Unit 2 - Cells and Immunity

AQA GCE A-Level Biology - Unit 2 - Cells

\[3.2.1.1\] What is a **EUKARYOTIC** cell?

Cells which have a **membrane-bound nucleus** and **membrane-bound organelles**.

\[3.2.1.1\] What does a typical **animal** cell look like?

\[3.2.1.1\] What does a typical **plant** cell look like?

\[3.2.1.1\] What does a typical **algal** cell look like?

Algal cells can be **unicellular** or **multicellular**.

\[3.2.1.1\] What does a typical **fungal** cell look like?

\[3.2.1.1\] What is the **CELL-SURFACE MEMBRANE**?

A membrane found on the surface of animal cells and below the cell wall of other cells.

It has a ***fluid-mosaic*** structure.

* Contains a **phospholipid bilayer**.

\[3.2.1.1\] What is the **NUCLEUS**?

* The Nucleus​ is a membrane bound structure that contains the cell’s hereditary information and controls the cell’s growth and reproduction.

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* The nuclear envelope is a double membrane that separates the nucleus from the cytoplasm. The nuclear envelope is perforated with nuclear pores.

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* When a cell is not dividing, the chromosomes are organised into long entangled structures called chromatin and not into individual chromosomes.

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* Nucleoplasm is the gelatinous substance within the nuclear envelope.

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* The nucleolus is not surrounded by a membrane.

[3.2.1.1] What is the function of the nucleus?

* It is responsible for protein synthesis, cell division, growth and differentiation.

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* It is a site for the transcription process in which mRNA is produced for protein synthesis.

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* Regulates the integrity of genes and gene expression.

\[3.2.1.1\] What are **LYSOSOMES**?

* Round organelles surrounded by a membrane, with no clear internal structure.
* Contain hydrolytic enzymes called lysozymes.

→ Used to digest invading cells or break down components of the cell.

\[3.2.1.1\] What is the **MITOCHONDRIA**?

* The site of **aerobic** **respiration** where **ATP** is produced.

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* Mitochondria have a double membrane - the inner one is highly folded into **cristae** which contain **electron carrier** proteins.

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* Inside is the **matrix**, which contains **enzymes** involved in **respiration**.

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* Contains its own **DNA** and **ribosomes** in order to duplicate in cell division.

\[3.2.1.1\] What are **CHLOROPLASTS**?

* Small, flattened structures found in plant and algal cells which are the site of **photosynthesis**.

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* They are surrounded by a double membrane, with an inner membrane called the **thylakoid** **membrane**.

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* Thylakoid membranes are stacked up to form **grana**, which are linked together by **lamellae**.

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* The fluid inside is the **stroma**.

\[3.2.1.1\] What are **RIBOSOMES**?

* Very small organelles found attached to the **rough endoplasmic reticulum** or **free** in the cytoplasm.

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* They are made from **proteins** and **RNA**.

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* They do not have a membrane.

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__***RESPONSIBLE FOR PROTEIN SYNTHESIS***__.

[3.2.1.1] What is the ENDOPLASMIC RETICULUM?

* A series of thin intricate channels with a folded membrane enclosing a fluid filled space.

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→ **Rough Endoplasmic Reticulum** have ribosomes on the surface and are responsible for *folding and processing proteins*.

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→ **Smooth Endoplasmic Reticulum** are responsible for *lipid and steroid synthesis*.

\[3.2.1.1\] What is the **CELL WALL**?

* A **rigid** structure which surrounds cells. Made from **cellulose** in plants and algae and made from **chitin** in fungi.

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* It supports the cell and prevents change of shape.

[3.2.1.1] What is the VACUOLE?

A membrane-bound organelle found in the cytoplasm that contains **cell sap** (a *weak solution of sugars and salts*). The surrounding membrane is called the **tonoplast**.

* **Maintains interior pressure**.
* Keeps cell **rigid**.
* **Isolates unwanted chemicals**.

\[3.2.1.1\] What is the **GOLGI APPARATUS**?

