Anatomy of Cells

Introduction to Biology

The Discovery of Cells•  In Holland, Anton van

Leeuwenhoek examined pond water and a sample taken from a human mouth.

•  He drew the organisms he saw—which today we call bacteria.

•  Leeuwenhoek examined as many types of cells as he could.

Overview: The Importance of Cells

•  The early discoveries of cells are summarized in the cell theory, a fundamental concept of biology.

•  The cell theory states: o  All living things are made up of cells. o  Cells are the basic units of structure and function in living things. o  New cells are produced from existing cells.

Origin of Cellular Life•  The Earth formed about 4.6 billion years ago.

o  For about 500 million years, the Earth was continually bombarded by chunks of rock and ice in the solar system.

•  The early atmosphere of Earth contained: o  Water vapor H2O o  Nitrogen N2 o  Carbon dioxide CO2 o  Methane CH4 o  Ammonia NH3

Origin of Cellular Life•  How did life arise from such a harsh environment? •  Two scientists designed a model of what conditions

were like on Earth at this time. o  This is called the Miller-Urey Apparatus

Miller-Urey Apparatus

•  This apparatus simulated three important conditions on Earth: –  The high amount of lightning –  Heat and gases released by volcanic activity –  Water vapor present in the atmosphere.

Results of Miller-Urey Apparatus

•  Simple compounds including water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2) were used to simulate the atmosphere.

•  After 2 weeks, 10-15% of the carbon had been used to form sugars, amino acids, and parts of nucleic acids. o  These simple organic compounds could have produced the

proteins, lipids, and carbohydrates that make up life today.

The First Cells•  The first life forms on Earth were likely single-celled

prokaryotic organisms. o  Prokaryotic organisms are single-celled organisms that do not

have a nucleus. •  Their DNA or RNA is usually floating freely inside the cell.

o  Prokaryotic cells also do not have any membrane bound organelles.

A typical rod-shaped bacterium

A thin section through the bacterium Bacillus coagulans (TEM)

0.5 µm

Pili

Nucleoid

Ribosomes

Plasma membrane

Cell wall

Capsule

Flagella

Bacterial chromosome

Parts of a Prokaryotic Cell•  Nucleoid – Area where DNA or RNA is located. Not

enclosed in a membrane like a nucleus. •  Ribosomes – Small structures that use DNA or RNA

instructions to produce proteins. •  Pili – Hollow, hair-like structures that can be used to

exchange genes. •  Flagella – Spin to produce movement. •  Cell membrane – Controls what leaves or enters the

cell

Antibiotics•  Antibiotics are anti-bacterial chemicals that

originally came from mold. •  Each antibiotic works in different ways.

o  Penicillin disrupts the bacteria’s ability to produce a cell wall, causing it to burst due to an influx of water into its cytoplasm.

Antibiotic Resistance•  Bacteria can mutate and evolve quickly, due to

their small size and fast reproduction rate. •  Sometimes, a mutation will result in their ability to

resist the action of antibiotics. o  Over time, this mutation can spread throughout an entire colony, creating

a strain of antibiotic-resistant bacteria.

•  Resistant bacteria will not be affected by the antibiotics in the same way.

•  Eukaryotes are organisms with much larger and more complex cells than prokaryotes. •  DNA is in a nucleus that is

bounded by a nuclear membrane.

•  Have membrane-bound organelles

•  The largest eukaryotic cells are 0.1mm to 1.0mm in size. Why haven’t they evolved any larger?

Eukaryotic Cells

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Total surface area

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5

Total volume

Surface-to-volume ratio

Surface area increases while Total volume remains constant•  Volume represents the

size of the cell. •  Surface area represents

the amount of cell membrane to transport food, waste, water, and oxygen.

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Total surface area (height x width x number of sides x number of boxes)

6

1

11

5

6

Total volume (height x width x length X number of boxes)

Surface-to-volume ratio (surface area ÷ volume)

Surface area increases while Total volume remains constant•  A cell with a volume of

1mm3 will have a total surface area of 6mm2.

•  This provides plenty of area for the cell to absorb what it needs.

