A Tour of the Cell

By the end of this presentation, you should be able to:

Distinguish between prokaryotic and eukaryotic cells

Explain why cells are small (surface area to volume ratio)

Explain the function of the cell parts

Explain how organelles work together to carry out specific cellular functions

Cells Are the Fundamental Units of Life

All organisms are made of cells

Cells are descended from other cells

Cells share common features

Biologists use microscopes and the tools of biochemistry to study cells

Most cells are between 1 and 100 m in diameter, too small to be seen by the unaided eye

Electron microscopes can be used to view structures within cells

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Figure 4.3

Scanning electron microscopy (SEM)

Transmission electron microscopy (TEM)

Longitudinal section of cilium

Cross section of cilium

Cilia

2 m

2 m

50

m

10

m

50

m

Brightfield (unstained specimen)

Electron Microscopy (EM)

Fluorescence

Brightfield (stained specimen)

Differential-interference contrast (Nomarski)

Phase-contrast

Confocal

Light Microscopy (LM)

Prokaryotic and Eukaryotic Cells

All living organisms are composed of one of two types of cells: prokaryotic or eukaryotic

Prokaryotic: Domains Bacteria and Archaea

Eukaryotic: Protists, fungi, animals, and plants

Comparing Prokaryotic and Eukaryotic Cells

All cells have:

Plasma membrane

Cytosol

Chromosomes (DNA)

Ribosomes

Prokaryotic cells are characterized by having

No nucleus

DNA is located in a region called the nucleoid

No membrane-bound organelles

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Prokaryotic Cells

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(a) A typical rod-shaped bacterium

0.5 m

(b) A thin section through the bacterium Bacillus coagulans (TEM)

Bacterial chromosome

Nucleoid

Ribosomes

Cell wall

Plasma membrane

Capsule

Flagella

Prokaryotic Cells

Eukaryotic cells are characterized by having

DNA in a membrane-bound nucleus

Membrane-bound organelles

Eukaryotic cells are generally much larger than prokaryotic cells

Eukaryotic Cells

Metabolic requirements of the cell set limits on cell size

Increase in volume = increase in demand for material resources

Smaller cells have a more favorable surface area-to-volume ratio

Why are cells so small?

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Figure 4.7a

CYTOSKELETON:

NUCLEUS

ENDOPLASMIC RETICULUM (ER)

Smooth ER

Rough ER Flagellum

Centrosome

Microfilaments

Intermediate filaments

Microvilli

Microtubules

Mitochondrion

Peroxisome Golgi apparatus

Lysosome

Plasma membrane

Ribosomes

Nucleolus

Nuclear envelope

Chromatin

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Figure 4.7e

1 m

A single yeast cell (colorized TEM)

Mitochondrion

Nucleus

Vacuole

Cell wall

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Figure 4.7b

CYTO- SKELETON

NUCLEUS

Smooth endoplasmic reticulum

Chloroplast

Central vacuole

Microfilaments

Intermediate filaments

Cell wall

Microtubules

Mitochondrion

Peroxisome

Golgi apparatus

Plasmodesmata

Plasma membrane

Ribosomes

Nucleolus

Nuclear envelope

Chromatin

Wall of adjacent cell

Rough endoplasmic reticulum

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Figure 4.7f

5

m

Cell wall

Cell

Chloroplast

Mitochondrion

Nucleus

Nucleolus

Cells from duckweed (colorized TEM)

Selective barrier that allows sufficient passage of oxygen, nutrients, and waste to meet the needs of the cell

Consists of a double layer of phospholipids and embedded proteins

Plasma Membrane

The Nucleus: Information Central

Contains most of the cells genetic information (genes)

Enclosed by a nuclear envelope (contains pores to allow materials to pass in and out of the nucleus)

DNA is organized into chromosomes

Chromosomes consist of chromatin: DNA and proteins

Chromatin condenses to form chromosomes during cell division

The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis

Within the Nucleus

Ribosomes: Protein Factories

Ribosomes are composed of ribosomal RNA and protein

Ribosomes carry out protein synthesis

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Figure 4.9

TEM showing ER and ribosomes Diagram of a ribosome

Ribosomes bound to ER

Free ribosomes in cytosol

Endoplasmic reticulum (ER)

Ribosomes ER

0.25 m

Large subunit

Small subunit

Endomembrane System

Regulates protein traffic and performs metabolic functions in the cell

Components of the endomembrane system: nuclear envelope, ER, Golgi apparatus, lysosome, vacuoles, plasma membrane

Components are either continuous or connected through transfer by vesicles

Endoplasmic Reticulum: Biosynthetic Factory

The ER membrane is continuous with the nuclear envelope

There are two distinct regions of ER

Smooth ER: lacks ribosomes

Rough ER: ribosomes on surface

Functions of Smooth ER

Synthesizes lipids

Metabolizes carbohydrates

Detoxifies drugs and poisons

Functions of Rough ER

Provides site-specific protein synthesis

Distributes transport vesicles, proteins surrounded by membranes

Consists of a series of flattened membrane sacs (cisternae)

