2.2. Cell theory, cell structure, endosymbiotic theory

Question 1

What are the three points of cell theory? What is the evidence for each point?

Answer 1

1| Cells are the basic units of life (nothing smaller than the cell can survive independently).
- sub-cellular components have never been seen to perform functions of life whereas full cells have
2| All living things consist of cells.
- biologists examined tissues from all kingdoms, each specimen had at least one cell
3| New cells come from pre-existing cells by the process of cell division.
- we have observed cells coming from other cells but never observed spontaneous generation. Under current conditions on Earth, cells can be made from pre-existing cells only but on pre-biotic Earth, it should have been made spontaneously from non-living matter.

Question 2

What are the discrepancies to the cell theory? Outline each one.

Answer 2

1| Striated (skeletal) muscles – more than 1 nucleus per cell – long cells, around 30 mm
2| Giant algae, Actabularia – single-celled organism but up to 20 cm long
3| Aseptate Funghi – fungal hyphae (narrow thread-like structures) are made up of rows of cells usually separated with septa but some funghi lack this septa -> each hyphae is a continuous multinucleated tube-like structure.

Question 3

List and define the functions of life.

Answer 3

Metabolism - all chemical reactions in a cell (catalyzed)
Response - the ability to react to stimuli from the environment
Nutrition - obtaining food to get energy
Excretion - removal of waste products
Reproduction - production of offspring sexually or asexually
Growth - irreversible increase in size
Homeostasis - regulating internal conditions (keeping them within limits)

Question 4

Contrast anucleate and multinucleate cells (what are their functions)?

Answer 4

Anucleate cells don’t have a nucleus (cannot transcribe DNA to make mRNA and synthesize proteins - red blood cells (erythrocytes)) and multinucleate cells have multiple nuclei (more mRNA and proteins), e.g. skeletal muscle/aseptate fungi.

Question 5

Compare and contrast prokaryotic and eukaryotic cells in terms of structure, size, division, specialization, and complexity.

Answer 5

Common features: Plasma membrane, Cytoplasm, Present DNA, Present ribosomes
Differences: Size (prokaryotic - 0.2-10 microm, eukaryotic - 10-100 microm), Complexity – eukaryotic cells are compartmentalized (many differences arise from this fact like type of cell division, DNA structure…)

Question 6

Describe the process of binary fission.

Answer 6

in ideal conditions happens every 20 minutes – phospholipids get added to the existing plasma membrane, the offspring produced is genetically identical to the parent cell - cloning is a type of asexual reproduction

Question 7

List all organelles in a prokaryotic cell and outline their function.

Answer 7

Cell wall - provides shape, allows the cell to withstand turgor pressure without bursting, principal component of it is peptidoglycan
Plasma membrane - controls transport in/out of the cell
Nucleoid - region where circular, naken prokaryotic DNA is situated, usually paler in micrographs because containing carbohydrates and nucleotides
Mesosomes - infoldings of the plasma membrane, site of CR
Thylakoids - site of photosynthesis
Pilli - adhesion to the surface, the exchange of DNA between cells (sexual reproduction - bacterial conjugation, genetically changing another cell, male and female bacterium, cytoplasmic bridge)
Plasmoid - additional genetic info, promotes variation
Cytoplasm - cytosol, fluid substance, site of all metabolic reactions, contains enzymes that catalyze many metabolic reactions, has a high protein content (dark in EM micrographs)
Flagellum - a thread that moves the cell by propelling it through its aqueous environment

Question 8

Define organelles.

Answer 8

bodies inside eukaryotic cells surrounded by their own membranes, each specialized for a certain function

Question 9

What are the advantages of being compartmentalized?

Answer 9

1| pH, enzyme, and substrate levels can be kept at optimal levels for each specific process happening inside the cell (processes cannot intervene with each other).
2| Free radicals, peroxide, and other strong oxidizing agents cannot intoxicate/damage other parts of the cell (free radicals in the mitochondria).
3| Organelles can be moved around the cell (mitochondria for example).

Question 10

List all organelles in an eukaryotic cell and outline their functions/structure.

