Cell Structure & Transport

Concept 1: Cell Structure & Function

Success Criteria & Vocabulary

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  • Draw and label each main organelle, and describe their function.

      • Cell membrane

      • Cell wall

      • Vacuole

      • Nucleus

      • Mitochondrion

      • Chloroplast

  • Communicate the similarities and differences between plant and animal cells.

Click this drop-down menu to see the list of Vocabulary.

Cell: Most basic unit of life.

Cell membrane: Semipermeable barrier surrounding the cytoplasm of a cell.

Cell wall: Rigid structural organelle that surrounds the cell membrane and provides support to the plant cell.

Chlorophyll: Green pigment in plants responsible for capturing light energy. This light energy is used to split water molecules during photosynthesis.

Chloroplast: Organelle found only in plants, that is the site of photosynthesis.

Chromatin: DNA in its loose, inconspicuous form. Visible during the interphase of the cell cycle.

Cristae: Infoldings of the inner mitochondrial membrane that is the site of the electron transport chain.

Cytoplasm: Jelly-like fluid organelle inside the cell in which the organelles are suspended.

Extracellular: Term for something located outside of the cell.

Grana: Stack of thylakoids.

Matrix: Fluid contained within the inner membrane of mitochondria, that is the site of the Krebs cycle.

Mitochondria: Organelle found in both plants and animals that converts the chemical energy stored in glucose into ATP.

Nuclear membrane/envelope: Double phospholipid bilayer of the nucleus.

Nuclear pore: Passageway through the nuclear membrane/envelope, made of proteins.

Nucleus: Semipermeable organelle that contains the genetic material of a cell, and is the site of DNA replication.

Organelle: Functional units inside a cell that perform specific tasks required by the cell.

Phospholipid: Major component of the cell membrane that has a hydrophilic head and hydrophobic tails.

Phospholipid bilayer: Two layers of phospholipids.

Semi-permeable: Ability of cell membranes to allow some substances to pass it but not others.

Stroma: Colourless fluid surrounding grana/thylakoids.

Surface area: Total area occupied by the surface of an object.

Thylakoid: Flattened membranes that contain chlorophyll.

Turgor pressure: The pressure that the content of the cell exerts on the cell membrane, pushing it against the cell wall

Vacuole: Fluid-filled organelle in both animal and plant cells that stores water and minerals.

Tasks

Watch my teaching video on Cell Structure & Function

Complete Education Perfect:

Task called '2.4 Concept 1'.

  • Animal cell organelles

  • Plant cell organelles

  • Animal and Plant Cell Comparison

Complete sciPad:

  • Page 10 - Animal Cell Organelles

  • Page 11 - Identifying Animal Cell Structures

  • Page 12 - Plant Cell Organelles

  • Page 13 - Identifying Plant Cell Structures

Mark your own work using the sciPad online answers.

Learn the 24 keywords using Quizlet:

Concept 1: Support Notes

Introduction to Cell Structure & Function

ORGANELLES are functional units within CELLS that carry out specific tasks that are required by the cell.

The term 'organelle' comes from the idea that these structures are similar to organs in a body, only they’re so much smaller. Organelles carry out specific tasks necessary for the function of a cell, just as organs perform specific tasks necessary for the body.

Although some organelles are very important for specific cells in the body, you DON'T need to know about all organelles. You just need to know about the structure and function of: CELL MEMBRANE, NUCLEUS, MITOCHONDRIA, VACUOLE, CELL WALL (plants only), and CHLOROPLASTS (plants only).

Animal cells come in a wide variety of shapes and sizes because they perform specialised tasks all over the body.

So the composition of organelles in cells will be different depending on what type of cell it is, whether it’s a muscle cell, a sperm cell, a nerve cell and so on, and the role that cell has in the body.

Video: Introduction to Cells: The Grand Cell Tour (Amoeba Sisters) + WORKSHEET. Comprehensive introduction to cells. More information than you need to know. Let me know if you want the worksheet that goes with this video.
Video: Specialised Cells: Significance and Examples (Amoeba Sisters) + WORKSHEET. Describes different types of plant and animal cells. However, not a lot on comparing organelle structure and distribution.

Cell Membrane (plant & animal cells)

The CELL MEMBRANE is the ORGANELLE that surrounds and protects the fluid inside a CELL (called the CYTOPLASM) from the environment outside the cell (called the EXTRACELLULAR environment).

