Chapter 3: The Cellular Level of Organization

Figure 3–1

The Cell

• Performs all life functions

Sex Cells

• Sex cells (germ cells):– reproductive cells – male sperm– female oocytes (eggs)

Somatic Cells

• Somatic cells (soma = body):– all body cells except sex cells

Organelle Functions

Table 3–1 (1 of 2)

Organelle Functions

Table 3–1 (2 of 2)

Functions of Cell Membrane (1 of 2)

• Physical isolation• Monitors & Regulates exchange

with environment:– extracellular fluid composition– chemical signals– ions and nutrients enter– waste and cellular products released

• Structural support: – anchors cells and tissues

Structures and functions of the

cell membrane

The Cell Membrane

• Contains lipids, carbohydrates, and functional proteins

• Double layer of phospholipid molecules:– hydrophilic heads—toward watery

environment, both sides– hydrophobic fatty-acid tails—inside

membrane – barrier to ions and water soluble

compounds

6 Functions of Membrane Proteins (1 of 2)

1. Anchoring proteins (stabilizers):– attach to inside or outside structures

2. Recognition proteins (identifiers): – label cells normal or abnormal

3. Enzymes: – catalyze reactions

6 Functions of Membrane Proteins (2 of 2)

4. Receptor proteins:– bind and respond to ligands (ions,

hormones)

5. Carrier proteins: – transport specific solutes through

membrane

6. Channels: – regulate water flow and solutes through

membrane

Membrane Carbohydrates

• Proteoglycans, glycoproteins, and glycolipids:– extend outside cell membrane– form sticky “sugar coat” (glycocalyx)

Functions of Membrane Carbohydrates

• Lubrication and protection• Anchoring and locomotion• Specificity in binding (receptors)• Recognition (immune response)

Cytoplasm

• All materials inside the cell and outside the nucleus: – cytosol (fluid):

• dissolved materials:– nutrients, ions, proteins, and waste products

– organelles: • structures with specific functions

What are cell organelles and their functions?

Types of Organelles

• Nonmembranous organelles: – no membrane– direct contact with cytosol

• Membranous organelles: – covered with plasma membrane– isolated from cytosol

Nonmembranous Organelles

• 6 types of nonmembranous organelles: – cytoskeleton – microvilli – centrioles – cilia – ribosomes – proteasomes

Figure 3–3a

The Cytoskeleton

• Structural proteins for shape and strength

Microfilaments

• Thin filaments composed of the protein actin: – provide additional mechanical

strength – interact with proteins for consistency– Pairs with thick filaments of myosin

for muscle movement

Intermediate Filaments

• Mid-sized between microfilaments and thick filaments:– durable (collagen)– strengthen cell and maintain shape– stabilize organelles– stabilize cell position

Microtubules

• Large, hollow tubes of tubulin protein:– attach to centrosome– strengthen cell and anchor organelles– change cell shape– move vesicles within cell (kinesin and

dynein)– form spindle apparatus

Figure 3–3b

Microvilli

• Increase surface area for absorption

• Attach to cytoskeleton

Centrioles in the Centrosome

• Centrioles form spindle apparatus during cell division

• Centrosome: cytoplasm surrounding centriole

Figure 3–4a

Cilia Power

• Cilia move fluids across the cell surface

Figure 3–4b,c

Ribosomes

• Build polypeptides in protein synthesis

• Two types: – free ribosomes in cytoplasm:

• proteins for cell

– fixed ribosomes attached to ER:• proteins for secretion

Proteasomes

• Contain enzymes (proteases)• Disassemble damaged proteins for

recycling

Membranous Organelles

• 5 types of membranous organelles:– endoplasmic reticulum (ER)– Golgi apparatus– lysosomes– peroxisomes– mitochondria

Endoplasmic Reticulum (ER)

Figure 3–5a

Endoplasmic Reticulum (ER)

