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Eukaryotic cells are organised around systems of membranes Cells are surrounded by a plasma membrane within which is contained a complex fluid called the cytosol Suspended in the cytosol are numerous specialised membrane-bound structures called organelles The cytosol together with the organelles make up the cytoplasm The plasma membrane serves as a boundary between the cell and its environment It retains the cell as a unit and, due to its partial permeability, regulates the movement of materials into and out of the cell Plasma membranes vary in thickness between 7.5 and 10 nm (nanometres m) Plasma Membrane Cytosol

Eukaryotic cells are organised around systems of membranes Plasma membranes vary in thickness between 7.5 and 10 nm (nanometres m) Plasma Membrane Cytosol The plasma membranes of animals cells are often highly folded and form microvilli Microvilli increase the surface area available for the exchange of materials Membranes are composed primarily of phospholipid and protein molecules Microvilli At magnifications of around , membranes are seen to have a three-layered structure magnify

three-layered membrane structure The three-layered structure is seen as two thin dense outer layers surrounding a thicker but less dense middle region cytosol extra- cellular fluid

Animal cells, with the exception of mammalian red blood cells, possess a nucleus The nucleus is a large spherical or oval organelle surrounded by a double membrane called the nuclear envelope The nuclear envelope is perforated by nuclear pores The fluid contained within the nucleus is called nucleoplasm Chromatin appears as granular material in the nucleoplasm During cell division chromatin condenses into more tightly coiled threads called chromosomes The nucleus contains the hereditary material and controls the activities of the cell Nuclear Envelope Nuclear Pore The Nucleolus is a small densely staining disc within the nucleus Nucleoplasm The nucleus contains the hereditary material in the form of chromatin in non-dividing cells

DNA is the material of heredity; DNA associated with histone protein molecules forms the chromatin of the non-dividing cell Nuclear Envelope (double membrane) Nuclear pore Nucleoplasm Nucleolus Chromatin - granular appearance of the hereditary material in non-dividing cells Coiled and condensed chromatin forms distinctive thread-like chromosomes during cell division

The nuclear envelope is a double-layered structure consisting of two adjacent membranes, nm apart, forming a perinuclear space Ribosomes are often found attached to the external membrane The pores enable communication to occur between the nucleus and the cytoplasm perinuclear space two membranes (envelope) The envelope is perforated by pores, each about nm in diameter ribosomes

mitochondrion nuclear envelope nucleolus chromatin

Each mitochondrion consists of an outer smooth membrane and an inner folded membrane Much of the process of aerobic respiration occurs within the mitochondria Mitochondria are loosely termed the ‘powerhouses of the cell’ MORE DETAIL MORE DETAIL Mitochondria are rod-shaped organelles concerned with energy production

The folds are called cristae Inner folded membrane Outer smooth membrane Fluid matrix of outer compartment Fluid matrix of inner compartment The surfaces of the cristae are studded with minute spheres

outer membrane cristae fluid matrix endoplasmic reticulum

MORE DETAIL MORE DETAIL Extending from the nuclear membrane and continuous with it is the rough endoplasmic reticulum Rough endoplasmic reticulum neatly compartmentalises the cell and provides a transport system for newly synthesised proteins Rough endoplasmic reticulum is studded with ribosomes which are the sites of protein synthesis Free ribosomes are also found within the cytosol Reticulum means network and the rough endoplasmic reticulum is an extensive network of flattened sacs that extend throughout the cell

Proteins are synthesised at the ribosomes and engulfed into the cavities of the endoplasmic reticulum Newly synthesised proteins are transported to different parts of the cell Lamellae or flattened sacs of the endoplasmic reticulum, each made up of two membranes Cavity of endoplasmic reticulum Ribosomes - sites of PROTEIN SYNTHESIS junction between the lamellae of the endoplasmic reticulum

cavity of endoplasmic reticulum ribosome

A second type of endoplasmic reticulum also occurs within the cytosol Smooth endoplasmic reticulum consists of a system of branching tubes and is concerned with the synthesis of steroids and fatty acids Unlike the rough endoplasmic reticulum, there are no ribosomes associated with the smooth endoplasmic reticulum SMOOTH ENDOPLASMIC RETICULUM This is the smooth endoplasmic reticulum

Proteins and lipids are modified and packaged within the Golgi Apparatus in preparation for distribution within the cell or export from the cell Materials are concentrated and packaged into vesicles that appear as ‘pinched off’ portions of the Golgi Apparatus The vesicles form a satellite around the stacked membranes The principal role of the Golgi Apparatus is that of secretion from the cell Secretory cells possess well developed Golgi Bodies The Golgi Apparatus or Golgi Body is a stack of 4 to 8 membrane-bound sacs located close to the nucleus

The Golgi Apparatus exists as stacks of closely packed membrane bounded sacs or cisternae Clusters of ‘pinched off’ portions of the Golgi body form a satellite of vesicles around the array of cisternae The Golgi Apparatus is highly developed in cells secreting proteins or complex carbohydrates cisternae satellite of vesicles

stacked membranes Golgi vesicles

Destruction of bacteria and other debris taken into the cell Destruction of ageing, redundant cell organelles Destruction of the entire cell when damaged or diseased MORE DETAIL MORE DETAIL Release of the digestive enzymes FROM the cell to degrade other cells Lysosomes perform a number of functions Lysosomes are membrane bound organelles that contain digestive enzymes known as acid hydrolases

DESTRUCTION OF BACTERIAL CELLS The plasma membrane invaginates and engulfs the bacterial cell by a process known as ENDOCYTOSIS

