Chapter 4: Physiology of Cells

Chapter 4: Physiology of Cells

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT
Passive transport processes do not require any energy expenditure of the cell membrane (Table 4-1) Diffusion: a passive process (Figure 4-1) Molecules spread through the membranes Molecules move from an area of high concentration to an area of low concentration down a concentration gradient (Figure 4-2) As molecules diffuse, a state of equilibrium occurs

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: PASSIVE TRANSPORT (cont
Simple diffusion (Figure 4-3) Molecules cross the phospholipid bilayer Solutes permeate the membrane; therefore the membrane is called permeable

Osmosis (Figure 4-4) Diffusion of water through a selectively permeable membrane; limits diffusion of at least some solute particles Water pressure that develops as a result of osmosis is called osmotic pressure Potential osmotic pressure is the maximal pressure that could develop in a solution when it is separated from pure water by a selectively permeable membrane

Osmosis (cont.) Knowledge of potential osmotic pressure allows prediction of the direction of osmosis and the resulting change in pressure Isotonic: when two fluids have the same potential osmotic pressure (Figure 4-5) Hypertonic (higher pressure): cells placed in solutions that are hypertonic to intracellular fluid always shrivel as water flows out of them; if medical treatment causes the extracellular fluid to become hypertonic to the cells of the body, serious damage may occur Hypotonic (lower pressure): cells placed in a hypotonic solution may swell as water flows into them; water always osmoses from the hypotonic solution to the hypertonic solution

Osmosis results in gain of volume on one side of the membrane and loss of volume on the other side of the membrane

Facilitated diffusion (mediated passive transport) A special kind of diffusion in which 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

Channel-mediated passive transport (Figure 4-6) Channels are specific; allow 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 Aquaporins are water channels that permit rapid osmosis Carrier-mediated passive transport (Figure 4-7) Carriers attract and bind to the solute, change shape, and release the solute on the other side of the carrier Carriers are usually reversible depending on the direction of the concentration gradient

Role of passive transport processes Move substances down a concentration gradient, thus maintaining equilibrium and homeostatic balance Types of passive transport: simple and facilitated diffusion (channels and carriers); osmosis is a special example of channel-mediated passive transport of water

filtration This form of transport involves the passing of water and permeable solutes through a membrane by the force of hydrostatic pressure. Hydrostatic pressure is the force, or weight, of a fluid pushing against a surface Results in the separation of large and small molecules Shown here is the filtration system of kidneys

Filtration Another example of filtration can be seen in capillaries as blood goes from being oxygenated to deoxygenated

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: ACTIVE TRANSPORT
Active transport processes require the expenditure of metabolic energy by the cell (Table 4-2) Transport by pumps Pumps are membrane transporters that move a substance against their concentration gradient; opposite of diffusion Examples: calcium pumps (Figure 4-8) and sodium-potassium pumps (Figure 4-9)

MOVEMENT OF SUBSTANCES THROUGH CELL MEMBRANES: ACTIVE TRANSPORT (cont
Transport by vesicles allows substances to enter or leave the interior of a cell without moving through its plasma membrane Endocytosis: the plasma membrane “traps” some extracellular material and brings it into the cell in a vesicle Two basic types of endocytosis (Figure 4-10) Phagocytosis (“condition of cell eating”): large particles are engulfed by the plasma membrane and enter the cell in vesicles; the vesicles fuse with lysosomes, which digest the particles Pinocytosis (“condition of cell drinking”): fluid and the substances dissolved in it enter the cell Receptor-mediated endocytosis: membrane receptor molecules recognize substances to be brought into the cell (Figure 4-11)

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 and then pulled to the plasma membrane by the cytoskeleton, where the contents are released Exocytosis also provides a way for new material to be added to the plasma membrane

Role of active transport processes Active transport requires energy use by the membrane Pumps concentrated substances on one side of membrane, such as when storing an ion inside an organelle Vesicle-mediated (endocytosis, exocytosis): move large volumes of substances at once, such as in secretion of hormones and neurotransmitters

