THE CELL

Mariejim Diane O. Payot, RMT, MSMT

THE CELL

What is a Cell? • Basic organizational unit of life • Fundamental unit of life • Functional unit of life • Simplest organization of matter • Smallest independent units of life

What, Exactly, Is a Cell? • Basic building blocks of all living things • Functions:

– Metabolism and Energy Use – Synthesis of molecules – Communication – Reproduction and Inheritance

The Fundamental Theory of Biology 1. All organisms consist of one or more cells. 2. The cell is the smallest unit of life. 3. Each new cell arises from another cell. 4. A cell passes hereditary information to its offspring.

Overview of Cells

Measuring Cells: Bacteria on the Tip of a Pin • Bacteria are the smallest and simplest cells

“Animalcules and Beasties” • No one knew cells existed until microscopes were invented

The Discovery of the Cell

Modern Microscopes

10 μm

A Light micrograph. A phase-contrast microscope yields high-contrast images of transparent specimens, such as cells.

B Light micrograph. A reflected light microscope captures light reflected from opaque specimens.

C Fluorescence micrograph. The chlorophyll molecules in these cells emitted red light (they fluoresced) naturally.

D A transmission electron micrograph reveals fantastically detailed images of internal structures.

E A scanning electron micrograph shows surface details of cells and structures. SEMs may be artificially colored to highlight certain details.

Relative Sizes

Fig. 3-5b, p. 47

human eye (no microscope)

largest organisms

small animals humans

frog eggs

100 µm 1 mm 1 cm 10 cm 1 m 10 m 100 m

light microscopes

electron microscopes most eukaryotic cells

molecules of life viruses mitochondria, chloroplasts

most bacteria

complex carbohydrates lipids DNA (width)

proteins

small molecules

0.1 nm 1 nm 10 nm 100 nm 1 µm 10 µm

What, Exactly, Is a Cell? • All cells start life with a plasma membrane, cytoplasm, and a region

of DNA which, in eukaryotic cells only, is enclosed by a nucleus.

Cell (Plasma) Membrane • Outermost component of a cell • Functions

– Selective barrier – Encloses cytoplasm

• Extracellular • Intracellular

CELL MEMBRANE

The Fluid Mosaic Model • Considered as a two-dimensional fluid of mixed composition • 1972: Jonathan Singer and Garth Nicolson • Components:

– Phospholipids (polar and non-polar) – Cholesterol (less permeable; stabilize membrane) – Membrane Proteins – Glycocalyx (cell coat)

• Carbohydrates + proteins = glycoproteins • Carbohydrates + lipids = glycolipids

Membrane Lipids

Membrane

Proteins

Membrane Carbohydrates

The Fluid Mosaic Model • Lipid bilayer balloon filled with fluid

– Structural foundation of cell membranes – Mainly phospholipids arranged tail-to-tail

in a bilayer

• Polar Regions – Heads – Hydrophilic – Exposed to water

• Non-polar Regions – Tails – Hydrophobic – Away from water

The Fluid Mosaic Model

The Fluid Mosaic Model

The Fluid Mosaic Model

Fatty Acid Tails

Polar Heads

Phospholipid Molecules

Membrane Proteins • Transport Proteins

– Passively or actively assist ions or molecules across a membrane

• Enzymes – Speed up chemical processes

• Adhesion Proteins – Help cells stick together

• Recognition Proteins – Tag cells as “self”

• Receptor Proteins – Bind to a particular substance

outside the cell

The Fluid Mosaic Model

Membrane Lipids: Cholesterol and Glycolipids

Membrane Proteins and Transmembrane Proteins

Channel Pore and Peripheral Proteins

Membrane Carbohydrate (Glycocalyx) and Glycoprotein

MOVEMENT THROUGH THE CELL MEMBRANE

Maintaining Homeostasis Maintenance of relatively constant internal environment despite fluctuations in the external environment

“For metabolism to work, a cell must keep its internal

composition stable – even when conditions outside are

greatly different.”

