Cell Division (Cellular Reproduction) Part 1 of 4

The ability of all life to replicate itself for future generations originates in the reproduction of cells.

All cells arise from other cells.

Cells have a limited life span.

During cell division, one cell becomes split into 2 cells. The original cell is called the PARENT CELL. The 2 cells resulting from the division are called DAUGHTER CELLS.

CELL CYCLE
– The cell cycle is the entire lifespan of a cell, starting with its production from a previous parent cell and ending with its division into 2 new daughter cells.
– Compared to the rest of the cells’ life, cell division is a brief and distinct stage in the cells’ life.
– The cell cycle is composed of an orderly sequence of phases that are controlled by the DNA of the cells’ nucleus.
– It is composed of an INTERPHASE where the cell is growing larger and replicating its DNA. Then there is nuclear division, called MITOSIS, that has 4 stages; prophase, metaphase, anaphase and telophase. Mitosis ends with the division of the cell into 2 separate daughter cells.

INTERPHASE
– Interphase is not part of cell division. It is a stage when the cell is growing, metabolizing and replicating its DNA.
– All cells spend most of their lives (about 90%) in Interphase. Some cells never leave the stage of interphase.
– Interphase provides enough time for the cell to grow large enough to eventually divide into 2 daughter cells.
– Interphase is divided into 3 stages; G1 phase, S phase and G2 phase.

1. G1 phase or Gap phase
– The cell experiences growth in volume and carries on its normal processes.
– If centrioles (small granules outside the nuclear membrane) are present, they begin to replicate.
– It is the longest phase.

2. S phase
– During Interphase the nucleus exist as a distinct organelle, bound by the nuclear membrane.
– Inside the nucleus are long, thin, unwound strands of chromosomes. These chromosomes influence the activity of the cell.
– This single set of chromosomes replicates itself.
– The genetic information (DNA) is doubled, providing the correct amount of this material for equal distribution during cell division.

3. G2 phase
– In the final stage of Interphase, the nucleus is still well defined.
– Replication of the centrioles is completed.
– The spindle apparatus that helps move the chromosomes during mitosis begins to be assembled.
– The cell chemically prepares for the cell division by replicating organelles and creating the chemicals needed for the actual division process.

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Types of Transport: Movement through the Cell Membrane

Transport systems within the cell are like a highway system, they provide for the constant movement of molecules, in and out of the cell.

Transport systems are needed because the cells membrane is “selectively permeable”. Some molecules can pass through (permeate) the cell membrane, while others cannot.

PASSIVE TRANSPORT
– Because cells naturally move and collide, passive transport requires no energy to move molecules into or out of the cell.
– They move on their own.

1. DIFFUSION (Simple)
– The movement of molecules from an area where they are highly concentrated to an area where they are less concentrated or more spread out.
– Molecules move across the cell membrane from an area of high concentration to an area of low concentration. It stops when the concentration is evenly distributed or equal.

2. OSMOSIS
– It is simple diffusion of water ONLY.
– -OSOMOTIC PRESSURE (the pressure exerted by dissolved particles in water) moves the water across the cell.
– Water moves from where the osmotic pressure is low to where the osmotic pressure is high.
– The osmotic pressure is always working to make the solution on both sides of the cell membrane equal.
– When two solutions of different concentrations are compared the solution with the higher concentration is called HYPERTONIC and the solution with the lower concentration, HYPOTONIC. When two solutions have the same concentration they have EQUILIBRIUM.
– Equilibrium: when the osmotic pressure is equal on both sides. When the osmotic pressure is the same on both sides of the cell membrane it is ISOTONIC or ISOSMOTIC. Both sides of the cell membrane have the same amount of particles and pressure, so there is no movement on either side.
– HYPERTONIC or HYPEROSMOTIC: when a cell is in an area of higher concentration. A higher osmotic pressure is placed on the cell, so the water will move out of the cell into the surrounding area.
– HYPOTONIC or HYPOOSMOTIC: when a cell has a higher concentration of particles inside the cell then are in the solution that the cell is in. The water will move into the cell to try to make both sides of the membrane equal.

3. FACILITATED DIFFUSION
– It is diffusion that is helped by the use of protein carrier molecules.
– It works in both directions, in and out of the cell.
– It is similar to simple diffusion, but it allows larger molecules, that need extra help to get across the cell membrane.
– Ex: Glucose molecules are too large to cross the cell membrane. So a glucose carrier protein, that is located in the cell membrane, combines with the glucose molecule and helps it cross the cell membrane.
– The molecule is picked up on the one side of the membrane and released on the other side.

ACTIVE TRANSPORT
– Movement of molecules, which do not normally move in this direction, from an area of low concentration to an area of high concentration.
– The cell must use energy to move these molecules.
– Like facilitated diffusion, active transport uses protein carrier molecules.
– The active transport system uses the high energy molecule (ATP) ADENOSINE TRIPHOSPHATE.
– ATP is split during active transport providing free energy to power the transportation process.

1. ENDOCYTOSIS
– A way for cells to move very large molecules into a cell.
– The cell membrane surrounds the molecule and forms a vacuole. The vacuole will move the molecule into the cell.

2. EXOCYTOSIS
– The active transport that moves molecules out of a cell.
– The molecule is surrounded and pushed out of the cell.
– This process is the exact reverse of endocytosis.