M2 Basic Components of Living Organisms

Question 1

Describe the difference between prokaryotic and eukaryotic cells

Answer 1

Prokaryotic cells are single-called organisms with no true nucleus, genetic material is ‘naked’ DNA, average diameter of cell is 0.5 - 5 μm, contains 70S ribosomes.
Eukaryotic cells are multicellular organisms, genetic material is in a membrane bound nucleus and has DNA as chromosomes, average diameter of the cell is 10 - 100 μm, and has 80S ribosomes.

Question 2

What is metabolism?

Answer 2

Chemical reactions are the fundamental processes of life and in cells they require enzymes and specific reaction conditions.
Metabolism involves the synthesis and breaking down of molecules.
Reactions take place in the cytoplasm, which is separated from the external environment by a cell-surface membrane. Membranes are selectively permeable and control the movement of substances into and out of the cell and organelles - membranes are barriers.

Question 3

Describe the nucleus (eukaryotic)

Answer 3

• The nucleus contains coded genetic information in the form of DNA molecules. DNA directs the synthesis of all proteins required by the cell.
• Therefore DNA controls the metabolic activities of the cell, as many of the proteins are necessary for metabolism to occur.
• DNA is contained within a double membrane called a nuclear envelope to protect it from damage.
• The nuclear envelope contains nuclear pores that allow molecules to move into and out of the nucleus.
• DNA is too large to leave the nucleus to the site of protein synthesis in the cytoplasm so it is transcribed into smaller RNA molecules which are exported via the nuclear pores.
• DNA associated with proteins called histones to form a complex called chromatin. Chromatin coils and condensed to form chromosomes which become visible when cells are preparing to divide.

Question 4

Describe the nucleolus (eukaryotic)

Answer 4

• The nucleolus is an area within the nucleus and is responsible for producing ribosomes.
• It is composed of proteins and RNA. RNA is used to produce ribosomal RNA (rRNA) which is then combined with proteins to form ribosomes necessary for protein synthesis.

Question 5

Describe mitochondria (eukaryotic)

Answer 5

• Mitochondria are the site of the final stages of cellular respiration, where the energy stored in the bonds of complex, organic molecules is made available for the cell to use by the production of ATP.
• Mitochondria have a double membrane. The inner membrane is highly folded to form structures called cristae and the fluid interior is called the matrix. The membrane forming cristae contains enzymes used in aerobic respiration.
• Mitochondria contain a small amount of DNA, called mitochondrial (mt)DNA. Mitochondria can produce their own enzymes and reproduce themselves.

Question 6

Describe vesicles (eukaryotic)

Answer 6

Vesicles are membranous sacs that have storage and transport roles. They consist of a single membrane with fluid inside. Vesicles are used to transport materials inside the cell.

Question 7

Describe lysosomes (eukaryotic)

Answer 7

• Lysosomes are specialised vesicles containing hydrolytic enzymes.
• They are responsible for breaking down waste material in cells, including old organelles. They play an important role in the immune system as they are responsible for breaking down pathogens ingested by phagocytic cells.
• They play an important role in programmed cell death.

Question 8

Describe the cytoskeleton (eukaryotic)

Answer 8

• The cytoskeleton is a network of fibres necessary for the shape and stability of a cell.
• Organelles are held in place by the cytoskeleton and it controls cell movement and the movement of organelles within cells.
• The cytoskeleton has three components: microfilaments, microtubules and intermediate fibres.

Question 9

Describe microfilaments (eukaryotic cytoskeleton)

Answer 9

• Microfilaments are contractile fibres formed from the protein actin.
• Actin works with another protein called myosin to produce muscle movement.
• They are responsible for a cell contraction during cytokinesis, the process in which a single eukaryotic cell is divided into two daughter cells.

Question 10

Describe microtubules (eukaryotic cytoskeleton)

Answer 10

• Microtubules are globular tubulin proteins that polymerise to form a scaffold-like structure that determines the shape of a cell.
• They act as tracks for the movement of organelles, including vesicles, around the cell, they are then moved by motor proteins.
• Spindle fibres, which have a role in the physical segregation of chromosomes in cell division, are composed of microtubules.

Question 11

Describe intermediate filaments (eukaryotic cytoskeleton)

Answer 11

• Intermediate filaments consistir of entwined dimers of various proteins.
• These fibres give mechanical strength to cells and help maintain their integrity of the cytoskeleton.

Question 12

Describe centrioles (eukaryotic)

Answer 12

• Centrioles are composed of microtubules.
• Two associated centrioles form the centrosome, which is involved in the assembly and organisation of the spindle fibres during cell division.

Question 13

Describe flagella (eukaryotic)

Answer 13

• Flagella is a whip-like extension that protrudes from some cell types. Flagella are longer than cilia.
• Flagella are used primarily to enable cells motility.
• In some cells they are used as a sensory organelle detecting chemical changes in the cell’s environment.

