Chapter 1 - Cells

Question 1

Cell theory

Answer 1

• All organisms are composed of cells (and the products of cells).
• All cells come from pre-existing cells.
• The cell is the smallest living organisational unit.

Question 2

Why are cells so small?

Answer 2

To increase their surface area to volume ratio.

• object gets larger = volume increases more rapidly than surface area.
• cells = small = bigger SA than volume.

Question 3

Distinguish between prokaryotes and eukaryotes

3 points

Answer 3

• prokaryotes lack membrane bound organelles and a nucleus, whereas eukaryotes have membrane bound organelles and a nucleus.
• prokaryotes divide by binary fission, whereas eukaryotes divide by mitosis and meiosis.
• prokaryotes have free floating, single, circular thread of DNA, whereas eukaryotes have linear DNA held in the nucleus.

Question 4

Plasma membrane

1. Structure
2. Describe the fluid-mosaic model

• draw a diagram of a plasma membrane

Answer 4

1. Two layers of phospholipid molecules with associated protein and carbohydrate molecules.
2. Termed mosaic as it has an appearance of a mosaic, and is termed fluid as the membrane can move - it is not rigid.

Question 5

What can cross the membrane?

Answer 5

• small polar molecules: repelled by the interior of the membrane, but not too strongly. So, very small polar molecules can cross, although with difficulty. E.g. water.
• small non-polar molecules: not repelled by the fatty acid interior of the membrane, so able to diffuse readily across the membrane. E.g. carbon dioxide and oxygen gas.
• large non-polar molecules: lipid soluble, able to dissolve in the lipids, so can diffuse freely. E.g. alcohol.

Question 6

What cannot cross the plasma membrane?

Answer 6

• small ions: their electrical charge is repelled from the non-polar interior of the phospholipid bilayer. E.g. sodium ions (Na+)
• large polar molecules: unable to cross by simple diffusion because they are polar and too large - must diffuse by facilitated diffusion. E.g. glucose, amino acids.

Question 7

6 ways of crossing the membrane

Answer 7

1. Simple diffusion: the passive movement of molecules from an area of high concentration to an area of lower concentration until equilibrium is reached.
2. Osmosis: the passive movement of free water molecules through a semi-permeable membrane from an area of high concentration to an area of lower concentration until equilibrium is reached.
3. Facilitated diffusion: the passive movement of molecules via a protein channel or carrier protein from an area of high concentration to an area of lower concentration until equilibrium is reached.
4. Active transport: the active (requires energy) movement of molecules from an area of low concentration to an area of higher concentration via a carrier protein.
5. Endocytosis: a form of bulk transport in which a cell transports molecules into a cell in an energy-using process.
6. Exocytosis: a form of bulk transport in which vesicles containing cellular molecules fuses with the plasma membrane and then material is expelled in an energy-using process.

Question 8

Organelles involved in the exportation of a protein and functions

9 organelles

Answer 8

1. Nucleus: contains the DNA, controls cell functions.
2. Cell membrane: controls what enters and leave the cell.
3. Ribosome: site of protein synthesis, attached to the R.E.R or free-floating.
4. R.E.R: transport and modification of proteins.
5. S.E.R: lipid synthesis.
6. Golgi apparatus: modification and packaging of proteins, a stack of flattened membrane sacs.
7. Secretory vesicle: transports protein to cell membrane, fuses, exists cell by exocytosis.
8. Mitochondria: site of ATP production by aerobic cellular respiration.
9. Cell wall (plant cell only): provides protection for the cell, rigid cellulose layer.

Question 9

Difference between a plant cell and animal cell

• list 4
• draw/label a diagram of each

Answer 9

1. Plant cells have a cell wall, animal cells don’t.
2. Chloroplasts are found in some plant cells, but no animal cells.
3. Plant cells have a large vacuole, animal cells don’t.
4. Animal cells have a centriole, plant cells don’t.