A network of fluid-filled, membrane-bound flattened sacs which ***processes and packages new lipids and proteins into vesicles for secretion from the cell***.

\[3.2.1.1\] What are **GOLGI VESICLES**?

Small fluid-filled sacs in the cytoplasm, surrounded by a membrane. They ***store lipids and proteins and transport them out of the cell***.

[3.2.1.1] How do cells become specialised?

The cell **expresses different genes** which produces different proteins.

[3.2.1.1] How are epithelial cells adapted to their function?

* Many **microvilli** to **increase surface area**.

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* Many mitochondria to produce **ATP** for **active transport**.

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* Lots of golgi or rough endoplasmic reticulum in order to ***produce carrier and channel proteins for facilitated diffusion and active transport***.

\[3.2.1.2\] What is a **prokaryotic** cell?

Cells which ***do not have a nucleus or any membrane-bound organelles***.

\[3.2.1.2\] What does a typical **prokaryote** look like?

\[3.2.1.2\] What is the **CAPSULE**?

An organelle made from secreted slime that protects the cell from an immune response.

[3.2.1.3] What are the two types of microscope?

Optical and Electron.

\[3.2.1.3\] What is meant by '**magnification**'?

How large an image is compared to real life.

\[3.2.1.3\] What is the equation for **magnification**?

==**Magnification**== = %%**Image Size**%% / ^^**Actual Size**^^

\[3.2.1.3\] What is meant by the term '**resolution**'?

The ability of a microscope to distinguish detail.

\[3.2.1.3\] What are **OPTICAL** microscopes?

Microscopes that use light to view objects.

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* Maximum resolution of **0.2** micrometres.
(Cannot view ribosomes, endoplasmic reticulum or lysosomes).
* Maximum useful magnification of **x1500**.

\[3.2.1.3\] What are **ELECTRON** microscopes?

Electron microscopes use a __beam of electrons that are focused by electromagnets inside a vacuum environment__.

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* Maximum resolution of **0.0002** micrometres.
* Maximum useful magnification of **x1500000**.
* Produces **black and white** images.

\[3.2.1.3\] Why do electron microscopes use a **vacuum** environment?

So that particles in the air *do not deflect electrons* out of the beam alignment.

[3.2.1.3] What are the two types of electron microscope?

**Transmission** and **Scanning**.

\[3.2.1.3\] What are **TRANSMISSION** electron microscopes?

A beam of electrons passes through a __**THIN**__ section of specimen.
\--> Areas that **absorb** the electrons appear darker on the electron micrograph that is produced.

\[3.2.1.3\] What are **SCANNING** electron microscopes?

A beam of electrons passes across the surface and **scatters**.
\--> The pattern of scattering builds up a **3D** image depending on the contours of the specimen.

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*LOWER RESOLVING POWER THAN TRANSMISSION ELECTRON MICROSCOPES*.

[3.2.1.3] What are the limitations to electron microscopes?

* The whole system must be in a __**vacuum**__, so living specimens __cannot__ be observed.
* A complex straining process is required which may introduce **artefacts** into the image.
* Specimens have to be __very thin__.

\[3.2.1.3\] What is an **ARTEFACT**?

Artefacts look like part of the microscope sample but are actually a side-effect of sample preparation or the conditions in the microscope.

\[3.2.1.3\] What is step one of **CELL FRACTIONATION**?

**HOMOGENISATION**.

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This process releases the organelles by breaking open the cell membranes without damaging them. This is done with a __homogeniser__ and it produces a ***homogenate*** (a liquid of broken up cells).

\[3.2.1.3\] What are the characteristics of the solution in which **homogenisation** is carried out?

* **ICE COLD**

→ Reduces enzyme activity.
* **ISOTONIC**

→ Same **water** **potential** as the organelles so water is not taken in or lost by **osmosis**.
* **BUFFERED**

→ Maintains **pH**.

\[3.2.1.3\] What is step two of **CELL FRACTIONATION**?

