Cell Structure

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Don’t Freak Out

• Test on cell organelle on Friday!

• This test should be a buffer test and help raise your overall test score.

• All information will come from this week!

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Cells Provide Compartments for Biochemical Reactions

Cells contain

• water, small and large molecules

• Each cell contains at least 10,000 different molecules (many copies of most)

• Cells use these molecules to transform matter and energy, respond to their environments and to reproduce

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Cell Theory

1. The cell is the basic unit of life.

2. All living things are composed of cells.

3. All cells come from preexisting cells.

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First Unifying Theme of Biology

Two Conceptual Implications of Cell Theory

• Studying cell biology is in some sense the same as studying life– The principles that cause bacterial functioning are similar to those governing the approx. 60 trillion cells in an adult human.

• Life is continuous– All those human cells came from a single fertilized cells which were cells from parents whose cells came from parents and so on– all the way back to the evolution of the first living cells. 5

Cell size– limited by surface area to volume ratio

• As a living cell grows larger, its metabolic activity and need for resources increases faster than its surface area.

• Cells must maintain a large SA/V ratio in order to function.

• This explains why multicellular organisms must be composed of many small cells rather than a few large ones.

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Cell Membrane forms the outer surface of every cell

• Remember, a key to the origin of life was the enclosure of biochemical functions within a cell membrane.

• Phospholipid bilayer with proteins.

– Selectively permeable

– Communication with other cells and gets signals from environment

– Proteins can bind and adhere to surrounding environment

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Cells are classified as prokaryotic or eukaryotic

• Prokaryotes

– Does not have membrane-enclosed internal compartments (organelles); in particular, no nucleus

• Eukaryotes- contain membrane-bound compartments (organelles!) where specific metabolic functions occur

– Organelles allow a “division of labor” that was important for the evolution of more complex organisms

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Prokaryotic Cells Do Not Have a Nucleus (Review)

• 1-10 micrometers in length (smaller than eukaryotes)

• Have cell membrane

• Have nucleoid region where DNA is stored

• Have cytoplasm- consists of a liquid component, other particles, and ribosomes

• Has ribosomes- RNA and proteins– sites of protein synthesis.

• “Sticky” cell wall (different than plant cell wall)9

Prokaryotes

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Prokaryote

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Compartmentalization is key to eukaryotic cell function

• Each type of organelle has a specific role in the cell.

• Plant and animal cells have organelles in common as well as differences in organelles.

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Eukaryotic Animal Cell

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Ribosomes are factories for protein synthesis (making)

• Translate mRNA into a polypeptide chain

• Consists of a large subunit and a small subunit

• Consists of rRNA (ribosomal RNA) and proteins

• Not membrane-enclosed

• In prokaryotes, float freely in cytoplasm

• In eukaryotes, float in cytoplasm and on rough ER

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Nucleus contains cell’s DNA

• DNA- hereditary information (chromosomes/chromatin)

• Nucleus is usually the largest organelle.

• Functions:

– Location of DNA and DNA replication

– Where DNA is transcribed to RNA

– Contains the nucleolus, a region where ribosomes assembly begins

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Nucleus

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Nucleus

• Two lipid bilayers (two membranes) that form the nuclear envelope.

• Separates DNA transcription from translation

• Has nuclear pores which regulate movement of molecules into and out of the nucleus.

• Nucleus is continuous with the endoplasmic reticulum.

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Nuclear Envelope and E.R. Connection.

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Endomembrane System

• A group of interrelated organelles

• And interconnected system of membrane-enclosed compartments and contains the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and lysosomes.

• Vesicle shuttle substances between the various components of the endomembrane system and the cell membrane.

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The Endomembrane System

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Endoplasmic Reticulum

• Network of interconnected membranes branching through the cytoplasm

• Tubes and flattened sacs

• Collectively called the ER

• Interior is distinct from the outer cytoplasm

• Two types!

