Animal, plant and bacterial cells: structures and functions
Every living thing is made of cells, and every cell has the same basic problems to solve.
It must keep its contents together, control what goes in and out, store the instructions that run it, release energy, and build the proteins it is made of.
Inside each cell, small structures share out these jobs.
Learning the cell is really learning who does which job, and why each structure is built the way it is.
Two kinds of cell
You need to be able to explain how the sub-cellular structures of eukaryotic and prokaryotic cells are related to their functions, including: (a) animal cells – nucleus, cell membrane, mitochondria and ribosomes; (b) plant cells – nucleus, cell membrane, cell wall, chloroplasts, mitochondria, vacuole and ribosomes; (c) bacteria – chromosomal DNA, plasmid DNA, cell membrane, ribosomes and flagella.
Sub-cellular structures are the parts inside a cell.
Biologists sort all cells into two kinds by one big difference:
- Eukaryotic cells keep their genetic material inside a nucleus. Animal cells, plant cells, fungi and protists are eukaryotic. Most are about 10 to 100 µm across.
- Prokaryotic cells have no nucleus. Their DNA is free in the cytoplasm. Bacteria are prokaryotic. They are much smaller: most are about 1 to 5 µm long.
A micrometre (µm) is a thousandth of a millimetre; the size and scale section later in this unit shows how to convert.
The cytoplasm is the jelly-like liquid that fills a cell.
Most of the cell's chemical reactions happen in it, controlled by enzymes.
It is not one of the structures in the list you must know, and it is not a structure in the cytoplasm: an exam answer that names "the cytoplasm" when asked for a structure in the cytoplasm scores nothing.
Animal cells
The diagram below shows a typical animal cell, such as a cell from the lining of your cheek, as it would look with a light microscope and an electron microscope together.
1. nucleus
2. cell membrane
3. cytoplasm
4. mitochondrion (plural: mitochondria)
5. ribosome
structure
function
how the structure fits the job
nucleus
contains the genetic material (DNA, in chromosomes) that controls the activities of the cell
the DNA carries the instructions for making the cell's proteins, so the nucleus decides what the cell makes and does; a membrane round it keeps the DNA in one place
cell membrane
controls which substances enter and leave the cell
it is partially permeable: small molecules such as oxygen and water pass through, but large ones such as starch do not
mitochondria
the site of aerobic respiration, which releases energy for the cell
each has a folded inner membrane, giving a large surface for the enzymes of respiration; cells that use a lot of energy have many mitochondria
ribosomes
the site of protein synthesis: they join amino acids together to make proteins
they are tiny and very numerous, because a cell makes thousands of proteins; many enzymes, antibodies and hormones are proteins
Recent papers have asked for the roles of mitochondria and ribosomes, and for a structure found in the cytoplasm with its role.
Notice the link between structure and function.
A muscle cell contracts all day, so it is packed with mitochondria.
A cell that makes and releases enzymes, such as a cell in the pancreas, is packed with ribosomes.
Plant cells
Plant cells have everything an animal cell has, plus three structures animal cells never have: a cell wall, chloroplasts and a permanent vacuole.
1. nucleus
2. cell membrane
3. cell wall
4. chloroplast
5. mitochondrion
6. vacuole
7. ribosome
8. cytoplasm
structure
function
how the structure fits the job
cell wall
strengthens and supports the cell
it is made of cellulose fibres, which are strong; when water enters the cell, the wall stops it bursting
chloroplasts
the site of photosynthesis
they contain the green pigment chlorophyll, which absorbs light energy; leaf cells near the light have many chloroplasts
vacuole
a permanent space filled with cell sap, a solution of sugars and salts
when it is full of water it pushes the cytoplasm against the cell wall, keeping the cell firm, which supports the plant
nucleus, cell membrane, mitochondria, ribosomes
the same jobs as in an animal cell
plant cells respire all the time, day and night, so they need mitochondria as well as chloroplasts
Two facts catch many students out:
- The cell membrane is in both animal and plant cells. In a plant cell it lies just inside the cell wall. The plant-only structures are the cell wall, the permanent vacuole and the chloroplasts.
- Not every plant cell has chloroplasts. A root cell is underground in the dark, so it has none. It is still a plant cell, because it has a cell wall and a permanent vacuole.
Recent papers have asked why plant cells do not burst when they are placed in water.
