CH 3 - Cells: The Living Units

CH 3 - Cells: The Living Units

Section 1: Overview of the Cellular Basis of Life (p. 62)

• Cell- Basic structural & functional unit of living organisms.

• Diversity of Cells- Over 200 different types of human cells- Cells vary greatly in size, shape, & function

Cellular Diversity

• All human cells have some common structures

• Three basic human cell parts:1) Plasma membrane

- Flexible outer boundary2) Cytoplasm

- Intracellular fluid containing organelles3) Nucleus

- Control center

Generalized Cell

Even though all cells have the same basic internal parts, it’s the number & combination of

those parts that gives each cell it’s specific function.


Section 2: The Plasma Membrane – Structure (pp. 63-

67)

• Plasma Membrane- flexible, double membrane surrounding every cell- sometimes referred to as a “cell membrane”- composed of lipids & proteins- plays critical role in cellular activity- separates intracellular fluid from extracellular fluid

Plasma Membrane: Structure

• Phospholipids- 75% of membrane; lipid bi-layer- Phosphate heads; hydrophilic- Fatty acid tails; hydrophobic

• Glycolipids- 5% of membrane- Act as markers for cell identification

• Cholesterol- 20% of membrane- Maintains membrane stability & flexibility


• Membrane Proteins- Responsible for the specialized membrane functions

• Membrane protein functions:- Transport of molecules in/out of cell- Act as receptors for signals to/from the cell- Provide ability to recognize & attach to adjacent cells


Transport of molecules…


Sending & receiving signals…


Recognizing & attaching to other cells…


• Membrane Junctions- Bind individual cells w/ other cells - Allows cells to communicate w/ others to function correctly

• Three main types of junctions:1) Tight junctions 2) Desmosomes3) Gap junctions


• Tight Junctions- Prevent fluids/most molecules from moving between cells- Cells fused together by proteins- Found in places where you don’t want fluids to leak out of

tissues (e.g., digestive tract, blood vessels, etc.)


• Desmosomes- Act as “rivets” or “spot-welds” that anchor cells together- Prevent cells from separating- Reduce chances of tearing when subjected to pulling forces- Found in areas under mechanical stress (Skin, Heart, etc.)


• Gap Junctions- “Communication” junction between cells- Hollow cylinders; allow molecules to pass from cell to cell- Found in areas that need to move ions & other substances

between cells (Cardiac cells, Smooth muscle)



Section 3: The Plasma Membrane – Transport (pp. 68-

79)

• Interstitial Fluid- water-based fluid surrounding all cells in our body- “Soup” of amino acids, sugars, fatty acids, vitamins,

hormones, neurotransmitters, & salts

• Membrane Transport- Plasma membranes are selectively permeable- Some molecules easily pass through; others do not- Substances pass to/from inside of cell & interstitial fluid

Plasma Membrane: Transport

Animation: Membrane Permeability

• Types of Membrane Transport1) Passive processes

- No cellular energy (ATP) is required

2) Active processes- Cellular energy (ATP) is always required- Require specialized carrier proteins

Plasma Membrane: Transport

• Types of passive processes:

1) Diffusion (aka “Simple Diffusion”)- Movement of molecules from high to low concentration- DOWN the concentration gradient- Molecules inherently WANT to move apart- Speed is influenced by temperature & particle size

(Higher temps = faster diffusion; Smaller particles = faster diffusion)

Examples = Oxygen, Carbon dioxide, Fat-soluble vitamins

Plasma Membrane: Passive Transport

Animation: Diffusion


2) Facilitated Diffusion- Molecules move down concentration gradient- Must have either carrier proteins or channel proteins- Particles are either too large to pass through the bi-layer

or they are charged particles that are repelled- Rate of diffusion is limited by number of carriers/channels



2) Facilitated Diffusion- Channels may be open all the time (“Leakage” channels)- Channels may be controlled by chemical/electrical signals

(“Gated” channels)


Leakage channel Gated channel


3) Osmosis- Diffusion of water thru selectively permeable membranes- Water moves very freely through lipid bi-layers- Water concentration determined by solute concentration

**In this case, b/c solutes cannot diffuse, water will instead. It moves from high water concentration (low solute concentration) to

low water concentration (high solute concentration).


