Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

Question 1

large molecules—such as proteins and polysaccharides, as well as larger particles—generally cross the membrane in bulk, packaged in

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vesicles.

Question 2

the cell secretes certain molecules by the fusion of vesicles with the plasma membrane; this process is called

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exocytosis

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the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane

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endocytosis

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three types of endocytosis

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“cellular eating”), pinocytosis (“cellular drinking”), and receptor-mediated endocytosis.

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Human cells use receptor-mediated endocytosis to take in

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cholesterol for membrane synthesis and the synthesis of other steroids

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Cholesterol travels in the blood in particles called

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low-density lipoproteins (LDLs)

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is the movement of materials into a cell via vesicles that form from the plasma membrane

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endocytosis

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is the movement of materials out of a cell by the fusion of vesicles with the plasma membrane

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exocytosis

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these processes enable patches of membrane to flow back and forth between the

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interior cell and the plasma membrane

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exocytosis is the movement of vesicles in the cell to the plasma membrane, where they fuse with the membrane and release their contents into the

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surrounding fluid

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this process occurs predominantly in

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secretory cells, such as mucus producing cells or pancreatic cells that secrete enzymes into the digestive tract

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endocytosis, the plasma membrane invaginates, or pinches in, forming a vesicles that moves the enclosed materials inside the

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cell

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there are three types of endocytosis each involving its own

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specific cell machinery

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the cell engulfs debris, bacteria, or other particles by extending pseudopodia around the material

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phagocytosis or cellular eating

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produces a food vacuole.

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invagination

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the material inside the vacuole will be digested after the food vacuole fuses with a

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lysosome containing hydrolytic enzymes

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occurs in amoebas and in immune cells such as macrophages and neutrophils in mammals

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phagocytosis

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the cell engulfs extracellular fluid, including molecules such as sugars and proteins

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pinocytosis, or cellular drinking

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sugars and proteins materials enter the cell inside a

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vesicle

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epithelial cells in capillaries use pinocytosis to engulf the liquid portion of

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blood at the capillary surface

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the resulting vesicles travel across the capillary cells and release their contents to surrounding

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tissues while blood cells remain in blood

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receptor-mediated endocytosis is very specific, it triggered when

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membrane receptors bind to specific external molecules such as protein-cholesterol complexes or protein bound to iron

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membrane vesicles pinch off, and the external receptor and its cargo are brought into the

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cell

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the plasma membrane is selectively permeable, some molecules can move across it , while others

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cannot

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lipids can dissolve in the

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lipid bilayer

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lipids move down their concentration gradient from where they are more concentrated to where they are

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less concentrated

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is a form of passive transport- it does not require energy from the cell

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diffusion

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most molecules can’t cross the

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lipid bilayer

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facilitated diffusion doesn’t require energy from the cell so it’s also a form of

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passive transport

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water crosses the plasma membrane by facilitating

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diffusion or diffusing across the lipid bilayer directly

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the diffusion of water across a membrane is called

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osmosis

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the sodium-potassium pump moves ions against their concentration gradient, from where they are

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less concentrated to where they are more concentrated

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this requires energy from the cell and is known as

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active transport

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energy from ATP is used to move

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sodium ions out of the cell and potassium ions in

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another type of active transport is

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cotransport

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both sodium ions and glucose move into the cell through a

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cotransport protein

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sodium ions move down the concentration gradient created by the

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sodium-potassium pump and glucose moves against its concentration gradient

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in intestinal cell materials can be exported in vesicles that fuse with the plasma membrane and release their contents outside the cell, this process is called

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exocytosis

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the plasma membrane pinches in, forming a vesicle that contains material from outside the cell

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endocytosis

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we can also see oxygen and carbon dioxide on the side of the cell diffusing across the

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lipid bilayer