Chapter 4 Physiology of Cells

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Chapter 4Physiology of Cells
Department of Applied Science King Saud
University/ Community College By Murad Sawalha
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Movement of Substances through Cell Membranes

• Passive transport processesdo not require any
  energy expenditure of the cell membrane
• Diffusiona passive process
• Molecules spread through the membranes
• Molecules move from an area of high concentration
  to an area of low concentration, down a
  concentration gradient
• As molecules diffuse, a state of equilibrium will
  occur
• 2 types simple facilitated diffusion

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Movement of Substances through Cell Membranes

• Simple diffusion
• Molecules cross through the phospholipid bilayer
• Solutes permeate the membrane therefore, we call
  the membrane permeable
• Osmosis
• Diffusion of water through a selectively
  permeable membrane, which limits the diffusion of
  at least some of the solute particles
• Water pressure that develops as a result of
  osmosis is called osmotic pressure

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Movement of Substances through Cell Membranes

• Facilitated diffusion (mediated passive
  transport)
• A special kind of diffusion whereby movement of
  molecules is made more efficient by the action of
  transporters embedded in a cell membrane
• Transports substances down a concentration
  gradient
• Energy required comes from the collision energy
  of the solute

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Movement of Substances through Cell Membranes

• Facilitated diffusion (cont.)
• Channel-mediated passive transport
• Channels are specificallow only one type of
  solute to pass through
• Gated channels may be open or closed (or
  inactive)may be triggered by any of a variety of
  stimuli
• Channels allow membranes to be selectively
  permeable

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Movement of Substances through Cell Membranes

• Facilitated diffusion (cont.)
• Carrier-mediated passive transport
• Carriers attract and bind to the solute, change
  shape, and release the solute out the other side
  of the carrier
• Carriers are usually reversible, depending on the
  direction of the concentration gradient

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Movement of Substances through Cell Membranes

• Active transport processesrequire the
  expenditure of metabolic energy by the cell
• Transport by pumps
• Pumps are membrane transporters that move a
  substance against its concentration gradientthe
  opposite of diffusion
• Examples calcium pumps and sodium-potassium
  pumps
• Transport by vesiclesallows substances to enter
  or leave the interior of a cell without actually
  moving through its plasma membrane

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Movement of Substances through Cell Membranes

• Active transport processes (cont.)
• Endocytosisthe plasma membrane traps some
  extracellular material and brings it into the
  cell in a vesicle
• Two basic types of endocytosis
• Phagocytosiscondition of cell-eating large
  particles are engulfed by the plasma membrane and
  enter the cell in vesicles vesicles fuse with
  lysosomes, where the particles are digested
• Pinocytosiscondition of cell-drinking fluid
  and the substances dissolved in it enter the cell
  
• Receptor-mediated endocytosismembrane receptor
  molecules recognize substances to be brought into
  cell

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Movement of Substances through Cell Membranes

• Active transport processes (cont.)
• Exocytosis
• Process by which large molecules, notably
  proteins, can leave the cell even though they are
  too large to move out through the plasma membrane
• Large molecules are enclosed in membranous
  vesicles that are then pulled by the cytoskeleton
  to the plasma membrane, where the contents are
  released
• Exocytosis also provides a way for new material
  to be added to the plasma membrane

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Cell Metabolism

• Metabolism is the set of chemical reactions in a
  cell
• Catabolismbreaks large molecules into smaller
  ones usually releases energy
• Anabolismbuilds large molecules from smaller
  ones usually consumes energy

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Cell Metabolism

• Role of enzymes
• Enzymes are chemical catalysts, reducing
  activation energy needed for a reaction
• Enzymes regulate cell metabolism
• Chemical structure of enzymes
• Proteins of a complex shape
• The active site is where the enzyme molecule fits
  the substrate moleculethe lock-and-key model

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Cell Metabolism

• Classification and naming of enzymes
• Enzymes usually have an -ase ending, with the
  first part of the word signifying the substrate
  or the type of reaction catalyzed
• Oxidation-reduction enzymesknown as oxidases,
  hydrogenases, and dehydrogenases energy release
  depends on these enzymes
• Hydrolyzing enzymeshydrolases digestive enzymes
  belong to this group

