Cell Structures

• Components of cells:

  All cells contain;

  • a cell-membrane based on a phospholipid bilayer, enclosing cell contents. It holds the cell together and is semi-permeable, therefore controls what enters and leaves the cytoplasm
  • cytoplasm - which comprises of the liquid cytosol and all the organelles suspended in it. This is the site of many chemical reactions including anaerobic respiration
  • genetic material, made of DNA
• Types of cells:

  Cells can be divided into 2 groups;

  • prokaryotes - bacteria
  • eukaryotes - cells found in plants, fungi, algae and animals that have internal membranes, forming organelles

  an organelle is a small structure in the cytoplasm of a cell that has a specific function
• Eukaryotic cells:

  • includes animal, plant, algae and fungal cells
  • can form multicellular or unicellular organisms
  • eukaryotic cells have membrane-bound organelles. Being in a compartment, the chemicals involved in a particular process are kept separate from the rest of the cytoplasm. This allows chemical reactions to take place quickly and efficiently
• Components of a eukaryotic cells:

  • membrane-bound organelles include nuclei, endoplasmic reticulum, mitochondria, chloroplasts, Golgi and lysosomes
  • they are more complex than prokaryotic cells
  • DNA is stored in the nucleus of these cells
  • These cells are larger than prokaryotic cells
  • eukaryotic cells form new cells by mitosis and meiosis
• Components of an animal cells:

  • mitochondria
  • ribosome
  • cytoplasm
  • lysosome
  • rough endoplasmic reticulum
  • centriole
  • Golgi apparatus
  • plasma membrane
  • smooth endoplasmic reticulum
  • nucleus
• Components of a plant cell:

  • smooth endoplasmic reticulum
  • nucleus
  • rough endoplasmic reticulum
  • ribosome
  • cell wall
  • Golgi body
  • chloroplast
  • mitochondria
  • cell membrane
  • large central vacuole
  • amyloplast

  these contain the same as animal cells with the additional chloroplasts, vacuole and cell wall. Some also contain starch grains
• Components of an algal cell:

  • nucleus
  • Golgi apparatus
  • ribosome
  • cell membrane
  • cell wall
  • centrioles
  • cytoplasm
  • starch vacuole
  • mitochondrion
  • flagellum
  • chloroplast

  this is similar to a plant cell, but algae are usually unicellular organisms. They also contain chloroplasts and a cellulose cell wall like a plant cell. Not all algal cells have flagella
• Components of a fungal cell:

  • ribosome
  • cell-surface membrane
  • rough endoplasmic reticulum
  • nucleus
  • mitochondria
  • Golgi apparatus
  • smooth endoplasmic reticulum
  • cytoplasm
  • chitin cell wall
  • vacuole

  fungi can be multicellular or unicellular organisms. Their cell walls are made of chitin
• Nucleus:

  • this includes; the outer membrane, inner membrane, nucleoplasm, nucleolus, chromatin, nuclear envelope and a pore in the nuclear envelope
  • the nuclear envelope is a double membrane that surrounds the nucleus. Nuclear pores are found through the nuclear envelope
  • nucleoplasm is a granular jelly like material that makes up the bulk of the nucleus
  • chromatin is the DNA and associates histone proteins found within the nucleoplasm
  • the nucleolus is a small spherical body within the nucleoplasm
  • the nucleus stores DNA for coding proteins and the nucleolus makes ribosomes
• Plasma/cell membrane:

  • this surrounds the cell. It's made of a phospholipid bilayer and contains transport proteins
  • this controls what enters and exists the cell
• Mitochondria:

  • a double-membrane surrounds the organelle. The inner membrane is folded to form extensions known as cristae
  • the matrix makes up the remainder of the mitochondria
  • the fluid contains proteins, lipids and traces of DNA. This DNA resembles circular prokaryotic DNA
  • aerobic respiration to produce ATP happens here
• Chloroplast:

  • found in plant/algal cells
  • a double membrane surrounds it. There are also internal membranes forming structures called thylakoids which stack into grana
  • contains solution called the stroma that contains enzymes for photosynthesis and starch grains
  • contains DNA that resembles circular prokaryotic DNA
  • chloroplast absorb sunlight for photosynthesis
• Ribosome:

  • made of rRNA and protein
  • found in the cytoplasm or attached to the rough endoplasmic reticulum
  • contain a large and a small subunit
  • 80S type found in eukaryotic cells
  • it's function is synthesising proteins
• Rough endoplasmic reticulum:

  • system of membranes that interconnect
  • ribosomes present on the outside
  • Functions: synthesising proteins and folds proteins
• Smooth endoplasmic reticulum:

  • similar to the rough endoplasmic reticulum without ribosomes on the surface
  • this synthesises lipids
• Golgi apparatus:

  • it consists of a stack of membranes that form flattened sacks, or cisternae
  • it produces small rounded hollow structures called vesicles which are usually seen coming off the edges
  • it modifies and packages proteins and lipids. It also produces lysosomes
• Lysosome:

  • membrane-bound vesicle containing enzymes
  • circular in shape but larger than ribosomes
  • often found near the Golgi where they are produced
  • it stores hydrolytic enzymes (lysozymes)
• Vacuole:

  • found in plant cells
  • filled with cell sap - a weak solution of sugars and salts
  • it helps give the cell rigidity and therefore prevents the plant wilting
• Cell wall:

  • not in animal cells
  • the wall surrounds the cell membrane and has high tensile strength
  • plant and algal cell walls = cellulose
  • fungal cell walls = chitin
  • they provide strength and support, therefore preventing osmotic lysis
• Cell differentiation:

  as cells specialise, they will have more or fewer of certain organelles to help them carry out their role. In order to do this, they all have the same DNA, but switch certain genes on or off

