B3 Cell Structure

Question 1

What is the equation for magnification?

Answer 1

magnification = size of image / actual size of object

Question 2

What is the difference between optical microscopes and electron microscopes

Answer 2

Light microscopes use light rays but electron microscopes use a beam of electrons.

Question 3

What are the similarities and differences between TEMs and SEMs?

Answer 3

Both produce images in black and white compared to light microscopes. The specimen must be dead to observe

SEMs produce a 3D image but TEMs produce images with higher resolution(more detail)

Question 4

What is resolution?

Answer 4

It is the minimum distance apart that close objects can be in order for them to be distinguished as separate items.

Greater resolution means greater clarity - the image produced is more clear and precise

Question 5

What is Cell Fractionation

Answer 5

The process where cells are broken up and different organelles they contain are separated out

Question 6

Why does the cell first have to be placed in a cold, isotonic, buffered
solution?

Answer 6

The coldness - reduces enzyme action so they don’t break down the organelles

buffered solution - maintains a constant pH

isotonic - prevents organelles from bursting or shrinking due to osmosis

Question 7

Describe the process of Cell Fractionation?

Answer 7

A sample of tissue is taken , cut into smaller pieces and kept in a cold , isotonic and buffered solution

The cut-up tissue is further broken up by a homogeniser (blender) to release the organelles from the cells

The resultant fluid, homogenate is then filtered to remove any complete cells and large cell fragments

The filtrate is then spun in an ultracentrifuge at a low speed to remove large cell fragments. This also causes a pellet to form at the bottom with the heavy organelles(nuclei) with the supernatant of lighter organelles suspended above the pellet.

The supernatant is drained off and placed into another tube, which is sun at a higher speed. This will form a new pellet with lighter organelles and a new supernatant. The new supernatant is drained off and placed into another tube, which is sun at an even higher speed. This process is repeated until the organelles present are separated out.

The organelles collected can now be studied under an electron microscope

Question 8

How would you measure the size of an object on a microscope?

Answer 8

You would use a slide called an eyepiece graticule

Question 9

How would you calibrate the eyepiece graticule?

Answer 9

You would use a slide called a stage micrometer. The length of each division on it is usually 10um

Question 10

How do you use a calibrated eyepiece graticule to measure the length of an object/cell?

Answer 10

number of graticule divisions x length of a graticule division = measurement (um)

Question 11

Describe the features of the nucleus

Answer 11

It has a double membrane known as a nuclear envelope, which separates it from the cytoplasm. It has many pores

Nuclear pores are important channels to allow mRNA and ribosomes to travel out of the nucleus and enzymes and signalling molecules to travel into the nucleus

It also has chromatin (material from which chromosomes are made)

It has one or more dark dense regions known as nucleoli or nucleolus which are where ribosomes are made

It also has nucleoplasm which is jelly like material that makes the bulk of the nucleus

Question 12

Describe the rough endoplasmic reticulum

Answer 12

It has ribosomes on its outer surface

It provides a large surface area for the synthesis of proteins. It also provides a pathway for the transport of materials especially proteins throughout the cell

Question 13

Describe the smooth endoplasmic reticulum

Answer 13

It is more tubular in appearance and lacks ribosomes

Its function is to synthesise, store and transport lipids and carbohydrates

Its membrane buds off to form vesicles (packages of proteins and other molecules) for transport to the Golgi Apparatus

Question 14

Describe the Golgi Apparatus

Answer 14

It is similar to the ER with cisternae (flattened sacs of membranes but it is more compact

Its function is to modify lipids and proteins, package them into vesicles such as lysosomes which transport them to specific destinations inside or outside of the cell

Question 15

Describe lysosomes

Answer 15

Lysosomes contain enzymes that hydrolyse dead cells and worn out organelles so that the useful chemicals they are made from can be re-used

Question 16

Describe ribosomes

Answer 16

They are sites of protein synthesis containing one large and one small subunit. 80S ribosomes are found in eukaryotes and 70S (smaller) ribosomes are found in prokaryotes

Question 17

Describe mitochondria

Answer 17

They are surrounded by a double membrane with the inner membrane folded to form cristae.

The inner membrane is coated in enzymes which catalyse the reactions of aerobic respiration to produce ATP

Cristae provide a large surface area for the attachment of enzymes and other proteins involved in respiration

The matrix (fluid) formed by the cristae makes up the remainder of the mitochondrion. It contains proteins, lipids , ribosomes and DNA that allows mitochondria to control some of the production of some of their own proteins. Many enzymes involved in respiration are found in the matrix

Question 18

Describe chloroplasts

Answer 18

They are surrounded by a double membrane. It is filled with a liquid called the stroma

It also has membrane bound compartments called thylakoids containing chlorophyll

These thylakoid membranes stack up to form grana

Grana are joined together by lamellae (thin and flat thylakoid membranes)

They also contain circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Question 19

Describe vacuoles

Answer 19

Permanent vacuoles only exist in plants. Animal cells can have temporary vacuoles but they aren’t common features.

