2.1.1 Cell structure

Question 1

Nucleus

Answer 1

-DNA directs the synthesis of all proteins in the cell so controls all metabolic activites.
-DNA contained within a double envelope (nuclear envelope) with nuclear pores to let molecules in and out.
-DNA is too large to leave to the site of protein synthesis so is transcribed into mRNA.
DNA associates with histone proteins to form a compelx called chromatin which coils and condenses into chromosomes.

Question 2

Nucleolus

Answer 2

• Responsible for producing ribosomes and is composed of proteins and RNA.
  -RNA is used to produce ribosomal RNA (rRNA), which, when combined with proteins, forms the ribosomes necessary for protein synthesis.

Question 3

Mitochondria

Answer 3

• Site of the final stages of cellular respiration where the energy stored in organic molecules is made available to the cell by the production of ATP.
• Double membrane: The inner membrane is folded to form cristae, and the fluid interior is the matrix.
  The membrane forming the cristae contains enzymes used in aerobic respiration.
  -Contains a small amount of mitochondrial (mt) DNA to produce their own enzymes and reproduce.

Question 4

Vesicles

Answer 4

• Membranous sacs that have storage and transport roles consisting of a single membrane with fluid inside.

Question 5

Lysosomes

Answer 5

• Specialised form of vesicles that contain hyrdolytic enzymes and responsible for breaking down waste material, including old organelles.
• Play an important role in the immune system by breaking down pathogens ingested by phagocytic cells.
• Play a role in apoptosis or programmed cell death.

Question 6

Centrioles

Answer 6

• Component of the cytoskeleton composed of microtubules.
• Two centrioles form a centrosome which is involved in the assembly and organisation of the spindle fibres used in cell division.
• Used to position the cillia and the flagella.

Question 7

Flagella

Answer 7

-Primarily used to enable a cells mobility.
-9+2 arrangment of microtubules.
-9×3 arrangment of microtubules for the basal body.
-In some cells, they are used as sensory organelle detecting chemical changes in the cell’s environment.

Question 8

Cilia

Answer 8

-Stationary cila are present on the surface of many cells and have important functions in sensory organs such as the nose.
-Mobile cila beat in a rhythmic manner, creating a current and causing fluids or objects adjacent to the cell to move, e.g, lungs or trachea.

Question 9

Rough Endoplasmic reticulum

Answer 9

-Network of membranes enclosing folded membrane bound sacs called cisternae and is connected to the outer membrane of the nucleus
-The RER has ribosomes on its surface and is responsible for the synthesis and transport of proteins.

Question 10

Ribosomes

Answer 10

-Can be free floating in the cytoplasm or attached to the endoplasmic reticulum.
-They are not surrounded by a membrane.
-Constructed of RNA that are made in the nucleolus and are the site of protein synthesis.
-Mitochondria and chloroplasts contain ribosomes as do prokaryotic cells.

Question 11

Golgi apparatus

Answer 11

-Compact structure formed of cisternae and doesn’t contain ribosomes.
-Role of modifying and ‘packaging’ proteins into vesicles, which may be secretory vesicles if the proteins are destined to leave the cell or lysosomes that stay in the cell.

Question 12

Protein Production

Answer 12

1. The length of DNA that codes for a protein is called a gene.
2. The gene is transcribed onto an mRNA molecule, which exits the nucleus through the nuclear pores.
3. The mRNA travels via the transport function of the cytoskeleton and attached itself to a ribosome of the RER, which synthesises its proteins by ordering the correct amino acids and making a polypeptide chain.
4. The polypeptide chain moves into the cisternae of the RER and is packaged into transport vesicles.
5. It travels to the golgi appparatus via the cytoskeleton transport and enters via its cis face.
6. The polypeptide chain is then structurally modified (e.g, adding a sugar molecule) before being packaged into secretory vesicles and leaving via its trans face.
7. The secretory vesicles move towards and fuse with the plasma membrane and secrete their proteins through exocytosis.

Question 13

Plasmodesmata

Answer 13

Provides cytoplasmic connections between 2 plant cells to allow for molecules to move between cells.

Question 14

Cellulose Cell Wall

Answer 14

• Made of cellulose, a complex carbohydrate, and are freely permeable so substances can pass in and out.
  -They give the cell shape as the contents of the cell push against it, making it rigid. It supports both the cell and the plant.
  -Acts as a defence mechanism against pathogens.

Question 15

Vacuole

Answer 15

-Vacuoles are membrane lined sacs in the cytoplasm containing cell sap.
-Plant cells often have a large permanent vacuole to maintain tugor by pushing the cell contents against the cell wall.
-The plant cell vacuole membrane is called the tonoplast and is selectivly permeable, allowing small molecules through.
-If vacuoles appear in animal cells, they are small and transient (not permanent)

Question 16

Chloroplast

Answer 16

-They are responsible for photosynthesis in plant cells and are found in the leaves and stems but not the roots.
-They have a double membrane structure, and the fluid enclosed is called the stroma.
-Also, they have an internal network of membranes that form flatterned sacs called thylakoids, when stacked thylakoids become a granum.
-Granum are joined by membranes called lamellae and contain the chlorophyll pigments.
-Starch produced is stored as starch grains.
-Chloroplasts contain DNA and ribosomes to make proteins.

Question 17

DNA (prokaryotic cells)

Answer 17

-Generally only has 1 circular loop of DNA, a chromosome, which is super 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.
-DNA is naked and not associated with any proteins.

Question 18

Ribosomes (prokaryotic cells)

Answer 18

-Ribosomes in prokaryotic cells are smaller than eukaryotic cells’ ribosomes.
-Their relative size is determined by the rate at which they settle or form a sediment in a solution.
-Eukaryotic ribosomes=80S
-Prokaryotic ribosomes=70S

Question 19

Cell Wall (prokaryotic cells)

Answer 19

-Prokaryotic cells have a cell wall made of peptidogylcan, also known as murein. It is a complex polymer formed from amino acids and sugars.

Question 20

Flagella (prokaryotic cells)

Answer 20

-The flagella of prokaryotic cells are thinner than in eukaryotes and don’t have the 9+2 arrangment.
-Energy to rotate the filament that forms the flagellum is from chemiosmosis, not ATP as in eukaryotic cells.
-The basal body attaches the filament comprising the flagellum to the plasma membrane of a bacterium.
-A molecular motor causes the hook to rotate, giving the filament a whip like movement that propels the cell.

Question 21

Microtubule

Answer 21

Diameter:
-25nm
Made of:
-A tube of tubulin protein subunits.
Function:
-Form eukaryotic cilia and flagella.
-Form tracks for the movement of vesicles
-Form spindle fibres, which cause the movement of chromosomes in cell division.
-Form a scaffold providing structure and mechanical strength to the cell.

Question 22

Microfilament

Answer 22

Diameter:
-7nm
Made of:
-Helical strands of actin subunits.
Function:
-Cause cell movement and changes in cell shape.
-Involved in cytokinesis.
-Lengthen and shorten (treadmilling)

Question 23

Intermediate Filament

Answer 23

Diameter:
-8nm-10nm
Made of:
-Many different types of proteins (e.g. keratin) in a variety of flexible filaments.
Function:
-Form part of the scaffold inside the cell providing mechanical strength.

Question 24

Smooth Endoplamsic Reticulum

Answer 24

-A series of tubular membrane bound sacs.
-Responsible for the synthesis of lipids and carbohydrates.