The answer is the cell wall: water enters by osmosis and the cell swells, but the strong cellulose wall stops it bursting.
The transport section of this unit explains osmosis.
Bacterial cells
Bacteria are prokaryotes.
They have no nucleus, no mitochondria and no chloroplasts, but they still have to store DNA, make proteins and, for many, move.
1. chromosomal DNA
2. plasmid DNA
3. cell membrane
4. cell wall
5. ribosome
6. flagellum (plural: flagella)
7. cytoplasm
structure
function
how the structure fits the job
chromosomal DNA
carries most of the genes and controls the activities of the cell
it is one large loop of DNA, free in the cytoplasm because there is no nucleus
plasmid DNA
carries a few extra genes, such as genes for resistance to an antibiotic
plasmids are small rings of DNA; they can be copied and passed between bacteria, so a useful gene spreads quickly
cell membrane
controls which substances enter and leave the cell
partially permeable, as in an animal cell
ribosomes
the site of protein synthesis
smaller than the ribosomes of eukaryotic cells, but they do the same job
flagella
move the bacterium
a flagellum is a long whip-like tail that rotates to push the cell through liquid, towards food or away from harm
Bacteria also have a cell wall, but it is not made of cellulose.
Plasmids matter far beyond this unit: in the genetic modification unit, scientists use plasmids to carry a human gene into bacteria, and in the health unit you will see how resistance genes on plasmids help bacteria survive antibiotics.
Comparing the three cells
structure
animal cell
plant cell
bacterial cell
cell membrane
yes
yes
yes
ribosomes
yes
yes
yes (smaller)
nucleus
yes
yes
no: chromosomal DNA free in the cytoplasm
mitochondria
yes
yes
no
cell wall
no
yes (cellulose)
yes (not cellulose)
chloroplasts
no
yes, in many cells
no
permanent vacuole
no
yes
no
plasmids
no
no
yes, in many
flagella
no
no
in some
When you compare cells, write a full comparative sentence that names both cells: "A plant cell has a cell wall but an animal cell does not."
"A bacterial cell is smaller than an animal cell."
Quoting the table back, or writing "plant: cell wall; animal: none", earns less, because examiners want the comparison made.
Going further (not examined): mitochondria and chloroplasts contain their own small loops of DNA and their own small ribosomes, rather like bacteria.
Most biologists think they began as bacteria that lived inside larger cells long ago.
- Each marking point earns one mark, shown "(1)". A 2-mark question needs two separate creditworthy points.
- A "/" in a mark scheme separates answers that are accepted as alternatives, and words in bold inside a marking point must appear in your answer.
- Examiners use "accept", "ignore" and "reject" lines. An ignored word neither gains nor loses; a rejected word loses the mark. "Allow" is rare.
- "Any two from" means a longer list is printed and any two correct points score. Two answers that are really the same point earn one mark.
- The list rule: if a question asks for one answer and you give two, a wrong one cancels a right one.
- In a calculation the correct answer on the answer line scores full marks with no working, but working earns marks for each stage if the answer slips, and a wrong value carried forward correctly from an earlier part still scores.
- A requested rounding or form (2 decimal places, 3 significant figures, standard form) has its own mark: write the full calculator value, then round as asked.
- Information printed in the question earns nothing when you copy it back.
- A 6-mark question with an asterisk (*) is levels-marked. The examiner reads the whole answer and places it in Level 1 (1-2 marks), Level 2 (3-4) or Level 3 (5-6) by how many of the named areas it covers, then picks the mark by its detail, correct terms and links. The asterisk also means a logically structured answer is rewarded.
- Phonetic spellings are accepted unless the word becomes another biology term: a spelling of mitochondria that reads as "mitosis" would not be credited.
- Eukaryotic cell: a cell with its genetic material inside a nucleus (animal, plant, fungal and protist cells).
- Prokaryotic cell: a cell with no nucleus; its DNA is free in the cytoplasm (bacteria).
- Mitochondria: the site of aerobic respiration, which releases energy for the cell.
- Ribosomes: the site of protein synthesis.
- Plasmid: a small ring of DNA in a bacterial cell, separate from the chromosomal DNA.
- Flagellum: a long whip-like structure that rotates to move a bacterial cell.
Draw one straight line from each structure to its function.