A phagocytic vacuole containing the engulfed bacterial cell is formed A primary lysosome now fuses with the phagocytic vacuole DESTRUCTION OF BACTERIAL CELLS The plasma membrane invaginates and engulfs the bacterial cell by a process known as ENDOCYTOSIS

A secondary lysosome forms within which digestion of the bacterial cell takes place A phagocytic vacuole containing the engulfed bacterial cell is formed A primary lysosome now fuses with the phagocytic vacuole DESTRUCTION OF BACTERIAL CELLS The plasma membrane invaginates and engulfs the bacterial cell by a process known as ENDOCYTOSIS

Some of the products of digestion are absorbed into the cytosol A secondary lysosome forms within which digestion of the bacterial cell takes place A phagocytic vacuole containing the engulfed bacterial cell is formed A primary lysosome now fuses with the phagocytic vacuole DESTRUCTION OF BACTERIAL CELLS The plasma membrane invaginates and engulfs the bacterial cell by a process known as ENDOCYTOSIS

Some of the products of digestion are absorbed into the cytosol A secondary lysosome forms within which digestion of the bacterial cell takes place A phagocytic vacuole containing the engulfed bacterial cell is formed A primary lysosome now fuses with the phagocytic vacuole DESTRUCTION OF BACTERIAL CELLS The plasma membrane invaginates and engulfs the bacterial cell by a process known as ENDOCYTOSIS The undigested remains are retained within the lysosome which is now termed the RESIDUAL BODY The residual body migrates to the plasma membrane releasing its contents by EXOCYTOSIS

Some of the products of digestion are absorbed into the cytosol A secondary lysosome forms within which digestion of the bacterial cell takes place A phagocytic vacuole containing the engulfed bacterial cell is formed A primary lysosome now fuses with the phagocytic vacuole DESTRUCTION OF BACTERIAL CELLS The plasma membrane invaginates and engulfs the bacterial cell by a process known as ENDOCYTOSIS The undigested remains are retained within the lysosome which is now termed the RESIDUAL BODY The residual body migrates to the plasma membrane releasing its contents by EXOCYTOSIS This example of lysosome activity is a form of Intracellular Digestion

DESTRUCTION OF AGEING ORGANELLES Another function of lysosomes concerns cytoplasmic turnover When a cell organelle ages or becomes redundant, it becomes surrounded by a membrane to form a vacuole A primary lysosome fuses with this vacuole and forms a secondary lysosome

DESTRUCTION OF AGEING ORGANELLES Another function of lysosomes concerns cytoplasmic turnover When a cell organelle ages or becomes redundant, it becomes surrounded by a membrane to form a vacuole A primary lysosome fuses with this vacuole and forms a secondary lysosome The organelle is degraded and some of the products of digestion are absorbed into the cytosol The undigested remains are retained within the lysosome which is now termed the RESIDUAL BODY The residual body migrates to the plasma membrane releasing its contents by EXOCYTOSIS

DESTRUCTION OF AGEING ORGANELLES Another function of lysosomes concerns cytoplasmic turnover When a cell organelle ages or becomes redundant, it becomes surrounded by a membrane to form a vacuole A primary lysosome fuses with this vacuole and forms a secondary lysosome The organelle is degraded and some of the products of digestion are absorbed into the cytosol The undigested remains are retained within the lysosome which is now termed the RESIDUAL BODY The residual body migrates to the plasma membrane releasing its contents by EXOCYTOSIS

EXTRACELLULAR ACTION OF LYSOSOMES Sometimes the enzymes of primary lysosomes are released from the cell and therefore act EXTRACELLULARLY Primary Lysosomes

EXTRACELLULAR ACTION OF LYSOSOMES Sometimes the enzymes of primary lysosomes are released from the cell and therefore act EXTRACELLULARLY Certain types of bone cell (osteoclasts) release such enzymes to destroy cartilage tissue which can then be replaced by bone This activity occurs during development THE DIGESTIVE ENZYMES OF THE LYSOSOMES MAY ACT OUTSIDE THE CELL AND PERFORM EXTRACELLULAR FUNCTIONS Degradative Enzyme Molecules

DESTRUCTION OF THE ENTIRE CELL When a cell becomes damaged or diseased, all the lysosomal membranes become leaky Digestive enzymes flood the entire cell and cell death follows

DESTRUCTION OF THE ENTIRE CELL This function of lysosomes in bringing about cell death is called AUTOLYSIS Lysosomes are loosely termed the SUICIDE BAGS of the cell When a cell becomes damaged or diseased, all the lysosomal membranes become leaky Digestive enzymes flood the entire cell and cell death follows

Many animal cells display a dense area of cytoplasm close to the nucleus called the centrosome The centrosome contains a pair of cylindrical structures called centrioles positioned at right angles to one another Centrioles are involved in cell division when they generate spindle fibres, which are responsible for the separation of chromosomes MORE DETAIL MORE DETAIL centrioles

The centriole cylinder consists of nine sets of tube-like structures that together form the wall of the cylinder Each tube-like structure is composed of three tiny tubes called MICROTUBULES The microtubules are arranged in TRIPLETS CENTRIOLES PLAY A PART IN CELL DIVISION IN ANIMAL CELLS TRIPLET OF MICROTUBULES

Plasma membrane MicrovilliCytosol Nucleus Rough endoplasmic reticulum Golgi body Mitochondrion Smooth endoplasmic reticulum Lysosome Free ribosomes Centrioles Microtubules

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