• endo = within, plasm = cytoplasm, reticulum = network

• Cisternae are storage chambers within membranes

Functions of ER

• Synthesis of proteins, carbohydrates, and lipids

• Storage of synthesized molecules and materials

• Transport of materials within the ER

• Detoxification of drugs or toxins

Smooth Endoplasmic Reticulum (SER)

• No ribosomes attached• Synthesizes lipids and carbohydrates:

– phospholipids and cholesterol (membranes)

– steroid hormones (reproductive system)– glycerides (storage in liver and fat cells)– glycogen (storage in muscles)

Rough Endoplasmic Reticulum (RER)

• Surface covered with ribosomes:– active in protein and glycoprotein

synthesis– folds polypeptides protein structures– encloses products in transport

vesicles

Golgi Apparatus

Figure 3–6a

Golgi Apparatus

• Vesicles enter forming face and exit maturing face

Functions of the Golgi ApparatusPLAYPLAY

Vesicles of the Golgi Apparatus

• Secretory vesicles:– modify and package products for

exocytosis

• Membrane renewal vesicles:– add or remove membrane

components

• Lysosomes:– carry enzymes to cytosol

Transport Vesicles

Figure 3–7a

• Carry materials to and from Golgi apparatus

Figure 3–7b

Exocytosis

• Ejects secretory products and wastes

Lysosomes

Figure 3–8

• Powerful enzyme-containing vesicles:– lyso = dissolve, soma = body

Lysosome Structures

• Primary lysosome: – formed by Golgi and inactive

enzymes

• Secondary lysosome: – lysosome fused with damaged

organelle– digestive enzymes activated– toxic chemicals isolated

Lysosome Functions

• Clean up inside cells:– break down large molecules– attack bacteria– recycle damaged organelles– ejects wastes by exocytosis

Autolysis

• Self-destruction of damaged cells:– auto = self, lysis = break– lysosome membranes break down– digestive enzymes released– cell decomposes– cellular materials recycle

Peroxisomes

• Are enzyme-containing vesicles:– break down fatty acids, organic

compounds

– produce hydrogen peroxide (H2O2)

– replicate by division

Membrane Flow

• A continuous exchange of membrane parts by vesicles:– all membranous organelles (except

mitochondria)– allows adaptation and change

KEY CONCEPT

• Cells: basic structural and functional units of life– respond to their environment– maintain homeostasis at the cellular

level– modify structure and function over

time

Mitochondrion Structure

Figure 3–9a

Mitochondrion Structure

• Have smooth outer membrane and folded inner membrane (cristae)

• Matrix: – fluid around cristae

Mitochondrial Function

• Mitochondrion takes chemical energy from food (glucose):– produces energy molecule ATP

Figure 3–9b

Aerobic Cellular Respiration

• Aerobic metabolism (cellular respiration):– mitochondria use oxygen to break

down food and produce ATP

The Reactions

glucose + oxygen + ADP carbon dioxide + water + ATP

• Glycolysis: – glucose to pyruvic acid (in cytosol)

• Tricarboxylic acid cycle (TCA cycle):– pyruvic acid to CO2 (in matrix)

KEY CONCEPT

• Mitochondria provide cells with energy for life:– require oxygen and organic

substrates– generate carbon dioxide and ATP

How does the nucleus control the cell?

Figure 3–10a

The Nucleus

• Is the cell’s control center

Structure of the Nucleus

• Nucleus:– largest organelle

• Nuclear envelope:– double membrane around the nucleus

• Perinuclear space:– between 2 layers of nuclear envelope

• Nuclear pores:– communication passages

Within the Nucleus

• DNA:– all information to build and run

organisms

• Nucleoplasm:– fluid containing ions, enzymes,

nucleotides, and some RNA

• Nuclear matrix:– support filaments

Nucleoli in Nucleus

• Are related to protein production• Are made of RNA, enzymes, and

histones• Synthesize rRNA and ribosomal

subunits

Organization of DNA

• Nucleosomes:– DNA coiled around histones

• Chromatin:– loosely coiled DNA (cells not dividing)

• Chromosomes:– tightly coiled DNA (cells dividing)

What is genetic code?

