Cell Energy
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
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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