• Importance of Osmosis- When osmosis occurs, water enters or leaves cell- Changes in cell volume disrupt cell function

• Tonicity- Ability of a solution to cause a cell to shrink or swell


Animation: Osmosis

• Isotonic solution- Solution with same solute

concentration as the cytoplasm- Cells in these solutions maintain

the same volume- Our extracellular fluid is isotonic

Plasma Membrane: Tonicity

• Hypertonic solution- Solution with greater solute

concentration than cytoplasm- Cells in these solutions lose

water & shrink (“crenate”)- Dehydration leads to this


• Hypotonic solution- Solution with lower solute

concentration than cytoplasm- Cells in these solutions gain

water & can burst (“lyse”)


• Types of active processes:

1) Active transport- Requires carrier proteins & ATP- Moves molecules against concentration gradient- Solutes “pumped” from low to high concentration

Example = Na+-K+ pump (found in all cells)

Plasma Membrane: Active Transport

• Types of active processes:

2) Vesicular transport- Requires ATP- Cell uses vesicles (hollow capsules) to move large substances in/out- Also called “bulk” transport


Endocytosis:- bulk transport of substances INTO the cell

2 Types: 1) Phagocytosis

- Using pseudopods to engulf solids - “Eating” - Macrophages, white blood cells


Endocytosis:- bulk transport of substances INTO the cell

2 Types: 2) Pinocytosis

- Membrane infolds, bringing in extracellular fluid

- “Drinking” - Nutrient absorption in small intest.


Exocytosis:- bulk transport of substances OUT of the cell

Examples:1) Hormone secretion2) Neurotransmitter release3) Mucus secretion



Section 4: The Cytoplasm, Organelles, & Nucleus (pp.81-

95)

• Cytoplasm- located between the plasma membrane & nucleus- site where most cellular activities are accomplished

Composed of: 1) Cytosol - water w/ solutes (proteins, salts, sugars, etc.) in it 2) Organelles - metabolic machinery of the cell 3) Inclusions - glycogen, pigments, lipid droplets, crystals, vacuoles

The Cytoplasm

• Cytoplasmic Organelles- “little organs” - specialized components that perform specific jobs in cell- work together to help cell carry out its specific function

1) Mitochondria- Power plants of cell; provide cell with ATP- Contain their own DNA & RNA- Found in abundance in cells requiring huge quantities of

energy (kidney, liver, muscle, etc.)

Cytoplasmic Organelles

2) Ribosomes- Sites of protein synthesis- Cells are protein factories; all proteins made by the cell

are built here- May be floating freely in cytoplasm or attached to endoplasmic reticulum


Ribosomes

3) Endoplasmic Reticulum (ER)- Interconnected network of passageways through cell

Two Types: a) Rough ER

- surface is studded w/ ribosomes - make all proteins that are to be secreted by cell - build proteins that will be incorporated into lipid bi-layer - particularly abundant in secretory cells & liver cells


3) Endoplasmic Reticulum (ER)- Interconnected network of passageways through cell

Two Types: b) Smooth ER

- surface is smooth - involved in lipid/cholesterol/glycogen breakdown - deals w/ detoxification of drugs & carcinogens - builds steroid-based hormones - abundant in liver, kidney, & intestinal cells


4) Golgi Apparatus- modifies, concentrates, & packages proteins- proteins pass from ER into Golgi apparatus- proteins placed in vesicles & transported throughout cell

Cytoplasmic OrganellesAnimation: Endomembrane System

5) Lysosomes- membranous bags containing digestive enzymes- break down ingested bacteria, viruses, & toxins- degrade nonfunctional organelles- break down bone to release calcium- destroy cells in injured tissue


6) Peroxisomes- membranous sacs containing oxidases/catalases- function to detoxify alcohol- most importantly, they neutralize free radicals (chemicals

that scramble biological molecules)


7) Cytoskeleton- elaborate series of “rods” running throughout the cytosol- provides a framework that supports all cellular structures

2 Main Types:a) Microfilaments - involved in cell motility, change in cell shape, &

endocytosis/exocytosis


7) Cytoskeleton- elaborate series of “rods” running throughout the cytosol- provides a framework that supports all cellular structures

2 Main Types:b) Microtubules - hollow tubes that can change in shape, size, & location - determine overall cell shape & placement of organelles


10) Cilia- small, hair-like extensions on the surface of cells - move in a wavelike motion propelling substances across

the surfaces of cells (cells lining the respiratory system)

11) Flagella- long, tail-like extensions that rotate & propel the entire

cell through a solution (sperm)


12) Microvilli- fingerlike extensions of plasma membrane- increase surface area for absorption- very extensive in the lining of the small intestine


13)Nucleus- genetic library w/ blueprints for all cellular proteins- responds to various signals & determines type & amount

of proteins to be made- most cells have 1 nucleus- red blood cells have no nucleus- skeletal muscle cells have multiple nuclei


14) Nuclear Envelope- double membrane surrounding nucleus; porous- pores regulate transport of molecules into/out of nucleus

15) Nucleoli- spherical bodies in nucleus- produce new ribosomes that are sent out thru pores in

nuclear envelope into the cytoplasm



Section 5: Cell Growth & Reproduction (pp.95-107)