DNA and Genes

• DNA:– instructions for every protein in the

body

• Gene:– DNA instructions for 1 protein

Genetic Code

• The chemical language of DNA instructions:– sequence of bases (A, T, C, G)– triplet code:

• 3 bases = 1 amino acid

KEY CONCEPT

• The nucleus contains chromosomes

• Chromosomes contain DNA• DNA stores genetic instructions for

proteins• Proteins determine cell structure

and function

How do DNA instructions become proteins?

Protein Synthesis

• Transcription:– copies instructions from DNA to

mRNA (in nucleus)

• Translation:– ribosome reads code from mRNA (in

cytoplasm)– assembles amino acids into

polypeptide chain

Protein Synthesis

• Processing:– by RER and Golgi apparatus produces

protein

mRNA Transcription

• A gene is transcribed to mRNA in 3 steps:– gene activation– DNA to mRNA– RNA processing

Step 1: Gene Activation

• Uncoils DNA, removes histones• Start (promoter) and stop codes on

DNA mark location of gene:– coding strand is code for protein– template strand used by RNA

polymerase molecule

Step 2: DNA to mRNA

• Enzyme RNA polymerase transcribes DNA:– binds to promoter (start) sequence– reads DNA code for gene– binds nucleotides to form messenger

RNA (mRNA)– mRNA duplicates DNA coding strand,

uracil replaces thymine

Step 3: RNA Processing

• At stop signal, mRNA detaches from DNA molecule:– code is edited (RNA processing)– unnecessary codes (introns) removed– good codes (exons) spliced together– triplet of 3 nucleotides (codon)

represents one amino acid

Codons

Table 3–2

Translation (1 of 6)

• mRNA moves: – from the nucleus– through a nuclear

pore

Figure 3–13

Translation (2 of 6)

• mRNA moves:– to a ribosome in

cytoplasm– surrounded by amino

acids

Figure 3–13 (Step 1)

Translation (3 of 6)

• mRNA binds to ribosomal subunits

• tRNA delivers amino acids to mRNA

Figure 3–13 (Step 2)

Translation (4 of 6)

• tRNA anticodon binds to mRNA codon

• 1 mRNA codon translates to 1 amino acid

Figure 3–13 (Step 3)

Figure 3–13 (Step 4)

Translation (5 of 6)

• Enzymes join amino acids with peptide bonds

• Polypeptide chain has specific sequence of amino acids

Protein Synthesis: Sequence of Amino Acids in the Newly Synthesized Polypeptide

PLAYPLAY

Figure 3–13 (Step 5)

Translation (6 of 6)

• At stop codon, components separate

KEY CONCEPT

• Genes: – are functional units of DNA – contain instructions for 1 or more

proteins

• Protein synthesis requires:– several enzymes– ribosomes– 3 types of RNA

KEY CONCEPT

• Mutation is a change in the nucleotide sequence of a gene:– can change gene function

• Causes:– exposure to chemicals– exposure to radiation– mistakes during DNA replication

Overcoming the Cell Barrier

• The cell membrane is semipermeable: – nutrients must get in– products and wastes must get out

Permeability

• Permeability determines what moves in and out of a cell:

• A membrane that: – lets nothing in or out is impermeable– lets anything pass is freely permeable– restricts movement is selectively

permeable

Selective Permeability

• Cell membrane is selectively permeable:– allows some materials to move freely– restricts other materials

Membrane Transport: Fat- and Water-Soluble MoleculesPLAYPLAY

Restricted Materials

• Selective permeability restricts materials based on:– size– electrical charge– molecular shape– lipid solubility

Transport

• Transport through a cell membrane can be:– active (requiring energy and ATP)– passive (no energy required)

3 Categories of Transport

• Diffusion (passive)• Carrier-mediated transport

(passive or active)• Vesicular transport (active)

