Section 1, Chapter 3Cells

CellsOverview

Basic unit of life75 Trillion cells in human bodyVary in size and shapes

Over 260 types of cells in body i.e. neurons, muscle, bone, blood cells All types are derived from just 1 fertilized egg

DifferentiationForming specialized cells from unspecialized cells

Major Parts of a CellNucleus

Cell Membrane

CytoplasmCytosol = fluidOrganelles

Cell Membrane

OverviewThe cell membrane maintains integrity of cell

Membrane is a fluid structure – flexible

Selectively permeable- allows only select substances into and out of cell

Signal transduction – communication between cell & environment

The cell membrane is composed of three major components

1. Bilayer of phospholipids 2. Cholesterol 3. Membrane proteins

Polar region (hydrophilic)

Nonpolar region(hydrophobic)

Phospholipid bilayer

Phospholipid Bilayer

Nonpolar Hydrocarbon tailsPhospholipids align so that tails are hidden from water

Nonpolar tails form the interior of membrane

Polar Phosphate headsPolar groups align to face water

Polar heads are exposed to surfaces of membrane

The phospholipid bilayer forms a fluid, yet stable boundary.

The interior the cell membrane is hydrophobic which means it is

Permeable to nonpolar molecules Steroids, Oxygen, Carbon Dioxide can easily cross the membrane

Impermeable to polar moleculesProteins, Water, Sugars, ect. Cannot easily cross membrane

Cholesterol= rigid rings of hydrocarbons

Contributes to the nonpolar nature of membrane

Stabilizes the cell membrane

Increases rigidity of cell membrane

Membrane Proteins

1. Integral ProteinsSpans cell membrane

Forms ion channels & pores

Examples include: Na+ channels and K+ channels Aquaporins = water channels

Transmembrane ProteinsIntegral protein, where one end extends outside cell, and the other end dips into cytoplasm.

Many function as receptors

Transmembrane Protein Receptor

A molecule (ligand) binds to portion of receptor outside cell

Transmembrane Protein changes conformation

Portion inside cell sends a signal into cell

Membrane Proteins2. Peripheral Proteins

Are loosely associated with the membrane.

May be a glycoprotein (protein + sugar)

Includes Cellular Adhesion Molecules (CAM)

Cellular Adhesion Molecules (CAM)

Peripheral proteins that guide moving cells to targets

Establishes cell-to-cell connectionsForms new neural connections in learning and memoryGuides cells surrounding an embryo towards uterusGuides white blood cells to injury

CAM ProteinSelectin

Covers the surface of white blood cells (WBC)

Selectin binds to carbohydrates on surface of damaged capillaries

Allows circulating WBCs to anchor near site of injury

1. Nuclear Envelope

2. Nucleolus

3. Chromatin

3 Parts of the nucleus

Nuclear Envelope

Double-layered membrane = 2 lipid bilayers.

Nuclear PoresChannel proteins allow specific molecules across nuclear envelope

Ribosomes & RNA leaves nucleus through nuclear pores

Nucleolus “little nucleus”

• Dense body of RNA & Proteins

• Produces Ribosomes

Chromatin “Colored substance”

Chromosome “colored body”

Chromatin condenses into chromosomes during mitosis.

DNA wrapped around histone proteins

End of Section 1, Chapter 3

Section 2, Lecture 3

Movements into & out of Cells

The cell membrane creates a barrier through which molecules must cross into and out of the

cell.

Passive movementsRequires no energy from cell

DiffusionFacilitated DiffusionOsmosisFiltration

Active MovementsRequires energy from cell

Ion pumpsEndocytosisExocytosis

DiffusionTendency of atoms or molecules in water or air to move from areas of higher concentration to lower concentration

Moving substances eventually become diffuse, or evenly distributed

Diffusion occurs because all substances are in constant motion

Diffusion of sugar In water

DiffusionSubstances move down their concentration gradient (from high to lower concentration)

Concentration gradient = difference in concentrations

Requirements for diffusion across cell membrane

2. A concentration gradient must exist across the cell membrane

1. Cell membrane must be permeable to substance.• Oxygen, Carbon Dioxide, and Steroid Hormones easily diffuse across

the cell membrane.

Oxygen enters cells & Carbon Dioxide leaves cells by diffusion

Illustrating diffusion across cell

Substances move down their concentration gradient.

Diffusion through carrier proteins within the cell membrane

Carrier proteins include ion channels and other proteins that “carry” substances across the cell membrane.

Facilitated diffusion transports ions, glucose, and some hormones across the cell membrane.

facilitated diffusion

Diffusion of water across a semipermeable membrane

Osmosis

Water freely crosses the membrane, but solutes (sugars, salts, and proteins) cannot cross the membrane.

Remember: Water follows salts!

water moves down its concentration gradient, i.e. from area of higher [water] into area of lower [water]

60% H2O40% protein

80% H2O20% protein

Higher H2O concentration

A B

Higher soluteconcentration

Membrane is impermeable to proteins

Osmotic Pressure = Pressure generated by osmosis.