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1. Rough ER

• Called rough because of attached ribosomes to outer surface

• Proteins are chemically modified in the rough ER and “tagged” for delivery

• The rough ER participates in transporting these proteins to other locations in the cell; vesicles pinch off to do this

• Most membrane-bound proteins made in rough ER

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2. Smooth ER• Lack ribosomes

• Four important roles

–Chemical modification of small molecules that may be toxic to the cell in order to remove easier

– Site for glycogen degradation in animal cells

– Site where lipids and steroids are synthesized

– Stores calcium ions; when release trigger a number of cell reactions, such as muscle contractions

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Smooth ER

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Golgi apparatus

• Two Components: 1) Flattened sacs called cisternae and 2) small membrane enclosed vesicles

• Protein containing vesicles from rough ER release into Golgi apparatus cisternae, where they are further modified.

• Vesicles coming off the Golgi apparatus go to lysosomes or the cell membrane.

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Golgi Functions

1. Concentrates, packages, and sorts proteins before they are sent to their cellular or extracellular destinations

2. It adds some carbohydrates to proteins

3. It is where some polysaccharides for the plant cell wall are synthesized (made).

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Golgi Apparatus

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3-D Illustration of the Golgi Apparatus

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Electron Micrograph of the Golgi Apparatus

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Golgi Apparatus Directing Secretions

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Lysosomes

• Originate from the Golgi apparatus

• Contain digestive enzymes and they are the site where macromolecules are hydrolyzed into their monomers

• R1+R2 (linked monomers) + H2O R1-OH + R2-H

• **Hydrolysis

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Lysosomes

• Some things hydrolyzed by lysosomes enter by phago and pinocytosis

• Autophagy is the programmed destruction of cell components.

• An entire class of diseases called lysosomal storage diseases can happen if autophagy does not work properly. Ex: Tay-sachs– a lipid not broken down and accumulates in brain cells, damaging them.

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Lysosomes

Plant cells do not have lysosomes

• But their central vacuole seems to play a similar role and has many digestive enzymes.

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Mitochondria

The mitochondrion is used to phosphorylate ADP to ATP (energy). Two membranes- outer and inner. Inner is folded in.

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Mitochondria and Endosymbiosis

Mitochondria evolved from ancient prokaryotic cells through endosymbiosis. The evidence includes:

1. Has its own DNA that is circular like prokaryotes

2. Has prokaryotic like ribosomes

3. Replicates like a prokaryote

4. Does protein synthesis like a prokaryote

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3-D Illustration of the Mitochondria

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In cells, a higher the energy demand, corresponds with an

increase in the number of mitochondria in the cell.

Eukaryotic Plant Cell- What is different?

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Plastids-- Chloroplasts

• In plants and algae and can differentiate into different types of organelles

• Chloroplasts- contains the green pigment chlorophyll and is the site of photosynthesis.

• Photosynthesis– converts light into food

• Two membranes.

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Chloroplasts

• Thylakoids- stacks of hollow disks. Light energy converted to chemical energy

• Stroma- aqueous fluid surrounding the thylakoids. Carbohydrates synthesized here.

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Vesicles and Vacuoles

Vacuoles and vesicles are membrane bound compartments used for storage. Vacuoles are larger than vesicles. Plants use a large central water vacuole to maintain their turgor and structure.

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Contractile Vacuoles

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The Cytoskeleton Provides Strength and Movement

• Interior of cell– network of protein filaments

• Filament- each type is a polymer made up of protein monomers

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Cytoskeleton-- Functions1. Supports cell and maintains its shape

2. Holds cell organelles and other particles in position within the cell

3. Moves organelles an other particles around within the cell

4. Involved in movements of the cytoplasm called cytoplasmic streaming

5. Interacts with extracellular structures, helping anchor the cell in place.

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Three components of eukaryotic cytoskeleton

1.Microfilaments

2.Intermediate filaments

3.Microtubules

Different functions…48

Microfilaments

• Usually in bundles

• Help the entire cell or parts move.

• Determine and stabilize the shape.

• Made up of actin!

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Intermediate Filaments

• Anchor cell structures in place.

• They resist tension!

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Microtubules

• Long, hollow, unbranched cylinders.

• Form rigid internal skeleton for some cells or cell regions.

• Act as a framework along which motor proteins can move structures within a cell.

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