1.2- Cell structure and organisation

THE NUCLEUS:

• Contains the organism's hereditary material and controls the cell's activities.
• Usually spherical and between 10 and 20 micrometres in diameter, the nucleus has a number of parts:
• The nuclear envelope, nuclear pores, chromosomes, and the nucleolus.

Functions:

• Act as the control centre of the cell through the production of mRNA and tRNA and hence protein synthesis.
• Retain the genetic material of the cell in the form of DNA and chromosomes.
• Manufacture rRNA and ribosomes.

Nuclear Envelope:

• A double membrane that surrounds the nucleus.
• Its outer membrane is continuous with the endoplasmic reticulum of the cell and often has ribosomes on its surface.
• It controls the entry and exit of materials in and out of the nucleus and contains the reactions taking place within it.

Nuclear pores:

• Allow the passage of large molecules, such as mRNA, out of the nucleus.
• There are typically around 3000 pores in each nucleus, each 40-100nm in diameter.

Nucleolus:

• It manufactures rRNA and assembles the ribosomes.
• There may be more than 1 nucleolus in a nucleus.

• Usually rod-shaped and 1-10 micrometres in length.
• Made up of the following structures:
• Double membrane, cristae, the matrix (contains a strand of DNA and 70s ribosomes)

Functions:

• The sites of aerobic stages of respiration 
• They are therefore responsible for the production of energy-carrier molecule, ATP.
• Because of this, the number and size of the mitochondria, are high in cells that have a high level of metabolic activity and therefore require a plentiful supply of ATP.

Double membrane:

• Around the organelle that controls the entry and exit of material.
• The inner of the 2 membranes are folded to form extensions known as cristae.

Cristae:

• Extensions of the inner membrane, which in some species extend across the whole width of the mitochondrion.
• These provide a large surface area for the attachment of enzymes and other proteins involved in respiration.

The matrix:

• Makes up the remainder of the mitochondrion.
• It contains protein, lipids, ribosomes, and DNA that allows the mitochondria to control the production of some of their own proteins.
• Many enzymes involved in respiration are found in the matrix.

• Organelles that carry out photosynthesis.
• They vary in shape and size but are typically disc-shaped, 2-10 micrometres long and 1 micrometre in diameter.
• The following are their main features:
• The chloroplast envelope, the grana, the stroma (contain 70s ribosomes, circular DNA, thylakoids (which make granum))

Functions:

• Photosynthesis (In many plants, the highest concentration of chloroplast is in the palisade mesophyll cells, just below the upper surface of the leaf)

The chloroplast envelope:

• A double plasma membrane that surrounds the organelle.
• It is highly selective in what it allows to enter and leave the chloroplast.

The grana:

• Stacks of up to 100 disc-like structures called thylakoids.
• Within the thylakoids is the photosynthetic pigment called chlorophyll.
• The grana are where the first stage of photosynthesis (light absorption) takes place.

The stroma:

• A fluid-filled matrix where the second stage of photosynthesis (synthesis of sugars) takes place.
• Within the stroma are a number of other structures, such as starch grains.

• The theory that describes the origin of chloroplasts and mitochondria
• Division of chloroplasts was seen to closely resemble that of free-living cyanobacteria
• It was suggested that mitochondria were once independent bacteria
• Mitochondria and chloroplasts both have 70s ribosomes and circular DNA, therefore, it was suggested that some ancient bacteria with very fluid membranes engulfed others and maintained a symbiotic relationship (special interactions between species). 
• Those good at turning glucose and oxygen into ATP were thought to evolve into mitochondria.
• Those that could turn carbon dioxide and water into glucose were thought to have evolved into chloroplasts. 

Contains circular DNA                                    

Contains circular DNA

Mitochondria have two chambers: matrix and the cristae.

Chloroplast also has two chambers stroma and thylakoid.

The inner membrane of mitochondria is folded into cristae.

The inner membrane of the chloroplast rises into flattened sacs called as thylakoids.