Question 14

Describe cilia (eukaryotic)

Answer 14

• Cilia are hair-like extensions that protrude from some cell types, cilia are shorter than flagella but are usually present in greater numbers.
• Cili can be stationary or mobile.
• Stationary cilia are present on the surface of many cells and have an important function in sensory organs, eg. in the trachea they move mucus away from the lungs
• Each cilium contains two central microtubules surrounded by nine pairs of microtubules arranged like a ‘wheel’, which is known as the 9 + 2 arrangement. Pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion.

Question 15

Describe the endoplasmic reticulum (eukaryotic)

Answer 15

The endoplasmic reticulum is a network of membranes inclosing flattened sacs called cisternae. It is connected to the outer membrane of the nucleus.

Question 16

Describe the smooth endoplasmic reticulum (eukaryotic)

Answer 16

Associated with the production and metabolism of fats and steroid hormones
Has a role in detoxifying chemicals

Question 17

Describe the rough endoplasmic reticulum (eukaryotic)

Answer 17

Studded with ribosomes and involved in protein production and folding

Question 18

Describe ribosomes (eukaryotic)

Answer 18

• Ribosomes can be free floating in the cytoplasm or attached to the ER, forming the rough ER
• They are not surrounded by a membrane
• They are constructed of RNA molecules made in the nucleolus of the cell
• Ribosomes are the site of protein synthesis
• Mitochondria and chloroplasts also contain ribosomes, as do prokaryotic cells

Question 19

Describe the Golgi apparatus (eukaryotic)

Answer 19

• The Golgi apparatus has a flattened disc-like structure, formed of cisternae that does not contain ribosomes.
• Located near the nucleus
• It has a role in modifying proteins (from rough ER) and modifies and packages them into vesicles for the transport of products.
• It has the same role for lipids (from smooth ER)

Question 20

Describe protein production

Answer 20

1. Proteins are synthesised on the ribosomes bound to the endoplasmic reticulum
2. They then pass into its cisternae and are packaged into transport vesicles
3. Vesicles containing the newly synthesised proteins move towards the Golgi apparatus via the transport function of the cytoskeleton
4. The vesicles fuse with the cis face of the Golgi apparatus and the proteins enter. The proteins are structurally modified before leaving the Golgi apparatus in vesicles from its trans face.
5. Secretory vesicles carry proteins that are to be released from the cell. The vesicles move towards and fuse with the cell surface membrane, releasing their contents by exocytosis. Some vesicles form lysosomes - these contain enzymes for use in the cell

Question 21

Describe the cell wall (plant cells)

Answer 21

• Plant cells are rigid structures, they have a cell wall surrounding the cell-surface membrane
• Plant cells are made of cellulose, a complex carbohydrate. They are freely permeable so substances can pass into and out of the cell through the cellulose wall.
• The cell walls of a plant cell gives it shape. The contents of the cell press against the cell and the plant as a whole.
• The cell wall also acts as a defence mechanism, all plant cells have a cellulose cell wall.

Question 22

Describe vacuoles (plant cell)

Answer 22

• Vacuoles are membrane lined sacs in the cytoplasm containing cell sap.
• Many plant cells have large permanent vacuoles which are very important in the maintenance of turgor, so that the contents of the f the cell push against the cell wall and maintain a rigid framework for the cell.
• The membrane of a vacuole is called the tonoplast. It is selectively permeable, meaning some small molecules can pass through it but others cannot.
• If vacuoles appear in animal cells, they are small and not permanent.

Question 23

Describe chloroplasts (plant cell)

Answer 23

• Chloroplasts are the organelles responsible for photosynthesis. They are found in the cells of green parts of a plant.
• They have a double membrane structure. The fluid enclosed in the chloroplast is called the stroma.
• Chloroplasts have an internal network of membranes, which form flattened sacs called thylakoids. Several thylakoids stacked together are called a granum. The grana are joined by membranes called lamellae. The grana contain the chlorophyll pigments, where light-dependant reactions occur during photosynthesis.
• Starch produced by photosynthesis is present as starch grains. Chloroplasts also contain DNA and ribosomes, there for are able to make their own proteins.

Question 24

Describe DNA (prokaryotic)

Answer 24

• Prokaryotes generally only have one molecule of DNA (a chromosome) which is coiled to make it more compact.
• The genes on the chromosome are often grouped into operons, meaning a number of genes are switched on or off at the same time.

Question 25

Describe ribosomes (prokaryotic)

Answer 25

• Ribosomes are smaller in prokaryotic cells (70S) their relative size is determined by the rate at which they settle.
• Both 70S ribosomes and 80S ribosomes are necessary for protein synthesis, but larger 80S ribosomes are involved in the formation of more complex proteins.

