Cell Membranes Valencia college

Transcription

1 6 Cell Membranes Valencia college

2 6 Cell Membranes Chapter objectives: The Structure of a Biological Membrane The Plasma Membrane Involved in Cell Adhesion and Recognition Passive Processes of Membrane Transport Active Processes of Membrane Transport How Large Molecules Enter and Leave a Cell Some Other Functions of Membranes

3 6.1 What Is the Structure of a Biological Membrane? The general structure of membranes is known as the fluid mosaic model composed of lipids, proteins & carbohydrates Phospholipids form a bilayer which is like a lake in which a variety of proteins float.

4 Figure 3.20 Phospholipids (Part 2)

5 Figure 3.20 Phospholipids (Part 1)

6 Figure 6.1 The Fluid Mosaic Model carbohydrate Peripheral membrane protein Integral membrane protein

7 6.1 What Is the Structure of a Biological Membrane? Membranes may vary in lipid composition. Phospholipids vary in fatty acid chain length, degree of saturation, and phosphate groups. Membranes may be up to 25 percent cholesterol. Important in membrane integrity Situated next to an unsaturated fatty acid

8 6.1 What Is the Structure of a Biological Membrane? Phospholipid bilayers are flexible, and the interior is fluid, allowing lateral movement of molecules. This Fluidity depends on temperature and lipid composition.

9 6.1 What Is the Structure of a Biological Membrane? Membranes also contain proteins; the number of proteins varies with cell function. Typically plasma membranes have 1 protein to 25 lipids In mitochondrial membrane 1 protein to 15 lipids In some neuron membranes 1 protein to 70 lipids Two types of membrane proteins: Peripheral membrane proteins lack exposed hydrophobic groups and do not penetrate the bilayer. Integral membrane proteins have hydrophobic and hydrophilic regions or domains. Some extend across the lipid bilayer; others are partially embedded.

10 Figure 6.3 Interactions of Integral Membrane Proteins

11 6.1 What Is the Structure of a Biological Membrane? Transmembrane proteins extend all the way through the phospholipid bilayer. They have one or more transmembrane domains, and the domains on the inner and outer sides of the membrane can have specific functions.

12 6.1 What Is the Structure of a Biological Membrane? The proteins and lipids in the membrane are independent of each other and only interact noncovalently and can move freely within the bilayer membrane But some membrane proteins have fatty acids or other lipid groups covalently attached and are referred to as anchored membrane proteins.

13 6.1 What Is the Structure of a Biological Membrane? Membranes are dynamic and are constantly forming, transforming, fusing, and breaking down.

14 6.1 What Is the Structure of a Biological Membrane? Membranes also have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules. Glycolipids carbohydrate + lipid Glycoproteins carbohydrate + protein

15 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? Cell to Cell adhesion & recognition Cells can arrange themselves in groups involving the plasma membrane by: 1. Cell recognition Like cells bind 2. Cell adhesion. Connection between two cells is strengthened

16 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? Molecules involved in cell recognition and binding are glycoproteins (glycocalyx) Binding of cells is usually homotypic: The same molecule sticks out from both cells and forms a bond. Some binding is heterotypic: The cells have different proteins; example, sperm & egg

17 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? Cell junctions are specialized structures that hold cells together: Tight junctions Desmosomes Gap junctions

18 Figure 6.7 Junctions Link Animal Cells Together (A) Tight junctions help ensure directional movement of materials.

19 Figure 6.7 Junctions Link Animal Cells Together (B) Desmosomes are like spot welds.

20 Figure 6.7 Junctions Link Animal Cells Together (C) Gap junctions allow communication.

21 6.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition? Cell membranes also adhere to the extracellular gelatinous matrix composed of collagen protein arranged in fibers The transmembrane protein integrin binds to the matrix outside epithelial cells, and to actin filaments inside the cells. The binding is noncovalent and reversible. Important in maintaining tissue integrity

22 Figure 6.8 Integrins Mediate the Attachment of Animal Cells to the Extracellular Matrix

23 6.3 What Are the Passive Processes of Membrane Transport? Membranes have selective permeability some substances can pass through, but not others. 1.Passive transport no outside energy required (diffusion). 2.Active transport energy required.

24 6.3 What Are the Passive Processes of Membrane Transport? Diffusion: The process of random movement of particles toward equilibrium. Equilibrium is achieved when particles can continue to move, but there is no net change in distribution.

25 Figure 6.9 Diffusion Leads to Uniform Distribution of Solutes Addition of dyes equally Amount of each colored dye Molecules of each dye

26 6.3 What Are the Passive Processes of Membrane Transport? Net movement is directional until equilibrium is reached. Diffusion is the net movement from regions of greater concentration to regions of lesser concentration.

