Biology notes
Characteristics of Living Things and Cells
1. Objectives
By the end of this topic, you should be able to:
- Describe the characteristics of living organisms.
- Compare unspecialised plant and animal cells with microbes.
- State the functions of cell structures and organelles.
- Explain how the structure of specialised cells helps them perform their functions.
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2. Characteristics of Living Things
Living things are organisms that carry out processes necessary to stay alive. The main characteristics are often remembered using MRS GREN.
M – Movement
Movement is a change in position or place.
- Animals can move their whole bodies or parts of their bodies.
- Plants show movement by growing towards light or responding to gravity.
R – Respiration
Respiration is the chemical process that releases energy from food.
- Energy is needed for movement, growth, repair and other life processes.
- Aerobic respiration uses oxygen.
Word equation:
Glucose + Oxygen → Carbon dioxide + Water + Energy
S – Sensitivity
Sensitivity is the ability to detect and respond to changes in the environment.
Examples:
- Humans pull their hand away from something hot.
- Plants grow towards light.
- Pupils become smaller in bright light.
G – Growth
Growth is a permanent increase in size and dry mass.
- Plants grow throughout much of their lives.
- Animals grow mainly during certain stages of their lives.
R – Reproduction
Reproduction is the process by which organisms produce new organisms of the same species.
There are two main types:
- Sexual reproduction – involves genetic material from two parents.
- Asexual reproduction – involves one parent and usually produces genetically similar offspring.
E – Excretion
Excretion is the removal of waste products made by the body.
Examples:
- Carbon dioxide is removed during breathing.
- Urea is removed in urine.
- Excess water and salts can be removed through sweat.
Important: Excretion is different from egestion.
- Excretion: removal of metabolic waste.
- Egestion: removal of undigested food from the digestive system.
N – Nutrition
Nutrition is the taking in of substances needed for energy, growth and repair.
- Animals obtain nutrients by eating food.
- Green plants make their own food by photosynthesis.
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3. Cells
A cell is the basic structural and functional unit of a living organism.
This means that:
- All living organisms are made of cells.
- Cells carry out life processes.
- New cells come from existing cells.
Unicellular organisms
Organisms made up of one cell.
Examples:
- Amoeba
- Bacteria
- Some algae
Multicellular organisms
Organisms made up of many cells.
Examples:
- Humans
- Dogs
- Trees
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4. Unspecialised Cells
An unspecialised cell is a cell that has not yet developed a specific structure or function.
They can develop into different types of specialised cells.
Examples include:
- Stem cells in animals.
- Meristematic cells in plants.
Unspecialised cells are important because they can divide and develop into specialised cells.
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5. Animal Cell Structure
The main structures of a typical animal cell are:
Cell membrane
- Thin outer boundary of the cell.
- Controls what enters and leaves the cell.
- Helps maintain the cell's internal environment.
Cytoplasm
- Jelly-like substance inside the cell.
- Where many chemical reactions occur.
Nucleus
- Contains the cell's genetic material (DNA).
- Controls the activities of the cell.
- Important for cell division.
Mitochondria
- Site of aerobic respiration.
- Releases energy for the cell.
Ribosomes
- Site of protein synthesis.
- Make proteins needed for growth and repair.
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6. Plant Cell Structure
Plant cells have the structures found in animal cells, plus some additional structures.
Cell wall
- Made mainly of cellulose.
- Gives the cell strength and support.
- Helps maintain the cell's shape.
Chloroplasts
- Contain the green pigment chlorophyll.
- Absorb light energy for photosynthesis.
- Usually found in cells exposed to light, such as leaf cells.
Permanent vacuole
- Contains cell sap.
- Helps maintain the cell's internal pressure.
- Helps keep the plant cell firm.
Important comparison
Animal cells:
Cell membrane, cytoplasm, nucleus, mitochondria, ribosomes.
Plant cells:
All of the above + cell wall, chloroplasts and large permanent vacuole.
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7. Specialised Cells
A specialised cell is a cell that has a particular structure adapted to perform a specific function.
The structure of a specialised cell is related to its job.
Examples of specialised animal cells
Red blood cell
- Carries oxygen around the body.
- Has a biconcave shape to provide a large surface area.
- Contains haemoglobin.
- Has no nucleus when mature, leaving more room for haemoglobin.
Nerve cell
- Carries electrical impulses around the body.
- Has a long axon to transmit impulses over long distances.
- Branched ends help it connect with other cells.
Sperm cell
- Delivers male genetic material to the egg.
- Has a flagellum for movement.
- Contains many mitochondria to provide energy.
- Has a streamlined shape.
Muscle cell
- Contracts and relaxes to produce movement.
- Contains many mitochondria because it needs lots of energy.
Examples of specialised plant cells
Root hair cell
- Absorbs water and mineral ions from the soil.
- Has a long extension that increases surface area.
- Thin cell wall allows substances to enter more easily.
- Contains many mitochondria for active transport of mineral ions.