The cell membrane is mostly made of tiny molecules called PHOSPHOLIPIDS. Phospholipids are made up of two parts, a head part and a tail part. Their head part is attracted to water and their tail part is repelled by water they are water hating.

Hydrophillic and hydrophobic - In case you're interested...
For the chemists out there, the head is hydrophilic because it has a polar phosphate group, and the tails are hydrophobic because it has a non-polar lipid group). The phospholipids form an inner and an outer layer. And together, these layers are called a bilayer, (bi- meaning two, so bilayer means two layers). The water-loving heads of the phospholipids line both surfaces of the bilayer, while the tails are in the middle because they want to be shielded away from water.

The cell membrane is called a PHOSPHOLIPID BILAYER because it is made up of two layers of phospholipids. Embedded within this phospholipid bilayer are different types of membrane proteins (proteins that are within the membrane).

The cell membrane itself and transmembrane proteins only allows certain things through, making it "selectively permeable" as it is very selective in terms of what molecules it lets in and out. This is advantageous as it is both a barrier to harmful materials, and a gateway to desired materials.

For this reason, the cell membrane is said to be SEMIPERMEABLE.

Nucleus (plant & animal cells)

The NUCLEUS stores and protects the organism's DNA. Most of the time, DNA is stored as loose strands called CHROMATIN.

The nucleus is a large, usually round ORGANELLE. The nucleus is surrounded by the NUCLEAR MEMBRANE, which can also be called the NUCLEAR ENVELOPE. The nucleus’s phospholipid membrane also has many holes called NUCLEAR PORES. These nuclear pores allow the passage of selected materials in and out of the nucleus (therefore, it is a SEMIPERMEABLE membrane).

The clear nuclear membrane protects the DNA from chemicals and enzymes in the cytoplasm that could damage it and create errors in this code.

DNA - In case you're interested....
The nucleus contains most of the cell’s genetic information in the form of DNA. Sections of DNA are called genes and these code for proteins that are produced by the cell. These proteins control the function of the cell and the physical traits or phenotype of the organism. DNA is the master copy of instructions for cells and the organisms that possess them. That’s why DNA is so important. The clear nuclear membrane protects the DNA from chemicals and enzymes in the cytoplasm that could damage it and create errors in this code.
Most of the time, cells store DNA as loose strands within the nucleus called chromatin. In this form, DNA is easy to access so genes can be transcribed and the DNA can be replicated. The way DNA is stored changes a lot during cell division, forming structures called chromosomes.

Mitochondria (plant & animal cells)

MITOCHONDRIA are small oval-shaped ORGANELLES that are the sites for aerobic respiration.

Mitochondria have an inner membrane and an outer membrane. The inner membrane has a lot of folds (or invaginations/inward-folds) called CRISTAE. These folds increase the SURFACE AREA of the inner membrane to maximise the rate of aerobic respiration.

The space within the inner membrane is called the MATRIX. This is a viscous or thick fluid that contains a lot of enzymes and substrates or reactants used for aerobic respiration.

Aerobic respiration - In case you're interested...
Mitochondria produce energy in a process called aerobic respiration. They use (glucose) from food we eat, and oxygen from the air we breathe to turn it into energy in the form of ATP (plus water and carbon dioxide as byproducts).
ATP (adenosine triphosphate) is the molecule cells use as a source of energy to perform various functions. This is why mitochondria are commonly referred to as the “powerhouse of the cell”.

Vacuole (plant & animal cells)

Plant cells have a large central VACUOLE, while animal cells only have a small, peripheral one. It is primarily filled with water but also contains dissolved materials.

The most important role of the plant vacuole is to control the TURGOR PRESSURE within a cell. Turgor pressure is the pressure that the content of the cell exerts on the cell membrane, pushing it against the cell wall.

Vacuoles are also important for the storage of dissolved materials. These can be materials important for the cell to function, or they can be waste from toxic materials that could harm the cell if allowed to enter the cytoplasm.

Large, central vacuole.

Small, peripheral vacuole.