Functions of the Golgi apparatus

Modifies products of the ER

Manufactures certain macromolecules

Sorts and packages materials into transport vesicles

Golgi Apparatus: Shipping and Receiving Center

Lysosomes: Digestive Compartments

Contains hydrolytic enzymes that digest macromolecules

Recycles old organelles (autophagy), digests molecules within food vacuoles (phagocytosis)

Lysosomes carry out intracellular digestion in a variety of ways

Contain oxidative enzymes

Performs essential metabolic functions, including:

Decomposition of long fatty acids

Decomposition of hydrogen peroxide into water and oxygen

Peroxisomes: Oxidation

Vacuoles: Diverse Maintenance Compartments

Vacuoles are large vesicles – three types:

Food vacuoles formed by phagocytosis

Contractile vacuoles in protists (pump water0

Central vacuoles in plant cells

Mitochondria and chloroplasts change energy from one form to another

Mitochondria - sites of cellular respiration, a metabolic process that uses oxygen to generate ATP

Chloroplasts - found in plants and algae, sites of photosynthesis

Mitochondria and chloroplasts have similarities with bacteria

Enveloped by a double membrane

Contain free ribosomes and circular DNA molecules

Grow and reproduce somewhat independently in cells

The Evolutionary Origins of Mitochondria and Chloroplasts

An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an endosymbiont relationship with its host

The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion

At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts

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Endosymbiont Theory

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Figure 4.16

Mitochondrion

Mitochondrion

Nonphotosynthetic eukaryote

Photosynthetic eukaryote

At least one cell Chloroplast

Engulfing of photosynthetic prokaryote

Nucleus

Nuclear envelope

Endoplasmic reticulum

Ancestor of eukaryotic cells (host cell)

Engulfing of oxygen- using nonphotosynthetic prokaryote, which becomes a mitochondrion

Mitochondria: Chemical Energy Conversion

Smooth outer membrane and an inner membrane folded into cristae

Cristae present a large surface area for enzymes that synthesize ATP

The inner membrane creates two compartments: intermembrane space and mitochondrial matrix

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Figure 4.17

Free ribosomes in the mitochondrial matrix

Mitochondrion

Intermembrane space

Matrix

Cristae

DNA

Outer membrane

Inner membrane

0.1 m

Chloroplasts: Capture of Light Energy

Contain the green pigment chlorophyll

Function in photosynthesis

Found in plants, algae and other photosynthetic protists

Chloroplasts are a type of plant plastid

Thylakoids, membranous sacs, stacked to form a granum (light reactions of photosynthesis)

Stroma, the internal fluid (Calvin-Benson cycle of photosynthesis (“dark” reactions))

Structure of a Chloroplast

Cytoskeleton

A network of fibers extending throughout the cytoplasm

Organizes the cell’s structures and activities, anchoring many organelles, supports and maintains shape of cell

Roles of the Cytoskeleton

Interacts with motor proteins to transport vesicles and other organelles along the “tracks” of the cytoskeleton

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Table 4.1

Centrosome is a “microtubule-organizing center”

The centrosome has a pair of centrioles which plays a role in cell division

Centrioles and Centrosomes

Cell projections that are composed of microtubules

Flagella are limited to one or a few per cell, while cilia occur in large numbers on cell surfaces

Cilia and Flagella

Cell Walls of Plants

An extracellular structure that distinguishes plant cells from animal cells

Prokaryotes, fungi, and some protists also have cell walls

Protects the plant cell, maintains its shape, and prevents excessive uptake of water

Plant cell walls are composed mostly of cellulose

The Extracellular Matrix (ECM) of Animal Cells

Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM)

Cell Junctions

Neighboring cells in an animal or plant often adhere, interact, and communicate through direct physical contact

Types of intercellular junctions:

Plasmodesmata (plants)

Tight junctions

Desmosomes

Gap junctions

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Channels between adjacent plant cells

Allow for the exchange of materials (water, solutes) between cells

Plasmodesmata of Plant Cells (Cell Walls)

Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells

Animal cells have three main types of cell junctions

Tight junctions

Desmosomes

Gap junctions

All are especially common in epithelial tissue (skin, lines organs and body cavities)

Tight Junctions

Between the cell membranes of adjacent animal cells

Form continuous seals around cells to prevent leakage of extracellular fluid

Example: between skin cells

Desmosomes

“Anchoring junctions”, fasten cells together in strong sheets

Composed of tough keratin proteins

Example: muscle cell attachments

Gap Junctions

“Communicating” junctions

Channels that directly connect the cytoplasm of two cells

Allow for the passage of small molecules such as ions, sugars and amino acids

Example: between heart muscle cells

The Cell: A Living Unit Greater Than the Sum of Its Parts

Cellular functions arise from cellular order

Example: a macrophage’s ability to destroy bacteria involves the whole cell, coordinating components such as the cytoskeleton, lysosomes, and plasma membrane

Macrophage (red) Consuming Bacteria (green)

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Summary of Key Organelles

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Summary of Key Organelles

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Summary of Key Organelles