Cell Membranes and Organelles-2

Cell Membranes and Organelles-2

• Cells are constantly changing, exhibiting various movements like chromosome and vesicle translocation, and shape changes from crawling to swimming.

• Eukaryotic cells contain a nucleus and many other organelles.

• Different organelles have their unique membranes.

• Unique proteins within each organelle determine its function.

• Nucleus, mitochondria, and chloroplasts have two bilayer membranes.

• Other organelles are surrounded by a single membrane.

• Plant and fungal cells lack lysosomes but contain a large central vacuole performing lysosome functions.

• Plant cell walls are primarily cellulose, providing shape and rigidity.

• Animal cells lack cell walls and have an extracellular matrix made of collagen, glycoproteins, and other components, providing support and structure.

Internal Organisation of the Cell

• Materials move to lysosomes through three pathways: endocytic, phagocytic, and autophagic.

• The endocytic pathway transports soluble molecules into the cell.

• The phagocytic pathway moves whole cells or large insoluble particles.

• The autophagic pathway moves worn-out organelles and cytoplasm.

• Hydrolytic enzymes within lysosomes break down proteins, nucleic acids, and other large molecules in the acidic lysosomal lumen.

Endosomes

• Endosomes sort and return plasma membrane proteins and soluble materials from the extracellular medium.

• They sort materials back to the membranes or to lysosomes for degradation.

• Endocytic pathway involves invagination of a plasma membrane section into a coated pit, lined by proteins like clathrin.

Lysosomes

• Lysosomes are digestive sacs with an acidic lumen, breaking down materials internalized by the cell and worn-out parts.

• Vary in size and shape and are numerous in animal cells.

• Contain hydrolytic enzymes (lysosomal enzymes) for breaking down macromolecules.

• Digest worn-out cellular membranes and organelles.

• Recycle membrane proteins back to the plasma membrane.

Autophagy

• Autophagy (eating oneself) is a process where an aged organelle is degraded within a lysosome.

• Materials taken in by endocytosis or phagocytosis can also be broken down in lysosomes.

• In phagocytosis, bacteria and other large insoluble particles are engulfed and internalized.

Deficiency in Lysosomal Enzymes

• Tay-Sachs disease is caused by a defect in an enzyme that breaks down certain glycolipids (gangliosides).

• These glycolipids are abundant in nerve cells. The condition results in a buildup of harmful substances.

Vacuoles and Vesicles

• Vacuoles store water, ions, and nutrients.

• They break down macromolecules and help with cell elongation.

• In plant cells, vacuoles are large, and the osmotic flow of water generates turgor pressure.

Peroxisomes

• Peroxisomes detoxify molecules and break down fatty acids to produce acetyl groups for biosynthesis.

• Found in most animal cells and some plant cells (except erythrocytes).

• Contain oxidases that use molecular oxygen to break down organic substances and form hydrogen peroxide (H₂O₂).

• Also contain catalase, which breaks down hydrogen peroxide.

Oxidation of Very Long Fatty Acids (VLFAs)

• Peroxisomes are involved in the oxidation of very long fatty acids (VLFAs).

• Mitochondria are responsible for oxidizing short, medium, and long-chain fatty acids.

Endoplasmic Reticulum (ER)

• Smooth ER synthesizes lipids, detoxifies hydrophobic compounds, and stores calcium.

• Rough ER synthesizes, processes, and sorts proteins (secreted, lysosomal, or membrane-bound proteins). It is especially important in cells secreting proteins.

Ribosomes

• Ribosomes are the site of protein synthesis.

• They are found free in the cytoplasm or attached to the rough ER.

• Ribosomes translate mRNA into proteins. Proteins are critical to a cell's function.

Protein Targeting. Signal Sequences

• Proteins' final destination depends upon their signal sequences.

• N-terminal hydrophobic signal sequences direct proteins to the rough ER.

• Lysosomal enzymes are tagged with mannose-6-phosphate in the Golgi complex.

Trafficking of Proteins synthesized on Bound/Free Ribosomes

• Proteins destined for various destinations are transported appropriately using different methods.

• Proteins destined for secretion or lysosomes will have initial targeting to organelles followed by modification.

• Proteins that have no targeting or initiating sequence remain in the cytoplasm.

The Nucleus

• The nucleus houses a cell's genetic material (DNA).

• The nuclear envelope has pores allowing exchange with the cytosol.

• The nucleolus is a region within the nucleus that produces rRNA.

Mitochondria

• Mitochondria are critical for energy production (ATP).

• They have two membranes, an outer and inner.

• They contain their own DNA independently from the nuclear DNA.

• The inner membrane folds extensively (cristae) to increase surface area.

Chloroplasts

• Chloroplasts are found only in plant cells.

• Essential in photosynthesis, converting carbon dioxide, water, and sunlight into glucose, providing the plant and other organisms with energy and food.

• Surrounded by two membranes including grana, thylakoids membranes, and stroma.

Cytoskeleton

• Provides structural support and aids in cell movement and division (mitosis).

• Includes intermediate filaments, microtubules, and microfilaments.

Golgi Apparatus and Vesicles

• Processes and sorts proteins secreted by the rough ER including lysosomal proteins and membrane proteins.

• Packages proteins into vesicles which move to other cell locations.

• Also modifies proteins before they are transported.

Cell Membranes and Organelles - Review Questions

• Questions regarding the major organelles, their functions, protein type, and location within cells are addressed.

• These questions focus on the roles of ribosomes, protein trafficking, types of secretions, targeting, phosphorylation of lysosomal proteins, and examples of diseases related to these processes.