**Filtration** of large cell debris.

\[3.2.1.3\] What is step three of **CELL FRACTIONATION**?

**Ultracentrifugation**.

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Cell Fractionation involves spinning samples at *progressively higher speeds* (**differential ultracentrifugation**) to separate the organelles.

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Different types of organelles have different **masses** or **densities**.

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When the **homogenate** is spun in a **centrifuge** the heaviest organelle will sink to the bottom. The density of the organelle relates to the speed it falls.

\[3.2.1.3\] What is the **SUPERNATANT**?

The **fluid above the pellet** which may contain less dense material.

\[3.2.1.3\] What is the **PELLET/FRACTION**?

The ***mass of dense material*** at the **bottom of the centrifuge** tube after spinning.

\[3.2.2\] What are **autosomes**?

**Autosomes** are ***chromosomes which are* __*not*__ *sex chromosomes***.

\[3.2.2\] How are **chromosomes** formed?

**Chromosomes** are formed when **DNA** winds around **histone proteins** to form **nucleosomes**. They then **supercoil** to form the dense chromosomal structure. This then replicates to form a **double-stranded chromatid**.

\[3.2.2\] What are **SISTER CHROMATIDS**?

**Sister** **chromatids** are formed when two identical chromatids are joined together at the **centromere**.

\[3.2.2\] What is the **centromere**?

The centromere is a **constricted** **region** where two sister chromatids are in contact with each other.

\[3.2.2\] What is **chromatin**?

**Chromatin** is the material that chromosomes are made up of.

\[3.2.2\] What is meant by the term ‘**haploid**’?

Cells with only **23 chromosomes** (e.g. gametes).

\[3.2.2\] What is meant by the term ‘**diploid**’?

Cells which have **46 chromosomes** (23 from each parent).

\[3.2.2\] What is **MITOSIS**?

Mitosis is the division of a diploid cell to produce two genetically identical daughter cells which have the same number of chromosomes as eachother.

\[3.2.2\] Why do cells need to divide?

* **Growth** (*all over in animals, in the meristems of plants*)

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* **Repair** of Damaged Tissues

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* **Asexual** **Reproduction** (*genetically identical offspring*)

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* **Maintaining** the number of **Chromosomes**

\[3.2.2\] What are the stages of **mitosis**?

I - Interphase

P - Prophase

M - Metaphase

A - Anaphase

T - Telophase

(c) - Cytokinesis

\[3.2.2\] What is the first stage of **interphase**?

__**G1 (Growth or Gap)**__

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→ Cell **Grows** and **Develops** whilst undergoing normal cell processes.

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→ Makes **proteins** for cell division.

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→ **ATP** is produced and stored.

\[3.2.2.\] What is the second stage of **interphase**?

__**S (Synthesis)**__

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→ **DNA Replication.**

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→ Chromosomes form chromatin.

\[3.2.2\] What is the third stage of **interphase**?

__**G2 (Gap or Growth)**__

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→ **ATP** is produced.

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→ **Organelles** **duplicate**.

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→ **More cytoplasm**.

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→ DNA is **error-checked**, the cell will **self-terminate** if something is massively wrong.

\[3.2.2.\] What is **prophase**?

* The **Nucleolus** breaks down and the **Nuclear** **Envelope** disappears.

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* Chromosomes coil and become visible under a microscope.

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* Chromosomes consist of **two sister chromatids** joined at the **centromere**.

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* **Centrosomes** move to **opposite** **poles**.

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* **Spindle Fibres** (***protein*** ***microtubules***) emerge from **centrosomes** (two **centrioles**).

\[3.2.2.\] What is **metaphase**?

* The **spindle** is completely formed.

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* Chromosomes line up along the **equator** of the spindle so that they are **equidistant** to the centrosome poles.
* *The microtubules attach to the centromere of each chromatid*.

\[3.2.2\] What is **anaphase**?

* The sister chromatids **divide** at the **centromere**.

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* The spindle fibres **contract** and pull the sister chromatids to opposite poles of the spindle (they are now called **chromosomes**).

\[3.2.2\] What is **telophase**?

