Chapter 3

Chapter 3The Cellular Level of

Organization

Lectures by Dr. Yukti Sharma

Learning Objectives Define a cell, and discuss modern cell theory.

Discuss structure and function of plasma membrane.

Discuss transport of substances across plasma membrane.

Describe cytoplasm and its components.

Cytosol and organelles: cytoskeleton, centrosome, cilia and flagella, ribosomes, endoplasmic reticulum, golgi complex, lysosomes, peroxisomes, proteasomes, mitochondria.

Define a nucleus and discuss its parts: nuclear membrane, nucleoplasm, nucleolus, chromosomes.

Discuss structure of chromosomes, and processes of transcription and translation.

Describe cell division

Discuss cellular diversity.

Discuss medical terminology related to changes in normal structure, shape and function of the cells

A cell is the basic unit of all living things. Prokaryotic cells Eukaryotic cells

The Cell

All eukaryotic cells are composed of three main parts:

1. Plasma membrane or “plasmalemma”

2. Cytoplasm - a gelatin-like substance, plus

structural fibers and organelles (but not the

nucleus)

3. Nucleus - contains the genetic library of the cell

A Generalized Cell

1. The plasma membrane forms the cell’s outer boundary and separates the cell’s internal environment from the outside environment.

A Generalized Cell

2. The cytoplasm contains all the cellular contents between the plasma membrane and the nucleus.

A Generalized Cell

3. The nucleus is a large organelle that contains DNA in molecules called chromosomes.

Each chromosome consists

of a single molecule of DNA

and associated packaging

proteins. A chromosome contains

thousands of hereditary

units called genes.

A Generalized Cell

A Generalized Cell Figure shows a generalized body cell labeled with

the plasma membrane, cytoplasm (and organelles) and nucleus.

The plasma membrane is much more than just a “fence” – it

is a flexible yet sturdy, “intelligent” semipermeable regulator

that:

Covers and protects the cell

Controls what goes in and

comes out

Links to other cells

The Plasma Membrane

The Plasma Membrane The Fluid Mosaic Model describes the

arrangement of molecules within the membrane: They resemble a sea of phospholipids with protein “icebergs” floating in it.

The Plasma Membrane The structure of the membrane

Phospholipids form a lipid bilayer - cholesterol

and glycolipids (sugar-lipids) also contribute.

Integral proteins - extend into or through the

bilayer.

• Transmembrane proteins

• Peripheral proteins

The Plasma Membrane Glycoproteins are membrane proteins with

a carbohydrate group attached that protrude

into the extracellular fluid.

The Glycocalyx

The Plasma Membrane

The Functions of the membrane proteins Transporters - selectively move substances

through the membrane.

Receptors - for cellular recognition; a ligand is a molecule that binds with a receptor.

Enzymes - catalyze chemical reactions

Others act as cell-identity markers.

Functions of Membrane Proteins1. Transport

2. Receptors for signal transduction

3. Attachment to cytoskeleton and extracellular matrix

(1) Transport (2) Receptors (3) Attachment

Functions of Membrane Proteins

4. Enzymatic activity

5. Intercellular joining

6. Cell-cell recognition

CAMs

Enzymes

GlycoproteinEnzymatic activity Intercellular joining Cell-cell recognition

The Plasma Membrane Examples of different membrane proteins

include

Ion channels

Carriers

Receptors

The Plasma Membrane

Examples of different

membrane proteins

include

Enzymes

Linkers

Cell identity markers

Because of the distribution of lipids and the proteins embedded in it, the membrane allows some substances across but not others; this is called Selective permeability

The Plasma Membrane

courtesy of Dr. Jim Hutchins

Membrane Permeability For those substances that are needed by the cell but

for which the membrane is impenetrable (impermeable), transmembrane proteins act as channels and transporters.

Transport Processes Passive processes involve substances

moving across the cell membranes without the input of any energy - they are said to move “with” or “down” their concentration gradient ([gradient] , where [ ] indicates “concentration”).

Active processes involve the use of energy, primarily from the breakdown of ATP, to move a substance against its [gradient].

Transport Processes Passive processes

Diffusion of solutes Diffusion of water (called osmosis) Facilitated diffusion (requires a specific channel

or a carrier molecule, but no energy is used) Active processes

Various types of transporters are used, and energy is required.

Concentration gradients

Concentrations of some key ions are very different on the inside versus the outside of cells creating a gradient

IN:[Na+] = low[K+] = high[Ca2+] = very

low[Cl-] = low

OUT:[Na+] = high[K+] = low[Ca2+] = low[Cl-] = high

(blood, interstitial fluid)

Passive Transport Processes Diffusion is the passive spread of particles

through random motion, from areas of high

concentration to areas of low concentration.