27 6.3 What Are the Passive Processes of Membrane Transport? Diffusion rate depends on: Diameter of the molecules or ions Smaller molecules diffuse faster Temperature of the solution Higher temps faster diffusion Concentration gradient Greater concentration more rapid the diffusion

28 6.3 What Are the Passive Processes of Membrane Transport? Simple diffusion: Small molecules pass through the lipid bilayer. Water and lipid-soluble molecules can diffuse across the membrane. Oxygen and Carbon dioxide Electrically charged and polar molecules can not pass through easily amino acids, sugars, ions

29 6.3 What Are the Passive Processes of Membrane Transport? Osmosis: The diffusion of water. Osmosis depends on the number of solute particles present, not the type of particles.

30 Figure 6.10 Osmosis Can Modify the Shapes of Cells (Part 1)

31 Figure 6.10 Osmosis Can Modify the Shapes of Cells (Part 2)

32 Figure 6.10 Osmosis Can Modify the Shapes of Cells (Part 3)

33 6.3 What Are the Passive Processes of Membrane Transport? If two solutions are separated by a membrane that allows water, but not solutes to pass through: Water will diffuse from the region of higher water concentration (lower solute concentration) to the region of lower water concentration (higher solute concentration).

34 6.3 What Are the Passive Processes of Membrane Transport? Water will diffuse (net movement) from a hypotonic solution across a membrane to a hypertonic solution. Animal cells may burst when placed in a hypotonic solution. Plant cells with rigid cell walls build up internal pressure that keeps more water from entering turgor pressure.

35 6.3 What Are the Passive Processes of Membrane Transport? Facilitated diffusion Polar molecules; such as, water, ions, a.a., sugars do not readily pass diffuse across membranes, but can cross the hydrophobic phospholipid bilayer passively in two ways: Channel proteins have a central pore lined with polar amino acids. Carrier proteins are membrane proteins that bind to some substances and speed their diffusion through the bilayer.

36 6.3 What Are the Passive Processes of Membrane Transport? Channel Proteins Ion channels: Specific channel proteins with hydrophilic pores. Most are gated can be closed or open to ion passage. Gate opens when protein is stimulated to change shape. Stimulus can be: a molecule (ligand-gated) or electrical charge resulting from many ions (voltage-gated).

37 Figure 6.11 A Ligand Gated Channel Protein Opens in Response to a Stimulus Polar substance

38 6.3 What Are the Passive Processes of Membrane Transport? All cells maintain an imbalance of ion concentrations across the plasma membrane; thus a small voltage potential exists termed membrane potential Rate and direction of ion movement through channels depends on the concentration gradient and the distribution of electrical charge.

39 6.3 What Are the Passive Processes of Membrane Transport? Membrane potential is a charge imbalance across a membrane. Measured membrane potential of animal cells: 70 mv (lots of potential energy)! Membrane potential is related to the concentration imbalance of K + Whose concentration is greater inside the cell then outside because of active transport

40 6.3 What Are the Passive Processes of Membrane Transport? The potassium channel allows K + but not Na + to pass through. K + passes through in the unhydrated state; hydrated Na + is too large to pass.

41 Figure 6.12 The Potassium Channel Hydrated Na + ions K +

42 6.3 What Are the Passive Processes of Membrane Transport? Aquaporins Water can cross a membrane by hitchhiking with hydrated ions, or moving through special water channels called aquaporins Red blood cells, kidney cells & plant cells

43 6.3 What Are the Passive Processes of Membrane Transport? Carrier proteins In facilitated diffusion, carrier proteins can transport polar molecules such as glucose across membranes in both directions. Glucose binds to the carrier protein, which causes it to change shape and release glucose on the other side.

44 Figure 6.14 A Carrier Protein Facilitates Diffusion (Part 1)

45 6.4 What Are the Active Processes of Membrane Transport? Active transport: Can move substances against a concentration and/or electrical gradient, but requires energy. The energy source is most often adenosine triphosphate (ATP), which is produced in the mitochondria

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47 6.4 What Are the Active Processes of Membrane Transport? Active transport across the membrane is directional. It involves three kinds of proteins: Uniporters Symporters Antiporters

48 Figure 6.15 Three Types of Proteins for Active Transport Uniporter Symporter Antiporter Ca +2 Na + a.a. Na-K pump

49 6.4 What Are the Active Processes of Membrane Transport? Two types of Active Transport Primary active transport: Requires direct hydrolysis of ATP. (ATP + H 2 O ADP + energy) i.e.: Sodium-Potassium pump Secondary active transport: Energy comes from an ion concentration & electrical gradient that is initiated by primary active transport. i.e.: absorption of glucose from the digestive system into the blood stream

50 6.4 What Are the Active Processes of Membrane Transport? The Sodium Potassium (Na + K + ) pump is a primary active transport system. Found in all animal cells. The pump is an integral membrane glycoprotein (an antiporter). Helps maintain the electrochemical gradient

51 Figure 6.16 Primary Active Transport: The Sodium Potassium Pump 1) 3 Na + 1 ATP bind to protein pump

52 6.5 How Do Large Molecules Enter and Leave a Cell? Macromolecules (proteins, polysaccharides, nucleic acids) are too large to cross the membrane. They can be taken in or secreted by means of membrane vesicles.