Turgor pressure - In case you're interested...
Usually this vacuole occupies between 30% and 90% of the plant cell’s internal space, but it changes depending on the turgor pressure needed by the plant. This turgor pressure gives the cell strength in the same way that the air pressure inside a soccer ball makes it firm. This allows plants to grow upwards, against the pull of gravity.
If a cell has high turgor pressure, the cell membrane will be pushed against the cell wall, and make the cell turgid and be able to hold its shape. If the vacuole has too little water to maintain turgor pressure, the cell membrane will pull away from the cell wall. The cell will become flaccid and lose structural integrity.
If you have ever seen a wilted plant, then you have seen the effects of a loss of turgor pressure. When you forget to water your houseplants and they are not able to store enough water i their vacuoles, their cells will become flaccid and the plant will wilt.

Cell Wall (plants cells only)

Plants are surrounded by a CELL WALL, which is a thick wall made of cellulose which gives the cell protection, shape, and strength.

It is non-selective and completely surrounds the cell membrane of a plant cell. The presence of a cell wall is one of the easiest ways to tell a plant cell from an animal cell.


Cell wall - In case you're interested...
Cell walls give plants the strength to form solid upright structures without the need for bones. Even when a plant cell has died, its cell wall can remain. This is how plants form wood. Typically only around 1% of a mature tree is made of living cells. The other 99% is made of cell walls from old dead cells!
Just like in animal cells, the cell membrane is responsible for the transport of materials into and out of the cell. If a plant cell is healthy, the cell membrane should be pushed hard up against the cell wall by the turgor pressure inside the cell.

Chloroplasts (plants cells only)

A CHLOROPLAST is a membrane-bound organelle that is the site of PHOTOSYNTHESIS in plant cells.

They are surrounded by a double membrane (one membrane is called the outer membrane, the other membrane is called inner membrane. The space between them is called the intermembrane space).

The outer membrane is permeable, meaning molecules such as carbon dioxide (CO2) are easily able to pass through the outer membrane. The inner membrane is SEMIPERMEABLE, meaning it allows some substances to pass through it but not others.

The fluid inside the inner membrane is called the STROMA. Within the stroma are many flattened membranes called THYLAKOIDS. These thylakoids are arranged into stacks called GRANA. This stacking dramatically increases the SURFACE AREA available for the chemical reactions of photosynthesis. Thylakoids contain CHLOROPHYLL, the green pigment responsible for capturing light energy.

It’s important to note that the stroma is transparent, so that the sunlight can easily pass through and be absorbed by chlorophyll.


Photosynthesis - In case you're interested...
The light energy captured by the chlorophyll is used to split water molecules into hydrogen and oxygen atoms. The hydrogen is then combined with carbon dioxide to produce glucose (sugar) for the plant. Oxygen is released as a by-product. The glucose that chloroplasts produce with photosynthesis is then used by mitochondria to create ATP.
Because plants create glucose through the process of photosynthesis, they usually have no need to eat at all. The few plants that are carnivorous consume prey as a source of nutrients such as proteins and amino acids, rather than glucose.

Concept 2: Diffusion

Success Criteria & Vocabulary

Click this drop-down menu to see the Success Criteria.

  • Describe the composition of the cell membrane and how it acts as a semipermeable membrane barrier.

  • Discuss diffusion in the context of different cellular processes.

  • Describe the importance of a high SA : V ratio, in terms of cell processes.

Click this drop-down menu to see the list of Vocabulary.

Active transport: Movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.

Aerobic respiration: Enzyme controlled process which requires oxygen to produce 38 ATP from the breakdown of glucose.

Carrier proteins: Transmembrane protein that is only open to one side of the membrane at a time, and changes shape to transport molecules through the cell membrane.

Cell membrane: Semipermeable barrier surrounding the cytoplasm of a cell.

Channel proteins: Transmembrane protein that is open to both ends.

Concentration gradient: When a solute is more concentrated in one area compared to another.

Diffusion: Passive movement of molecules down a concentration gradient.

Facilitated diffusion: Transport of substances across a membrane from an area of higher concentration to an area of lower concentration by means of a transmembrane protein.

Osmosis: Movement of water molecules from an area of high concentration to an area of low concentration through a semipermeable membrane.

Passive transport: Transport of a substance across a cell membrane by diffusion; Energy is not required.

Photosynthesis: Enzyme-controlled process occurring in plants that use energy from sunlight to combine carbon dioxide and hydrogen to produce glucose.

