Cells: The Living Units

Transcription

1 Chapter 3 Part A Cells: The Living Units Annie Leibovitz/Contact Press Images PowerPoint Lecture Slides prepared by Karen Dunbar Kareiva Ivy Tech Community College

2 Why This Matters Understanding the structure of the body s cells explains why the permeability of the plasma membrane can affect treatment

3 Video: Why This Matters

4 3.1 Cells: The Living Units Cell theory A cell is the structural and functional unit of life How well the entire organism functions depends on individual and combined activities of all of its cells Structure and function are complementary Biochemical functions of cells are dictated by shape of cell and specific subcellular structures Continuity of life has cellular basis Cells can arise only from other preexisting cells

5 3.1 Cells: The Living Units Cell diversity Over 200 different types of human cells Types differ in size, shape, and subcellular components; these differences lead to differences in functions

6 Figure 3.1 Cell diversity. Erythrocytes Epithelial cells Fibroblasts Skeletal muscle cell Smooth muscle cells Cells that connect body parts, form linings, or transport gases Cells that move organs and body parts Macrophage Fat cell Nerve cell Cell that stores nutrients Cell that fights disease Cell that gathers information and controls body functions Cell of reproduction Sperm

7 3.1 Cells: The Living Units Generalized cell All cells have some common structures and functions Human cells have three basic parts: 1. Plasma membrane: flexible outer boundary 2. Cytoplasm: intracellular fluid containing organelles 3. Nucleus: DNA containing control center

8 Figure 3.2 Structure of the generalized cell. Chromatin Nucleolus Nuclear envelope Nucleus Smooth endoplasmic reticulum Cytoplasm Plasma membrane Mitochondrion Lysosome Centrioles Centrosome matrix Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis

9 Extracellular Materials Substances found outside cells Classes of extracellular materials include: Extracellular fluids (body fluids), such as: Interstitial fluid: cells are submersed (bathed) in this fluid Blood plasma: fluid of the blood Cerebrospinal fluid: fluid surrounding nervous system organs Cellular secretions (e.g., saliva, mucus) Extracellular matrix: substance that acts as glue to hold cells together

10 Part 1 Plasma Membrane Acts as an active barrier separating intracellular fluid (ICF) from extracellular fluid (ECF) Plays dynamic role in cellular activity by controlling what enters and what leaves cell Also known as the cell membrane

11 3.2 Structure of Plasma Membrane Consists of membrane lipids that form a flexible lipid bilayer Specialized membrane proteins float through this fluid membrane, resulting in constantly changing patterns Referred to as fluid mosaic (made up of many pieces) pattern Surface sugars form glycocalyx Membrane structures help to hold cells together through cell junctions

12 Figure 3.3 The plasma membrane. Extracellular fluid (watery environment outside cell) Glycocalyx (carbohydrates) Lipid bilayer containing proteins Outward-facing layer of phospholipids Inward-facing layer of phospholipids Polar head of phospholipid molecule Nonpolar tail of phospholipid molecule Cholesterol Glycolipid Glycoprotein Functions of the Plasma Membrane: Mechanical barrier: Separates two of the body s fluid compartments. Selective permeability: Determines manner in which substances enter or exit the cell. Electrochemical gradient: Generates and helps to maintain the electrochemical gradient required for muscle and neuron function. Integral proteins Communication: Allows cell-to-cell recognition (e.g., of egg by sperm) and interaction. Cell signaling: Plasma membrane proteins interact with specific chemical messengers and relay messages to the cell interior. Filament of cytoskeleton Peripheral proteins Cytoplasm (watery environment inside cell)

13 Animation: Membrane Structure

14 Membrane Lipids Lipid bilayer is made up of: 75% phospholipids, which consist of two parts: Phosphate heads: are polar (charged), so are hydrophilic (water-loving) Fatty acid tails: are nonpolar (no charge), so are hydrophobic (water-hating) 5% glycolipids Lipids with sugar groups on outer membrane surface 20% cholesterol Increases membrane stability