CELL METABOLISM Metabolism is the set of chemical reactions in a cell
Catabolism: breaks large molecules into smaller ones; usually releases energy Anabolism: builds large molecules from smaller ones; usually consumes energy

CELL METABOLISM (cont.)
Role of enzymes Enzymes are chemical catalysts that reduce the activation energy needed for a reaction (Figure 4-12) Enzymes regulate cell metabolism Chemical structure of enzymes Proteins of a complex shape The active site is where the enzyme molecule fits the substrate molecule—the lock-and-key model (Figure 4-13)

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 Enzymes are a type of protein Oxidation-reduction enzymes: known as oxidases, hydrogenases, and dehydrogenases; energy release depends on these enzymes Hydrolyzing enzymes: hydrolases; digestive enzymes belong to this group Phosphorylating enzymes: phosphorylases or phosphatases; add or remove phosphate groups Enzymes that add or remove carbon dioxide: carboxylases or decarboxylases Enzymes that rearrange atoms within a molecule: mutases or isomerases Hydrases add water to a molecule without splitting it

General functions of enzymes Enzymes regulate cell functions by regulating metabolic pathways (Figure 4-14) Enzymes are specific in their actions Various chemical and physical agents known as allosteric effectors affect enzyme action by changing the shape of the enzyme molecule (Figure 4-15)

General functions of enzymes (cont.) Examples include (Figure 4-16): Temperature Hydrogen ion (H+) concentration (pH) Ionizing radiation Cofactors End products of certain metabolic pathways (Figure 4-17) Most enzymes catalyze a chemical reaction in both directions Enzymes are continually being destroyed and replaced Many enzymes are first synthesized as inactive proenzymes

CELL METABOLISM: CATABOLISM
Cellular respiration: the pathway by which glucose is broken down to yield its stored energy; an important example of cell catabolism Cellular respiration has three pathways that are chemically linked (Figure 4-21): Glycolysis (Figure 4-18) Citric acid cycle (Figure 4-19) Electron transport system (Figure 4-20)

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 adenosine triphosphate [ATP] and reduced nicotinamide adenine dinucleotide [NADH]) Includes many chemical steps (reactions that follow one another), each regulated by specific enzymes Is anaerobic (requires no oxygen) Occurs within cytosol (outside the mitochondria)

Citric acid cycle (Krebs cycle) Pyruvic acid (from glycolysis) is converted into acetyl coenzyme A (CoA) and enters the citric acid cycle after losing carbon dioxide (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 to yield waste CO2 and energy (in the form of energized electrons), which is transferred to ATP, NADH, and reduced flavin adenine dinucleotide (FADH2)

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 Protons flow back into the inner chamber through pump molecules in the cristae, and their energy of movement is transferred to ATP Low-energy electrons coming off the ETS bind to oxygen and rejoin their protons to form water

CELL METABOLISM: ANABOLISM
Protein synthesis is a central anabolic pathway in cells (Table 4-4) Deoxyribonucleic acid (DNA) A double-helix polymer (composed of nucleotides) that functions to transfer information, encoded in genes, to direct the synthesis of proteins (Figure 4-22) Gene: a segment of a DNA molecule that consists of approximately 1000 pairs of nucleotides and contains the code for synthesizing one RNA molecule, which then may be translated into one polypeptide (Figure 4-23) Ribonucleic acid (RNA) (Table 4-3) Coding RNA: mRNA, which is a transcript of a code for one polypeptide Noncoding RNA: rRNA and tRNA, which are copies of a DNA gene but regulate processes rather than code for a polypeptide

Copy this diagram These are all processes of anabolism

CELL METABOLISM: ANABOLISM (cont.)
Replication-occurs in the nucleus where DNA is located. First step in protein synthesis. DNA is copied by DNA polymerase to create a strand of DNA identical to the original, or the template.