SA : V ratio: Ratio of the cell’s surface area and its volume.

Semi-permeable: Ability of cell membranes to allow some substances to pass it but not others.

Surface area: Total area occupied by the surface of an object.

Volume: Amount of space in a 3D object.

Tasks

Watch my teaching video on Cell Transport Part 1 - Diffusion

Complete Education Perfect:

Task called '2.4 Concept 2'.

  • The cell membrane

  • Diffusion

Complete sciPad:

  • Page 14 - A Closer look at Organelles - Plasma Membrane.

  • Page 41 - An Introduction to Cell Transport Processes

  • Page 42 & 43 - Diffusion & Concentration Gradients

  • Page 44 - Facilitated Diffusion

  • Page 52 & 53 - Cell Size & Shape - Diffusion in a Model Cell

Mark your own work using the sciPad online answers.

Learn the 15 keywords using Quizlet:

Concept 2: Support Notes

Transport Across the Cell Membrane

Cell transport is the movement of molecules across the CELL MEMBRANE. Cell transport is important because molecules like oxygen and glucose need to be transported across the cell membrane before they can enter a plant or animal cell and be used.

But remember from Concept 1 that the cell membrane is only SEMIPERMEABLE, therefore only certain molecules can enter and exit the cell.

There are two ways they can do this:

  1. PASSIVE TRANSPORT where energy/ATP is NOT required

      • DIFFUSION (via cell membrane)

      • OSMOSIS (via cell membrane)

      • FACILITATED DIFFUSION (via transmembrane proteins like protein channels and carrier proteins)

  2. ACTIVE TRANSPORT where energy IS required

      • Via transmembrane proteins

        1. Protein pumps

        2. Protein channels

        3. Carrier proteins

This concept focuses on passive transport.

Why is it important to transport molecules across the cell membrane? - In case you're interested...Transportation means “movement from one place to another.” This includes everything from flying around the world, to molecules such as oxygen moving into your cells. Every day we depend on the movement of molecules within our bodies to digest food, remove toxic wastes and create the energy we need to move, grow, and survive.
How does transport happen? - In case you're interested...Well, the phospholipid bilayer is studded with proteins that go through the bilayer, and create a pathway for the transport of materials into and out of a cell. These proteins are called transmembrane proteins. There are two basic forms of transmembrane proteins: channels and carriers.
Proteins channels are open to both ends (both extracellular space and intracellular space) and allow molecules to pass freely. For example, this is how molecules such as glucose enters a cell (through a protein channel called GLUT1).
On the other hand, carrier proteins are only open to one side of the membrane at a time and they change shape to transport molecules through the cell membrane. Carrier proteins often require energy to transport molecules. The most famous carrier protein is known as the Na+/K+-ATPase pump.

Diffusion

DIFFUSION is the movement of molecules from an area of high concentrations to an area of low concentrations (so they are moving down a concentration gradient) until they are equally distributed. Diffusion is a form of PASSIVE TRANSPORT, because it does not require energy in the form of ATP.

The difference in concentration between the area of high concentration and the area of low concentration is called the CONCENTRATION GRADIENT. Diffusion doesn’t require energy because it is driven by this concentration gradient.

A concentration gradient only exists until the molecules are evenly distributed on both sides of the cell membrane. So the concentration gradient is directly proportional to the rate of diffusion.

Example of Diffusion: Aerobic Respiration

Oxygen concentration is lower in the muscle cells than in the bloodstream because that is where it is being used. So oxygen diffuses from the bloodstream to the muscle cell.

Carbon dioxide concentration is higher in the muscle cells than in the bloodstream because that is where it is being produced. So carbon dioxide diffuses from the muscle cell to the blood stream.

Diffusion is needed for aerobic respiration - In case you're interested...
Aerobic respiration is the process of producing the energy or ATP plants and animals use to grow, move and survive. It happens in organelles called mitochondria and uses glucose and oxygen to release energy, carbon dioxide and water.
Oxygen diffuses from the bloodstream and through the cell membranes of muscle cells, where it is used in the mitochondria to carry out aerobic respiration. This is because the concentration of oxygen is much higher in the bloodstream, and much lower in the cells as it is constantly used up inside the muscle cells to produce energy.

