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

Effect on plant cells

Animal veryThirsty!

Animal Healthy

Animal CellsDamaged!

SeriousWilting!

TemporaryWilting

Plant cellsfirm

• Video of Osmosisand egg experiment

anOsmometer

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 non-science 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• 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 • 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 Answers6. B Molecules tend to move down their concentration

gradients (ie. from higher concentration to lower concentration)

7. D The cell membrane helps maintain homeostasis

8. C Ion channels help ions move across the cell membrane

9. A Glucose often enters the cell by facilitated diffusion using carrier molecules.

10. C Low turgor pressure causes a plant to wilt.

11. B The Sodium-Potassium pump usually pumps sodium out of the cell and potassium into it.

12. A A cell expends energy on membrane pumps (a form of active transport)

13. B Phagocytosis is when cells engulf and destroy other cells

14. D Carrier proteins are most important in facilitated diffusion

15. 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

• Importance of Water and Carbon

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids• 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 OrganellesKnow the Structure & Function!

• Mitochondiron

• Ribosome

• Endoplasmic Reticulum

• Golgi Apparatus (Golgi Body)

• Vacuoles

• Lysosomes