Cell Structure

Question 1

Nucleus, nuclear envelope and nucleolus:
Structure

Answer 1

•Nucleus surrounded by double membrane called the nuclear envelope.
•Nucleolus has no membrane, it contains RNA. Chromatin threads are the genetic material, consisting of DNA wound around histone proteins. When about to divide, chromatin condenses and coils tightly into chromosomes, normally is spread out or extended.

Question 2

Nucleus, nuclear envelope and nucleolus:
Functions

Answer 2

•Nuclear envelope separates contents of nucleus from rest of cell.
In some regions outer and inner membranes fuse together – dissolved substances can pass through.
•Nuclear pores enable larger substances (mRNA/ ribosomes) to leave nucleus and some substances (steroid hormones) to enter from cytoplasm. Ribosomes are made in nucleolus.
•Chromosomes contain organism’s genes.
•Summary: Stores genome, transmits genetic info, provides instructions for protein synthesis.

Question 3

Rough endoplasmic reticulum (RER):
Structure

Answer 3

System of membranes, fluid filled cavities (cisternae) that continue from nuclear membrane.
Studded with ribosomes.

Question 4

Rough endoplasmic reticulum (RER):
Function

Answer 4

•Intracellular transport system: cisternae form channels for transporting substances.
It provides large SA for ribosomes, which assemble amino acids into proteins (protein synthesis). These proteins then actively pass through the membrane into the cisternae and are transported to the Golgi apparatus for further modification and packaging.

Question 5

Smooth endoplasmic reticulum (SER):
Structure

Answer 5

Similar to rough ER but with NO ribosomes

Question 6

Smooth endoplasmic reticulum (SER):
Function

Answer 6

Contains enzymes that catalyse reactions involved with lipid metabolism (e.g. synthesis of cholesterol, synthesis of lipids/phospholipids needed by the cell, synthesis of steroid hormones).
Involved with absorption, synthesis and transport of lipids.

Question 7

Golgi apparatus:
Structure

Answer 7

Stack of membrane-bound fattened sacs called cisternae. Secretory vesicles bring materials to and from.

Question 8

Golgi apparatus:
Function

Answer 8

Proteins are modified by:
Adding sugar molecules - glycoproteins.
• Adding lipid molecules - lipoproteins.
Being folded into their 3D shape.
Proteins are packaged into vesicles that are pinched off then: stored in the cell, or
• moved to pasma membrane to
be incorporated into it or exported out of the cell. Golgi is also responsible for making lysosomes.

Question 9

Mitochondrion:
Structure

Answer 9

Spherical, rod-shaped or branched, 2-5 μm long. Double membrane with fluid-filled cavity in- between. The inner membrane is highly folded into cristae.
The inner part of the mitochondrion is the fluid- filled mitochondrial matrix.
They possess 70S ribosomes – prokaryotic origins.

Question 10

Mitochondrion:
Function

Answer 10

The site of ATP production during aerobic respiration. Self-replicating so can be made if cell’s energy needs to increase.
Abundant in cells where much metabolic activity takes place
(e.g. liver cells, synapses between neurons where neurotransmitters are synthesised and released).

Question 11

Chloroplasts:
Structure

Answer 11

Large organelles, 4-10 μm long.
In plant cells and some protoctists.
Double membrane, inner membrane is continuous with stacks of flattened membrane sacs called thylakoids (plates) which contain chlorophyll. Each stack of thylakoids is called a granum (grana pl). Joined by the intergranal lamallae.
The fluid-filled matrix is called the stroma. Chloroplasts contain loops of DNA and starch grains.They also possess 70S ribosomes.

Question 12

Chloroplasts:
Function

Answer 12

Chloroplasts are the site of photosynthesis.
1st stage of photosynthesis: light energy trapped by chlorophyll and used to make ATP – occurs in grana. Water is also split to make hydrogen ions (light dependent reaction).
2nd stage of photosynthesis: hydrogen reduces CO2 using energy from ATP, to
make carbohydrates – occurs in the stroma (light independent). Chloroplasts are abundant in leaf cells, particularly to
palisade mesophyll layer.

Question 13

Vacuole:
Structure

Answer 13

Surrounded by a membrane called the tonoplast, and it contains fluid (water, waste products & other substances).

Question 14

Vacuole:
Function

Answer 14

Only plant cells have large permanent vacuole. Filled with water and solutes, maintains cell stability by pushing against cell wall when full – turgor.
If plant cells are turgid this helps support the plant (especially non- woody plants).

Question 15

Lysosomes: Structure

Answer 15

Small bags formed from the Golgi apparatus, each surrounded by a single membrane.
Contain powerful hydrolytic (digestive) enzymes. Abundant in phagocytic cells that can ingest and digest invading pathogens.

Question 16

Lysosomes: function

Answer 16

Keep powerful hydrolytic enzymes separate from rest of the cell.
Can engulf old cell organelles and foreign matter (e.g. bacteria), digest them and return digested components to cell for reuse. Can also digest self for self-destruction (autolysis).