Selectively Permeability of Cell Membranes • Membrane property that allows some substances in and keep others out • Enzymes, glycogen and potassium: high concentrations INSIDE • Sodium, calcium and chloride: high concentrations OUTSIDE

WAYS MOLECULES PASS THROUGH CELL MEMBRANE

1. Directly through (DIFFUSION) – Small molecules

2. Membrane channels – proteins

3. Carrier molecules – Bind to molecules, transport and drop

off – Glucose

4. Vesicles – Can transport a variety of materials – Fuse with cell membrane

Gated Na+

channel (open)

Gated Na+

channel (closed)

K+ leak channel (always open)

K+

Na+

Diffusion • Net movement of molecules or ions from a region of higher concentration

to a region of lower concentration within a solvent • At equilibrium: uniform distribution of molecules • Terminologies:

– Solution • Any mixture of liquids, gases, or solids in which the substances are uniformly

distributed with no clear boundary between the substances

– Solute • Dissolves in a solvent to form a solution

– Solvent • Predominant liquid or gas

Diffusion 1. Lipid-soluble molecules diffuse

directly through the plasma membrane

2. Most non-lipid-soluble molecules and ions do not diffuse through the plasma membrane

3. Some specific non-lipid-soluble molecules and ions pass through membrane channels or other transport proteins

Diffusion 1. Lipid-soluble molecules diffuse

directly through the plasma membrane

2. Most non-lipid-soluble molecules and ions do not diffuse through the plasma membrane

3. Some specific non-lipid-soluble molecules and ions pass through membrane channels or other transport proteins

Diffusion 1. Lipid-soluble molecules diffuse

directly through the plasma membrane

2. Most non-lipid-soluble molecules and ions do not diffuse through the plasma membrane

3. Some specific non-lipid-soluble molecules and ions pass through membrane channels or other transport proteins

Diffusion

Mediated Transport Mechanisms • Facilitated Diffusion

– Diffusion with aid of a carrier molecule – Requires no ATP

• Active Transport – Moves substances from low to high concentration – Requires ATP

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Na+–K+ pump Three sodium ions (Na+) and adenosine triphosphate (ATP) bind to the sodium–potassium (Na+–K+) pump.

Na+–K+ pump changes shape (requires energy).

The ATP breaks down to adenosine diphosphate (ADP) and a phosphate (P) and releases energy. That energy is used to power the shape change in the Na+–K+ pump.

The Na+–K+ pump changes shape, and the Na+ are transported across the membrane and into the extracellular fluid.

Two potassium ions (K+) bind to the Na+–K+ pump.

The phosphate is released from the Na+–K+ pump binding site.

P

K +

Na+–K+ pump resumes original shape.

The Na+–K+ pump changes shape, transporting K+

across the membrane and into the cytoplasm. The Na+–K+ pump can again bind to Na+ and ATP.

P

ATP Na+

Na+

K+

ADP

K+ Na+

K+

A Na+–K+ pump maintains a concentration of Na+ that is higher outside the cell than inside.

Na+ move back into the cell by a carrier molecule that also moves glucose. The concentration gradient for Na+ provides the energy required to move glucose, by cotransport, against its concentration gradient.

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Na+–K+

pump Na+

Carrier molecule

Glucose

Glucose Na+

Osmosis and Tonicity • Concentration of water depends on the total number of molecules or

ions dissolved in it

• Osmosis – Net diffusion of water molecules across a selectively permeable membrane

between two fluids with different water concentrations

• Osmotic Pressure – Force required to prevent movement of water across cell membrane

Osmosis and Tonicity

Osmosis and Tonicity • Tonicity

– Describes relative concentrations of solutes in fluids separated by a selectively permeable membrane

– Hypotonic – Hypertonic – Isotonic

Osmosis and Tonicity

Endocytosis • Process that brings

materials into cell using vesicles

• Two Types – Phagocytosis

• Cell eating (solid particles)