• Cell Cycle- series of changes cells go through from formation to reproduction- includes interphase & cell division/mitosis

Cell Growth

• Interphase- period from cell formation to beginning of cell division- cell is essentially doing all routine activities & growing

Subphases:1) G1 phase (Gap 1) – cell is metabolically active, building proteins, & growing vigorously2) S phase (synthetic) – DNA replication3) G2 phase (Gap 2) – enzymes & other proteins needed for division are made

G0 phase – only seen in cells that never divide; continue to function normally until death

Cell Growth

• Cell Division- period of time where the cell splits into 2 smaller cells- essential for body growth & tissue repair- very short amount of time compared to interphase

Two distinct events:1) Mitosis - nuclear division - prophase, metaphase, anaphase, telophase

2) Cytokinesis - division of the cytoplasm

Cell Growth

Stages of Mitosis (Quick overview)

1) Prophase- chromosomes become visible- nuclear envelope breaks down- spindle fibers begin to form

Mitosis

Stages of Mitosis2) Metaphase

- chromosomes line up at the cell’s equator- spindle fibers attach to the chromosomes

Mitosis

Stages of Mitosis3) Anaphase

- spindle fibers pull chromosomes to opposite poles of cell- poles of cell itself begin to be pushed apart- cell starts to elongate

Mitosis

Stages of Mitosis4) Telophase

- begins when chromosome movement stops- nuclear membrane forms around each set of DNA- spindle fibers disappear- cytokinesis can now be completed

Mitosis

WHY cells divide…- Cells function most efficiently at a very specific size- when they become too large, surface area of the plasma

membrane isn’t large enough to match the cell’s volume- large cells have difficulty taking in nutrients & getting rid

of wastes fast enough to maintain life- when cells grow above the optimum size, they divide

Control of Cell Division

HOW cells divide…- various factors involved in control of cell division

• “Go” signals- chemicals that respond to large cell size - growth factors, growth hormones, cyclins

• “Stop” signals- p53 gene creates proteins that inhibit cell division (more

than half of all cancers have defective p53 genes)- contact inhibition; cell grows until it touches another cell

Control of Cell Division

Remember…- cells are protein factories- DNA holds the information for building proteins

Gene- segment of DNA that codes for an individual protein- can be broken down into triplet groups (3 bases each)- each triplet (“codon”) specifies an individual amino acid

Protein Synthesis

Central Dogma of Genetics

*The ultimate goal of the cell is to turn DNA into protein. In order to do that, the cell must use an intermediate step

involving RNA.

Protein Synthesis

DNA → RNA → proteinTranscription

Translation

Three main types of RNA:1) Messenger RNA (mRNA)

- carries the re-written instructions for building proteins from the nucleus to the ribosome

- disposable copy of the blueprint

2) Ribosomal RNA (rRNA)- structural component of the ribosome- helps to physically combine amino acids into a protein

Protein Synthesis

Three main types of RNA:3) Transfer RNA (tRNA)

- carries amino acids from the cytoplasm to the ribosome- ribosome takes amino acid; releases tRNA back into cytoplasm to pick up new amino acid

Protein Synthesis

Steps of protein synthesis:1) Transcription

- process of converting (“rewriting”) DNA blueprint into mRNA so that it can be “read” by ribosomes.

2) Translation- process of converting (“translating”) mRNA into an amino

acid sequence (a.k.a. “protein”)

Protein Synthesis

Protein Synthesis - Transcription

Protein Synthesis - Translation

Genetic Code:- each mRNA codon specifies individual amino acids- there are 64 letter combinations for codons & 20 different

amino acids- some amino acids have more than one codon

Protein Synthesis

So here’s how it works…- All proteins start with the mRNA codon “AUG”- All proteins end with 1 of 3 mRNA codons (“UAA”, “UAG”,

“UGA”)

Protein Synthesis

Role of Rough ER…

Protein Synthesis

• Body cells contain the same DNA; but aren’t all identical

• Cell differentiation - Chemical signals in the embryonic stage of development channel cells down certain pathways by turning off certain genes - Gives each cell a specific function/role within the body

Developmental Aspects of Cells

Why do we get old??? - various theories exist explaining why our cells age & die

1) Wear & Tear Theory- over time, chemical “attacks” & free radicals have a cumulative effect, wearing the cell out

2) Immune System Disorders- Autoimmune responses & progressive weakening of immune response damages cells

Theories of Cell Aging

Why do we get old??? - various theories exist explaining why our cells age & die

3) The Genetic Theory- Termination of mitosis & cell aging are programmed into

genes- Number of times a cell can divide may be pre-determined

Theories of Cell Aging

Young Katie Holmes

Not-so-young Katie Holmes

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CH 3 - Cells: The Living Units