Topic 1: Cell Biology

Question 1

Cell Theory

Answer 1

• Cells are the smallest possible units of life
• All living organisms are made of cells
• Cells consist of cytoplasm, cell membrane.
• In plant cells and animal cells there is a nucleus containing genes.

Question 2

Exception to the cell theory: Skeletal muscle

Answer 2

• Made of muscle fibres
• Like cells, they are enclosed in a membrane, but they are much larger than most cells
• Contain hundreds of nuclei

Question 3

Exception to the cell theory: Giant algae

Answer 3

• Single cell + single nucleus

- Large - can grow to a length of as much as 100mm

Question 4

Exception to the cell theory: Aseptate fungi

Answer 4

• Consist of thread-like structures called hyphae
• Not divided up
• Long undivided sections of hypha which contain many nuclei

PS. - Still a strong overall trend for living organisms to be composed of cells, so the cell theory has not been abandoned.

Question 5

Functions of life in Unicellular organisms

Answer 5

• Unicellular organisms consist of only one cell.
• Carry out all functions of life in that cell.
  EXAMPLES:
  Paramecium
  Chlamydomonas

Question 6

Functions of life (7)

Answer 6

• Nutrition
• Growth
• Sensitivity / Response
• Excretion
• Metabolism
• Homeostasis
• Reproduction

MR SHENG

Question 7

Multicellular organisms

Answer 7

• A larger cell has a smaller surface area to volume ratio.
• The rate at which materials enter or leave depends on the surface area.
• The rate at which materials are used or produced depends on the volume.
• If a cell becomes too large it may not be able to take in essential materials or excrete waste quickly enough.
• Advantage of multicellular- allows division of labour.

Question 8

Emergent properties

Answer 8

Arise from the interaction of the component parts of a complex structure.

Question 9

Differentiation

Answer 9

• An organism’s entire set of genes is its genome.
• During differentiation a cell uses only the genes that it needs to follow its pathway of development.
• Other genes are unused.
• The genes for making haemoglobin are only expressed in developing red blood cells.
• Once a pathway of development has begun in a cell, it is usually fixed and the cell cannot change to a different pathway.

Question 10

Stem cells

Answer 10

Stem cells- undifferentiated cells that have the capacity to divide and differentiate to have a specific function and be committed.
Human embryos consist entirely of stem cells in their early stages, but gradually the cells in the embryo commit themselves to a pattern of differentiation. Once committed, a cell may still divide several more times.
Small nr of cells persist as stem cells and are still present in the adult body- most human tissues such as bone marrow, skin and liver.
They give some human tissues considerable powers of regeneration and repair, though they do not have as great a capacity to differentiate in different ways as embryonic stem cells.
Other tissues lack the stem cells needed for effective repair- brain, kidney and heart.
Therapeutic use of embryonic stem cells- Parkinson’s disease, tissue repair and other degenerative conditions.

Question 11

Ethics of stem cells

Answer 11

• Moral principles
• Advantages: helps to cure issue, embryonic are little more than balls of cells, lack a nervous system, if embryos are produced deliberately, large nr of IVF ones are never implanted.
• Disadvantages: Embryonic- cannot give informed consent.

Question 12

Examples of therapeutic stem cell use

Answer 12

• Stargardt’s macular dystrophy- a genetic disease that develops in children between the ages of 6 and 12. Most cases are due to a recessive mutation of a gene called ABCA4. This causes a membrane protein used for active transport in retina cells to malfunction, so photoreceptive cells degenerate and vision becomes progressively worse. Loss of vision.
• Leukemia- a type of cancer in which abnormally large nr of white blood cells are produced in the bone marrow. A normal white blood cell count is 4000-11000 per mm squared of blood and with leukemia the count rises above 30 000-100 000 per mm squared. A large needle is inserted into a large bone usually the pelvis and fluid is removed from the bone marrow, stem cells are extracted from this fluid, stored, a high dose of chemotherapy drugs is given, the stem cells then returned to the patient’s body.

