Structure of Cells

Question 1

How are phospholipids arranged in a plasma membrane? [3]

Answer 1

1. Bilayer;
2. Hydrophobic / fatty acid / lipid (tails) to inside;
3. Polar / phosphate group / hydrophilic (head) to outside;

Question 2

Describe the structure of a plasma membrane

Answer 2

Phospholipids forming a bilayer/two layers;

The phosphate heads are on the outside of the bilayer the fatty-acid tails are on the ‘inside’ of the bilayer;

The membrane contains intrinsic proteins (which pass right through the
membrane) and extrinsic proteins (which are confined to one layer)

The membrane has channel proteins and carrier proteins

There are cholesterol molecules to regulate the fluidity of the membrane

There are glycoproteins which are involved in cell recognition and signalling

Question 3

Explain how three features of a plasma membrane adapt it for its functions

Answer 3

1) phospholipid bilayer (as a barrier);
2) forms a barrier to water soluble / charged substances / allows non-polar substances to pass OR maintains a different environment on each side / compartmentalisation;

3) the bilayer is fluid;
4) so it can bend to form different shapes during phagocytosis / form vesicles / self repair;

5) intrinsic channel proteins through the bilayer;
6) let water soluble/charged substances through by facilitated diffusion;

7) carrier proteins (through the bilayer);
8) allow facilitated diffusion or active transport;

9) surface proteins / extrinsic proteins, glycoproteins / glycolipids;
10) cell recognition / act as antigens / receptors;

11) cholesterol;
12) regulates fluidity / increases stability;

Question 4

The structure of a cholera bacterium is different from the structure of an epithelial cell from
the small intestine. Describe how the structure of a cholera bacterium is different.

Answer 4

1) Cholera bacterium is prokaryote (and epithelial cell is eukaryotic);

2) Cholera does not have a nucleus/has DNA free in cytoplasm/has loop
of DNA;

3) Cholera has no membrane-bound organelles / no mitochondria / no
golgi / no endoplasmic reticulum / etc;

4) Cholera has small ribosomes only;
5) Cholera has a capsule / flagellum / plasmid / cell wall ;

Question 5

Contrast the structure of a bacterial cell and

the structure of a human cell. [5]

Answer 5

Bacterial cell is much smaller than a human cell;

Bacterial cells have a slime capsule but human cells do not;

Bacterial cell has a cell wall but human cell does not;

Bacterial cell lacks a nucleus but human cell has a nucleus;

Bacterial cell lacks membrane-bound organelles but human cell has membrane-
bound organelles;

Bacterial ribosomes smaller than human ribosomes / bacteria have 70S
ribosomes whereas humans have 80S ribosomes;

Bacterial DNA is circular but human DNA is linear;

Bacterial cells have plasmids but human cells do not;

Bacterial DNA is ‘naked’ whereas human DNA is bound to histones/proteins;