What are cells?
Introduction
Where in the world?
How does a cell resemble a city?
produce their own power
represented by pink/red buildings (mitochondria)
have a central controlling organisation
represented by tall building in centre (nucleus)
have border controls
represented by perimeter wall (membrane/wall)
have storage facilities
represented by the harbour and lakes (vacuoles)
import food and gas
represented by oil tankers
some can produce their own food
represented by green building (chloroplast)
export waste
represented by dumper trucks
have good communication links with neighbours
represented by communications satellites (neurones and hormones)
have good defence
represented by heavily-armoured ships and white boats (white blood cells)
What is a cell?
A cell is the basic unit of life, from which larger structures such as tissue and organs are made.
How big is a cell?
Most plant and animal cells are between 0.025 µm and 60 µm in size – around half the diameter of a human hair – and too small to see without a microscope.
The largest cell in the human body is the female egg cell, (ovum) at around 1,000 µm in diameter.
The smallest human cell is the sperm cell – the head is around 5 µm long.
Specialized cells
Most plants and animals are multicellular. The human body is made up of a round 200 different types of cell, all working together.
Most cells are specialized, meaning that each type of cell has a specific structure and function.
All cells with a nucleus contain the same genes, but different cells activate different genes so they only produce the proteins they need.
However, all cells have certain common features and structures called organelles.
What do cells contain?
Animal or plant?
Exam Question #1
Yeasts are microorganisms that are used in the brewing and baking industries.
The diagram shows a yeast cell.
State two ways in which the structure of this yeast cell differs from the structure of a bacterial cell.
(2 marks)
State
Recall one or more pieces of information.
Microorganism
microscopic living organisms that are too small to be seen with the naked eye and can only be observed using a microscope
Any two of the following points:
(Yeast cell)
(Bacterial cell)
2 Marks Max
Animal Cells
A closer look at animal cells
Exploring animal cells
How do animal cells specialize?
In animals, the first type of cells in the developing embryo are stem cells. These are unspecialized cells that go on to form all the different cell types in the adult.
How are animal cells adapted?
Animal cells: fit for a purpose
Plant Cells
A closer look at plant cells
Exploring plant cells
Unlike animals, many plant cells retain the ability to differentiate and specialize throughout their life. These cells are found in tissues called meristems.
How are plant cells adapted?
Exam Question #2
Figure 8 shows part of a root as seen using a light microscope.
Information about the two types of cell labelled in Figure 8.
| type of cell | surface area in μm2 | volume in μm3 | surface area to volume ratio |
|---|---|---|---|
| root surface cell | 5000 | 250000 | 1:50 |
| root hair cell | 36000 | 288000 | ? |
(i) Calculate the surface area to volume ratio of the root hair cell. (2 marks)
(ii) Explain the benefit to the plant of having root hair cells. (2 marks)
Explain
Learners’ explanations require a justification/exemplification of a point. The answer must contain some element of reasoning/justification – this can include mathematical explanations.
Root hair cell
a specialised cell found on the surface of plant roots.
Its primary function is to absorb water (by osmosis) and mineral ions (by active transport) from the soil.
(i) Calculate the surface area to volume ratio of the root hair cell. (2 marks)
- Substitution:
- 288 000 ÷ 36 000 (= 8) (1)
- Evaluation:
- 1:8 (1)
award full marks for correct ratio with no working
(ii) Explain the benefit to the plant of having root hair cells. (2 marks)
An explanation including:
4 Marks Max
Plant cells: fit for a purpose
What is a cell wall?
All plant cells have a cell wall – a rigid layer that surrounds the cell membrane.
The plant cell wall is made from cellulose, a carbohydrate polymer. The purpose of the cell wall is to:
Unlike the cell membrane, the cell wall is freely permeable to water and other molecules.
What is a vacuole?
The vacuole is a fluid–filled sac found within plant cells and some bacteria.
The vacuole has a range of functions, including:
The site of vacuoles depend on how much water the plant has absorbed.
What are chloroplasts?
Chloroplasts are the site of photosynthesis in plant cells
A green pigment in chloroplasts called chlorophyll absorbs the energy in sunlight.
This energy is used to convert carbon dioxide and water into glucose and oxygen.
Chlorophyll is embedded in disk-like structures called thylakoids, which are arranged into stacks.
Which organelle?
Cell processes
How do cells get their energy?
All organisms need energy to survive.
