Physiology of Cells

Passive Transport1. Diffusion

– Tendency of small particles to spread out evenly within a given space

– Occurs down a concentration gradient until equilibrium is reached• Measurable difference between one area to

another

– Diffusion or permeability dependent upon presence of specified channels

– Selectively permeable – channels open and close based on cell’s needs

Diffusion

Passive Transport2. Dialysis (fig 4-4, pg. 94)

– Selectively permeable membrane separates smaller and larger particles

Passive Transport3. Osmosis (fig 4-5, pg. 95)

– Diffusion of water through a selectively permeable membrane

Osmotic Pressure

• Osmotic pressure – pressure that develops in a solution as a result of osmosis– Important concept for maintaining homeostasis

• Isotonic – two fluids have the same osmotic pressure

• Hypotonic – lower concentration of solute outside of the cell (ECF)

• Hypertonic – higher concentration of solute outside of cell (ECF)

Effects of Osmosis on a Cell

Passive Transport4. Facilitated

Transport – (also carrier-

mediated passive transport)

– Movement of a molecule is facilitated by a carrier mechanism in the cell membrane

– Still requires concentration gradient

Passive Transport

5. Filtration– Passing of water and permeable solutes

through a membrane by the force of hydrostatic pressure

– Hydrostatic pressure: force or weight of a fluid pushing against a surface

– Occurs down a hydrostatic pressure gradient

– Most often associated with movement across a sheet of cells (ex: capillaries)

Active Transport

• Carrier-mediate process• Moves molecules “uphill” (against

concentration gradients)• Ex: Sodium-potassium pump

– Operates in all human cells

Sodium-Potassium Pump

• Transports Na+ out of the cell and K+ into the cell

• Requires energy (sodium-potassium ATPase) – made in mitochondria

• 3 Na+ bind on the intracellular side and are exchanged for 2 K+ from the ECF

Sodium-Potassium Pump

Endocytosis 1. Receptors in the plasma membrane bind

to molecules in the ECF2. Cytoskeleton pulls a portion of the

membrane inward, creating a pocket3. Edges of the “pocket” fuse forming a

vesicle4. Vesicle is pulled inside the cell by the

cytoskeleton 5. Vesicle fuses with the membrane walls of

a lysosome

Exocytosis

Process by which large molecules (proteins) exit the cell

1. Wastes/proteins enclosed by a membrane vesicle

2. Cytoskeleton moves vesicle to plasma membrane

3. Fuses with membrane & releases contents to ECF

Cell Life Cycle• Two main

processes:– Growth – Reproduction

Cell Growth

• During cell growth additional cytoplasm and plasma membrane are produce through protein synthesis– Includes structural proteins and

enzymes– Anabolic process

DNA Replication

• Prior to cell reproduction, DNA must be replicated:

1. DNA unzips2. Nucleotides attached to exposed

base pairs according to base pair rule (A-T; G-C)

3. DNA polymerase binds nucleotides4. Two identical DNA strands are forms

DNA Replication

Cell Reproduction

• One parent cell (diploid) produces two identical daughter cells (also diploid)

• Two steps:– Mitosis– Cytokinesis

Mitosis• Cell organizes replicated DNA

into two identical sets and distributes one set to each daughter cells

• 4 phases of mitosis (PMAT)1. Prophase

– Chromosomes shorten & thicken– Centrioles move to poles of cell– Spindle fibers appear– Nuclear membrane disappears

Mitosis

2. Metaphase– Chromosomes lined up at the equator

(middle)– Spindle fibers attached to centromere

3. Anaphase– Centromeres break apart separating

sister chromatids

Mitosis

4. Telophase– Spindle fibers disappear– Nuclear envelope reappears

Cytokinesis:• Completes cellular reproduction by

splitting the plasma membrane and cytoplasm into two

Meiosis

• Cell division which occurs in primitive sex cells

• The end result is mature gametes– Males sperm– Females ova (egg)

• Gametes are haploid (contain half the number of chromosomes of a somatic cell)– Reduction division

Deoxyribonucleic Acid (DNA)

• Double Helix (twisted ladder)• Monomer: nucleotide

1. 5 Carbon sugar (deoxyribose)2. Phosphate group3. Nitrogen base

• Nitrogen bases:• Purines: adenine, guanine• Pyrimidines: cytosine, thymine

• Chargaff’s rule

Deoxyribonucleic Acid (DNA)

Deoxyribonucleic Acid (DNA)

• Three base pairs = codon

• Sequence of base pairs determines each gene

• Genes control the production of proteins (therefore determine individual traits

Ribonucleic Acid (RNA)

• Single helix• Smaller than DNA• 5 carbon sugar = ribose• Uracil replaces thymine

which binds with adenine

Protein Synthesis

• Occurs in 2 steps:– Translation: DNA

RNA– Transcription: RNA

Proteins

Translation

• Occurs in the nucleus• DNA “unzips” exposing base pairs• RNA nucleotides (already present in the

nucleus) attach themselves to the exposed bases along one side of the DNA molecule

• These RNA nucleotides bind to each other with the help of RNA polymerase

• The chain that results is called messenger RNA (mRNA)

Preparation of mRNA

• Occurs in the nucleus• The mRNA pulles away from the DNA strand• The result is a copy or “transcript” of a

gene• Non-coding portions of mRNA (introns) are

removed• Remaining portions (exons) are spliced

back together• mRNA travels to the nucleus via nuclear

pores

Translation

• Occurs in the cytoplasm• Ribosomal subunits attach to one of the

mRNA• Transfer RNA (tRNA) bring specific amino

acids to the ribosome site– Which amino acid is based upon each mRNA

codon– The tRNA must have a complementary

anticodon to fit onto the binding site of each condon

Translation

• Amino acids are joined by peptide bonds

• Forms long molecules called polypeptides

• Several polypeptides are needed to make a protein

After Translation…

• Enzymes within the endoplasmic reticulum, Golgi apparatus and/or cytosol line polypeptides to form larger structures– May also form lipoproteins or

glycoproteins

Translation Video

http://www.dnatube.com/video/1947/Protein-Synthesis-Translation-1