The Cell & Its Environment

Chapters 4 & 5

Objectives for this Unit• Explain Cell Theory

• Compare and contrast prokaryotic and eukaryotic organisms

• Describe the fluid mosaic model of the cell membrane and the different ways that materials are transported into and out of the cell

• Predict what will happen to a cell in various environments (isotonic, hypertonic, & hypotonic)

• Compare and contrast plant cells and animal cells

• Explain the structures and functions of cell organelles and how they operate together

The Cell Theory

• The cell theory states that:• All living things are composed of one or more cells.• The cell is the basic unit of structure and function in

organisms.• All cells are produced from other cells.

Prokaryotes vs. Eukaryotes

Karyon = nucleus (from Greek for “kernel”)

• Pro = before - Prokaryotes evolved before the cell nucleus• DNA in prokaryotes floats freely in the nucleoid region

of the cytoplasm• Prokaryotes do not have nuclei or other membrane-

bound organelles• All prokaryotes are unicellular organisms.

• Eu = true• The DNA in a eukaryotic cell is contained within a

nucleus.• Eukaryotes tend to also have other membrane-bound

organelles (internal compartmentalization)• There are both unicellular and multicellular eukaryotes

Characteristics of Prokaryotes

• Capsule – made of proteins and sugars • allows prokaryotes to attach

to things like teeth, skin, and food

• Flagella (singular: flagellum)• Enable movement

• Cell wall - provides structure and supportProkaryotes do not have a cytoskeleton, so they rely on the cell wall for protection and structure

• Compartmentalization• Other organelles are also enclosed

by their own membranes, which allows different parts of the cell to carry out different functions.

• Cytoskeleton – provides inner framework• Structural support allows eukaryotic

cells to be much larger than prokaryotic cells

• May also contain flagella and cilia for movement

• Nucleus – houses DNA• The nucleus is enclosed by a nuclear envelope, which

provides extra protection for DNA

Characteristics of Eukaryotes

Common Features of Cells

• Cell membrane (plasma membrane) – outer boundary• regulates what enters & leaves the cell

• Cytoplasm – interior of cell• Consists of fluid called cytosol• Contains dissolved solutes & organelles

• Ribosomes – tiny organelles that can float freely in the cytoplasm or be attached to endoplasmic reticulum• Where proteins are made

• ALL CELLS HAVE DNA

Cell Size

• As the size of a cell increases, its volume grows more than its surface area.

• Cells have to be very small so the plasma membrane has enough surface area to exchange materials fast enough to keep up with cellular activities.

• If nutrients cannot get in and wastes cannot get out fast enough, the cell will die.

• Eukaryotic cells are about 100X larger than prokaryotic cells

Swimming Pool Analogy

• A swimming pool can be used to model a cell. As the size of the pool increases, it takes longer to transport pool noodles and beach balls between the edge of the pool and the center. Similarly, as the size of a cell increases, it takes longer to transport nutrients and wastes between the cell membrane and the rest of the cell.

The Cell Membrane (Plasma Membrane)

• Outer border of the cell made of a phospholipid bilayer embedded with cholesterol, proteins, and carbohydrates.

• Selective permeability – only certain molecules can pass directly through the phospholipid bilayer

• Protein channels are needed to get other molecules and ions through the cell membrane

The Phospholipid Bilayer

• Phospholipids are composed of 2 fatty acid tails and a phosphate head attached to a glycerol molecule. • The phosphate group is

polar/hydrophilic (it likes water)• The fatty acids are

nonpolar/hydrophobic (they do not like water)

• As a result, the phospholipids line up in 2 layers, with the phosphates on the outside and the fatty acids on the inside

• Phospholipids move rapidly! The cell membrane is more like a fluid than a solid.

• Cholesterol helps stabilize the cell membrane.• Membrane-bound proteins are also able to move around

the cell membrane.• Peripheral proteins are bound to one side of the

membrane or the other• Marker proteins – tell what type of cell it is• Receptor proteins – bind to specific substances outside the

cell (often hormones)• Integral proteins pass all the way through the lipid bilayer• Transport proteins aid movement into and out of the cell

• Carbohydrates also aid in cell recognition and communication

The Fluid Mosaic Model

The Fluid Mosaic Model

How Do Materials Get Through the Cell Membrane?

• Passive transport – does not require energy• Diffusion• Simple diffusion• Facilitated diffusion

• Osmosis

• Active transport – requires energy• Protein pumps• Endocytosis• Exocytosis

Diffusion

• Diffusion is the movement of particles from an area of high concentration to an area of low concentration (down their concentration gradient)• Results from the random movement of particles• Eventually particles will become evenly spread out• Random movements continue (dynamic equilibrium)

Diffusion

• Simple diffusion is the diffusion of particles directly through the phospholipid bilayer• Happens with very small and lipid-

soluble molecules

• Facilitated diffusion is the diffusion of particles through integral protein channels in the plasma membrane• Happens with ions and large

molecules

Osmosis

• Osmosis is the movement of water across a semipermeable membrane

• Salt sucks! In osmosis, water moves from an area of lower solute concentration to an area of higher solute concentration.• The salt or other solutes do not

move.

Osmosis• Osmosis can cause a cell to swell or shrink,

depending on its environment.

• Plant cells are more resistant to this change because of their cell walls.

Type of Solutions

• In a hypertonic solution, there is a higher solute concentration outside the cell than inside, so water moves out of the cell.• This causes the cell to shrivel

and shrink, and possibly dry out.

