Cell Structures

Question 1

What is the structure of the Nucleus, Nuclear Envelope and Nucleolus?

Answer 1

The structure is surrounded by a double membrane, called the nuclear enevelope. There are pores in the nuclear envelope.
The nucleoulus does not have a membrane around it. It contains RNA.
Chromatin is the genetic material, consisting of DNA wound around histobe proteins. When the cell is not dividing, chromatin is spread out or extended. When the cell is baout to divide, chromatin condensed and coils tightly into chromosomes. These make up all the organisms genome.

Question 2

What is the function of the Nucleus, Nuclear Envelope and Nucleolus?

Answer 2

The nuclear envelope separated the contents of the nucleus from the rest of the cell.
In some regions the outer and inner nuclear membranes fuse togther. At these points some dissolved substances and ribosomes can pass through.
The pores enable larger substances, such as mRNA to leave the nucleus. Substances such as some steroid hornines, may enter the nucleus, from the cytoplasm, via these pores.

Question 3

What are the main functions of the Nucleus?

Answer 3

Is the control centre of the cell.
Stores the organism’s genome.
Transmits genetic information.
Provided the instructions for Protein Synthesis.

Question 4

What is the Strucutre of the RER?

Answer 4

This is a system of membranes, containing fluid-filled cavities (cisternae) that are continuous woth the nuclear membrane.
It is coated with ribosomes.

Question 5

What it the function of the RER?

Answer 5

RER is the intracellular transport system: the cisternae form channels for transporting substances from one area of a cell to another.
It porvided a large surface area for ribosomes, which assemble amino acids into proteins. These proteins then actively pass through the Golgi Apparatus for modification and packaging.

Question 6

What is the strucutre of the SER?

Answer 6

This is a system of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane.
There are no ribosomes on its surface.

Question 7

What is the function of the SER?

Answer 7

SER contains enzymes that catalyse reactions involved with lipid metabolism such as:
Synthesis of Cholesterol
Synthesis of lipids/phosphlipids needed by the cell
Synthesis of steroid hormones
It is involved with absorption, synthesis and transport of lipids from the guy.

Question 8

What is the function of the Golgi Apparatus?

Answer 8

This consists of a stack of membrane-bound flattened sacs.

Secretory vesicles bring materials to and from the Golgi Apparatus.

Question 9

What is the function of the Golgi Apparatus?

Answer 9

Proteins are modified for example by:
Adding sugar molecules to make glycoproteins.
Adding lipid molecules to make lipoproteins
Being folded into their 3D shape.
Then the proteins are packaged into vesicles that are pinched off and then.
Stored in the cell or moved to the plasma membrane, either to be incorporated into the plasma membrane, or exported outisde the cell.

Question 10

What is the structure of Mitochondria?

Answer 10

These may be spherical, rod-shaped or branched, and are 2-5 um long.
They are surrounded by two membranes with a fluid-filled soace between them. The inner membrane is highly folded into cristae.
The inner part og the mitochondrion is a fluid-filled matrix.

Question 11

What is the function of the Mitochondria?

Answer 11

Mitochondria are the site of ATP production during aerobic respiration.
They are self-replicating, so more can be made if the cells energy needs to increase.
They are abundant in cells where much metabolic activity takes place, for example in liver cells and at synpses between neutones where neurotransmitters is synthesised and released.

Question 12

What is the structure of the Chloroplasts?

Answer 12

These are large organelle, 4-10um long.
They are found only in plant cells and is some protocists
They are surrounded by a double membrane or envelope. The inner membrane is continuous with wtacks of flattened membrane sacs called thylakoids which contain chlorophyll. Each pile of thylakoids is called a granum. The fluid-filled matrix is called the stroma.
Chloroplasts contain loops of DNA and starch grains.

Question 13

What is the function of Chloroplasts?

Answer 13

Chloroplasts are the site of photosynthesis.
The first stage of photosynthesis, when light energy is trapped by chlorophyll and used to make ATP, occurs in the grana. Water is also split to supply hydrogen ions.
The second stage when hydrogen reduces carbon dioxide, using energy from ATP, to make carbohydrates, occurs in the stroma.
Chloroplasts are abundant is leaf cells, particularly the palisade mesophyll layer.

Question 14

What the structure of the Vacuole?

Answer 14

The vacuole is surrounded by a membrane called the tonoplasts, and contains a fluid.

Question 15

What is the function of the Vacuole?

Answer 15

Only plant cells gave a large permanent vacuole.
It is filled with water and solutes and maintains cell stability, because when it pushed against the cell wall, making the cell turgid.
If all the plant cells are turgid then this helps to support the plant, especially in non-woody plants.

Question 16

What is the strucutre of Lysosomes?

Answer 16

These are small bags, formed from the Golgi Apparatus. Each is surrounded by a single membrane.
They contain powerful hydrolytic enzymes.
They are abundany in phagocytic cells such as neutrophils and macrophages that can ingestt and digest inavding pathogens such as bacteria.

Question 17

What is the function of Lysosomes?

Answer 17

Lsysosomes keep the powerful hydrolytic enzymes separate from the rest of the cell.
Lysosomes can engulf old cell organelles and foreign matter, digest them and return the digested components to the cell for reuse.

Question 18

What is the structure of Cilia and Undulipodia?

Answer 18

These are protrusions from the cell and are surrounded by the cell surface membrane.
Each contains microtubules.
They are formed from centrioles.

Question 19

What is the function of Cilia and Undulipodia?