Example of Diffusion: Photosynthesis

Carbon dioxide concentration is lower in the leaf cells than in the air because that’s where it is being used up.

Therefore, carbon dioxide diffuses from the air into the leaf (via the stomata).


Diffusion is needed for photosynthesis - In case you're interested...
Photosynthesis produces the sugar glucose from water and carbon dioxide. Glucose can be broken down during respiration into energy the plant can use to grow, reproduce, and survive.
Carbon dioxide diffuses from the air and into the leaf through small pores called stomata. This is because the concentration of carbon dioxide is much higher in the air, and much lower in the leaf and plant cell because it is constantly being used up within the leaves during photosynthesis.
Once it is inside the leaf, carbon dioxide diffuses through the plant cell membrane and into the chloroplasts where photosynthesis happens.
Video: Fluid Mosaic Model. Old school video, but a good overview of the fluid mosaic model.
Video for Experts: Cell Membranes are Way More Complicated than You Think (TED-Ed). More informative video of the fluid mosaic model, but more complex. You do not need to know about cell signalling and endocytosis.
Video: Inside the Cell Membrane (Amoeba Sisters). Describes surface area to volume ratio in cells, and fluid mosaic model of cell membrane. Don’t need to know about cholesterol.
Video: Diffusion (Amoeba Sisters). Comprehensive video on diffusion.
Interest Only Video: Oxygen's surprisingly complex journey through your body (TED-Ed). Puts diffusion into the context of the transport of oxygen through the human organ systems.

Facilitated Diffusion

FACILITATED DIFFUSION, which is the movement of molecules from an area of high concentration to an area of low concentration (down a concentration gradient) through CHANNEL or CARRIER PROTEIN in the CELL MEMBRANE.

Its only difference from simple diffusion is that these molecules are unable to diffuse through a cell membrane without the help of their specific carrier protein or protein channel.

Channel and carrier proteins help the diffusion of molecules, such as:

  • Glucose and amino acids (too large; carrier protein)

  • Amino acids (some are polar/charged; carrier protein)

  • Ions (charged; carrier protein)

  • Bulk flow of water (polar; channel called ‘aquaporin’)

Without aquaporins, water takes a long time to diffuse across the cell membrane.

Channel proteins and carrier proteins - In case you're interested...The channel and carrier proteins that span the phospholipid bilayer facilitate the diffusion of these molecules into or out of a cell.
CHANNEL PROTEINS: Large molecules, polar molecules (e.g. water) and ions (e.g. Na+, K+ or Cl-) use channel proteins to diffuse across a cell membrane. The channel proteins are open at both ends of the membrane. They act as a corridor, allowing these molecules to diffuse in and out of a cell from areas of high concentration to areas of low concentration.
CARRIER PROTEINS: Carrier proteins are only open at one end at a time. Molecules such as glucose and amino acids bind to the carrier proteins, which then change shape to allow these molecules to diffuse into a cell. Proteins are highly specific, meaning they can only transport one kind of molecule through the cell membrane.
Video: How Facilitated Diffusion Works. Good overview of facilitated diffusion and carrier proteins

Simulation 1

Explore how molecules can cross a cell membrane and learn about the nature of their movement. Set up the model with high oxygen and low carbon dioxide outside the cell and low oxygen and high carbon dioxide inside the cell.

Factors Affecting the Rate of Diffusion

There are many different factors that can affect the rate of DIFFUSION. We will focus a little on temperature, but the most important factor is SURFACE AREA TO VOLUME RATIO (SA : V).

Temperature

Molecules will diffuse quicker at higher temperatures than at lower temperatures, because increasing the temperature causes the molecules to move faster.

Simulation 2: If you place a drop of food colouring into a glass of water, eventually the entire glass is coloured evenly with that dye. Use this model to observe how food colouring diffuses throughout the water.

Simulation 3: Observe the model of diffusion below. Notice the position of blue, green, and white particles. In this simplified model, the blue molecules represent perfume molecules, and the white and green molecules represent the nitrogen and oxygen molecules in the air.

Surface Area to Volume Ratio (SA : V)

When a cell grows, the contents of the cell (its volume) increases at a much greater rate than the CELL MEMBRANE (its SURFACE AREA). This is because the surface area increases by a square factor (cm^2) and the VOLUME increases by a cube factor (cm^3).

