Cellular transport and the Cell Cycle

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. Diffusion results because of the random movement of particles.

Ex. Food coloring (solute) and water (solvent)

Three key factors---- Concentration, temperature, and pressure-----affect the rate of diffusion.

Osmosis: Diffusion of water

Remember the plasma membrane is a semipermeable so it regulates what goes in and out of the cell. So how does water move in and out of the cell? The diffusion of water across a selectively permeable membrane is called osmosis.

Regulating the water flow through the plasma membrane is an important factor in maintaining homeostasis within the cell.

Osmosis

• Water diffuses across the membrane it is in equal concentration on both sides of the membrane.

• It is important to understand how osmosis affects cells.

• In isotonic solution, the concentration of dissolved substance in the solution is the same as the cell. This means that osmosis is occurring but because water diffuses into and out of the cells at the same rate, the cells retain their normal shape.

In an a hypotonic solution, the concentration of dissolved substances is lower in the solution outside the cell than the concentration inside the cell. Meaning there is more water outside the cell than inside.

Water moves through the plasma membrane into the cell. The cell swells and its internal pressure increases.

The effect on this on an animal cell is if to much water enters the cell, the pressure increases greatly and the cell will burst. Due to the cell wall in plant cells, the cells expand but do not burst.

• In a hypertonic solution, the concentration of dissolved substances outside the cell is higher than the concentration inside the cell.

• Cells in a hypertonic solution experience osmosis that causes water to flow out.

• The effect of hypertonic solution on an animal cell is that water flows out of the cell and causes the cell to shrivel because of the decreased pressure in the cell. Plant cells mainly lose water from the vacuole and the plasma membrane and cytoplasm shrink away from the cell wall. Pressure drops.

Passive Transport• When no energy is required to move particles

across the membrane is classified as passive transport.

• Passive transport that uses transport proteins is called facilitated diffusion.

• Channel proteins and carrier proteins allow the movement of substances across the membrane with the concentration gradient (high to low)and requires no energy input from the cell.

Active Transport

• Movement of materials through a membrane against a concentration gradient is called active Transport and requires energy from the cell.

• What happens is a carrier protein binds with a molecule or ion to be transported. When the proper molecule or ion binds with the protein, chemical energy allows the cell to change the shape of the carrier protein so that the particle to be moved is released on the other sided of the membrane.

• Once the particle is released, the protein’s original shape is restored.

• Active transport allows movement into or out of a cell against a concentration gradient. (Low to high)

• The transport of substances across the cell membrane is required for cells to maintain homeostasis.

Transport of Large particles

• Some cells can take in large molecules, groups of molecules, or even whole cells.

• Endocytosis is a process by which a cell surrounds and takes in material from its environment.

• How it works. The material does not pass the membrane directly, instead it is engulfed and enclosed by a portion of the plasma membrane.

• Exocytosis is the expulsion or secretion of materials from a cell. Cells use exocytosis to expel wastes or hormones produced by cell.

• Because endocytosis and exocytosis both move masses of material, they both require energy.

Cell growth and reproduction

All cells grow and carry out a their function. The function of a cell depends on what kind of cell it is. Ex. Liver cells, skin cells, muscle cells.

Once a cell reaches a maximum size it must divide. Remember all cells come from preexisting cells.

Cells grow, divide then die. So it is important to replace the dying cells.

Cell Growth and Reproduction

• Prokaryotic organisms grow until reaching a critical size and then divide. This process is repeated over and over. The rate of growth of bacteria cells depend greatly on environmental factors such are nutrient availability and temperature.

• Prokaryotic cells divide in the process of fission.

Prokaryote: Fission

Eukaryotic cell

• The cell cycle (life of a cell) is more complex than a prokaryotic cell.

• The cell cycle is the sequence of growth and division of cells.

• The cell cycle is divide into phase. Interphase, prophase, metaphase, anaphase, telophase.

Interphase

Cells spend most of their time in interphase.

3 phases in Interphase.

G1 is the first phase. Cell grows rapidly and carries out metabolic activity (doing its job)

S is the second phase and in this phase DNA is synthesized and replicated. (A copy of DNA is made)

G2 is the last phase of interphase and in these phase the cell assemble cell parts needed for cell division. (Cell is preparing itself to divide)

The Phases of Mitosis

• Mitosis is the process by which two daughter cells are formed, each containing a complete set of chromosomes.

• Mitosis is cell division.

• Prophase, Metaphase, Anaphase and Telophase all occur during mitosis.

• End result of mitosis is 2 new cells.

Prophase

• Prophase is the first phase of mitosis.

1. Chromatin coils and makes a chromosome.

2. Centrioles move to opposites ends of cell.

3. As prophase continues, the nucleus begins to disappear as the nuclear envelope and the nucleolus disintegrate.

The chromosomes is made up by sister chromatids. Sister chromatids are held together by a structure called a centromere.

Metaphase

Chromosomes become attached to spindle fibers by their centromere.

The chromosomes are pulled by the spindle fibers and begin to line up on the midline, or equator, of the spindle.

Anaphase

• The separation of sister chromatids marks the beginning of anaphase.

• The centromeres split apart and chromatid pairs from each chromosome separate from each other. Chromatids make their way to opposite ends of the cell.

Telophase

• Chromatids are at opposite poles.

• Spindle fibers break down, the nucleolus reappears, and a new nuclear envelope forms around each set of chromosomes.

• A membrane starts form between the 2 nuclei.

Cytokinesis

• The cell’s cytoplasm divides in a process called cytokinesis.

• Animal cells the plasma membrane pinches in along the equator and the two new cells are separated.

Control of the cell cycle

• Proteins and enzymes control the cell cycle

• The interaction of these molecules, based in the cell’s environment and inside the cell, controls the cell cycle.

• Occasionally, cells lose control of the cell cycle.

So what can happen is uncontrolled dividing of cells can result from the failure to produce certain enzymes, overproduction of enzymes, or the production of other enzymes at the wrong time.

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• Enzymes production is directed by genes located on the chromosomes.

• A Gene is a segment of DNA that controls the production of a protein.

Cancer: A mistake in the cell cycle.

Cancer results of changes in one or more of the genes that produce substances that are involved in controlling the cell cycle.

Cancerous cells form masses of tissue call tumors that deprive normal cells of nutrients.

• Cancer is the second leading cause of death in the United States.

• The causes of cancer are difficult to pinpoint because both genetic and environmental factors are involved.

• Environmental factors such as cigarette smoke, air and water pollution, and exposure to ultraviolet radiation from the sun, are all known to damage the genes that control the cell cycle.

• Cancer may also be caused by viral infection that damage the genes.