Structure of eukaryotic cells

Question 1

Structure & Function of cell-surface membrane

Answer 1

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 10 nm

Question 2

Structure & Function of nucleus

Answer 2

Present in all eukaryotic cells, the nucleus is relatively large and separated from the cytoplasm by a double membrane (the 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 allowing enzymes (eg. DNA polymerases) 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 can be observed – these regions are individually termed ‘nucleolus’ (plural: nucleoli) and are the sites of ribosome production

Question 3

Structure & Function of mitochondria

Answer 3

The site of aerobic respiration within eukaryotic cells, mitochondria are just visible with a light microscope

Surrounded by double-membrane with the inner membrane folded to form cristae

The matrix 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 4

Structure & Function of chloroplasts

Answer 4

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 thylakoids
The light-independent stage (Calvin Cycle) takes place in the stroma

Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Question 5

Structure & Function of golgi apparatus

Answer 5

Flattened sacs of membrane similar to the smooth endoplasmic reticulum

Modifies proteins and lipids before packaging them into Golgi vesicles

Question 6

Structure & Function of golgi vesicles

Answer 6

The 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 7

Structure & Function of lysosomes

Answer 7

Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down)

Break down waste materials such as worn-out organelles, used extensively by cells of the immune system and in apoptosis (programmed cell death)

Question 8

Structure & Function of ribosomes

Answer 8

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) in prokaryotes, mitochondria and chloroplasts

Site of translation (protein synthesis)

Question 9

Structure & Function of rough endoplasmic reticulum

Answer 9

Surface covered in ribosomes

Formed from continuous folds of membrane continuous with the nuclear envelope

Processes proteins made by the ribosomes

Question 10

Structure & Function of smooth endoplasmic reticulum

Answer 10

Does not have ribosomes on the surface, its function is distinct to the RER

Involved in the production, processing and storage of lipids, carbohydrates and steroids

Question 11

Structure & Function of cell wall

Answer 11

Cell walls are formed outside of the cell membrane and offer structural support to cell

Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan in most bacterial cells

Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells

Question 12

Structure & Function of cell vacuole

Answer 12

Sac in plant cells surrounded by the tonoplast, selectively permeable membrane

Vacuoles in animal cells are not permanent and small

Question 13

In complex multicellular organisms, eukaryotic cells become?

Answer 13

Specialised