Unit 2: Cellular Structure & Function

Honors Biology

The Cell Theory

• Cell = the smallest unit of life
• Invention of the microscope allowed scientists to see & study cells
• Robert Hooke was the first person to see cells (dead cork cells) & gave them their name

• As microscopes improved scientists could identify cell organelles
• Organelle = part of a cell that performs a job or function for the cell
• Acts like the organs in your body (ex. Stomach)
• Further study eventually lead to the creation of the cell theory

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Cell Theory

• All living things are made of cells
• The cell is the basic unit of structure and function in living things
• All cells come from pre-existing (other living) cells

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All Cells Have These 5 Things

• Cell membrane = layer of molecules that creates the outside of the cell
• Function: control what enters and exits the cell

• Cytoplasm = a jelly-like liquid that fills the inside of the cell
• Function: holds the organelles & space where many chemical reactions occur

• DNA = the genetic material of the cell
• Function: controls what the cell does

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• Ribosome = tiny organelle that does not have a membrane
• Function: make proteins
• Protein = large molecule used for many things in cells
• Have nitrogen (N) in them

• Cytoskeleton = protein fibers (sticks) that act like the bones of the cell
• Functions:

- Supports & shapes the cell

- Provides strength

- Helps the cell to move

- Helps in cell division

- Holds organelles in place

2 Types of Cells

Prokaryotic Cells

• No nucleus (does have DNA)
• No membrane-bound organelles (not formed by a membrane)
• Only unicellular (single-celled) organisms
• Example = bacteria

Eukaryotic Cells

• Has a nucleus
• Has membrane-bound organelles (covered in a skin-like layer)
• Can be multicellular or unicellular (single-celled) organisms
• Example = plant, animal, & fungi cells

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Organelles Unique To Eukaryotic Cells

• In eukaryotic cells membrane-bound organelles create different “rooms” that allow the cell to perform different functions at the same time
• This allows eukaryotic cells to be larger than prokaryotic cells

• Nucleus = Protects the DNA
• Nuclear envelope = double membrane that makes the nucleus
• Nuclear pores = holes in the nuclear envelope
• Function: Lets things move in and out of the nucleus
• Nucleolus = dense area inside the nucleus
• Function: Makes ribosomes

• Rough ER = membrane folds that have ribosomes attached to them
• Function: Makes proteins
• Smooth ER = membrane folds that do not have ribosomes attached to them
• Functions:

- Makes lipids (fats and oils) & hormones

- Breaks down toxins (ex. drugs & alcohol)

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• Golgi apparatus = closely layered stacks of membrane envelopes
• Looks like a stack of pancakes
• Function: Receive, modify, sort, and deliver proteins
• Vesicle = membrane-bound sac (container)
• Function: Moves materials around the cell
• Made from a pinched off part of a membrane
• Cells can change the number of vesicles they have

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• Mitochondria = bean shaped organelle with two membranes
• Inner membrane has many folds
• Has its own DNA & ribosomes
• Function: Breaks down food to release usable energy (ATP)

• Centrosome = area of the cytoplasm where parts of the cytoskeleton are made

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• Centrioles = cylinder-shaped organelles that are at 90° to each other
• Found in the centrosome
• Only in animal cells
• Function is not known
• Might help with cell division

• Vacuole = liquid filled membrane sac (larger than vesicles)
• Function: Stores food, water, & waste
• Animal cells have many small vacuoles
• Plant cells have one large vacuole
• Central vacuole = the large vacuole in plant cells that stores water, pigments, waste, & other materials
• Takes up most of the space in the cell

• Lysosome = membrane sac that has enzymes (special proteins) inside it
• Function: breaks down and recycles cell waste & invaders
• Ex. old & damaged cell parts, large food molecules, disease causing bacteria
• Mostly found in animal cells, but also in some plant cells
• Flagella = tail like structure in some animal cells & prokaryotes
• Function: movement

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Flagella

• Cell wall = the thick outermost layer in plant cells
• Function: gives extra protection, support, and shape to the cell
• Fungi & bacteria also have cell walls, but they are made of different molecules
• Chloroplast = has two outer membranes and contains chlorophyll (green color)

• Has its own DNA & ribosomes
• Only found in plant cells
• Function: uses light to build sugars (food)

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Viruses & Vaccines

• Virus = an infectious agent made up of nucleic acid (DNA or RNA) surrounded by a capsid (a protein coat)
• Some viruses also have a membrane envelope

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vs

• Viruses depend on other organisms (their host cell) to reproduce
• Viruses cannot reproduce on their own!
• Viruses inject their genetic material into a cell & use the cell’s machinery to create new viruses
• The host cell is harmed in the process and eventually dies as a result

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HIV Infected Cell

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Viruses Are NOT Living Things!

• Viruses are not made of cells
• They do not have a nucleus, organelles, cytoplasm, or cell membrane
• Since they don’t possess all the characteristics of life they are not considered living things!
• Viruses only have two of the characteristics of life
• Genetic material
• Change over time

Viruses Are Specific

• A virus can only attack certain cell types
• Ex. Rabies virus only attacks brain or nervous cells
• Ex. Bacteriophage is a virus that only infects bacteria cells
• A virus recognizes cells it can infect by matching its surface marker with a receptor site on a cell
• Like a lock and key or the pieces of a puzzle!