Solutions

• All molecules are constantly in motion

• Molecules in solution move randomly

• Random motion causes mixing

Concentration Gradient

• Concentration is the amount of solute in a solvent

• Concentration gradient: – more solute in 1 part of a solvent

than another

Function of Concentration Gradient

• Diffusion: – molecules mix randomly – solute spreads through solvent – eliminates concentration gradient

Diffusion

• Solutes move down a concentration gradient

Factors Affecting Diffusion Rates

• Distance the particle has to move• Molecule size:

– smaller is faster

• Temperature: – more heat, faster motion

Factors Affecting Diffusion Rates

• Gradient size: – the difference between high and low

concentration

• Electrical forces: – opposites attract, like charges repel

Diffusion and the Cell Membrane

Figure 3–15

• Diffusion can be simple or channel-mediated

Simple Diffusion

• Materials which diffuse through cell membrane:– lipid-soluble compounds (alcohols,

fatty acids, and steroids)– dissolved gases (oxygen and carbon

dioxide)

Channel-Mediated Diffusion

• Materials which pass through transmembrane proteins (channels):– are water soluble compounds– are ions

Factors in Channel-Mediated Diffusion

• Passage depends on:– size– charge– interaction with the channel

Osmosis

Figure 3–16

• Osmosis is the diffusion of water across the cell membrane

How Osmosis Works

• More solute molecules, lower concentration of water molecules

• Membrane must be freely permeable to water, selectively permeable to solutes

Osmosis Water Movement

• Water molecules diffuse across membrane toward solution with more solutes

• Volume increases on the side with more solutes

Osmotic Pressure

• Is the force of a concentration gradient of water

• Equals the force (hydrostatic pressure) needed to block osmosis

Tonicity

• The osmotic effect of a solute on a cell: – 2 fluids may have equal

osmolarity, but different tonicity

Figure 3–17a

Isotonic Solutions

• A solution that does not cause osmotic flow of water in or out of a cell

• iso = same, tonos = tension

Hypotonic Solutions

• hypo = below• Has less solutes• Loses water through osmosis

Cells and Hypotonic Solutions

• A cell in a hypotonic solution:– gains water– ruptures (hemolysis of

red blood cells)

Figure 3–17b

Hypertonic Solutions

• hyper = above • Has more solutes• Gains water by osmosis

Cells and Hypertonic Solutions

• A cell in a hypertonic solution:– loses water– shrinks (crenation of red

blood cells)

Figure 3–17c

KEY CONCEPT (1 of 2)

• Concentration gradients tend to even out

• In the absence of membrane, diffusion eliminates concentration gradients

KEY CONCEPT (2 of 2)

• When different solute concentrations exist on either side of a selectively permeable membrane, osmosis moves water through the membrane to equalize the concentration gradients

Special transport mechanisms

Special Transport Mechanisms

• Carrier-mediated transport of ions and organic substrates:– facilitated diffusion – active transport

Characteristics of Carrier-Mediated Transport

• Specificity: – 1 transport protein, 1 set of

substrates

• Saturation limits: – rate depends on transport proteins,

not substrate

• Regulation: – cofactors such as hormones

Special Transport Mechanisms

• Cotransport– 2 substances move in the same

direction at the same time

• Countertransport– 1 substance moves in while another

moves out

Facilitated Diffusion

• Passive• Carrier mediated

Figure 3–18

How Facilitated Diffusion Works

• Carrier proteins transport molecules too large to fit through channel proteins (glucose, amino acids):– molecule binds to receptor site on

carrier protein– protein changes shape, molecules pass

through– receptor site is specific to certain

molecules

Active Transport

• Active transport proteins:– move substrates against

concentration gradient– require energy, such as ATP – ion pumps move ions (Na+, K+, Ca+,