Animals obtain their energy from the food they eat, but plants can make their own food by photosynthesis.
In both cases, however, energy must first be converted into a form that can easily be used by cells. This process is called respiration.
Where does respiration take place?
Mitochondria are cellular organelles in which respiration takes place.
Mitochondria use enzymes to convert the energy from glucose into ATP – the basic energy source for all cells.
Mitochondria have an inner membrane on which the enzymes are embedded.
This membrane is highly folded to increase the surface area on which respiration can take place.
What is aerobic respiration?
Aerobic respiration is the process of releasing energy through the oxidation of glucose molecules.
Aerobic respiration is summarized by the equation:
glucose + oxygen → carbon dioxide + water (+ energy)
C6H12O6 + 6O2 → 6CO2 + 6H20 (+ ATP)
This reaction releases energy in the form of ATP – a compound that can readily be used in cellular processes.
What is anaerobic respiration?
Anaerobic respiration takes place without oxygen, and releases less energy than aerobic respiration because glucose molecules are only partially broken down.
During strenuous exercise, cells are deprived of oxygen but still need energy to work. The body responds by converting glucose into lactic acid and energy, leading to an oxygen ‘debt’.
Lactic acid causes muscle cramps. When exercise stops, oxygen levels rise, paying off the oxygen debt and oxidising the lactic acid.
How is energy used?
The chemical energy produced by respiration, ATP, is used by cells to undertake work.
Where might ATP be used?
Required Practical – Osmosis
Exam Question #3
Diffusion, active transport and osmosis can be used to move substances into and out of cells.
A student was investigating osmosis in potato cubes.
He used the following method:
cut a potato into equal-sized cubes
- record the mass of each potato cube
- place each potato cube into different concentrations of salt solution
- remove the potato cubes after 30 minutes
- dry the potato cubes and record the final mass of each cube.
He plots his results on the graph opposite
The method controls a number of variables.
State one other variable that needs to be controlled during the student’s investigation.
(1 mark)
State
Recall one or more pieces of information.
Osmosis
Diffusion of water molecules across a membrane in response to differences in solute concentration. Water moves from areas of high-water/low-solute concentration to areas of low-water/high-solute concentration.
Any one variable from
1 Mark Max
What is photosynthesis?
Photosynthesis is a chemical reaction where light energy is used to convert carbon dioxide and water into glucose and oxygen.
This reaction can be summarized by the equation:
|
carbon dioxide |
+ |
water |
|
light energy |
↓ |
chlorophyll |
|
glucose |
+ |
oxygen |
Using energy from photosynthesis
The glucose produced by photosynthesis has many uses
in plants, such as for:
Exam Question #4
The diagram shows a section through a leaf.
(i) Complete the sentence by putting a cross (X) in the box next to your answer.
The cell that will make the most glucose is
(1 mark)
(ii) Describe how carbon dioxide enters the leaf.
(2 marks)
(iii) Describe the process of photosynthesis that takes place in the leaf to produce oxygen.
(3 marks)
Describe
Learners give an account of something. Statements in the response need to be developed as they are often linked but do not need to include a justification or reason.
Photosynthesis
The chemical reaction in which light energy is used to convert carbon dioxide and water into glucose and oxygen.
(i) The cell that will make the most glucose is
(ii) Describe how carbon dioxide enters the leaf.
Any two from the following:
Any three from the following:
6 Marks Max
Protein synthesis
Summary activities
Glossary
aerobic respiration
The process of releasing energy through the oxidation of glucose molecules.
anaerobic respiration
The process of releasing energy from glucose molecules in the absence of oxygen.
ATP
Adenosine triphosphate, the major form of energy used by cells.
cell
The basic structural and functional unit of life.
cell membrane
The partially-permeable barrier that regulates substances entering and leaving a cell.
cell wall
The rigid external coat that protects and supports plant cells.
chlorophyll
The green pigment found in chloroplasts.
chloroplast
The site of photosynthesis in plant cells.
cytoplasm
The jelly-like material in which all a cell’s organelles are found, and in which most cellular processes and reactions occur.
mitochondria
The site of energy release by respiration.
nucleus
The location of a cell’s DNA.
photosynthesis
The chemical reaction in which light energy is used to convert carbon dioxide and water into glucose and oxygen.
ribosome
The site of protein synthesis.
vacuole
The fluid-filled cavity found in plant cells that stores water and nutrients.