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Viral Life Cycles

• After a virus invades a cell, there are 2 possible life cycles
1. Lytic cycle = the viral genetic material immediately takes over the cell and begins making copies of itself
• The new viruses are released when the cell bursts

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2. Lysogenic cycle = the viral genetic material becomes integrated into the host cell’s DNA
• When the cell replicates the new cells will have the viral DNA as well
• When the viral DNA gets a signal it enters the lytic cycle & makes new copies of the virus
• This is happening in many cells at the same time!

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Vaccines

• Pathogen = something that causes a disease or sickness
• Ex. virus and bacteria can be pathogens
• Vaccine = a substance that teaches your immune system to recognize and attack a specific pathogen
• Allows your body to build up immunity to the pathogen without getting sick
• Vaccines are effective against both viruses and bacteria!
• Antibiotic = a substance that can only treat diseases caused by bacteria

• Major types of vaccines
• Live-attenuated vaccine = vaccine made from a weakened pathogen
• Usually gives you lifelong immunity
• Inactivated vaccine = vaccine made from a “killed” pathogen
• May require that you get multiple doses to maintain immunity (booster shots)
• Vaccines can also be made from parts of pathogens or toxins produced by pathogens
• Ex. the HPV vaccine is made from proteins found on the capsid of the virus

Cell Membranes

• Also called the plasma membrane
• Function: Controls what enters and exits the cell
• Is selectively permeable
• Only allows some things to cross the cell membrane
• Made of phospholipids
• Phospholipid = molecule made of a phosphate group, a glycerol, and two fatty acids

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Phospholipid Structure

Membrane Structure

• Phospholipid bilayer = double-layer of phospholipids
• Middle of the membrane is nonpolar because…
• Polar heads point outwards
• Hydrophobic tails point inward

• Small, nonpolar molecules can move through the membrane easily on their own
• Ex. CO₂ and O₂
• Large, polar, or charged molecules can’t get through the nonpolar middle
• Ex. H₂O and sugars

• Lipids can move past each other easily
• It’s fluid-like (flexible)
• Proteins are embedded (stuck) in the membrane
• Some move molecules across the membrane (transport proteins)
• Cholesterol embedded in the membrane (a steroid/hormone) keeps it fluid and flexible

• The Fluid Mosaic Model describes the membrane’s structure & behavior
• The membrane is flexible and is made of many different molecules

Cellular Transport

• Passive transport = molecules move across the cell membrane without the cell using energy
• Active transport = the cell uses energy (ATP) to move molecules across the cell membrane

Passive Transport

• Molecules move from an area of high concentration to low concentration
• Can also say that molecules move down a concentration gradient
• Concentration gradient = the difference in the amount of a substance between two places
• Molecules move until equilibrium is reached
• Equilibrium = the same amount in all areas

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• There are 3 types of passive transport
1. Diffusion = Molecules move across the cell membrane on their own
• Also called simple diffusion
2. Facilitated diffusion = molecules diffuse through protein channels in the cell membrane
• Molecules can’t cross on their own because they are polar, charged, or too big
3. Osmosis = the passive transport of water across the cell membrane

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• Example of simple diffusion
• Molecules are moving down a concentration gradient & crossing the membrane on their own

Active Transport

• Cells use active transport to maintain homeostasis (stable internal condition)
• Doesn’t always want to be equal with the environment
• Cells want their pH, temp., & concentrations of molecules to remain stable

• Molecules move from an area of low concentration to an area of high concentration
• Molecules move against (or up) their concentration gradient

• All types of active transport require the cell to use ATP
1. The cell moves molecules using protein pumps
• Protein pumps work like a lock & key system, each protein only moves certain molecules
2. Endocytosis = the cell uses vesicles to move molecules into the cell
• Once inside the cell the vesicle can fuse with lysosomes to break down the materials
3. Exocytosis = the cell uses vesicles to move molecules out the cell
• Once the vesicle fuses with the cell membrane the materials are released

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• Endocytosis & Exocytosis

Osmosis

• Osmosis = the diffusion of water across the cell membrane
• Is a type of passive transport
• Gets its own name because water is so important to cells!
• The cytoplasm of a cell is mostly water
• Water can move through the cell membrane on its own or through special proteins called aquaporins

Types of Cell Environments

1. Hypertonic
• Environment = a lot of solutes, a little water
• Cell = few solutes, a lot of water
• Water moves…OUT of the cell
• The cell…SHRINKS

2. Hypotonic
• Environment = few solutes, a lot of water
• Cell = a lot of solutes, a little water
• Water moves…INTO the cell
• The cell…SWELLS, eventually can burst

3. Isotonic
• Equal amounts of solutes and water in the cell and the environment
• Water moves…IN and OUT of the cell equally
• The cell…STAYS THE SAME

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Plant vs. Animal Cells

• Animal cells want to be in an isotonic environment
• Hypotonic environment can cause the cell to lyse (burst)
• Hypertonic environment can cause the cell to shrink
• Plant cells want to be in a hypotonic environment
• The cell wall prevents the cell from bursting
• The water entering the plant cell makes it firm & allows the plant to stand upright so it can get sunlight

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