  • specialised cells are more efficient and can carry out their functions more effectively than undifferentiated ones
• Factors of cell specialisation:

  • how many mitochondria - lots will mean it requires a lot of ATP
  • large surface area - does it have microvilli (folding of the membrane). This will mean it is good at absorption/exchange. This is often found in the small intestine
  • large surface area to volume ratio - shapes that are long and thin lead to faster diffusion
  • lots of ribosomes - this means the cell synthesises a lot of proteins
  • lots of chloroplasts - the cell will have a high rate of photosynthesis
• Cell organisation:

  in multi-cellular organisms, cells can be organised to form tissues, organs and organ systems

  • tissue - a group of similar cells organised into a structural unit which serves a particular function (muscle, nervous, elastic)
  • organ - a group of tissues organised into a structure which serves a particular physiological function or functions (heart, leaf)
  • organ system - a group of interacting organs forming a complex functional whole (circulatory system)
  • organism - organ systems that work together to perform a particular function
• Prokaryotic cells:

  • these usually make up basic, single celled organisms
  • as they are simple, it makes them very versatile and they are found in every habitat in the world. Bacteria are prokaryotes
  • they are much smaller than eukaryotic cells
  • they do not contain any membrane-bound organelles in their cytoplasm, including having no nucleus - the DNA is found free in the cytoplasm
  • Prokaryotes form new cells by binary fission
• Features of prokaryotes:

  • no membrane-bound organelles - no mitochondria, chloroplasts, endoplasmic reticulum, lysosomes, nucleus
  • ribosomes - smaller than in eukaryotes (70S)
  • cell wall - contains murein
  • plasmids - small loops of DNA containing genes for antibiotic resistance
  • slime capsule (some bacterial cells only) - protection
  • flagellum (some bacterial cells only) - movement of cell
• Differences in DNA in prokaryotes:

  • DNA in prokaryotic cells is circular, whereas it is linear in eukaryotic cells
  • DNA in prokaryotes is not associated with histone proteins, whereas in eukaryotes it is
  • DNA in prokaryotic cells is shorter than in eukaryotic cells - it does not contain non-coding regions (introns)
  • not stored in nucleus - free-floating in the cytoplasm
• Binary fission:

  • circular DNA and plasmids replicate (plasmids can be replicated loads of times)
  • the cell gets bigger, DNA loops move to the opposite poles of the cell
  • the cytoplasm begins to divide and new cell walls begin to form
  • the cytoplasm divides into 2 daughter cells. Each cell has one copy of circular DNA but a variable number of copies of plasmids

  Bacterial cells divide at an exponential rate. Their growth is often plotted on graphs using a log scale to make the scale numbers more manageable
• Viruses:

  • viruses are not cells and so are non-living. They are described as acellular
  • as they are not cells, viruses cannot undergo cell division. Instead, they need to attach to and enter a host cell and use that cell's machinery to replicate

  virus particles consist of 3 components;

  • genetic material - a strand of nucleic acids, either DNA or RNA
  • capsid - a protein coat
  • attachment protein - allows the virus to attach to a host cell (complementary to receptors on host cell membrane). Different viruses have different attachment proteins and therefore some viruses can only fit one type of cell
• Viruses:

  • some virus particles may contain other structures such as a lipid envelope that surrounds the capsid (HIV)
  • some viruses are adapted to only infect bacterial cells - these are called bacteriophages and are more complex
• Viral replication:

  • viruses attach to a host cell surface - their attachment proteins bind to a complementary receptor on the cell membrane of the host cell
  • the virus injects its genetic material (DNA or RNA) into the host cell
  • viral genetic material and proteins are produced by the host cell machinery (e.g ribosomes)
  • these viral components then assemble to make new viruses which then are released from the host cell
• Cell fractionation:

  • a TEM shows us the detailed structure of the cell, and its organelles, but this does not give us any information about the function of these structures. As the sample is dead when it is viewed, we are not able to observe anything happening inside the cell
  • to determine organelle function, organelles must be isolated from the cell in a 3-stage process called cell fractionation
  • this includes homogenisation, filtration and ultracentrifugation
• Homogenisation:

  • this means breaking open the cells by breaking the plasma membrane to release the organelles into solution. This can be done by vibrating cells or grinding them up in a blender. The solution for this process is;
  • ice cold - reduces kinetic energy of enzymes (deactivates them) so they do not damage organelles
  • isotonic - water potential of solution should be the same as inside the organelles to prevent osmotic lysis (bursting)
  • buffered - maintains pH so proteins and enzymes in organelles do not become denatured
• Filtration:

  • homogenised cell solution is filtered through a gauze
  • this separates large cells or tissue debris from the organelles (it doesn't separate out different organelles)
  • the organelles are much smaller than the debris so they pass through the gauze
• Ultracentrifugation:

  • this process now separates out particular organelles from the solution of all other organelles. The solution of cell organelles is poured into a tube, placed into a centrifuge and then spun at a low speed
  • the heaviest/most dense organelles get flung to the bottom, forming a thick sediment called a pellet
  • the rest of the organelles are suspended in the fluid above called the supernatant
• Ultracentrifugation:

  • the supernatant is drained off and poured into a fresh tube to be spun at a higher speed
  • the heaviest of the remaining organelles will form the next pellet
  • the process is repeated at higher speeds until all the organelles are separated out
  • each time the pellet at the bottom of the tube contains lighter and lighter organelles
• Order of organelles in centrifugation:

  (most dense to least dense)

  • nucleus
  • chloroplasts
  • mitochondria
  • lysosomes
  • endoplasmic reticulum
  • ribosomes

  ultracentrifugation can also be used to separate out even smaller molecules (DNA)