2.1.5 Eukaryotic Cells

Question 1

Cell Surface Membrane

Answer 1

All cells are surrounded be a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment
- the membrane is described as being ‘partially permeable’
The cell membrane is formed from a phospholipid bilayer of phospholipids spanning a diameter of around 10nm

Question 2

Cell Wall

Answer 2

Found in plant cells but not in animal cells
Cell walls are formed outside of the cell membrane and offer structural support to cells
Structural support is provided by the polysaccharide cellulose in cells, and peptidoglycan in most bacterial cells
Narrow threads of cytoplasm (surrounded be a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells

Question 3

Nucleus

Answer 3

Present in all eukaryotic cells (expect red blood cells)
Relatively large and separated from the cytoplasm by a double membrane (nuclear envelope) which has many pores
Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as letting enzymes (e.g. DNA polymerase) and signalling molecules to travel in
The nucleus contains chromatin (the material from which chromosomes are made)
- chromosomes are made of sections of linear DNA tightly wound around proteins called histones
Usually, at least one or more darkly stained regions cam be observed - these regions are individually called nucleolus (plural=nucleoli) and they are the sites of ribosome production

Question 4

Mitochondria

Answer 4

Mitochondria are the site of aerobic respiration within all eukaryotic cells
They are visible with a light microscope
Surrounded by a double-membrane with the inner membrane folded to form cristae
The matrix are formed by the cristae contains enzymes needed for aerobic respiration, producing ATP
Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)

Question 5

Chloroplasts

Answer 5

Found in plant cells
Larger than mitochondria
Also surrounded by a double membrane
Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana
Grana are joined together by lamellae (thin and flat thylakoid membranes)
Chloroplasts are the site of photosynthesis
The light-dependent stage takes place in the thylakoid
The light-independent stage (Calvin Cycle) takes part in the stroma
Also contains small circular pieces of DNA and ribosomes used to synthesis proteins needed in chloroplast replication and photosynthesis

Question 6

Ribosomes

Answer 6

Found in all cells
Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells
Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells
70S ribosomes (composed of 50S and 30S subunits) are found in prokaryotes, mitochondria and chloroplasts
Site of translation (protein synthesis)

Question 7

Rough Endoplasmic Reticulum (RER)

Answer 7

Found in plant and animal cells
Surface covered in ribosomes
Formed in continuous folds of membrane continuous with bathe nuclear envelope
Processes proteins made by the ribosomes

Question 8

Smooth Endoplasmic Reticulum (ER)

Answer 8

Found in plant and animal cells
Does not have ribosomes on the surface
Function distinct from RER
Involved in the production, processing and storage lipid, carbohydrates and steroids

Question 9

Golgi Apparatus

Answer 9

Found in plant and animal cells
Flattened sacs of membrane similar to the ER
Modifies proteins and lipids before packaging them into golgi vesicles
- these vesicles then transport the proteins and lipids to their required destination
- proteins that go through the Golgi apparatus are usually exported (e.g. hormones such as insulin), put into lysosomes (such as hydrolytic enzymes) or delivered to membrane-bound organelles

Question 10

Large Permanent Vacuoles

Answer 10

A sac in plant cells surrounded by the tonoplast, a selectively permeable membrane
Vacuoles in animal cells are not permanent and small

Question 11

Vesicles

Answer 11

Found in plant and animal cells
A membrane-bound sac for transport and storage

Question 12

Lysosomes

Answer 12

Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break down biological molecules)
Break down waste materials such as worn-out organelles
Used extensively by cells of the immune system and in apoptosis (programmed cell death)

Question 13

Centrioles

Answer 13

Hollow fibres made of microtubules
Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division
Not found in flowering plants and fungi

Question 14

Microtubles

Answer 14

Found in all eukaryotic cells
Makes up the cytoskeleton of the cell about 25 no in diameter
Made of alpha and beta tubulin combined to dimers
The dimers are then joined into protofilaments
13 protofilaments in a cylinder make a microtubule
The cytoskeleton is used to provide support and movement of the cell

Question 15

Microvilli

Answer 15

Found in specialised animal cells
Cell membrane projections
Used to increase the surface area of the cell surface membrane in order to increase the rate of exchange of substances

Question 16

Cilia

Answer 16

Hair-like projections made from microtubules
Allows the movement of substances over the cell surface

Question 17

Flagella

Answer 17

Found in specialised cells
Similar in structure of cilia, made of longer microtubules
Contract to provide cell movement
For example, in sperm cells