Question 13

Structure of prokaryotic cells

Answer 13

Cells are divided into two types according to their structure, prokaryotic and eukaryotic. The first cells to evolve were prokaryotic and many organisms still have prokaryotic cells, including all bacteria. Prokaryotic cells have a relatively simple cell structure, are not compartmentalised, do not have a nucleus, mitochondria or any other membrane-bound organelles within their cytoplasm.

Question 14

Surface area to volume ratios

Answer 14

As size increases- S.A to Volume ratio decreases.

Question 15

Binary fission in Prokaryotes

Answer 15

Divide by binary fission- simply means splitting in two. The bacterial chromosome is replicated so there are two identical copies. These are moved to opposite ends of the cell and the wall and plasma membrane are then pulled inwards so the cell pinches apart to form two identical cells. Some prokaryotes can double in volume and divide by binary fission every 30 minutes.

Question 16

Structure of Eukaryotic cells

Answer 16

Cytoplasm
Plasma membrane
Nucleus - double membrane
Mitochondria - double membrane
Chloroplast (in plant cell) - double membrane
Vacuole (in plant cell)
Cell wall (in plant cell)
80S ribosomes
Golgi apparatus
Lysosome
Rough ER
Smooth ER
Vesicles

Question 17

The Davson-Danielli model

Answer 17

A bilayer of phospholipids in the middle with layers of protein on either side. Proposed in 1930s.

1. Chemical analysis provided evidence.
2. Evidence suggested that the plasma membrane of red blood cells has enough phospholipids in it to form an area twice as large as the area of the plasma membrane suggesting a phospholipid bilayer.
3. Experiments showed that membranes form a barrier to the passage of some substances, despite it being very thin, and layers of protein could act as the barrier.

Question 18

The Singer-Nicolson/Fluid mosaic model

Answer 18

In 1950s and 60s evidence arose that did not dit the Davson-Danielli model.

1. Freeze-fracture electron micrographs showed that globular proteins were present in the centre of the phospholipid bilayer.
2. Analysis of membrane proteins showed that parts of their surfaces were hydrophobic so they would be positioned in the bilayer and in some cases would extend from one side to the other.
3. Fusion of cells with membrane proteins tagged with fluorescent markers showed that these proteins can move within the membrane as the colours became mixed.

Question 19

Phospholipids

Answer 19

Basic components of all biological membranes. Are amphipathic- have both hydrophobic and hydrophilic qualities. Phosphate head- hydrophilic, hydrocarbon fatty acid tails- hydrophobic.

Question 20

Cholesterol

Answer 20

A component of animal cell membranes.
Most is hydrophobic but like phospholipids there is one hydrophilic end so it fits between phospholipids in the membrane.
It restricts movement of the phospholipid molecules and therefore reduces the fluidity of the membrane and the permeability of the membrane.

Question 21

Diffusion

Answer 21

Passive movement of particles from an area of a high concentration to an area of low concentration as a result of random motion of particles.
Simple and facilitated diffusion exist.

Question 22

Osmosis

Answer 22

Passive movement of water molecules from an area of higher concentration of water to an area of lower concentration of water across a partially permeable membrane.

Question 23

In osmosis experiments

Answer 23

Volume of water should be measured with a volumetric flask, the initial and final mass should be measured with the same balance accurate to 0.01 grams.

Question 24

Active transport

Answer 24

The movement of substances across membranes using energy from ATP.
Against the concentration gradient.
Protein pumps in the membrane are used - particular to particles and work in a specific direction.

Question 25

Sodium-potassium pumps in Axons

Answer 25

Antiporter- sodium and potassium moved in opposite directions.
ATPase- energy for pumping is from converting ATP to ADP and phosphate.
One ATP provides enough energy to pump two potassium ions in and three sodium ions out of the cell.
The concentration gradients generated are needed for the transmission of electric nerve impulses in axons.

Question 26

Transport using vesicles

Answer 26

Vesicles are used to move materials from one part of the cell to another- for ex. move proteins from the rough ER to the Golgi apparatus.
The fluidity of membranes allows them to move and change shape.
Small pieces of membrane can be pinched off the plasma membrane to create a vesicle containing some material from the outside of the cell- endocytosis. Vesicles also move the plasma membrane and fuse with it, releasing the contents outside the cell- exocytosis.