H2O moved down its concentration gradient

Osmotic Pressure exerted on Cells

Intracellular fluid (fluid inside cells) = 0.9% NaCl

Extracellular Solutions (fluid outside cells) may exert osmotic pressure onto cells.

Isotonic = extracellular solution of 0.9%NaCl

Hypertonic = extracellular solution above 0.9%NaCl

Hypotonic = extracellular Solution below 0.9%NaCl

Red Blood Cell in isotonic solutionNo osmotic pressure = normal shape & size

Isotonic Solution

Extracellular [NaCl] is equal to Intracellular [NaCl].

Results in no osmotic pressure

H2O

H2O

Water moves into and out of the cell

Red Blood Cell in hypertonic solution.Water leaves the cell causing it to shrink.

Hypertonic Solution

Extracelluar [NaCl] is greater than Intracellular [NaCl]

Water moves out of cell & cell may shrink

H2O

net water movement out of the cell. Cell shrinks

Red Blood Cell in hypotonic solution.Water enters the cell causing it to swell.

Hypotonic Solution

Extracellular [NaCl] is less than Intracellular [NaCl].

Water moves into cell & the cell swells

Cell may lyse (burst)

H2O

net water movement into the cell. Cell swells and my lyse (burst)

In filtration of water and solids, gravity forces water through filter paper.

FiltrationMolecules are forced through membranesForce created by hydrostatic pressure, such as blood pressure.Commonly separates solids from liquids.

Filtration in body

Blood pressure forces water and smaller solutes through tiny openings in capillary wall.

Larger molecules, proteins, and cells remain inside the capillaries.

Active Transport

Up to 40% of a cell’s energy supply is used for active transport.

i.e. From lower concentration to higher concentration.

Movement against a concentration gradient.

Requires cellular energy (usually in the form of ATP).

Active transport uses energy to “pump” particles through a carrier protein

ATP = currency of energy for cell

Sodium/Potassium Pump: example of active transport

3 Na+ are pumped out of the cell, while 2 K+ are pumped into the cell.

The Na+/K+ pump creates a high extracellular [Na+] and a high intracellular [K+]

endocytosisCells take in larger substances by engulfing them.

The cell membrane surrounds the substance and forms a vesicle from a portion of the cell membrane.

Pinocytosis –cell takes in fluids

Phagocytosis –cell takes in solid particles

Receptor-Mediated Endocytosis

Cell takes in specific molecules

Removes substances even in very low concentrations

Reverse of Endocytosis

Cell organelles produce chemicals and proteins, and then package them in vesicles.

The vesicles fuse with the cell membrane releasing the chemicals from the cell.

Exocytosis

Example of Exocytosis: Neurotransmitters are secreted from neurons by exocytosis

Transcytosis

Combines endocytosis & exocytosis

Quickly transports substances across cell

Trancytosis transports HIV across linings of the mouth, anus, and female reproductive tract

End of Section 2, Chapter 3Transcytosis of HIV

Section 3, Chapter 3

The Cell Cycle

The Cell Cycle: series of changes that a cell undergoes from the time it forms until it divides

Stages of the Cell Cycle

Interphase

Mitosis

Cytokinesis

Cell Differentiation

Interphase is a very active period of the cell’s life

During interphase the cell grows and performs its routine functions

Interphase can be divided into 3 sub-phasesG1 phase (G = gap)S phase (S = synthesis)G2 phase

G1 PhaseThe cell is active & grows during G1 phase

G1 phase is followed by a restriction “checkpoint” that determines the cell’s fate:

The cell may grow & divide

The cell may remain active, but not divide

The cell may undergo apoptosis = programmed cell death

S PhaseDNA synthesis occurs during S phase

The cell replicates its DNA in preparation of cell division

unreplicatedchromosome

S phase

replicatedchromosome

centromere

sister chromatids

The 46 chromosomes are paired (diploid) 23 paternal (father) + 23 maternal (mother)

Each cell in the body contains 46 chromosomes

Cell Division - produces 2 daughter cells from 1 cell

Mitosis specifically refers to the division of a cell’s chromosomes. Karyokinesis is the division of the nucleusCytokinesis is the division of the cytoplasm

Mitosis occurs in Somatic (non-sex) cells – sex cells are formed from Meiosis

Both daughter cells from mitosis retain the complete genome (46 chromosomes)

ProphaseChromatin condenses into chromosomes

Centrosomes move towards opposite poles of cell

The nuclear envelope breaks down

Spindle fibers arise from the centrosomes = asters

There are 4 phases of mitosis1. Prophase2. Metaphase3. Anaphase4. Telophase

Use PMAT to remember the order!

MetaphaseSpindle fibers attach to centromeresChromosomes align along equator of cell

AnaphaseSpindle fibers shorten & pull on chromosomes

Chromosomes separate & move towards opposite poles of cell

Cytokinesis usually begins during anaphase

Telophase

Chromosomes complete migration

Nuclear envelopes reform

Chromosomes begin to unwind

Cytokinesis completes

CytokinesisDivision of Cytoplasm

Begins during Anaphase & continues through Telophase

Microfilament rings pinches off two cells from one

End of Chapter 3