Download Unit 2: The Cell

Unit 2: The Cell
1. Membrane Structure:
a. Cell Theory:
 1665 Robert Hooke examines pieces of cork and records his
observations of empty compartments he calls “cells”.
 1838 Matthias Schleiden states that plants are composed of cells and
that cells are the basis of plant function.
 1839 Theodor Schwann states the same thing about animals.
 1858 Virchow states that cells come from living cells.
 Cell Theory states that:
o Organisms are made of cells.
o Cells are the basic unit of structure and function.
o Cells come from other cells.
b. Plasma Membrane
 The outer boundary of the cell is the plasma membrane.
 It regulates passage of material in and out of the cell.
 Selective permeability means that only certain substances are able to
pass through the membrane.
c. Fluid Mosaic Model:
 The cell membrane is a phospholipid bilayer: two layers composed of
Phospholipids.
 There are proteins found within the membrane as well that are used to
facilitate the movement of particles into and out of the cell.
d. Proteins:
 Proteins embedded in the cell membrane serve different functions:
i.
Transport Proteins: Regulate the movement of
substances.
ii. Channel Proteins: Form small openings for molecules to
diffuse through.
iii. Carrier Proteins: Binding site on the surface grabs
molecules and pulls them into the cell.
iv.
Gated Proteins: Similar to carrier proteins, but are not
always open.
v.
Receptor Proteins: Molecular triggers set off cell
responses.
vi.
Recognition Proteins: Nametags to identify cells to the
immune system
2. Membrane Function:
Passive Transport
a. Diffusion:
 The random movement of particles is called diffusion.
 Diffusion occurs from an area of high concentration to an area of low
concentration, until equilibrium is reached.
 Particles that diffuse across the cell membrane are lipids and particles
that dissolve in lipids like O2 and CO2
b. Osmosis:
 The name given to the diffusion of water in and out of the cell.
 Cells must be in osmotic balance with their surroundings.
 Blood: If a blood cell is placed in a solution of pure water the cell
bursts as water concentration outside the cell is higher than the
concentration inside the cell (inside contains solutes). IV’s thus
contain a mixture of solutes so that cell damage does not occur!
i.
Isotonic Solutions: H2O concentration outside the cell
matches the concentration inside the cell. H2O does not
move in or out.
ii. Hypotonic Solutions: H2O outside the cell is greater than
the concentration inside the cell. H2O  into the cell
iii. Hypertonic Solutions: H2O outside the cell is less than the
concentration inside the cell. Water  out of the cell
 Plants: Water tends to move into plant cells as well, the cell wall helps
counter the rush of water into the cell by increasing the pressure within
the cell. If the cell is placed into a salt water solution then there is a net
movement of water out of the cell.
Diffusion and Osmosis are forms of Passive Transport
Active Transport:
 Sometimes cells need to move molecules from an area of low
concentration to an area of high concentration. This is called moving
against the concentration gradient.
 In order to do this the cell must expend energy by using “pumps”.
 The pump functions by binding to a specific molecule and then using
chemical energy to change its shape in order to move the molecule.
 Once the molecule is released then the shape of the pump is restored.
c. Endocytosis:
 Endocytosis is the movement of materials into the cell when the cell
membrane engulfs packets of material.
 Exocytosis occurs when the cell membrane transports materials out of
the cell.
i.
Phagocytosis: Cell eating. The process by which food is
engulfed and then the membrane forms a packet around the
food called a vesicle, which digests food later.
ii. Pinocytosis: The same process but liquid is taken in rather
than solid materials
iii.
Receptor aided Endocytosis: Molecules out of the cell bind
with proteins that fit specific receptors on the membrane
surface. The membrane forms a vesicle around the material
and then the material enters the cell to be released.
Section Questions:
1. What are the functions of the cell membrane?
2. Draw a model of the cell membrane showing the various parts along with a legend
that makes the model easy to understand.
3. What is diffusion? Use one specific example.
4. Explain the concept of concentration gradient and use diagrams to clarify.
5. What is difference between osmosis and diffusion?
6. You are a cell membrane physician. You job is to determine the part of the
membrane that is missing or what is wrong with each of the following:
a. The bumps in the phospholipids bilayer are missing.
b. The bilayer is unrestricted in movement.
c. Membrane is weak.
d. Polypeptides are not formed within the cell.
e. Cell respiration stops.
f. There is no polar part to the membrane.
g. Specific chemicals cannot recognize the cell membrane.