Answer 19

The epithelial cells lining the airways each have hundreds of cilia that beat and move the band of mucus.
Nearly all cell types in the body have one cilium that acts as an antenna. it contains receptors and allows the cell to detect signals about its immediate environment.
The only type of human cell to have an undulipodium is a spermatozoon. The undulipodium enables the spermatozoon to move.

Question 20

What is the structure of Ribosomes?

Answer 20

Small spherical organelles, about 20nm in diameter.
Made of ribosomal RNA.
Made in the nucleolus, as two separate subunits, which pass through the nuclear envelope into the cell cytoplasm and then combine.
Some remain free in the cytoplasm and some attach to the endoplasmic reticulum.

Question 21

What is the function of the Ribosomes?

Answer 21

Ribosomes bound to the exterior of RER are mainly for synthesizing proteins that will be exported outside the cell.
Ribosomes that are free in the cytoplasm, either singly or in clusters, are primarily the site of assembly of proteins that will be used inside the cell.

Question 22

What is the structure of Centrioles?

Answer 22

The centriole consists of two bundles of microtubules at right angles to each other. The Microtubules are made of tubulin protein subunits, and are arranged to form a cylinder.

Question 23

What is the function of Centrioles?

Answer 23

Before the cell divides, the spindle, made of threads of tubulin, forms the centrioles.
Chromosomes attach to the middle part of the spindle and motor proteins walk along the tubulin threads, pulling the chromosomes to opposite ends of the cell.
Centrioles are involved in the formation of cilia and undulipodia:
Before the cilia form, the centrioles multiply and line up beneath the cell surface membrane.
Microtubules then sprout outwards from each centriole, forming a cilium or undulipodium.
Centrioles are usually absent from cells of plants but may be present in some unicellular green algae, such as Chlamydomonas.

Question 24

What is the structure of the Cytoskeleton?

Answer 24

A network of protein structures within the cytoplasm.
It consists of:
Rod-like microfilaments made of subunits of the protein actin; they are polymers of actin and each microfilament is about 7nm in diameter.
Intermediate filaments are about 10nm in diameter.
Straight, cylindrical microtubules, made of protein subunits called tubulin; about 18-30nm in diameter.
The cytoskeleton motor proteins, myosins, kinesins and dyneins, are molecular motors. They are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy source.

Question 25

What is the function of the Cytoskeleton?

Answer 25

The protein microfilaments within the cytoplasm give support and mechanical strength, keep the cell’s shape stable and allow cell movement.
Microtubules also provide shape and support to cells, and help substances and organelles to move through the cytoplasm within a cell. They form the track along which motor proteins (dynein and kinesin) walk and drag organelles from one part of the cell to another. They form the spindle before a cell divides. These spindle threads enable chromosomes to be moved within the cell. Microtubules also make up the cilia, undulipodia and centrioles.
Intermediate filaments are made of a variety of proteins. They anchor the nucleus within the cytoplasm and extend between the cells in some tissues, between special junctions, enabling cell-cell signalling and allowing cells to adhere to a basement membrane, therefore stabilizing tissues.

Question 26

What is the structure of the Cellulose Cell Wall?

Answer 26

The cell wall of plants is on the outside of the plasma membrane. It is made from bundles of cellulose fibres.

Question 27

What is the function of the Cellulose Cell Wall?

Answer 27

Absent from animal cells, the cell wall is strong and can prevent plant cells from bursting when turgid (swollen).
The cell walls of plant cells:
- provide strength and support
- maintain the cell’s shape
- contribute to the strength and support of the whole plant
- are permeable and allow solutions (solute and solvent) to pass through
Fungi have cell walls that contain chitin, not cellulose.

Question 28

What is the process of making and secreting a protein?

Answer 28

Exocytosis is an active process so energy is needed and it is a type of bulk transport.

1. the gene that has the coded instructions for a proteins such as insulin, housed on chromatin in the nucleus, is transcribed into a length of RNA, called mRNA.
2. many copies of this mRNA are made and they pass out of the pores in the nuclear envelope to the ribosomes.
3. at the ribosomes, the instructions are translated and insulin molecules are assembled.
4. the insulin molecules pass into the cisternae of the rough endoplasmic reticulum (RER) and along these hollow sacs.
5. vesicles with insulin inside are pinched off from the RER and pass, via microtubules and motor proteins, to the Golgi apparatus.
6. the vesicles fuse with the Golgi apparatus, where the insulin protein molecules may be modified for release.
7. inside vesicles pinched off from the Golgi apparatus, these molecules pass to the plasma membrane.
8. the vesicles and plasma membrane fuse, and the insulin is released to the outside of the cell.

Question 29

Similarities between Prokaryotic and Eukaryotic cells:

Answer 29

They both have a plasma membrane
They both have cytoplasm
They both have ribosomes for assembling amino acids into proteins
They both have DNA and RNA

Question 30

Differences between Prokaryotic and Eukaryotic cells:

Answer 30

Prokaryotic cells are much smaller
Prokaryotic cells have a much less well-developed cytoskeleton with no centrioles
Prokaryotic cells do not have a nucleus
Prokaryotic cells do not have membrane-bound organelles such as mitochondria, endoplasmic reticulum, chloroplasts or Golgi Apparatus,
Prokaryotic cells have a wall that is made of peptidoglycan and not cellulose
Prokaryotic cells have smaller ribosomes.
Prokaryotic cells have naked DNA that is not wound around histone proteins but floats free in the cytoplasm, as a loop.
Prokaryotic cells divide by binary fission whereas Eukaryotic cells divide by mitosis.