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Total surface area (height x width x number of sides x number of boxes)

125

150

11

5

1.2

Total volume (height x width x length X number of boxes)

Surface-to-volume ratio (surface area ÷ volume)

Surface area increases while Total volume remains constant•  A larger cell with a

volume of 125mm3 will only have a surface area of 150mm2.

•  This cell will not be able to transport wastes and nutrients fast enough.

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Total surface area (height x width x number of sides x number of boxes)

6

125 125

150 750

1

11

5

1.2 66

Total volume (height x width x length X number of boxes)

Surface-to-volume ratio (surface area ÷ volume)

Surface area increases while Total volume remains constant•  If the larger cell is instead

broken down into 125 smaller cells, it will once again have enough surface area.

•  This is why multicellular organisms exist!

Cell Organization•  The eukaryotic cell can be divided into two major parts: the

nucleus and the cytoplasm. •  The cytoplasm is the fluid portion of the cell outside the

nucleus. •  Prokaryotic cells have cytoplasm as well, even though they

do not have a nucleus.

Eukaryotic Cell Anatomy•  A eukaryotic cell has internal membranes that

partition the cell into organelles. o  Organelles are small structures within cells that have specific jobs.

•  Plant and animal cells have most of the same organelles, although there are a few differences.

Flagellum

Centrosome

CYTOSKELETON

Microfilaments

Intermediate filaments

Microtubules

Peroxisome

Microvilli

ENDOPLASMIC RETICULUM (ER

Rough ER Smooth ER

Mitochondrion Lysosome

Golgi apparatus

Ribosomes:

Plasma membrane

Nuclear envelope

NUCLEUS

In animal cells but not plant cells: Lysosomes Centrioles Flagella (in some plant sperm)

Nucleolus

Chromatin

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Rough endoplasmic reticulum

In plant cells but not animal cells: Chloroplasts Central vacuole and tonoplast Cell wall Plasmodesmata

Smooth endoplasmic reticulum

Ribosomes (small brown dots)

Central vacuole

Microfilaments Intermediate filaments Microtubules

CYTOSKELETON

Chloroplast

Plasmodesmata Wall of adjacent cell

Cell wall

Nuclear envelope

Nucleolus Chromatin

NUCLEUS

Centrosome

Golgi apparatus

Mitochondrion

Peroxisome

Plasma membrane

The Nucleus•  The nucleus contains most of the cell’s genes and is

usually the largest organelle. •  The nuclear envelope is a membrane that encloses

the nucleus, separating it from the cytoplasm. •  In the same way that the main office controls a large

factory, the nucleus is the control center of the cell. •  The nucleus contains nearly all the cell’s DNA and,

with it, the coded instructions for making proteins and other important molecules.

The Nuclear Membrane•  The nuclear envelope is dotted with thousands of

nuclear pores, which allow material to move into and out of the nucleus.

•  The nucleus mainly contains chromatin— the cell’s DNA instructions joined with proteins.

The Nuclear Membrane•  The nucleus also contains a small

dense region called the nucleolus.

•  The nucleolus produces ribosomes, which are needed to build proteins.

Organelles that Build Proteins

•  Because proteins carry out so many of the essential functions of living things, a big part of the cell is devoted producing and transporting them.

•  Proteins are synthesized on ribosomes, which can be found in two places: o  Freely floating in the cytoplasm o Attached to the endoplasmic reticulum

Ribosomes: Protein Factories

•  Ribosomes are particles made of RNA and protein o  Ribosomes produce proteins by following coded instructions

that come from DNA. o  Each ribosome is like a small machine in a factory, turning out

proteins on orders that come from its DNA “boss.”

Endoplasmic Reticulum•  The function of the endoplasmic reticulum (ER) is to

assist in the production, processing, and transport of proteins and in the production of lipids.

•  The endoplasmic reticulum (ER) is a huge membrane that is connected to the nuclear membrane.

•  There are two distinct regions of ER: o  Smooth ER, which lacks ribosomes o  Rough ER, with ribosomes studding its surface

Smooth Endoplasmic Reticulum

•  The smooth endoplasmic reticulum: o  Synthesizes lipids o  Metabolizes carbohydrates o  Stores calcium o  Detoxifies poison

•  The smooth endoplasmic reticulum does not contain any ribosomes, so it is unable to synthesize proteins.