Types of Solutions

• In a hypotonic solution, there is a lower solute concentration outside the cell than inside, so water moves into the cell.• This causes the cell to swell

and possibly burst.

Types of Solutions

• In an isotonic solution, the solute concentration is the same on the inside and outside of the cell.• There is no change in the cell.

Active Transport

• Active transport requires energy in the form of ATP (adenosine triphosphate)

• Needed to:• Transport molecules or ions from an area of low

concentration to an area of high concentration (against concentration gradient)

• Transport negative ions into a negatively charged area or positive ions into a positively charged area

• Transport very large molecules or objects into and out of the cell (endocytosis & exocytosis)

Sodium-Potassium Pumps

• Sodium-potassium pumps are needed to transport sodium ions (Na+) out of the cell and potassium ions (K+) into the cell• 3 Na+ transported out of cell• 2 K+ transported into cell• ATP is used to power the pump• Extremely important for nerve cells!

How it works

Endocytosis• During endocytosis, the plasma membrane engulfs

and then takes in substances from the cell’s environment.• Unicellular organisms use this process to eat food.• White blood cells do this to protect the body

• Phagocytosis – solid materials are taken in

• Pinocytosis – liquid droplets are taken in

Endocytosis

Exocytosis

• Exocytosis gets rid of waste or other materials (such as hormones) that need to be excreted from the cell• Materials are enclosed by a vesicle (made of

phospholipids) in the cell’s interior• The vesicle travels to the edge of the cell and fuses

with the cell membrane• Contents are expelled out of the cell

Exocytosis

Levels of Organization

• Cell – There are many different types of cells that all have their own unique structure & function

• Tissue

• Organ

• Organ System

• Organism

The Animal Cell

Nucleus• In animal cells (as in all eukaryotic cells), the nucleus is

where DNA is stored.

• The nucleus is enclosed by the nuclear envelope• Helps protect DNA from mutagens like UV rays.

• RNA is also found inside the nucleus. • DNA provides the instructions for building proteins within

the cell• Different types of RNA work together to build those proteins

• The nucleolus is a specialized area of the nucleus where ribosomes are made • Ribosomes are made of RNA & proteins• Ribosomes exit the nucleus via pores in the nuclear

envelope.

Nucleus

Nuclear envelope

Pore

DNA & RNA

Nucleolus

Ribosomes

• Ribosomes are made of RNA and proteins. Their job is to attach amino acids together to build long polypeptide chains (they form peptide bonds through condensation reactions!)

• May float freely in the cytoplasm or attach to the rough endoplasmic reticulum (RER).

Free ribosomes

Attached to RER

Endoplasmic Reticulum (ER)

• Endoplasmic = inside the cytoplasm

• Reticulum = meshlike or netlike structure

• The ER is a system of membranes made of a phospholipid bilayer and other membrane components.

• It consists of:• Smooth ER

Rough ER

Smooth Endoplasmic Reticulum (SER)

• Does not contain ribosomes (appear smooth under a microscope)

• Produces lipids

• Breaks down toxins into less harmful substances • there is lots of SER in liver

cells

Rough Endoplasmic Reticulum (RER)

• Appears rough under a microscope because it is studded with ribosomes

• Polypeptide chains made by ribosomes enter the rough endoplasmic reticulum, where protein synthesis continues

• Proteins are pinched off into vesicles and carried to the Golgi apparatus

Rough ER

Rough ER (covered with ribosomes)

Vesicle carrying proteins from RER to Golgi apparatus

Golgi Apparatus (Golgi body, Golgi complex)

• Vesicles containing proteins from the RER fuse with the membrane of the Golgi apparatus

• The Golgi apparatus is often referred to as the “post office” of the cell. Here, proteins are packaged into vesicles labeled with protein markers.

• These vesicles travel to the cell membrane, where the proteins are expelled from the cell via exocytosis.

Golgi Apparatus

Vesicle from RER to Golgi body

Golgi apparatus

Vesicle from Golgi complex to cell membrane

Mitochondria

• Energy is produced in the mitochondria – glucose is used to make ATP (adenosine triphosphate).

• Mitochondria are often referred to as the “powerhouses” of the cell.

• Have their own DNA and double membrane

Lysosomes

• Lysosomes are often compared to recycling centers.

• Where large molecules and old cell parts are broken down so their building blocks can be reused.

Cytoskeleton

• The cytoskeleton is a network of protein filaments within the cell.

• Supports the inside of the cell and helps prevent the cell membrane from collapsing.

• Aids in cell movement and the transport of materials within the cell.

• Main reason why eukaryotic cells can be larger than prokaryotic cells

Protein microfilaments

Centrioles

• Centrioles aid with organization during cell division

• The spindle that forms and separates the cell in 2 originates at the centrioles.

The Plant Cell Cell wall

• Outside of cell membrane• Helps regulate intake &

retention of water• Protects plant cell

Large vacuole• Stores water and breaks

down waste products Chloroplasts

• Where photosynthesis takes place

• Like mitochondria, they have their own DNA and a double membrane

The Plant Cell• **Plant cells do not have lysosomes or

centrioles!!

Endosymbiotic Theory

• It is believed that mitochondria and chloroplasts originated as prokaryotic cells that were ingested by other cells.

• Both of these prokaryotes benefited their host cells, and a symbiotic relationship developed.

• Only plants, animals, and fungi consist of multicellular organisms• (Some fungi

are unicellular)

• Most living things on Earth are too small for us to see without a microscope!

Domains of Life