Chapter 8 and 10b

Cell Processes

Cells do What?

Osmosis• What is diffusion? (review)

• Osmosis is the diffusion of water across a selectively permeable membrane– This maintains homeostasis in the cell

Cells in an Isotonic Solution• Isotonic solution - the concentration of

dissolved substances in the solution is the same as the concentration of dissolved substances inside the cell

• Water does go in and out of the cell, but there is no net change in concentration

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Cells in a Hypotonic Environment• Hypotonic solution - the concentration of

dissolved substances is lower in the solution outside the cell than the concentration inside the cell

• Therefore, water moves through the plasma membrane into the cell

• Pressure increases inside against the cell membrane and – Animal cells may burst– Plant cells become more “firm” as they expand

against the cell wall. (grocery stores)

Hypotonic

Cells in a Hypertonic Solution• Hypertonic solution - the concentration of

dissolved substances outside the cell is higher than the concentration inside the cell

• Water moves out of the cell– In animals, cells shrivel– In plants, membrane shrink away from cell wall

Osmosis Lab

Passive Transport• The movement of molecules across a

membrane by diffusion (no energy required)

• Facilitated Diffusion is passive transport across the cell membrane using proteins– Transport proteins form a channel to allow specific

molecules to flow across– Carrier proteins change shape to allow a substance

to pass through the plasma membrane

Animation

Facilitated Diffusion

Active Transport

• The movement of materials through a membrane against a concentration gradient– Requires energy

• Carrier proteins have a specific shape to allow them to bind with specific molecules

• Then then change shape (with energy) so molecule can be transported across

• Carrier protein resumes original shape

Active Transport

Transport of Large Molecules• Endocytosis - process by which a cell

surrounds and takes in material from its environment– Membrane forms around material and engulfs it

• Exocytosis - the expulsion of materials from a cell (wastes, hormones)– Membrane surrounds material and takes it to

membrane, opens up.

Endocytosis

Exocytosis

Section 1 Review

• What factors affect osmosis?

• How do animal and plant cells react differently in a hypotonic solution?

• How are facilitated diffusion and active transport similar? How different?

• How do carrier molecules facilitate transport of molecules across a membrane?

• A paramecium expels water when in freshwater. What kind of internal environment does it have?

Cell Size Limitations• Most cells are between 2 and 200 m in diamater

– Red blood cells - 8 m

– Yolk of ostrich egg - 8 cm

• Why can’t we have big cells?

• Diffusion works slowly over large distances

• DNA can only work so fast

• Surface to volume ratio

– As size increases, the volume increases faster than the surface area

– Cell membrane would back up with all the nutrients coming into and out of a large cell

Surface to Volume Ratio

1 mm

1 mm2 mm

2 mm4 mm

4 mmWhat is the surface area of each?

What is the volume of each?

2 mm1 mm

4 mm

Cell Division & Chromosomes• Cell Division is the process by which new cells are

produced from one cell– Two new cells identical to original

• Early biologists discovered that chromosomes appear during cell division and then disappear – Later, found importance of chromosomes (DNA)

• When not dividing, chromosomes exist as chromatin - long strands of DNA wrapped around proteins called histones (groups of histones are called nucleosomes) – Upon division, they condense and organize into

chromosomes

Chromosomes

back

The centromere plays an important role in the movement of chromosomes during mitosis

The Cell Cycle• The cell cycle is the sequence of growth and

division of a cell

• The majority of a cell’s life is spent in a period of growth and normal function called interphase

• Following interphase, a cell enters its period of division, called mitosis – Mitosis is the process by which two daughter cells are

formed, each containing a complete set of chromosomes

• Following mitosis, is cytokinesis, where the cytoplasm divides, separating the two daughter cells

The Cell Cycle

Phases of Mitosis - Prophase• Prophase - 1st and longest phase• Chromatin condenses into chromosomes• Each duplicated chromosome has two halves,

called sister chromatids• The nucleus begins to disappear

– The nucleolus and nuclear envelope disintegrate

• Centrioles migrate to opposite corners– Centrioles are small, dark, cylindrical structures

made of microtubules that play a role in chromatid separation

We’re still on Prophase• The spindle starts to form between the

centrioles– The spindle is a football-shaped, cagelike

structure consisting of thin fibers made of microtubules that helps separate sister chromatids later on

Interphase Prophase

The Phases of Mitosis - Metaphase• Shortest phase

• The doubled chromosomes (sister chromatids) become attached to the spindle fibers by their centromeres

• Chromosomes are pulled and line up at the midline or equator of the cell

• Each sister chromatid is attached to a spindle fiber at one pole

The Phases of Mitosis - Anaphase• The spindles pull the sister chromatids apart

– This marks the beginning of anaphase

• The centromeres split• The chromatids are pulled

apart by the shortening of the microtubules in the spindle fibers

The Phases of Mitosis -Telophase• Begins as the chromatids reach the opposite

poles of cell

• Chromosomes begin to unwind

• Spindle breaks down

• Nucleolus reappears and nuclear envelope forms around chromosomes

• A new double membrane begins to form between two new nuclei

Cytokinesis

• Differs in plants and animals

• In animals, the plasma membrane pinches in along the equator

Cytokinesis• In plants, a cell plate is laid down across the

cell equator– Then, a cell membrane forms around each new

cell and new cell walls form on each side of the cell plate

Results of Mitosis

• In unicellular organisms, the organism simply multiplied

• In multicellular organisms, two new cells are created to replenish dead cells or contribute to the growth of the organism

• Guarantees genetic continuity (new cells do things the same way as the old cells)

Cell Cycle Quiz

Levels of Organization• Cell

– Ex: muscle cell

• Tissue - groups of cells that work together to perform a specific function– Ex: muscle tissue

• Organs - tissues organized in various combinations that perform complex roles– Ex: stomach

• Organ System - multiple organs that work together– Ex: digestive system

• Organism all the organ systems working together – Ex: Florida panther

Section 2 Review• Describe how a cell’s surface-to-volume ratio

limits its size.

