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Cell is the base of life

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The Endomembrane systemA membranous system of interconnected tubules and cisternae

Membranes of the endomembrane system vary in structure, composition, thickness and behavior

The endomembrane system includes:

Nuclear envelopeEndoplasmatic reticulumGolgi apparatusLysosomesVacuolesPlasma membrane (related to endomembrane)

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ER manufactures membranes

Endoplasmatic reticulum (ER) – network within the cytoplasm – extensive membranous network of tubules and sacs (cisternae) which sequesters its internal lumen (cisternal space) from the cytosol.

Consist of smooth and rough ER.

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Smooth ER

Participates in the synthesis of lipids, phospholipids and steroids

Participates in carbohydrate metabolism

Detoxifies drugs and poisons

Stores calcium ion necessary for muscle contraction

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Rough ERManufactures secretory proteins and membranes

Proteins to be secreted are synthesized by ribosomes attached to rough ER

Polypeptide chain is threaded through ER membrane into the lumen or cisternal space

Protein folds into its native conformation

Undergo modification: oligosaccharide are added to the proteins in order to make glycoprotein

Proteins departs in a transport vesicle pinched off from transitional ER adjacent to the rough ER site production

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Rough ER

Glycoproteins – protein covalently bonded to carbohydrate

Oligosaccharide – small polymer of sugar units

Transport vesicle – membrane vesicle in transit from one part of the cell to another

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Rough ER and membrane production

Membrane proteins are produced by ribosomes.

Growing polypeptide anchors by hydrophobic regions into the ER membrane

Enzymes within the ER membrane synthesize phospholipids from raw materials in the cytosol

Newly expanded ER membrane can be transported as a vesicle to other parts of the cell

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Apparatus Golgi

Golgi apparatus – organelle made of stacked, flattened membranous sacs (cisternae), that modifies, stores and routes products of the ER

Has a distinct polarity. Membranes of cisternae at opposite ends differ in thickness and composition.

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Apparatus GolgiTwo poles are called the cis face (forming face) and the trans face (maturing face)

Cis face, which is closely associated with transitional ER, receives products by accepting transport vesicles from the ER.

Trans face pinches off vesicles from the Golgi and transports molecules to other sites

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Apparatus Golgi

Golgi products in transit from one cisternae to the next, are carried in transport vesicles.

The Golgi:alters some membrane phospholipids

modifies the oligosaccharide portion of glycoproteins

target products for various parts of the cell

sorts products for secretion

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Apparatus Golgi

Many polysaccharides including hyaluronic acid are Golgi products

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LysosomesLysosomes are relatively large vesicles formed by the Golgi:

- organelles which are membrane-enclosed bag of hydrolytic enzymes that digest all major classes of macromolecules.

Enzymes include lipases, carbohydrases, proteases, and nucleases

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LysosomesLysosomal membrane performs two important functions:

Sequesters potentially destructive hydrolytic enzymes from the cytosol

Maintains the optimal acidic environment for enzyme activity by pumping H+s inward from the cytosol to the lumen

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Function of the lysosomes

a. Intracellular digestion

Phagocytosis – cellular process of ingestion, in which the plasma membrane engulfs substances and pinches off to form a particle-containing vacuole

Lysosomes may fuse with food-filled vacuoles, and their hydrolytic enzymes digest the food:

• Amoeba and other protists• Human macrophages

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Function of the lysosomes

b. Recycle cell’s own organic material

Lysosomes may engulf other cellular organelles or part of the cytosol and digest them (autophagy)

Resulting monomers are released into the cytosol

where they can be recycled into new macromolecules

c. Programmed cell destruction This process is important during metamorphosis and

development

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The formation and functions of lysosomes

The cell encloses food in a vacuole.

The food vacuole fuses with a lysosome, and

hydrolytic enzymes digest the food.

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The formation and functions of lysosomesAfter hydrolysis, simple sugars, amino acids, and other

monomers pass across the lysosomal membrane into the cytosol as nutrients for the cell.

Lysosomes recycle the molecular ingredients of organelles (autophagy).

The cell continually renews itself

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Lysosomes and human diseaseSymptoms of inherited storage diseases result from impaired lysosomal function.

Lack of a specific lysosomal enzymes causes substrate accumulation which interferes with lysosomal metabolism and other cellular functions

Pompe’s disease – the missing enzyme is a carbohydrase that breaks down glycogen – glycogen accumulation damages the liver

Tay-Sachs disease – brain impairment by accumulation of lipids

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The formation and functions of lysosomes

Transformation of a tadpole into a frog

and

Disappearance of tissue between the hands fingers of human embryos

are done by digestion with lysosomes

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Diverse function of vacuoles

Food vacuoles – vacuole formed by phagocytosis

Contractile vacuoles – pump water excess out of the cell (in protozoa)

Central vacuole enclosed by a membrane (tonoplast) exist in mature plants.