15 Membrane Proteins Allow cell communication with environment Make up about half the mass of plasma membrane Most have specialized membrane functions Some float freely, and some are tethered to intracellular structures Two types: Integral proteins; peripheral proteins

16 Membrane Proteins (cont.) Integral proteins Firmly inserted into membrane Most are transmembrane proteins (span membrane) Have both hydrophobic and hydrophilic regions Hydrophobic areas interact with lipid tails Hydrophilic areas interact with water Function as transport proteins (channels and carriers), enzymes, or receptors

17 Membrane Proteins (cont.) Peripheral proteins Loosely attached to integral proteins Include filaments on intracellular surface used for plasma membrane support Function as: Enzymes Motor proteins for shape change during cell division and muscle contraction Cell-to-cell connections

18 Figure 3.4 Membrane proteins perform many tasks. Transport A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. Enzymes Enzymatic activity A membrane protein may be an enzyme with its active site exposed to substances in the adjacent solution. A team of several enzymes in a membrane may catalyze sequential steps of a metabolic pathway as indicated (left to right) here. ATP Receptor Signal Receptors for signal transduction A membrane protein exposed to the outside of the cell may have a binding site that fits the shape of a specific chemical messenger, such as a hormone. When bound, the chemical messenger may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell. CAMs Intercellular joining Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (cell adhesion molecules or CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions. Attachment to the cytoskeleton and extracellular matrix Elements of the cytoskeleton (cell s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may anchor to membrane proteins, which helps maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars which help to make up the glycocalyx) serve as identification tags that are specifically recognized by other cells. Glycoprotein

19 Figure 3.4a Membrane proteins perform many tasks. Transport A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. ATP

20 Animation: Transport Proteins

21 Figure 3.4b Membrane proteins perform many tasks. Receptor Signal Receptors for signal transduction A membrane protein exposed to the outside of the cell may have a binding site that fits the shape of a specific chemical messenger, such as a hormone. When bound, the chemical messenger may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell.

22 Animation: Receptor Proteins

23 Figure 3.4c Membrane proteins perform many tasks. Attachment to the cytoskeleton and extracellular matrix Elements of the cytoskeleton (cell s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may anchor to membrane proteins, which helps maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together.

24 Animation: Structural Proteins

25 Figure 3.4d Membrane proteins perform many tasks. Enzymes Enzymatic activity A membrane protein may be an enzyme with its active site exposed to substances in the adjacent solution. A team of several enzymes in a membrane may catalyze sequential steps of a metabolic pathway as indicated (left to right) here.

26 Animation: Enzymes

27 Figure 3.4e Membrane proteins perform many tasks. Intercellular joining Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (cell adhesion molecules or CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions. CAMs

28 Figure 3.4f Membrane proteins perform many tasks. Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars which help to make up the glycocalyx) serve as identification tags that are specifically recognized by other cells. Glycoprotein

29 Glycocalyx Consists of sugars (carbohydrates) sticking out of cell surface Some sugars are attached to lipids (glycolipids) and some to proteins (glycoproteins) Every cell type has different patterns of this sugar coating Functions as specific biological markers for cellto-cell recognition Allows immune system to recognize self vs. nonself

30 Clinical Homeostatic Imbalance 3.1 Glycocalyx of some cancer cells can change so rapidly that the immune system cannot recognize the cell as being damaged. Mutated cell is not destroyed by immune system so it is able to replicate

31 Cell Junctions Some cells are free (not bound to any other cells) Examples: blood cells, sperm cells Most cells are bound together to form tissues and organs Three ways cells can be bound to each other Tight junctions Desmosomes Gap junctions

32 Cell Junctions (cont.) Tight junctions Integral proteins on adjacent cells fuse to form an impermeable junction that encircles whole cell Prevent fluids and most molecules from moving in between cells Where might these be useful in body?

33 Figure 3.5a Cell junctions. Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Interlocking junctional proteins Intercellular space Tight junctions: Impermeable junctions that form continuous seals around the cells prevent molecules from passing through the intercellular space.