Contains 70s ribosomes                                  

Contains 70s ribosomes

• An elaborate, 3D system of parallel double membranes, spreading through the cytoplasm of the cells.
• It is continuous with the outer nuclear membrane.
• The membranes enclose a network of tubules and flattened sacs called cisternae.
• Connected to nuclear envelope
• Two kinds: rough ER and smooth ER

RER

• Rough as it has membrane-bound proteins. Regular, fixed structure in electron micrograph image 

Functions:

• Provide a large surface area for the synthesis of proteins and glycoproteins.
• Provide a pathway for the transport of materials (proteins in particular)

SER

• Lacks ribosomes on its surface and is often more tubular in appearance.

Functions:

• Synthesise, store and transport lipids and carbohydrates

• Consists of a stack of membranes that make up flattened sacs, or cisternae, with small rounded hollow structures called vesicles.
• The proteins and lipids produced by the ER are passed through the Golgi apparatus in a strict sequence.
• The Golgi modifies these proteins adding other molecules such as carbohydrate, to 
• Once sorted, the modified proteins and lipids are transported in Golgi vesicles which are regularly pinched off from the ends of the Golgi cisternae.
• These vesicles may move to the cell surface, where they fuse with the membrane and release their contents on the outside.
• Functions:
  • Add carbohydrates to proteins to form glycoproteins.
  • Produce secretory enzymes
  • Secrete carbohydrates
  • Transport, modify and store lipids.
  • Form lysosomes.

• Formed when the vesicles produced by the Golgi apparatus contain enzymes such as proteases and lipases.
• They also contain lysozymes, enzymes that break down the cell walls of certain bacteria. Lysosomes isolate these enzymes from the rest of the cell before releasing them
• Up to 1.0 micrometres in diameter,
• Functions:
  • Break down material ingested by phagocytic cells, such as white blood cells and bacteria.
  • Release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell.
  • Digest worn out organelles so that the useful chemicals they are made of can be re-used.
  • Completely break down cells after they have died 

• There are 2 types, depending on the cells in which they are found:
  • 80S: Found in eukaryotic cells, is around 25nm in diameter.
  • 70S: Found in prokaryotic cells, is slightly smaller.
• Have 2 subunits - 1 large and 1 small - each of which contains rRNA and protein.
• Functions:
  • Site of protein synthesis.

• Characteristic of all plant cells, the cell wall consists of microfibrils of the polysaccharide cellulose, embedded in a matrix called pectin.
• Cellulose microfibrils have considerable strength, therefore, allows the cell to maintain its turgidity
• Cell walls have the following features:
  • They consist of a number of polysaccharides, such as cellulose.
  • Passing of plasmodesmata- cytoplasm strands pass through cell to cell. Network of cytoplasm in connected cells is called the symplast.
• Functions:
  • To provide mechanical strength in order to prevent the cell bursting under the pressure created by the osmotic entry of water.
  • To give mechanical strength to the plant as a whole.
  • To allow water to pass along it and so contribute to the movement of water through the plant. (Apoplast- space outside the cells through which solution moves.)

• Within mature plant cells there is usually one large central vacuole.
• The single membrane around it is called the tonoplast.
• A plant vacuole contains a solution of mineral salts, sugars, amino acids and wastes 
• Functions:
  • They support plants by making cells turgid.
  • The sugars and amino acids may act as a temporary food store.

Common organelles:

• Nucleus,
• Endoplasmic reticulum,
• Golgi bodies,
• lysosomes,
• Mitochondria,
• Cell membrane
• Ribosomes
• Vacuole (In animal cells they are temporary, small and scattered throughout the cell)

Organelles in plants:

• Cell wall
• Plasmodesmata
• Chloroplast (Only in cells above ground)

Organelles in animals:

• Centrioles

Centrioles occur in all animal cells and mosy protoctistans but not in cells of higher plants. They are located just outside of the nucleus. Centrioles are tow rings of microtubules, making hollow cylinders positioned at right angles against each other. Together they are called centrosomes. 

Functions:

During cell division, they are responsible for the organistaion of the microtubules that make the spindle. 

