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

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)

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

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

 

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

Cell Size Limitations

•      Most cells are between 2 and 200 mm in diamater

–   Red blood cells - 8 mm

–   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

 

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

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

 

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)

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

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

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

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

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)