Question 26

Describe the cell wall (prokaryotic)

Answer 26

• Prokaryotic cells have a cell wall made from peptidoglycan (a complex polymer formed from amino acids and sugars).

Question 27

Describe the flagella (prokaryotic)

Answer 27

• The flagella of prokaryotes is thinner than in eukaryotes and do not have the 9 + 2 arrangement.
• The energy to rotate the filament that forms the flagellum is supplied from the process of chemiosmosis, not ATP as in eukaryotic cells.
• The flagellum is attached to the cell membrane of a bacterium by a basal body and rotated by a molecular motor.
• The basal body attaches the filament comprising the flagellum to the cell-surface membrane of a bacterium.
• A molecular motor causes the hook to rotate giving the filament a whip like movement, which propels the cell.

Question 28

Describe DNA (eukaryotic)

Answer 28

• In eukaryotic cells their DNA is present within a nucleus and exists as multiple chromosomes which are super coiled, each one wraps around a number of proteins called histones, forming a complex for efficient packaging.
• This complex is called chromatin and chromatin coils and condenses to form chromosomes.
• Eukaryotic genes are generally switched on and off individually.

Question 29

Similarities and differences in prokaryotic cells and eukaryotic cells

Answer 29

Nucleus: not present in prokaryotes, present in eukaryotes
DNA: circular in prokaryotes, linear in eukaryotes
DNA organisation: proteins fold and condense DNA in prokaryotes, DNA is associated with proteins called histones in eukaryotes.
Extra chromosomal DNA: circular DNA called plasmids in prokaryotes, only present in chloroplasts and mitochondria in eukaryotes
Organelles: non membrane-bound in prokaryotes, membrane bound and non-membrane bound in eukaryotes
Cell wall: made of peptidoglycan in prokaryotes, made of chitin in fungi, cellulose in plants and not present in animals
Ribosomes: smaller 70S in prokaryotes, larger 80S in eukaryotes
Cytoskeleton: present in both, more complex in eukaryotes
Reproduction: binary fission in prokaryotes, asexual or sexual in eukaryotes
Cell type: unicellular in prokaryotes, multi and unicellular in eukaryotes
Cell-surface membrane: present in both

Question 30

How does a light microscope work

Answer 30

• A compound light microscope has two lenses, the objective lens, which is placed near the specimen, and an eyepiece lens, through which the specimen is viewed.
• The objective lens produces a magnified image, which is magnified again by the eyepiece lens.
• The two lenses allows for much higher magnification and reduced chromatic aberration.

Question 31

Calculation for magnification

Answer 31

Image size ÷ actual size

Question 32

Define magnification

Answer 32

How many times bigger the image produced by the microscope is compared to the actual size of the object.

Question 33

Define resolution

Answer 33

The ability to see two objects such a close together as separate objects

Question 34

Magnification and resolution of a light microscope

Answer 34

Magnification: 2000x
Resolution: 200nm

Question 35

Magnification and resolution of a laser scanning microscope

Answer 35

Magnification: 2000x
Resolution: difficult to determine

Question 36

Magnification and resolution of a transmission electron microscope (TEM)

Answer 36

Magnification: 500,000x
Resolution: 0.2nm

Question 37

Magnification and resolution of a scanning electron microscope (SEM)

Answer 37

Magnification: 100,000x
Resolution: 0.2nm

Question 38

Describe a compound light microscope

Answer 38

• Uses visible light to illuminate a thin sample
• Low resolution compared to electron microscopes
• Useful for looking at living things, cells and tissues, and getting an overview of a sample

Question 39

Describe a confocal laser scanning microscope

Answer 39

• Look at specific parts of a cell by labelling them with proteins
• Low resolution compared to electron microscopes, can only see fluorescent objects, and fluorescence can cause artefacts
• Useful for looking at living cells, understanding the relationship between cells and highlighting individual components of cells

Question 40

Describe a scanning electron microscope (SEM)

Answer 40

• Look at the surface of objects at high resolution
• Resolution is often not as high as transmission electron microscopes, cannot be used to look at living things and are expensive
• Useful for looking at surfaces of objects in 3d

Question 41

Describe a transmission electron microscope (TEM)

Answer 41

• Look at a very thin cross-section of a leaf
• Can’t be used to look at living things, expensive
• Useful for looking at the internal structure of objects, objects in high resolution and the relationships between structures in high resolution

Question 42

Define an artefact

Answer 42

Damage caused by a preparation technique and can be easily confused with the sample’s microstructure
Artefacts can be due to mechanical, chemical, ionic or physical attraction

Question 43

Why would you stain a cell?

Answer 43

• Makes cells become viable/easier to see by providing contrast between structures
• Differential staining can distinguish between two types of organisms, or can help you identify certain parts of a cell

Question 44

What length is one eyepiece division equal to at different magnifications

Answer 44

X40 = 2.5 μm
X100 = 1 μm
X400 = 0.25 μm