So we say that as the cell grows larger, the SURFACE AREA TO VOLUME RATIO (SA : V) decreases.

Because the SA : V ratio plays a vital role in the efficiency of DIFFUSION, cells are dependent on an optimal SA : V ratio to function properly.

If cells get too big, there is comparatively less membrane per volume available for essential molecules such as oxygen to diffuse through and more cell contents that require these molecules in order to function. So diffusion is less efficient for larger cells because essential molecules will not be able to reach the centre of the cell fast enough. The cell that’s too large will also be overloaded with toxic waste products like carbon dioxide in plant cells because waste molecules are unable to diffuse out of the cell quickly enough.

Therefore, when a cell gets too large, the cell will divide to keep high surface area to volume ratio.

The cubes in the diagram above represent a cell. The larger 2 cm cube represents a large cell above has a much lower surface area to volume ratio than the small 1 cm cube which represents a small cell.

Different cells have developed different adaptations to increase their surface area to volume ratio. The most common adaptation are tiny hair-like projections called MICROVILLI.


Specific adaptations to increase surface area - In case you're interested...Some cells, such as root hair cells, increase their surface area by having an elongated shape. This increases the amount of membrane available for diffusion while reducing the volume of the cell.
Other cells, such as red blood cells, have a biconcave shape. This is a disc-like shape with a flattened centre, almost like a donut! For red blood cells, this increases the surface area available for oxygen to diffuse into the cell.
Other cells and organelles have many folds in their cell membranes to increase the surface area. For example, the cells that line our small intestine are responsible for absorbing nutrients from food that we eat. To do this, they require a large surface area. They are covered with many folds called microvilli, which are long, finger-like extensions on the top of the cells. They create a very large area, allowing for maximum diffusion.
Video: Surface Area to Volume Ratio Explained (Science Sauce). Easy to understand video explaining surface area to volume ratio.
We will be doing an experiment very similar to this phenolphthalein experiment in this video above. The experiment demonstrates how diffusion becomes less efficient as the surface area to volume ratio decreases.

Concept 3: Osmosis & Active Transport

Success Criteria & Vocabulary

Click this drop-down menu to see the Success Criteria.

  • Compare the similarities and differences between diffusion, facilitated diffusion, osmosis, and active transport.

  • Explain different examples of cell transport in plants and animals.

Click this drop-down menu to see the list of Vocabulary.

Active transport: Movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.

Aerobic respiration: Enzyme controlled process which requires oxygen to produce 38 ATP from the breakdown of glucose.

Carrier proteins: Transmembrane protein that is only open to one side of the membrane at a time, and changes shape to transport molecules through the cell membrane.

Diffusion: Passive movement of molecules down a concentration gradient.

Facilitated diffusion: Transport of substances across a membrane from an area of higher concentration to an area of lower concentration by means of a transmembrane protein.

Flaccid: State of a plant cell in a hypertonic solution.

Hypertonic: Solution that has a higher concentration of solute than the cell.

Hypotonic: Solution that has a lower concentration of solute than the cell.

Isotonic: Solution that has an equal concentration of solute compared with the cell.

Lysis: Animal cell bursting in a hypotonic solution.

Osmosis: Movement of water molecules from an area of high concentration to an area of low concentration through a semipermeable membrane.

Passive transport: Transport of a substance across a cell membrane by diffusion; Energy is not required.

Semi-permeable: Ability of cell membranes to allow some substances to pass it but not others.

Solute: A substance dissolved in water.

Tonicity: Difference in relative concentration of solutes of two solutions, which determines the direction of osmosis.

Turgidity: Pressure created by water pushing up against the cell wall

Turgor pressure: The pressure that the content of the cell exerts on the cell membrane, pushing it against the cell wall

Tasks

Complete Education Perfect:

Task called '2.4 Concept 3'.

  • Osmosis

  • Active Transport

Complete sciPad:

  • Page 45 - Osmosis

  • Page 46 - Tonicity

  • Page 47 - Tonicity and Cells

  • Page 50 & 51 - Active Transport - Carrier Protiens

  • Page 56 & 57 - Cell Transport Processes Chapter Review

Mark your own work using the sciPad online answers.

Concept 3: Support Notes

Osmosis

OSMOSIS is the movement of water across a SEMIPEREABLE membrane, from an area of high water concentration to an area of low water concentration.