Vacuoles have membranes called tonoplasts which is filled with cell sap - a watery solution of different substances including sugars ,enzymes and pigments

When the vacuole is full of sap, the cell is said to be turgid

Question 20

Describe cell walls in plants

Answer 20

It is made of the polysaccharide cellulose in plants

The cell wall doesn’t completely seal off a cell completely from its neighbours. There are pores within the walls called plasmodesmata which connect two cells together by their cytoplasm , enabling exchange and transport of substances

Question 21

Describe features of a prokaryote

Answer 21

They have simpler structures than eukaryotic cells and lack a nucleus, ER and Golgi. They only contain one cell

They have a mucus-protective layer called a capsule which protects the bacteria from viruses or an immune response from the body. The capsule contains water to prevent the cell from drying out

Some prokaryotes have a long, helical tube called a flagella which rotate to provide locomotion. They are powered by protein motors

Many bacteria also feature pili - hollow protein structures used for bacterial conjugation ( the transfer of genetic material from one bacterium to another)

They also contain small loops of DNA called plasmids which are independent to the bacteria’s main circular DNA. They contain genes that can be passed between bacteria e.g antibiotic resistance

They contain circular (nucleoid) DNA which has the genetic information for the replication of bacterial cells

They have a cell membrane and a cell wall (made from murein - peptidolycan)

Question 22

Describe viruses

Answer 22

They are acellular , non-living particles.

They contain nucleic acids such as DNA and RNA as genetic material but can only multiply inside living host cells. The nucleic acid is enclosed in a protein coat called the capsid

Some viruses like HIV are surrounded by a lipid envelope. If this isn’t present, the capsid has attachment proteins which are essential to allow the virus to identify and attach to a host cell

Question 23

How do viruses replicate?

Answer 23

They replicate by attaching to their host cell with the attachment proteins on their surface. They then inject their nucleic acid into the host cell. The genetic info on the injected viral nucleic acid then provides instructions for the host cell’s metabolic processes to start producing the viral components , nucleic acid, enzymes and structural proteins which are then assembled into new viruses

Question 24

How is the body organised?

Answer 24

A group of cells with the same function work together to form tissues

Different tissues work together to form organs

Different organs work together to form organ systems

Different organ systems work together to form an organism

Question 25

What is the cell cycle

Answer 25

It is a series of events that cells go through as they grow and divide. It has three stages: Interphase(takes up the majority of the cell cycle), Mitosis, Cytokinesis

Question 26

Describe Interphase

Answer 26

During G1(First Gap) phase, the cell increases in size , protein and organelle synthesis takes place

During the S phase, chromosomes are replicated and DNA synthesis happens.

During G2 (Second Gap) phase, organelles and molecules required for cell division are produced

Once G2 phase of Interphase is complete, the cell is ready for the M phase - mitosis

Question 27

Define mitosis

Answer 27

The process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus.

It has 4 stages:
Prophase
Metaphase
Anaphase
Telophase

Question 28

Describe the stages of mitosis

Answer 28

Prophase - The chromosomes shorten and condense. nucleolus and nuclear envelope break down. Centrioles move towards opposite poles of the nucleus. Spindle fibres begin to emerge and extend from the centrioles

Metaphase - Chromosomes line up where they are equidistant to the two centriole poles. The spindle fibres reach and attach to the centromeres of the chromosomes. Each sister chromatid is attached to a spindle fibre originating from opposite poles

Anaphase - the sister chromatids are separated at the centromere (which divides into two). The chromatids are pulled to opposite poles by the spindle fibres. This causes the spindle fibres to shorten

Telophase - Chromosomes arrive at opposite poles and begin to decondense back to chromatin. Nucleoli and Nuclear Envelopes begin to reform around each set of chromosomes. The spindle fibres break down

Question 29

Describe cytokinesis

Answer 29

The cytoplasm divides to form two genetically identical daughter cells

Question 30

Define Binary Fission and describe the process

Answer 30

The process of cell division that takes place in prokaryotic cells

The circular DNA molecule replicates and both copies, each attach to side of the cell membrane. Plasmids also replicate. The cell membrane begins to grow between the two DNA molecules and begins to pinch inwards , dividing the cytoplasm into two. A new cell wall forms between the two molecules of DNA , dividing the original cell into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids.

Even if the daughter cells have different numbers of plasmid copies, they are still genetically identical as they still contain the same genetic code (DNA base sequences) but they just have more or less identical copies of that code