8 Cellular Transport and the Cell Cycle
· You will discover how molecules are transported across the plasma membrane.
· You will sequence the stages of cell division.
· You will identify the relationship between the cell cycle and cancer.
8.1 Cellular Transport Pg 194
• Explain how the processes of diffusion, passive transport, and active transport occur and why they are important to cells.
• Predict the effect of a hypotonic, hypertonic, or isotonic solution on a cell.
Plasma Membrane
- Acts as dam or pump (does not limit diffusion of water
Diffusion- movement from high conc. To lower conc. To reach dynamic equilibrium
Osmosis: Diffusion of Water
• The diffusion of water across a selectively permeable membrane
• Regulating the water flow through the plasma membrane is an important factor in maintaining homeostasis within a cell.
• Sugar molecules on each side of membrane remain the same- but water does change
• Water flows to side where the water conc. is lower.
What controls osmosis? One factor - Unequal distribution of particles, called a concentration gradient
Cells in an isotonic solution
• occurs in most cells because they are surrounded by water
• Doesn’t matter if multicellular or unicellular organisms-have osmosis
• , the concentration of dissolved substances in the solution is the same as the concentration of dissolved substances inside the cell.
• In an isotonic solution, water molecules move into and out of the cell at the same rate, and cells retain their normal shape.
• A plant cell has its normal shape and pressure in an isotonic solution.
Cells in a hypotonic solution- pictures pg. 197
• Conc. Of dissolved substances is lower in solution outside cell that inside-
• Water enters a cell by osmosis, causing the cell to swell.
• Animal cells- lysis; absorb too much water & can burst
• Plant cells swell beyond their normal size as pressure increases but cell walls prevent bursting
Cells in a hypertonic solution
• Conc. Of dissolved substances outside cell is higher than inside
• In a hypertonic solution, water leaves a cell by osmosis, causing the cell to shrink.
• Plant cells lose pressure as the plasma membrane shrinks away from the cell wall. Wilting
• Animal cells just shrink
Passive transport types
Movement of materials across membrane without energy and Uses Transport Proteins which link & guide molecules through membrane.
With the concentration gradient = no energy required
1. Facilitated Diffusion- passive transport using transport proteins in membrane wall
2. Transport proteins called channel protein form channels that allow specific molecules to flow through
3. Carrier Proteins- change shape to allow passage-
Active Transport
• Need energy to move particles opposite the normal diffusion direction (from high to low).
• A carrier proteins binds with particles (by shape), and energy allows the cell to change the shape of the carrier protein, releasing particle on other side of membrane (protein returns to original shape.
• Works going in and out
Movement of materials through a membrane against a concentration gradient is called active transport and requires energy from the cell
Transport of Large Particles
Endocytosis
• is a process by which a cell surrounds and takes in material from its environment
• The material is engulfed and enclosed by a portion of the cell’s plasma membrane.
• The resulting vacuole with its contents moves to the inside of the cell.
Exocytosis - is the expulsion or secretion of materials from a cell.
Endocytosis and exocytosis both move masses of material and both require energy.
Transport of substances across the cell membrane is required for cells to maintain homeostasis.
8.2 Objectives
• Sequence the events of the cell cycle.
• Relate the function of a cell to its organization in tissues, organs, and organ systems.
Cell Size Limitations
• The cells that make up a multicellular organism come in a wide variety of sizes and shapes.
• Considering this wide range of cells sizes, why then can’t most organisms be just one giant cell?
Diffusion limits cell size
• Although diffusion is a fast and efficient process over short distances, it becomes slow and inefficient as the distances become larger.
• Because of the slow rate of diffusion, organisms can’t be just one giant-sized cell.
DNA limits cell size
• The nucleus contains the blueprints (DNA) for protein formation. Proteins perform critical cell functions. Limit on speed the blueprint can be copied and moved outside the nucleus
• The cell cannot survive unless there is enough DNA to support the protein needs of the cell
• In many large cells, more than one nucleus is present.
• Large amounts of DNA in many nuclei ensure that cell activities are carried out quickly and efficiently.
Surface area-to-volume ratio
• As a cell’s size increases, its volume increases much faster than its surface area.
• If cell size doubled, the cell would require eight times more nutrients and would have eight times more waste to excrete.
• The surface area, however, would increase by a factor of only four.
• The cell would either starve to death or be poisoned from the buildup of waste products.
• Cells divide before they become too large to function properly.
Cell Reproduction
Cell division is the process by which new cells are produced from one cell. Cell division results in two cells that are identical to the original, parent cell
The discovery of chromosomes
• Early biologist observed structures in the nucleus that seemed to change, vanish and reappear.
• Structures, which contain DNA and become darkly colored when stained, are called chromosomes.
• Chromosomes are the carriers of the genetic material that is copied and passed from generation to generation of cells. Accurate transmission of chromosomes during cell division is critical.
• One parent cell splits into 2 identical daughter cells.
• All organisms grow and change; worn out tissues are repaired or are replaced by new cells.