Amount of substance and the steepness of the

concentration gradient.

Temperature

Surface area

Diffusion distance

What can/can’t diffuse through the cell membrane?

Passive Transport Processes

Channel-Mediated Facilitated Diffusion Carrier-Mediated Facilitated Diffusion

Passive Transport Processes An example of Channel-Mediated Facilitated Diffusion is

the passage of potassium ions through a gated K+

Channel

An example of Carrier-Mediated Facilitated Diffusion is the

passage of glucose across the cell membrane.

Diffusion Through the Plasma Membrane

Figure 3.7

Extracellular fluid

Cytoplasm

Lipid-solublesolutes

Lipidbilayer

Lipid-insolublesolutes

Watermolecules

Small lipid-insolublesolutes

(a) Simple diffusion directly through the phospholipid bilayer

(c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge

(b) Carrier-mediated facilitated diffusion via protein carrier specific for one chemical; binding of substrate causes shape change in transport protein

(d) Osmosis, diffusion through a specific channel protein (aquaporin) or through the lipid bilayer

Passive Transport Processes Osmosis is the net movement of water

through a selectively permeable membrane from an area of high water concentration to one of lower water concentration.

Water can pass through plasma membrane in 2 ways: through lipid bilayer by simple diffusion through aquaporins (integral membrane proteins)

Effect of Membrane Permeability on Diffusion and Osmosis

Figure 3.8a

Effect of Membrane Permeability on Diffusion and Osmosis

Figure 3.8b

Osmosis in Cells

KEY CONCEPT Concentration gradients tend to even out due

to random motion of particles In the absence of a membrane, diffusion

eliminates concentration gradients When different solute concentrations exist on

either side of a selectively permeable membrane, either:

1. diffusion of permeable molecules equalizes concentrations OR

2. osmosis moves water through the membrane to equalize the concentration gradients

Active Transport Processes

Cytoplasm

Extracellular fluidK+ is released andNa+ sites are ready tobind Na+ again; thecycle repeats.

Cell ADP

Phosphorylationcauses theprotein tochange its shape.

Concentration gradientsof K+ and Na+

The shape change expels Na+ to the outside, and extracellular K+ binds.

Loss of phosphaterestores the originalconformation of thepump protein.

K+ binding triggersrelease of thephosphate group.

Binding of cytoplasmic Na+ to the pump proteinstimulates phosphorylationby ATP.Na+

Na+

Na+

Na+Na+

K+K+

K+

K+

Na+

Na+

Na+

ATPP

P

Na+

Na+Na+

K+

K+

P

Pi

K+

K+

Secondary Active Transport Mechanisms

Transport in Vesicles Vesicle - a small spherical sac formed by budding off

from a membrane Endocytosis - materials move into a cell in a vesicle

formed from the plasma membrane three types: receptor-mediated endocytosis phagocytosis bulk-phase endocytosis (pinocytosis)

Exocytosis - vesicles fuse with the plasma membrane,

releasing their contents into the extracellular fluid Transcytosis - a combination of endocytosis and

exocytosis

Receptor-Mediated Endocytosis

Phagocytosis

Bulk-phase Endocytosis

Cytoplasm - 2 Components1. Cytosol - intracellular fluid, surrounding the

organelles

- The site of many chemical reactions

- Energy is usually released by these reactions.

- Reactions provide the building blocks for cell

maintenance, structure, function and growth.

2. Organelles

- Specialized structures within the cell

Network of protein filaments throughout the cytosol

Provides structural support for the cell

The Cytoskeleton

Types Microfilaments

Intermediate

filaments

Microtubules

The Cytoskeleton

Organelles Centrosome - located near the nucleus, consists of two

centrioles and pericentriolar material

Organelles Cilia - short, hair-

like projections from the cell surface, move fluids along a cell surface

Flagella - longer than cilia, move an entire cell; only example is the sperm cell’s tail

Organelles

Organelles Ribosomes - sites of protein synthesis

Organelles Endoplasmic

reticulum - network of membranes in the shape of flattened sacs or tubules

- Rough ER - connected to the nuclear envelope, a series of flattened sacs, surface is studded with ribosomes, produces various proteins

-Smooth ER - a network of membrane tubules, does not have ribosomes, synthesizes fatty acids and steroids, detoxifies certain drugs

Organelles Golgi complex - consists of 3–20 flattened,

membranous sacs called cisternae.

