IBDP Biology - Cell Structure

In this IBDP Biology chapter, we will understand the ultrastructure of cells

• An organism made out of one cell is unicellular
• An organism made out of more then one cell is multicellular

• Light microscopes- use glass lenses to refract light rays and produce a magnified image.
• The specimen needs to be flat
• A stain is often put onto specimens to make them easier to see.

• Condenser lens - Focuses the light onto the specimen held between the cover slip and slide
• Objective lens - Collects light passing through the specimen and produces a magnified image
• Eyepiece lens - Magnifies and focuses the image from the objective onto the eye

• This removes the problem of resolution as it uses a different type of wave.
• Electron beams have much smaller wavelengths then light rays. They have a resolution 400 times higher.
• With a light microscope we can only magnify something 1400 times. With an electron microscope we can magnify something 300,000 times.

• They pass electrons through a thin specimen.
• Specimens need to be thin and stained.
• The stains used are normally heavy metals such as lead.
• Ions of these metals are taken up by some parts of the cells more then others.
• The ions are large and positively charged.
• The negatively charged electrons do not pass through them and so don’t arrive on the screen.
• The screen stays dark in these areas.

• Bounce electrons off the surface of an object.
• Give a 3D image.
• The original images are in black and white but colour is often added to them.
• Because we can see so much more detail with electron microscopes, we call the image we can see with them the ultrastructure.

• The size of an image divided by the real size of the object.

Magnification = size of image / real size of object

• We use two units when working out magnification
• Micrometres (one 1000th  of a mm) 1x10-3 mm 
• Nanometres (one 1000th of a micrometre) 1x10-6 mm

•Light microscopes are unable to show objects smaller then 200nm across.

•The degree of detail that can be seen in an image is called the resolution.

•This is determined by the wavelength of the radiation that is uses.

•Shorter wavelengths give the best resolution.

• Normally the largest organelle in the cell.
• Has a tendency to take up stains most easily
• Surrounded by two membranes (the nuclear envelope)
• There are small gaps all over the envelope (nuclear pores)
• The nucleus contains chromosomes.
• When chromosomes form a tangle they are called chromatin.
• Carries a code which instructs the cell on making proteins.
• Parts of the chromatin are used for the first stage of protein synthesis, transcription.
• During transcription, information on DNA is copied onto molecules of messenger RNA, which travels out of the nucleus and into the cytoplasm.
• An especially darkly staining area is the nucleolus where DNA used for making ribosomes is kept.

• A network of membranes found in every eukaryotic cell.
• If they have ribosomes attached to them they are rough endoplasmic reticulum (RER)
• If they do not then they are smooth endoplasmic reticulum (SER)
• The RER is normally continuous with the nuclear envelope.
• The enclosed spaces formed by the membranes are called cisternae.
• RER is where most protein synthesis takes place.
• As protein molecules are made they collect inside the cisternae.
• SER has many different roles. It’s sometimes the sight of the production of steroid hormones, or where toxins are broken down.

•A stack of curved membranes enclosing a series of flattened sacs.

•Not a stable structure, constantly changing.

•One side- tiny vesicles move towards the Golgi and fuse together, forming a new layer to the stack.

•At the other side- the sacs break down, forming vesicles that move away from the Golgi.

•The vesicles that have fused with the Golgi have come from the endoplasmic reticulum. They contain proteins that were made there.

•In the Golgi proteins are packaged and processed, turning them into the desired product.

•Some of the proteins are then transported in vesicles to the plasma membrane.

•The vesicles fuse with the membrane and deposit the proteins outside the cell (exocytosis) 

•The production of useful substances in a cell, and their release is called secretion

•some vesicles remain in the cell. Some of these contain proteins that function as digestive enzymes, these vesicles are called lysosomes .

• Tiny bags of digestive enzymes
• Surrounded by a single membrane.
• Main function is to fuse with other vesicles in the cell that have something needed to be digested e.g. A bacterium brought in by an endocytosis.
• Help destroy worn out or unwanted organelles.
• The enzymes break down the large molecules into something soluble that can disperse into the cytoplasm.

• Found in plant cells but never animal cells. The sight of photosynthesis.
• Has a double membrane called an envelope.
• Inside the chloroplast there are membranes called grana. When these form stacks they are called thylakoids.
• The grana contain chlorophyll and this is where the light dependant reaction in photosynthesis takes place.
• Light energy is used to split water molecules to provide hydrogen ions. These are used to make ATP and reduced NADP.
• These are used to make carbohydrates using carbon dioxide in the light independent reaction.
• The light independent reactions take place in the stroma, the background material of the chloroplast.
• Chloroplasts often contain starch grains. These are used as energy stores in plants.

• Found in both plant and animal cells
• Surrounded by a double membrane, an envelope.
• Site of aerobic respiration.
• Where ATP is made.
• The inner membrane of the mitochondria are folded to make cristae. Where ATP is made.
• The background material is called the matrix. This is where the Krebs cycle takes place.

• A membrane bound organelle that contains liquid.
• Plants often have vacuoles that contain cell sap.
• He membrane surrounding the vacuole is known as the tonoplast.

• Plant cells are always surrounded by cell walls.
• Not an organelle as it is not inside the cell.
• Made up of long strands of cellulose.
• These fibres are arranged in a criss-cross manner held together by the matrix that contains pectin.
• Pectin is also found in the middle lamella that cements one cell to another.

• Every cell is surrounded by a plasma membrane.
• A thin layer made up of lipid molecules and protein molecules.
• Controls what leaves and what enters a cell.

• Are found in animal cells but not plant cells.
• They make and organise tiny structures called microtubules.
• These are made from a protein called tubulin
• During cell division microtubules form the spindle and move the chromosomes around the cell.

• Long thin extensions of the surface of a cell, which can produce movement.
• They are found in some animal cells and rarely in plant cells.
• They have the same basic structure.
• ‘Cilia’ is used for short structures found in large numbers.
• ‘flagella’ are longer and normally found in ones or twos.
• They both contain microtubules. They are always arranged with two in the middle surrounded by nine pairs of microtubules.
• Movement is made by these microtubules moving over each other.
• The movement causes them to bend and then straighten.
• The movement of cilia can move fluids over the surface of a cell.
• Flagella causes the cell to swim through a liquid.

• All plant and animal cells contain a network of filaments called microfilaments. These act as a skeleton helping to support the cell and determine its shape.
• Together with microtubules these filaments make up the cytoskeleton.
• Provides mechanical strength for the cell.
• Provides ‘tracks’ by which organelles can be moved.
• The microtubules can act as motors using energy from ATP to pull organelles along.

• Means ‘before nucleus’
• The DNA lies free in the cytoplasm
• The DNA is always attached to the plasma membrane.
• They do not have complex membrane bound organelles.
• Have ribosomes but these are smaller.
• Surrounded by a cell wall. But this one is made up of peptidoglycan.

This is the end of the topic

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Drafted by Eva (Biology)

Photo references:

1. https://www.bulbapp.com/u/microscope~10
2. https://www.jeolbenelux.com/JEOL-BV-News/Tag/transmission-electron-microscope
3. https://www.thinkymixer.com/en-gl/library/report/evaluation-of-materials-using-scanning-electron-microscope-sem/
4. https://link.springer.com/chapter/10.1007/978-3-319-26197-3_1
5. https://biologydictionary.net/plant-cell/
6. https://byjus.com/biology/difference-between-cilia-and-flagella/
7. https://www.proprofs.com/quiz-school/story.php?title=prokaryotic-vs-eukaryotic-cells