Palisade cell
- Found near the upper surface of leaves.
- Contains many chloroplasts.
- Performs photosynthesis efficiently.
Guard cells
- Surround stomata.
- Control the opening and closing of stomata.
- Help regulate gas exchange and water loss.
Xylem cells
- Transport water and mineral ions from roots to the rest of the plant.
- Form long tubes.
- Have thick, strengthened walls.
- Are dead when fully developed.
Phloem cells
- Transport dissolved sugars and other organic substances around the plant.
- Transport food from areas where it is produced or stored to areas where it is needed.
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8. Cells → Tissues → Organs → Organ Systems → Organism
In multicellular organisms, cells are organised into levels.
Cells → Tissues → Organs → Organ systems → Organism
Cell
The basic unit of life.
Tissue
A group of similar cells working together to perform a particular function.
Example:
- Muscle tissue
Organ
A structure made from different tissues working together.
Examples:
- Heart
- Stomach
- Leaf
Organ system
A group of organs working together to perform major functions.
Examples:
- Digestive system
- Respiratory system
- Circulatory system
Organism
A complete living thing.
Example:
- Human
- Dog
- Mango tree
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9. Microbes
Microorganisms (microbes) are organisms that are generally too small to be seen clearly without a microscope.
Examples include:
- Bacteria
- Protozoa
- Microscopic fungi
- Some algae
Bacteria
- Usually single-celled.
- Have a cell membrane, cytoplasm and genetic material.
- Do not have a nucleus enclosed by a membrane.
- Some have a cell wall and flagellum.
Protozoa
- Usually single-celled organisms.
- Often found in water or moist environments.
- Some can move using structures such as cilia or flagella.
Fungi
Some fungi are microscopic and may consist of individual cells or networks of microscopic structures.
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Key Terms to Remember
Cell: Basic unit of life.
Organelle: A specialised structure inside a cell that performs a particular function.
Unspecialised cell: A cell that has not yet developed a specific function.
Specialised cell: A cell adapted to perform a particular function.
Tissue: A group of similar cells working together.
Organ: Different tissues working together to perform a function.
Organ system: A group of organs working together.
Microorganism: A very small organism that generally requires magnification to be seen clearly.
MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition. Cell Specialization, Cell Structure & Transport Across Cells
1. Cell Specialization
A specialised cell is a cell that has a particular structure adapted to perform a specific function.
Cell specialization
- During growth, unspecialised cells develop into specialised cells.
- Specialised cells have structures that help them perform their jobs efficiently.
- Different cells have different shapes, sizes and structures depending on their function.
Examples of specialised animal cells
Red blood cell
- Transports oxygen around the body.
- Contains haemoglobin.
- Has a large surface area for absorbing oxygen.
- Has no nucleus when mature, leaving more space for haemoglobin.
Nerve cell
- Carries electrical impulses around the body.
- Has a long fibre called an axon.
- Branched ends connect with other cells.
Sperm cell
- Fertilises the egg.
- Has a flagellum for movement.
- Contains many mitochondria to provide energy.
- Has a streamlined shape.
Muscle cell
- Contracts and relaxes to produce movement.
- Contains many mitochondria because it requires lots of energy.
Examples of specialised plant cells
Root hair cell
- Absorbs water and mineral ions from the soil.
- Has a long hair-like extension that increases surface area.
- Thin wall allows substances to enter easily.
Palisade cell
- Found near the upper surface of leaves.
- Contains many chloroplasts.
- Carries out photosynthesis.
Guard cells
- Surround the stomata.
- Control the opening and closing of stomata.
- Help control gas exchange and water loss.
Xylem cells
- Transport water and mineral ions through the plant.
- Form long tubes.
- Have thick, strengthened walls.
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2. Cell Structure and Function
A cell is the basic structural and functional unit of life.
Main cell structures
Cell membrane
- Surrounds the cell.
- Controls what enters and leaves the cell.
- It is selectively permeable, meaning some substances can pass through more easily than others.
Cytoplasm
- Jelly-like material inside the cell.
- Site of many chemical reactions.
Nucleus
- Contains genetic material (DNA).
- Controls the activities of the cell.
- Important for cell division.
Mitochondria
- Site of aerobic respiration.
- Releases energy for the cell.
Ribosomes
- Make proteins.
- Protein production is called protein synthesis.
Structures mainly found in plant cells
Cell wall
- Made mainly of cellulose.
- Provides strength and support.
- Helps maintain the shape of the cell.
Chloroplasts
- Contain chlorophyll.
- Absorb light energy.
- Site of photosynthesis.
Large permanent vacuole
- Contains cell sap.
- Helps keep the plant cell firm.
- Helps maintain pressure inside the cell.