(2)
structure
nucleus
mitochondrion
function
absorbs light energy
contains genetic material that controls the cell
joins amino acids to make proteins
releases energy in respiration
strengthens the cell
Answer:
- nucleus to "contains genetic material that controls the cell" (1)
- mitochondrion to "releases energy in respiration" (1)
Draw exactly one line from each box.
Two lines from one box score zero for that box, even if one of them is right.
Cells in the leg muscles of a sprinter contain many mitochondria.
Explain why these muscle cells contain many mitochondria.
(2)
Full-mark answer: "Mitochondria are where aerobic respiration takes place (1), so many mitochondria release more energy for the muscle cells to contract (1)."
- Marking: an identified point (the function of mitochondria), then a linked consequence (more energy released for contraction).
- Weaker answer: "Mitochondria produce energy for the muscles." This scores at most 1. Energy is released by respiration; it is never produced or created, and examiners reject "produce energy".
- Weaker answer: "Because they need a lot of mitochondria." This scores 0: it repeats the question.
This kind of 2-mark Explain is sat by both tiers.
A scientist looks at a cell from pond water.
It has a cell membrane, ribosomes and a flagellum.
It has no nucleus.
(a) Give one reason why this cell is prokaryotic. (1)
(b) State the function of the flagellum. (1)
(c) State where the genetic material is found in this cell. (1)
Answers:
- (a) "It has no nucleus." (1) Accept "its DNA is free in the cytoplasm". Ignore "it is small": size alone does not decide.
- (b) "Movement / to move the cell." (1)
- (c) "In the cytoplasm, as a loop of chromosomal DNA / and in plasmids." (1) Reject "in the nucleus".
*Describe the similarities and differences between the structure of an animal cell and the structure of a bacterial cell.
(6)
A Level 3 answer:
"Both cells have a cell membrane that controls what enters and leaves, both have cytoplasm where reactions take place, and both have ribosomes where proteins are made.
Both contain DNA.
In the animal cell the DNA is in chromosomes inside a nucleus, but the bacterial cell has no nucleus: its DNA is a single loop of chromosomal DNA free in the cytoplasm, and it may also have small rings of DNA called plasmids, which the animal cell does not have.
The animal cell has mitochondria for respiration, but the bacterial cell has none.
The bacterial cell has a cell wall, but the animal cell does not, and some bacteria have a flagellum for movement.
The bacterial cell is much smaller than the animal cell, and its ribosomes are smaller."
Why it is Level 3: it covers both areas the question names, similarities (membrane, cytoplasm, ribosomes, DNA) and differences (nucleus, plasmids, mitochondria, cell wall, flagellum, size), with correct names and each difference stated for both cells.
- A Level 2 answer (3-4 marks) gives a few correct differences ("the bacterial cell has no nucleus and has plasmids and a flagellum; the animal cell has mitochondria") but no similarity. Covering only one of the two areas caps the answer at Level 2.
- A Level 1 answer (1-2 marks) gives one or two isolated facts ("bacteria have no nucleus") with no comparison.
- To reach the top of Level 3: several similarities and several differences, each written as a comparison that names both cells ("the bacterial cell has a cell wall but the animal cell does not").
Figure 1 shows the mean number of mitochondria in four types of human cell.
type of cell
mean number of mitochondria per cell
heart muscle cell
5000
liver cell
2000
sperm cell
70
red blood cell
0
Figure 1
(a) Describe the data in Figure 1. (2)
"The heart muscle cell has the most mitochondria, 5000 (1).
The red blood cell has none, and the sperm cell has only 70 (1)."
(b) Explain why the heart muscle cell has more mitochondria than the sperm cell. (2)
"The heart muscle cell contracts all the time, so it needs a lot of energy (1).
More mitochondria means more aerobic respiration, which releases more energy (1)."
In (a), adding a reason earns nothing extra: Describe wants what the data show.
In (b), a description with no "because" or "so" ("the heart cell has 5000 and the sperm 70") earns nothing.
When a question asks for the function of a structure, use the exact verb the mark scheme wants: mitochondria release energy in respiration; ribosomes make proteins (protein synthesis); the nucleus contains genetic material that controls the cell; the cell membrane controls what enters and leaves; the flagellum is for movement.
Chromosomes are found in the nucleus: "in the DNA" or "in the double helix" names no place in the cell.
- "Mitochondria produce energy." Energy cannot be made. Write "release energy by respiration".