Transcription: mRNA forms along a segment of one strand of DNA (Figure 4-24) Editing the transcript (Figure 4-25) Noncoding introns are removed and the remaining exons are spliced together to form the final, edited version of the mRNA copy of the DNA segment Spliceosomes are ribosome-sized structures in the nucleus that splice mRNA transcripts (Box 4-4)

Translation (Figure 4-26) 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; type of amino acid is determined by the fit of a specific tRNA’s anticodon with mRNA’s codon (Figure 4-27) As amino acids are brought into place, peptide bonds join them, eventually producing an entire polypeptide chain Translation of genes can be inhibited by RNA interference (RNAi), which protects the cell against viral infection (Box 4-5)

Processing: chaperone molecules and other enzymes in the cytosol, endoplasmic reticulum, and Golgi apparatus help polypeptides fold and then possibly combine into larger protein molecules or hybrid molecules Proteome All the proteins synthesized by a cell make up the cell’s proteome All the proteins synthesized in the whole body are called the human proteome

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 (Figure 4-28; Table 4-5) Cell growth: depends on using genetic information in DNA to make the structural and functional proteins needed for cell survival Cell reproduction: ensures that genetic information is passed from one generation to the next

GROWTH AND REPRODUCTION OF CELLS (cont.)
Cell growth: a newly formed cell produces a variety of molecules and other structures necessary for growth by using the information contained in the genes of DNA molecules; this stage is known as interphase Production of cytoplasm: more cell material is made, including growth and/or replication of organelles and plasma membrane; a largely anabolic process

DNA replication (Table 4-6) Replication of the genome prepares the cell for reproduction; the mechanics are similar to RNA synthesis DNA base paring (Figure 4-29) The DNA strand uncoils and the strands come apart Along each separate strand a complementary strand forms The two new strands are called chromatids instead of chromosomes Chromatids are attached pairs, and the centromere is the name of their point of attachment

The 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) Cell reproduction: cells reproduce by splitting themselves into two smaller daughter cells (Table 4-7)

Mitotic cell division: the process of organizing and distributing nuclear DNA during cell division has four distinct phases (Figure 4-31) Prophase (“before phase”) After the cell has prepared for reproduction during interphase, the nuclear envelope falls apart as the chromatids coil up to form chromosomes joined at the centromere (Figure 4-30) As chromosomes form, the centriole pairs move toward the poles of the parent cell and spindle fibers are constructed between them

Mitotic cell division (cont.) Metaphase (“position-changing phase”) Chromosomes move so that one chromatid of each chromosome faces its respective pole Each chromatid attaches to a spindle fiber Anaphase (“apart phase”) The centromere of each chromosome has split to form two chromosomes, each consisting of a single DNA molecule Each chromosome is pulled toward the nearest pole to form two separate but identical pools of genetic information

GROWTH AND REPRODUCTION OF CELLS: MIOTIC CELL DIVISION (cont.)
Mitotic cell division (cont.) Telophase (“end phase”) DNA returns to its original form and location within the cell After completion of telophase, each daughter cell begins interphase to develop into a mature cell Meiosis (Figures 4-31 and 33-1)

THE BIG PICTURE: TISSUES, MEMBRANES, AND THE WHOLE BODY
Tissues and membranes maintain homeostasis Epithelial tissues Form membranes that contain and protect the internal fluid environment Absorb nutrients Secrete products that regulate functions involved in homeostasis Connective tissues Hold organs and systems together Form structures that support the body and permit movement

THE BIG PICTURE: TISSUES, MEMBRANES, AND THE WHOLE BODY (cont.)
Muscle tissues Work with connective tissues to permit movement Nervous tissues Work with glandular epithelial tissue to regulate body function

Regulating the cell life cycle Cyclin-dependent kinases (CDKs) are activating enzymes that drive the cell through the phases of its life cycle Cyclins are regulatory proteins that control the CDKs and “shift” them to start the next phase

CYCLE OF LIFE: CELLS Different types of cells have different life cycles Advancing age creates changes in cell numbers and in their ability to function effectively Examples of decreased functional ability include muscle atrophy; loss of elasticity of the skin; and changes in the cardiovascular, respiratory, and skeletal systems

Key points for chapter 4 Most cell processes occur at the same time in all the cells throughout the body The processes of normal cell function result from the coordination dictated by the genetic code

Chapter 4: Physiology of Cells

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