Answer 10

Nucleus - largest and heaviest organelle (animal), controls processes inside the cell by controlling the process of gene expression (protein synthesis), surrounded by a nuclear membrane (phospholipid bilayer) with nuclear pores, contains nucleoplasm (= nucleolus + chromatin), nucleolus = densely packed DNA + RNA + proteins, chromatin = 46 uncoiled DNA molecules associated with histone (chromosome formation) - more densely packed at the edge of the nucleus
rER - organelle with a single membrane, made out of cisternae (flattened membrane sacs), bound ribosomes on cisternae, refine proteins by giving them quaternary structure (prosthetics…)
sER - production of carbohydrates and lipids
Golgi apparatus - made our of cisternae shorter and more curved than those in ER, have no ribosomes, final modification/refinement of proteins created in ER - packed into vesicles and secreted (many vesicles next to GA)
Vesicles - organelles with a single membrane with fluid inside, can be formed as a result of budding of GA or after endocytosis, contain proteins (dark in EM picture), lysosomes (lysozyme) and peroxisomes (catalase), non-permanent and small in animal cells
Vacuole - a big vesicle, largest organelle in plant cells, stores water and other juices with dissolved compounds (like nicotine in tobacco plants)
Ribosomes - nor organelles (no membrane), composed of rRNA and proteins (formed in the nucleus), big and small subunit, 70S and 80S type
Plastid - a family of double-membraned organelles, like chloroplast (green) also exist red, orange, yellow… - only in plants cells
Chloroplasts - surrounded by a double membrane, thylakoid membrane containing chlorophyll and other pigments, site of photosynthesis, stroma, outer membrane, inner membrane, granum, thylakoids
Centrioles - elongated cylindrical structures – only in animal cells
Mitochondria - site of CR, surrounded by a double membrane, ribosomes, matrix, inner membrane, outer membrane, intermembrane space, starch grains, cristae, mDNA
Cilia and flagella - project from the cell surface, cilia only in animal cells, cilia have the same function as pilli in prokaryotic cells (attachment), flagella used for locomotion (movement)
Plasma membrane - composed of phospholipids and proteins, controls the transport in and out of the cell (endocytosis and exocytosis)

Question 11

What is the function of lysosomes?

Answer 11

contains lysozyme which is used in the internal digestion of damaged cell particles/organelles - only animal cells have them- APOPTOSIS

Question 12

What is Svedberg?

Answer 12

the unit indicating how fast something sediments when centrifuged

Question 13

Which proteins are transported in and which out of the cell?

Answer 13

Proteins synthesized on free ribosomes stay inside the cell and those synthesized on bound ribosomes get transported outside of the cell.

Question 14

How do cells adapt to their function?

Answer 14

hormone-producing cells have an extensive rER for all that protein production (protein usually seen on the photo of the cells)

Question 15

What are cell walls in different cells made of (plant, fungi, prokaryotes)?

Answer 15

plant - cellulose
fungi - chitin
prokaryotes - peptidoglycan

Question 16

From what kind of a common ancestor did eukaryotes evolve?

Answer 16

an anaerobic, one-celled common ancestor (with a nucleus) that reproduced either asexually by mitosis or sexually by meiosis and fertilization

Question 17

Describe the endosymbiotic theory for both organelles involved. How did both organisms benefit from endosymbiosis?

Answer 17

Mitochondria - endocytosis of aerobic heterotrophic bacteria by the common ancestor
Chloroplasts - endocytosis of photoautotrophic bacteria by the CA
Both organisms benefit from endosymbiosis (mitochondrion): bacteria was giving the CA a supply of ATP produced more efficiently than by anaerobic CR, and the bacteria was provided with food allowing it to grow and divide, being passed on to daughter cells (they eventually became codependent)

Question 18

What is the evidence for endosymbiotic theory?

Answer 18

1| 70S ribosomes
2| Double membrane (vacuole)
3| Circular, naked DNA
4| Divide by binary fission (mitochondria)

Question 19

What is cell differentiation (what is the reason for it – tissue formation)?

Answer 19

• when cells develop along different pathways despite the same genome because of different gene expression due to different chemical signals in the cells’ environment that impact it.
  Tissues - when group of cells in the same environment get influenced by the same chemicals and express the same genes (specialize for the same function).

Question 20

What are the housekeeping genes? What are the other ones? Give an example.

Answer 20

• genes active in all cells that are required for basic life processes
• other genes are expressed in only some cells because they cause the development of specialized structures
• e.g. genes for the synthesis of hemoglobin are only expressed during the development of red blood cells