• Why is it necessary for a cell’s chromosomes to be distributed to its daughter cells in such a precise matter?

• Describe what happens in prophase. In metaphase. In telophase. In anaphase.

• Describe the phases of the cell cycle.

• Give an example at each level of organization in an organism.

Normal Control of the Cell Cycle• Proteins called cyclins and a set of enzymes that attach to

the cyclin control the cell cycle

• Research points to the portion of interphase just before DNA replication as a key period in the cell cycle

• Enzyme production is directed by genes located on chromosomes

– A gene is a segment of DNA that controls the production of a protein

• These control how fast or how slow the cell cycle needs to proceed.

– When would the cell cycle need to speed up or slow down?

Cancer• Cancer is a malignant growth resulting from

uncontrolled cell division• This failure to control cell growth may result

from not producing enzymes, overproducing enzymes, or producing the wrong enzymes at the wrong times.

• Currently, scientists think cancer starts because of changes in one or more of the genes that produce substances to control the cell cycle

Cancer• Tumors grow and deprive normal cells of

nutrients

• In later stages, the cancer spreads throughout the body (called metastasis)

• Leading cause of death in the United States (heart disease now #2)

• Can be genetically caused

• Can be environmentally caused– Cigarette smoke, air and water pollution,

exposure to UV rays from the sun, viral infections

Animation

Cancer Prevention• Diets low in fat and high in fiber can reduce the risk

of cancer

– Ex: fruits, vegetables and grain products for fiber

• Vitamins and minerals may also help prevent cancer

– Expecially carotenoids (found in yellow, orange, and green leafy vegetables), vitamins A, C, and E, and calcium

• Exercise, not smoking and other healthy habits are known to reduce the risk also

Section 3 Review• Do all cells complete the cell cycle in the

same amount of time?

• What controls the cell cycle?

• How can disruption of the cell cycle result in cancer?

• How does cancer affect normal cell functioning?

• What can be done to help reduce the risk of cancer?

Diploid Cells• In animals and most plants, chromosomes

occur in pairs – one from each parent– A cell with two of each kind of chromosome is called a

diploid cell and contains a diploid, or 2n, number of chromosomes

Haploid Cells• Organisms produce

gametes that contain one of each kind of chromosome – Called haploid, and has a

haploid, or n, number of chromosomes

Homologous Chromosomes

• The two chromosomes of each pair in a diploid cell are called homologous chromosomes

• The chromosomes contain the same genes in the same order, but might not be identical (unless parents genotypes are identical)

Why Meiosis?• Why won’t mitosis work for making gametes?• Cell division which produces gametes

containing half the number of chromosomes as a parent’s body cell is called meiosis– Why is this good?

• Meiosis consists of two separate divisions– Meiosis I– Meiosis II

Why Meiosis?• By the end of meiosis II, there are four

haploid cells called sex cells or gametes– Male gametes are called sperm or pollen– Female gametes are called eggs

• Fertilization is when sperm and egg unite – produces a zygote (fertilized cell)– Fusion of haploid sex cells is called sexual

reproduction

• Example: Male is 2n=46, gamete is n=23– Female is 2n=46, gamete n=23– After fertilization, new zygote is 2n=46

• Mitosis then takes over and organism grows

Phases of Meiosis

• Other than what happens to the chromosomes, everything else in meiosis is essentially the same as in mitosis

• Interphase– Cell replicates its

chromosomes

– End up with two identical sister chromatids, held together by a centromere

Prophase I• Chromosomes coil, spindle forms, etc.• Homologous chromosomes line up with other to form a

four-part structure called a tetrad– A tetrad is two homologous chromosomes (one from mom, one

from dad), each made of two sister chromatids • The chromatids are paired so tightly that chromosomes

can actually break and exchange genetic material– Process called crossing over– This can occur at any part of a chromatid and can occur several

times• Crossing over results in the new combinations of alleles

resulting in different people– Each pair of homologous chromosomes averages 2-3

crossovers in humans

Metaphase I

• Centromeres become attached to spindle fiber

• Tetrads pulled to equator of cell

Anaphase I• Homologous chromosomes, each with its two

chromatids, separate and move to opposite ends of the cell

• Centromeres don’t split• Ensures that each new cell will only receive one

chromosome from each homologous pair – Errors happen here often

back

Telophase I• Spindle broken down• Chromosomes uncoil• Cytoplasm divides • Each cell has half the

genetic information of the original cell– This is because it has only

one chromosome from each homologous pair

– This chromosome is still doubled

Meiosis II

• Some cells rest between stages

• Other cells go from Anaphase I straight to metaphase of meiosis II

• PMAT proceed much like mitosis

• End up with one copy of each chromosome (haploid)

Mitosis vs. Meiosis

Meiosis Animation

Section 2 Review

• How are the cells at the end of meiosis different from the cells at the beginning of meiosis?

• What does haploid mean? Diploid?• If the diploid number of a plant is 10, how

many chromosomes would you expect to find in its triploid offspring?