34 Cell Junctions (cont.) Desmosomes Rivet-like cell junction formed when linker proteins (cadherins) of neighboring cells interlock like the teeth of a zipper Linker protein is anchored to its cell through thickened button-like areas on inside of plasma membrane called plaques Keratin filaments connect plaques intercellularly for added anchoring strength Desmosomes allow give between cells, reducing the possibility of tearing under tension Where might these be useful in body?

35 Figure 3.5b Cell junctions. Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Plaque Intermediate filament (keratin) Linker proteins (cadherins) Desmosomes: Anchoring junctions that bind adjacent cells together act like molecular Velcro and also help form an internal tension-reducing network of fibers.

36 Cell Junctions (cont.) Gap junctions Transmembrane proteins (connexons) form tunnels that allow small molecules to pass from cell to cell Used to spread ions, simple sugars, or other small molecules between cells Allows electrical signals to be passed quickly from one cell to next cell Used in cardiac and smooth muscle cells

37 Figure 3.5c Cell junctions. Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Channel between cells (formed by connexons) Gap junctions: Communicating junctions that allow ions and small molecules to pass are particularly important for communication in heart cells and embryonic cells.

38 How do substances move across the plasma membrane? Plasma membranes are selectively permeable Some molecules pass through easily; some do not Two ways substances cross membrane Passive processes: no energy required Active processes: energy (ATP) required

39 3.3 Passive Membrane Transport Passive transport requires no energy Two types of passive transport Diffusion Simple diffusion Carrier- and channel-mediated facilitated diffusion Osmosis Filtration Type of transport that usually occurs across capillary walls

40 Diffusion Collisions between molecules in areas of high concentration cause them to be scattered into areas with less concentration Difference is called concentration gradient Diffusion is movement of molecules down their concentration gradients (from high to low) Energy is not required Speed of diffusion is influenced by size of molecule and temperature

41 Figure 3.6 Diffusion. Dye pellet Diffusion occurring Dye evenly distributed

42 Diffusion (cont.) Molecules have natural drive to diffuse down concentration gradients that exist between extracellular and intracellular areas Plasma membranes stop diffusion and create concentration gradients by acting as selectively permeable barriers

43 Clinical Homeostatic Imbalance 3.2 If plasma membrane is severely damaged, substances diffuse freely into and out of cell, compromising concentration gradients Example: burn patients lose precious fluids, proteins, and ions that weep from damaged cells

44 Diffusion (cont.) Nonpolar, hydrophobic lipid core of plasma membrane blocks diffusion of most molecules Molecules that are able to passively diffuse through membrane include: Lipid-soluble and nonpolar substances Very small molecules that can pass through membrane or membrane channels Larger molecules assisted by carrier molecules

45 Diffusion (cont.) Simple diffusion Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through phospholipid bilayer Examples: oxygen, carbon dioxide, fat-soluble vitamins

46 Animation: Diffusion

47 Figure 3.7a Diffusion through the plasma membrane. Extracellular fluid Lipidsoluble solutes Cytoplasm Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer

48 Diffusion (cont.) Facilitated diffusion Certain hydrophobic molecules (e.g., glucose, amino acids, and ions) are transported passively down their concentration gradient by: Carrier-mediated facilitated diffusion Substances bind to protein carriers Channel-mediated facilitated diffusion Substances move through water-filled channels

49 Diffusion (cont.) Carrier-mediated facilitated diffusion Carriers are transmembrane integral proteins Carriers transport specific polar molecules, such as sugars and amino acids, that are too large for membrane channels Example of specificity: glucose carriers will carry only glucose molecules, nothing else Binding of molecule causes carrier to change shape, moving molecule in process Binding is limited by number of carriers present Carriers are saturated when all are bound to molecules and are busy transporting

50 Figure 3.7b Diffusion through the plasma membrane. Lipid-insoluble solutes (such as sugars or amino acids) Shape change releases solutes Carrier-mediated facilitated diffusion via protein carrier specific for one chemical; binding of substrate causes transport protein to change shape