Processing and Packaging

Organelles Lysosomes - vesicles that form from the Golgi

complex and contain powerful digestive enzymes

Organelles Peroxisomes

Smaller than lysosomes

Detoxify several toxic substances such as alcohol

Abundant in the liver

Proteasomes

Continuously destroy unneeded, damaged, or

faulty proteins

Found in the cytosol and the nucleus

Organelles

Mitochondria - the “powerhouses” of the cell Generate ATP

Have inner and outer mitochondrial membranes similar in

structure to the plasma membrane

Cristae - the series of folds of the inner membrane

Matrix - the large central fluid-filled cavity

Self-replicate during times of increased cellular demand or

before cell division

Contain own DNA

• Inherited only from your mother

Mitochondria

Nucleus

Packing of DNA into a Chromosome of a Dividing Cell

Overview of Gene Expression

The Central DogmaDNA > RNA > Protein.

Translation

Translation

Translation

Translation

Translation

Translation

Translation

Transcription

Somatic Cell Division - Mitosis

The cell cycle is a sequence of events in which a body

cell duplicates its contents and divides in two

Human somatic cells contain 23 pairs of chromosomes

(total = 46)

The two chromosomes that make up each pair are called

homologous chromosomes (homologs)

Somatic cells contain two sets of chromosomes and are

called diploid cells

Cell Division

Interphase - the cell is not dividing

- The cell replicates its DNA

- Consists of three phases, G1, S, and G2, replication of DNA occurs in the S phase Mitotic phase - consists of a nuclear division (mitosis) and a cytoplasmic division (cytokinesis) to form two identical cells

The Cell Cycle

DNA Replication

Nuclear Division: Mitosis

Prophase - the chromatin fibers change into chromosomes.

Metaphase - microtubules align the centromeres of the

chromatid pairs at the metaphase plate.

Anaphase - the chromatid pairs split at the centromere and

move to opposite poles of the cell; the chromatids are now

called chromosomes.

Telophase - two identical nuclei are formed around the

identical sets of chromosomes now in their chromatin form.

Cytoplasmic Division: Cytokinesis

Division of a cell’s cytoplasm to form two identical cells

Usually begins in late anaphase

The plasma membrane constricts at its middle, forming a cleavage furrow

The cell eventually splits into two daughter cells.

Interphase begins when cytokinesis is complete .

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

Mitosis

1

Pericentriolar material

NucleolusNuclear envelopeChromatin

Plasma membraneCytosol

(a) INTERPHASE

CentriolesCentrosome:

all at 700xLM

1

LateEarly




Chromosome(two chromatidsjoined atcentromere

(a) INTERPHASE

(b) PROPHASE


Fragments ofnuclear envelope

Mitotic spindle(microtubules)

Kinetochore

2

all at 700xLM

Centromere

1




Metaphase plate

(a) INTERPHASE


(c) METAPHASE

2

3

LateEarly (b) PROPHASE



Kinetochore

all at 700xLM


Centromere

1

EarlyLate(d) ANAPHASE




Chromosome

(a) INTERPHASE


(c) METAPHASE

2

3

4

Cleavage furrow




Kinetochore

Metaphase plate

all at 700xLM


Centromere

1





(a) INTERPHASE


Cleavage furrow

(e) TELOPHASE

(c) METAPHASE

2

3

4

5

Cleavage furrow




Kinetochore

Metaphase plate

Chromosome

all at 700xLM


Centromere

1



NucleolusNuclear envelopeChromatinPlasma membraneCytosol

(a) INTERPHASE


(f) IDENTICAL CELLS IN INTERPHASE

Cleavage furrow

(e) TELOPHASE

(c) METAPHASE

Cleavage furrow

2

3

4

5

6




Kinetochore

Metaphase plate

Chromosome

all at 700xLM

Centromere

Chromosome(two chromatidsjoined atcentromereMitosis

Reproductive Cell Division During sexual reproduction, each new

organism is the result of the union of two gametes (fertilization), one from each parent.

Meiosis - reproductive cell division that occurs in the gonads (ovaries and testes) that produces gametes with half the number of chromosomes.

Haploid cells - gametes contain a single set of 23 chromosomes.

Fertilization restores the diploid number of chromosomes (46).

Reproductive Cell Division Meiosis occurs in two successive stages: meiosis I and

meiosis II .

Each of these two stages has 4 phases: prophase, metaphase, anaphase, and telophase.

Summary - Meiosis I begins with a diploid cell and ends with two cells having the haploid number of chromosomes; in Meiosis II, each of the two haploid cells divides, and the net result is four haploid gametes that are genetically different from the original diploid starting cell.

Meiosis and Cytokinesis

Comparison of Mitosisand Meiosis

Cellular Diversity The average adult has

nearly 100 trillion cells.

There are about 200

different types of cells.

Cells come in a variety of

shapes and sizes.

Cellular diversity permits

organization of cells into

more complex tissues and

organs.

Chapter 3

Science

membrane phospholipids

membrane permeability

function of plasma membrane

plasma membrane courtesy

plasma membrane glycoproteins

nuclear membrane

membrane proteins transporters

functions of membrane