53 6.5 How Do Large Molecules Enter and Leave a Cell? Three types of Endocytosis 1. Phagocytosis A mechanism to bring solid particles into the cell by means of vesicles by pinching off from the plasma membrane 2. Pinocytosis Processes that bring liquids or dissolved substances into cells into a eukaryotic cell. 3. Receptor mediated endocytosis Molecules on the cell surface recognize & triggers uptake of specific materials

54 Figure 6.18 Endocytosis and Exocytosis (A)

55 6.5 How Do Large Molecules Enter and Leave a Cell? Phagocytosis: Molecules or entire cells are engulfed. Some protists feed in this way. Some white blood cells engulf foreign substances A food vacuole or phagosome forms, which fuses with a lysosome.

56 6.5 How Do Large Molecules Enter and Leave a Cell? Pinocytosis: A vesicle forms to bring small dissolved substances or fluids into a cell. Vesicles are much smaller than in phagocytosis. Pinocytosis is constant in endothelial (capillary) cells.

57 6.5 How Do Large Molecules Enter and Leave a Cell? Receptor mediated endocytosis is highly specific: Depends on receptor proteins integral membrane proteins to bind to specific substances. Sites are called coated pits coated with other proteins such as clathrin.

58 Figure 6.19 Receptor-Mediated Endocytosis (Part 1)

59 Figure 6.19 Receptor-Mediated Endocytosis (Part 2)

60 6.5 How Do Large Molecules Enter and Leave a Cell? Mammalian cells take in cholesterol by receptor-mediated endocytosis. In the liver, cholesterol is packaged into lowdensity lipoprotein, or LDL, and secreted to the bloodstream. (Cholesterol & triglycerides are water insoluble) Cells that need cholesterol have receptors for the LDLs in clathrin-coated pits.

61 6.5 How Do Large Molecules Enter and Leave a Cell? Exocytosis: Material in vesicles is expelled from a cell. Indigestible materials are expelled. Important in the secretion of digestive enzymes from the pancreas and neurotransmitters from neurons.

62 Figure 6.18 Endocytosis and Exocytosis (B)

63 6.6 What Are Some Other Functions of Membranes? Other Membrane Functions are electrically excitable: The plasma membrane of neurons are electrically excitable allowing nerve impulse conduction. In muscle cells, electrical excitation results in muscle contraction.

64 6.6 What Are Some Other Functions of Membranes? Some membranes transform energy: Inner mitochondrial membranes energy from fuel molecules is transformed to ATP. Thylakoid membranes of chloroplasts transform light energy to chemical bonds.

65 Figure 6.20 Other Membrane Functions (Part 1) A pigment attached to a membrane protein absorbs energy Energy Transformation

66 6.6 What Are Some Other Functions of Membranes? Some membrane proteins process information: Binding of a specific ligand can initiate, stop, or modify cell functions.

67 Figure 6.20 Other Membrane Functions (Part 3) Signal binding induces change in receptor protein causing some effect inside the cell

68 7 Cell Signaling and Communication Valencia College

69 7 Cell Signaling and Communication Chapter Objectives: What Are the Signals, and How Do Cells Respond to Them? How Do Signal Receptors Initiate a Cellular Response? How Is the Response to a Signal Transduced through the Cell? How Do Cells Change in Response to Signals? How Do Cells Communicate Directly?

70 7.1 What Are Signals, and How Do Cells Respond to Them? All cells, prokaryote & eukaryote, process information from the environment. The information can be a chemical, or a physical stimulus such as light, smell, taste, temperature, sound. Signals can come from outside the organism, or from neighboring cells.

71 7.1 What Are Signals, and How Do Cells Respond to Them? To respond to a signal, a cell must have a specific receptor that can detect it. A signal transduction pathway is the sequence of molecular events and chemical reactions that lead to a cell s response to a signal.

72 7.1 What Are Signals, and How Do Cells Respond to Them? In a large multicellular organism, signals reach target cells by diffusion or by circulation in the blood. Autocrine signals affect the cells that made them. Paracrine signals affect nearby cells. Hormones travel to distant cells, usually via the circulatory system.

73 Figure 7.1 Chemical Signaling Systems Autocrine Paracrine Hormone

74 7.1 What Are Signals, and How Do Cells Respond to Them? A signal transduction pathway involves: 1. Signal 2. Receptor 3. Response

75 Figure 7.2 A Signal Transduction Pathway Signals released from other tissues Signal binding changes the three dimensional shape of the receptor & exposes its active site Protein kinase activity

76 7.1 What Are Signals, and How Do Cells Respond to Them? A signal transduction pathway: The signal causes a receptor protein to change conformation Conformation change gives it protein kinase activity Phosphorylation alters function of a responder protein

77 7.1 What Are Signals, and How Do Cells Respond to Them? The signal is amplified A protein that binds to DNA is activated Expression of one or more genes is turned on or off Cell activity is altered