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Cells: The Living Units

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Presentation on theme: "Cells: The Living Units"— Presentation transcript:

1 Cells: The Living Units
Chapter 3 : Part A Cells: The Living Units

2 Cell Theory The cell is the smallest structural and functional living unit Organismal functions depend on individual and collective cell functions Biochemical activities of cells are dictated by their specific sub cellular structures called organelles

3 Cell Diversity Over 200 different types of human cells
Types differ in size, shape, subcellular components, and functions

4 (a) Cells that connect body parts, form linings, or transport gases
Erythrocytes Fibroblasts Epithelial cells (a) Cells that connect body parts, form linings, or transport gases Nerve cell Skeletal Muscle cell Smooth muscle cells (e) Cell that gathers information and control body functions (b) Cells that move organs and body parts Sperm Macrophage (f) Cell of reproduction Fat cell (c) Cell that stores nutrients (d) Cell that fights disease Figure 3.1

5 Generalized Cell All cells have some common structures and functions
Human cells have four basic parts: Plasma membrane - flexible outer boundary Cytoplasm - intracellular fluid Organelles - multiple functions Nucleus - control center

6 Chromatin Nuclear envelope Nucleolus Nucleus Smooth endoplasmic
reticulum Plasma membrane Mitochondrion Cytosol Lysosome Centrioles Centrosome matrix Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements • Microtubule • Intermediate filaments Peroxisome Figure 3.2

7 Plasma Membrane Bimolecular layer of lipids and proteins in a constantly changing fluid mosaic Plays a dynamic role in cellular activity Separates intracellular fluid (ICF) from extracellular fluid (ECF) Interstitial fluid (IF) = ECF that surrounds cells

8 Extracellular fluid (watery environment) Cholesterol Polar head of
phospholipid molecule Cholesterol Glycolipid Glycoprotein Carbohydrate of glycocalyx Outward- facing layer of phospholipids Integral proteins Filament of cytoskeleton Inward-facing layer of phospholipids Bimolecular lipid layer containing proteins Peripheral proteins Nonpolar tail of phospholipid molecule Cytoplasm (watery environment) Figure 3.3

9 Membrane Lipids Phospholipids (lipid bilayer)
Phosphate heads: polar and hydrophilic Fatty acid tails: nonpolar and hydrophobic (Review Fig. 2.16b)

10 Functions of Membrane Proteins
Transport Receptors for signal transduction Attachment to cytoskeleton and extracellular matrix

11 A protein (left) that spans the membrane
(a) 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. Figure 3.4a

12 (b) Receptors for signal transduction Signal
A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell. Receptor Figure 3.4b

13 (c) Attachment to the cytoskeleton and extracellular matrix (ECM)
Elements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. Figure 3.4c

14 Functions of Membrane Proteins
Enzymatic activity Intercellular joining Cell-cell recognition

15 (d) Enzymatic activity
Enzymes A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here. Figure 3.4d

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

17 (f) Cell-cell recognition
Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Glycoprotein Figure 3.4f

18 Membrane Transport Plasma membranes are selectively permeable
Some molecules easily pass through the membrane; others do not

19 Types of Membrane Transport
Passive processes No cellular energy (ATP) required Substance moves down its concentration gradient Active processes Energy (ATP) required Occurs only in living cell membranes

20 Passive Processes What determines whether or not a substance can passively permeate a membrane? Lipid solubility of substance Channels of appropriate size Carrier proteins PLAY Animation: Membrane Permeability

21 Passive Processes Simple diffusion
Carrier-mediated facilitated diffusion Channel-mediated facilitated diffusion Osmosis

22 Passive Processes: Simple Diffusion
Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer Diffusion is the movement of solutes from a solution of higher concentration to that of a lower concentration PLAY Animation: Diffusion

23 (a) Simple diffusion of fat-soluble molecules
Extracellular fluid Lipid- soluble solutes Cytoplasm (a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer Figure 3.7a

24 Passive Processes: Facilitated Diffusion
Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which: Exhibit specificity (selectivity) Are saturable; rate is determined by number of carriers or channels Can be regulated in terms of activity and quantity

25 Facilitated Diffusion Using Carrier Proteins
Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids) Binding of substrate causes shape change in carrier

26 (b) Carrier-mediated facilitated diffusion via a protein
Lipid-insoluble solutes (such as sugars or amino acids) (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein Figure 3.7b

27 Facilitated Diffusion Using Channel Proteins
Aqueous channels formed by transmembrane proteins selectively transport ions or water Two types: Leakage channels Always open Gated channels Controlled by chemical or electrical signals

28 (c) Channel-mediated facilitated diffusion
Small lipid- insoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Figure 3.7c

29 Passive Processes: Osmosis
Movement of solvent (water) from a solution of low concentration to that of a higher concentration, across a selectively permeable membrane Water diffuses through plasma membranes: Through the lipid bilayer Through water channels called aquaporins (AQPs)

30 (d) Osmosis, diffusion of a solvent such as
Water molecules Lipid billayer Aquaporin (d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3.7d

31 Passive Processes: Osmosis
Water concentration is determined by solute concentration because solute particles displace water molecules Osmolarity: The measure of total concentration of solute particles When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

32 Solute and water molecules move down their concentration gradients
(a) 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 H2O Solute Solute molecules (sugar) Membrane Figure 3.8a

33 (b) 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. Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move Left compartment Right compartment H2O Solute molecules (sugar) Membrane Figure 3.8b

34 Importance of Osmosis When osmosis occurs, water enters or leaves a cell Change in cell volume disrupts cell function PLAY Animation: Osmosis

35 Tonicity Tonicity: The ability of a solution to cause a cell to shrink or swell Isotonic: A solution with the same solute concentration as that of the cytosol Hypertonic: A solution having greater solute concentration than that of the cytosol Hypotonic: A solution having lesser solute concentration than that of the cytosol

36 Figure 3.9 (a) Isotonic solutions (b) Hypertonic solutions
(c) 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 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 solutes than are present in cells). Figure 3.9


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