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Cell Metabolism

• Classification and naming of enzymes (cont.)
• Phosphorylating enzymesphosphorylases or
  phosphatases add or remove phosphate groups
• Enzymes that add or remove carbon dioxide
  carboxylases or decarboxylases
• Enzymes that rearrange atoms within a
  moleculemutases or isomerases
• Hydrases add water to a molecule without
  splitting it

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Cell Metabolism

• General functions of enzymes
• Enzymes regulate cell functions by regulating
  metabolic pathways
• Enzymes are specific in their actions

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Cell Metabolism

• General functions of enzymes (cont.)
• Various chemical and physical agents known as
  allosteric effectors affect enzyme action by
  changing the shape of the enzyme molecule
  examples of allosteric effectors include the
  following
• Temperature
• Hydrogen ion (H) concentration (pH)
• Ionizing radiation
• Cofactors
• End products of certain metabolic pathways

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Cell Metabolism

• General functions of enzymes (cont.)
• Most enzymes catalyze a chemical reaction in both
  directions
• Enzymes are continually being destroyed and are
  continually being replaced
• Many enzymes are first synthesized as inactive
  proenzymes

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Cell Metabolism

• Catabolism
• Cellular respiration, the pathway in which
  glucose is broken down to yield its stored
  energy, is an important example of cell
  catabolism cellular respiration has three
  pathways that are chemically linked
• Glycolysis
• Citric acid cycle
• Electron transport system (ETS)

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Cell Metabolism

• Catabolism (cont.)
• Glycolysis
• Pathway in which glucose is broken apart into two
  pyruvic acid molecules to yield a small amount of
  energy (which is transferred to ATP and NADH)
• Is anaerobic (requires no oxygen)
• Occurs within cytosol (outside the mitochondria)

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Cell Metabolism

• Catabolism (cont.)
• Citric acid cycle (Krebs cycle)
• Pyruvic acid (from glycolysis) is converted into
  acetyl CoA and enters the citric acid cycle after
  losing CO2 and transferring some energy to NADH
• Citric acid cycle is a repeating (cyclic)
  sequence of reactions that occurs inside the
  inner chamber of a mitochondrion. Acetyl splits
  from CoA and is broken down, yielding waste CO2
  and energy (in the form of energized electrons),
  which is transferred to ATP, NADH, and FADH2

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Cell Metabolism

• Catabolism (cont.)
• Electron transport system (ETS)
• Energized electrons are carried by NADH and FADH2
  from glycolysis and the citric acid cycle to
  electron acceptors embedded in the cristae of the
  mitochondrion
• As electrons are shuttled along a chain of
  electron-accepting molecules in the cristae,
  their energy is used to pump accompanying protons
  (H) into the space between mitochondrial
  membranes

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Cell Metabolism

• Anabolism
• Protein synthesis is a central anabolic pathway
  in cells
• Deoxyribonucleic acid (DNA)
• A double-helix polymer (composed of nucleotides)
  that functions to transfer information, encoded
  in genes, that directs the synthesis of proteins
• Genea segment of a DNA molecule that consists of
  approximately 1000 pairs of nucleotides and
  contains the code for synthesizing one
  polypeptide

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Cell Metabolism

• Anabolism (cont.)
• 1) TranscriptionmRNA forms along a segment of
  one strand of DNA
• 2) Editing
• 3) Translation
• After leaving the nucleus and being edited, mRNA
  associates with a ribosome in the cytoplasm
• tRNA molecules bring specific amino acids to the
  mRNA at the ribosome the type of amino acid is
  determined by the fit of a specific tRNAs
  anticodon with mRNAs codon
• As amino acids are brought into place, peptide
  bonds join themeventually producing an entire
  polypeptide chain

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Growth and Reproduction of Cells

• Cell growth and reproduction of cells are the
  most fundamental of all living functions and
  together constitute the cell life cycle
• Cell growthdepends on using genetic information
  in DNA to make the structural and functional
  proteins needed for cell survival
• Cell reproductionensures that genetic
  information is passed from one generation to the
  next

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Growth and Reproduction of Cells

• Cell growth (cont.)
• Growth phase of the cell life cycle can be
  subdivided into the first growth phase (G1), the
  DNA synthesis phase (S), and the second growth
  phase (G2)