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Cell Membrane Structure & Function

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Presentation on theme: "Cell Membrane Structure & Function"— Presentation transcript:

1 Cell Membrane Structure & Function
Chapter 4 Cell Membrane Structure & Function

2 4.1 – How Is the Structure of a Membrane Related to its Function?

3 The plasma membrane isolates the cell while allowing communication with its surroundings
Three General Functions: Selectively Permeable Regulates exchange Communication

4 Membranes are Fluid Mosaics in Which Proteins Move
Fluid Mosaic Model was developed in 1972 Phospholipids act as grout for membrane proteins, which represent tiles

5 Phospholipid Bilayer is the Fluid Portion of the Membrane
Phospholipid Review Have a polar head Hydrophilic Have 2 nonpolar tails Hydrophobic Double bonds in the tail increase fluidity of membrane

6 The Plasma Membrane is a Phospholipid Bilayer
The tails point inward away from the watery environment The heads point outward toward the watery environment Membrane is “fluid” because the phospholipds are not bonded together

7 Phospholipids bilayer selectively isolates internal environment from external environment
Most biological molecules are hydrophilic and cannot pass through the membrance easily Some molecules can freely pass through the membrane

8 Cholesterol in animal cell membranes make the bilayer stronger, more flexible, less fluid & less permeable to water soluble substances Flexibility and fluid nature of the bilayer is important to its function

9 The Plasma Membrane

10 A Mosaic of Proteins is Embedded in the Membrane
Proteins embedded within or attached to the surface of the bilayer regulate the movement of substance across the membrane and communicate with the environment Many membrane proteins are glycoproteins

11 Three Categories of Membrane Proteins
Transport Proteins – regulate the movement of hydrophilic molecules through the membrane Channel Proteins – form pores and channels for small water-soluble molecules Carrier Proteins – bind molecules and move them across the membrane

12 Channel proteins do not change shape
Carrier proteins change shape

13 Three Categories of Membrane Proteins (con’t)
2. Receptor Proteins – bind molecules in the environment, triggering changes in the metabolism of the cell 3. Recognition Proteins – serve as identification tags and cell-surface attachment sites

14 4.2 – How Do Substances Move Across the Membrane

15 Molecules Move in Response to Gradients
Characteristics of a fluid Fluid – any substance that can move or change shape in response to external forces without breaking apart Concentration – number of molecules in a given unit of volume Gradient – physical difference in properties such as temperature, pressure, or concentration

16 Diffusion The movement of molecules from regions of high concentration to regions of low concentration Movement down the concentration gradient

17 Diffusion Con’t Molecules move randomly and continuously, colliding with each other, until a dynamic equilibrium exists in which there is no concentration gradient The greater the concentration gradient, the faster the rate of diffusion

18 Example of Diffusion A drop of food coloring in a glass of water

19 Movement Across Membranes Occurs by Both Passive & Active Transport
There are significant concentration gradients of ions and molecules across the plasma membrane because the cytoplasm is very different from the extracellular fluid

20 Movement Across the Membrane Occurs by:
Passive Transport Substances move down conc. gradient No energy is required Active Transport Substance move up conc. gradient Energy is required

21 Types of Passive Transport
Simple Diffusion Facilitated Diffusion Osmosis

22 Simple Diffusion Membranes are selectively permeable to diffusion of molecules Lipid-soluble molecules & very small molecules can easily diffuse across the membrane Rate of simple diffusion depends on conc. gradient, the size of the molecule & its lipid solubility

23 Facilitated Diffusion
Molecules cross the membrane with the help of membrane transport proteins Channel proteins Carrier proteins No energy required molecules move down the conc. gradient

24 Osmosis Osmosis is the diffusion of water
Water moves down the conc. gradient across a selectively permeable membrane Dissolved substances reduce the concentration of water molecules in solution

25 Osmosis is Important in the Life of Cells
Water balance between cells and their surroundings is crucial to organisms Three environments exist due to varying water concentrations Isotonic Hypertonic Hypotonic

26 Isotonic Cell Environment
Water concentration around the cell is the same as the water concentration inside the cell No net movement of water occurs Cell remains the same size The type of dissolved particles does not have to be the same, but the total concentration of all dissolved particles is equal

27 Isotonic Water is moving in to and out of the cell at an equal rate.

28 Hypertonic Cell Environment
The solution outside the cell has a higher concentration of solutes than the interior of the cell Lower water concentration Water will flow out of the cell by osmosis Cells shrivel and shrink

29 Hypertonic Net movement of water out of the cell Cell shrinks

30 Hypotonic Cell Environment
The solution outside the cell has a lower solute concentration than the solution inside the cell Higher water concentration Water will flow into the cell by osmosis Cells will swell and sometimes burst

31 Hypotonic Net movement of water into the cell Cells swell

32 Active Transport Uses Energy to Move Molecules
All cells need to move some substances against their conc. gradient Membrane proteins that require energy are used to move molecules against their conc. gradient Active transport proteins are sometimes called “pumps” because they move substances uphill

33 Active Transport Proteins
Active transport proteins span the width of the membrane and have 2 active sites One site binds the substance to be transported Second site binds an energy carrier molecule, usually ATP

34 Example of Active Transport

35 Cells Engulf Particles or Fluids by Endocytosis
Types of Endocytosis Pinocytosis Phagocytosis Receptor-Mediated Endocytosis

36 Pinocytosis Moves liquids into the cell Means “cell drinking”
A small patch of membrane dimples inward to form a vesicle surrounding the fluid The acquired material has the same concentration as extracellular fluid

37 Pinocytosis

38 Phagocytosis Moves large particles into the cell Means “cell eating”
Extensions of the membrane fuse around the large particle and carry it to the interior of the cell in a vacuole for intracellular digestion

39 Phagocytosis

40 Cells Move Material Out of the Cell by Exocytosis
A membrane-enclosed vesicle carrying the material to be expelled moves to the cell surface The vesicle then fuses with the plasma membrane and releases its contents

41 Exocytosis

42 4.3 – How Are Cell Surfaces Specialized?

43 Various Specialized Junctions Allow Cells to Connect and Communicate
Four types of connections occur between cells, depending on the organism and cell type Desmosomes Tight Junctions Gap Junctions Plasmodesmata

44 Desmosomes Membranes of adjacent cells are held together by proteins and carbohydrates Further strengthened by protein filaments that extend from inside the desmosome to the interior of each cell

45 Tight Junctions Membranes of adjacent cells are fused together to create leak-proof junctions

46 Gap Junctions Cell-to-cell cytoplasmic connections found in animal cells that need to communicate with each other

47 Plasmodesmata Cell-to-cell cytoplasmic connections between plant cells

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