Cells

Cells

There are two main types of cells, eukaryotic and prokaryotic cells. Eukaryotic cells are larger than prokaryotic cells, and their DNA is contained in a nucleus.

Cells, figure 1

Most eukaryotic cells are complex multicellular organisms containing a range of specialised cells to perform a variety of functions. Organisms are made up of organ systems, which contain a range of organs working together to perform a function. Organs are made up of a range of tissues working together to perform a function and specialised cells of a similar structure and functions are organised into tissues.

Eukaryotic Cells

Eukaryotic cells contain a range of organelles. Cells, figure 1

Organelle Structure Function
Cell-surface membrane Phospholipid bilayer with proteins and cholesterol embedded within it. Glycolipids and glycoproteins in the surface. The fluid-mosaic model of the membrane refers to the fluidity and range of molecules in the membrane. Cholesterol provides strength and reduces fluidity, proteins are for transport and the glycoproteins and glycolipids are for cell recognition and act as receptors.
Nucleus Surrounded by a double membrane nuclear envelope with nuclear pores. Containing chromosomes, consisting of protein-bound, linear DNA, and one or more nucleolus Nucleolus is the site of rRNA product and makes ribosomes. DNA replication and transcription occur in the nucleus.
Mitochondria Double membrane organelle. The inner membrane is folded to form cristae. Contains a fluid centre called the matrix Site of aerobic respiration and ATP production.
Chloroplasts Surrounded by a double membrane. Contains thylakoids, which are folded membrane containing chlorophyll pigments. Contains a fluid centre, the stroma. The site of photosynthesis. The stroma contains enzymes for the light-independent stage of photosynthesis
Golgi apparatus and Golgi vesicles Stacks of membranes creating flattened sacs called cisternae, surrounded by small round hollow vesicles. Proteins and lipids and modified here. Carbohydrates can be added to proteins to form glycoproteins. Finished products are transported in the Golgi vesicles.
Lysosomes Formed when the Golgi apparatus contains hydrolytic enzymes. A type of Golgi vesicle that releases lysozymes.
Ribosomes Small granules in cells made of protein and rRNA. Made up of a small and large subunit. 80s in size in eukaryotes. The site of translation in protein synthesis.
Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) Sheets of membranes linked to the nucleus. The membranes form a network of cisternae- a network of tubules and flattened sacs. The RER have ribosomes on the outer surface. RER – site of protein synthesis and glycoprotein synthesis. The proteins can then be transported through the RER. SER – create, store and transport lipids and carbohydrates.
Cell wall Found in plants, algae and fungi. Consists of polysaccharides, cellulose in plants and chitin in fungi. There is a thin boundary layer between adjacent cells called the middle lamella. Provides structural strength to cells and prevents cells from bursting when water enters by osmosis.
Cell vacuole Found in plants. A single membrane sac filled with fluid containing salts, sugars and amino acids. The membrane around a cell vacuole is called the tonoplast. To provide support to a cell, store amino acids and sugars and can contain pigments to attract pollinators.

Prokaryotic Cells

Bacterial cells are the key example of prokaryotic cells. In addition to them being smaller and not having a nucleus, there are other differences compared to a eukaryotic cell shown in the diagram below.

Cells, figure 1

The cytoplasm does not contain any membrane-bound organelles and the ribosomes are smaller, 70s in size.

Bacteria still contain DNA, but it is not stored in a nucleus. The DNA is found as a single circular molecule in the cytoplasm and is not associated with histone proteins.

Prokaryotic cells do have cells walls, but they do not contain cellulose or chitin. Instead they are made of a glycoprotein called murein.

Some bacteria also contain three additional features. They can contain plasmids, which are rings of DNA containing genes linked to survival such as antibiotic resistance. Surrounding the cell wall, some bacteria have a capsule which is to provide protection from other cells and to help bacteria stick together. Finally, they can have flagella which used for locomotion.

Viruses

Viruses are non-living and acellular. They are even smaller than bacteria and only contain genetic material, a capsid and an attachment protein.

Cells, figure 1

HIV is surrounded by a further lipid envelope which has attachment proteins on the outside. This is to enable the virus to identify the host cells to enter.

Cells, figure 2

Studying Cells

Knowing the internal structure of cells has been possible due to a range of methods. These include using microscopes, cell fractionation and ultracentrifugation.

Microscopes

There are three key types of microscopes; optical microscopes, transmission electron microscopes and scanning electron microscopes.

The magnification of a microscopes refers to how many times larger the image is compared to the object.

The resolution of a microscope is the minimum distant between two objects in which they can still be viewed as separate.

The resolution in an optical microscope is determined by the wavelength of light, and the wavelength of the beam of electrons determines the resolution in electron microscope.

Optical (light) Microscopes

Light microscopes have a poor resolution due to the long wavelength of light. Small organelles in a cell are not visible using an optical microscope, but living samples can be examined and a colour image is obtained.

Structures viewed under an optical microscope can be measured using the formula: Magnification = size of image size of real object

Cells, figure 1

Electron Microscopes (EM)

A beam of electrons has a very short wavelength and therefore EMs have a high resolution and small organelles can be visualised. An image is created using an electromagnet to focus the beam of negatively charged electrons.

Electrons are absorbed by air, which is why samples for an EM must be in a vacuum. For this reason only non-living specimens can be examined. The image is also black and white as the samples must be stained.

Transmission Electron Microscopes(TEM)

Extremely think specimens are stained and placed in a vacuum. An electron gun produces a beam of electrons that pass through the specimen. Some parts absorb the electrons and appear dark. The image produced is 2D and shows detailed images on the internal structure of cells.

Cells, figure 2

Scanning Electron Microscope (SEM)

The specimens do not need to be thin, as the electrons are not transmitting through. Instead, the electrons are beamed onto the surface and the electrons are scattered in different ways depending on the contours. This produces a 3D image.

Cells, figure 3

Cell Fractionation & Ultracentrifugation

These two techniques are used to separate cell components. These have enabled scientists to study the structure and function of all cell components further.

Cell Fractionation

Cells are broken down so that the organelles are free to be separated. This is done using a homogeniser, a blender. During homogenation the cells must be kept in a solution that is cold to reduce enzyme activity to prevent the breakdown of cell components. The solution must also be isotonic to prevent any movement of water by osmosis resulting in organelles shrivelling or bursting. Finally, the solution must be buffered to prevent pH changes. This is to prevent damage to organelles and enzymes. Once the cell has been broken open the solution must be filtered to remove large piece of debris.

Ultracentrifugation

Once filtered, the homogenate solution is ready to be centrifuged. The solution is placed into a centrifuge which spins at high speed to separate organelles depending on their density due to the centrifugal force.

Cells, figure 1

Which cell components are only sometimes found in prokaryotic cells?
Your answer should include: Flagellum / slime / capsule
What is cell fractionation?
Your answer should include: Breaking / open / homogenising
Why must the solution be kept cold in cell fractionation?
Your answer should include: Prevent / enzyme / action
Why must the solution have the same water potential?
Your answer should include: Prevent / lysis / shrivelling / organelles
Why must the solution be buffered?
Your answer should include: Prevent / enzyme / action