Question 27

Cell division and cell origins

Answer 27

Until the 19th century, some biologists believed that life could appear in non-living material- “theory of spontaneous generation”. There is no evidence, however.

Question 28

Pasteur’s experiments

Answer 28

Tested the idea of cells coming from pre-existing cells only.

• Louis Pasteur used swan-necked flasks.
• Placed samples of broth in flasks with long necks and melted the glass of the necks and bent it.
• Boiled the broth in some of the flasks to kill any organisms, others as controls.
• Fungi and other organisms soon appeared in the unboiled flasks but not in the boiled ones.
• Then snapped the necks of some of the flasks to leave a short neck- organisms appeared in those flasks.
• Concluded that the swan-necks prevented organisms from the air getting into the flasks and that no organisms appeared spontaneously.

Question 29

The endosymbiotic theory

Answer 29

Symbiosis - two organisms living together.

• A larger cell takes in a smaller cell by endocytosis, so the smaller cell is inside a vesicle in the cytoplasm of the larger cell.
• The smaller cell is kept alive and performs useful functions (like aerobic respiration) for the larger cell.
• EXAMPLE 1: A cell that respired anaerobically took in a bacterium that respired aerobically, supplying both itself and the larger cell with energy in the form of ATP. Useful for the larger cell. The aerobic bacterium evolved into mitochondria and the larger one into heterotrophic eukaryotes.
• EXAMPLE 2: A heterotrophic cell took in a smaller photosynthetic bacterium, which supplied it with compounds. The photosynthetic prokaryote evolved into chloroplasts and the larger cell into photosynthetic eukaryotes.

EVIDENCE: grow and divide like cells, have a naked loop of DNA like prokaryotes, 70S ribosomes like prokaryotes, double membranes.

Question 30

Mitosis

Answer 30

During mitosis, chromosomes become shorter and fatter- condensation. This occurs by a complex process called supercoiling.
Processes in mitosis - PMAT:
- Early prophase- supercoiling. + Late prophase- spindle microtubules extend from each pole to the equator.
- Metaphase- nuclear membrane has broken down and chromosomes move to the equator.
- Anaphase- centromeres have divided and the chromatids have become chromosomes.
- Early telophase- chromosomes reach the poles and nuclear membranes form around them. + Late telophase- cell divides to form two cells with genetically identical nuclei (cytokinesis).

Question 31

Mitotic index

Answer 31

The ratio between the nr of cells in mitosis in a tissue and the total nr of observed cells.

Question 32

Cell cycle in eukaryotes

Answer 32

The cell cycle - sequence of events between one cell division and the next.
Consists of interphase and cell division (mitosis + cytokinesis at the end).

Interphase:

• G1 phase
• S phase - replication of all genetic material in the nucleus.
• G2 phase

Question 33

Cyclins

Answer 33

A group of proteins that ensure that the tasks are performed at the correct time and that the cell only moves on to the next stage of the cycle when it is appropriate.
Cyclins bind to enzymes called cyclin-dependent kinases- kinases become active and attach to phosphate groups to other proteins in the cell.
A threshold concentration is required for progression into the next stage of cell cycle.
Discovery- known as serendipity- happy and unexpected findings. (Sea urchin eggs, a protein increased and decreased in concentration, corresponded with cell cycle phases),

Question 34

Tumour formation

Answer 34

Oncogenesis- formation of tumours.

Starts with mutations in genes involved in the control of cell cycle- oncogenes.
Have to occur in several oncogenes in the same cell.
Small chance, but many cells.
Some chemicals cause mutations.
Cell can undergo repeated uncontrolled cell divisions- produces a mass of cells- Primary tumour.
Primary tumours- often benign.
Others are malignant- cells become detached and get carried to other parts of the body, developing a Secondary tumour.
Secondary tumours- malignant, referred to as cancer.
Metastasis- spreading of cells to form tumours in a different part of the body.
Smoking increases the risk- correlation. Chemicals are carcinogenic.