Enzyme

The induced-fit hypothesis: The substrate molecule binds with the enzyme molecule and induces a change in shape of the enzyme molecule.

Enzyme Cofactors
A cofactor is the non-protein component of an enzyme essential for its catalytic activity.

Inorganic ions= Activators
can be loosely or firmly bound

Organic compounds:

1. firmly bound = prosthetic group
2. loosely bound = coenzymes

----Prosthetic group
----integral part, e.g. FAD

----Coenzymes
----carrier to transfer atom, e.g. NAD, NADP

Enzyme Inhibitions (nothing to do with cofactor)

Competitive inhibitors:
substance similar to substrate, compete for active site
+ substrate concentration, - degree of inhibition, vice versa
+ inhibitor concentration, + degree of inhibition, vice versa

Non-competitive inhibitors:
attach to parts other than active site, not compete for active site
+ substrate concentration, X degree of inhibition, vice versa
+ inhibitor concentration, + degree of inhibition, vice versa
binding affinity

Reversible inhibitors
Can be Competitive or Non-competitive
Effect on enzyme:
occupying active site/altering conformation of active site temporarily

Irreversible inhibitors
ONLY NON-COMPETITIVE
Effect on enzyme:
altering conformation of active site permanently

End-product inhibition

Metabolic pathway:
Step-wise reaction for

1. preventing vigorous reaction that a quick release of heat may damage the cells
2. allowing the energy released from the reactions to be utilized effectively
3. intermediate metabolites of pathway may be useful to other metabolic pathways

Negative feedback inhibition:
When end-product is in excess, end product itself act as an allosteric inhibitor and binds with the allosteric site of allosteric enzyme. This will temporary shut down the entire series of reaction.
All the intermediate metabolites and the end-product will not be formed.

Active transport

Unidirectional----substance are transported in one direction only
Carrier protein

For large particles
Phagocytosis

1. Invagination of the cell membrane/pseudopodia surround foreign particles
2. Cup-shaped cavity pinched off from cell membrane, it forms phagocytic vesicles/food vacuole
   
3. Vesicle migrates to the center of the cell
4. Lysosome fuses with the vesicle
5. Hydrolytic enzymes are discharged from lysosome to vesicle for digestion
6. Digested products are absorbed by the cytoplasm
7. Vesicle containing indigested matter moves towards the edge of the cell
8. Indigested matter removed when vesicle fused with the cell membrane

Pinocytosis
Similar to phagocytosis but differs in

1. liquids instead of solid are digested
2. pinocytic vesicles are much smaller than phagocytic vesicles

blood+neurone --71

animal cells and tissues: Epithelia

They are tissues that cover the surface of internal and external organ.
They give protection, secretion, absorption, gaseous diffusion, excretion etc.
connective tissue= basement membrane

Simple epithelia: Single layer of epithelial cell, e.g. Squamous, ciliated epithelium
compound epithelia: more than one layer, e.g. stratified epithelium

Squamous epithelium
Consist of flattened, plate-like cells with disc shape nucleus and wavy boundary.

features:
The thinness of the epithelium allows rapid diffusion/filtration of materials

Place found:
alveolar membrane, bowman's capsule

Ciliated epithelium
Consist of columnar cells with numerous cilia on the free surface. Goblet cells interspersed among columnar cells.
Columnar cells: tall and narrow shape, containing elongated nucleus.
Goblet cells: specialized epithelial cell that secret mucus

features:
Coordinated beating action produced facilitate movement of material(dust/ cell)

Place found:
inner surface of the oviduct, inner lining of trachea

Stratified epithelium:
Consist of basement membrane--germinative layer and layers of old cells

• Germinative layer is a site where active cell division takes place.
• Newly divided cells are pushed upwards to replace the old ones,while the old cells become gradually flattened near the surface.
• The outermost cells are usually dead, flattened and gradually sloughed off because they are kept away from the nutrient supply

features:
Protect the underlying tissues against abrasion.

Place found:
inner lining of virgina
epidermis of skin

• the outer cells are keratinized (layer of keratin)
• horny and waterproof layer provides excellent protection from mechanical injury and dehydration

Phloem and Xylem

Phloem
composed of sieve tube elements, companion cells, parenchyma, sclereids and fibres

sieve tube element:
tubular cell
fuse together end to end to form continuous tube--sieve tube
Its maturity:

• nuclear degenerates
• cytoplasm restricted to the edge of the cell
• end wall perforated--sieve pore
• keep alive by companion cell<---connected via plasmodesmata

Xylem
composed of tracheary elements, fibres and parenchyma

Tracheary elements:
Elongated cells that transport water and minerals.
provide high mechanical strength to withstand the high tension of the uprising columns of water give support to the plant

two kinds: -----vessel element -------------- tracheid
different-aaaa------shorter-------------------longer
aaaaaaaaaaaaa---larger lumen---------------smaller

common- Thick and highly lignified
----------When extensively lignified, it would be deprived of water,
----------nutrient supply and die off, cell content degenerates
---------- leaving empty lumen to facilitate the transport of water
-----------and minerals.

vessel element:
align as hollow tubes called xylem vessel
primary wall area covered by secondary wall material--

• first occur as rings or helices called annular thickening and spiral thickening
• next more extensive deposit of lignin reticulate thickening and pitted thickening

Tracheid:
similar to sclerenchyma
tapered end wall overlapping each other
passage of water from cell to cell is facilitate by bordered pits, through pit membranes