– Pinocytosis • Cell drinking (liquid

particles)

Exocytosis

• Process that carries materials out of the cell

CELL STRUCTURES

Organelles: specialized metabolic functions Cytoplasm: holds the organelles

Nucleus: INFORMATION CENTER

Nucleus • Double membrane • Contains DNA • Functions

– Directs chemical reactions in cells • Transcribe (DNA to RNA) Æ translates into proteins

– Stores genetic information and transfers during cell division

Nuclear envelope: GATEWAY TO THE NUCLEUS Nuclear pores: surface of nucleus

Nuclear Envelope • Edge of nucleus or outer boundary • Nuclear membrane • Double membrane • Controls the passage of molecules

• Pores, receptors and transport proteins in the nuclear envelope

control the movement of molecules into and out of the cell

Chromosomes: GENETIC CONTAINERS • Contains hereditary information (genes)

• Nucleoplasm: inner mass of nucleus

• Chromatin: genetic material in non-dividing

cell – Combination of DNA and protein

• During cell division, each chromosomes coil

tightly, making it visible through a light microscope

Nucleolus: PRE-ASSEMBLY POINT FOR RIBOSOMES • Non-membrane bound in

the nucleoplasm • Present in non-dividing

cells • 2-3 or thousands • Contains proteins and RNA • Assembly of ribosomes is

completed after leaving the nucleus through the pores of the nuclear envelope

Ribosomes • Produce proteins; membrane-bound and free

Ribosomal proteins, produced in the cytoplasm, are transported through nuclear pores into the nucleolus.

The small and large ribosomal subunits leave the nucleolus and the nucleus through nuclear pores.

rRNA, most of which is produced in the nucleolus, is assembled with ribosomal proteins to form small and large ribosomal subunits.

The small and large subunits, now in the cytoplasm, combine with each other and with mRNA during protein synthesis.

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rRNA

Ribosomal proteins from cytoplasm

Small ribosomal unit

Large ribosomal unit

Nuclear pore

mRNA Ribosome

Nucleolus

Nucleus

DNA (chromatin)

Endoplasmic Reticulum: PRODUCTION & TRANSPORT • Continuous system of sacs and tubes • Extension of nuclear envelope • Storage for enzymes and proteins • Point of attachment for ribosomes • Rough ER: protein production • Smooth ER: no ribosomes

– Lipid synthesis; detoxification; Ca storage

Rough Endoplasmic Reticulum

Smooth Endoplasmic Reticulum

Golgi Apparatus: PACKAGING, SORTING, EXPORT • 1898 Camille Golgi • Closely, packed stacks of membranes • Modifies polypeptides and lipids • Sorts and packages the finished products into transport vesicles • Distributes proteins and lipids • Produces lysosomes

• Cisternae: flattened sacks (fluid reservoirs) • Apparatus or complex: collection of membranes associated with ER

Golgi Apparatus

Secretory Vesicle • Small, membrane-

enclosed, sac-like organelle

• Contains enzymes or secretory products

• Site of intracellular degradation

• Stores, transports or degrades its contents

Lysosome: DIGESTION AND DEGRADATION • Membrane-bound spherical organelles • Vesicle with enzymes for intracellular digestion (acid hydrolases)

– Digest organic molecules

Mitochondria: POWERHOUSE • Double-membrane • Produces ATP • Contains folds (cristae)

Peroxisome

• Enzyme-filled vesicle that breaks down amino acids, fatty acids, and toxic substances

Vacuole: CELL MAINTENANCE • Membrane-surrounded • Sac in the cytoplasm • Fluid-filled • Isolates or disposes wastes, debris, or toxic materials • Storage site of food • Pumps water out of cell (contractile vacuole)

Cytoskeleton • Framework of cell • Dynamic network of protein filaments • Interacts with accessory proteins (motor proteins)