* Chromosomes **decondense** (they become long and thin again and can no longer be seen clearly under a microscope).

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* New Nuclear Envelopes form around each set of chromosomes, but in the same cytoplasm space.

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* The spindle begins to break down.

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* The nucleolus reforms.

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* **Cytokinesis** occurs.

\[3.2.2\] What is **cytokinesis**?

The **division of the cytoplasm** by constriction from the edges of the cell.

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* In **ANIMAL CELLS**:

→ The plasma membrane is *pulled inwards* around the *equator* to form a **cleavage furrow**. This is done using a **contractile ring of protein** of **actin** and **myosin** **fibres**.

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* In **PLANT CELLS**:

→ A new cell wall is formed across the **equator**.

→ A **middle lamella** forms and **cellulose** is deposited.

\[3.2.2\] What is the formula for **mitotic index**?

**NUMBER OF CELLS UNDERGOING MITOSIS**

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**TOTAL NUMBER OF CELLS IN THE FIELD OF VIEW**

\[3.2.2\] What causes **cancer**?

**Uncontrollable Cell Division**.

\[3.2.2\] What is a **tumour**?

A large group of cells that are **dividing uncontrollably**.

\[3.2.2\] How can **drugs treat cancer** by ***blocking part of the cell cycle***?

* **Chemotherapy** prevents the **synthesis of enzymes** needed for **DNA Replication**. This means that cells cannot enter the **S phase** and therefore self-destruct.

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* **Radiation** and **Drugs** **damage** **DNA** so much that the cell cannot fix it which means the cell has to self-destruct.

\[3.2.2\] Why can drugs that are used to treat **cancer** cause **hair loss**?

Rapidly dividing cells like hair cells are killed which leads to hair loss as there is nothing to keep producing the proteins needed for hair growth.

\[3.2.3\] Why is the **cell membrane** described as ‘**fluid-mosaic**’?

The molecules can **move** within the membrane and there are other molecules (such as **proteins**) embedded within the **phospholipid bilayer**.

\[3.2.3\] What is the structure of the cell membrane?

\[3.2.3\] What is the **phospholipid bilayer**?

A **phospholipid** **bilayer** is composed of two layers of polar phospholipids; their **hydrophobic** **tails** facing inwards and **hydrophilic** **heads** outwards.

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* They *regulate the movement of substances* through the membrane (which is **selectively permeable**).

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* Internally, **membrane-bound compartments** formed from phospholipid bilayers provide the basic structure of organelles. An example of a membrane-bound organelle is the **lysosome**. The **lysozymes** need to be kept **compartmentalised** otherwise they would breakdown most of the cellular components.

\[3.2.3\] What is **cholesterol**?

Cholesterol is a type of **lipid**.

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* Cholesterol binds to **hydrophobic** tails of phospholipids, causing them to pack together. This restricts movement, making the membrane ***less fluid and more rigid***.

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* It has **hydrophobic** regions so acts as a **barrier to polar substances**.

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* Gives the membrane **stability**.

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* **Maintains the shape** of animal cells.

\[3.2.3\] How does **TEMPERATURE** affect the **permeability** of cell membranes?

__**BELOW 0°C:**__

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* Phospholipids **don’t have much energy** so are unable to move and they are **packed** tightly together, membrane is **rigid**.
* Channel and carrier proteins **denature**, **increasing** **permeability**.
* **Ice** **Crystals** may form and **pierce** the membrane, **increasing** **permeability** once it is melted.

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__**BETWEEN 0°C AND 45°C:**__

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* Phospholipids can **move** around and aren’t as packed, the membrane is **selectively permeable**.
* ==*Increased Temperature = Increased Movement = Increased Permeability*==

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__**ABOVE 45°C:**__

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* The phospholipid bilayer begins to **break** **down**, **increasing** **permeability**.
* Water expands, increasing the **pressure**.
* Channel and carrier proteins **denature**, **increasing permeability**.

\[3.2.3\] How do **SOLVENTS** affect membrane **permeability**?