- "Plant cells have a cell membrane and animal cells do not." Both have a cell membrane. The plant-only structures are the cell wall, chloroplasts and the permanent vacuole.
- "Animal cells have chloroplasts." They never do. Only some plant cells and some protists have chloroplasts.
- Naming the cytoplasm as a structure in the cytoplasm. It is the liquid the structures sit in.
- "Bacteria have a nucleus." Their DNA is free in the cytoplasm. Say "chromosomal DNA" or "a loop of DNA", not "nucleus".
- "The flagellum is for protection." It is for movement.
- "Respirate." The verb is "respire": cells respire. Examiners treat "respirate" as wrong science, not as a spelling slip.
- Calling the large vacuole the nucleus. In a plant cell, such as a root hair cell, the big space filled with cell sap is the permanent vacuole. The nucleus is much smaller and sits in the cytoplasm.
Check your understanding
1. Which structure is found in a bacterial cell but not in a plant cell?
- A cell membrane
- B chloroplast
- C plasmid
- D ribosome
(1)
Total [1]
2. Where are the chromosomes found in an animal cell?
- A in the cell membrane
- B in the cytoplasm
- C in the mitochondria
- D in the nucleus
(1)
Total [1]
3. Complete the sentences using words from the box. Each word may be used once, more than once or not at all.
cellulose
chlorophyll
glucose
light
protein
sound
A plant cell wall is made of ................
.
Chloroplasts contain a green pigment called ................
that absorbs ................
energy.
(3)
Total [3]
4. Complete the table. Write yes or no in each box. (3)
structure
animal cell
plant cell
bacterial cell
cell membrane
cell wall
nucleus
Total [3]
5. Cells lining the small intestine take in glucose by active transport, which needs energy.
(a) State the function of the mitochondria. (1)
(b) Explain why the cells lining the small intestine contain many mitochondria. (2)
Total [3]
6. A student looks at a cell from a carrot root. The cell has a cell wall and a large vacuole but no chloroplasts. The student says: "This cannot be a plant cell, because it has no chloroplasts."
Explain why the student is wrong.
(2)
Total [2]
7. (a) State one difference between the DNA in a bacterial cell and the DNA in an animal cell. (1)
(b) Some bacteria carry a gene for resistance to an antibiotic. Name the small rings of DNA that often carry this gene. (1)
(c) A pancreas cell makes large quantities of a digestive enzyme, which is a protein. Explain which structure is present in very large numbers in this cell. (2)
Total [4]
8. Use Figure 1 in Worked example 5.
A mature red blood cell has no mitochondria and no nucleus.
Suggest one advantage to the red blood cell of having no nucleus.
(1)
Total [1]
Answers
1. The only correct answer is C, plasmid (1).
- A is not correct because plant cells also have a cell membrane.
- B is not correct because bacteria have no chloroplasts.
- D is not correct because plant cells also have ribosomes.
2. The only correct answer is D, in the nucleus (1).
- A is not correct because the cell membrane controls what enters and leaves; it holds no chromosomes.
- B is not correct because the chromosomes of an animal cell are kept inside the nucleus, not free in the cytoplasm.
- C is not correct because mitochondria are the site of respiration.
3. cellulose (1); chlorophyll (1); light (1)
4. One mark per correct row:
- cell membrane: yes, yes, yes (1)
- cell wall: no, yes, yes (1)
- nucleus: yes, yes, no (1)
- every box must be filled: a blank box is not "no"
5. (a) site of (aerobic) respiration / releases energy (1)
- reject "produces energy" / "makes energy"
(b) An explanation linking:
- active transport needs energy (1)
- many mitochondria release more energy by respiration (for absorbing glucose) (1)
6. An explanation linking:
- it has a cell wall and a (permanent) vacuole, which only plant cells have (1)
- root cells are underground / get no light, so they do not photosynthesise and need no chloroplasts (1)
7. (a) any one from: bacterial DNA is not in a nucleus / is free in the cytoplasm; bacterial DNA is a single loop; bacteria also have plasmids (1)
(b) plasmids (1)
(c) ribosomes (1); because ribosomes are the site of protein synthesis / make proteins, and the enzyme is a protein (1)
8. any one from: more space for haemoglobin / to carry more oxygen; the cell is flexible / can change shape to squeeze through capillaries (1)