• Functions

– Support – Holds organelles in place (organize) – Enable cell to change shape

Cytoskeletal Elements • Microtubules

– Hollow filaments of tubulin subunits – Dynamic scaffolding; structural support

• Microfilaments – Reinforcing cytoskeletal elements; movement – Fibers of actin subunits – Strengthen/change shape of cell

• Intermediate Elements – Lock cells and tissues together – Most stable; maintain shape

Cytoskeletal Elements

Intermediate Filaments Microtubules Microfilaments

Cilia • Short, hair-like structures • Project from the plasma

membrane of some cells • Propels materials across

cell’s surface • Moved by organized arrays of

microtubules • Example: clears pathways

from airways

Microvilli • Shorter than cilia • Increases surface area

Flagella

• Whip-like structure • Propels cell through fluid • Sperm cell

Fig. 3-9, p. 52

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CELL DIVISION

Cell Division

• Formation of 2 daughter cells from a single parent cell

• Each cell (except sperm and egg) contains 46 chromosomes

• Egg and sperm contain 23 chromosomes

• Thread-like structures inside the nucleus of animals and plants

• Each is made up of: – Protein – Single molecule of DNA

• “chroma” (color) and “soma” (body)

– Cell structures strongly stained by colorful dyes

What is a Chromosome?

• Unique structure: – DNA tightly wrapped around histones

• “For an organism to grow and function properly,

cells must constantly divide to produce new cells to replace old, worn-out cells.”

• Ensures DNA is accurately copied and

distributed

What do Chromosomes do?

Mitosis • Cell division that occurs in all cells except sex cells • Forms 2 daughter cells • Components

–Chromatids: 2 strands of chromosomes genetically identical

–Centromere: where 2 chromatids are connected –Centrioles: small organelle composed of 9 triplets

What are Centromeres? • Constricted region of linear

chromosomes

• Center or at the end of the chromosome

• Help chromosomes aligned properly during cell division

• Attachment site for sister chromatids

• Repetitive stretches of DNA • Protect the ends of linear chromosomes

• Lose a bit of their DNA every time a cell divides

– When telomere is gone Æ cell cannot replicate – WBCs have special enzyme

• Role in cancer

– Do not lose telomeres but help to fuel the uncontrolled growth of malignant cells

What are Telomeres?

How many chromosomes do humans have?

Do males have different chromosomes than females?

Human Chromosome

Mitosis • Nuclear division that maintains the chromosome number

• Basis of:

– Body growth – Tissue repair and replacement

• Cell cycle starts when new cell forms, & ends when the cell reproduces

Cell Cycle

• Series of events from the time a cell forms until its cytoplasm divides

• Three Phases – Interphase – Mitosis – Cytoplasmic Division

Interphase • Most of cell’s activities occur • DNA replication • G1

– 1st interval (gap) of growth before DNA replication

• S – Interval of synthesis (DNA

replication)

• G2 – 2nd interval; cell prepares to divide – Make proteins for mitosis

Stages in Mitosis: Prophase

• Chromatin condenses into chromosomes • Centrioles move to opposite ends

The chromosomes become visible as distinct structures as they condense further. Microtubules assemble and move one of the two centrosomes to the opposite side of the nucleus, and the nuclear envelope breaks up.

Stages in Mitosis: Metaphase • Chromosomes align

Fig. 8-5b (4), p. 141

4 Metaphase

All of the chromosomes are aligned midway between the spindle poles. Microtubules attach each chromatid to one of the spindle poles, and its sister to the opposite pole.

Stages in Mitosis: Anaphase

• Chromatids separate to form 2 sets of chromosomes • Chromosomes move toward the centrioles

Motor proteins moving along spindle microtubules drag the chromatids toward the spindle poles, and the sister chromatids separate. Each sister chromatid is now a separate chromosome.

Stages in Mitosis: Telophase

• Chromosomes disperse • Nuclear envelopes and nucleoli form • Cytoplasm divides to form 2 cells

The chromosomes reach the spindle poles and decondense. A nuclear envelope forms around each cluster. Mitosis is over.