As with simple DIFFUSION and FACILITATED DIFFUSION, the water molecules are moving down the concentration gradient. Osmosis does not require any energy from RESPIRATION to move water molecules down a concentration gradient, therefore it is a type of PASSIVE TRANSPORT.

For example: Osmosis in root hair cells

In plants, water is taken up from the soil by special cells on a plant’s roots called root hair cells.

For OSMOSIS to happen, the soil must have a higher concentration of water than inside the root hair cell in order for water to move into the root cell’s SEMIPERMEABLE membrane and into the root cell.

This osmosis happens after rainfall, when there is a higher concentration of water in the soil than inside a plant’s cells. The water moves down the concentration gradient and into the plant, through the cell membrane of the root hair cells. All of this happens passively, meaning it doesn’t require energy.

Video: Osmosis and Water Potential (Amoeba Sisters). More in-depth than you need to know, use as a general overview. This has a worksheet associated with it. Let me know if you'd like to have access to this worksheet.

Tonicity

There are many different substances that can be dissolved in water. These dissolved substances are called SOLUTES.

The concentration of these solutes on either side of a cell’s semipermeable membrane will affect how water moves into our out of the cell by OSMOSIS. The measure of the concentration of these solutes on either side of the membrane is called TONICITY.

HYPERTONIC Solution

Hypertonic solutions are very concentrated solutions. E.g. very salty or sugar water.

The water concentration is higher in the cell, so the water is going to want to move out of the cell and into the extracellular environment.

The cell shrinks in size and becomes FLACCID (plant) or PLASMOLYSED (animal).

HYPOTONIC Solutions

Hypotonic solutions are very dilute solutions. E.g. pure water.

The water concentration is higher in the extracellular environment, so the water is going to want to move into the cell.

The cell grows in size and becomes TURGID (plant). The animal cell keeps growing until the cell membrane ruptures (LYSIS).

ISOTONIC Solutions

Isotonic solutions are neither concentrated nor dilute. E.g. saline.

The water concentration is the same inside the cell and in the extracellular environment, so there is no net movement of water.

The cell does NOT change size.

Active Transport

ACTIVE TRANSPORT is the complete opposite to PASSIVE TRANSPORT.

Active transport moves molecules across the CELL MEMBRANE against a concentration gradient, from an area of low concentration to an area of high concentration.

You can think of active transport as pushing a large object up a hill. It takes up a lot of energy, as you are pushing the object against the natural slope.

Active transport only ever takes place through transmembrane proteins.

In active transport, molecules can bind to a specific CARRIER PROTEIN, which changes shape to then release the molecules to the other side of the cell membrane. This process requires energy, as the carrier proteins are moving molecules against the concentration gradient.

The Na+/K+ Pump

The most well-known example of a carrier protein is the sodium-potassium pump (also called the Na+/K+-ATPase).

This pump is a carrier protein that transport charged sodium ions out of the cell and potassium ions into a cell, against their concentration gradient.

Sodium or potassium ions bind to the carrier protein, then ATP is used to change the protein’s shape. This allows the ions to be released to the other side of the cell, through the cell membrane, against the concentration gradient.

Below: A carrier protein that uses ATP to transport molecules against its concentration gradient.

Examples of Active Transport

Uptake of minerals by root hair cells

Root hair cells are special cells found on the roots of plants. Root hair cells use active transport to help the plant take up minerals that are found in the soil. Generally, the concentration of minerals is lower in the soil than the root hair cell itself.

So root hair cells have to actively move these mineral ions into the cell against its concentration gradient using carrier proteins embedded in the cell membrane. To be able to cope with all of this active transport, root hair cells need a lot of energy from respiration, so they have many mitochondria.

If the plant root hair cell can’t do this active transport of minerals, it won’t be able to survive.

Removal of salt by fish gills

Fish that live in salt water take up high concentrations of salt ions dissolved in sea water. This level of salt is actually harmful to fish. Fish remove these ions using cells in the gills. But because the ions are in a much higher concentration in the ocean compared to the cells in the gills, these cells use carrier proteins to actively pump the ions out of the fish.

Like with root hair cells, these cells in the gills need a lot of energy to do this, so they have a lot of mitochondria.

Comics Corner

osmosis.mp4
fluid mosaic model.mp4