2. Ribosomes: build the cell’s proteins Lecture 4: The Cell 2. Ribosomes: build the cell’s proteins Ribosomes contain rRNA and protein. A ribosome is composed of two subunits وحدتين that combine تتحد to carry out protein synthesis تخليق البروتين. مرتبطة حرة

In the nucleolus, ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits. The subunits pass from the nuclear pores to the cytoplasm where they combine to form ribosomes. Cell types that synthesize large quantities of proteins (e.g., pancreas) have large numbers of ribosomes. Types of Ribosomes:- Free ribosomes are suspended معلق in the cytosol and synthesize proteins that function within the cytosol. Bound ribosomes are attached to ملتصق بـ the outside of the endoplasmic reticulum. These synthesize proteins that are either included into membranes or for secretion outside the cell.

3- The Endomembrane System The endoplasmic reticulum الشبكة الإندوبلازمية manufacturers membranes and performs many other biosynthetic functions The Golgi apparatus جهاز جولـﭽـىfinishes, sorts, and ships cell products The Lysosomes اليسوزوماتare digestive compartments The Vacuoles الفجواتhave diverse functions in cell maintenance

A)- The endoplasmic reticulum (ER) manufacturers membranes and performs many other biosynthetic functions The ER includes membranous tubules There are two types of ER that differ in structure and function. Smooth ER looks smoothناعمة because it lacks ribosomes. Rough ER looks rough خشنة because ribosomes (bound ribosomes) are attached to its outside.

The smooth ER: الشبكة الإندوبلازمية الناعمة - It is rich in enzymes and plays a role in metabolic processes. - Its enzymes synthesize lipids دهون (oils, phospholipids, and steroids) including the sex hormones الهرمونات الجنسية. - Extensive توجد بكثرة in the liver, it helps detoxify إبطال الأثر السام drugs المخدرات and poisons السموم The rough ER: الشبكة الإندوبلازمية الخشنة - is especially abundant متوفرة in those cells that secrete proteins as it contains ribosomes. - These secretory proteins are packaged in transport vesicles that carry them to their next stage.

B)- Golgi apparatus: finishes, sorts تـُجَهز, packaging and ships تغلف وتنقل cell products Many transport vesicles أوعية ناقلة from the ER travel to the Golgi apparatus for modification تطوير of their contents. The Golgi function is manufacturing تصنيع, warehousing تغليف, sorting تـَجْهيز (Packaging), and shipping نقل materials to outside the cell. The Golgi apparatus is especially extensive in cells specialized for secretion خلايا إفرازية. The Golgi also manufactures pectin and polysaccharides.

وعاء ناقل

C)- Lysosomes الأجسام المُحللة are digestive components The lysosome is a membrane-bounded sac كيسof enzymes that digests macromolecules.

Lysozomal enzymes Function of Lysozomal enzymes Lysosomal enzymes work best at pH = 5 (acidic). The lysosomal enzymes are synthesized by rough ER and then transferred to تنقل إلى the Golgi then to lysosomes. Function of Lysozomal enzymes They hydrolyse يحللproteins, fats, polysaccharides, and nucleic acids. Can destroy the cell by autodigestion (autophagy) الهضم الذاتى. Can fuse with food vacuoles الفحجوة الغذائية to digest food, (when a food item is brought into the cell by phagocytosis). Can also fuse with another organelle or part of the cytosol. This process of autophagy called recycling which renews the cell

D)- Vacuoles: الفجوات have diverse functions وظائف متنوعةin cell maintenance للحفاظ على الخلية Vesicles أوعـية and vacuoles فجوات are membrane-bound sacs أكياس ذات أغشية with varied functions. Food vacuoles فجوة غذائية, from phagocytosis, fuse with lysosomes. Contractile vacuoles فجوة منقبضة , found in freshwater protists, pump excess water out of the cell. Central vacuoles فجوة مركزية are found in many mature plant cells.

الفجوة المركذية خلية نباتية خلية حيوانية

Other Membranous Organelles A)- Peroxisomes contain enzymes that transfer hydrogen from various substrates to oxygen Functions of peroxisomes 1- Hydrogen peroxide (H2O2) is a poison, but the peroxisome has enzyme that converts H2O2 to water. 2- Some peroxisomes break fatty acids down to smaller molecules that are transported to mitochondria for fuel لإنتاج الطاقة. 3- They detoxify يبطل المفعول السُّمى alcohol and other harmful compounds. Thus, it exists extensively in the liver cells

Other Membranous Organelles B)- Mitochondria: Thy are the main energy transformers of cells as they convert energy to fuel الطاقة that cells can use for work. Mitochondria are the sites of cellular respiration, generating تـُنتِج ATP from the catabolism هدم of sugars, fats, and other fuels مصادر طاقة in the presence of oxygen. Almost all eukaryotic cells have mitochondria. Mitochondria and chloroplasts are mobile and move around the cell along tracks in the cytoskeleton.

Mitochondria have a smooth outer membrane and a highly folded inner membrane forming cristae نتوءات. The inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA, ribosomes, and enzymes.