The structure of eukaryotic chromosomes
• Chromatin- during interphase- DNA is wrapped around proteins called histones. (Each bead on the string is a group of histones called a nucleosome. The Chromatin strands must reorganize and become tightly packed before the cell can divide.
The Cell Cycle
• A sequence of growth and division of a cell.
• The majority of a cell’s life is spent in the growth period known as interphase
• Following interphase, a cell enters its period of nuclear division called mitosis
• Following mitosis, the cytoplasm divides, separating the two daughter cells.
Interphase, the busiest phase, is divided into three parts.
• 1. Rapid growth and metabolic activity- protein production is high.
• 2. The cell copies its chromosomes.
• 3. another shorter growth period in which mitochondria and other organelles are manufactured and cell parts needed for cell division are assembled
The Phases of Mitosis
The four phases - prophase, metaphase, anaphase, and telophase
Prophase
• the chromatin coils to form visible chromosomes & nucleus disappears
• centrioles in animal cells appear and migrate to ends (poles)
• spindle forms between (thin fibers of microtubules (plants do it w/o centriole)
• sister chromatids - The two halves of the doubled structure
• Sister chromatids are held together by a structure called a centromere, which plays a role in chromosome movement during mitosis.
Metaphase:
• Chromosomes move to the equator
• Spindle fibers reach from poles to centromere of the spindle
Anaphase:
• the centromeres split and the sister chromatids are pulled apart to opposite poles of the cell
Telophase:
• Chromosomes unwind to direct metabolic activities
• Spindle breaks down, nucleolus reappears, new envelope and double membrane forms between 2 new cells
• Two distinct, identical daughter cells are formed.
• The cells separate as the cell cycle proceeds into the next interphase
Cytokinesis
• Following telophase, the cell’s cytoplasm divides in a process called cytokinesis.
• Cytokinesis differs between plants and animals.
• Toward the end of telophase in animal cells, the plasma membrane pinches in along the equator.
• Plant cells have a rigid cell wall, so the plasma membrane does not pinch in.
• A structure known as the cell plate is laid down across the cell’s equator.
• A cell membrane forms around each cell, and new cell walls form on each side of the cell plate until separation is complete.
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Results of Mitosis
• When mitosis is complete, unicellular organisms remain as single cells.
• In multicellular organisms, cell growth and reproduction result in groups of cells that work together as tissue to perform a specific function.
• Tissues organize in various combinations to form organs that perform more complex roles within the organism
• Multiple organs that work together form an organ system.
8.3 Section Objectives
• Describe the role of enzymes in the regulation of the cell cycle.
• Distinguish between the events of a normal cell cycle and the abnormal events that result in cancer. Identify ways to potentially reduce the risk of cancer.
Normal Control of the Cell Cycle
Proteins and enzymes control the cell cycle
• The cell cycle is controlled by proteins called cyclins and a set of enzymes that attach to the cyclin and become activated. Occasionally, cells lose control of the cell cycle.
• This uncontrolled dividing of cells can result from the failure to produce certain enzymes, the overproduction of enzymes, or the production of other enzymes at the wrong time.
• Cancer is a malignant growth resulting from uncontrolled cell division.
• Enzyme 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 is a result of changes in one or more of the genes that produce substances that are involved in controlling the cell cycle.
• tumors - Cancerous cells which form masses of tissue that deprive normal cells of nutrients
• metastasis - In later stages, cancer cells enter the circulatory system and spread throughout the body, forming new tumors that disrupt the function of organs, organ systems, and ultimately, the organism.
• 2nd leading cause of death in US
• Can affect any tissue in body
The causes of cancer
• 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 infections that damage the genes.
• Cancer prevention
• Physicians and dietary experts agree that diets low in fat and high in fiber content can reduce the risk of many kinds of cancer.
• Vitamins and minerals may also help prevent cancer.
• daily exercise and not using tobacco also are known to reduce the risk of cancer.
Summary
Cellular Transport
• Osmosis is the diffusion of water through a selectively permeable membrane.
• Passive transport moves a substance with the concentration gradient and requires no energy from the cell.
• Active transport moves materials against the concentration gradient and requires energy to overcome the flow of materials opposite the concentration gradient.
• Large particles may enter a cell by endocytosis and leave by exocytosis.
Cell Growth and Reproduction
• Cell size is limited largely by the diffusion rate of materials into and out of the cell, the amount of DNA available to program the cell’s metabolism, and the cell’s surface area-to-volume ratio.
• The life cycle of a cell is divided into two general periods: a period of active growth and metabolism known as interphase, and a period that leads to cell division known as mitosis.
Cell Growth and Reproduction
• Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase.
• The cells of most multicellular organisms are organized into tissues, organs, and organ systems.
Control of the Cell Cycle
• The cell cycle is controlled by key enzymes that are produced at specific points in the cell cycle.
• Cancer is caused by genetic and environmental factors that change the genes that control the cell cycle.
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