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Diverse function of vacuolesCentral vacuole

-is the major food storage (protein storage in seeds);-stores inorganic ions (K+ and Cl-);

-sequesters dangerous metabolic by-products from the cytoplasm

-contains soluble pigments in some cells;

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Diverse function of vacuolesCentral vacuole

-helps against predators by containing poisonous compounds;

- plays a role in plant growth by absorbing water and elongating the cell;

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Relationships between endomembranesMembrane and secretory proteins produced by the ER flows in the form of transport vesicles to the Golgi.Golgi pinches off vesicles:

Vesicles give rise to lysosomes and vacuoles andfuse with and add to plasma membrane. The membrane expends and releases secretory proteins

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Other membranous organellesMitochondria and chloroplasts

– the main energy transformers of cells

Mitochondria and chloroplasts are organelles that transduce energy acquired from the surroundings into forms useable for cellular work

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Other membranous organelles

Mitochondria are the sites of cellular respiration:catabolic process that generates ATP by extracting energy from sugars, fats and other molecules

Chloroplasts the sites of photosynthesis:they convert solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds from CO2 and H2O

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Mitochondria Enclosed by double membranes that are not part of endomembrane system (the membrane proteins are synthesized by free ribosomes)

Contain ribosomes and some DNA that programs a small portion of their own protein synthesis

Are semiautonomous organelles that grow and reproduce within the cell

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Mitochondriaouter membrane

inner membrane

Cristae

Matrix

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Mitochondria

Found in nearly all eukaryotes cells Number of mitochondria depends on the cell’s metabolic activityAre about 1 μm in diameter and 1-10 μm in lengthAre dynamic structures that move, change their shape and divide

Mitochondria contain their own DNA (termed mDNA) and

are thought to represent bacteria-like organisms incorporated into eukaryotic cells over 700 million years ago (perhaps even as far back as 1.5 billion years ago).

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MitochondriaThey function as the sites of energy release (following glycolysis in the cytoplasm) and ATP formation (by chemiosmosis).

Smooth outer membrane is highly permeable to small solutes, but it blocks passage of proteins and other macromolecules

Convoluted inner membrane contains embedded enzymes that are involved in cellular respiration. It folds into a series of cristae, which are the surfaces on which ATP is generated.

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Mitochondria

Intermembrane space – a narrow region between the inner and outer mitochondrial membranes

Reflects the solute composition of the cytoplasm, because the outer membrane is permeable

Mitochondrial matrix – compartment enclosed by the inner membrane, contains enzymes that catalyze many metabolic steps of cellular respiration. Some enzymes of respiration and ATP production are actually embedded in the inner membrane.

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Muscle Cell Mitochondria

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PlastidsPlastids are also membrane-bound organelles that only occur in plants and photosynthetic eukaryotes.

They include amyloplasts, chromoplasts and chloroplasts.

Amyloplasts – colorless plastids that store starch in roots and tubersChromoplasts – plastids containing pigments other than chlorophyll; responsible for fruits and flowers color.

Chloroplasts – chlorophyll-containing plastids which are the sites of photosynthesis in eukaryotes.

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PlastidsChloroplasts are found in eukaryotic algae, leaves and other green plant organsAre lens-shaped and measure about 2-5mAre dynamic structures that change shape, move and divide.

Functional compartments:Intermembrane space – separates the two membranes

Inside the chloroplast is another membranous system – thylakoids – segregates the interior of the chloroplast into two compartments: thylakoid space and stroma.

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Plastids

Thylakoids function in the steps of photosynthesis that initially convert light energy to chemical energy

Collectively a stack of thylakoids are a granum [plural = grana]) floating in a fluid termed the stroma.

Photosynthetic reactions that use chemical energy to convert carbon dioxide to sugar occur in the stroma

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Chloroplasts

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Chloroplasts

Like mitochondria, chloroplasts have their own DNA, termed cpDNA.

Chloroplasts of Green Algae (Protista) and Plants (descendants of some Green Algae) are thought to have originated by endosymbiosis of a prokaryotic alga similar to living Prochloron (Prochlorobacteria).

Chloroplasts of Red Algae (Protista) are very similar biochemically to cyanobacteria (also known as blue-green bacteria.

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PeroxisomesPeroxisomes are roughly spherical and often have a granular or crystalline core that is probably a dense collection of enzymes. This peroxisome is in a leaf cell.

Notice its proximity to two chloroplasts and a mitochondrion.

These organelles cooperate with peroxisomes in certain metabolic functions (TEM).

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PeroxisomesPeroxisomes do not bud from the endomembrane system.They grow by incorporating proteins and lipids made in the cytosol.

They increase in number by splitting in two when they reach a certain size.

Peroxisomes in liver detoxify alcohol by transferring H to O and producing H2O2

Peroxidase destroys toxic H2O2 by converting it to H2O

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PeroxisomesPeroxisomes convert fatty acids to smaller molecules that can be used by mitochondria in the process of cellular respiration.

In plant seeds glyoxysomes, special peroxisomes, convert fatty acids to sugar.

This provides growing seedlings with energy and carbon source.

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Cytoplasm

The cytoplasm was defined earlier as the material between the plasma membrane (cell membrane) and the nuclear envelope.

Fibrous proteins that occur in the cytoplasm, referred to as the cytoskeleton maintain the shape of the cell.

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Cytoplasm

Microtubules function in cell division and serve as a "temporary scaffolding" for other organelles.

Actin filaments are thin threads that function in cell division and cell motility.

Intermediate filaments are between the size of the microtubules and the actin filaments.

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Reading

Ch. 6 pp. 108-111