All prokaryotic cells:

• DNA molecule loose in cytoplasm
• Murein cell wall
• 70S ribosomes
• Cytoplasm 
• Cell membrane

Some prokaryotes:

• Slime coat
• Flagellum
• Mesosome
• Plasmids

• Described as 'acellular'
• No organelles
• Able to take over a cell's metabolism and multiple inside host cell
• Can be crystallised 
• Each virus particle is made up of a core of nucleic acid, either DNA or RN, surrounded by a protein coat

Human: Flu, chickenpox, mumps

Plants: tobacco mosaic virus

Birds: Avian flu

Mammals: swine flu

• Usually forms a lining of the internal and external surfaces of the body. They have no blood vessels but some may have nerve endings. The cell sits on a basement membrane made from collagen and protein. 

Cuboidal epithelial cell:

• Cells have a cuboidal shape and are only one cell thick. It occurs in the proximal convoluted tub of the kidney nephron and the ducts of salivary glands. 

Columnar epithelial cell: 

• Cells are elongated, some of which have cilia and therefore, are found in the oviduct and trachea. 

Squamous epithelial cell:

• Cells are flattened against the basement membrane and form the wall of the alveoli and line the renal capsule of the nephron.

Skeletal muscle:

• Attached to bones, fibres can give powerful contractions but the muscles tire quickly. They are voluntary, muscles, therefore you are able to control the contractions. Also known as striated muscle tissue.

Smooth muscle:

• Involuntary contractions, contract rhythmically and are found in skin, blood vessels, digestive and respiratory tracts. Also known as unstriated muscle. 

Cardiac muscle:

• Only found in the heart, contract rhythmically and do not tire. Stimulation from nerves and hormones can modify the contractions.

Connective tissues:

• Connects or supports tissues and organs. Contains elastic and collagen fibres in fluid or matrix. Between fibres are fat-storing cells and cells of the immune system.

Similarities and differences between Prokaryotic and Euakryotic cell [9]

Enclosed by cell wall. [1] 

Presence of cell/plasma membrane. [1]

photosynthetic membrane. [1] 

DNA in tangled nucleoid / single chromosome / loop 

(not if looks like plasmid) [1] 

Additional rings of DNA – plasmids / food reserve granules.   [1] 

Infoldings of cell membranes – mesosome. [1] 

Ribosomes in cytoplasm. [1] 

Ribosomes (very) much larger in eukaryote/70S. [1] 

Prokaryote (1 -10 μm), eukaryote (10 – 100 μm). [1]                                        

Cell wall of eukaryote made of cellulose/chitin and 

Prokaryote wall murein / peptidoglycan. [1] 

Mitochondria in eukaryotes and mesosomes in prokaryotes [1] 

Eukaryotes have nucleus / nuclear membrane / more  

genetic information. [1] 

(Containing several paired) chromosomes / linear Chromosomes/DNA. [1]            

Eukaryote compartmentalised by membranes /contains organelles or 2 examples. [1] 

Eukaryote chromosomes have protein / histones  [1] 

(half marks max. if pro and eu wrong way round or no diagram.)

Mitochondrion

A. folded inner membrane/cristae;

B. increases surface area;

C. for (attachment of){enzymes/ ATP synthetase} / location of {electron transport chain/ stalked particles};

D. site of (aerobic) respiration/ATP production;

E. good diagram/ description of location of any four from -cristae, matrix, stalked particles, inter membrane space,inner/outer membrane, (circular)DNA, ribosomes ;

Chloroplast

F. thylakoids contain {photosynthetic pigments/ chlorophyll}; 

G. thylakoids stacked into grana;

H. function is photosynthesis;

I. Starch grains store {photosynthetic products / glucose}

J. good diagram/ description of location of any four from -stroma, {thylakoid/grana},ribosomes, (circular)DNA, starch grain, lipid droplet, lamellae, inner/ outer membrane;

similar to prokaryotic cell

K. contain ribosomes;

M. (circular) DNA;

O. comparison of mesosome with cristae