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3. Comparison of Typical Plant and Animal Cells
genui{"learning_viz":{"type_id":"PLANT_VS_ANIMAL_CELL"}}Structure Animal Cell Plant Cell Function
Cell membrane ✓ ✓ Controls movement of substances into and out of the cell
Cytoplasm ✓ ✓ Site of chemical reactions
Nucleus ✓ ✓ Controls cell activities and contains DNA
Mitochondria ✓ ✓ Releases energy through respiration
Ribosomes ✓ ✓ Makes proteins
Cell wall ✗ ✓ Supports and strengthens the cell
Chloroplasts ✗ ✓ Carry out photosynthesis
Large permanent vacuole ✗ ✓ Contains cell sap and maintains firmness
Main difference
Animal cells have a cell membrane but do not have a cell wall or chloroplasts.
Plant cells have a cell membrane, cell wall, chloroplasts and large permanent vacuole.
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4. Movement of Substances
Cells need substances to enter and leave them.
Examples of substances that move across cell membranes include:
- Oxygen
- Carbon dioxide
- Water
- Mineral ions
- Glucose
Movement across cell membranes occurs mainly by:
1. Diffusion
2. Osmosis
3. Active transport
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5. Transport Across Cells
Diffusion
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.
In simple terms:
High concentration → Low concentration
genui{"learning_viz":{"type_id":"DIFFUSION"}}Important points
- Diffusion does not require energy from respiration.
- It is a passive process.
- It continues until particles become evenly distributed.
- The difference in concentration is called a concentration gradient.
Examples
- Oxygen diffuses from the air in the lungs into the blood.
- Carbon dioxide diffuses from the blood into the lungs.
- Carbon dioxide diffuses into leaves through stomata.
Factors affecting diffusion
Diffusion happens faster when:
- The concentration gradient is larger.
- The temperature is higher.
- The surface area is larger.
- The distance substances must travel is shorter.
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6. Osmosis
Osmosis is the net movement of water molecules from a region of higher water concentration to a region of lower water concentration, through a partially/selectively permeable membrane.
In simple terms:
More dilute → More concentrated
genui{"learning_viz":{"type_id":"OSMOSIS"}}Important points
- Osmosis involves water only.
- It requires a partially permeable membrane.
- It is a passive process.
- It does not require energy from respiration.
In plant cells
Water enters a plant cell:
- The cell becomes swollen and turgid.
- The cell wall prevents it from bursting.
Water leaves a plant cell:
- The cell becomes less firm.
- If a large amount of water leaves, the cell membrane pulls away from the cell wall.
- This is called plasmolysis.
In animal cells
Animal cells do not have a cell wall.
Too much water enters:
- The cell may swell and burst.
Too much water leaves:
- The cell shrinks.
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7. Active Transport
Active transport is the movement of substances from a low concentration to a high concentration, using energy.
In simple terms:
Low concentration → High concentration
Important points
- It moves substances against the concentration gradient.
- It requires energy.
- The energy comes from respiration.
- It uses proteins in the cell membrane to move substances.
Example: Root hair cells
Root hair cells absorb mineral ions from the soil.
Sometimes the concentration of mineral ions is higher inside the root cell than in the soil.
Therefore, the ions must move:
Low concentration → High concentration
Active transport allows the plant to absorb the mineral ions despite the concentration gradient.
Another example
The small intestine can use active transport to absorb some nutrients when their concentration in the intestine is lower than inside the cells.
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8. Diffusion vs Osmosis vs Active Transport
Feature Diffusion Osmosis Active Transport
What moves? Particles/substances Water molecules Substances/ions
Direction High → Low concentration High water concentration → Low water concentration Low → High concentration
Energy needed? No No Yes
Membrane needed? Not always Yes Yes
Passive or active? Passive Passive Active
Easy way to remember
Diffusion:
High → Low
Osmosis:
Water moves through a partially permeable membrane.
Active transport:
Low → High + Energy
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9. Levels of Organization
Multicellular organisms are organised into different levels:
Cell → Tissue → Organ → Organ System → Organism
Cell
The basic unit of life.
Example:
- Muscle cell
Tissue
A group of similar cells working together to perform a particular function.
Examples:
- Muscle tissue
- Epithelial tissue
Organ
A structure made of different tissues working together.
Examples:
- Heart
- Stomach
- Kidney
- Leaf
Organ system
A group of organs working together to perform a major function.
Examples:
- Digestive system
- Respiratory system
- Circulatory system
Organism
A complete living thing.
Examples:
- Human
- Dog
- Mango tree
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Quick Summary to Memorize
Cell specialization: Cells become adapted to perform specific functions.
Cell membrane: Controls movement of substances into and out of the cell.
Cell wall: Supports and strengthens plant cells.
Chloroplast: Site of photosynthesis.
Mitochondria: Releases energy through respiration.
Nucleus: Controls cell activities and contains DNA.
Diffusion: High concentration → Low concentration.
Osmosis: Water moves from higher water concentration to lower water concentration through a partially permeable membrane.
Active transport: Low concentration → High concentration using energy.
Levels of organization:
Cell → Tissue → Organ → Organ System → Organism