H2 Biology - Organelles and Cellular Structures

Under the Singapore-Cambridge GCE A-Level Syllabus

What are the widths/diameters/lengths of the nucleus, lysosome, mitochondrion, chloroplast, and ribosome?

Nucleus: 5-20 μm in diameter

Lysosome: 0.2-0.5 μm in diameter

Mitochondrion: 0.5-1.5 μm in width, 3-10 μm in length

Chloroplast: 5-10μm in length

Ribosome: 20nm in diameter

State all 3 diagnostic features of the nucleus.

1. Enclosed by nuclear envelope

2. Contains chromatin

3. Contains nucleolus

Describe the 5 structures.of the nuclear envelope.

• Double membrane separated by perinuclear space

• Outer membrane continuous with endoplasmic reticulum

• Perforated by nuclear pores

• Each nuclear pore is lined with proteins to form nuclear pore complex

• Encloses nucleoplasm

Describe the two types of chromatin.

Heterochromatin:

• more tightly packed

• electron-dense, appears as coarse granules under EM

• often transcriptionally inactive

Euchromatin:

• less compact

• not electron-dense

• often transcriptionally active

Describe the 4 structures of the nucleolus.

• Dense

• Spherical

• Made up of large loops of DNA coming from a number of chromosomes

• Loops contain genes coding for rRNA

State the 3 functions of the nucleus.

1. Stores hereditary material (DNA)

2. Nuclear envelope protects DNA from metabolically active cytoplasm

3. Site for transcription to synthesise mRNA and rRNA

State the 2 functions of the nucleolus.

1. Site of rRNA synthesis

2. Site of assembly of ribosomal subunits from proteins and rRNA

State the function of nuclear pores.

Regulates movement of substances between nucleus and cytoplasm, thus controlling processes in the cell

State the shape of the nucleus.

Spherical to ovoid

Describe the 4 structures of the endoplasmic reticulum.

• extensive network of folded membranes forming sheets and tubules called cisternae

• ER membrane separates the ER lumen from the cytosol

• ER membrane is continuous with outer nuclear envelope

• ER lumen is continuous with perinuclear space

Describe the 2 structures of the RER.

• continuous network of membranous sheets (cisternae)

• outer surface of membrane studded with ribosomes

State the 2 functions of the RER.

• Site for synthesis of proteins destined for secretion or incorporation into membranes

• Facilitation of folding of polypeptide chain

Describe the function of the RER as a site for protein synthesis.

• RER is the site for synthesis of proteins destined for secretion or incorporation into membranes

• these proteins are transported to other compartments in the cell via transport vesicles that bud off from RER membrane

Describe the function of the RER in facilitating the folding of polypeptide chain.

• polypeptide chain grows from bound ribosome

• it enters ER lumen through a pore in ER membrane

• it folds into its specific 3-dimensional shape as it enters, guided by ER proteins and enzymes

Describe the 2 structures of the SER.

• network of membranous tubules (cisternae)

• lacks ribosomes

State the 2 functions of the SER.

• synthesis of lipids

• detoxification of drugs and poisons

Describe how detoxification is carried out in SER.

• hydroxyl groups are added to drugs and poisons

Describe the 5 structures of the Golgi Apparatus.

• consists of stack of flattened membrane-bound sacs (cisternae)

• has distinct structural polarity

• cis face, nearest to the ER receives transport vesicles containing newly synthesised proteins and lipids from the ER

• trans face, nearest to the plasma membrane ships vesicles containing mature proteins and lipids to other parts of the cell

• contains golgi vesicles that transfer material between cisternae as well as between GA and other parts of the cell

State and describe the 6 functions of the GA.

• further modifies, sorts and packages products of the ER into vesicles

• vesicles transport products to other parts of the cell or releases them out of the cell.

• synthesis of secretory polysaccharides

• synthesis of lysosomes via budding from trans face

• glycosylation of proteins

• phosphorylation of proteins

Compare Golgi vesicles to transport vesicles to secretory vesicles.

