower resolution images than TEMs
-requires a vacuum so cannot be used to observe living structures
-no colour image

Question 18

what two main stains are used during microscopy?

Answer 18

-eosin is used to highlight the cytoplasm
-iodine contained in potassium iodine solution hihglights starch grains

Question 19

explain why electron microscopes show more detail than a optical microscope

Answer 19

-electron microscope has a greater resolving power
-electron beams have a shorter wavelength

Question 20

describe how to prepare a temporary mount of a root tip

Answer 20

-cut 1 cm from the tip of growing root as this is where growth occurs and therefore mitosis occurs
-place root in hydrochloric acid in order to halt cell division and hydrolyse the middle lamella
-stain the root tip with dye that binds to the chromosomes
-macerate tissues in water using mounted needle in order to break the tip open and spread the cells out thinly
-use mounted needle at 45 degree angle to press down the coverslip and obtain a single layer of cells. push hard to spread the tissue so its thin enough to allow light through but not smeared sideways as this will damage chromosomes

Question 21

describe what happens in interphase

Answer 21

-in G1 the cell synthesises proteins for replication and the cell grows in size
-in S the DNA replicated
-in G2 the organelles divide

Question 22

describe what happens in prophase

Answer 22

-chromosomes condense, getting shorter and fatter which makes them more visible
-the centrioles move to the poles of the cell and mitotic spindle fibres form
-the nuclear envelope breaks down so the chromosomes are free in the cytoplasm

Question 23

describe what happens in metaphase

Answer 23

-the chromosomes, each with two chromatid, line up in the cell equator
-they attach to the mitotic spindle fibres by their centromeres

Question 24

describe what happens in anaphase

Answer 24

-the spindle fibres contract causing the centromere to divide
-sister chromatids are sperated from each other and chromatids are pulled to opposite poles of the cell, making them appear V shaped

Question 25

describe what happens in telophase

Answer 25

chromatids reach opposite poles and become indistinct as they are longer and thinner
-the nuclear envelope reforms around each set of chromosomes, so their are now two nuclei
-the cytoplasm divides during cytokinesis and forms two daughter cells that are genetically identical

Question 26

how do you calculate the mitotic index?

Answer 26

mitotic index = number of cells with visible chromosomes/total number of cells

Question 27

how do prokaryotic cells divide?

Answer 27

-process called binary fission
-DNA loop replicates so both copies stay attached to the cell membrane
-plasmids will replicate in the cytoplasm
-cell elongates which separates the two DNA loops
-the cell membrane contracts and forms a septum
-cell splits into two identical cells each with 1 copy of the DNA loop but with variable plasmids

Question 28

differences between prokaryotes and eukaryotes?

Answer 28

-prokaryotes have a cytoplasm that lacks membrane-bound organelles
-prokaryotes have smaller ribosomes
-prokaryotes do not have a nucleus but have a singular, circular DNA molecule which is not associated with proteins
-prokaryotes have a cell wall composed of murein

Question 29

why is mitosis important in the life of an organism

Answer 29

-causes growth of cells
-replaces cells and repairs tissue
-produces genetically identical cells
-allows for asexual reproduction