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Biology
Homeostasis, Passive and Active
Transport in Cells
Homeostasis
• The balance maintained within a living
thing
• Specifically, with respect to cells:
– how a cell maintains a balance of materials
(food, water, waste, minerals, salts, etc.)
inside.
Transport
• How a cell brings needed materials (food,
water, mineral etc.) in across the cell
membrane
• How a cell sends other materials (waste,
excess water, secreted hormones) out
across the membrane
• How a cell moves materials from place to
place within the cell.
Two Types of Transport
• Passive Transport: requires no energy
expenditure by the cell
– Examples: diffusion, osmosis, facilitated
diffusion, diffusion through ion channels
• Active Transport: requires the cell to use
energy to transport materials
– Examples: Membrane pumps, endocytosis,
exocytosis, pinocytosis.
Passive Transport: Diffusion
• In a solution, materials will move from an
area of high concentration to an area of
lower concentration, until an equilibrium is
reached
• The difference in concentration between
two areas is called the concentration
gradient or diffusion gradient.
• In diffusion, materials always move from
higher concentration to lower
concentration.
Passive Transport (diffusion)
• Normal diffusion. The solute particles
gradually spread out evenly through the
solution.
• Note: The water molecules also spread
into the solute.
Passive Transport: Osmosis
• The cell membrane is selectively
permeable– some materials can cross it
more easily than others.
• Remember: Water also diffuses. It goes
into the places that have less water (and
more solutes)
• Water can easily move across the cell
membrane
Osmosis
• Osmosis is when water diffuses across a
membrane into, or out of, a cell.
– Which direction osmosis takes depends on
the solution the cell is placed in
• Hypotonic solutions contain less dissolved solutes
than the cell. Osmosis is inwards (water enters the
cell)
• Hypertonic solutions contain more dissolved
solutes than the cell. Osmosis is outwards (water
leaves the cell)
• Isotonic solutions contain the same amount of
dissolved solutes as the cell. There is no net
osmosis.
Effects of Osmosis on cells
Effect on
animal cells
Animal very
Thirsty!
Animal
Healthy
Animal Cells
Damaged!
Serious
Wilting!
Temporary
Wilting
Plant cells
firm
Effect on plant
cells
• Video of Osmosisand egg experiment
an
Osmometer
Passive Transport: Facilitated
• Carrier proteins may assist certain
materials across the cell membrane
• Ion Channels provide small pathways
through the membrane for some ions
to move through.
– Ions couldn’t otherwise pass through
the lipid bilayer.
Ion channels allow some
small ions to pass, but block
larger molecules
Carrier proteins help
some materials pass
through the
membrane 
Active Transport
• Materials are pumped, pulled or
pushed through the membrane.
• Energy is required.
Ion Pumps
• Ion pumps push ions across the membrane.
• Example (Na+/K+ pump)
– A protein embedded in the membrane binds to three
sodium ions from inside the cell
– ATP causes the protein to change shape, pushing the
sodium through the membrane
– Potassium from outside the cell can now bind to the
protein
– The protein changes shape again, pushing the
potassium into the cell.
Ion Pumps
• Classroom video on active transport.(9:00)
• Sodium/Potassium pump video.(1:36)
• Sodium/Potassium pump video by nonscience students ( 56 sec)
• Another short animation (24 sec)
Endo-, Exo-, and Pinocytosis
• Endocytosis and
exocytosis are both
variations of a process
called pinocytosis.
• In all forms of pinocytosis,
materials cross the cell
membrane in small vesicles
Exocytosis and Endocytosis
• Exocytosis pushes things out of the cell.
• Endocytosis brings things into the cell.
Phagocytosis
• Phagocytosis is endocytosis on a larger
scale. Large molecules or even entire cells
are surrounded and brought inside the
cell.
• In humans, phagocytosis mostly occurs in white
blood cells, when they engulf bacteria.
Video of Phagocytosis
Edible Video of Phagocytosis
• Endocytosis video (35 sec)
• Endocytosis & exocytosis animation made
by students using string. (27 sec)
Diagram of Onion Cell
Onion Skin Cells (low power)
Onion Cells (high power)
Test Friday
• Chapters 1, (2), 3, 4 and 5 in textbook
• Topics
– Chapter 1: What is Life?
• Features and properties of life.
– Chapter 3: Biochemistry
• Proteins, Carbohydrates, Lipids, Nucleic Acids.
• Their properties, sub-units, and biological uses.
– Chapter 4: The Cell
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Membrane: The lipid bilayer, other features of the cell membrane
Cytoplasm: cytosol; organelles-- their structures and functions
The Nucleus: Nuclear membrane, nucleoplasm, chromosomes and nucleolus
Eukaryotic vs. Prokaryotic cells
– Chapter 5: Transport in Cells
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Passive Transport: Diffusion and Osmosis
Osmosis and the Cell Membrane: ion channels, facilitated diffusion.
Effects of osmosis on cells.
Active Transport: ion pumps, endo and exocytosis, phagocytosis
Typical Test Format
• About 50% fact based questions (multiple
choice / fill in blank / short answer)
• About 30 % identification and brief
explanation (labelling diagrams,
identifying structures and describing
functions)
• About 20% longer answer questions:
Explaining concepts and justifying
opinions
Multiple Choice Answers
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
B Molecules tend to move down their concentration gradients
(ie. from higher concentration to lower concentration)
D The cell membrane helps maintain homeostasis
C Ion channels help ions move across the cell membrane
A Glucose often enters the cell by facilitated diffusion using
carrier molecules.
C Low turgor pressure causes a plant to wilt.
B The Sodium-Potassium pump usually pumps sodium out of the
cell and potassium into it.
A A cell expends energy on membrane pumps (a form of active
transport)
B Phagocytosis is when cells engulf and destroy other cells
D Carrier proteins are most important in facilitated diffusion
C The solution outside the cell is probably hypertonic, because
the membrane appears to have pulled back from the cell walls.
Review One
• Describe the most important features of living things
• Be able to list characteristics of living things
• Be able to justify opinions about what should and
should not be considered alive.
• You should be able to describe the scientific
methods, and distinguish between experiment,
hypothesis and theory.
• You should be able to identify good experimental
procedures, including use of controls and single
variable procedures.
Review 2
• There will be no test questions from
chapter 2,
• HOWEVER: you will be expected to know
the difference between atoms, molecules
and ions, and to know what mixtures and
solutions are.
Review 3
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Importance of Water and Carbon
Carbohydrates
Lipids
Proteins
Nucleic Acids
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Constituents (monomers or subunits)
Structure (how are they bonded together)
Sources (where do we get them?)
Uses (what does our body do with them?)
Review 4
• Cells
• Membrane
– Lipid Bilayer (hydrophilic, hydrophobic)
– Protein inclusions
• Cytoplasm
– Organelles
• Mitochondrion
• Ribosomes
• ER
• Nucleus
The Organelles
Know the Structure & Function!
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Mitochondiron
Ribosome
Endoplasmic Reticulum
Golgi Apparatus (Golgi Body)
Vacuoles
Lysosomes