Rough Endoplasmic Reticulum

•  The rough ER o  Holds ribosomes

o  Produces any proteins needed by the cell.

The Golgi Apparatus•  The Golgi apparatus is a series of flattened membrane

sacs in the cytoplasm. •  Functions of the Golgi apparatus:

o  Modifies, sorts, and packages materials into transport vesicles for storage or transport out of the cell.

o  A typical path for a protein produced by the cell: o  Rough ER → Golgi → Cell membrane → Released by cell

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Nuclear envelope

Nucleus

Rough ER

Smooth ER

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Nuclear envelope

Nucleus

Rough ER

Smooth ER

Transport vesicle

cis Golgi

trans Golgi

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Nuclear envelope

Nucleus

Rough ER

Smooth ER

Transport vesicle

cis Golgi

trans Golgi

Plasma membrane

Organelles that Store, Clean Up, and Support

•  These are organelles that help the cell maintain its shape, clean up wastes, and store material needed later. o  Vacuoles o  Lysosomes o  Cytoskeleton

Vacuoles•  Vesicles and vacuoles are membrane-bound sacs

that store many materials. •  Plant cells often have one large central vacuole. This

fills with water, making the cell rigid. o  When they are empty and dry, plants wilt!

Lysosomes•  Lysosomes serve as the cell’s cleanup crew. •  A lysosome is full of enzymes that can digest proteins,

lipids, polysaccharides, and nucleic acids. o  Can also breakdown old organelles so they can be re-used.

Animation: Lysosome Formation

Cytoskeleton•  The cytoskeleton is a network of protein filaments that

give the cell shape. o  Can also help transport materials across the cell.

•  Centrioles are part of the cytoskeleton that help move chromosomes during cell division.

Organelles that Capture and Release Energy

•  All life requires energy. •  Organisms either can get their energy from sunlight

via photosynthesis, or by eating other organisms via cell respiration.

•  Photosynthesis occurs in chloroplasts. •  Cell respiration occurs in mitochondria.

Mitochondria•  Mitochondria are the power plants of the cell. •  They convert the chemical energy stored in food into

smaller molecules for the cell to use. •  Mitochondria have two membranes, outer and inner. •  The inner membrane is folded up to increase the

amount of surface area to do chemical reactions.

Chloroplasts•  Chloroplasts contain the green pigment

chlorophyll, as well as enzymes and other molecules that function in photosynthesis

•  Chloroplasts are found in leaves and other green organs of plants and in algae

Plasma Membrane•  The plasma membrane is a selective barrier.

o Allows passage of oxygen, nutrients into the cell, and waste out of the cell.

•  The general structure of a biological membrane is a double layer of phospholipids o This allows the cell to control what goes in and

out.

Cell Wall•  The cell wall is made of cellulose and serves as

support and protection for the cell. •  Animals do not have cell walls, but plants, fungi, and

algae do. •  The cell wall is outside of the cell membrane.

Plants: Plasmodesmata•  The cell wall is so thick that oxygen, nutrients, water,

and waste cannot travel easily through. •  Plasmodesmata are channels that perforate plant cell

walls •  Through plasmodesmata, water and other small

molecules can enter the cell.

Animals: Tight Junctions, Desmosomes, and Gap Junctions

•  Although animal cells do not have cell walls, they also have special structures within their cell membranes.

•  At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid. o  Example: Lining of small intestines

•  Desmosomes (anchoring junctions) fasten cells together into strong sheets o  Example: Layers of outer skin cells

•  Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells o  Example: Cardiac muscle cells

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Tight junctions prevent fluid from moving across a layer of cells

Tight junction

0.5 µm

1 µm

0.1 µm

Gap junction Extracellular matrix

Space between cells

Plasma membranes of adjacent cells

Intermediate filaments

Tight junction

Desmosome

Gap junctions

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

•  Cells rely on the integration of structures and organelles in order to function

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