Unit 1: Cells Refresher, Selectively Permeable

Membranes, Cell Regulation, and Enzymes

Parts of the Cell

• Cell Membrane: made of a phospholipid bilayer with proteins that function as channels, markers, and receptors• It is a selectively permeable boundary between the inside of

the cell and its external environment.

Parts of the Cell

• Nucleus: An organelle in eukaryotic cells that contains the cell’s DNA.• It also manages cell functions.• Contains the nucleolus.

Parts of the Cell

• Cell wall: *ONLY IN PLANTS*• Rigid wall made of cellulose (sugar), proteins, and

carbohydrates• Forms a boundary around plant cells and provides structure

and support

Parts of the Cell

• Cytoplasm: gel-like fluid that lies inside of the cell and around the organelles• Contains salts, minerals, and organic molecules

Parts of the Cell

• Cytoskeleton: A network of support made of• Microtubules• Microfilaments

• Helps to support the organelles and maintain cell shape

Parts of the Cell

• Ribosomes: made up of two subunits made of protein and RNA• They are the location of protein synthesis.

Parts of the Cell

• Endoplasmic Reticulum: “cell highway”• System of tubules and sacs that moves molecules around the

cell• TWO types:

• Rough ER• Smooth ER

Parts of the Cell

• Rough Endoplasmic Reticulum: covered with ribosomes

Parts of the Cell

• Smooth Endoplasmic Reticulum: has no ribosomes• Involved in the synthesis of lipids and the breakdown of toxins

Parts of the Cell

• Golgi Apparatus: stacks of flattened sacs• Receives proteins from the Rough ER and distributes them• Like the post office: receiving, processing, packaging, shipping

Parts of the Cell

• Mitochondria: “powerhouse of the cell”• Made of an outer membrane and folded inner membrane

(cristae)• Converts energy from food into usable energy (ATP)

Parts of the Cell

• Lysosomes: sphere-shaped organelles that contain hydrolytic enzymes• These enzymes break down food particles, invading objects, or

worn out cell parts

Parts of the Cell

• Chloroplasts: a double membrane containing stacked sacs (thylakoids) that contain chlorophyll• Involved in photosynthesis

Selectively Permeable Membrane

• The cell membrane is selectively permeable.• What does this mean?• It will let some substances cross but not others, according to

the needs of the cell.

• The cell membrane is made of a phospholipid bilayer. (hydrophilic heads, hydrophobic tails)

Solutions

• Solutions are made of two components:

• Solute: the substance that is dissolved or put into the solvent (ex. Salt)

• Solvent: the liquid into which the solute is dissolved (ex. Water)

• The concentration of the solution depends on how much solute is present.• Higher amount of solute = Higher concentration

Tonicity

• Hypotonic Solution: One solution has a lower concentration of solute than another

• Hypertonic Solution: one solution has a higher concentration of solute than another

• Isotonic Solution: both solutions have the same concentrations of solute

Methods of Transport Across the Membrane

1. Diffusion

2. Osmosis

3. Facilitated Diffusion

4. Active Transport

Diffusion

• PASSIVE transport (no energy required)• Movement of molecules from an area of high concentration to an

area of low concentration

Osmosis

• PASSIVE transport (no energy)• Transport of water across a membrane• Water moves from a high concentration of water (low

concentration of solute) to a low concentration of water (high concentration of solute)

Facilitated Diffusion

• PASSIVE transport• Uses proteins to carry polar molecules or ions across the

membrane• Proteins are embedded in the membrane as channels.

The molecules pass through the channels.

Active Transport

• Requires energy (ATP)• Transports molecules AGAINST a concentration gradient

(low to high)

Cell Cycle Refresher

• The growth and division of cells is called the cell cycle.• The cell cycle is made up of stages• Interphase• Mitosis• Cytokinesis

Interphase

• The cell grows, carries out functions, and replicates.• Made of three stages• G1: the cell grows, carries out normal functions, and

prepares to replicate DNA• S (synthesis): the cell copies its DNA to prepare for

cell division• G2: the cell prepares for the division of the nucleus

Mitosis

• Made up of 4 stages• Prophase• Metaphase• Anaphase• Telophase

Prophase

• Chromatin appears• Sister chromatids are

attached at the centromere• Spindle fibers form• Nuclear envelope

disappears• Spindle fibers attach to

sister chromatids

Metaphase

• Sister chromatids are pulled along the spindle apparatus and line up in the middle of the cell.

Anaphase

• The microtubules of the spindle apparatus begin to shorten.• Sister chromatids

separate.• The chromosomes

move toward the poles (ends) of the cell.

Telophase

• The chromosomes reach the poles. • Two new nuclear

membranes begin to form and the nucleoli reappear.• The spindle

apparatus breaks down.