51 Diffusion (cont.) Channel-mediated facilitated diffusion Channels with aqueous-filled cores are formed by transmembrane proteins Channels transport molecules such as ions or water (osmosis) down their concentration gradient Specificity based on pore size and/or charge Water channels are called aquaporins Two types: Leakage channels Always open Gated channels Controlled by chemical or electrical signals

52 Figure 3.7c Diffusion through the plasma membrane. Small lipidinsoluble solutes Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge

53 Diffusion (cont.) Osmosis Movement of solvent, such as water, across a selectively permeable membrane Water diffuses through plasma membranes Through lipid bilayer (even though water is polar, it is so small that some molecules can sneak past nonpolar phospholipid tails) Through specific water channels called aquaporins (AQPs) Flow occurs when water (or other solvent) concentration is different on the two sides of a membrane

54 Figure 3.7d Diffusion through the plasma membrane. Water molecules Lipid bilayer Aquaporin Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer

55 Diffusion (cont.) Osmolarity: measure of total concentration of solute particles Water concentration varies with number of solute particles because solute particles displace water molecules When solute concentration goes up, water concentration goes down, and vice versa Water moves by osmosis from areas of low solute (high water) concentration to high areas of solute (low water) concentration

56 Diffusion (cont.) When solutions of different osmolarity are separated by a membrane permeable to all molecules, both solutes and water cross membrane until equilibrium is reached Equilibrium: Same concentration of solutes and water molecules on both sides, with equal volume on both sides

57 Figure 3.8a Influence of membrane permeability on diffusion and osmosis. Membrane permeable to both solutes and water Solute and water molecules move down their concentration gradients in opposite directions. Fluid volume remains the same in both compartments. Left compartment: Solution with lower osmolarity Right compartment: Solution with greater osmolarity Both solutions have the same osmolarity: volume unchanged H 2 O Solute Freely permeable membrane Solute molecules (sugar)

58 Diffusion (cont.) When solutions of different osmolarity are separated by a membrane that is permeable only to water, not solutes, osmosis will occur until equilibrium is reached Same concentration of solutes and water molecules on both sides, with unequal volumes on both sides

59 Figure 3.8b Influence of membrane permeability on diffusion and osmosis. Membrane permeable to water, impermeable to solutes Solute molecules are prevented from moving but water moves by osmosis. Volume increases in the compartment with the higher osmolarity. Left compartment Right compartment Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move H 2 O Selectively permeable membrane Solute molecules (sugar)

60 Diffusion (cont.) Movement of water causes pressures: Hydrostatic pressure: pressure of water inside cell pushing on membrane Osmotic pressure: tendency of water to move into cell by osmosis The more solutes inside a cell, the higher the osmotic pressure

61 Animation: Osmosis

62 Diffusion (cont.) A living cell has limits to how much water can enter it Water can also leave a cell, causing cell to shrink Change in cell volume can disrupt cell function, especially in neurons

63 Diffusion (cont.) Tonicity Ability of a solution to change the shape or tone of cells by altering the cells internal water volume Isotonic solution has same osmolarity as inside the cell, so volume remains unchanged Hypertonic solution has higher osmolarity than inside cell, so water flows out of cell, resulting in cell shrinking Shrinking is referred to as crenation Hypotonic solution has lower osmolarity than inside cell, so water flows into cell, resulting in cell swelling Can lead to cell bursting, referred to as lysing

64 Figure 3.9 The effect of solutions of varying tonicities on living red blood cells. Isotonic solutions Hypertonic solutions Hypotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of nonpenetrating solutes than are present inside the cells). Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of nonpenetrating solutes than are present inside cells).

65 Clinical Homeostatic Imbalance 3.3 Intravenous solutions of different tonicities can be given to patients suffering different ailments Isotonic solutions are most commonly given when blood volume needs to be increased quickly Hypertonic solutions are given to edematous (swollen) patients to pull water back into blood Hypotonic solutions should not be given because they can result in dangerous lysing of red and white blood cells