Surrounding cells in an **increasing concentration** of a solvent **increases membrane permeability** because the **solvent dissolves the lipids**, causing a loss of structure.

\[3.2.3\] What are the four ways that molecules can be transported across a membrane?

* Diffusion
* Facilitated Diffusion
* Active Transport
* Osmosis

\[3.2.3\] What is the definition of **diffusion**?

The **net movement** of molecules or ions from a **region of high concentration** to a region of **low concentration** until they are **equally distributed**. It is a **random** and **passive** process.

\[3.2.3\] What type of molecules are transported through the phospholipid bilayer by **simple diffusion**?

**Small** molecules or **uncharged particles**.

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* e.g. H₂O, CO₂, O₂

\[3.2.3\] What are the factors which affect the rate of diffusion across membranes?

* **Steepness** of the **Concentration Gradient**

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* **Distance** over which diffusion occurs (**DIFFUSION PATHWAY**)

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* **Surface Area** between the two regions.

→ an increased surface area = an increased rate of diffusion

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* **Temperature**

→ an increased temperature increases the kinetic energy of the molecules so they move faster across the membrane

\[3.2.3\] What is the definition of **facilitated diffusion**?

The transport of molecules from a **region of high concentration** to a **region of low concentration** across a membrane using **channel** and **carrier proteins**. It relies on the **inbuilt motion (kinetic energy) of diffusing molecules** and occurs **down a concentration gradient**.

\[3.2.3\] What particles are transported via facilitated diffusion in **CHANNEL proteins**?

**Charged particles** (**ions**).

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* e.g. Na⁺, PO₄³⁻, O²⁻

\[3.2.3\] How do **channel proteins** work in facilitated diffusion?

Particles can diffuse in/out of the cell dependent on the *direction* of the **concentration gradient**.

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* Some channel proteins may open or close in response to a **specific messenger** or a **change in voltage** (*gated channel proteins*).

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* Some channels are always open but others open when a ***hormone*** is attached.

\[3.2.3\] What particles are transported via facilitated diffusion in **CARRIER proteins**?

**Large Molecules**.

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* e.g. Glucose and Amino Acids.

\[3.2.3\] How do **carrier proteins** work in facilitated diffusion?

The particle binds to the protein, which then **changes shape** and transports the particle across the membrane.

\[3.2.3\] What are the types of **passive** transport?

Osmosis, Diffusion, Facilitated Diffusion.

\[3.2.3\] What are the types of **active** transport?

Active Transport, Bulk Transport (e.g. *Endocytosis*).

\[3.2.3\] What is the definition of **active transport**?

The **active movement** of molecules or ions from a **region of low concentration** to a region of **high concentration** using **energy** and **intrinsic carrier molecules**. This occurs **AGAINST** a **concentration gradient**.

\[3.2.3\] How does **direct active transport** work?

* A **phosphate group** from an **ATP** molecule attaches to the carrier protein, causing it to change shape.

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* As it changes shape, it *pumps the molecule into the cell*.

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* The phosphate group is then released and the protein reverts back to its original shape.

\[3.2.3\] What do cells that carry out active transport need many of?

**Mitochondria** to produce **ATP** by **aerobic respiration**.

\[3.2.3\] Which factors affect the **rate of active transport**?

* The **speed** of individual carrier proteins.

→ *The faster they work, the faster the rate of active transport.*

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* The **number** of carrier proteins.

→ *More carrier proteins = faster rate of active transport.*

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* **Rate of Respiration**

→ *If respiration is inhibited, active transport doesn’t occur.*

\[3.2.3\] What is **co-transport**?

**Co-transport** is the **coupled** movement of substances across a cell membrane via **carrier proteins**.

\[3.2.3\] How does **glucose** enter cells by co-transport?

1) Sodium Ions are **actively transported out of the epithelial cells** and into the blood.