Cytokinesis

• ANIMAL cells: microfilaments pinch the cytoplasm to split the cells

• PLANT cells: a cell plate forms to divide the cells

Cell Cycle Regulation

• The rate and timing of cell division in different parts of an organism are VERY IMPORTANT for normal growth, development, and maintenance.

• Different types of cells divide at different rates.• Skin cells divide very often, while liver cells do not. Nerve cells and

muscle cells do not divide at all after they reach maturity.

Cell Cycle Regulation

•Different combinations of cyclin and CDK signal…• DNA replication• Protein synthesis• Nuclear division

Cell Cycle Regulation

• The cell cycle has built-in checkpoints that monitor the cycle and can stop it if something goes wrong.• The checkpoints are controlled by stop and go signals.• Cancer is caused by the uncontrolled division of cells.

Cell Cycle Regulation

• There are three checkpoints during the cell cycle.

1. G1 checkpoint2. G2 checkpoint3. M checkpoint

• The checkpoints are directed by a cell cycle control system.• The control cycle has a

built-in clock, but is also regulated by internal and external controls.

G1 Checkpoint

• Most important for many cells• If the cell receives a go-ahead signal, it will complete

the cell cycle and divide.• If the cell does not receive a go-ahead signal, it will exit

the cycle and switch to a nondividing state.• Most human cells are in this phase.

Cell Cycle Control Molecules

• The cell cycle is paced by control molecules.• Protein kinases activate or deactivate proteins by

phosphorylating them. • Cyclin activates protein kinases.• Cyclin + kinases form cyclin-dependent kinases (Cdk’s)

Cell Cycle Control Molecules

• MPF= “maturation-promoting factor” or “M-phase-promoting factor”• MPF pushes the cell past the G2 checkpoint and into

the M phase (mitosis)• MPF promotes mitosis by phosphorylating other

protein kinases

M Phase Checkpoint

• The M checkpoint makes sure that all of the chromosomes are attached to the spindle during metaphase and before anaphase.• Why do you think this is important??• If some chromosomes are not attached to the spindle,

their kinetochores send out a signal to delay anaphase.

Growth Factors

• Growth factors are proteins released by cells that stimulate other cells to divide.• Example: PDGF (platelet-derived growth factor) is

produced by platelets (blood cells). PDGF stimulates cell division in fibroblasts, which help to heal wounds.

Cell Cycle Regulation

• Apoptosis is programmed cell death• This is useful when cells

become worn out and unusable.• During apoptosis, cells shrink

and shrivel in a controlled process.

ENZYMES

Chemical reactions of life..

• Building molecules: synthesis

• Breaking down molecules: digestion

Nothing works without enzymes!• All chemical reactions in living organisms require enzymes to work• Building molecules = synthesis enzymes enzyme

• Breaking down molecules = digestive enzymes enzyme

• Enzymes speed up reactions = catalysts

Enzymes are Proteins

• Each enzyme is the specific helper to a specific reaction• Each enzyme needs to be the right shape for the

job• Enzymes are named for the reaction they help• Sucrase breaks down sucrose• Proteases break down proteins• Lipases break down lipids• DNA polymerase builds DNA

Enzymes aren’t used up…

• Enzymes are not changed by the reaction• Used only temporarily• Re-used again for the same reaction with other

molecules• Very little enzyme needed to help in many reactions

substrate

Active site enzyme

product

It’s shape that matters!

• Lock & Key model• Shape of protein allows enzyme & substrate to fit• Specific enzyme for each specific reaction

Enzyme Vocabulary• Enzyme• Helper protein molecule

• Substrate• Molecule that enzymes work

on• Products• What the enzyme helps

produce from the reaction• Active site• Part of enzyme that substrate

molecule fits into

What affects enzyme action?

• Correct protein structure• Correct order of amino acids

• Temperature• pH (acids & bases)

• These all affect the shape of an enzyme. Remember: shape is the most important part!

Order of Amino Acids

• Wrong order = wrong shape = can’t do its job!

DNA

Chain of amino acids

Right shape!

DNA

Wrong order of amino acids

Wrong shape!

Temperature

• Effect on rates of enzyme activity• Optimum temperature• Greatest number of collisions between enzyme & substrate• Human enzymes• 35 degrees-40 degrees Celsius (Body temp=37 degrees

Celsius)• Raise temperature (boiling)• Denature protein (unfold, lose shape) *EGGS*

• Lower temperature• Molecules move slower• Fewer collisions between enzyme & substrate

Temperature

Temperature

Reacti

on

Rate

Human enzymes

37°C

pH

• Effect on rates of enzyme activity• Changes in pH change protein shape• Most human enzymes = pH 6-8• Depends on where in the body• Pepsin (stomach) = pH 3• Trypsin (small intestines) = pH 8

7

pH

pH

reac

tio

n r

ate

20 1 3 4 5 6 8 9 10

stomachpepsin

intestinestrypsin

11 12 13 14