Question 17

Cilia and Undulipodia
(flagella in prokaryotes):
Structure

Answer 17

Protrusions from the cell. Each composed of microtubules and are formed from centrioles.
Structured by a ring of 9 pairs of microtubules surrounding 2 central pairs, surrounded by cell surface membrane.
Called ‘9 + 2’ formation.

Question 18

Cilia and Undulipodia
(flagella in prokaryotes):
Function

Answer 18

The epithelial cells lining your airways each have many hundreds of cilia that beat and move the band of mucus.
Most cell types have one cilia that acts as an antenna. It contains receptors that allow it to detect signals about its immediate environment. Only human cell with undulipodium (longer cilium) is spermatozoon, to enable it to move.

Question 19

Organelles without membrane

Answer 19

Ribosomes
Centrioles
Cellulose cell wall
Cytoskeleton

Question 20

Ribosomes:
Structure

Answer 20

small spherical organelles ZUnm diameter).
Made of ribosomal RNA (rRNA) in the nucleolus as two separate subunits that pass through the nuclear envelope into the cytoplasm, then combine, when protein synthesis is about to begin.
Remain free in cytoplasm or attach to RER.

Question 21

Ribosomes
(Function)

Answer 21

Ribosomes bound to exterior of RER are for synthesising proteins to be exported out of the cell. Ones that are free in cell (singly/clusters) are for the assembly of proteins for use in the cell. Eukaryotes have 80S, prokaryotes have 70S generally.

Question 22

Centrioles:
Structure

Answer 22

Centrosomes consist of two bundles of microtubules (2 centrioles) at right angles.
Microtubules are made of tubulin protein subunits and are arranged in nine triplets to form a cylinder
- a centriole.

Question 23

Centrioles:
Function

Answer 23

Before cell division the spindle, made of threads of tubulin, forms from the centrioles.
Chromosomes attach to the middle part of the spindle, and motor proteins walk along the tubulin threads, pulling chromosomes to opposite ends of the cell- in cell division.

Centrioles are involved in the formation of cilia and undulipodia:
• Before cilia form, centrioles multiply and line up beneath the cell surface membran
• Microtubules then sprout outwards from each centriole, forming a cilium/undulipodium.
Centrioles are usually absent from cells of more complicated plants.

Question 24

Cellulose cell wall:
Structure

Answer 24

The cell wall in plant cells is made from bundles of cellulose fibres.
Some have plasmodesmata - channels for exchanging substances with adjacent cells.

Question 25

Cellulose cell wall:
Function

Answer 25

The cell wall is strong and can prevent plant cells from bursting when turgid.
The cell wall of plant cells: provides
strength/support, maintains cell’s shape, contribute to strength and support of whole plant, and are permeable and allow solutions (solutes dissolved in solvents) to pass through.
Fungi’s cell walls are made of chitin.

Question 26

Cytoskeleton
(Structure)

Answer 26

Network of protein structures within cytoplasm, consists of:

•Rod-like microfilaments made of subunits of protein actin.
•Intermediate filaments slightly bigger.
Straight, cylindrical microtubules, made of protein subunits called tubulin.
Larger than filaments.
•The cytoskeletal motor proteins, myosins, kinesins and dyneins, are molecular motors. They are also enzymes and have a site that binds to and allow hydrolysis of ATP as their energy source.

Question 27

Cytoskeleton
(Function)

Answer 27

Microtubules and Microfilaments:
• Provide shape and support/mechanical
strength to cells.
• Help substances and organelles move
through the cytoplasm within the cell.
• Microtubules form the track along with
motor proteins (dynein and kinesin) walk
and ‘drag’ organelles.
• Microtubules form the spindle before cell division – move chromosomes.
• Make up cilia (microfilaments allow movement – movement of cell),
undulipodia and centrioles.

Intermediate filaments are made of a variety of proteins.
They:
• Anchor the nucleus within the cytoplasm.
• Extend between cells in some tissues,
between special junctions, enabling cell-cell signalling and allowing cells to adhere (stick) to a basement membrane, therefore stabilising tissues.

Question 28

Interrelationship between organelles making/secreting proteins:

Answer 28

• The gene that has coded instructions for a protein (e.g. insulin), housed on chromatin in the nucleus, is transcribed into a length of messenger RNA (mRNA).
• Many copies of this mRNA are made, and they pass out of the nuclear pores to the ribosomes.
• At the ribosomes, the instructions are translated, and insulin molecules are assembled.
• The insulin molecules pass into the cisternae of the RER and along these hollow sacs.
• Vesicles with insulin inside are ‘pinched off’ from the RER and pass, via microtubules and motor
proteins, to the Golgi apparatus.
• The vesicles fuse with the Golgi apparatus, where insulin protein molecules may be modified for
release.
• Inside vesicles pinched off from the Golgi apparatus, these molecules then pass to the plasma membrane.
• The vesicles and the plasma membrane fuse, and the insulin is released to the outside of the cell.