→ *The concentration of Na⁺ in the cell* ***decreases****. There is now a* ***higher*** *concentration of Na*⁺ *in the intestine than in the epithelial cell.*

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2) Na⁺ ions can now diffuse into the cell through the **sodium-glucose cotransporter protein**. *[They carry the glucose with them.]*

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3) Glucose is transported into the cell **against its concentration gradient** by the cotransporter.

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4) The concentration of glucose in the epithelial cell is **higher** than in the blood. Glucose diffuses out of the cell by **facilitated diffusion** through a carrier protein.

\[3.2.3\] How is the **ileum** adapted for absorption?

* **Villi** increases **surface area**.

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* **Very thin walls** so there is a **short diffusion pathway**.

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* Walls contain **muscle** so can move.

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* **Good blood supply** (*maintains concentration gradient*).

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* **Microvilli** increase **surface area to volume ratio**.

\[3.2.3\] What is **osmosis**?

**Osmosis** is the **net movement of water molecules** from a region of **less negative water potential** to a region of **more negative water potential** through a **partially permeable membrane**. It is a **random** and **passive** process.

\[3.2.3\] What is **water potential**?

The **tendency of water molecules** to **leave** a solution via **osmosis**.

\[3.2.3\] What is the **water potential scale**?

The water potential of **pure water** is **0KPa**, which is the highest value.

→ *This is because adding a solute makes it harder for water molecules to move, reducing the water potential (making it more negative)*

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\[***A less negative water potential = a higher water potential = a lower concentration of solute]***

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\[***A more negative water potential = a lower water potential = a higher concentration of solute]***

\[3.2.3\] What is a **hypotonic solution**?

Solutions with a **less negative** water potential compared with the inside of the cell.

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* Animal Cells in hypotonic solutions undergo cell **lysis**.


* Plant Cells in undergo **turgor** (become **turgid**).

\[3.2.3\] What is an **isotonic solution**?

Solutions with the **same water potential** as the cell.

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* Animal Cells and Plant Cells have no net movement of water.

\[3.2.3\] What is a **hypertonic solution**?

Solutions with a **more negative** water potential than the cell.

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* Animal Cells undergo **crenation**.
* Plant Cells undergo **plasmolysis**.

\[3.2.4\] What is a **pathogen**?

A **microorganism** that **causes disease**.

\[3.2.4\] What are the body’s **defense mechanisms**?

__**NON-SPECIFIC:**__

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* **Physical** **Barrier** (e.g. Skin, Mucus, Stomach Acid, Cilia)
* **Phagocytosis**

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__**SPECIFIC:**__

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* **Cell** **Mediated** Response (*T-Lymphocytes*)
* **Humoral** Response (*B-Lymphocytes*)

\[3.2.4\] What is an **antigen**?

Molecules (usually *proteins*) which trigger an **immune response**, i.e. trigger the production of antibodies. They are found on the surface of body cells and pathogens.

\[3.2.4\] What do antigens allow our immune systems to identify?

* Bacterium
* Viruses
* Transplanted Cells
* Abnormal Cells

\[3.2.4\] How does **phagocytosis** work?

* The phagocyte surrounds the pathogen and **engulfs** it, forming a **phagocytic vacuole**.

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* The phagocyte’s **lysosomes** fuse with the phagocytic vacuole and the **lysozymes** digest the pathogen.

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* **Antigens** from the pathogen appear on the surface of the body cell, meaning it has become an **antigen-presenting cell**.

\[3.2.4\] What are **T-Lymphocytes**?

A type of white blood cell which have receptors on their membrane which are complementary to the shape of a specific antigen. The T-cell will become activated when an antigen binds to the receptor.

\[3.2.4\] How do **Helper T Cells** work?

* Receptors on the surface bind to antigens on antigen-presenting cells.

→ This activates helper T cells to replicate via **mitosis**.

→ Some of these T cells then differentiate into other cells.

→ Some remain as T cells which active **B Cells**.

→ Some activate **macrophages** to perform **phagocytosis**.

→ Some form **cytotoxic T cells**.