• Golgi vesicles transport products between GA cisternae, transport vesicles transport products from one cellular compartment to another, secretory vesicles transport products to the cell surface membrane.

State the 4 diagnostic features of lysosomes.

1. Spherical sac bound by single membrane

2. Uniformly granular electron-dense appearance under EM

3. Contains lysosomal enzymes that digest various macromolecules

4. Acidic internal pH of 4-5 at optimum acidic pH of lysosomal enzymes

State the 3 functions of lysosomes.

1. Digestion by endocytosis

2. Autophagy

3. Autolysis

Describe the function of lysosomes which is digestion by endocytosis.

• lysosomes fuse with vacuoles formed from endocytosis

• lysosomal enzymes release into vacuole and digest its material inside, which could be food or harmful substances/organisms

Describe the function of lysosomes which is autophagy.

• unwanted structures are enclosed by membrane from the ER

• structure fuses with lysosome to form autophagic vacuole

• lysosomal enzymes hydrolyse ingested material and return products to cytosol for reuse

Describe the function of lysosomes which is autolysis.

• many lysosomes in the cell release their contents together, destroying the cell

State the shape of mitochondria.

Cylindrical or rod shaped

Describe the 3 diagnostic features of the mitochondria.

1. Wall comprises 2 membranes separated by intermembrane space. Outer membrane is smooth, inner membrane is highly convoluted with numerous infoldings called cristae

2. Mitochondrial matrix, interior of mitochondria, contains 70S ribosomes, circular DNA and various enzymes

3. Mitochondrial space divided into intermembrane space and mitochondrial matrix

Explain the function of the mitochondria.

Function: Main site of ATP production during aerobic respiration.

Explanation:

Matrix contains enzymes of Krebs cycle.

Cristae maximise surface area to accommodate the proteins and enzymes responsible for ATP synthesis.

State the shape of chloroplasts.

Disc-shaped

Describe the 3 diagnostic features of chloroplasts.

1. Enclosed by double membrane called the chloroplast envelope, where the two membranes are separated by intermembrane space.

2. In stroma is another membranous system in the form of flattened, interconnected disc-like sacs called thylakoids.

3. Membranes divide chloroplast space into intermembrane space, stroma and thylakoid space.

State the function of chloroplasts.

Site of photosynthesis

State the shape of ribosomes.

Small, spherical and dense.

Describe the 3 diagnostic features of ribosomes.

1. Comprises rRNA and proteins.

2. 80S eukaryotic ribosome consists of small 40S and large 60s ribosomal subunits.

3. Located in the RER as bound ribosomes, cytosol as free ribosomes, mitochondrial matrix and chloroplast stroma.

State the 2 functions of ribosomes.

1. Site of polypeptide synthesis.

2. Facilitates translation of mRNA base sequence into specific amino acid sequence of polypeptide chain.

Define the cytoskeleton.

Cytoskeleton is a dynamic network of fibrous protein structures throughout the cytoplasm.

Describe the diagnostic feature of microtubules.

Walls are composed of globular proteins called tubulin.

Describe the 5 functions of microtubules.

1. Maintain cell shape.

2. Serve as tracks along which organelles and materials move within cells.

3. Forms spindle fibres which separate chromosomes during cell division.

4. Makes up centrioles which function as MTOCs in lower plants and animals.

5. Major structural components of cilia and flagella.

Describe the 3 diagnostic features of centrioles.

1. Centrosome, near the nucleus, comprises 2 centrioles

2. Exist as pair of rod-like structure at right angles to each other.

3. Comprises 9 triplets of microtubules arranged in a ring.

Explain the function of centrioles.

• During prometaphase, centrioles produce system of microtubules called spindle fibres that radiate towards metaphase plate of cell.

• Spindle fibres attach to kinetochore proteins found in centromere of chromosomes.

What components of the cell does the endomembrane system comprise? State in order of flow from nucleus to CSM.

Outer nuclear membrane, endoplasmic reticulum, Golgi apparatus, lysosome and vacuoles, cell surface membrane.