Fibres
similar to sclerenchyma but with thicker cell walls
do not involved in conduction of water
provide additional mechanical support

Ground tissues:Parenchyma, Collenchyma, Sclerenchyma

hardness:
P < C < S PCS

Parenchyma
Place: Found in cortex and pith of stem and cortex of root and mesophyll of leaves.

features:made up of thin-walled living cell, metabolically active and capable of division, capable of elongation, potentially meristemic(modified to form epidermal cell)

functions:
1.mechanical support of non-woody plant,
2. facilitate gas exchange,
3. contains chloroplast for photosynthesis

Collenchyma
place: under the epidermis of stems and around the midrib of leaves.

features: similar to parenchyma, with additional deposits of cellulose at the coners of the cell walls .

functions:
1. mechanical support to young, actively glowing stems and leaves.

Sclerenchyma
features: dead cells with cell contents lost----thickened and impermeable to water, evenly thickened cell wall

secondary thickening: laying down of secondary cell wall which is highly lignified on the primary cell wall except at the pit. It formed when the cell growth is stopped.

functions:
1. high mechanical strength provided by high tensile strength and compressional strength of lignin.

Fibres:
elongated and tapering cells
found in vascular tissues(phloem...), behind epidermas

Sclereids:
shorty and relatively spherical
behind epidermas, cortex, fruits, seeds

Lysosome

Single membrane bounded vesicle pinched off from the releasing face of Golgi Apparatus.

NOT secretory vesicle! They are retained in the cytoplasm.

Function:

1. Breaking down unwanted molecules.
2. Digesting material taken up from outside
   
3. Breaking down worn -out organelles
4. Destroying the whole cell when the cell is severely damaged
   
5. reabsorbing unnecessary tissue by releasing enzyme outside the cell.
   

Functional relationshiop between nucleus, ER, Ribosomes and Golgi apparatus

Golgi apparatus

Structure:
Forming face----convex end next to ER----form new cisternae by vesicle budding off rER
Releasing face----concave end----secretary vesicles and lysosomes are pinched off

Function:

1. Storage, chemical modification, packing and export of proteins
2. enzyme production
3. lysosome formation
4. synthesizing and exporting of polysaccharides, e.g. celloluse

Ribosomes

Made up of ribosomal RNA and proteins.

They are attached to rough ER or suspended freely in cytoplasmic matrix.

They synthesis protein

Endoplasmic reticulum

Structure:

• single membrane-bounded

• highly folded in to flattened sacs and tubes which is
  

----cisternae

system of channel extends throughout cytoplasm, from outer nuclear membrane to cell
membrane. It is an extension of the outer nuclear membrane without pore. It filled with fluid containing enzymes for metabolic activities.
Tubular cisternae continuously pinched off to form Golgi Apparatus.

	Rough ER	Smooth ER
Appearance	ribosome attached	no
cisternae	nothing special	more tubular
function	Synthesize protein(secretary,membrane)	synthesize lipid and steroids

Function of all ER:

1. Extensive membrane system acts as cytoskeleton for support
2. Intricate channel of ER facilitate transport .
3. ER membrane provide a large surface area for attachment of enzyme

Example:
rough ER: pancreatic islet cells
smooth ER:sebaceous gland cells

AL Biology: Cell membrane

Made up of phospholipid bilayer, hydrophobic hydrocarbon chain face inward to each other----suit aqueous environment on both sides of the cell membrane and to accommodate the hydrophobic hydrocarbon chains.

it is in the form of fluid mosaic model----phospholipid and protein molecules can move side way within the bimolecular leaflet, movement is restricted by cholesterol but still in liquid form.
Function
Permeable to fat soluble substances
Carrier protein permeable to aqueous substance

The membrane protein:
outer or inner surface----peripheral protein(pp)
extended into leaflet----integral protein(ip)
penetrate both surface----transmembrane protein(tp)

type	location	function
enzymes	pp	[speeding up metabolic reactions]
recognition proteins	pp(outer)	[cell recognition(distinguishing foreign molecules from self molecules)]
receptor proteins	tp	[allowing the cell to recognize a signal and trigger corresponding reactions ]
channel proteins	tp	[forming channels through which small polar molecules may pass]
carrier proteins	tp	[for active transport and facilitated diffusion]

ERRCC

The surface carbohydrates:
mostly conjugated with membrane protein to form glycoprotein.

Differentiation in cell membrane:
Microvilli---- finger-liked projections which increases the surface area of the cell for absorption of material, facilitate absorption and secretion due to presence of contractile filament eg. intestine epithelial cell.
pseudopodia----foot-like membrane extensions found in cell capable of changing shape. It facilitate phagocytosis, pinocytosis or movement for itself.

Functions of the cell membrane
1. Maintains the integrity of the cell.
2.Controls the movement of material in and out of the cell by channel protein etc.
3.Protects the cell from harmful material outside such as hydrolytic enzyme
4.Recognition proteins perform cell recognition.
5.Receptor proteins allow the cell to recognize a signal and trigger corresponding reaction.
6.subcellular membranes divide the cell into compartments so that metabolic reactions can take place without interference.
7. Provides surface for attachment of enzyme
8. Provide electrical insulation(myelin sheath)
9. Enable movement, phagocytosis and pinocytosis.

resources from Advance level biology for Hong Kong