Cell Processes Middle School Science

Moving materials in cells Cells need energy 24/7 They cannot make their own, but can change energy from one form to another Cells obtain energy from their environment and convert it to a usable form Complex process

Moving materials in cells Metabolism The sum of all activities within a cell Cannot just happen Need raw materials Need to eliminate waste All material needs to enter and exit through the cell membrane

Moving materials in cells Materials enter and leave a cell by one of three methods: Diffusion Osmosis Active transport

Diffusion Cell membranes in living things need to allow materials to pass through Selectively-permeable membrane Substances pass through the pores Driving force behind the movement of many substances into or out of the cell is diffusion

Diffusion The process by which molecules of a substance move from areas of higher concentration of that substance to areas of lower concentration of that substance

Diffusion What causes diffusion to occur? Mainly small molecules Driving force behind movement All molecules are in motion Solids – molecules move slowly Liquids – move more quickly Gases – move quickly Molecules collide with each other, pushing away from one another

Diffusion What causes diffusion to occur? Driving force behind movement Collisions will continue as molecules spread out evenly

Diffusion Why don’t the organelles and cytoplasm pass through the cell membrane? Cell membrane is selectively permeable Permits only certain substances to diffuse Oxygen, water, and food molecules are permitted to diffuse into the cell Carbon dioxide and other waste materials are permitted to diffuse out of the cell

Osmosis A special kind of diffusion The diffusion of water across a selectively permeable membrane = osmosis Water is the most important substance that passes through a cell membrane About 80% of the cell is water

Diffusion and Osmosis Diffusion and osmosis are types of passive transport They do not require energy to be able to happen – they just happen

Active Transport What if the cell needs materials that cannot diffuse through the membrane? Active transport “carries” the materials into or out of the cell Cells have several forms of active transport Transport proteins can “pick up” molecules and carry them into or out of the cell Requires energy Ex: calcium, potassium, and sodium

Active Transport What if the cell needs materials that cannot diffuse through the membrane? Transport by engulfing Another method of active transport Cell membrane surrounds, or engulfs, a particle Once surrounded, the cell membrane wraps around the particle and forms a vacuole within the cell Requires energy

Cell Growth and Division Limits on Cell Growth Why don’t cells get bigger and bigger? Has to do with the transportation of materials into and out of the cell If a cell were to get too large, its membrane would not be able to handle the flow of materials passing through it The amount of raw materials needed by a large cell couldn’t enter fast enough The wastes produced couldn’t leave fast enough

Cell Division In order for an organism to grow, the total number of cells must increase Cell division One cell divides into two new daughter cells Occurs in a series of stages, or phases The process of cell division = mitosis

Mitosis Phase 1: Chromosomes are copied Interphase Cell is performing life functions, not dividing Chromosomes appear as threadlike coils called chromatin In animal cells, the two centrioles can be seen outside the nucleus Most plant cells do not have centrioles

Mitosis Phase 1 Near the end, the process of cell division begins The chromosomes are duplicated Doubling the normal chromosome number in the cell Each chromosome and its sister chromosome (copy) are attached at an area called the centromere At this time the sister chromosomes are called chromatids

Mitosis Phase 2: Mitosis begins Prophase Mitosis begins Mitosis = the process by which the nucleus of a cell divides into two nuclei and the formation of two new daughter cells begins Threadlike chromatin shorten and form rodlike chromosomes Centrioles begin to move to opposite ends of cell Meshlike spindle develops, forming a bridge between opposite ends of the cell

Mitosis Phase 3: Chromosomes attach to the spindle Metaphase The chromosomes begin to attach to the spindle Chromosomes are attached to the spindle by the centromere, which still connects each chromatid to its identical sister chromatid

Mitosis Phase 4: Chromosomes begin to separate Anaphase The centromere splits The sister chromatids separate from each other Chromatids move to opposite ends of cell along the spindle The chromatids are again called chromosomes at this point

Mitosis Phase 5: Two new nuclei form Telophase The chromosomes begin to uncoil and lose their rodlike appearance A nuclear membrane forms around the chromatin at each end of the cell In each nucleus, a nucleolus reappears At this point, mitosis is complete but the cell still has one phase to go through

Mitosis Phase 6: Two daughter cells form Cytokinesis Final phase of cell division Involves the division of the cytoplasm The membrane surrounding the cell begins to move inward until the cytoplasm is pinched in two Each part contains a nucleus with identical chromosomes The cell membrane (or cell wall) complete the division

Mitosis

Mitosis