Cell Organelles Continuo……..

5)- Peroxisomes: generate and degrade H2O2 in performing various metabolic functions Peroxisomes contain enzymes that transfer hydrogen from various substrates to oxygen An intermediate product of this process is hydrogen peroxide (H2O2), a poison, but the peroxisome has another enzyme that converts H2O2 to water. Some peroxisomes break fatty acids down to smaller molecules that are transported to mitochondria for fuel. Others detoxify alcohol and other harmful compounds. Thus, it exists extensively in the liver cells

The Cytoskeleton الهيكل الخلوى Microtubules and microfilaments The Cytoskeleton الهيكل الخلوى A network of fibres شبكة من الالياف that provide structural support تدعيم to the cell. The cytoskeleton also functions in cell motility تحرك الخلية and regulation Pages 126 - 131

The Cytoskeleton الهيكل الخلوى The cytoskeleton is a network of fibers extending throughout تمتد عبر the cytoplasm. The cytoskeleton organizes يرتب the structures and activities of the cell. Fig. 7.20, 126

Cytoskeleton A-Microtubules & filaments Microfilaments Intermediate filaments Microtubules Thick Thin Middle سميكة رفيعة متوسطة Responsible for cell motility, and separation of chromosome during cell division. (Tubulin protein) Support cell motility and transport materials within the cell. (Actin protein) Reinforcing the cell shape and fixing position of organelles. (Fibrous protein) Page 126 - 131

Page 127

The Cytoskeleton الهيكل الخلوى Cytoskeleton provide structural support to the cell, and also functions in cell motility and regulation The cytoskeleton provides mechanical support and maintains shape of the cell. The cytoskeleton is dynamic, dismantling in one part and reassembling in another to change cell shape. The cytoskeleton also plays a major role in cell motility by interacting with motor proteins. In cilia and flagella motor proteins pull components of the cytoskeleton past each other. This is also true in muscle cells. Fig. 7.21a, Page 126

The Cytoskeleton الهيكل الخلوى Cytoskeleton provide structural support to the cell, and also functions in cell motility and regulation Interactions of motor proteins and the cytoskeleton circulates materials within the cell. The cytoskeleton may transmit mechanical signals that rearrange the nucleoli and other structures. Motor molecules also carry vesicles or organelles to various destinations w إلى أماكن مختلفة provided by the cytoskeleton. Fig. 7.21b, Page 126

Microtubule Organelle P Motor Protein P Energy Microtubules functions as tracks قضيب that guide motor proteins carrying organelles to their destination المكان المستهدف. They move chromosomes during cell division

During cell division the centrioles replicate. In many cells, microtubules grow out from a centrosome near the nucleus. In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. During cell division the centrioles replicate. Fig. 7.22, Page 128

Cilia and Flagella Microtubules are the central structural supports both cilia الأهداب and flagella الأسواط. Both can move unicellular and small multicellular organisms by propelling water outside the organism. Cilia usually occur in large numbers on the cell surface. Flagella usually occur in just one or a few per cell. Cilia move more like oars مجادبف with alternating power and recovery strokes. Flagella have an undulatory movement حركة تموجية. So, They differ in their beating pattern أسلوب الحركة. Fig. 7.23, Page 129

cilia flagellum

Both cilia and flagella have the same ultrastructure التركيب الدقيق. Both have a core مركز of microtubules sheathed by the plasma membrane. 9-doublets (9 + 2 pattern) تسعة مجموعات كل منها مكون من أنيبيبتان of microtubules arranged around a pair at the center. Flexible “wheels” of proteins connect outer doublets to each other and to the core. The outer doublets are also connected by motor proteins. The structure of cilium and flagellum is identical to that of centriole. Fig. 7.24

Cilia and flagella are formed of arms of a motor protein (dynein بروتين الداينين). Dynein arms alternately grab, move, and release the outer microtubules. Protein cross-links limit sliding and the force is expressed as bending إلتواء. Fig. 7.25

Thus, Many prokaryotes are motile About half of all prokaryotes are capable of directional movement الحركة الموجهة by the following:- By the flagella الأســواط, scattered مبعثرة over the entire surface, is the most common شائعة method of movement. By two or more filaments خيوط. By threads خيوط that anchors تربط the cells to the substratum السطح الذى تعيش عليه. Fig. 27.7, Page 530 Page 529

In uniform environment بيئة متجانسة, a flagellated cell may wander randomly. In a heterogenous environment بيئة مختلطة, many prokaryotes are capable of taxis حركة موجهة, movement toward or away from a stimulus عامل مؤثر. There are two kinds of taxis: Chemotaxis, تحت تأثير كيميائى حركة موجهة : movement toward the source (positive chemotaxis) or away (negative chemotaxis). Phototaxis تحت تأثير ضوئى حركة موجهة : movement toward the source of light (negative or positive).

7- Cell membrane The plasma membrane functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the whole volume of the cell. Fig. 7.6, Page 113, and Fig. 7.29 Page 133

Cell membrane Composed of lipids (phospholipids) and proteins Lipid layer contains hydrophilic and hydrophobic regions Carbohydrate chains Hydrophilic محب للماء Phospholipid Hydrophobic كاره للماء Proteins Fig. 7.6 Page 113, and Fig. 7.29 Page 138

Prokaryotes Eukaryotes Comparison between Prokaryotes and Eukaryotes Term Prokaryotes Eukaryotes Size 1-10 um in diameter 10-100 um in diameter Cell wall Existed In plant cell (not animal cell) nucleus No nuclear envelope but Nucleoid True nucleus exists with nuclear envelope DNA As fibre in the nucleoid region (plasmids in some cases) As Chromatin (DNA and protein) Specialized Organells Most of them are absent All are existed Cell division By Binary Fission Meiotic and/or Mitotic