Mg2+) – exchange pump countertransports 2

ions at the same time

Sodium-Potassium Exchange Pump

Figure 3–19

Receptor-Mediated Endocytosis

• Receptors (glycoproteins) bind target molecules (ligands)

• Coated vesicle (endosome) carries ligands and receptors into the cell

Figure 3–22a

Pinocytosis

• Pinocytosis (cell drinking) • Endosomes “drink” extracellular

fluid

Phagocytosis

• Phagocytosis (cell eating)– pseudopodia (psuedo =

false, podia = feet) – engulf large objects in

phagosomes

Figure 3–22b

Figure 3–7b

Exocytosis

• Is the reverse of endocytosis

Summary

Table 3–3

• The 7 methods of transport

What is transmembrane potential?

Electrical Charge

• Inside cell membrane is slightly negative, outside is slightly positive

• Unequal charge across the cell membrane is transmembrane potential

• Resting potential ranges from —10 mV to —100 mV, depending on cell type

Cell Life Cycle

Figure 3–3

Cell Life Cycle

• Most of a cell’s life is spent in a nondividing state (interphase)

3 Stages of Cell Division

• Body (somatic) cells divide in 3 stages:– DNA replication duplicates genetic

material exactly– Mitosis divides genetic material

equally – Cytokinesis divides cytoplasm and

organelles into 2 daughter cells

Interphase

• The nondividing period: – G-zero phase—specialized cell

functions only – G1 phase—cell growth, organelle

duplication, protein synthesis – S phase—DNA replication and histone

synthesis– G2 phase—finishes protein synthesis

and centriole replication

DNA Replication

Figure 3–24

• DNA strands unwind • DNA polymerase attaches

complementary nucleotides

Mitosis

• Mitosis divides duplicated DNA into 2 sets of chromosomes:– DNA coils tightly into chromatids– chromatids connect at a centromere– protein complex around centromere is

kinetochore

Features of Prophase

• Nucleoli disappear • Centriole pairs move to

cell poles• Microtubules extend

between centriole pairs• Nuclear envelope disappears• Spindle fibers attach to

kinetochore

Features of Metaphase

• Chromosomes align in a central plane (metaphase plate)

Features of Anaphase

• Microtubules pull chromosomes apart

• Daughter chromosomes groups near centrioles

Features of Telophase

• Nuclear membranes reform

• Chromosomes uncoil• Nucleoli reappear• Cell has 2 complete

nuclei

KEY CONCEPT

• Mitosis duplicates chromosomes in the nucleus for cell division

Features of Cytokinesis

• Division of the cytoplasm

• Cleavage furrow around metaphase plate

• Membrane closes, producing daughter cells

Long Life, Short Life

• Muscle cells, neurons rarely divide• Exposed cells (skin and digestive

tract) live only days or hours• Normally, cell division balances cell

loss

Factors Changing Cell Division

• Increases cell division:– internal factors (MPF) – extracellular chemical factors (growth

factors)

• Decreases cell division:– repressor genes (faulty repressors cause

cancers)– worn out telomeres (terminal DNA

segments)

Cell Differentiation

• Cells specialize or differentiate:– to form tissues (liver cells, fat cells,

and neurons) – by turning off all genes not needed by

that cell

KEY CONCEPT

• All body cells, except sex cells, contain the same 46 chromosomes

• Differentiation depends on which genes are active and which are inactive

SUMMARY (1 of 4)

• Structures and functions of human cells

• Structures and functions of membranous and nonmembranous organelles

SUMMARY (2 of 4)

• ATP, mitochondria, and the process of aerobic cellular respiration

• Structures and functions of the nucleus:– control functions of nucleic acids– structures and replication of DNA– DNA and RNA in protein synthesis

SUMMARY (3 of 4)

• Structures and chemical activities of the cell membrane:– diffusion and osmosis – active transport proteins– vesicles in endocytosis and exocytosis– electrical properties of plasma

membrane

SUMMARY (4 of 4)

• Stages and processes of cell division:– DNA replication– mitosis– cytokinesis