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Cells
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Characteristic #2 of all living things: All living organisms are composed of cells
Some organisms have many cells and some have only one cell
Humans have ~100 trillion cells
The Cell is the basic unit of all living things
Robert Hooke was a monk who discovered cells in 17th century while looking at cork
through his microscope. He named the small objects he saw “cells” because they
reminded him of the sleeping cells in a monastery.
Cell Definition: cells are the basic units of all living things.
Examples of types of cells: red blood cells, one-celled bacteria, nerve cells, bone cells
Two types of Cells:
o Prokaryote: A one-celled organism that has no nucleus or organelles. A bacteria
is a prokaryote
o Eukaryote: A cell that groups with other cells and has organelles and a nucleus.
Human and plant cells are eukaryotic.
The Cell Membrane
The Cell Membrane is a double layered barrier that
protects the inside of the cell.
The membrane is made of phospholipids (a
phosphate molecule attached to two fatty acids)
Phospholipid Bilayer: the phospholipids form two
layers that are like a fatty acid oreo– the phosphates
are the chocolate cookie layer, and the fatty acids
are the cream inside
When placed in water, the phospholipids form a
double layered membrane around the cell
Phosphates are on the outside of the oreo because
they are attracted to their watery environment.
Fatty acids are repelled by water, so they face inside
towards each other
This bilayer construction acts like a wall which keeps
things from entering and exiting the cell through the
membrane
Proteins stuck within the membrane act like doors that lets particles pass into and out of
the cell.
The cell membrane is selectively permeable (permeable means that things can pass
through). It lets some things pass through (small particles, gases, water) but not others.
Solutions
Solutions are a mixture of water and solute (stuff)
Solute can be sugar, salt, or any other substance
When solutions are put together, then solute, stuff, tries to move to the area that has the
least amount of solute. Solute hates a crowd.
When solutions are put together, water tries to move to the area that has the most solute
(stuff) – water follows the stuff.
Just remember – (1) solute hates a crowd, and (2) water follows the stuff.
When solutions are placed next to one another with only a selectively permeable
membrane between them, only some things will be able to move from one side of the
membrane to the other. Usually, only water can move and the stuff is left behind. In this
case, water wants to follow the stuff, so it goes to the side with the most solute.
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Two types of movement through the cell membrane
Passive transport - movement through the cell membrane that doesn’t require energy
from the cell
o Dropping a weight from the roof is passive – no energy is required
o In passive transport, water/solute go where they want to – solute goes away from
the crowd, and water follows the stuff
o Two examples of passive transport through the cell membrane: osmosis and
diffusion
o Diffusion: movement of solute (particles) from areas
of high solute concentration to areas of low solute
concentration until they are evenly distributed –
solute is trying to spread out
 Perfume spreads out when sprayed into the
air
 Chocolate particles spread out in milk
 Some gases and small particles diffuse
through the cell membrane until the inside
and outside of the cell are the same.
o Osmosis: the diffusion of water through a selectively permeable membrane.
 The water moves from low solute concentrations to high solute
concentrations – it follows the stuff!
 The water moves into the area with the most
solute
o Osmosis Through the Cell Membrane
 The cell membrane is permeable to water but
not large particles
 Water moves from low solute concentration to
high (water follows the stuff)
 When water leaves the cell, the cell shrinks
 When water enters the cell, it gets larger
o
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Channel Proteins: Some proteins act as doors in the cell membrane. Particles
diffuse from areas of high to low concentrations by passing through these
proteins.
Active Transport – materials move through the cell membrane against a concentration
gradient. The water or solute is moving where it does not
want to go.
o Any transport that requires ATP energy is active
transport
o Movement of the solute from the area of low
concentration to the area of high concentration
requires ATP energy – in other words, it takes
energy to push water away from the stuff, or to force
solute to go into a crowd
o One type of active transport uses carrier proteins to
carry unwilling particles across the membrane
The Cell and its Organelles
The cell contains various organelles, each with a different function
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Nucleus
o Control center of the cell
Plant Cell
o Contains chromosomes
o Contains nucleolus
Nucleolus
o Is in the nucleus
o Makes ribosomes
Endoplasmic Reticulum
o The ER transports materials within the cell
o The surface of the rough ER is covered with ribosomes
Ribosomes
o Sites of protein synthesis
o Can be on the surface of the ER or in the cytoplasm
Golgi Apparatus
o Modifies, packages, and ships proteins & lipids within and outside of the cell
Cytoplasm
o The fluid that fills the inside of the cell
Vacuole
o Stores water, salts, and waste
o Many small vacuoles in animal cells
o One large vacuole in plant cells
o When the vacuole fills with water in a plant, cells become rigid and the plant can
stand upright
Mitochondria
o Site of cellular respiration
o Plugs into molecules and supplies energy
Lysosome
o Bag filled with acids
o Digests old cell parts
Chloroplast (Plant cell only)
o Site of photosynthesis
o Production of food (glucose)
Cell Wall (Plant cell only)
o Surrounds the cell membrane of a plant cell